Shining light on dinitrogen cleavage: Structural features, redox chemistry, and photochemistry of the key intermediate bridging dinitrogen complex

The key intermediate in dinitrogen cleavage by Mo(N[t-Bu]Ar)₃, 1 (Ar = 3,5-C₆H₃Me₂), has been characterized by a pair of single crystal X-ray structures. For the first time, the X-ray crystal structure of (μ-N₂)[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°C. 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-N₂)[Mo(N[t-Bu]Ar) ₃]₂[B(Ar^F)₄], 2[B(Ar ^F)₄] (Ar^F = 3,5-C₆H ₃(CF₃)₂) and (μ-N₂)[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-N₂ 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 2²⁺ 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 N₂ 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.