Nickel(II) Complexes with Electron-Rich, Sterically Hindered PNP Pincer Ligands Enable Uncommon Modes of Ligand Dearomatization

We report the reactivity
and characterization of hydride, methyl,
and bromo NiII complexes with a new class of electron-rich
and sterically hindered PNP pincer ligands, Me4PNPR (R = iPr, tBu), in which a classical metal–ligand cooperative mode
of reactivity via CH2 arm deprotonation is blocked by methylation.
This enables new, uncommon modes of PNP ligand dearomatization that
involve reactivity in the para position of the pyridine ring. In particular,
the reduction of [(Me4PNPiPr)­NiIIMe]­B­(ArF)4 with KC8 leads to the formation of a new C–C bond via dimerization
of two complexes through the para position. This reactivity stands
in sharp contrast to the previously reported bromo or chloro complexes,
where stable NiI halogen moieties are formed. Computational
analysis showed a greater propensity for ligand-centered radical formation
for the presumed intermediate one-electron-reduced species. UV-induced
homolysis of the NiII–Me bond in [(Me4PNPiPr)­NiIIMe]­B­(ArF)4 leads to the formation of a Me radical detected by
radical traps and NiI intermediates that can be trapped
in the presence of halide ions to give previously characterized, stable
NiI halogen complexes. In addition, treatment of the bromo
complexes [(Me4PNPR)­NiIIBr]­BPh4 with a powerful hydride source, LiBEt3H, leads
to the reduction of the pyridine ring and the formation of NiII complexes with an anionic amide donor reduced pincer ligand,
although aromatic NiII hydride complexes could also be
obtained with a weaker hydride source. We have observed that steric
bulk at the phosphine donors controls the reactivity of the resulting
NiIIH complexes. While t-Bu-substituted
[(Me4PNPtBu)­NiIIH]Y (Y = BPh4, B­(ArF)4) does not
react with O2, the less sterically hindered iPr-substituted [(Me4PNPiPr)­NiIIH]Y reacts instantaneously to give an unstable
superoxide adduct that can be observed by EPR.