Thermal Expansion Behavior of MI[AuX2(CN)2]-Based Coordination

Apr 4, 2017 - Synopsis. Two new coordination polymer systems, M[AuX2(CN)2] (M = Ag; X = Cl, Br or M = Cu; X = Br) and M[Au(CN)4] (M = Ag, Cu), were sy...
1 downloads 9 Views 2MB Size
Download PDF
Article pubs.acs.org/IC

Thermal Expansion Behavior of MI[AuX2(CN)2]‑Based Coordination Polymers (M = Ag, Cu; X = CN, Cl, Br) Jeffrey S. Ovens and Daniel B. Leznoff* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada S Supporting Information *

ABSTRACT: Two sets of trans-[AuX2(CN)2]−-based coordination polymer materialsM[AuX2(CN)2] (M = Ag; X = Cl, Br or M = Cu; X = Br) and M[Au(CN)4] (M = Ag, Cu)were synthesized and structurally characterized and their dielectric constants and thermal expansion behavior explored. The M[AuX2(CN)2] series crystallized in a tightly packed, mineral-like structure featuring 1-D trans-[AuX2(CN)2]−-bridged chains interconnected via a series of intermolecular Au···X and M···X (M = Ag, Cu) interactions. The M[Au(CN)4] series adopted a 2-fold interpenetrated 3-D cyanobound framework lacking any weak intermolecular interactions. Despite the tight packing and the presence of intermolecular interactions, these materials exhibited decreased thermal stability over unbound trans-[AuX2(CN)2]− in [nBu4N][AuX2(CN)2]. A significant dielectric constant of up to εr = 36 for Ag[AuCl2(CN)2] (1 kHz) and a lower εr = 9.6 (1 kHz) for Ag[Au(CN)4] were measured and interpreted in terms of their structures and composition. A systematic analysis of the thermal expansion properties of the M[AuX2(CN)2] series revealed a negative thermal expansion (NTE) component along the cyano-bridged chains with a thermal expansion coefficient (αCN) of −13.7(11), −14.3(5), and −11.36(18) ppm·K−1 for Ag[AuCl2(CN)2], Ag[AuBr2(CN)2], and Cu[AuBr2(CN)2], respectively. The Au···X and Ag···X interactions affect the thermal expansion similarly to metallophilic Au···Au interactions in M[Au(CN)2] and AuCN; replacing X = Cl with the larger Br atoms has a less significant effect. A similar analysis for the M[Au(CN)4] series (where the volume thermal expansion coefficient, αV, is 41(3) and 68.7(19) ppm·K−1 for M = Ag, Cu, respectively) underscored the significance of the effect of the atomic radius on the flexibility of the framework and, thus, the thermal expansion properties.

■

INTRODUCTION Coordination polymer materials have become an intense area of research due to the ability to impart properties such as porosity,1−6 luminescence,7−14 vapochromism,15−22 and thermal expansion23−32 via their modular nature, which allows for the strategic incorporation of metal centers, bridging units, and ancillary ligands.33−38 Such materials have been targeted for applications such as vapor sensing,10,20−22,39 gas storage,1,40,41 and optical components.42−45 In some of the more typical applications, flexible, porous coordination polymers (metal− organic frameworks in particular) are desirable.1 On the other hand, nonporous mineral-like coordination polymer materials have displayed interesting properties and potential for useful applications.10,20−22 Cyano-bound frameworks have a particularly distinguished place in the pantheon of coordination polymer building blocks, starting with archetypical Prussian Blue analogues46−49 and Hofmann clathrates,50−52 which are generally readily synthesized and show useful properties and applications. One of the more unique properties of cyano-based materials is their tendency to exhibit negative thermal expansion (NTE);23,24,28,53−56 the presence of transverse vibrational modes of the cyano bridges is a widely accepted origin for their NTE properties (as well as in oxide materials exhibiting M−O−M′ linkages).29−31 Prussian Blue analogues such as © 2017 American Chemical Society

Cd[Pt(CN)6]·2H2O and Fe[Co(CN)6] in particular have been studied extensively for their thermal expansion properties, although the magnitudes of their thermal expansion coefficients (αl = d ln l/dT; l: lattice parameter, T: temperature) are usually low in these cases.25 On the other end of the spectrum, Ag3[Co(CN)6] showed “colossal” (|α| > 100 ppm·K−1) thermal expansion, with thermal expansion coefficient magnitudes ranging from 130 to 150 ppm·K−1.55 More recently, [M(CN)2]−-based (M = Ag, Au) coordination polymers such as In[M(CN)2]3 also showed colossal thermal expansion with αa = 104 ppm·K−1 and αc = −84 ppm·K−1 for M = Ag and αa = 86 ppm·K−1 and αc = −63 ppm·K−1 for M = Au.23 In the latter examples, both materials exhibit metallophilic M···M interactions between the Ag or Au centers: the difference in the strength of these interactions is what accounts for the significant difference in the thermal expansion coefficients observed.57−60 In another related example, AuCN and the isomorphous complexes Ag[Au(CN)2] and Cu[Au(CN)2] (all containing chains connected by metallophilic interactions) were found to exhibit a slight degree of NTE along the cyanide bonds of αCN = −8.9 ppm·K−1 for AuCN, −9.0 ppm·K−1 for Ag[Au(CN)2], and −13.8 ppm·K−1 for Cu[Au(CN)2 ], Received: December 30, 2016 Published: April 4, 2017 7332

