Roles of Hydrogen Bonding in Proton Transfer to κ(P),κ(N),κ(P)-N(CH(2)CH(2)P(i)Pr(2))(2)-Ligated Nickel Pincer Complexes

[Image: see text] The nickel PNP pincer complex (((i)Pr)PNP)NiPh (((i)Pr)PNP = κ(P),κ(N),κ(P)-N(CH(2)CH(2)P(i)Pr(2))(2)) was prepared by reacting (((i)Pr)PNP)NiBr with PhMgCl or deprotonating [(((i)Pr)PN(H)P)NiPh]Y (((i)Pr)PN(H)P = κ(P),κ(N),κ(P)-HN(CH(2)CH(2)P(i)Pr(2))(2); Y = Br, PF(6)) with KO(t)Bu. The byproducts of the PhMgCl reaction were identified as [(((i)Pr)PN(H)P)NiPh]Br and (((i)Pr)PNP′)NiPh (((i)Pr)PNP′ = κ(P),κ(N),κ(P)-N(CH=CHP(i)Pr(2))(CH(2)CH(2)P(i)Pr(2))). The methyl analog (((i)Pr)PNP)NiMe was synthesized from the reaction of (((i)Pr)PNP)NiBr with MeLi, although it was contaminated with (((i)Pr)PNP′)NiMe due to ligand oxidation. Protonation of (((i)Pr)PNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C(6)D(6). Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(((i)Pr)PN(H)P)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 (((i)Pr)PNP)NiMe, whereas at 70 °C with (((i)Pr)PNP)NiPh, both resulting in [(((i)Pr)PN(H)P)NiCl]Cl as the final product. Protonation of (((i)Pr)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 (((i)Pr)PNP)NiPh and (((i)Pr)PNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (((i)Pr)PNP′)NiBr, (((i)Pr)PNP)NiPh, (((i)Pr)PNP′)NiPh, (((i)Pr)PNP)NiMe, [(((i)Pr)PN(H)P)NiPh]Y (Y = Br, PF(6), BPh(4)), [(((i)Pr)PN(H)P)NiPh](2)[NiCl(4)], [(((i)Pr)PN(H)P)NiMe]Y (Y = Cl, Br, BPh(4)), [(((i)Pr)PN(H)P)NiBr]Br, and [(((i)Pr)PN(H)P)NiCl]Cl, were characterized by X-ray crystallography.