Multidentate Lewis Acids. Synthesis and Structure of Complex Aluminum Chloride Alkoxides

Complexes of simple aluminum chloride alkoxides with THF can be prepared by treating the corresponding alcohols or trimethylsilyl ethers with aluminum chloride and THF. Low-temperature ¹H, ¹³C, and ²⁷Al NMR spectra suggest that the products of these reactions are inhomogeneous and exist in solution as mixtures resulting from redistribution. More complex aluminum chloride alkoxides and their adducts can be prepared from the bis(trimethylsilyl) ethers of cyclic 1, 2-diols. These chloride alkoxides also appear to undergo redistribution in solution, but some adopt well-defined structures in the solid state. An X-ray crystallographic study of representative chloride alkoxide 8b, derived from a trans-1, 2-cyclohexanediol, reveals a characteristic pentacyclic structure organized around a central Al₂O₂ ring containing approximately trigonal oxygens and trigonal-bipyramidal aluminums. Compound 8b crystallizes in the triclinic space group [formula omitted] with unit cell parameters a = 8.710 (3) Å, b = 9.309 (5) Å, c = 9.600 (3) Å, a = 88.82 (3)°, β = 82.02 (3)°, γ = 75.31 (4)°, V = 745.6 ų, = 1.333 g cm^-3, and Z = 1 (dimer). Full-matrix least-squares refinement of 1425 reflections (I ≥ 3.00σ(I)) converged at R = 0.054 and R_w = 0.050.