Highly birefringent cyanoaurate coordination polymers: the effect of polarizable C-X bonds (X = Cl, Br).

Four new coordination polymers, Mn(Clterpy)Au(CN)(2) (Clterpy = 4'-chloro-2,2';6',2''-terpyridine), Mn(Brterpy)Au(CN)(2) (Brterpy = 4'-bromo-2,2';6',2''-terpyridine), Pb(Clterpy)Au(CN)(2), and Pb(Brterpy)(mu-OH(2))(0.5)Au(CN)(2) were synthesized and structurally characterized, and their birefringence values were measured. The supramolecular structures of the two Mn(II) polymers are the same: they form one-dimensional (1D) chains of Mn(Xterpy)Au(CN)(2) units (X = Cl, Br), each having one bridging and one terminal Au(CN)(2)(-). The Pb(II) analogues form 1D polymers containing chains of Pb(Xterpy)Au(CN)(2) linked via Au(CN)(2)(-) units. Pb(Brterpy)(mu-OH(2))(0.5)Au(CN)(2) also contains a bridging water unit. In the plane of the primary crystal growth direction, the birefringence values of the four coordination polymers were found to be 0.378(19), 0.50(3), 0.38(2), and 0.26(3), respectively. The birefringence values are related to the supramolecular structures in terms of maximizing the alignment of the terpyridine-based units and the maximum off-axis positioning of the C-X bonds. With the exception of that for the Pb(Brterpy)(mu-OH(2))(0.5)Au(CN)(2) polymer, the birefringence values are either as large as or significantly larger than in the related M(2,2';6',2''-terpyridine)Au(CN)(2) systems. These polymers illustrate the utility of adding polarizable carbon-halogen bonds as a design element in highly birefringent coordination polymers.