Theoretical investigations of NMR chemical shifts and reactivities of oxovanadium(v) compounds

Abstract

Employing gradient-corrected levels of density-functional theory (DFT), medium-sized basis sets, and optimized geometries, chemical shifts are calculated for [VOClnF3-n] (n = 0-3), VF5, [VO(OCH2CH2)3N], [V(CO)6]-, [V(CO)5(N2)]-, as well as for the model compounds [VO(OMe)nMe3-n] (n = 0-3) and their AlH3 adducts. Experimental trends in δ(51V) are well reproduced with DFT-based methods; for example, the slopes of the δ(51V)calc vs. δ(51V)expt linear regression lines are 0.92 and 1.03 at the GIAO-BP86 and GIAO-B3LYP levels, respectively. Ethylene polymerization observed with [V(O ⋯ AlX3)(OR)nR′3-n] (X, R, R′ = bulky alkyl, aryl, or silyl groups) is shown for model systems (X = H, R = R′ = Me) to proceed by insertion of the olefin into a V - C bond via a transition state with approximate square-pyramidal coordination about vanadium. For the tri- and dialkyl derivatives (n = 0, 1), similar activation barriers of ca. 19 kcal/mol are computed (BP86 level including zero-point energies), whereas that of the monoalkyl species (n = 2) is predicted to be much higher, ca. 30 kcal/mol. The relevance of these results for the apparent relationship between δ(51V) and catalytic activities is discussed. © 1998 John Wiley & Sons, Inc.