VAPOCHROMIC COORDINATION POLYMERS FOR USE IN ANALYTE DETECTION
Vapochromic coordination polymers useful for analyte detection are provided. The vapochromism may be observed by visible color changes, changes in luminescence, and/or spectroscopic changes in the infrared (IR) signature. One or more of the above chromatic changes may be relied upon to identify a specific analyte, such as a volatile organic compound or a gas. The chromatic changes may be reversible to allow for successive analysis of different analytes. The polymer has the general formula MW[M′X(Z)Y]N wherein M and M′ are the same or different metals capable of forming a coordinate complex with the Z moiety; Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides; W is between 1-6; X and Y are between 1-9; and N is between 1-5. One embodiment provides [Metal(CN)2]-based coordination polymers with vapochromic properties, such as Cu[Au(CN)2]2 and Zn[Au(CN)2]2 polymers.
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This application is a continuation of U.S. patent application Ser. No. 12/169,406 filed 8 Jul. 2008 which is a continuation-in-part of U.S. patent application Ser. No. 11/577,299 filed 17 Oct. 2005, now U.S. Pat. No. 8,008,090, which claims the benefit of U.S. provisional patent application Ser. No. 60/618,573 filed 15 Oct. 2004. The disclosure of each of the previously referenced patent applications and patent is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTIONThis application relates to coordination polymers having vapochromic properties useful for analyte detection.
BACKGROUND OF THE INVENTIONThe controlled design and synthesis of metal-organic coordination polymers from the self-assembly of simple molecular building blocks is of intense interest due to the promise of generating functional materials.1,2 Vapochromic materials, which display optical absorption or luminescence changes upon exposure to vapors of analytes, such as volatile organic compounds (VOCs), have been a focus of attention due to their potential applications as chemical sensors.3-9 For example, when exposed to certain organic solvents, the extended Prussian Blue Co2+—[Re6Q8(CN)6]4− (Q=S, Se) system yields dramatic changes in the visible spectrum that are attributable to the sensed solvent impacting the geometry and hydration around the CoII centers.9
Several vapochromic compounds based on AuI, PdII, and PtII coordination polymers have also been reported.3-8 The vapochromism in these systems is based on changes in both the visible absorption and emission spectra. In the linear {(T1[Au(C6Cl5)2]}n polymer, weak interactions between the T1 atoms and the adsorbed VOC molecules modify slightly the color, and more significantly the emission spectra.6 On the other hand, changes in the emission spectra of [Pt(CN—R)4][M(CN)4] (R=iso-C3H7 or C6H4—CnH2n+1; n=6, 10, 12, 14 and M=Pt, Pd) occur when metal-metal distances are modified due to the presence of VOC molecules in lattice voids; small changes in the absorption spectrum can also be observed.7,10 Another example is the trinuclear AuI complex with carbeniate bridging ligands, for which its luminescence is quenched in the solid-state when C6F6 vapor is adsorbed due to the disruption of Au—Au interactions.5
Some of these vapochromic materials have recently been incorporated in chemical sensor devices. For example, [Au—(PPh2C(CSSAuC6F5)PPh2Me)2][ClO4] has been used in the development of an optical fiber volatile organic compound sensor.11 A vapochromic light emitting diode12 and a vapochromic photodiode13 have also been built using tetrakis(p-dodecylphenylisocyano) platinum tetranitroplatinate and bis(cyanide)-bis(p-dodecylphenylisocyanide)platinum(II), respectively.
In these previous discoveries, slight shifts in the νCN stretch are observed if hydrogen-bonding between the N(cyano) atoms and the VOC molecules present in the lattice occurs. Importantly, VOCs cannot be readily differentiated or identified via IR spectroscopy in this case since νCN shifts of only 0-10 cm−1 are usually observed.11,14,15
To overcome the shortcomings of the prior art, the need has arisen for coordination polymers having improved vapochromic properties for enhancing the sensitivity of analyte detection. The IR signatures achieved by the present invention are unusually diagnostic for a particular analyte, both in the number and position of the IR bands. In the case of some gases, the adsorption of the analyte to the polymer substantially enhances the IR response. That is, the response in the νCN or other pertinent region of the spectrum is extremely strong compared to the direct IR-signature of some gases, which is the current state-of-the-art in gas sensors. Moreover, in the present invention the vapochromism of polymers can be readily and reversibly observed both by multiple means, such as visible colour changes and luminescence changes in addition to IR spectroscopic changes.
SUMMARY OF THE INVENTIONIn accordance with the invention, a vapochromic polymer is described having the general formula MW[M′X(Z)Y]N wherein M and M′ are the same or different metals capable of forming a coordinate complex with the Z moiety; Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides; W is between 1-6; X and Y are between 1-9; and N is between 1-5. For example, in one embodiment W and X are 1 and Y and N are 2.
The vapochromic properties of the polymer change when the polymer is exposed to different analytes. The polymer may therefore be used for analyte detection. The vapochromism may be observed by visible color changes, changes in luminescence, and/or spectroscopic changes in the infrared IR signature. One or more of the above chromatic changes may be relied upon to identify a specific analyte, such as a volatile organic compound or a gas. The chromatic changes may be reversible to allow for successive analysis of different analytes using the same polymer.
In alternative embodiments of the invention M may be a transition metal, such as Cu and Zn, M′ may be a metal such as Au, Ag, Hg and Cu, and Z may be a pseuodohalide, such as CN, SCN, SeCN, TeCN, OCN, CNO and NNN. Optionally, an organic ligand may be bound to M. In one particular embodiment a new class of [Metal(CN)2]-based coordination polymers with vapochromic properties is described, such as Cu[Au(CN)2]2 and Zn[Au(CN)2]2 polymers.
In drawings which describe embodiments of the invention but which should not be construed as restricting the spirit or scope of the invention in any way,
Throughout the following description specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
This application relates to vapochromic polymers useful for detection of analytes. The polymers have the general formula MW[M′X(Z)Y]N wherein M and M′ are the same or different metals capable of forming a coordinate complex in conjunction with the Z moiety; Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides; W is between 1-6; X and Y are between 1-9; and N is between 1-5. As will be apparent to a person skilled in the art and as described herein, the vapochromic polymers of the invention may also comprise other constituents including ligands, counterbalancing ions and other metals. The invention encompasses polymers having the same empirical formula as set out above which exhibit vapochromic properties.
As described below, the vapochromism of the polymers may be observed by (1) visible changes, such as changes in colour or luminescence upon exposure to analytes, or by (2) infrared (IR) spectroscopic changes. The invention thus provides two means or “channels” to thereby achieve highly sensitive analyte detection. As used in this patent application the term “vapochromic” refers to a material that has a spectroscopic property change upon exposure to a liquid or the vapour of a volatile liquid or gas and the term vapochromism refers to such a spectroscopic property change. The spectroscopic property may include any wavelength of light including microwaves, infrared, visible colour and luminescence. As used in this patent application the process of “detecting chromatic changes” includes detecting a spectroscopic property change, including both visible and non-visible changes resulting from exposure to an analyte.
As exemplified by the examples described below, in some embodiments of the invention M is a transition metal such as copper (Cu) or zinc (Zn) and M′ is a metal such as gold (Au), silver (Ag), mercury (Hg) or copper. The Z moiety may be an ion or anionic ligand. Suitable Z moieties include pseudohalide ions such as CN—. As will be apparent to a person skilled in the art, other suitable pseudohaldides include SCN, SeCN, TeCN, OCN, CNO and NNN. In particular embodiments of the invention Cu[Au(CN)2]2 and Zn[Au(CN)2]2 polymers are described. The Cu[Au(CN)2]2 embodiment takes advantage of the unique chemical properties of gold (I) and copper (II) ions, such as attractive gold-gold interactions and luminescence for gold and a flexible coordination sphere for copper. The attractive interactions enable the formation of chemically stable, high-dimensionality materials and the gold-luminescence, cyanide-IR and copper(II) visible spectrum can all act as simultaneous sensory outputs. Similarly, with respect to the Zn[Au(CN)2]2 embodiment, distinctive luminescence and other photochromic qualities are exhibited.
In other embodiments of the invention the metal M may be a 1st row transition metal other than Cu or Zn, such as Sc, Ti, V, Cr, Mn, Fe, Co, or Ni, or some other transition-metal such as Zr, Nb or Ru. M may also be a lanthanide. Although the Mn (water)16, Fe (with K-salt)16, Co (none, with K-salt17,18, and DMF19), Zn (none)20 and a few lanthanides (Gd, Eu, Yb—all with no ligands)21,22-24 complexes are known in the prior art (ligands shown in brackets), no sensor or vapochromic properties for such complexes have been previously described.
Optionally, an organic ligand may be bound to M. The ligand may be any ligand capable of capping the metal cation, and may include nitrogen, oxygen, sulfur or phosphorus donors.
Depending upon the resultant charge of the MW[M′X(Z)Y]N structure, a charge-balancing ion, either a cation or anion, may also be present. For example, a charge balancing ion may be required where M′ is Hg.
In alternative embodiments of the invention the metal M′ may be selected to produce both linear metal cyanides or non-linear cyanides. For example, cyanometallate units such as [Au(CN)2]−, [Ag(CN)2]− or [Hg(CN)2] may be incorporated into polymers in conjunction with different transition metal cations and supporting ligands according to the following general equation:25-27,28-33
[METAL cation+organic ligandz]n++[M′(CN)x]y→[METAL(ligand)z][M′(CN)x]n
-
- (n=1-5, x=2-9, y=−5 to 0, z=0-9)
The synthesis may be readily accomplished in solvents such as water or alcohols. Compared to prior art approaches, the polymers and polymer-analyte compositions of the present invention can be prepared from extremely simple commercially available starting materials in minimal steps. As described in the Examples section below, the synthetic methodology, which has built-in design flexibility, low-cost and simple synthesis, is also a general advantage of coordination polymer systems over current zeolitic technology. The system is modular in that the metal cation and organic ligand can be chosen as desired to target a particular application or property.
An important advantage of embodiments of the invention described herein is that the vapochromic properties of the polymers may be reversible. For example, the Cu[Au(CN)2]2 embodiment shows reversible vapochromic sensor behaviour attributable to the Cu—Au pairing.34 Starting with a solid of any Cu[Au(CN)2]2(solvent)x, addition of a different solvent vapour (analytes) generates a new complex. As described below, exceptions may apply in the case of very strong donor solvents such as pyridine or ammonia, which bind strongly to the CuII center and are not easily displaced by other solvents. Similarly, the Zn[Au(CN)2]2 embodiment exhibits reversible vapoluminescent material qualities.35 The polymers of the invention may thus be employed in a dynamic system for successively detecting different analytes without the need for reinitialization (although reinitialization may still be required to repeatedly detect the same analyte).
