| Abstract
| - Pulsed 355 nm laser excitation of toluene or hexane solutions containing W−L (W = mer,trans-W(CO)3(PCy3)2; PCy3 = tricyclohexylphosphine; L = H2, D2, N2, C2H4, or CH3CN) resulted in thephotoejection of ligand L and the formation of W. A combination of nanosecond UV−vis flash photolysisand time-resolved step-scan FTIR (s2-FTIR) spectroscopy was used to spectroscopically characterize thephotoproduct, W, and directly measure the rate constants for binding of the ligands L to W to reform W−Lunder pseudo-first-order conditions. From these data, equilibrium constants for the binding of L to W wereestimated. The UV−vis flash photolysis experiments were also performed as a function of pressure inorder to determine the activation volumes, ΔV⧧, for the reaction of W with L. Small activation volumesranging from −7 to +3 cm3 mol-1 were obtained, suggesting that despite the crowded W center aninterchange mechanism between L and the agostic W···H−C interaction of one of the PCy3 ligands (or aweak interaction with a solvent molecule) at the W center takes place in the transition state. Density functionaltheory (DFT) calculations were performed at the B3LYP level of theory on W with/without the agostic C−Hinteraction of the PCy3 ligand and also on the series of model complexes, mer,trans-W(CO)3(PH3)2L(W‘−L, where L = H2, N2, C2H4, CO, or n-hexane) in an effort to confirm the infrared spectroscopicassignment of the W−L complexes, to simulate and assign the electronic transitions in the UV−vis spectra,to determine the nature of the HOMO and LUMO of W−L, and to understand the agostic C−H interactionof the ligand vs solvent interaction. Our DFT calculations indicate an entropy effect that favors agosticW···H−C interaction over a solvent σ C−H interaction by 8−10 kcal mol-1.
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