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Severely Bent Dinitrogen Bridging in Highly Preorganized Dinuclear Cobalt Complexes Featuring an Intricate Electronic
Yue Wang1, Shweta Singh2, Andreas Meyer3,4
1Institute of Inorganic Chemistry, University of Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany.
None:
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.
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