Structural Characterization of Dialkylaluminum Carboxylates: Models for Carboxylate Alumoxanes

Abstract

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.