The invention may be used for detecting a wide variety of analytes including volatile organic compounds (VOCs) and gases. The solid polymers adsorb (i.e. bind or trap) analyte, such as organic solvents, exposed to the polymers in a vapour (or liquid) phase. The detectable VOCs typically include a hetero (non-carbon) atom donor such as hydrogen, nitrogen, oxygen, sulfur and phosphorus donors. Examples of solvent vapours that will effectively adsorb to the polymers of the invention include pyridine, dioxane, water, ethanethiol and trimethylphosphine. Donor gases such as H2S and ammonia also readily bind and are detectable. The binding capacity and sensitivity of the polymers may be adjusted through altering the identity of the metals M and M′ to enable detection of a range of gases, including but not limited to NOx, SOx, COx and alkenes. For example, the zinc-based polymer described herein appears to bind CO2 and may have applications as a CO2-sensor.
As will be appreciated by a person skilled in the art, the polymers of the invention may find application in wide range of industrial and commercial applications, such as in the chemical, energy and environmental sectors. The polymers may be used in many different solid forms depending upon the vapochromic application, such as powders, crystals, thin films or combinations thereof. Exemplary industrial applications include: personal and badge monitors in chemical laboratories (e.g. industrial chemical or pharmaceutical research laboratories, paint and coatings manufacturing, cosmetics manufacturing) for hazardous vapour detection; portable or stationary threshold monitors for chemical vapours in laboratory environments or chemical storage facilities for hazardous vapour detection or regulated emission requirements; environmental sensor for volatile organic compounds or gases (“electronic noses”) for use at environmental remediation sites, landfills, air-quality monitoring etc.; and responsive coatings, art supplies, colour-changing paint and other related applications where a colour-changing material is desired.
As will be apparent to a person skilled in the art, the vapochromic polymers described herein may be deployed in various different forms and applications for specifically detecting ammonia. For example, the polymers may be used in medical applications for sensing ammonia in the breath of patients. In one embodiment, a polymer may be embedded in a paper strip, similar to litmus paper, or onto a binding agent such as silica, which a patient would be instructed to breathe on. Many medical conditions, such as ulcers, kidney disease and liver disease, are associated with abnormally high (but still low in an absolute sense) levels of ammonia in the breath of some affected patients. Many other potential applications for detecting low levels of ammonia may also be envisioned.
Although the present invention has been principally described in relation to analyte sensing and detection, the polymers and compositions described herein may be useful for other purposes such as extraction, purification and storage applications.
EXAMPLESThe following examples will further illustrate the invention in greater detail although it will be appreciated that the invention is not limited to the specific examples.
The following description of experimental details and experimental results is presented in multiple parts. Example 1.0 describes synthetic procedures and experimental results for the Cu[Au(CN)2]2(solvent)x system. Example 2.0 describes a similar synthetic procedure and experimental results for an analogous Zn[Au(CN)2]2(solvent)x system.
Example 1.0 Cu-Based Polymers 1.1 Cu[Au(CN)2]2(Solvent)x System 1.1.1 Experimental Apparatus and General ProcedureGeneral Procedure and Physical Measurements. All manipulations were performed in air. All the reagents were obtained from commercial sources and used as received. Infrared spectra were recorded as KBr pressed pellets on a Thermo Nicolet Nexus 670 FT-IR spectrometer. Microanalyses (C, H, N) were performed at Simon Fraser University. Magnetic susceptibilities were measured on polycrystalline samples at 1 T between 2 and 300 K using a Quantum Design MPMS-5S SQUID magnetometer. All data were corrected for temperature independent paramagnetism (TIP), the diamagnetism of the sample holder, and the constituent atoms (by use of Pascal constants).36 Solid-state UV-visible reflectance spectra were measured using an Ocean Optics SD2000 spectrophotometer equipped with a tungsten halogen lamp. Thermogravimetric analysis (TGA) data were collected using a Shimadzu TGA-50 instrument in an air atmosphere.
1.1.2 Synthetic ProceduresSynthesis of Cu[Au(CN)2]2(DMSO)2, 1: A 0.5 mL dimethylsulfoxide (DMSO) solution of Cu(ClO4)2.6H2O (0.037 g, 0.1 mmol) was added to a 0.5 mL DMSO solution of KAu(CN)2 (0.057 g, 0.2 mmol). Green crystals of Cu[Au(CN)2]2(DMSO)2 were obtained by slow evaporation over several days, filtered and air-dried. Yield: 0.050 g, 70%. Anal. Calcd. for C8H12N4Au2CuO2S2: C, 13.39; H, 1.69; N, 7.81. Found: C, 13.43; H, 1.72; N, 7.61. IR (KBr): 3005(w), 2915(w), 2184(s), 2151(m), 1630(w), 1426(w), 1408(w), 1321(w), 1031(m), 993(s), 967(m), 720(w), 473(m) cm−1. The same product can be obtained by absorption of DMSO by Cu[Au(CN)2]2(H2O)2.
Synthesis of Cu[Au(CN)2]2(DMSO)2, 2: A 0.2 mL DMSO solution of Cu(ClO4)2.6H2O (0.037 g, 0.1 mmol) was added to a 0.4 mL DMSO solution of KAu(CN)2 (0.057 g, 0.2 mmol). Blue needles of Cu[Au(CN)2]2(DMSO)2 formed after one hour and were filtered and dried under N2. Yield: 0.057 g, 80%. Anal. Calcd. for C8H12N4Au2CuO2S2: C, 13.39; H, 1.69; N, 7.81. Found: C, 13.50; H, 1.76; N, 7.62. IR (KBr): 3010(w), 2918(w), 2206(m), 2194(s), 2176(m), 2162(m), 1631(w), 1407(w), 1316(w), 1299(w), 1022(m), 991(s), 953(m), 716(w), 458(m) cm−1.
Synthesis of Cu[Au(CN)2]2(DMF), 3: A 2 mL N,N-dimethylformamide (DMF) solution of Cu(ClO4)2.6H2O (0.037 g, 0.1 mmol) was prepared. This solution was added to a 3 mL DMF solution of KAu(CN)2 (0.057 g, 0.2 mmol). A dark blue-green mixture of powder and crystals of Cu[Au(CN)2]2(DMF) was obtained after several days of slow evaporation and was filtered and air-dried. Yield: 0.033 g, 52%. Anal. Calcd for C7H7N5Au2CuO: C, 13.25; H, 1.11; N, 11.04. Found: C, 13.26; H, 1.11; N, 11.30. IR (KBr): 2927(w), 2871(w), 2199(s), 2171(shoulder), 1665(s), 1660(s), 1492(w), 1434(w), 1414(w), 1384(m), 1251(w), 1105(w), 674(w), 516(w), 408(w) cm−1. Single crystals of 3 were obtained by dissolving Cu[Au(CN)2]2(H2O)2 (5) in DMF and allowing the solution to evaporate very slowly. The single crystals and the crystal/powder mixture as prepared above had identical IR spectra. The same product can also be obtained by vapour absorption of DMF by several Cu[Au(CN)2]2(solvent)x complexes.
Synthesis of Cu[Au(CN)2]2(pyridine)2, 4: A 10 mL pyridine/water/methanol (5:47.5:47.5) solution of Cu(ClO4)2.6H2O (0.111 g, 0.3 mmol) was prepared. This solution was added to a 10 mL pyridine/water/methanol (5:47.5:47.5) solution of KAu(CN)2 (0.171 g, 0.59 mmol). A blue powder of Cu[Au(CN)2]2(pyridine)2 was obtained immediately and was filtered and air-dried. Yield: 0.163 g, 75%. Anal. Calcd for C14H10N6Au2Cu: C, 23.36; H, 1.40; N, 11.68. Found: C, 23.52; H, 1.44; N, 11.58. IR (KBr): 3116(w), 3080(w), 2179(s), 2167(s), 2152(s), 2144(m), 1607(m), 1449(m), 1445(s), 1214(m), 1160(w), 1071(m), 1044(w), 1019(m), 758(s), 690(s), 642(m) cm−1. Single crystals of 4 were obtained by slow evaporation of the remaining solution. The crystals and powder had identical IR spectra. The same product can also be obtained by vapour absorption of pyridine by several Cu[Au(CN)2]2(solvent)x complexes.
Synthesis of Cu[Au(CN)2]2(H2O)2, 5: A 10 mL aqueous solution of Cu(ClO4)2.6H2O (0.259 g, 0.7 mmol) was prepared and added to a 10 mL aqueous solution of KAu(CN)2 (0.403 g, 1.4 mmol). A pale green powder of Cu[Au(CN)2]2(H2O)2 formed immediately and was filtered and air-dried. Yield: 0.380 g, 91%. The same product can be obtained by vapour absorption of water by several Cu[Au(CN)2]2(solvent)x complexes. Anal. Calcd for C4H4N4Au2CuO2: C, 8.04, H, 0.67, N, 9.38. Found: C, 8.18; H, 0.71; N, 9.22. IR (KBr): 3246(m), 2217(s), 2194(vw), 2171(s), 1633(w) cm−1.
Synthesis of Cu[Au(CN)2]2, 6: Cu[Au(CN)2]2(H2O)2 was heated (150° C.) in vacuo to yield green-brown Cu[Au(CN)2]2. The yield is quantitative, with no νCN peaks for hydrated 5 observable. Anal. Calcd for C4N4Au2Cu: C, 8.56, H, 0, N, 9.98. Found: C, 8.68, H, trace, N, 9.80. IR (KBr): 2191(s), 1613(vw), 530(m) cm−1.
Synthesis of Cu[Au(CN)2]2(CH3CN)2, 7: A 1 mL CH3CN solution of Cu(ClO4)2.6H2O (0.037 g, 0.1 mmol) was prepared and added to a 2 mL CH3CN solution of KAu(CN)2 (0.057 g, 0.2 mmol). A green powder of Cu[Au(CN)2]2(CH3CN)2 precipitated immediately along with a white powder of KClO4. To prevent the replacement of CH3CN by atmospheric water, the solvent was removed under vacuum and the KClO4 side product was not removed through washing and filtering. Anal. Calcd for Cu[Au(CN)2]2(CH3CN)2+2(KClO4) (C8H6N6Au2Cl2CuK2O8): C, 10.44; H, 0.65; N, 9.12. Found: C, 10.99; H, 0.57; N, 8.69. IR (KBr): 2297(w), 2269(w), 2192(s), 1600(w), 1445(w), 1369(w), 1088(s), 941(w), 925(w), 752(w), 695(w), 626(m), 512(w), 468(w), 419(w) cm−1. The same product (without KClO4) can be obtained by vapour absorption of acetonitrile by Cu[Au(CN)2]2(DMSO)2 (1 or 2).
