Orientational and Stereochemical Preferences in [Hydridotris(3,5-dimethyl-1-pyrazolyl)borato](CO)2W(η3-allyl) Complexes
journal contributionposted on 19.08.1997, 00:00 by David S. Frohnapfel, Peter S. White, Joseph L. Templeton, Heinz Rüegger, Paul S. Pregosin
Irradiation of Tp‘(CO)3WH in the presence of alkynes or unconjugated dienes generates η3-allyl complexes, Tp‘(CO)2W(η3-CHRCHCHR‘). A kinetic preference for anti-alkyl substitution due to η2-vinyl intermediates exists for the reactions with terminal alkyne substrates. The monoalkyl-substituted allyl complexes are configurationally stable on the NMR time scale but show a thermodynamic preference for syn-substitution upon heating in solution. Separate crystal structures of the syn and anti isomers of the 1-methylallyl complex, Tp‘(CO)2W(η3-CH2CHCHCH3), have revealed isomers which differ by approximately 90° in allyl orientation (θ). The term meso is introduced to emphasize the orthogonal relationship between the anti (θ = 30°, exo) and the syn (θ = 120°, meso) isomers. The change in orientation of the allyl is accompanied by a change in the OC−W−CO angle from acute to obtuse in order to maximize donation from the π-nonbonding orbital of the allyl into the dπ metal orbitals. EHMO (extended Hückel molecular orbital) calculations on a model complex [H3W(CO)2(η3-C3H5)]2- show electronic minima at θ = 30° and 115° corresponding to the two orientations.