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

are highly insoluble in general, complete separation is very difficult; however, formation of the AgX byproduct can be minimized by decreasing the rate of addition of AgNO3 and by ensuring that the trans-[AuX2(CN)2]− building block is in a slight excess. The presence of the AgBr impurity especially increases when the synthesis of Ag[AuBr2(CN)2] is conducted at a scale of 1 mmol or higher. Due to the insolubility of these materials in solvents such as H2O, MeOH, MeCN, DMF, DMSO, or acetone, single crystals could not be grown by recrystallization methods, and hydrothermal techniques were discarded since higher temperatures are known to result in the reductive elimination of halogen from trans-[AuX2(CN)2]−-based building blocks.22,63,64 Attempts to grow crystals of Ag[AuX2(CN)2] by layering solutions of gels containing the dissolved starting materials were also unsuccessful. Thus, the solid-state structures were determined via Monte Carlo methods from powder X-ray diffraction (PXRD) data; indexing and Rietveld refinements of the PXRD profiles (Figures S1 and S2) revealed an isomorphous pair for Ag[AuCl2(CN)2] and Ag[AuBr2(CN)2]. The structure of Ag[AuX2(CN)2] can be described as a 1-D linear coordination polymer consisting of Ag(I) centers bridged by trans-[AuX2(CN)2]− building blocks as shown in Figure 1a

following the trend of increased NTE behavior with decreased metallophilic interaction strength (Au···Cu < Au···Ag < Au··· Au).61 In contrast to the Au(I)-containing materials above, Au(III)based trans-[AuX2(CN)2]−-containing (X = Cl, Br, I, CN) coordination polymers have not been investigated in terms of their thermal expansion properties, despite their ability to participate in halogen−halogen (X···X) interactions, which can play similar structural roles to metallophilic interactions and could thus substantially influence the thermal expansion of the materials.42,62 With this goal in mind, herein we report several new organic ligand-free, mineral-like Ag(I)- and Cu(I)-based materials containing trans-[AuX2(CN)2]− linkers and describe their structural features, explore their thermal expansion properties, and compare them to analogous cyanometalate materials, particularly those with metallophilic interactions.

■

RESULTS AND DISCUSSION Synthesis and Structures of Ag[AuX2(CN)2] (X = Cl, Br) and Cu[AuBr2(CN)2]. The addition of AgNO3 to [nBu4N][AuX2(CN)2] (X = Cl, Br) in methanolic solution resulted in an instant precipitate of Ag[AuX2(CN)2] in both cases. This change is observable in both the IR and Raman spectra via significant shifts in their respective νCN peaks from the [nBu4N][AuX2(CN)2] starting material (see Table 1). ParticTable 1. Comparison of IR and Raman νCN and νAuX (X = Cl, Br) Stretches for [nBu4N][AuX2(CN)2], Ag[AuX2(CN)2], and Cu[AuX2(CN)2] (X = Cl, Br, CN) peak position/cm−1 compound [nBu4N][Au(CN)4] [nBu4N][AuCl2(CN)2] [nBu4N][AuBr2(CN)2] Ag[AuCl2(CN)2] Ag[AuBr2(CN)2] Cu[AuBr2(CN)2]a Ag[Au(CN)4] Cu[Au(CN)4]

IR νCN

2165 2226 2218 2228 2189 2209

Raman νCN 2192, 2199 2185 2180 2237 2229

Raman νAuX 340 209 327 201

2215, 2229 2217, 2225, 2229

a

The high absorption in the visible region precludes Raman spectroscopy for Cu[AuBr2(CN)2].

ularly noteworthy is the change in the νAuX stretches in the Raman spectra. In prior reports of trans-[AuX2(CN)2]−containing materials (with organic ligands present), only minimal shifts in the νAuX peaks (less than 4 cm−1) were observed when compared to the [nBu4N][AuX2(CN)2] starting material;22 however in this case, there is a significant red-shift in these peaks from 340 to 327 cm−1 for X = Cl and from 209 to 201 cm−1 for X = Br. This suggests the existence of a substantial interaction between the halo moieties and another species in the structure. While most of the trans-[AuX2(CN)2]− molecules remained intact and were incorporated in the Ag[AuX2(CN)2] material (as evidenced from powder X-ray diffraction data and the presence of the νAuX peaks in the Raman spectrum), some AgX byproduct also formed, as confirmed by CHN elemental analysis and PXRD analysis. While the rapid precipitation of the highly insoluble Ag[AuX2(CN)2] products likely allowed for the formation of Ag[AuX2(CN)2], partial abstraction of halide moieties from trans-[AuX2(CN)2]− by free Ag+ cations nevertheless does occur to some extent. Since both products

Figure 1. Crystal structure of Ag[AuCl2(CN)2] as viewed (a) perpendicular and (b) parallel to the Ag[AuCl2(CN)2] chains. M··· Cl interactions are shown as dashed green lines. Ag, pink; Au, gold; C, gray; Cl, light green; N, blue.

for Ag[AuCl2(CN)2]. Each of these chains interacts with four neighboring chains via several Ag···X and Au···X interactions, as shown in Figure 1b. The Ag···X interactions have lengths of 3.03(1) and 3.26(1) Å for X = Cl and 3.127(9) and 3.15(3) Å for X = Br (cf. the sums of the van der Waals radii: 3.47 Å for dAg−Cl and 3.57 Å for dAg−Br),65 while the Au···X interactions have lengths of 3.21(1) Å for X = Cl and 3.57(1) Å for X = Br (cf. the sums of van der Waals radii: 3.41 Å for dAu−Cl and 3.51 Å for dAu−Br; see Table 2).65 This multitude of interactions is consistent with the significant shift in the νAuX stretch observed 7333

