Synthesis, Structure and Reactivity Studies of Nickel Pincer Complexes and Evaluating Redox Non-innocence of the Ligand

Abstract

Bimetallic cores are ubiquitous in natural enzymes and presently being used in several important chemical transformations such as epoxidation of olefins or stereoselective polymerization of oxiranes. To understand the nature of interaction between two metal centers in a bimetallic core we synthesized the diamond core, [Ni(2-PNP)]2 (PNP− = N[2-P(CHMe2)2-4-MeC6H3]2) and observed that the two Ni(I) metal centers are feebly interacting, particularly in an antiferromagnetic manner, despite an extremely short distance (2.3288(7) Å) between them. The molecule behaves as a masked Ni(I) synthon and can perform the cleavage of a plethora of homonuclear or heteronuclear bonds to incorporate rare functionalities onto nickel.

A current interest has been surged in group transfer reactions employing nickel reagents since its higher congeners, namely Pd and Pt are too expensive. Transition metal mediated transfer of boryls to aromatic and aliphatic hydrocarbons to engender corresponding aryl boronic esters appears to be very promising given the widespread utility of the ester in C−C bond coupling reactions. In the attempt of nickel mediated borylation we discovered a simple route to engender formation of a stable, square planar nickel boryl complex, (PNP)NiBcat, (cat = O2C6H4) hitherto unknown in literature. Theoretical calculations (DFT) suggested the single bond nature of the Ni−B bond and quite gratifyingly the boryl moiety was successfully transferred to bromobenzene in a cyclic manner. A mild protocol for the synthesis of relatively rare nickel silyl complexes has also been developed.

An important discovery along these investigations was the conclusive proof of redox non-innocence behavior of the pincer PNP backbone. Prior to our work, the PNP ligand was believed to be a redox inactive ancillary, but through extensive spectroscopies including multi-edge X-ray absorption spectroscopy we were able to show unequivocally its redox non-innocence. First, the redox non-innocence was manifested through a ligand based oxidation of (PNP)NiCl. Later we extend this study over other 3d transition metals to prove that the redox non-innocence of the ligand behaves as a function of metal.