Zwitterionic Cobalt(I)-NHC Complexes with Tetraphenylborate Ligation: Synthesis, Characterization, and Reactivity

Zwitterionic metal complexes of Rh and Ru featuring a tetraphenylborate ancillary ligand have been explored widely in organometallic chemistry. Analogous 3d metal complexes, however, are rarely known. From the oxidation reaction of cobalt(0)-N-heterocyclic carbene complexes [(NHC)Co(eta(2):eta(2)-(CH2CHSiMe2)(2)O)] (NHC = N-heterocyclic carbene) with [Cp2Fe][BPh4], we synthesized the zwitterionic cobalt(I)-NHC complexes [(IMes)Co((eta(6)-C6H5)BPh3)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazole-2-ylidene, 1) and [(IPr)Co((eta(6)-C6H5)BPh3)] (IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene, 2) in good yields. Characterization data and computational studies revealed the S = 1 ground spin state for 1 and 2. These zwitterionic cobalt(I) complexes can act as cobalt(I) synthons to prepare cobalt(I)-NHC complexes bearing other ancillary ligands. Their reactions to CO and CNBut form the zwitterionic cobalt(I) complexes [(IMes)Co((eta(6)-C6H5)BPh3)(CO)] (3), [(IPr)Co((eta(6)-C6H5)BPh3)(CO)] (4), and [(IMes)Co((eta(6)-C6H5)BPh3)(CNBut)] (5) and the ionic cobalt(I) complex [(IMes)Co(CNBut)(4)][BPh4] (6). In the reactions of 2 with pyridine, IPr, and IMes, the ionic cobalt(I)-NHC complexes [(IPr)Co(py)(3)][BPh4] (7), [(IPr)(2)Co][BPh4] (8) and [(IPr)Co(IMes)][BPh4] (9) were formed. The structures of these complexes were established by single-crystal X-ray diffraction studies.