Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP‑N(CH2CH2PiPr2)2‑Ligated Nickel Pincer Complexes

The nickel PNP pincer complex (iPrPNP)­NiPh (iPrPNP = κP,κN,κP-N­(CH2CH2PiPr2)2) was prepared by reacting (iPrPNP)­NiBr with PhMgCl or deprotonating [(iPrPNHP)­NiPh]Y (iPrPNHP = κP,κN,κP-HN­(CH2CH2PiPr2)2; Y = Br, PF6) with KOtBu. The byproducts of the PhMgCl reaction were identified as [(iPrPNHP)­NiPh]Br and (iPrPNP′)­NiPh (iPrPNP′ = κP,κN,κP-N­(CHCHPiPr2)­(CH2CH2PiPr2)). The methyl analog (iPrPNP)­NiMe was synthesized from the reaction of (iPrPNP)­NiBr with MeLi, although it was contaminated with (iPrPNP′)­NiMe due to ligand oxidation. Protonation of (iPrPNP)­NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(iPrPNHP)­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 (iPrPNP)­NiMe, whereas at 70 °C with (iPrPNP)­NiPh, both resulting in [(iPrPNHP)­NiCl]Cl as the final product. Protonation of (iPrPNP)­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 (iPrPNP)­NiPh and (iPrPNP)­NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (iPrPNP′)­NiBr, (iPrPNP)­NiPh, (iPrPNP′)­NiPh, (iPrPNP)­NiMe, [(iPrPNHP)­NiPh]Y (Y = Br, PF6, BPh4), [(iPrPNHP)­NiPh]2[NiCl4], [(iPrPNHP)­NiMe]Y (Y = Cl, Br, BPh4), [(iPrPNHP)­NiBr]­Br, and [(iPrPNHP)­NiCl]­Cl, were characterized by X-ray crystallography.