Structural Characterization of MAO and Related Aluminum Complexes. 1. Solid-State ²⁷Al NMR with Comparison to EFG Tensors from ab Initio Molecular Orbital Calculations

Abstract

Experimental and ab initio molecular orbital techniques are developed for study of aluminum species with large quadrupole coupling constants to test structural models for methylaluminoxanes (MAO). The techniques are applied to nitrogen- and oxygen-containing complexes of aluminum and to solid MAO isolated from active commercial MAO preparations. (Aminato)- and (propanolato)aluminum clusters with 3-, 4-, and 6-coordinate aluminum sites are studied with three (27)Al NMR techniques optimized for large (27)Al quadrupole coupling constants: field-swept, frequency-stepped, and high-field MAS NMR. Four-membered (aminato)aluminum complexes with AlN(4) coordination yield slightly smaller C(q) values than similar AlN(2)C(2) sites: 12.2 vs 15.8 MHz. Planar 3-coordinate AlN(2)C sites have the largest C(q) values, 37 MHz. In all cases, molecular orbital calculations of the electric field gradient tensors yields C(q) and eta values that match with experiment, even for a large hexameric (aminato)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, and predicts a spectrum that is too broad to match the field-swept NMR of methylaluminoxane, which shows at least three sites, all with C(q) values greater than 15 MHz but less than 21 MHz. Thus, the proposed hexaaluminum cluster, with its strained four-membered rings, is not a major component of MAO. However, calculations for dimers of the cage complex, either edge-bridged or face-bridged, show a much closer match to experiment. Also, MAO preparations differ, with a gel form of MAO having significantly larger (27)Al C(q) values than a nongel form, a conclusion reached on the basis of (27)Al NMR line widths in field-swept NMR spectra acquired from 13 to 24 T.

Tools and strategies for processing diffusion-ordered 2D NMR spectroscopy (DOSY) of a broad, featureless resonance: an application to methylaluminoxane (MAO)

“…The dominant, broad MAO resonance is multi-component, likely to consist of clusters and linked clusters [14,15]. Thus, analysis strategies which are based on peak integration cannot be used.…”

“…-the average molecular mass is 1100 Da [14]; -the empirical formula by chemical analysis is Al(CH 3 ) 1.4-1.5 O 0.8-0.75 , which can be approximated by monomer unit Al 4 (CH 3 ) 6 O 3 [14]; -aluminium chemistry supports bridging methyl groups [17]; -field-swept NMR does not indicate a high concentration of three-coordinate aluminium sites [15]; -calculated 1 H chemical shifts of model structures and motifs place most methyl resonances at -1 to 0 ppm and bridging methyls at 0 to 1 ppm (unpublished work); and -the slow diffusing species have resonances near 1 ppm, in the region of suspected bridging methyl groups (unpublished work).…”

Theoretical and 27Al CPMAS NMR investigation of aluminum coordination changes during aluminosilicate dissolution

“…A perfectly tetrahedral Al site would have C Q ϭ 0 MHz. Larger quadrupole coupling constants for four-fold Al have previously been attributed to strain in the structure (Bryant et al, 2001). The full relaxation of the Q 3 [4] Al cluster probably represents a more distorted AlO 4 site than found on the nepheline glass surface, resulting in a larger calculated C Q value.…”

Olefin oligomerization, homopolymerization and copolymerization by late transition metals supported by (imino)pyridine ligands

“…For the sake of simplicity, MAO is commonly referred to as linear chain or cyclic rings [-Al(Me)-O-] n containing three-coordinate aluminum centers, yet the true structure of MAO is still a matter of debate [79]. It may be a dynamic mixture of linear-, ringand cage-complexes, all formed from methyl aluminoxane subunits during the controlled hydrolysis of trimethyl aluminum [80][81][82]. Some proposed structures for MAO include one-dimensional linear chains and cyclic rings containing three-coordinate Al centers, two-dimensional structures, and three-dimensional clusters (Scheme 9) [83].…”