Shining Light on Dinitrogen Cleavage: Structural Features, Redox Chemistry, and Photochemistry of the Key Intermediate Bridging Dinitrogen Complex

摘要

The key intermediate in dinitrogen cleavage by Mo(N[t-Bu]Ar)_3,1 (Ar = 3,5-C_6H_3Me_2), has been characterized by a pair of single crystal X-ray structures. For the first time, the X-ray crystal structure of (μ-N_2)[Mo(N[t-Bu]Ar)_3]_2,2, and the product of homolytic fragmentation of the N-N bond, N≡Mo(N[t-Bu]Ar)_3, are reported. The structural features of 2 are compared with previously reported EXAFS data. Moreover, contrasts are drawn between theoretical predictions concerning the structural and magnetic properties of 2 and those reported herein. In particular, it is shown that 2 exists as a triplet (S = 1) at 20℃. Further insight into the bonding across the MoNNMo core of the molecule is obtained by the synthesis and structural characterization of the one- and two-electron oxidized congeners, (μ-N_2)[Mo(N[T-Bu]Ar)_3]_2[B(Ar~F)_4], 2[B(Ar~F)_4] (Ar~F = 3,5-C_6H_3(CF_3)_2) and (μ-N_2)[Mo(N[t-Bu]Ar)_3]_2[B(Ar~F)_4]_2, 2[B(Ar~F)_4]_2, respectively. Bonding in these three molecules is discussed in view of X-ray crystallography, Raman spectroscopy, electronic absorption spectroscopy, and density functional theory. Combining X-ray crystallography data with Raman spectroscopy studies allows the N-N bond polarization energy and N-N internuclear distance to be correlated in three states of charge across the MoNNMo core. For 2[B(Ar~F)_4], bonding is symmetric about the μ-N_2 ligand and the N-N polarization is Raman active; therefore, 2[B(Ar~F)_4] meets the criteria of a Robin-Day class III mixed-valent compound. The redox couples that interrelate 2,2~+, and 2~(2+) are studied by cyclic voltammetry and spectroelectrochemistry. Insights into the electronic structure of 2 led to the discovery of a photochemical reaction that forms N≡Mo(N[t-Bu]Ar)_3 and Mo(N[t-Bu]Ar)_3 through competing N-N bond cleavage and N_2 extrusion reaction pathways. The primary quantum yield was determined to be Φ_p = 0.05, and transient absorption experiments show that the photochemical reaction is complete in less than 10 ns.