THEORETICAL INVESTIGATIONS OF NMR CHEMICAL-SHIFTS AND REACTIVITIES OFOXOVANADIUM(V) COMPOUNDS

Employing gradient-corrected levels of density-functional theory (DFT) , medium-sized basis sets, and optimized geometries, chemical shifts a re calculated for [VOClnF3-n] (n = 0-3), VF5, [VO(OCH2CH2)(3)N], [V(CO )(6)](-), [V(CO)(5)(N2())](-), as well as for the model compounds [VO( OMe)(n)Me3-n] (n = 0-3) and their AlH3 adducts. Experimental trends in delta(V-51) are well reproduced with DFT-based methods; for example, the slopes of the delta(V-51)(calc) vs. delta(V-51)(expt) Linear regre ssion lines are 0.92 and 1.03 at the GIAO-BP86 and GIAO-B3LYP levels, respectively. Ethylene polymerization observed with [V(O ... AlX3)(OR) (n)R'(3-n)] (X, R, R' = bulky alkyl, aryl, or silyl groups) is shown f or model systems (X = H, R = R' = Me) to proceed by insertion of the o lefin into a V-C bond via a transition state with approximate square-p yramidal coordination about vanadium. For the tri-and dialkyl derivati ves (n = 0, 1), similar activation barriers of ca. 19 kcal/mol are com puted (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 betw een delta(V-51) and catalytic activities is discussed. (C) 1998 John W iley & Sons, Inc.