Structural Characterization of Dialkylaluminum Carboxylates:  Models for Carboxylate Alumoxanes

The reaction of Al(^tBu)₃ with carboxylic acids, RCO₂H, yields the dimeric di-tert-butylaluminum carboxylates [(^tBu)₂Al(μ-O₂CR)]₂, where R = ^tBu (1), CCl₃ (2), Ph (3), CH₂Ph (4), CHPh₂ (5), CPh₃ (6), C(H)=C(H)Ph (7), CH₂OCH₃ (8), CH₂OCH₂CH₂OCH₃ (9), and CH₂(OCH₂CH₂)₂OCH₃ (10), which have been characterized by ¹H and ¹³C NMR and IR spectroscopy and mass spectrometry. The molecular structures of compounds 1, 3, 4, and 9 have been determined by X-ray crystallography, the first structural determinations for any such compounds. The carboxylate groups act as bidentate bridging ligands consistent with spectroscopic characterization. Ab initio calculations on the model compounds H₂Al(λ²-O₂CH), eclipsed-H₂Al[OC(O)H], staggered-H₂Al[OC(O)H], [H₂Al(μ-O₂CH)]₂, and [H₃Al{OC(O)H}]^- indicate that the observed carboxylate-bridged dimer is thermodynamically favored over the hypothetical chelating monomer. The Al₂O₄C₂ cyclic core in compound 1 is flat while those in compounds 3, 4, and 9 adopts chairlike conformations. The extent of the puckering of the core in [(^tBu)₂Al(μ-O₂CR)]₂ is dependent on the steric bulk of the carboxylate substituent, R. The dimeric compounds, [(^tBu)₂Al(μ-O₂CR)]₂, are satisfactory as simplistic structural models for the carboxylate groups bound to the boehmite-like core of carboxylate alumoxanes, [Al(O)_x(OH)_y(O₂CR)_z]_n, as prepared from the reaction of boehmite with a carboxylic acid. Ab initio calculations on the model anion [(H₅Al)₂(μ-O₂CH)]^5- indicate that the optimum Al···Al distance for the carboxylate ligand bridging two six-coordinate aluminum centers constrained on a surface is 3.2−3.8 Å. The carboxylate ligand is therefore near perfectly suited to bind to the (100) surface of boehmite, Al···Al = 3.70 Å, and hence stabilize the boehmite-like core in carboxylate alumoxanes.