Relaxation and dissociation following photoexcitation of the (μ-N ₂)[Mo(N[t-Bu]Ar)₃]₂ dinitrogen cleavage intermediate

Frequency resolved pump-probe spectroscopy was performed on isolated (μ-N₂)[Mo(N[t-Bu]Ar)₃]₂ (Ar = 3,5-C ₆H₃Me₂), an intermediate formed in the reaction of Mo(N[t-Bu]Ar)₃ to bind and cleave dinitrogen. Evidence is presented for 300 fs internal conversion followed by subpicosecond vibrational cooling on the ground electronic state in competition with bond dissociation. Fast cooling following photoexcitation leads to a relatively low overall dissociation yield of 5%, in quantitative agreement with previous work [Curley, J. J.; Cooke, T. R.; Reece, S. Y.; Mueller, P.; Cummins, C. C. J. Am. Chem. Soc.2008, 130, 9394]. Coupling of vibrational modes to the excitation and internal conversion results in a nonthermal distribution of energy following conversion, and this provides sufficient bias to allow the nitrogen cleavage reaction to compete with breaking of the Mo-NN bond despite a higher energetic barrier on the ground state.