Synthesis, Structure, and Properties of [(tmeda)Cu[Hg(CN)₂]₂][HgCl₄]:  A Non-Centrosymmetric 2-D Layered System that Shows Strong Optical Anisotropy

Abstract

The first coordination polymer incorporating Hg(CN) 2 units as building blocks is reported. The reaction of the labile transition-metal halide CuCl 2, the soft Lewis acidic Hg(CN) 2, and N,N,N‘,N‘-tetramethylethylenediamine (tmeda) formed large single crystals of [(tmeda)Cu[Hg(CN) 2 ] 2 ][HgCl 4 ] ( 1 ). The migration of two labile chloride ligands from harder Cu(II) to softer Hg(II) drives the formation of 1, which self-assembles in high yield from several combinations of reagents. Complex 1 crystallizes in the tetragonal, noncentric P 4̄2 1 m space group, Z = 2, with a = b = 10.9576(12) Å and c = 9.8516(13) Å. The structure is composed of a 2-D cationic layer of [(tmeda)Cu[Hg(CN) 2 ] 2 ] 2+ units in which the six-coordinate Cu(II) centers are bridged by four Hg(CN) 2 groups and capped by a tmeda ligand. This array is interspersed with a layer of [HgCl 4 ] 2 - anions, which form bridging Hg−Cl bonds with the Hg(CN) 2 units. At 300 K, a - or b -oriented single-crystal plates of 1 are strongly birefringent with a birefringence value of Δ n = 6.38 × 10 - 2, as measured at λ = 546.1 nm. The strongly anisotropic crystal structure, including the highly polarizable Hg(II) ions, is responsible for this significant optical birefringence. Upon cooling, the birefringence of 1 decreases slightly to Δ n = 6.12 × 10 - 2 at 90 K.