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

material does not form; instead, a mixture of Cu(II)- and Au(I)-containing decomposition products is generated. The PXRD profile (see Figure S3) for Cu[AuBr2(CN)2] was similar to that of Ag[AuX2(CN)2], indicating an analogous structure; however the decreased number of peaks observed suggested a higher crystallographic symmetry. Indeed, Cu[AuBr 2 (CN) 2 ] adopts an isostructural motif to Ag[AuBr2(CN)2], but in a monoclinic crystal system rather than triclinic (see Table S1 for further information). As was observed in Ag[AuBr 2 (CN) 2 ], the structure of Cu[AuBr2(CN) 2] contains numerous Cu···Br and Au···Br interactions of 2.981(1)−3.546(1) Å and 3.112(1)−3.413(1) Å, respectively (cf. the sums of van der Waals radii: 3.25 Å for dCu−Br and 3.51 Å for dAu−Br; see Table 2).65 The structures of M[AuX2(CN)2] described above are related to the Au(I)- and Cu(I)-based materials Ag[Au(CN)2] and Cu[Au(CN)2]:61 all feature parallel, tightly packed linear chains of Ag(I) or Cu(I) centers bridged by trans[AuX2(CN)2]− or [Au(CN)2]− units. The contrasting feature is that in M[Au(CN)2] the chains interact primarily via Au···Au interactions, whereas those in M[AuX2(CN)2] interact with each other by M···X interactions (M = Cu, Ag, Au; X = Cl, Br). Synthesis and Structure of M[Au(CN)4] (M = Ag, Cu). To avoid formation of insoluble Ag(I)- and Au(I)-containing impurities, the replacement of trans-[AuX2(CN)2]− with K[Au(CN)4], which lacks any abstractable halides and is less prone to reductive elimination, was targeted. Accordingly, an aqueous or MeCN solution of K[Au(CN)4] was added to a solution of AgNO3 or [Cu(MeCN)4]OTf, resulting in a white powder of Ag[Au(CN)4] or Cu[Au(CN)4], respectively. While Ag[Au(CN)4] forms as an instant precipitate, Cu[Au(CN)4] precipitates slowly over several days. Ag[Au(CN)4] was found to be soluble only in MeCN, from which X-ray quality crystals were grown. In the solid-state structure, [Au(CN)4]− building blocks are bound through all four cyano groups to four Ag(I) centers, forming an overall 3-D network with tetrahedral nodes (Figure 2a), which can be classified as a (42·84) net.66 In order to more clearly illustrate the full structure, a 2-D “slice” of the framework is shown in Figure 2b, which exhibits a network resembling a brick wall. In this 2-D slice, the [Au(CN)4]− units alternate between having all cyano groups in the plane and having two cyano groups

Table 2. Selected Bond and Interaction Lengths (Å) in M[AuX2(CN)2] (M = Ag, Cu; X = Cl, Br) Ag[AuCl2(CN)2]

Ag[AuBr2(CN)2]

M−N M−X M···X

2.10(1) 2.28(1) 3.03(1), 3.26(1)

2.136(5) 2.44(1) 3.127(9), 3.15(3)

Au···X

3.21(1)

3.57(1)

Cu[AuBr2(CN)2] a 2.41(1) 2.981(1), 2.999(5), 3.242(1), 3.546(1) 3.112(1), 3.413(1)

a

Cu−N distances for Cu[AuBr2(CN)2] are not reliable due to the use of restraints and constraints in the Rietveld refinements.

in the Raman spectrum for these compounds. While these interactions are often characterized as van der Waals interactions, they can also be thought of as X−M donor− acceptor interactions, increasing the square-planar coordination sphere of the trans-[AuX2(CN)2]− building blocks to a pseudooctahedral geometry. It is noteworthy that despite the larger van der Waals radius for Br atoms, the Ag−X distances in Ag[AuBr2(CN)2] are similar in length (shorter in some cases) to those in Ag[AuCl2(CN)2], consistent with the softer nature of the Br atoms, which lends themsleves to either a stronger van der Waals interaction or a stronger donor−acceptor relationship with the Au centers. Despite this fact, the difference in Raman Ag[AuBr2(CN)2] peaks in νAuBr and for free trans[AuBr2(CN)2]− is smaller than that for the νAuCl peaks in Ag[AuCl2(CN)2] compared to free trans-[AuCl2(CN)2]−, suggesting a weaker correlation between νAuX peak position and interaction strength for M···Br interactions than for M···Cl interactions. Although Ag(I)-containing materials are often light sensitive, no noticeable degradation of Ag[AuCl2(CN)2] or Ag[AuBr2(CN)2] was observed when they were left in ambient light for several days. Exposing them to broadband UV light for short periods of time also appeared to have no effect. When [Cu(MeCN)4]OTf and CH2Cl2 are used instead of AgNO3 and MeOH, reaction with [nBu4N][AuBr2(CN)2] generates Cu[AuBr2(CN)2], with analogous νCN shifts in the IR spectrum to Ag[AuBr2(CN)2] (see Table 1; Raman spectra could not be obtained due to the high absorbance exhibited by Cu[AuBr2(CN)2]). On the other hand, when a trans[AuCl2(CN)2]− salt is used, the analogous Cu[AuCl2(CN)2]

Figure 2. Crystal structure of Ag[Au(CN)4] showing (a) the coordination environment about the Ag(I) centers and (b) a 2-D portion of the overall 3-D Ag[Au(CN)4] network. This network also propagates parallel to the view direction via cyano linkers. Ag, pink; Au, gold; C, gray; N, blue. 7334

