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.