Synthesis, Characterization and Catalytic Activity of Iron, Cobalt and Nickel Complexes Bearing an N‑Heterocyclic Carbene-Based PCP Pincer Ligand

Abstract

Reactions of the bis-((R2phosphanyl)-methyl)-1H-benzo-[d]-imidazole-3-ium hexafluorophosphate precursors [(PCP-R)-H]-PF6 (R = iPr, Ph) with zerovalent precursors [Fe3(CO)12], [Co2(CO)8], [Ni-(COD)2] and [Ni-(PPh3)4], respectively, gave rise to the cationic iron, cobalt and nickel complexes [Fe-(PCP-iPr)-(CO)2H]-PF6 (1), [Co-(PCP-iPr)-(CO)2]-PF6 (2), [Co-(PCP-Ph)-(CO)2]-PF6 (3), [Ni-(PCP-iPr)-(cyclooct-4-en-1-yl)]-PF6 (4) and [Ni-(PCP-iPr)-H]-PF6 (5), by oxidative addition of the benzimidazolium CH bond in [(PCP-R)-H]-PF6. The complexes bearing the bidentate ligand 3-((diisopropylphosphanyl)-methyl)-1-methyl-1H-benzo-[d]-imidazolidene PC-iPr [Fe-(PC-iPr)-(CO)3] (6) and [Fe-(PC-iPr)-(CO)3H]-BF4 (7) were also synthesized. All complexes were characterized by NMR and FTIR spectroscopies, high resolution mass spectrometry and selected cases by single-crystal X-ray diffraction. Cobalt complexes 2 and 3 were catalytically active in the hydroboration of styrene with pinacolborane (HBPin) using 1 mol % of precatalyst and 2 mol % of KOtBu in THF at 70 °C for 18 h with yields of 87-93%. In addition, complex 2 also catalyzed the hydroboration of terminal alkenes in good yields (68-88%). Reaction of complex 2 with 5 equivs of HBPin and 2 equivs of KOtBu in THF gave rise to the cobalt-(I) hydride complex [Co-(κ2-(P,C)-PCP-iPr)-H-(CO)2] (8), indicating that the mechanism of the catalytic process follows a cobalt-(I) hydride pathway.