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

Abstract

The low-coordinate monovalent cobalt complex (IPr)Co[N(SiMe 3 )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 9diazofluorene (FluN 2 ) adduct (IPr)Co[N(SiMe 3 )DIPP](FluN 2 ) (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 FluN 2 in benzene under ambient conditions to produce 1,2di(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 FluN 2 and cobalt catalyst (2) concentrations (monitored by 1 H 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.

(PNP)Cobalt-Catalyzed Olefination of Diazoalkanes

“…The order in substrate and catalyst were obtained using the method of initial rates (Figure b,c) due to the well-behaved kinetics at early conversion and the rapid disappearance of product, even at lower concentrations. The data support a first-order dependence for both the cobalt precatalyst and diazoalkane, similar to the reported kinetic profile of the olefination of diazoalkane E by (IPr)­Co­[N­(SiMe 3 )­DIPP] . Such a rate law is consistent with olefin formation from direct nucleophilic attack from the carbene of a second equivalent of diazoalkane to the cobalt–diazoalkane complex, as suggested by computational studies performed by Tilley and co-workers. , …”

“…The data support a first-order dependence for both the cobalt precatalyst and diazoalkane, similar to the reported kinetic profile of the olefination of diazoalkane E by (IPr)­Co­[N­(SiMe 3 )­DIPP] . Such a rate law is consistent with olefin formation from direct nucleophilic attack from the carbene of a second equivalent of diazoalkane to the cobalt–diazoalkane complex, as suggested by computational studies performed by Tilley and co-workers. , …”

“…Cobalt–diazoalkane complexes remain rare; only five examples have been isolated and structurally characterized. − ,, However, the isolation of cobalt diazoalkane complex Co-3 also provided insight into the coordination chemistry responsible for the lack of catalytic turnover and motivated the synthesis of other complexes that were active for olefination. Treatment of a benzene- d 6 solution of Co-1 with 1 equiv of tolyldiazomethane resulted in a rapid color change from red to green to deep orange (Figure ).…”

“…Recently, Tilley and co-workers reported the catalytic olefination of diazoalkanes with cobalt precatalysts . Both the N -heterocyclic carbene (NHC)-supported amide complex, (IPr)­Co­[N­(SiMe 3 )­DIPP], and the diazofluorene derivative, (IPr)­Co­[N­(SiMe 3 )­DIPP]­(FluN 2 ), produced olefin and azine products in an 89:11 ratio (Scheme a).…”

“…6 Recently, Tilley and co-workers reported the catalytic olefination of diazoalkanes with cobalt precatalysts. 7 Both the N-heterocyclic carbene (NHC)-supported amide complex, (IPr)Co[N(SiMe 3 )DIPP], and the diazofluorene derivative, (IPr)Co[N(SiMe 3 )DIPP](FluN 2 ), produced olefin and azine products in an 89:11 ratio (Scheme 1a). Density functional theory (DFT) calculations for the olefination suggest that both the redox noninnocent 9-diazofluorene and a low-coordinate cobalt compound are required for the diazoalkane coupling.…”