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(η2:η2-(CH2CHSiMe2)2O)] (NHC = N-heterocyclic carbene) with [Cp2Fe]­[BPh4], we synthesized the zwitterionic cobalt­(I)–NHC complexes
[(IMes)­Co­((η6-C6H5)­BPh3)] (IMes = 1,3-bis­(2,4,6-trimethylphenyl)-imidazole-2-ylidene, 1) and [(IPr)­Co­((η6-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((η6-C6H5)­BPh3)­(CO)] (3), [(IPr)­Co­((η6-C6H5)­BPh3)­(CO)] (4), and
[(IMes)­Co­((η6-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)2Co]­[BPh4] (8) and [(IPr)­Co­(IMes)]­[BPh4] (9) were formed. The structures of these complexes
were established by single-crystal X-ray diffraction studies.