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

Abstract

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) × 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 ) × 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.