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 Ni-II complexes with a new class of electron-rich and sterically hindered PNP pincer ligands, Me4PNPR (R = Pr-i, Bu-t), 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) (NiMe)-Me-II]B(Ar-F)(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 Ni-I 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 Ni-II-Me bond in [(Me4PNPiPr) (NiMe)-Me-II]B(Ar-F)(4) leads to the formation of a Me radical detected by radical traps and Ni(I )intermediates that can be trapped in the presence of halide ions to give previously characterized, stable Ni-I halogen complexes. In addition, treatment of the bromo complexes [(Me4PNPR)(NiBr)-Br-II]BPh4 with a powerful hydride source, LiBEt3H, leads to the reduction of the pyridine ring and the formation of Ni-II complexes with an anionic amide donor reduced pincer ligand, although aromatic Ni-II 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 Ni(II)H( )complexes. While t-Bu-substituted [(Me4PNPtBu) (NiH)-H-II]Y(Y=BPh4, B(Ar-F)(4)) does not react with O-2, the less sterically hindered Pr-i-substituted [(Me4PNPiPr)(NiH)-H-II]Y reacts instantaneously to give an unstable superoxide adduct that can be observed by EPR.