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

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.