Synthesis of Cu[Au(CN)2]2(dioxane)(H2O), 8: A 2 mL dioxane/water (2:1) solution of Cu(ClO4)2.6H2O (0.037 g, 0.1 mmol) was prepared. This solution was added to a 4 mL dioxane/water (2:1) solution of KAu(CN)2 (0.057 g, 0.2 mmol). A pale blue-green powder of Cu[Au(CN)2]2(dioxane)(H2O) was obtained immediately and was filtered and air-dried. Yield: 0.057 g, 85%. The same product can be obtained by vapour absorption of dioxane by several Cu[Au(CN)2]2(solvent)x complexes (the water molecule included in this case is from ambient moisture). Anal. Calcd for C8H10N4Au2CuO3: C, 14.39; H, 1.51; N, 8.39. Found: C, 14.31; H, 1.21; N, 8.43. IR (KBr): 2976(m), 2917(m), 2890(w), 2862(m), 2752(w), 2695(w), 2201(s), 2172(w), 1451(m), 1367(m), 1293(w), 1255(s), 1115(s), 1081(s), 1043(m), 949(w), 892(m), 871(s), 705(w), 610(m), 515(m), 428(m) cm−1.
Synthesis of Cu[Au(CN)2]2(NH3)4, 9: This product was obtained by vapour absorption of NH3 by several Cu[Au(CN)2]2(solvent)x complexes. The yield is quantitative as shown by IR. Anal. Calcd for C4H12N8Au2Cu: C, 7.63, H, 1.92, N, 17.80, found: C, 7.56; H, 1.98; N, 17.71. IR (KBr): 3359(s), 3328(s), 3271(s), 3212(m), 3182(m), 2175(m), 2148(s), 1639(m), 1606(m), 1243(s), 685(s), 435(w) cm−1.
1.1.3 X-ray Crystallographic AnalysisX-Ray Crystallographic Analysis. Cu[Au(CN)2]2(DMSO)2 1 and 2, Cu[Au(CN)2]2(DMF) 3 and Cu[Au(CN)2]2(pyridine)2 4: Crystallographic data for all structures are collected in Table 1. Crystals 1, 3 and 4 were mounted on glass fibers using epoxy adhesive and crystal 2 was sealed in a glass capillary. Crystal 1 was a green rectangular plate (0.09×0.12×0.3 mm3), crystal 2 was a pale blue needle (0.11×0.11×0.2 mm3), crystal 3 was a green needle (0.09×0.09×0.15 mm3) and crystal 4 was a dark blue platelet (0.02×0.06×0.15 mm3).
For 1, data in the range 4°<2θ<55° were recorded using the diffractometer control program DIFRAC37 and an Enraf Nonius CAD4F diffractometer. The NRCVAX Crystal Structure System was used to perform psi-scan absorption correction (transmission range: 0.0301-0.1726) and data reduction, including Lorentz and polarization corrections.38 All non-hydrogen atoms were refined anisotropically. Full matrix least-squares refinement (1231 reflections included) on F (93 parameters) converged to R1=0.042, wR2=0.047 (Io>2.5σ(Io)).
For 2, 3 and 4, data in the ranges 6.9°<2θ<136.1°, 9.2°<2θ<144.0° and 12.0°<2θ<142.6° respectively were recorded on a Rigaku RAXIS RAPID imaging plate area detector. A numerical absorption correction was applied (transmission range: 0.019-0.161, 0.0070-0.0199 and 0.3484-0.5826) and the data were corrected for Lorentz and polarization effects.39 For 2, the Au, Cu and S atoms were refined anisotropically, while the remainders were refined isotropically. For 3 and 4, all non-hydrogen atoms were refined anisotropically. Full matrix least-squares refinement on F was performed on 2, 3 and 4, the data converging to the following results: for 2, R1=0.062, wR2=0.082 (Io>3.0σ(Io), 2026 reflections included, 205 parameters); for 3, R1=0.0315, wR2=0.0456 (Io>3.0σ(Io), 1538 reflections included, 148 parameters); for 4, R1=0.0276, wR2=0.0401 (Io>3.0σ(Io), 1021 reflections included, 107 parameters).
All structures were refined using CRYSTALS.4° The structures were solved using Sir 92 and expanded using Fourier techniques. Hydrogen atoms were included geometrically in all structures but not refined. Diagrams were made using Ortep-3 (version 1.076)41 and POV-Ray (version 3.6.0)42. Selected bond length and angles for 1-4 are reported in Tables 2 to 5 respectively.
1.1.4 ResultsSynthesis. The reaction of CuII salts with KAu(CN)2 in dimethylsulfoxide (DMSO) produced two different compounds, depending on the total concentration of starting reagents. In dilute solution, green crystals of polymorph 1 formed slowly, whereas blue crystals of polymorph 2 were obtained rapidly in a highly concentrated solution. The IR spectra of 1 and 2 show different features (Table 6); the higher-energy bands likely correspond to bridging CN-groups, while the lower-energy bands are due to either free or loosely bound CN-groups.43 The X-ray crystal structures of 1 and 2 revealed two different polymeric networks, both with the same empirical formula Cu[Au(CN)2]2(DMSO)2, as confirmed by elemental analysis.
Crystal Structure of the Green Cu[Au(CN)2]2(DMSO)2 Polymorph, 1. The five-coordinate CuII center in 1 has a τ-value44 of 0.44, where τ=0 is pure square pyramidal and τ=1 is pure trigonal bipyramidal, suggesting that the coordination geometry could be considered equally distorted from either polyhedron. The CuII center is bound to two DMSO-O atoms (O—Cu—O=167.06°) and three N(cyano) atoms (
Crystal Structure of the Blue Cu[Au(CN)2]2(DMSO)2 Polymorph, 2. The structure of polymorph 2 contains CuII centers in a Jahn-Teller distorted octahedral geometry, with the two DMSO molecules bound in a cis-equatorial fashion (O—Cu—O=95.2°) rather than in the nearly 180°-arrangement in 1. Selected bond lengths and angles for 2 are found in Table 3. The four remaining sites (two axial and two equatorial) are occupied by N(cyano) atoms of bridging [Au(CN)2]− units, generating corrugated 2-D sheets (
Magnetic Properties. As polymorphs 1 and 2 clearly have significantly different solid-state structures, it follows that their physical and chemical properties may also vary; this is obviously the case for their solid-state optical reflectance spectra, which show λmax of 550±7 and 535±15 nm respectively (Table 6). To explore this key issue, a series of representative properties were investigated. For example, the magnetic susceptibilities of 1 and 2 were measured at temperatures varying from 300 to 2 K. At 300 K, μeff=1.98 and 1.93μB for 1 and 2 respectively, typical for CuII centers.46 As the temperature drops, μeff decreases and reaches 1.74 and 1.67μB at 2 K for 1 and 2 respectively. There is no maximum in either χM vs T plot. This behaviour is consistent with weak antiferromagnetic coupling, probably mediated by the diamagnetic AuI center.25-27,47 Thus, the two polymorphs have similar magnetic properties.
Thermal stability. Examining the thermal stabilities of 1 and 2 by thermogravimetric analysis (
Vapochromic Behavior. Interestingly, even though both polymorphs are thermally stable up to at least 100° C., the DMSO molecules can easily be replaced by ambient water vapour at room temperature to yield Cu[Au(CN)2]2(H2O)2 (5), as shown by elemental and thermogravimetric analysis. Despite the fact that both polymorphs have different solid-state structures, IR spectroscopy and powder X-ray diffraction show that both polymorphs convert to the same Cu[Au(CN)2]2(H2O)2 (5) complex (Table 6). This conversion is reversible. However, if DMSO vapour is added back to 5, only the green polymorph Cu[Au(CN)2]2(DMSO)2 (1) is formed, even if the original DMSO-complex to which H2O was added was the blue polymorph (2). The exchange of DMSO for H2O can be observed visually from the associated colour change (
Cu[Au(CN)2]2(DMSO)2 (either 1 or 2) also displays vapochromic behaviour when exposed to a variety of other donor solvent vapours (i.e. analytes) in addition to H2O. Each Cu[Au(CN)2]2(solvent)x complex can be distinguished easily by its colour (
Importantly, this solvent exchange is completely reversible, thus permitting dynamic solvent sensing. As indicated in the above synthetic examples, starting with a solid of Cu[Au(CN)2]2(solvent)x, addition of a different solvent vapour generates a new complex. The only exceptions occur in the case of very strong donor solvents such as pyridine or ammonia, which bind strongly to the CuII center and are not easily displaced by other solvents.
