Switchable Product Selectivity in Diazoalkane Coupling Catalyzed by a Two-Coordinate Cobalt Complex

The low-coordinate monovalent cobalt complex (IPr)Co[N(SiMe3)DIPP] [2, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; DIPP = 2,6-diisopropylphenyl], supported by bulky amide and N-heterocyclic carbene (NHC) ligands and its 9-diazofluorene (FluN2) adduct (IPr)Co[N(SiMe3)DIPP](FluN2) (3) are described. Complex 3 was characterized as possessing a high-spin divalent cobalt center antiferromagnetically coupled to a ligand-based radical, resulting in an overall triplet spin ground state (S = 1). Both 2 and 3 are catalyst precursors for the homocoupling of FluN2 in benzene under ambient conditions to produce 1,2-di(9H-fluoren-9-ylidene)hydrazine (8) and 9,9′-bifluorenylidene (9) in a ratio of 1:8.1. A switch in product selectivity was observed for the reaction in the polar solvent tetrahydrofuran (THF), or in the presence of exogenous good L-type ligands such as tert-butylnitrile, to generate the corresponding hydrazine 8 as the major product. A mechanistic study was carried out to rationalize the observed product distributions. The reaction exhibits first-order rate dependence on both the FluN2 and cobalt catalyst (2) concentrations (monitored by 1H NMR spectroscopy), and 3 was identified as the catalytic resting state. Theoretical calculations were carried out to simulate the production of hydrazine 8 and olefin 9. The result predicted turnover frequencies (TOFs) of 4.6 × 10–7 and 2.3 × 10–6 s–1 for the generation of 8 and 9 in benzene, respectively, in good agreement with the experimentally observed product ratio. Modeling the reaction in media with higher polarity such as THF resulted in a more favorable kinetic barrier toward the formation of hydrazine 8 due to the stabilization of the more polar C–N bond-forming transition state (8, TOF = 2.6 × 10–5 s–1 vs 9, TOF = 6.4 × 10–6 s–1, in THF). Moreover, simulation of the potential energy surface with a coordinated L-type donor, such as acetonitrile, suggests that the selectivity switch could also result from a modified ligand field, rendering diazoalkane adduct 3 more nucleophilic and lowering the barrier of rate-limiting C–N bond formation to give hydrazine 8.