Structural characterization of MAO and related aluminum complexes. 1. Solid-state Al-27 NMR with comparison to EFG tensors from ab initio molecular orbital calculations
Pl. Bryant et al., Structural characterization of MAO and related aluminum complexes. 1. Solid-state Al-27 NMR with comparison to EFG tensors from ab initio molecular orbital calculations, J AM CHEM S, 123(48), 2001, pp. 12009-12017
Experimental and ab initio molecular orbital techniques are developed for s
tudy of aluminum species with large quadrupole coupling constants to test s
tructural models for methylaluminoxanes (MAO). The techniques are applied t
o nitrogen- and oxygen-containing complexes of aluminum and to solid MAO is
olated from active commercial MAO preparations. (Aminato)- and (propanolato
)aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied
with three Al-27 NMR techniques optimized for large Al quadrupole coupling
constants: field-swept, frequency-stepped, and high-field MAS NMR. Four-me
mbered (aminato)aluminum complexes with AlN4 coordination yield slightly sm
aller C-q values than similar AlN2C2 Sites: 12.2 vs 15.8 MHz. Planar 3-coor
dinate AlN2C sites have the largest C-q values, 37 MHz. In all cases, molec
ular orbital calculations of the electric field gradient tensors yields C-q
and eta values that match with experiment, even for a large hexameric (ami
nato) aluminum cage. A D-3d symmetry hexaaluminum oxane cluster, postulated
as a model for MAO, yields a calculated C-q of -23.7 MHz, eta = 0.7474, an
d predicts a spectrum that is too broad to match the field-swept NMR of met
hylaluminoxane, which shows at least three sites, all with C-q values great
er than 15 MHz but less than 21 MHz. Thus, the proposed hexaaluminum cluste
r, with its strained four-membered rings, is not a major component of MAO.
However, calculations for dimers of the cage complex, either edge-bridged o
r face-bridged, show a much closer match to experiment. Also, MAO preparati
ons differ, with a gel form of MAO having significantly larger Al-27 C-q va
lues than a nongel form, a conclusion reached on the basis of Al-27 NMR lin
e widths in field-swept NMR spectra acquired from 13 to 24 T.