Structural characterization of dialkylaluminum carboxylates: Models for carboxylate alumoxanes

The reaction of Al((t)Bu)(3) with carboxylic acids, RCO(2)H, yields the dimeric di-tert-butylaluminum carboxylates [((t)Bu)(2)Al(mu-O(2)CR)](2), where R = (t)Bu (1), CCl3 (2), Ph (3), CH(2)Ph (4), CHPh(2) (5), CPh(3) (6), C(H)=C(H)Ph (7), CH2OCH3 (8), CH2OCH2CH2OCH3 (9), and CH2(OCH2CH2)(2)OCH3 (10), which have been characterized by H-1 and C-13 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(lambda(2)-O2CH), eclipsed-H2Al[OC(O)H], staggered-H2Al[OC(O)H], [H2Al(mu-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 I is flat while those in compounds 3, 4, and 9 adopts chairlike conformations. The extent of the puckering of the core in [((t)Bu)(2)Al(mu-O(2)CR)](2) is dependent on the steric bulk of the carboxylate substituent, R. The dimeric compounds, [((t)Bu)(2)Al(mu-O(2)CR)](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)(O(2)CR)(z)](n), as prepared from the reaction of boehmite with a carboxylic acid. Ab initio calculations on the model anion [(H5Al)(2)(mu-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 Angstrom. The carboxylate ligand is therefore near perfectly suited to bind to the (100) surface of boehmite, Al ... Al = 3.70 Angstrom, and hence stabilize the boehmite-like core in carboxylate alumoxanes.