Each Cu[Au(CN)2]2(solvent)x complex was also synthesized by reacting Cu(II) salts with [Au(CN)2]− in the appropriate solvent and each was found, by elemental analysis, IR spectroscopy, TGA, and crystallography, to be identical to the complex generated by solvent exchange. In every case, elemental analysis and TGA (Table 7) indicate that the number of solvent molecules incorporated into the complex per transition metal center is always the same as the number incorporated by vapour adsorption. This is easily rationalized by the fact that all adsorbed solvent molecules are ligated to the CuII center in a 1:1, 1:2 or, in the case of ammonia, a 1:4 ratio, with no additional loosely trapped solvent molecules in channels (as shown by TGA, Table 7), as is often observed in other porous systems that include solvent.49-52
Crystal Structure of Cu[Au(CN)2]2(DMF), 3. In order to better understand the structural changes that occur during a vapochromic response of the DMSO polymorphs, the structures of Cu[Au(CN)2]2(DMF) (3) and Cu[Au(CN)2]2(pyridine)2 (4) were investigated. The structure of 3 contains CuII centers with a square-pyramidal geometry, where the four basal sites are occupied by N(cyano) atoms of bridging [Au(CN)2]− units and the apical site is occupied by an O-bound DMF molecule. Selected bond lengths and angles for 3 are listed in Table 4. The alternation of CuII centers and [Au(CN)2]− units generates a 2-D square grid motif with all the DMF molecules pointing either above or below the plane of the sheet (
Crystal Structure of Cu[Au(CN)2]2(pyridine)2, 4. The structure of 4 is similar to that of 3, except that the CuII centers are surrounded by two solvent molecules, generating octahedrally coordinated metals. The axial sites and two of the equatorial sites are occupied by N(cyano) atoms of bridging [Au(CN)2]− units. Pyridine molecules occupy the two other equatorial sites. Selected bond lengths and angles for 4 are listed in Table 5. As observed for 3, infinite 2-D layers are obtained (
Solvent free Cu[Au(CN)2]2, 6. The green-brown solvent-free complex, Cu[Au(CN)2]2 (6), was also prepared by thermally removing in vacuo the water molecules from 5. Changes in the powder X-ray diffractogram and in the νCN peaks of 6 indicate that some rearrangement in the framework occurred. The IR spectrum only shows one stretching frequency (2191 cm−1), indicating that all CN groups are in a similar environment, reminiscent of the Cu[Au(CN)2]2(DMF) structure. This is also comparable with the results published for the Mn[Au(CN)2]2(H2O)216 and the Co[Au(CN)2]2(DMF)219 systems (which show stretches at 2150 and 2179 cm−1 respectively). In these two coordination polymers, the M[Au(CN)2]2 unit (M=Mn or Co) forms 2-D square grids, with solvent molecules hanging above and below the plane of the sheet. Although the three-dimensional topology of Cu[Au(CN)2]2 is not known, it likely forms a similar 2-D square grid network with all N(cyano) atoms equatorially bound to a square planar CuII center (
Concentration-controlled synthesis of structural isomers of coordination polymers Results obtained by X-ray crystallography and elemental analysis indicate that 1 and 2 of this Example are true polymorphs or supramolecular isomers, as opposed to pseudopolymorphs that differ by incorporation of varying amounts or identities of co-crystallized solvent molecules.53,54 As mentioned above, many factors contribute to the preferential formation of one polymorph over another and it can often be a challenge to control the synthesis of a desired isomer.53-57 Varying crystallization conditions, such as solvent type, starting materials, temperature and concentration are often important to ensure generation of just one polymorph. For example, crystallizing Ni[Au(CN)2]2(en)2 (en=1,2-ethylenediamine) from [Ni(en)3]Cl2.2H2O or [Ni(en)2Cl2] generates molecular and 1-dimensional polymorphic materials respectively.47 Also, it has been shown that metastable polymorphs can be obtained by rapid crystallization from a supersaturated solution, e.g., via a fast drop in temperature.56,58 For example, {Cu[N(CN)2]2(pyrazine)}n forms green/blue and blue polymorphs when crystallized from concentrated and dilute solution respectively.59
Similarly, in the Cu[Au(CN)2]2(DMSO)2 system described in this Example, if the total concentration of reagents is below 0.2 M, 1 is formed, while 2 is obtained exclusively from >0.5 M solutions. The concentration-controlled synthesis of structural isomers of coordination polymers is uncommon relative to examples with molecular systems.59,60 This concentration dependence suggests that green 1 is the thermodynamic product, while blue 2, which rapidly precipitates from solution, is likely a kinetic product. The fact that Cu[Au(CN)2]2(H2O)2 converts exclusively to the green polymorph 1 when adsorbing DMSO is further evidence that 1 is the most energetically favorable polymorph. Interestingly, the density of thermodynamically preferred 1 is actually lower than that of 2. This surprising situation has been observed in other polymorphs.61 Although it is unclear if this result can be attributed to entropic or enthalpic contributions, it is conceivable that the formation of shorter Au—Au bonds in 1 relative to 2 could be an important energetic factor.
Metal-ligand superstructures It has been recognized that a system does not need to be porous in order to undergo guest uptake.62 For example, a flexible metal-ligand superstructure can dynamically adapt in order to accommodate a variety of potential guests.62-68 In this light, the Jahn-Teller influenced flexible coordination sphere and the greater lability of CuII compared with other transition metals are likely important features of the Cu[Au(CN)2]2(solvent)x system. The related Mn[Au(CN)2]2(H2O)2 and Co[Au(CN)2]2(DMF)2 systems previously reported form more rigid frameworks.16,19 For these two systems, thermal treatment is required to remove the guest molecules and yield compounds exhibiting zeolitic properties. The lability of CuII in the system of the present invention facilitates the reversible exchange of adsorbed solvent molecules without any thermal treatment required. It also likely increases the flexibility of the framework by allowing the breaking and the reformation of Cu—N(cyano) bonds, thereby adapting to the solvent guest present. Gold-gold interactions are probably present in all the Cu[Au(CN)2]2(solvent)x complexes and help to stabilize the 3D-network as solvent exchange takes place.
Taking into account the varied structures of the Cu[Au(CN)2]2(solvent)x complexes, several modes of flexibility within the fundamental structural framework, i.e. the two-dimensional square-grid network of the Cu[Au(CN)2]2 moiety (
The source of the vapochromism in the Cu[Au(CN)2]2(solvent)x system differs from that of other AuI-containing systems.3-6 Cu[Au(CN)2]2(solvent)x shows vapochromism in the visible since each donor solvent molecule that is adsorbed binds to the CuII center and modifies differently the crystal field splitting. As a consequence, the colour of the vapochromic compound changes as the d-d absorption bands shift with donor. In addition to donor identity, the resulting coordination number (five or six) and specific geometry of the copper center also influences the colour of the complexes by altering the splitting of the d-orbitals.
The [Au(CN)2]− unit is also a key component of this system since it telegraphs the changes in solvent bound to the CuII centers via the νCN stretch. Each Cu[Au(CN)2]2(solvent)x has a different IR signature since every VOC modifies in a different manner the electron density distribution around the CuII center. This influences the amount of π-back bonding from the CuII center to the CN group, which in turn is observed in the IR spectrum due to the change in vibration frequency.43 Also, the number of bands observed is related to the symmetry and coordination number of the CuII centers, as described in detail above.
In summary, it has been illustrated in this Example that, despite their different solid-state structures, the two Cu[Au(CN)2]2(DMSO)2 polymorphs exhibit the same vapochromic behaviour with respect to sorption of analytes such as VOCs. The use of [Au(CN)2]− as a building block is important to the function of this vapochromic coordination polymer. First, it provides the very sensitive CN reporter group that can allow IR-identification of the solvent adsorbed in the materials. Also, Au—Au interactions via the [Au(CN)2]− units increase the structural dimensionality of the system in most cases and probably help provide stabilization points for the flexible Cu[Au(CN)2]2 framework.
1.2 Cu[Au(CN)2]—Based Sensing of Ammonia and AminesIn this example coordination polymer Cu[Au(CN)2]2(μ-H2O)2 (5) was shown to be effective in reversibly sensing ammonia and amines at low concentrations.
As indicated above, Cu[Au(CN)2]2(μ-H2O)2 5 is a green powder which, upon exposure to vapours of volatile organic compounds, exhibits visible vapochromism and changes in the νCN region of the IR spectrum. In this example, a finely ground powder of Cu[Au(CN)2]2(μ-H2O)2 (5) deposited onto a CaF2 plate was exposed to a series of solvent vapours in a range of concentrations in order to determine sensitivity levels. Use of the νCN IR signature was more sensitive than use of the colour change monitored by UV-Vis solid state reflectance for all analytes; sensitivity limits for a range of VOCs via both detection channels are reported in Table 8.
Consistent with the vapochromic mechanism, which requires that the analyte ligate to the copper(II) center, 5 was found to be insensitive to non-coordinating or very weakly coordinating classes of VOCs (with respect to copper(II)) and related analytes. A representative set of compounds (in brackets) covering a range of functional groups such as esters (ethyl acetate), ethers (tetrahydrofuran), ketones (acetone), thiols (tetrahydrothiophene), aldehydes (formaldehyde), saturated hydrocarbons (pentane), aromatic hydrocarbons (benzene), chloroalkanes (dichloromethane) and even, remarkably, concentrated acetic acid, all generated no response from 5, which remained unscathed after exposure to saturated atmospheres of the respective VOCs. Compound 5 generated vapochromic responses to weak donors such as DMSO, DMF and MeCN (Table 8), with IR-sensitivities in the range of 400-1000 ppm, while stronger nitrogen-based donor VOCs and ammonia had much higher sensitivities, from several-hundred ppm down to ppb levels.
Among a series of representative amines, 5 was found to be more sensitive to primary (1°) amines versus secondary and tertiary (2° and 3°) amines. This observation is likely attributable to the higher steric hindrance of 2° and 3° amines (or long-chain 1° amines such as butylamine) restricting their donor ability, rather than their increasing vapour pressures; the boiling point of pyridine (115° C.) and ethylenediamine (118° C.) are similar, yet their sensitivities are 400 ppm vs. 385 ppb respectively. Similarly, although the boiling point of ethylenediamine (118° C.) is significantly higher than diethylamine (55° C.), 5 was much more sensitive towards ethylenediamine. In the case of propylamine and butylamine, since their basicity and boiling points are fairly similar, the large decrease in sensitivity (140 ppb vs. 100 ppm respectively) probably reflects the limited ability of the flexible coordination polymer structure to readily adapt to accommodate longer chain alkyl groups.
Indeed, as shown in Table 8 below, 5 was found to be extremely sensitive to ammonia and low molecular-weight 1°-amines. For example, the detection limit with the naked eye (by watching the colour change) when 5 is exposed to ammonia is as low as 40 ppm; with visible reflectance spectroscopy and using the spectrum of 1 as a background, it drops to 230 ppb. By using the νCN IR spectroscopy channel, ammonia concentrations as low as 36 ppb could be detected. Similarly, propylamine could be detected down to 140 ppb.
Titration studies. Titration of 5 with consecutive equivalents of ammonia or propylamine showed that a series of intermediates were obtained en route to the final products observed upon exposure to a large excess. Monitored by IR spectroscopy, 5 interacts with ammonia and amines in a similar manner. Titration with two or less equivalents of NH3 per Cu(II) centre of 5 causes a decrease in the νCN bands attributable to 5 at 2216 and 2171 cm−1 and the formation of two new bands at 2181 and 2151 cm−1. Upon continuing the titration from two to four equivalents of ammonia, new νCN peaks at 2175 and 2148 cm−1 replaced the 2181/2151 cm−1 set. Finally, further titration (up to 35 equiv.) generated the final NH3-saturated film that exhibited a new νCN band at 2141 with a small band at 2136 cm−1. This titration data suggests that two intermediates of stoichiometry Cu[Au(CN)2]2(NH3)2 (9) and Cu[Au(CN)2]2(NH3)4 (10) are formed on the way towards the final saturated product. Note that the IR spectrum of 9 matches that observed for the low-concentration/high sensitivity tests of 5 with NH3 (Table 8). The presence of two νCN bands for all intermediates and the product also indicates that two different cyanide environments exist in the solids.
