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

Reactions of the bis-((R₂phosphanyl)-methyl)-1<i>H</i>-benzo-[<i>d</i>]-imidazole-3-ium hexafluorophosphate precursors [(PCP-R)-H]-PF₆ (R = <i>i</i>Pr, 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-<i>i</i>Pr)-(CO)₂H]-PF₆ (<b>1</b>), [Co-(PCP-<i>i</i>Pr)-(CO)₂]-PF₆ (<b>2</b>), [Co-(PCP-Ph)-(CO)₂]-PF₆ (<b>3</b>), [Ni-(PCP-<i>i</i>Pr)-(cyclooct-4-en-1-yl)]-PF₆ (<b>4</b>) and [Ni-(PCP-<i>i</i>Pr)-H]-PF₆ (<b>5</b>), 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-[<i>d</i>]-imidazolidene PC-<i>i</i>Pr [Fe-(PC-<i>i</i>Pr)-(CO)₃] (<b>6</b>) and [Fe-(PC-<i>i</i>Pr)-(CO)₃H]-BF₄ (<b>7</b>) 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 <b>2</b> and <b>3</b> were catalytically active in the hydroboration of styrene with pinacolborane (HBPin) using 1 mol % of precatalyst and 2 mol % of KO<i>t</i>Bu in THF at 70 °C for 18 h with yields of 87-93%. In addition, complex <b>2</b> also catalyzed the hydroboration of terminal alkenes in good yields (68-88%). Reaction of complex <b>2</b> with 5 equivs of HBPin and 2 equivs of KO<i>t</i>Bu in THF gave rise to the cobalt-(I) hydride complex [Co-(κ²-(P,C)-PCP-<i>i</i>Pr)-H-(CO)₂] (<b>8</b>), indicating that the mechanism of the catalytic process follows a cobalt-(I) hydride pathway.