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

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Abstract
Rapid quenching of a melt of CuCl and AlCl3 results in the formation of the metastable framework structure, β-CuAlCl4. The structure, determined by single-crystal X-ray crystallography (space group Pna21, 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 °C results in a phase transition to the more thermodynamically stable α-CuAlCl4. The solid solution CuAlCl4-xBrx is described for both α and β phases. The structures of α-CuAlCl4 and α-CuAlBr4, determined by single-crystal X-ray diffraction (space group P4̄2c, a = b 5.4409(1) Å and c = 10.1126(3) Å, V = 299.37(1) Å3, 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 α- and β-CuAlCl4 reveal large van der Waals channels, which are proposed to be important for the reversible adsorption of carbon monoxide and ethylene by these materials.