Thermal Expansion Behavior of M^I[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 ε_r = 36 for Ag[AuCl₂(CN)₂] (1 kHz) and a lower ε_r = 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 (α_CN) 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, α_V, 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.