A similar trend was observed for the propylamine titration of 5: one equivalent of propylamine vapour generated new peaks at 2185 and 2155 cm−1, along with the peaks of 5 at 2216 and 2171 cm−1. Addition of a second equivalent of propylamine greatly increased the peaks at 2185 and 2155 cm−1 with a concomitant loss of the peaks for 5. A third equivalent of propylamine generated three new peaks at 2182, 2142 and 2133 cm−1 with a decrease in the intensity of the 2185/2155 cm−1 set; these peaks became dominant upon addition of a fourth equivalent. Excess propylamine generates a peak at 2143 cm−1. The propylamine titration results also suggest the presence of two intermediates, namely Cu[Au(CN)2]2(CH3CH2CH2NH2)2 (11) and Cu[Au(CN)2]2(CH3CH2CH2NH2)3 (12) prior to saturation.
These titrations were also monitored by visible reflectance spectroscopy due to the drastic colour changes from green to blue-green to deep blue that occur upon exposure of 5 to ammonia or propylamine. When 5 is exposed to one to two equivalents of ammonia the λmax shifts from 535 to 510 nm with one isosbestic point at 522 nm suggesting that there are two compounds in equilibrium, most probably the starting material and the Cu[Au(CN)2]2[NH3]2 complex. When exposed to three to six equivalents of ammonia, the λmax shifts up to 470-485 nm with another crude isosbestic point at approximately 500 nm suggesting that another set of two compounds are in equilibrium, most probably the bis- and the tetrakis-ammine coordinated complexes 9 and 10. It was observed that the maxima obtained after exposing 5 to 1 and 2 equivalents of ammonia and allowing it to equilibrate are very close and the same trend is observed when exposing it to 3 to 4 equivalents of ammonia, suggesting that there are two intermediates formed Cu[Au(CN)2]2[NH3]x (x=2, 4) before it sees an excess of ammonia, which generates a maximum at 435 nm.
When 5 was titrated with propylamine vapour, the same blue-shifting trend was observed as for the ammonia analyte. Thus, the addition of one and two equivalents of propylamine shifted the visible reflectance spectrum from 535 to 502 nm, with a clear isosbestic point at 513 nm. Titration with a third equivalent of propylamine caused a further shift to 462 nm, with a new isosbestic point at 491 nm; a fourth equivalent initiated no further shift in this case. The presence of two separate isosbestic points and the titration equivalents needed to maximize the bands (two and three equiv.) are consistent with the proposed formation of intermediates 11 and 12. Addition of an excess of propylamine (14 equivalents) shifted λmax to 428 nm, generating a dark blue compound. When saturated with ammonia or propylamine, the observed UV-Vis maxima are unique to each analyte, since the ammonia/propylamine binds to the copper(II) centre and each has a different ligand field strength; the same can be said for the other strongly-bonding analytes in Table 8.
Kinetics of vapochromic response. Kinetic studies of analyte adsorption/desorption were also performed on the coordination polymer 5 by exposing it to an excess of ammonia and propylamine vapour over a period of 2 minutes. When exposed to ammonia, one isosbestic point was observed at 2175, suggesting that two compounds were in equilibrium. The νCN stretches were recorded with respect to time after the exposure of compound 5 to ammonia and it was observed that the band of the dry coating at 2171 and 2216 cm−1 shifts to 2181 and 2151 cm−1 first, which matches the value obtained for Cu[Au(CN)2]2[NH3]2 using both IR titration studies and synthetic studies. With time peaks at 2175 and 2148 cm− are formed, which matches the value obtained for Cu[Au(CN)2]2[NH3]4 as described above. After some time, compound 5 sees an excess ammonia and hence a peak at 2141 cm−1 was observed due to the presence of free Au(CN)2−.
A stepwise blue-shifting of the visible absorption band is observed over the same time-period for ammonia adsorption. However, the presence of dynamic mixtures of adducts renders the shifting conglomerate bands difficult to assign based on their maxima; however, the end-point is the fully saturated system at 435 nm.
Desorption of NH3 analyte from the copper center is also observed when the cell is opened to air at room temperature. Kinetic studies were also performed on the desorption of ammonia over two minutes from a mixture of 10 and the fully saturated species at 2175/2148 and 2141/2136 cm−1 respectively; the bis-ammonia adduct 9 rapidly grew in at 2181/2150 cm−1, confirming the loss of two ammonia molecules. A visible colour change is also observed from blue to green with the final visible maximum at 525 nm (a mixture of 5 and 9) over 2 minutes. Note that the desorption of ammonia is very quick and hence as soon as the cell is opened loss of ammonia commences.
Analogous kinetic studies with propylamine as analyte also showed the formation of intermediates. When compound 5 was exposed to a small excess of propylamine, a shift in the IR νCN stretches of the starting material with respect to time was observed. The νCN stretches shifts from 2171 and 2216 cm−1 to 2155 and 2187 cm−1, consistent with the formation of Cu[Au(CN)2]2[CH3CH2CH2NH2]2 (11). With time the peak at 2155 cm−1 decreases and the peaks at 2182, 2140 and 2133 cm−1 increase, analogous to Cu[Au(CN)2]2[CH3CH2CH2NH2]3 (12).
Desorption of propylamine from the copper centre was also observed when the cell containing compound 5 and propylamine vapour was opened to air at room temperature. The IR νCN stretches for Cu[Au(CN)2]2[CH3CH2CH2NH2]3 12 moved from 2182, 2140 and 2133 cm−1 to 2155 and 2187 cm−1, analogous to Cu[Au(CN)2]2[CH3CH2CH2NH2]2 11, as analysed by IR titration and synthetic studies. A peak at 2171 cm−1 attributable to the hydrate 5 also grew in over time; exposing the sample to a high humidity (68%) atmosphere for two hours, sharp peaks at 2171 and 2216 cm−1 were observed with residual peaks at 2155 and 2187 cm−1 corresponding to Cu[Au(CN)2]2[CH3CH2CH2NH2]2 11. However, at low humidity, the Cu[Au(CN)2]2[CH3CH2CH2NH2]2 11 compound is not reversible unless a burst of heat is applied (to give the anhydrous complex, Cu[Au(CN)2]2).
Upon absorption of propylamine by 5, blue shifts occur but, as with the NH3 case, the presence of a dynamic mixture of Cu[Au(CN)2]2[CH3CH2CH2NH2]x compounds, each with a different absorption band, precluded a detailed analysis of the changes; the final visible maximum (broad, 425-440 nm) after saturation corresponds to a mixture of 11 and the saturated species 12. Upon desorption, the band red-shifts and broadens but, as is evident from the IR-data, does not rapidly settle to a single compound—it represents a mixture of bis-propylamine 11 and hydrate 5.
In general, the response time of 5 depends greatly on the analyte (Table 8) and is longer with vapour-saturated dimethylformamide, dimethylsulfoxide, pyridine and acetonitrile (minutes to hours) vs. ammonia (<5 seconds). At lower vapour concentrations of analyte, the response times increase, to within a minute for NH3, for example.
Resetting the vapochromic material. Upon standing in an NH3-free environment at high humidity, 5 loses all bound ammonia and propylamine within 2 hours as determined by IR and UV-Vis spectroscopy. However, in order to quickly and fully liberate bound analyte and thereby fully reset the sensor, 5 was heated at 160° C. for 150 seconds. Rather than regenerate 5 exactly, the related anhydrous Cu[Au(CN)2]2 complex 6 was produced, as confirmed by the single νCN stretch at 2191 cm−1. This material reacts in the same fashion with NH3 as 5 to reform 9 but the sensitivity is three times lower than the original hydrated 5 and its response time is also slower. Thus, in the absence of high humidity or a heating reset, 5 acts primarily as a sensitive one-time dosimeter for ammonia and 1°-amines. Using a heating reset, the sensor was repeatedly cycled a further three times with no apparent degradation or change in performance.
On the other hand, when exposed to more weakly-bound analytes such as DMF, DMSO, pyridine and acetonitrile, the material will reset to the analyte-free hydrated 5 at room temperature in ambient atmosphere upon removal of the analyte vapour; the speed of this regeneration depends on the specific analyte and ambient humidity but range from seconds for acetonitrile to minutes for pyridine. Thus, for these analytes, 5 acts as a reversible sensor; repeating the cycle several times caused no apparent degradation of the material.
The potential cross-interference of different VOCs with ammonia was also analyzed by introducing mixtures of NH3 and another VOC to 5: only small alkyl chain 1°-amines interfere with the NH3 sensory response and even then only if they are present at a higher concentration than the ammonia. Other coordinating VOCs such as pyridine, acetonitrile, DMF and DMSO do not interfere. A high level of humidity has an impact on the vapochromic response of 5 to weaker donating solvents (DMSO, DMF, pyridine, acetonitrile etc.) but not with NH3 or small 1° amines.
As will be apparent to a person skilled in the art, the quantitative values referred to in this example are guidelines only and are subject to a margin of error. The detection sensitivity of a particular vapochromic polymer to a particular analyte will vary depending upon various factors including atmospheric conditions, the manner in which the polymer is deployed and the concentration of the polymer.
Example 2.0 Zn-Based Polymers 2.1 Zn[Au(CN)2]2(Solvent)x SystemSynthesis of Zn[Au(CN)2]2(DMSO)2. To a 1 mL DMSO solution of Zn(ClO4)2(H2O)6 (0.032 g, 0.086 mmol) was added KAu(CN)2 (0.050 g, 0.173). Slow evaporation yielded crystals of Zn[Au(CN)2]2(DMSO)2. Anal. Calcd. for C8H12N4Au2O2S2Zn: C, 13.35; H, 1.68; N, 7.79%. Found: C, 13.50; H, 1.72; N, 8.04%. IR (KBr, cm−1) 3009 (m), 2919(m), 2849(w), 2186(s), 2175(s), 1409(m), 1314(m), 1299(m), 1031(m), 1013(s), 1005(s), 957(m), 710(w).
Although the structure of a solvent-free Zn[Au(CN)2]2 polymer (13) is known, it is believed that no luminescence or analyte binding properties have previously been reported.
The zinc-based polymer described herein appears to bind CO2: Anal. Calcd. for C5N4Au2O2Zn: C, 9.89; H, 0.00; N, 9.22%. Found: C, 9.73; H, 0.00; N, 9.32%. IR (KBr, cm−1): 2192 (s).
2.2 Synthesis, Structure and Photoluminescent Properties of Four Polymorphs of Zn[Au(CN)2]2(13) 2.2.1 General Procedures and Physical MeasurementsAll manipulations were performed in air. [nBu4N][Au(CN)2].½ H2O was synthesized as previously described.69 All other reagents were obtained from commercial sources and used as received. Infrared spectra were recorded as pressed KBr pellets on a Thermo Nicolet Nexus 670 FT-IR spectrometer. Microanalyses (C, H, N) were performed at Simon Fraser University on a Carlo Erba EA 1110 CHN Elemental Analyzer. Thermogravimetric analysis (TGA) data were collected using a Shimadzu TGA-50 instrument heating at 1° C./min in an air atmosphere. Differential Scanning calorimetry (DSC) measurements were collected on a Perkin Elmer DSC 7 instrument with a Perkin Elmer TAC 7/DX controller heating at 2° C./min from 25-300° C.
