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

• Published in: Coordination chemistry reviews Vol. 250; no. 13; pp. 1681 - 1695
• Main Authors: Grills, David C., Huang, Kuo-Wei, Muckerman, James T., Fujita, Etsuko
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.07.2006
• Subjects: Binding rates; Dinitrogen complex; Flash photolysis; Nitrogen fixation; Time-resolved Infrared; Nitrogen fixation; Flash photolysis; Binding rates; Dinitrogen complex; Time-resolved Infrared
• ISSN: 0010-8545; 1873-3840
• DOI: 10.1016/j.ccr.2006.01.002

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.