α- and β-CuAlCl₄:  Framework Construction Using Corner-Shared Tetrahedral Metal−Halide Building Blocks

Rapid quenching of a melt of CuCl and AlCl(3) results in the formation of the metastable framework structure, beta-CuAlCl(4). The structure, determined by single-crystal X-ray crystallography (space group Pna2(1), a = 12.8388(5) Å, b = 7.6455(3) Å, and c = 6.1264(3) Å, Z = 2), can be derived from a distorted hexagonal closest packed anion sublattice. Annealing at temperatures above 100 degrees C results in a phase transition to the more thermodynamically stable alpha-CuAlCl(4). The solid solution CuAlCl(4)(-)(x)()Br(x)() is described for both alpha and beta phases. The structures of alpha-CuAlCl(4) and alpha-CuAlBr(4), determined by single-crystal X-ray diffraction (space group P2c, a = b 5.4409(1) Å and c = 10.1126(3) Å, V = 299.37(1) Å(3), Z = 2, and a = b = 5.7321(2) Å and c = 10.6046(8) Å, Z = 2, respectively), are derived from a distorted cubic closest packed anion sublattice. The mechanism for this phase transition is described in relation to that previously described for cristobalite-type structures. The structures of both alpha- and beta-CuAlCl(4) reveal large van der Waals channels, which are proposed to be important for the reversible adsorption of carbon monoxide and ethylene by these materials.