Kinetic studies of the photoinduced formation of transition metal-dinitrogen complexes using time-resolved infrared and UV-vis spectroscopy

摘要

Previous kinetic studies of photoinitiated transition metal-dinitrogen bond forming reactions are reviewed, with an emphasis on room temperature reactivity, and in particular, the techniques of time-resolved infrared (TRIR) spectroscopy and UV-vis flash photolysis. Our recent results on the reactivity of the formally 16-electron, but agostically stabilized, complex, mer,trans-W(CO)_3(PCy_3)_2 (W) (Cy = cyclohexyl) toward N_2 in toluene and n-hexane solution are then discussed. Laser flash photolysis of a toluene solution of W-N_2 in the presence of excess N_2 resulted in the photoejection of N_2. The back reaction of W with N_2 was followed by monitoring the decay of the transient absorption of W at 600 nm. The second-order rate constant for the reaction of N_2 with W in toluene to generate W-N_2 was found to be (3.0 ± 0.2) x 10~5 M~(-1) s~(-1). The rate of the reverse reaction was found to be 100 ± 10 s~(-1), allowing an estimation of the equilibrium constant, K_(N_2) = (3.0 ± 0.5) x 10~3 m~(-1). Time-resolved step-scan FTIR (s~2-FTIR) spectroscopy was also used to spectroscopically characterize the W intermediate and monitor its back-reaction with N_2 in n-hexane solution. The rate of formation of W-N_2 measured by s~2-FTIR agreed well with that measured by flash photolysis. Finally, density functional theory (DFT) calculations have been performed on the model complexes, mer,trans-W(CO)_3(PH_3)_2(L) (L = none and N_2) in order to understand the observed IR and UV-vis spectra of W and W-N_2 and to determine the nature of the frontier molecular orbitals of W and W-N_2, allowing their lowest energy excited states to be assigned.