Solid-state luminescence data were collected at room temperature on a Photon Technology International (PTI) fluorometer, using a Xe arc lamp and a photomultiplier detector. Finely ground powder samples were drop cast, from the synthesis solvent, on a quartz plate and placed at an approximately 45° angle in a quartz cuvette. Ammonia exposure experiments were conducted under the same conditions, sealing the top of the cuvette with parafilm to prevent rapid loss of ammonia gas. The headspace of a bottle of concentrated ammonia solution (29.4%) was used as the source of ammonia gas.
Emission lifetimes data were determined using a customized apparatus (Prof. K. Sakai, Kyushu University) equipped with an Iwatsu DS-4262 Digital Oscilloscope and a Hamamatsu R928/C3830 photomultiplier tube coupled to a Horiba H-20-VIS grating monochromator. The excitation source was an N2 laser (337 nm) (Usho KEN-1520).
Nitrogen porosity measurements at 77 K of 13α-13δ were carried out on a custom-built vacuum line (Prof. I. Gay, Simon Fraser University) using a gas-volumetric measurement technique. Samples of α-δ were pretreated by heating to 150° C. under vacuum in order to degas the sample and remove any surface-bound solvent. A maximum pressure of 80 Torr was used.
The vapochromic behaviour of the polymorphs 13α-13δ was quantified in a customized chamber of known volume. The visible emission spectrum was monitored using an Ocean Optics QE65000 spectrometer with an Ocean Optics deuterium/tungsten-halogen DH-2000-FHS source. Titration and sensitivity limit studies were both performed on the different polymorphs, by introducing a known amount of NH3 (as 2.0 M NH3 solution in 2-propanol) into the chamber through a septum using an air-tight syringe.
The sensitivity of the different polymorphs to NH3 was determined by sequentially introducing known concentrations of NH3 into the chamber and observing any change in visible emission at 520 nm. The 520 nm wavelength was chosen to ensure that no peak overlap was observed. The emission spectrum of the appropriate NH3-free polymorph starting material was used as the background; this yielded higher sensitivities than use of a MgO white background.
The titration studies were performed by adding sequential equivalents of NH3 per Zn(II) centre, allowing the material in the chamber to equilibrate for 30 seconds, and then measuring the IR spectrum.
2.2.2 Synthetic ProceduresAlthough the inventors have experienced no difficulties, perchlorate salts are potentially explosive and should only be used in small quantities and handled with care.
α-Zn[Au(CN)2]2 (13α). A 15 mL aqueous solution containing Zn(ClO4)2′6H2O (37 mg, 0.10 mmol) was added to a 15 mL aqueous solution of KAu(CN)2 (57 mg, 0.20 mmol). Crystals begin to form after two days of slow, partial, evaporation yielding colourless X-ray quality crystals of α-Zn[Au(CN)2]2, (α). Yield: 40 mg (72%). Anal. Calcd for C4N4Au2Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C, 8.84%; H, 0.00%; N, 10.15%. IR (KBr, cm−1): 2216 (w), 2198 (s), 2158 (w), 517 (m). A similar preparation (using a different stoichiometry) and the crystal structure have been previously reported.20
β-Zn[Au(CN)2]2 (13β). To a 3 mL acetonitrile solution containing Zn(NO3)2.6H2O (30 mg, 0.10 mmol), a 3 mL acetonitrile solution of [nBu4N][Au(CN)2].½ H2O (100 mg, 0.20 mmol) was added while stirring, yielding an immediate precipitate. The mixture was centrifuged, after which the solvent was removed and the powder allowed to dry overnight. The sample was washed with three portions of acetonitrile (6 mL) through filter paper to remove any KNO3 or unreacted starting material. The resulting powder was air dried, yielding a white powder of β-Zn[Au(CN)2]2, (13β). Yield: 40 mg (72%). Anal. Calcd for C4N4Au2Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C, 8.51%; H, 0.00%; N, 9.78%. IR (KBr, cm−1): 2221 (sh), 2199 (s), 2158 (w), 521 (m).
X-ray quality crystals of 13β can be grown by slow, partial, evaporation of a 20 mL methanol solution of KAu(CN)2 (10 mL, 57 mg, 0.20 mmol) and Zn(ClO4)2′6H2O (10 mL, 37 mg, 0.10 mmol). The crystals and powder had identical IR spectra and powder diffractograms. Crystals of KClO4 are interspersed with crystals of 13β.
γ-Zn[Au(CN)2]2 (13γ). Method 1: To a 1 mL acetonitrile solution containing Zn(ClO4)2.6H2O (37 mg, 0.10 mmol), a 1 mL acetonitrile solution of [nBu4N][Au(CN)2].½ H2O (150 mg, 0.30 mmol) was added dropwise while stirring, yielding an immediate precipitate. The mixture was centrifuged, after which the solvent was removed and the powder allowed to dry overnight. The sample was washed with three portions of acetonitrile (2 mL) through filter paper to remove any KClO4 and unreacted starting material. The resulting powder was air dried, yielding a white powder of γ-Zn[Au(CN)2]2, (13γ). Yield: 40 mg (72%). Anal. Calcd for C4N4Au2Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C, 8.62%; H, 0.00%; N, 9.82%. IR (KBr, cm−1): 2187 (s), 2149 (w), 526 (m).
Method 2: To a 5 mL (99:1) acetonitrile:water solution containing Zn(ClO4)2.6H2O (37 mg, 0.10 mmol), a 5 mL (99:1) acetonitrile:water solution of KAu(CN)2 (57 mg, 0.20 mmol) was added dropwise while stirring, yielding an immediate precipitate, which was filtered and washed with three portions of acetonitrile (3 mL), leaving a powder of 13γ. Yield: 43 mg (77%). The IR spectra and X-ray powder diffractograms of the products from both methods are identical.
δ-Zn[Au(CN)2]2 (13δ). A 15 mL aqueous solution containing KAu(CN)2 (114 mg, 0.40 mmol) and KCN (26 mg, 0.40 mmol) was covered with a watch glass and brought to a boil, after which HCl (0.80 mL, 0.100 N solution, 0.080 mmol) was added. The solution was cooled until the beaker was warm to the touch, after which a 15 mL aqueous solution of ZnCl2 (26 mg, 0.20 mmol) was added. Over several hours, crystals of δ-Zn[Au(CN)2]2 (13δ) deposited. The crystals were immediately filtered and dried. Yield 60 mg (56%). Anal. Calcd for C4N4Au2Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C, 8.44%; H, 0.00%; N, 10.27%. IR (KBr, cm−1): 2191 (s), 2188 (s), 2156 (w), 2151 (w), 526 (m). Further evaporation yields a mixture of 13α and 13δ crystals, after which a yellow powder of AuCN is produced.
Zn(NH3)2[Au(CN)2]2} (14). Ammonia gas (10 mL), from the saturated headspace of a bottle of concentrated ammonia solution (29.4%), was introduced to a vial containing 20 mg of 13β. The vial was sealed and left standing for 30 min, yielding a white powder. The vial was then opened and the ammonia allowed to escape for 5 min, generating a bright, yellow powder of {Zn(NH3)2[Au(CN)2]2} 14. Anal. Calcd. for C4H6N6Au2Zn: C, 8.04%; H, 1.01%; N, 14.07%. Found: C, 8.30%; H, 0.91%; N, 13.87%. IR (KBr, cm−1): 3290 (m), 3178 (w), 2158 (s), 2117 (sh), 1202 (s), 618 (br,m).
Using 13α, 13γ, or 13δ as the starting material still generated {Zn(NH3)2[Au(CN)2]2} 14.
If a sample of {Zn(NH3)2[Au(CN)2]2} 14 is left unsealed for an hour, a white powder of Zn[Au(CN)2]2 13 forms. Anal. calcd. for C4N4Au2Zn: C, 8.53%; H, 0.00%; N, 9.94%. Found: C, 8.86%; H, 0.00%; N, 9.80%. IR (KBr, cm−1): 2197 (s), 2160 (w), 518 (m).
2.3.3 X-ray Crystallographic AnalysisCrystallographic data for the four polymorphic compounds 13α-13δ and {Zn(NH3)2[Au(CN)2]2} 14 are tabulated in Table 9. Crystals of 13β and 13δ were mounted on glass fibers using epoxy adhesive. 13β was a colourless plate having dimensions 0.20×0.20×0.02 mm3 and 13δ was a colourless plate having dimensions 0.14×0.11×0.06 mm3. The data for compounds 13β and 13δ were collected at room temperature on a Bruker Smart instrument with an APEX II CCD area detector at a distance of 6.0 cm from the crystal. A Mo Kα fine-focus sealed tube operated at 1.5 kW power (50 kV, 30 mA) was utilized for data collection. The following data ranges were recorded: 13β=4°<2θ<57°; 13δ=7°<2θ<65°.
For compound 13β, a total of 776 frames were collected with a scan width of 0.5° in ω; all frames were collected with a 20 s exposure time. The frames were integrated with the Bruker SAINT software package. Data were corrected for absorption effects using a numerical face-indexed technique (SADABS) with a transmission range of 0.058-0.700. Final unit-cell dimensions were determined based on the refinement of the XYZ-centroids of 1969 reflections with ranges 4°<2θ<51°
Crystals of compound 13δ were determined to be two component non-merohedral twins (CELL_NOW) by an approximately 180° rotation about the [100] direction in real space. A total of 4079 frames were collected with a scan width of 0.5° in ω; all frames were collected with a 20 s exposure time. Frames were integrated with the Bruker SAINT software package using the appropriate twin matrix. Data were corrected for absorption effects (TWINABS) with a transmission range of 0.036-0.110. Final unit-cell dimensions were determined based on the refinement of the XYZ-centroids of 9904 reflections with ranges 7°<2θ<61°
The structures of compounds 13β and 13δ were solved in CRYSTALS70 using direct methods (SIR92) and expanded using Fourier techniques. Diagrams were prepared using Cameron.71
The coordinates and anisotropic temperature factors for all atoms of compounds 13β and 13δ were refined. For 13β, the final refinement using observed data (Io>2.50σ(Io)) included an extinction parameter, and statistical weights included 103 parameters for 1656 unique reflections. For 138, the final refinement using observed data (Io>2.50σ(Io)) and statistical weights included 104 parameters for 2105 unique reflections. Selected bond lengths and angles are given in Tables 10 and 11 respectively.
