Inorganic Chemistry, Vol.56, No.13, 7332-7343, 2017
Thermal Expansion Behavior of M'[AuX2(CN)(2)]-Based Coordination Polymers (M = Ag, Cu; X = CN, CI, Br)
Two sets of trans-[AuX2(CN)(2)]--based coordination polymer materialsM[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 AuX and MX (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 ?r = 36 for Ag[AuCl2(CN)(2)] (1 kHz) and a lower ?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 (?CN) of -13.7(11), -14.3(5), and -11.36(18) ppmK1 for Ag[AuCl2(CN)(2)], Ag[AuBr2(CN)(2)], and Cu[AuBr2(CN)(2)], respectively. The AuX and AgX interactions affect the thermal expansion similarly to metallophilic AuAu 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, ?V, is 41(3) and 68.7(19) ppmK1 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.