Cu[Au(CN)2]2(DMSO)2: golden polymorphs that exhibit vapochromic behavior

Two polymorphs of an Au(CN)2]-based coordination polymer, CuAu(CN)2]2(DMSO)2, one green (1) and one blue (2), have been identified. In polymorph 1, alternation of five-coordinate Cu(II) and Au(CN)2]- units generates 1-D chains, while 2-D corrugated sheets are obtained in polymorph 2, which contains six-coordinate Cu(II) centers. Both polymorphs form 3-D networks by virtue of aurophilic interactions of 3.22007(5) A and 3.419(3) A, respectively, and show similar weak antiferromagnetic coupling, but have different thermal decomposition temperatures. They both show vapochromic properties and, importantly, despite their significantly different solid-state structures, the vapochromic behavior of the two polymorphs is essentially identical. Upon solvent exchange, both polymorphs convert to the same CuAu(CN)2]2(solvent)x complex (solvent = H2O, CH3CN, dioxane, N,N-dimethylformamide, pyridine, NH3). The CuAu(CN)2]2(DMF) and CuAu(CN)2]2(pyridine)2 complexes have very similar 2-D square grid structures, comparable to that of 2. The solvent molecules adsorbed by CuAu(CN)2]2 bind to the Cu(II) centers, thereby altering the visible spectrum associated with the Cu(II) chromophores and the number and frequency of the nu(CN) as well. The network-stabilizing gold-gold interactions and the flexible coordination sphere of Cu(II) probably facilitate reversible solvent exchange at room temperature.