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

constants of coordination polymers have not been widely measured,71,72 and the materials covered in the present discussion potentially fit the criteria for having high dielectric constants (they consist mostly of densely packed Ag/Cu, Au, and Cl/Br atoms), the room-temperature dielectric constants of Ag[AuCl2(CN)2] and Ag[Au(CN)4] were measured at a variety of electronic frequencies from 800 Hz (below which excessive current loss results in unreliable values)70 to 1 MHz. The data shown in Figure 4 show the expected trend: an increase in

perpendicular to the plane of view. In a similar manner, the Ag(I) centers alternate between being puckered “inward” and “outward” from the plane of view. These perpendicular cyano groups bind to the puckered Ag(I) centers of the adjacent 2-D slices, forming the overall 3-D framework. All voids in this 3-D network are occupied by a second interpenetrating framework, precluding porosity. No notable weak intermolecular interactions are present in this structure. A suitable solvent in which Cu[Au(CN)4] is soluble could not be found; thus the PXRD data for this complex were collected. The data matched the simulated PXRD profile (based on the single-crystal solution) for Ag[Au(CN)4] (see Figure S4). Subsequent Rietveld refinements on the PXRD data for Cu[Au(CN)4] confirmed that these complexes are isomorphous (see Table S2). Thermal Stability. Previously reported thermogravimetric data for some trans-[AuX2(CN)2]−-based materials demonstrated decreasing stability moving from X = Cl to Br to I.22,67,68 However, most of those materials (which generally contained organic ancillary ligands) do not exhibit the M···X interactions and mineral-like structures of the Ag[AuX2(CN)2] system reported herein. Thus, thermogravimetric analyses (TGA) were performed on the M[AuX2(CN)2] system (Figure 3) to assess any potential differences in thermal stability attributable to these structural features.

Figure 4. Dielectric constants, εr, of Ag[Au(CN)4] (red) and Ag[AuCl2(CN)2] (blue) plotted against frequency.

dielectric constant at 1 kHz from 9.6 for Ag[Au(CN)4] to 36 for Ag[AuCl2(CN)2] (4.4 and 25, respectively at 1 MHz). The value recorded for Ag[Au(CN)4] is very similar to that of AgCN (εr = 5.6; 1 MHz);70 AgCN has a higher density than Ag[Au(CN)4], likely accounting for the slightly higher value. On the other hand, AgCl shows a value of only 11.15,73 which, though it is higher than AgCN, is significantly smaller than the value recorded here for Ag[AuCl 2 (CN) 2 ]. That Ag[AuCl2(CN)2] is so much higher is surprising, as the addition of cyano groups reduces the packing density of heavy elements, which should result in a lower value. Nevertheless, in the full gamut of inorganic dielectric materials, these dielectric constants are not remarkable, and are comparable toor even exceeded byseveral common inorganic materials such as NaBr (εr = 6.0−6.5),74 BaO (εr = 34)75 TiO2 (εr = 86− 170), 76,77 and BaTiO 3 (ε r > 3000), 78 although Ag[AuCl2(CN)2], with a value of 25−36, is in the upper range of most inorganic materials. Thermal Expansion. Coordination polymers bridged mainly by cyano units are known to often display negative thermal expansion properties due to their transverse vibrational modes.23−26,28,79,80 When excited thermally, these vibrational modes serve to contract the M−CN−M′ distances, resulting in negative thermal expansion along such bonds. In particular, the system M[Au(CN)2] (which is related to the materials discussed here) shows NTE along the M−NC−Au−CN−M bonds (c-axis) of αCN = −9.0 ppm·K−1 and −13.8 ppm·K−1 for M = Ag and Cu, respectively, and positive thermal expansion (PTE) along the M−M′ (M = Ag, Cu; M′ = Au) bonds (a- and b-axes) of αMM = 83−84 ppm·K−1.61 The Au(III)-based materials presented here also contain cyano units oriented either in a single dimension (M[AuX2(CN)2] series) like the

Figure 3. Thermogravimetric analysis curves for Ag[AuCl2(CN)2] (red), Ag[AuBr2(CN)2] (blue), and Ag[Au(CN)4] (green).

Consistent with the stability trend for trans-[AuX2(CN)2]−based materials mentioned above, Ag[Au(CN)4] was the most thermally stable complex, with the first loss of cyanogen occurring at approximately 300 °C, followed by Ag[AuCl2(CN)2] with chlorine loss at 185 °C, and Ag[AuBr2(CN)2] with bromine loss at 160 °C. In all cases, final loss of cyanogen occurs between 400 and 440 °C, leaving gold nanoparticles and metallic silver as per a previous study.69 For comparison, TGA data for [ nBu 4 N][AuCl 2(CN) 2] and [nBu4N][AuBr2(CN)2] were also collected (Figure S5), showing somewhat increased decomposition temperatures of 200 and 180 °C for Cl2 and Br2 loss, respectively, suggesting the highly interconnected structures of Ag[AuX2(CN)2] afford no increased thermal stability over free trans-[AuX2(CN)2]− salts; indeed, their apparent weakening of the Au−X bond appears to slightly facilitate halogen loss at lower temparatures. Dielectric Properties. Materials with densely packed, polarizable atoms such as heavy metals and halides often exhibit high dielectric constants (εr).70 Since dielectric 7335

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

Figure 5. (a) Change in unit cell length parameters (red squares: a, blue circles: b, green triangles: c) as temperature is varied for Ag[AuCl2(CN)2]. (b) Change in M···M distances (dMM; red squares) perpendicular to the (111) planes, the change in Au−Cl···Ag distances (dMXM; blue circles) along the [21̅1̅] direction, and the change in Au−CN−Ag−NC−Au distances (dCN; green triangles) along the [011̅] direction for Ag[AuCl2(CN)2]. Linear fit curves are shown as solid black lines. (c and d) Crystallographic features and directions corresponding to dCN, dMM, and dMXM.