X-ray powder diffractograms were collected on a Rigaku RAXIS rapid curved image plate area detector with a graphite monochromated Cu-Kα radiation source and a 0.5 μm collimator. Powder samples were adhered to a glass fiber with grease. Peak positions for 13γ and cell parameters were determined with Dicvol.72 Structural models for 13γ were produced and refined with Powder Cell73 using triclinic symmetry. The spacegroup and atomic positions of the carbon and nitrogen atoms were determined with CRYSTALS70 using distance and angle restraints. A final refinement cycle for y was conducted in Powder Cell73 in the tetragonal spacegroup P bar-4 b 2.
The powder pattern of {Zn(NH3)2[Au(CN)2]2} 14 was compared with simulated patterns of the coordination polymer {Cd(NH3)2[Ag(CN)2]2},74 which was found to have a similar diffraction pattern. Unit cell determination and further refinements were performed in Powder Cell73 using the atomic positions of the {Cd(NH3)2[Ag(CN)2]2} complex.74 The relative intensities of the 21l reflections for the observed powder pattern were consistently less intense and very broad then the intensities in the calculated powder diffractogram.
2.2.4 Synthesis and IR SpectraEquations 1-4 below summarize schemes identified by the inventors for controlled synthesis of polymorphs 13α-13δ by the reaction of Zn(II) salts with two equivalents of the linear anionic Au(CN)2− building block. All four materials have comparable elemental analyses and thus are true polymorphs, not pseudo-polymorphic solvent adducts. Elemental analysis indicated that when 13β was initially formed (eq 2), it contained at least one labile acetonitrile per Zn[Au(CN)2]2 unit. This complex readily desolvated when left in an unsealed container overnight. Powder X-ray diffractograms showed no difference between the solvated and desolvated forms of 13β.
Polymer 13 exhibits exquisite structural sensitivity to synthetic conditions. More particularly, the synthesis of each polymorph is sensitive to solvent choice,75-78 concentration,79 pH80,81 and even the counterions associated with either the zinc(II) or gold(I) starting material, despite the fact that these counterions are not incorporated into the final polymer. This last point is best exemplified in the synthesis of 13βvs. 13γ (Equation 2 & 3), where changing the gold counterion from K+ to [nBu4N]+ and the zinc counterion from NO3− to ClO4− generates 13γ instead of 13β, under similar reaction conditions. Furthermore, if only one counterion is changed, a mixture of polymorphs 13β and 13γ are obtained. The counterions in the synthesis of 13β and 13γ could play an important role as a templating agent, preferentially inducing the formation of one network versus another.
Conversely, the synthetic route to 13α is insensitive to counterions and moderately insensitive to concentration, although mixtures of 13α and 13δ are formed under extremely concentrated conditions (2 mL total). Changing the solvent in equation 1 from water to methanol produces crystals of 13β only when Zn(ClO4)2 is used (See experimental section above). The difference may be partially attributed to changes in the hydrogen bonding characteristics of water vs. methanol.82 The same effect is also observed in the synthesis of 13δ, which is done under acidic conditions (Equation 4) with ZnCl2.
While the rationale behind the preferential formation of a particular polymer under a defined set of conditions is unclear, it is obvious (Equation 1-4) that the formation constant for each polymorph is relatively similar. Changing the solvent, concentration, counterion, and/or pH is sufficient to easily shift the resultant energy minimum from one polymorphic form to another.
The IR spectra of all four polymorphs, 13α-13δ, are similar, having strong νCN stretches between 2187 and 2199 cm−1 and readily visible 13C-satellites between 2158 and 2149 cm−1. These bands are all shifted to higher energy relative to KAu(CN)2 (νCN=2141 cm−1), indicating that all of the cyanide groups are bound to a zinc centre.43 For the most part, the similarities in the IR spectra for 13α-13δ precluded the use of the IR signatures as a definitive polymorph identifier; this was accomplished on the basis of a combination of distinct crystal habit (
Crystal Structure of 13α. The synthesis, with a Zn:Au(CN)2 ratio of 3:1, and the crystal structure of the resulting hexagonal crystals of Zn[Au(CN)2]2 (13α) were previously reported.20 The inventors have determined that the stoichiometrically rational reaction of Zn(II) and two equivalents of KAu(CN)2 in water generates the same hexagonal crystals (Table 10,
Crystal Structure of 13β. Rectangular plate crystals of 13β (
Crystal Structure of 13γ. Although single crystals of 13γ could not be obtained, pure microcrystalline powder of 13γ was synthesized from Zn(ClO4)2 and [cation][Au(CN)2] (cation=K+,nBu4N+) in MeCN. The powder diffractogram of 13γ was observed to be similar to the previously reported Pb[Au(CN)2]2 structure.126 Using this Pb(II) structure as a starting model, the structure of 13γ was determined from powder X-ray diffraction data. An excellent match between predicted and experimental powder diffractograms (
Crystal Structure of 13δ. Twinned cross-shaped crystals of 13δ (
Polymorphism has been extensively investigated in coordination polymers, with structural differences between polymorphic forms attributed to connectivity, interpenetration, and degree of solvent inclusion (pseudo-polymorphism).58,78,88,92-94 Even the widely investigated prototypical Prussian Blue-like system, Mn[Fe(CN)6], has been observed in two polymorphic forms: the standard rock-salt structure, and the doubly interpenetrated form.95
In the case of the four polymorphs 13α-13δ all have a zinc centre in a tetrahedral geometry. The difference between the networks of 13α and 13δ, and the diamond-like networks of 13β and 13γ is due to the pathway connecting zinc centres. Furthermore, the four polymorphs differ in the degree of interpenetration, decreasing from six to three from 13α to 13δ respectively. In all of the polymers the interpenetration is supported by gold-gold bonds ranging from 3.11-3.34 Å. While it is generally believed that a lower degree of interpenetration is ideal for creating empty cavities,82 it is interesting to note that 13β has a significantly lower density then the other polymorphs, despite the relatively high degree of interpenetration.48,96,97
2.2.6 Thermal StabilityAll four polymorphs are stable until at least 350° C., after which they begin to decompose, losing all of the cyanide groups in one step between 350-390° C. Thus, despite the different levels of interpenetration and supporting gold-gold networks, the four polymorphs show very similar thermal stabilities. In addition, DSC measurements of 13α-13δ over the range 25-300° C. show no indication of a phase change from one polymorph to another.
2.2.7 PhotoluminesenceGold (I) centres which are separated by less then 3.6 Å (the sum of the Van der Waals radii for gold)98 are said to show gold-gold (aurophilic) bonding;99 such systems are known to be potentially luminescent. As a result of their interesting emission properties, compounds containing gold-gold bonds have received a great deal of attention.100-103 In the crystal structures of 13α-13δ, network interpenetration is supported via gold-gold bonds on the order of 3.11-3.34 Å and indeed, 13α-13γ are emissive when exposed to UV light at room temperature.
The photoluminescence spectra of 13α-13δ are summarized in Table 12. Compound 13α shows two emission bands at 390 and 480 nm (
In contrast, 13β and 13γ show only one emission band with similar energies at 450 and 440 nm respectively (Table 12,
It has been observed both theoretically and experimentally that the gold-gold distances in a structurally related series of metal-metal bonded gold(I) systems are inversely proportional to the emission energies. Indeed, the low energy, phosphoresence, emissions of 13α-13γ obey this trend; on average, shorter gold-gold bonds in α (3.11 and 3.16 Å) emit at lower energy than the gold-gold bonds in 13β (3.1471(11)-3.2702(6) Å) and 13γ (3.29 Å). A plot of average Au—Au distance vs. emission energy yields a straight line and also predicts that 13δ should emit at around 427 nm. However, a non-emissive decay pathway may be more prevalent in 13δ at room temperature, rendering it truly non-emissive.
2.2.8 Response to Ammonia ExposureBased on the sensitive and dramatic visible and IR spectral response of the related Cu[Au(CN)2]2(H2O)2 system to ammonia gas as described above, the response of the colourless polymorphs 13α-13δ to ammonia vapour was examined, with particular focus on possible emission changes acting as a sensory output. Ammonia detectors have a variety of applications in agriculture, the automotive industry, industrial refrigeration, medical diagnosis, and anti-terrorism.107 From a health perspective, the human nose is capable of sensing ammonia at a concentration of 50 ppm, but the permissible long term exposure limit of ammonia is below this, at 20 ppm.107,108 For these reasons, the design of materials for the detection of ammonia is of great interest.
When the four polymorphs 13α-13δ were exposed to a large excess of NH3 gas, a new white powder was formed, the IR spectrum of which contained only one νCN band at 2141 cm−1, indicative of free Au(CN)2− units. The IR spectrum also showed the presence of metal-bound ammonia,109 suggesting that the zinc(II) centre is either tetrahedral, coordinated by four ammonia units, or octahedral, coordinated by six ammonia units; the former is more likely on the basis of typical Zn(II)-ammine coordination chemistry110-112 and thus this ammonia-saturated powder is tentatively formulated as {Zn(NH3)4[Au(CN)2]2} 15. The room temperature luminescence spectrum of this ammonia-saturated complex shows a single emission peak at 430 nm with an excitation band at 365 nm (
Removal from the ammonia-rich atmosphere and subsequent rapid loss of excess ammonia from {Zn(NH3)4[Au(CN)2]2} 15 generates a yellow powder (
The TGA of {Zn(NH3)2[Au(CN)2]2} 14 shows a drop starting at room temperature and ending at 95° C. The mass loss is consistent with a loss of two equivalents of ammonia (% loss observed 6%; calculated 5.7%). As the temperature is increased a second loss from 350 to 390° C. is observed, consistent with decomposition of the polymer via cyanide loss and formation of ZnO and Au (% loss observed 15.2%; calculated 14.7%), as seen for 13α-13δ. However, this ammonia loss also occurs without heating: if {Zn(NH3)2[Au(CN)2]2} 14 is left open to air for 30 minutes all ammonia molecules are released, leaving behind a white powder of Zn[Au(CN)2]213. Interestingly, the X-ray powder diffractogram of this ammonia-free product indicated that a mixture of both 13α and 13δ polymorphs had been generated. However, the ratio of the two polymorphs is sample-dependent; cases ranging from pure 13α to pure 13δ and various ratios in between have all been observed (the controlling factor remains unclear). Photoluminescence spectra of the mixtures is consistent with the luminescence of 13α, and mixtures of 13α and non-emissive 13δ. It is important to note that reintroduction of ammonia vapour to any mixture of 13α:13δ, or even to pure δ, invariably regenerates the saturated {Zn(NH3)4[Au(CN)2]2} 15 complex, followed by the bis-ammonia complex {Zn(NH3)2[Au(CN)2]2} 14 (as confirmed by powder X-ray diffraction measurements in situ) upon removal of the ammonia-rich atmosphere.
