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Download fileStructural Characterization of Dialkylaluminum Carboxylates: Models for Carboxylate Alumoxanes
journal contribution
posted on 1997-02-04, 00:00 authored by Charles E. Bethley, Candice L. Aitken, C. Jeff Harlan, Yoshihiro Koide, Simon G. Bott, Andrew R. BarronThe 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.