Table 3. Thermal Expansion Coefficients for Ag[AuX2(CN)2] (X = Cl, Br) and M[Au(CN)4] (M = Ag, Cu) along Their Unit Cell Axes material

αa/ppm·K−1

αb/ppm·K−1

αc/ppm·K−1

αV/ppm·K−1

Ag[AuCl2(CN)2] Ag[AuBr2(CN)2] Cu[AuBr2(CN)2] Ag[Au(CN)4] Cu[Au(CN)4] Ag[Au(CN)2]61 Cu[Au(CN)2]61

49.4(13) 52.1(9) 45.9(5) −5.46(8) −18.3(13) 83.9 83.3

23.6(10) 25.3(6) −11.36(18) −10.9(12) 34.6(8)

−4.7(14) 5.1(5) 73.6(6) 75.3(17) 60.7(14) −9.0 −13.8

88(3) 97.7(9) 103.1(12) 41(3) 68.7(19) 161.0 154.6

heavier halogen or lighter metal center appears to result in higher overall thermal expansion. The latter result is surprising, as a positive correlation of atomic radius to thermal expansion magnitude has been recorded previously, in contrast to this observation.80 To more deeply probe these thermal expansion properties, a closer look at the thermal expansion coefficients of the individual cell axes and crystallographic directions is necessary. Since Ag[AuCl2(CN)2] and Ag[AuBr2(CN)2] are triclinic, all cell axes and angles vary independently, as shown in Figure 5a (see Figure S6 for Ag[AuBr2(CN)2] data). The a- and b-axes of both Ag[AuCl2(CN)2] and Ag[AuBr2(CN)2] undergo PTE with thermal expansion coefficients of approximately 24 and 50 ppm·K−1, respectively (Table 3), which are typical of many PTE materials.23−26,28,79,80 These compounds differ in the caxes, however, with Ag[AuCl2(CN)2] exhibiting NTE at αc =

Au(I)-based materials described above or in all three dimensions (M[Au(CN)4] series); thus the thermal expansion properties of each were measured using variable-temperature PXRD. Ag[AuX2(CN)2] (X = Cl, Br) and Cu[AuBr2(CN)2]. The structural motif of these M[AuX2(CN)2] systems is very similar to that of Ag[Au(CN)2] and Cu[Au(CN)2], with parallel M[AuX2(CN)2] chains interacting with each other via M···X interactions in place of M···M interactions. This allows for a nearly direct comparison of the thermal expansion properties of M[AuX2(CN)2] with M[Au(CN)2] (M = Ag, Cu), in particular regarding the effect of the M···X vs M···M interactions. Overall, the three materials Ag[AuCl 2 (CN) 2 ], Ag[AuBr2(CN)2], and Cu[AuBr2(CN)2] exhibit positive volume expansion with αV = 88(3), 97.7(9), and 103.1(12) ppm·K−1, respectively. From these values alone, it can be concluded that a 7336

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry −4.7(14) ppm·K−1 and Ag[AuBr2(CN)2] exhibiting PTE at αc = 5.1(5) ppm·K−1; such small values approach zero thermal expansion (ZTE) in this direction.28 Along with the cell dimensions, the cell angles undergo significant changes, resulting in a combination of positive and negative thermal expansion parameter magnitudes of up to 40 ppm·K−1. Due to the packing of Ag[AuCl 2 (CN) 2 ] and Ag[AuBr2(CN)2] in their unit cells, the M−NC−Au−CN−M chains do not align with any of the unit cell axes, precluding an effective analysis of the above results in terms of a structure− property correlation. On the other hand, the thermal expansion properties of select directions within the crystal structure lattice can be analyzed; this also allows for a more direct comparison between the nonisomorphous Ag(I) and Cu(I) systems (i.e., this analysis will account for the difference in space group between the Cu(I) and Ag(I) systems) and also serves to “build in” the effects of the thermal expansion of the unit cell angles. In particular, the distance between metal centers along the 1-D chains via the cyano bridgesthe [011̅] direction for Ag[AuX2(CN)2] (hereafter referred to as dCN; see Figure 5c)is of interest, as this is expected to show NTE as a result of the cyano transverse vibrational modes. The distance between the Ag and Au metal centers along the Ag···X−Au− X···Ag (X = Cl, Br) interactionsthe [21̅1̅] direction for Ag[AuX2(CN)2] (hereafter referred to as dMXM; see Figure 5c)and the distance between the (111) planes described in Figure 5d (hereafter referred to as dMM) were also analyzed in order to facilitate structure−property correlations. For clarity, the thermal expansion coefficients corresponding to the dCN, dMXM, and dMM distances will be denoted as αCN, αMXM, and αMM, respectively. Since Cu[AuBr2(CN)2] adopts a different crystal system, the equivalent directions and planes for Cu[AuBr2(CN)2] are the [010] and [201] directions and (102̅) planes, respectively (see Figure S7). Figure 5b shows the results of this method of analysis, with diagrams describing the relevant directions in the solid-state structure shown in Figure 5c and d. As expected, the direction along the cyano chainsdCNundergoes NTE for all materials, with αCN = −13.7(11) ppm·K−1 (M = Ag, X = Cl), −14.3(5) ppm·K−1 (M = Ag, X = Br), and −11.36(18) ppm· K−1 (M = Cu, X = Br) (see Table 4). The other directions, however, undergo substantial PTE, with values of 31−75 ppm· K−1 (see Table 4 for exact values). The values of αCN for M[AuX2(CN)2] are not remarkable (cyanide-based coordination polymer materials exhibiting NTE where |α| > 100 ppm·K−1 are known);23,24,55 however, they are similar to the αCN values reported for Au(I)/Cu(I)-based

