Roles of Hydrogen Bonding in Proton Transfer to κ,κ,κ-N(CHCHP Pr)-Ligated Nickel Pincer Complexes.

The nickel PNP pincer complex ( PNP)NiPh ( PNP = κ,κ,κ-N(CHCHP Pr)) was prepared by reacting ( PNP)NiBr with PhMgCl or deprotonating [( PNP)NiPh]Y ( PNP = κ,κ,κ-HN(CHCHP Pr); Y = Br, PF) with KO Bu. The byproducts of the PhMgCl reaction were identified as [( PNP)NiPh]Br and ( PNP')NiPh ( PNP' = κ,κ,κ-N(CH=CHP Pr)(CHCHP Pr)). The methyl analog ( PNP)NiMe was synthesized from the reaction of ( PNP)NiBr with MeLi, although it was contaminated with ( PNP')NiMe due to ligand oxidation. Protonation of ( PNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in CD. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [( PNP)NiX] with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl, HO, or MeO). Protonation of the Ni-C bond was observed at room temperature with ( PNP)NiMe, whereas at 70 °C with ( PNP)NiPh, both resulting in [( PNP)NiCl]Cl as the final product. Protonation of ( PNP)NiBr was complicated by site exchange between Br and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate ( PNP)NiPh and ( PNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including ( PNP')NiBr, ( PNP)NiPh, ( PNP')NiPh, ( PNP)NiMe, [( PNP)NiPh]Y (Y = Br, PF, BPh), [( PNP)NiPh][NiCl], [( PNP)NiMe]Y (Y = Cl, Br, BPh), [( PNP)NiBr]Br, and [( PNP)NiCl]Cl, were characterized by X-ray crystallography.