In order to probe the mechanism of adsorption/desorption:115 titration of all four polymorphs with sequential equivalents of NH3, monitoring the νCN stretch in the IR, was investigated. Titration of 13α and 13δ reveals that they bind ammonia in a stepwise fashion (Equation 5), first converting to {Zn(NH3)2[Au(CN)2]2} 14 (νCN 2158 cm−1) and then to {Zn(NH3)4[Au(CN)2]2} 15 (νCN 2141 cm−1). This reaction is completely reversible; i.e., loss of ammonia regenerates the same polymorph. However, titration of 13β and 13γ reveals that the tetraaminezinc(II) complex forms directly (Equation 6), with no intermediate of {Zn(NH3)2[Au(CN)2]2} observed. Apparently, sufficient ammonia must be present to break all the bonds between the zinc(II) and the four cyanides in order for initial NH3-uptake to occur. Due to the adsorption routes for the four polymorphs, it is clear that once {Zn(NH3)4[Au(CN)2]2} is formed from 13D and 13γ, these polymorphs cannot be regenerated upon NH3-desorption; mixtures of 13α and 13δ are produced instead.
(α/δ)Zn[Au(CN)2]2+2NH3
{Zn(NH3)2[Au(CN)2]2+2NH3
(β/γ)Zn[Au(CN)2]2+4NH3
{Zn(NH3)4[Au(CN)2]2}→{Zn(NH3)2[Au(CN)2]2}+2NH3
Porosity measurements on all four polymorphs showed no adsorption of nitrogen gas at 77 K. Despite this, 13α-13δ reversibly bind ammonia vapour. It is well-known that non-porous coordination polymers can be quite flexible in the solid-state.54,116,117 The d10 zinc(II) cation can adopt several geometries encompassing coordination numbers from two to six;102,118 undoubtedly, this flexibility facilitates the structural rearrangement which occurs as the ammonia interacts with the zinc(II) centre. Note that although NH3-binding occurs at Zn(II), it is the array of Au(CN)2− units that act as the sensor, as the structural rearrangement changes the gold-gold distance and thus the emission energy.
As discussed above, ammonia sensors are used in a wide range of settings.107 For each application there are different requirements for an ideal sensor, including detection limits (0.1 ppb-200 ppm), response times (seconds-minutes), and operating temperatures (0-500° C.).107 Due to the highly varying requirements there are several different types of sensors employed in the detection of ammonia. Some common ammonia-sensor materials include conducting compounds based on metal-oxides119-121 or polymers such as polypyrrole122 and polyaniline;123 other sensors are based on spectrophotometric determination.124,125 While metal oxide-based sensors are very common, they require a high operating temperature, making them unsuitable in some applications, e.g. medical diagnostics.107 Conversely, although atomic absorption-based sensing can be operated at room temperature with high sensitivity, it is extremely expensive.107
This example clearly shows that all four polymorphs of zinc polymer 13 act as vapoluminescent sensors for ammonia. However, in order to determine if the Zn[Au(CN)2]2 materials (which are relatively inexpensive and quite stable) could compete with current NH3-detection systems, quantitative detection limits for each polymorph were measured, by monitoring the intensity of the {Zn(NH3)2[Au(CN)2]2} emission band (λmax=500 nm, λex=400 nm) emission band at 520 nm. Although sensitivities of the polymers vary depending on polymorph, the lowest detection limit was observed for 13β, with a NH3-detection limit of 1 ppb. The response times are also fast: within seconds for all polymorphs. In comparison with other materials used as ammonia sensors, these four coordination polymers have some of the lowest NH3-detection limits and response times.
In summary, the reaction of various zinc salts with the linear, anionic Au(CN)2− bridging ligand formed four structurally unique polymorphs, three of which are luminescent at room temperature. Care must be taken when synthesizing these polymorphs since changing the concentration, counterion, pH, and/or solvent can redirect the synthesis from one polymorphic form to another. All four polymorphs reversibly act as very sensitive sensors for ammonia vapour, changing their emission energies as ammonia is bound. The emission can be correlated to the gold-gold array distance in each compound.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
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Claims
1. A vapochromic polymer comprising:
- MW[M′X(Z)Y]N
- wherein M and M′ are the same or different metals capable of forming a coordinate complex with the Z moiety;
- Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides;
- W is between 1-6;
- X and Y between 1-9; and
- N is between 1-5.
2. The polymer as defined in claim 1, wherein W and X are 1, Y is 2, and N is 2 or 3.
3. The polymer as defined in claim 2, wherein said polymer is reversibly vapochromic.
4. The polymer as defined in claim 1, wherein M is a transition metal.
5. The polymer as defined in claim 1, wherein M′ is a metal selected from the group consisting of Au, Ag, Hg and Cu.
6. The polymer as defined in claim 1, wherein Z is a pseudohalide selected from the group consisting of CN, SCN, SeCN, TeCN, OCN, CNO and NNN.
7. The polymer as defined in claim 1, wherein M is selected from the group consisting of Cu, Zn, Sc, Ti, V, Cr, Mn, Fe, Co and a lanthanide.
8. The polymer as defined in claim 7, wherein M is Cu or Zn.
9. The polymer as defined in claim 1, wherein M′ is selected from the group consisting of Au and Ag and wherein Z is CN.
10. The polymer as defined in claim 9, wherein M′ is Au and wherein X is 1 and Y is 2.
11. The polymer as defined in claim 1, further comprising an organic ligand bound to M.
12. The polymer as defined in claim 11, wherein said organic ligand comprises at least one nitrogen, oxygen, sulphur or phosphorus donor.
13. The polymer as defined in claim 1, further comprising a counterbalancing cation or anion.
14. The polymer as defined in claim 1, wherein said polymer is vapoluminescent.
15. The polymer as defined in claim 1, wherein X=1 and Y=2.
16. A composition comprising a polymer as defined in claim 1, and an analyte adsorbed to said polymer.
17. The composition as defined in claim 16, wherein said analyte is a volatile organic compound.
18. The composition as defined in claim 16, wherein said analyte is a gas having a donor hydrogen, nitrogen, oxygen, sulphur or phosphorus atom.
19. A solid-state vapochromic structure comprising a polymer as defined in claim 1.
20. A solid-state vapochromic structure comprising a composition as defined in claim 16.
21. The use of a structure a defined in claim 19 for vapochromically sensing the presence of an analyte.
22. An analyte-bound complex comprising an analyte adsorbed to a polymer as defined in claim 1.
23. A method of detecting an analyte comprising:
- (a) providing a polymer as defined in claim 1;
- (b) exposing said polymer to a supply of said analyte; and
- (c) detecting any chromatic changes in said polymer resulting from exposure to said analyte.
24. The method as defined in claim 23, wherein said detecting comprises sensing any changes in the colour of said polymer.
25. The method as defined in claim 23, wherein said detecting comprises sensing any changes in the luminescence of said polymer.
26. The method as defined in claim 23, wherein said detecting comprises spectroscopically identifying any changes in the infrared signature of said polymer.
27. The method as defined in claim 26, wherein said spectroscopically identifying comprises detecting the number and position of νCN spectroscopic bands.
28. The method as defined in claim 23, wherein said chromatic changes are reversible.
29. The method as defined in claim 28, wherein said supply comprises different analytes and wherein steps (b) and (c) are dynamically repeated in respect of successive ones of said analytes.
30. The method as defined in claim 23, wherein M is Cu or Zn and wherein M′ is Au.
31. The method as defined in claim 30, wherein said polymer is selected from the group consisting of Cu[Au(CN)2]2 and Zn[Au(CN)2]2.
32. The method as defined in claim 23, wherein said analyte is a volatile organic compound.
33. The method as defined in claim 23, wherein said analyte is a gas.
34. The method as defined in claim 29, wherein said gas is CO and CO2.
35. The use of a polymer as defined in claim 1 for vapochromically sensing the presence of an analyte.
36. A method of synthesizing a polymer as defined in claim 8 comprising reacting a Cu(II) or a Zn(II) salt with [Au(CN)2]− in a solvent.
37. A vapochromic composition comprising a metal complex represented by the general formula MW[M′X(Z)Y]N
- wherein M and M′ are the same or different metals capable of forming a coordinate complex with the Z moiety;
- Z is selected from the group consisting of halides, pseudohalides, thiolates, alkoxides and amides;
- W is between 1-6;
- X and Y between 1-9; and
- N is between 1-5.
38. The composition as defined in claim 37 wherein;
- M is selected from the group consisting of Cu and Zn;
- M′ is selected from the group consisting of Au, Ag, Hg and Cu;
- Z is a pseudohalide selected from the selected from the group consisting of CN, SCN, SeCN, TeCN, OCN, CNO and NNN;
- W and X are 1;
- Y is 1; and
- N is 2 or 3.
39. The method as defined in claim 23, wherein said analyte is selected from the group consisting of ammonia and amines.
40. The method as defined in claim 39, wherein said analyte is ammonia.
41. The method as defined in claim 39, wherein said polymer comprises Zn[Au(CN)2]2 in any of its polymorphic forms.
42. The method as defined in claim 39, wherein said polymer comprises Cu[Au(CN)2]2.
43. The method as defined in claim 39, wherein M is Cu or Zn, M′ is Au, Z is CN, W is 1 and X and Y is 2.
44. The method as defined in claim 39, wherein said detecting comprises sensing any changes in luminescence of said polymer.
45. The method as defined in claim 39, wherein said detecting comprises sensing any changes in colour of said polymer.
46. The method as defined in claim 39, wherein said detecting comprises spectroscopically identifying any changes in the infrared signature of said polymer.
47. The method as defined in claim 40, wherein said exposing comprises exposing said polymer to ammonia present in the breath of a patient.
48. The method ad defined in claim 41, wherein said polymer comprises Zn[Au(CN)2]2 in its β polymorphic form.
Type: Application
Filed: Oct 21, 2011
Publication Date: May 10, 2012
Applicant:
Inventors: Julie LEFEBVRE (Saint-Jean-sur-Richelieu), Michael Iacov KATZ (North Vancouver), Daniel B. LEZNOFF (Vancouver)
Application Number: 13/279,099
International Classification: G01N 21/75 (20060101); C01C 3/11 (20060101); G01N 21/78 (20060101); G01N 21/76 (20060101); C07F 1/12 (20060101); C07F 19/00 (20060101);