M[Au(CN)2] (Table 3).61 The mechanism reported for NTE observed along the cyano chains in M[Au(CN)2] involves a spring-like action, where the 1-D chains bend into a slight zigzag with increasing temperatures, resulting in a contraction along the chains. Such a mechanism thus depends greatly on the flexibility of the overall network, which in turns depends on the metal−cyano bond strength and the strength of the interchain interactions (weaker interactions would result in higher flexibility and, thus, a potentially higher degree of thermal expansion). In the present case, changing the metal from Ag(I) to Cu(I) results in an observable decrease in αCN. A decrease in the radius of the metal center is well-known to result in a decrease in αCN for cyano-bridged metals due to an increase in M− N(cyano) bond strength for smaller atoms, resulting in a more rigid framework and thus restricting the ability to expand or contract with changing temperatures.80 However, in the Au(I)based system (M[Au(CN)2]), the trend is reversed: the Cu(I)based analogue exhibits stronger NTE along the cyano chains. The increase in αCN from Ag(I) to Cu(I) in M[Au(CN)2] was reportedly due to the differing arrangement of M···M interactions; whereas both Au···Ag and Ag···Ag interactions are still significant in Ag[Au(CN)2], Au···Cu interactions are less so, and Cu···Cu interactions are nonexistent (or very weak) in Cu[Au(CN)2]. This “loosening” of the weak interactions gives more freedom for the spring-like action described above, thus resulting in the larger negative αCN for the Cu(I) system and explaining why interaction strength orthogonal to the 1-D chains directly affects the thermal expansion along the chains. A more extreme example of this effect can be seen in the materials MCN (M = Au, Ag, Cu), which are nearly isostructural to M[Au(CN)2]. While AuCN exhibits a αCN value of −9.0 ppm· K−1 (similar to Ag[Au(CN)2]), AgCN and CuCN exhibit much higher values of −24.8 and −30.7 ppm·K−1, respectively.61 Since AuCN chains are very tightly bound by many Au···Au interactions, and AgCN and CuCN do not contain such M···M interactions (Ag···Ag and Cu···Cu interactions are known, although much weaker than Au···Au interactions), much higher α values are observed for AgCN and CuCN than in AuCN. These observations show that drastically varying the strength (or the absence or presence thereof) of intermolecular interactions can easily overcome the opposing impact of increasing the atomic radius of the metal centers. Relating this to the M[AuX2(CN)2] materials reported here, the similarity between the αCN values for M[AuX2(CN)2], AuCN, and M[Au(CN)2] suggests that the M···X interactions impact the thermal expansion in a similar manner to metallophilicity (i.e., the binding action of these interactions serves to restrict the magnitude of the thermal expansion parameters). However, the αCN values for the M[AuBr2(CN)2] materials reported here do follow the typical atomic radius trend (i.e., larger αCN for M = Ag than for M = Cu), contrasting the observations for M[Au(CN)2]; this indicates there there is a negligible difference between the overall strengths of the intermolecular interactions in Ag[AuBr2(CN)2] and Cu[AuBr2(CN)2], allowing the metal radius effects to take hold instead. More specifically, this suggests that either the Ag···Br and Cu···Br interactions are similar in strength to each other or the Au···Br interactions (which are present in both systems) play a much more significant role in the materials’ properties than either of the Ag···Br or Cu···Br interactions, overshadowing their effect. This lack of effect of changing the interactions types on αCN is further seen in the statistically equivalent αCN values observed

Table 4. Thermal Expansion Coefficients for Ag[AuX2(CN)2] (X = Cl, Br) and Cu[AuBr2(CN)2] along the Cyano Bridges (αCN), the Intersheet M···M Interactions (αMM), and along the Au−X···M Distances (αMXM) material

αCN/ppm·K−1a

αMXM/ppm·K−1b

αMM/ppm·K−1c

Ag[AuCl2(CN)2] Ag[AuBr2(CN)2] Cu[AuBr2(CN)2]

−13.7(11) −14.3(5) −11.36(18)

31.1(14) 35.8(18) 39.4(3)

58.9(13) 69.4(10) 74.8(12)

αCN occurs along the [011̅] direction for Ag[AuX2(CN)2] and along the [010] direction for Cu[AuBr2(CN)2]. bαMXM occurs along the [21̅1̅] direction for Ag[AuX2(CN)2] and along the [201] direction for Cu[AuBr2(CN)2]. cαMM occurs perpendicular to the (111) planes for Ag[AuX2(CN)2] and the (102̅) planes for Cu[AuBr2(CN)2]. a

7337

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

Figure 6. (a) Change in unit cell length parameters (red squares: a, blue circles: b, green triangles: c) as temperature is varied for Ag[Au(CN)4]. (b) Change in the interplanar distances corresponding to the (11̅3̅) (red squares) and (101) (blue circles) planes for Ag[Au(CN)4]. Linear fit curves are shown as solid black lines. (c) How these planes relate to the Ag[Au(CN)4] crystal structure. Alternating (11̅3̅) planes are removed for clarity.

