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.