Structure-property relationships of copper(I) tetrachloroaluminate and related compounds

Copper(I) tetrachloroaluminate, a metal halide analog of the corner-shared tetrahedral aluminophosphate framework materials, is exploited to explore the dynamic physical properties and reactivity of metal halide crystalline lattices. Upon irradiation with UV light, heating, or exposure to small molecule liquids and gases, CuAlCl₄ undergoes dynamic physical and chemical changes. The α- and β-CuAlCl₄ phases show brilliant blue to blue-green luminescence. The electronic structure of the CuAlCl ₄ corner shared tetrahedral framework is explored by fluorimetric and diffuse reflectance measurements on the isomorphous series α-CuAlBr _xCl_4−x (x = 0–4). The photoluminescence is demonstrated to be a bulk property of the electronically isolated CuCl_4/2 tetrahedra within the framework matrix. The sorption/desorption of small molecule gases further results in the reversible quenching of the photoluminescence. The β- and α-phases of CuAlCl₄ are further characterized by solid-state 27Al and 63Cu magic angle spinning nuclear magnetic resonance. The very short spin-lattice relaxation times of the copper spins, and the sensitivity of the I = 3/2 63Cu nucleus to the small differences in the local structure of Cu in the two phases, allow 63Cu spectra to be acquired in very short time periods (1 minute), in which the β and α phases are clearly resolved. This time resolution is exploited to follow the phase transition from the pseudo-hexagonal close-packed β-CuAlCl ₄ into the pseudo-cubic close-packed α-CuAlCl₄, which occurs above 100°C. In situ time-resolved 63Cu MAS NMR and synchrotron x-ray diffraction experiments were used to measure the kinetics of this phase transition as a function of temperature. The transformation is shown to be a first-order phase transition involving no intermediate phases with an activation energy of 138 kJ/mol. The kinetic data obey a first order Avrami-Erofe'ev rate law. A one-dimensional growth mechanism is proposed that involves a combination of Cu+ ion self-diffusion and a translational reorganization of the close-packed anion layers imposed by the periodic rotations of [AlCl₄−] tetrahedra. Ethylene reversibly reacts with the solid phases of CuAlCl₄ to yield one and two equivalent adduct phases. Upon sorbtion the three dimensional metal halide framework is deconstructed into loosely associated one-dimensional chains, which upon desorbtion reconstruct the three-dimensional α-CuAlCl₄ framework. The structures of the adduct phases and reactive pathways that interconvert them are probed by single-crystal X-ray diffraction, time-resolved in situ synchrotron powder X-ray, multinuclear solid state NMR spectroscopy, and UV-vis diffuse reflectance spectroscopy. Three crystalline phases are reported. At ambient temperature, under ethylene pressures below 100 Torr, formation of α-(C₂H₄)CuAlCl₄ and β-(C ₂H₄)CuAlCl₄ is favored. When the initial pressure of ethylene is greater than 1 atmosphere, the (C₂H₄) ₂CuAlCl₄ phase is observed to form without going through a one equivalent intermediate. The structure reactivity relationship of the ethylene copper aluminum chloride system suggests some general understandings of how the coordination environment of Cu(I) affects its ability to act as σ-acceptor and π-donor in olefin separations and catalysis.