coordinated to one metal center, but only weakly interacting with the other, its ability to counteract positive thermal expansion in the Au−X···Ag direction is extremely limited, and thus the αMXM values do not differ as greatly as αMM. M[Au(CN)4] (M = Ag, Cu). The thermal expansion data for Ag[Au(CN)4] and Cu[Au(CN)4] are shown in Figures 6a and S8 (thermal expansion parameters are summarized in Table 3). Like the M[AuX2(CN)2] system above, the Cu(I) material exhibits greater positive thermal volume expansion than the Ag(I) material. This result is more surprising in this case, since all intermetal connections are made by cyano linkages. As for the thermal expansion parameters for the individual crystal axes, Ag[Au(CN)4] exhibits NTE in both the a- and b-axes, while Cu[Au(CN)4] exhibits NTE only in the b-axis and PTE in the a-axis; both materials exhibit PTE in the c-axis. To better understand the observed thermal expansion behavior, thermal expansion parameters in more structurally relevant directions in M[Au(CN)4] can be examined in the same manner as M[AuX2(CN)2]. In particular, the distances between the (113̅ )̅ and (101) planes shown in Figure 6c are of interest, as these planes define two different 2-D networks built of Au and Ag/Cu centers interconnected by cyano moieties, potentially providing useful insight into how each set of cyano bridges contributes to the overall thermal expansion properties. Figure 6b shows the thermal expansion behavior of the distances between these planes, and Table 5 summarizes the corresponding thermal expansion coefficients.

for both Ag[AuCl2(CN)2] and Ag[AuBr2(CN)2] (i.e., the halide does not have a significant effect on this value). In the directions orthogonal to the cyano-bridged chains, the larger Br atoms in Ag[AuBr2(CN)2] result in larger αMM and αMXM values than the smaller Cl atoms in Ag[AuCl2(CN)2]. This effect is most significant in αMM (i.e., in the direction perpendicular to the (111) planes) in Ag[AuX2(CN)2] (see Figure 5). This is due to a combination of two factors: (1) the M···X interactions between the (111) planes are further apart than those in the Au−X···Ag direction, presenting less restriction to expansion, and (2) not only is the spring-like motion responsible for the NTE along the cyano-bridged chains likely restrained to flexing in and out of the (111) planes due to the bulk of the halides along the planes, the aforementioned Au−X···Ag transverse vibrational modes also occur in and out of this plane. Together, these factors would naturally result in larger thermal expansion in this direction. The larger parameter observed for X = Br compared to X = Cl is thus likely due more to geometric constraints than to the strength of any M···X interactions. The Au−X···Ag bonding system (i.e., αMXM) in the third direction (along the [21̅1̅] direction) can be analyzed in a similar way to M−O−M′ systems, as they both exhibit a single transverse vibrational mode. In the latter case, since the O atom is strongly bound to both of the metal centers, its transverse vibrational mode often results in negative thermal expansion. However, since the halogen in the present system is 7338

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

flexibility, which better facilitates the mechanism shown in Figure 7 than does the Cu(I) centers in Cu[Au(CN)4]. While examining the distances between the (11̅3̅) and (101) planes was beneficial in understanding the thermal expansion mechanism of the M[Au(CN)4] materials, the question of why αV is much greater for Cu[Au(CN)4] than for Ag[Au(CN)4] is still outstanding. To understand why this is the case, the variation of the cell angle, β, with temperature is worth examining. In terms of the structure, this angle represents how the sheets shown in Figure 2 and 7 are oriented with respect to one another, independent of the expansions and contractions for the (11̅3̅) and (101) interplanar distances discussed above. Since β > 90°, any increase in this angle with temperature would partially negate any effect on αV from any expansion exhibited by the a- and c-axes. Indeed, αβ is appreciably higher for Ag[Au(CN)4] than for Cu[Au(CN)4] (46 vs 31 ppm·K−1), which ultimately leads to a lower αV value for Ag[Au(CN)4], as observed.

Table 5. Thermal Expansion Coefficients for M[Au(CN)4] (M = Ag, Cu) Perpendicular to the (113̅ )̅ and (101) Planes material

α(11̅3̅)/ppm·K−1

α(101)/ppm·K−1

Ag[Au(CN)4] Cu[Au(CN)4]

47.1(9) 38.0(5)

−30.2(10) −28.1(15)

This analysis reveals a significant expansion of the distance between the (11̅3̅) planes (α(11̅3̅) = 38.0(5)−47.1(9) ppm·K−1) concomitant with a similarly large contraction of the distance between the (101) planes (|α(101)| = 28.1(15)−30.2(10) ppm· K−1). NTE occurring simultaneously in all independent directions is rare, even in systems featuring only one major binding mode; thus this result is not surprising.25 As shown in Figure 6c, the (101) planesthe distances between which show NTEare connected by infinite, unbroken −Au−CN− Ag− chains, whereas the (11̅3̅) planes, which show PTE, are connected by discrete Ag−NC−Au−CN−Ag segments. In other words broken cyano-bridged chains are poor contributers to NTE properties when compared to complete, continuous cyano-bridged chains. This property can be observed structurally by examining the out-of-plane displacement of the Ag/Cu atoms relative to the (11̅3̅) planes (see Figure 6c): the displacement of the Ag(I) atoms out of the (11̅3̅) planes increases with increasing temperature, resulting in overall PTE in the direction perpendicular to the (113̅ )̅ planes. For clarity, Figure 7 shows how these processesthe contraction of the

