Structure-Property Relationships of Copper(I) Tetrachloroaluminate and Related Compounds.

Abstract

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, CuAlCl&#8324; undergoes dynamic physical and chemical changes. The a- and a-CuAlCl&#8324; phases show brilliant blue to blue-green luminescence. The electronic structure of the CuAlCl&#8324; corner shared tetrahedral framework is explored by fluorimetric and diffuse reflectance measurements on the isomorphous series a-CuAlBr[subscript x]Cl&#8324;[subscript-x] (x=0-4). The photoluminescence is demonstrated to be a bulk property of the electronically isolated CuCl[subscript 4/2] tetrahedra within the framework matrix. The sorption/desorption of small molecule gases further results in the reversible quenching of the photoluminescence. The a- and a-phases of CuAlCl&#8324; are further characterized by solid-state &#178;&#8311;Al and &#8310;&#179;Cu 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 &#8310;&#179;Cu nucleus to the small differences in the local structure of Cu in the two phases, allow &#8310;&179;Cu spectra to be acquired in very short time periods (1 minute), in which the b and a phases are clearly resolved. This time resolution is exploited to follow the phase transition from the pseudo-hexagonal close-packed b-CuAlCl&#8324; into the pseudo-cubic close-packed a-CuAlCl&#8324;, which occurs above 100 &deg;C. In situ time-resolved &#8310;&#179;Cu 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[superscript+] ion self-diffusion and a translational reorganization of the close-packed anion layers imposed by the periodic rotations of AlCl&#8324;- tetrahedra. Ethylene reversibly reacts with the solid phases of CuAlCl&#8324; 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 a-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 <font face="symbol">a</font>-(C&#8322;H&#8324;)CuAlCl&#8324; and b-(C&#8322;H&#8324;)CuAlCl&#8324; is favored. When the initial pressure of ethylene is greater than 1 atmosphere, the (C&#8322;H&#8324;)&#8322;CuAlCl&#8324; 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) effects its ability to act as s-acceptor and p-donor in olefin separations and catalysis.