59th Annual Report on Research 2014

In parallel with studying the reactivity of high-valent organometallic Pd complexes, we began to synthesize, characterize, and evaluate the chemical properties of related high-valent Ni complexes. Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. In addition to Ni(0) and Ni(II) intermediates, several Ni-catalyzed reactions are pro-posed to also involve odd-electron Ni(I) and Ni(III) oxidation states. In this context, we have recently reported the isolation and characterization of a series of Ni^III(aryl)halide complexes stabilized by the tetradentate ligand N, N’-di-tert-butyl-2,11-diaza[3.3](2,6) pyridinophane (^tBuN4), which was recently employed by us to stabilize uncommon mononuclear Pd^III complexes. The described organometallic Ni^III species exhibit structural and electronic properties that suggest a metal-based radical description, in line with the presence of a Ni^III center. These complexes are stable at low temperature, yet they undergo rapid C-halide bond formation at room temperature, providing evidence that such Ni^III species are the active intermediates in oxidatively induced C-heteroatom bond formation reactions (Scheme 1). The characterized Ni^III(aryl)halide complexes undergo rapid transmetallation with Grignard or organozinc reagents to yield detectable Ni^III(aryl)alkyl species followed by C-C bond formation, strongly supporting their role as intermediates in Kumada and Negishi cross coupling reactions. In addition, one-electron oxidation of an isolated Ni^II(aryl)alkyl complex leads to rapid formation of reductive elimination products (Scheme 1), and both (^tBuN4)Ni^II and (^tBuN4)Ni^III species are active catalysts for Kumada and Negishi cross-coupling reactions. Overall, these studies provide for the first time strong evidence for the direct involvement of organometallic Ni^III complexes in Ni-mediated cross-coupling reactions and oxidatively induced C-heteroatom bond formation reactions.

Scheme 1

While a Ni^III(aryl)alkyl intermediate was spectroscopically characterized in the study mentioned above, that complex undergoes rapid reductive elimination and could not be isolated. In order to further stabilize such organometallic Ni^III complexes, we hypothesized that the use of perfluoroakyl ligands will slow down the reductive elimination reactivity of these species. Such an approach has been elegantly used recently by Vicic et al. to detect spectroscopically a terpyridine-supported bis(trifluoromethyl)Ni^III complex. In this context, we have also recently reported the isolation and detailed characterization of uncommonly stable mononuclear bis(trifluoromethyl)Ni^III complexes supported by the pyridinophane ligands ^MeN4 and ^tBuN4. Interestingly, these Ni^III complexes can be generated upon facile oxidation of the Ni^II precursors, including aerobic oxidation, and surprisingly they exhibit a very limited reactivity. Overall, these studies strongly suggest the ^RN4 tetradentate ligand system is capable of stabilizing various bis(hydrocarbyl)Ni^III complexes that would allow for their isolation and detailed investigation of their reactivity relevant to cross-coupling reactions.

Scheme 2