■

CONCLUSIONS This contribution has focused on two new coordination polymer systems: M[AuX2(CN)2], with 1-D chains interconnected by M···X interactions, and the 3-D network, M[Au(CN)4]. Although the M[AuX2(CN)2] system is tightly bound, its thermal stability was found to be lower than other trans[AuX2(CN)2]−- and [Au(CN)4]−-containing materials, suggesting that the overall thermal stability may in fact be hindered by the presence of weak, halogen-based interactions. The dielectric constants measured for Ag[Au(CN)4] and Ag[AuCl2(CN)2] are unremarkable when compared to a wide array of other common inorganic materials, although Ag[AuCl2(CN)2], compared to the similar material AgCl (εr = 11.5), exhibits a surprisingly high value despite a lower density of heavy, polarizable atoms. To probe this, further dielectric studies, including an examination of the effect of current loss, are in progress. Dielectric constant measurements on coordination polymer materials remain rare. The thermal expansion properties of Ag[AuBr2(CN)2] and Cu[AuBr2(CN)2], exhibiting thermal expansion parameters along the −Au−CN−M− chains (αCN) of −14.3(5) and −11.36(18) ppm·K−1, respectively, were compared to the related M[Au(CN)2] system. While the values themselves are similar, a reversal in their trend with respect to the metal center is observed (i.e., the M[AuBr2(CN)2] system shows an increase in αCN when replacing Cu with Ag, while the reverse is observed in the M[Au(CN)2] system). Since both systems consist of a similar structure type (parallel, linear cyano-bound chains held together by a variety of weak interactions), two major contributors to the αCN thermal expansion properties are possible. (1) Atomic radius of the metal center: In cyano-bound systems such as these, the effect of increasing the atomic radius of the metal centers is well-documented80 as resulting in an increase in the magnitude of the thermal expansion coefficient. This is explained as arising from a weaker M−CN bond, thus affording a higher degree of flexibility, allowing the network to expand and contract more easily. (2) Weak interactions: The effect of weak interactions on the thermal expansion properties is a more complicated discussion; previously reported theoretical and experimental studies have suggested that increasing the strength or quantity of these interactions generally decreases the magnitude of thermal expansion properties.23,55 This rationalizes the difference in αCN

Figure 7. Graphical representation of the thermal expansion mechanism of Ag[Au(CN)4].

(11̅3̅) interplanar distances and the expansion of the (101) interplanar distanceswork in concert, resulting in a “breathing” mechanism, giving the thermal expansion parameters tabulated in Table 5. In comparing Ag[Au(CN)4] and Cu[Au(CN)4], it was observed that while the NTE exhibited by the distance between the (101) planes (α(101)) does not differ appreciably, α(11̅3̅) for Ag[Au(CN)4] is significantly larger than that for Cu[Au(CN)4]. Returning to the effects of atomic radii discussed earlier, the larger Ag(I) center results in a higher degree of 7339

DOI: 10.1021/acs.inorgchem.6b03153 Inorg. Chem. 2017, 56, 7332−7343

Article

Inorganic Chemistry

Synthetic Procedures. Ag[AuCl2(CN)2]. A 20 mL 1:1 MeOH/ acetone solution of AgNO3 (303 mg; 1.78 mmol) was added to a 15 mL colorless 2:1 MeOH/acetone solution of [nBu4N][AuCl2(CN)2] (1126 mg; 2.00 mmol), resulting in an immediate cream-colored suspension of Ag[AuCl2(CN)2]. The yellow solid was collected by vacuum filtration, washed three times with water, and stored in a dark location (532 mg; 70% yield). IR (cm−1): 2226 (m; νCN). Raman (cm−1): 2237 (m; νCN), 659 (w), 520 (s), 327 (vs; νAuCl), 145 (s). Anal. Calcd for C2N2AgAuCl2: C 5.62, H 0.00, N 6.55. Found: C 5.66, H 0.19, N 6.15. Bulk purity was further demonstrated by PXRD (see Figure S1). Ag[AuBr2(CN)2]. A 5 mL aqueous solution of AgNO3 (32 mg; 0.19 mmol) was added to a 5 mL yellow aqueous solution of K[AuBr2(CN)2] (68 mg; 0.22 mmol), resulting in an immediate yellow suspension of Ag[AuBr2(CN)2]. The yellow solid was collected by vacuum filtration, washed three times with water, and stored in a dark location (80 mg; 81% yield). IR (cm−1): 2218 (m; νCN). Raman (cm−1): 2229 (m; νCN), 518 (s), 403 (w), 368 (w), 201 (vs; νAuBr), 137 (s). Anal. Calcd for 3C2N2AgAuBr2 + AgBr: C 4.15, H 0.00, N 4.84. Found: C 4.13, H 0.22, N 4.76. Bulk purity was further demonstrated by PXRD (see Figure S2). Cu[AuBr2(CN)2]. A 4 mL CH2Cl2 solution of [Cu(MeCN)4]OTf (0.05 mol·L−1; 0.2 mmol) was added to a yellow 2 mL CH2Cl2 solution of [nBu4N][AuBr2(CN)2] (132; 0.203 mmol), resulting in an immediate nearly black precipitate in a green solution. This mixture was centrifuged and washed with CH2Cl2 three times and allowed to air-dry overnight. The resulting intense red powder of Cu[AuBr2(CN)2] was then collected (82 mg; 86% yield). IR (cm−1): 2228 (νCN; m). The presence of a small (