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

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.