Thermal Expansion Behavior of MI[AuX2(CN)2]‑Based Coordination Polymers (M = Ag, Cu; X = CN, Cl, Br)

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 cyano-bound 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.