Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a Novel Diphosphonite Pyridinophane Ligand

Abstract

A series of (N2P2)Ni<sup>II</sup> complexes (N2P2 = P,P’-ditertbutyl-2,11-diphosphonito[3.3](2,6)pyridinophane) stabilized by a modified tetradentate pyridinophane ligand containing two phosphonite groups were synthesized and characterized. Cyclic voltammetry (CV) studies revealed the accessibility of the Ni<sup>I</sup> oxidation state at moderate redox potentials for these Ni<sup>II</sup> complexes. <i>In situ</i> EPR, low-temperature UV-vis, and electrochemical studies were employed to detect the formation of Ni<sup>I</sup> species during the reduction of Ni<sup>II</sup> precursors. Furthermore, the [(N2P2)Ni<sup>I</sup>(CNtBu)](SbF<sub>6</sub>) complex was isolated upon reduction of the Ni<sup>II</sup> precursor with 1 equiv of CoCp<sub>2</sub>, and was characterized by EPR and X-ray photoelectron spectroscopy (XPS). Finally, the (N2P2)Ni<sup>II</sup>Br<sub>2</sub> complex acts as an efficient catalyst for the Kumada cross-coupling of an aryl halide with an aryl or alkyl Grignard, suggesting that the N2P2 ligand can support the various Ni species involved in the catalytic C-C bond formation reactivity.