Severely Bent Dinitrogen Bridging in Highly Preorganized Dinuclear Cobalt Complexes Featuring an Intricate Electronic Structure

Preorganized bimetallic complexes could open up new avenues of cooperative substrate activation and transformation of inert dinitrogen (N₂). While the most common structural motif in synthetic dinuclear N₂ complexes is a linear M-N-N-M unit, only a few examples of bent geometries reminiscent of the proposed N₂ binding modes in the Haber-Bosch process or the M-cluster of Nitrogenase have been reported. Exploiting the structural constraints imposed by a compartmental pyrazolato/β-diketiminato hybrid ligand platform (L^3̵-), we here report a series of dicobalt-(I) complexes [LCo₂(N₂)]^- hosting N₂ within the preorganized bimetallic cleft with extremely acute Co-Ct_N2-Co angles of around 123.5° (Ct_N2 is the N₂ centroid). A detailed electronic structure analysis using wave function methods shows that the ground state of diamagnetic [LCo₂(N₂)]^- may not be a simple closed-shell singlet, but rather of multiconfigurational nature subject to spin-orbit coupling. Co-N-N-Co bending significantly decreases overlap of metal d-orbitals with the in-plane p-(N) orbitals, yet the N₂ substrate is substantially more activated (<i>ν̃</i> _NN ≈ 1900 cm^-1) than in most Co^I complexes with end-on bound N₂ or linear Co-N-N-Co arrangement; no coactivation by the alkali cation K⁺ is observed for [LCo₂(N₂)]^-. Reversible oxidation gives an unusual mixed-valent complex [LCo^ICo^II(N₂)] in which the highly bent Co-N-N-Co core is retained. [LCo₂(N₂)]-[K-(THF)₂] is found to cocrystallize with KBEt₃H, indicating that the putative [LCo₂(N₂H)]^2- has a very low hydricity. In presence of KC₈ and Me₃SiCl, complex [LCo₂(N₂)]^- serves as (pre)-catalyst for the reductive silylation of N₂ into N-(SiMe₃)₃. We discuss the implications of the highly exposed, "<i>cis</i>-bent" N₂ unit for onward reactivity.