Nickel-Mediated Carbon-Heteroatom Bond Formation and Efforts Towards High-Oxidation State Nickel Complexes.

Abstract

The formation of carbon-heteroatom bonds via reductive elimination from late transition metals is a topic that has recently garnered great interest in the field of organometallic chemistry. The Sanford lab has done a great deal of research in this field, especially with regards to palladium and platinum. Given the high cost and relative rarity of these metals, investigations were extended to nickel. Ni(phpy)2 (phpy = 2-phenylpyridine) was the initial target due to the success of the analagous PdII(phpy)2 complex at supporting isolable high oxidation state PdIV complexes. The Ni synthesis was accomplished using a metal-assisted deprotonation to install the second phpy ligand onto the metal, following an oxidative addition into the C–Br bond of 2-(2-bromophenyl)pyridine. NiII(phpy)2 was reacted it with a wide variety of electrophilic oxidants. In all cases, reductive elimination of the C–C homocoupling of the ligands was the major organic product, making further elucidation and identification of the intermediates difficult. The reaction also proceeded too quickly to detect either NiIII or NiIV species spectroscopically, even at low temperatures. Interestingly, however, there were low yields of products resulting from C–X bond reductive elimination. Van Koten’s isolation of an organometallic NiIII complex with a nickel-carbon bond and multiple bromide ligands provided an initial point of investigation into the reductive elimination of C–X bonds directly from an isolable NiIII organometallic complex. Under forcing conditions desired product was observed in 33% yield. Ni(phpy)(Pic)(Br) (Pic = 2-methylepyridine) was reacted with various brominating reagents, which resulted in our desired C–X reductive elimination product (2-(2-bromophneyl)pyridine) being generated in high yields with minimal competing C-C bond formation. Mechanistic investigations suggested that this transformation was possibly taking place via a transient high oxidation state intermediate. A number of strategies to stabilize a high oxidation state organometallic Ni complex were pursued, including using perhalogenated aryl ligands, trifluoromethyl ligands, and tridentate chelating ligands. Unfortunately, the desired scaffolds were not isolated.