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 (L3̵-), we here report a series of dicobalt-(I) complexes [LCo2(N2)]- hosting N2 within the preorganized bimetallic cleft with extremely acute Co-CtN2-Co angles of around 123.5° (CtN2 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 (ν̃ NN ≈ 1900 cm-1) than in most CoI 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 [LCoICoII(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, "cis-bent" N2 unit for onward reactivity.