AmericanChemicalSociety.com

The activation and functionalization of inert C-H bonds is still widely regarded as one of the “holy grails” of organometallic chemistry. These reactions target the conversion of inexpensive hydrocarbon precursors into value-added fine chemicals. Most research on C-H bond activation to date has utilized complexes of the 2nd and 3rd row transition elements. Our research focus is to overcome the thermodynamic and kinetic difficulties associated with the use of less costly Ni complexes for C-H bond activations, and is based on our serendipitous discovery that some Ni(I) complexes perform facile intramolecular C-H bond activations. The mechanisms of C-H bond activation documented in the literature typically occur with complexes bearing an even number of d-electrons, and so the odd number of d-electrons, as found in Ni(I), may indicate that an unprecedented mechanism operates. Our mechanistic studies are aimed at providing details of the C-H bond activation step in these reactions, which will provide fundamental insight into the factors which render these C-H bond activations kinetically facile. The organometallic chemistry of Ni(I) is poorly developed, compared to that of Ni(0) and Ni(II), and thus another area of interest is the design of facile synthetic routes to Ni(I) complexes designed to undergo C-H bond rearrangements. A long-term goal of this work is the development of catalytic C-H bond functionalization using Ni complexes as catalysts.

One of our intial potential routes to the synthesis of Ni(I) organometallic complexes was via the reaction of a highly reactive source of the reactive (PEt3)2Ni moiety with a suitable organometallic Ni(II) precursor. We have prepared the phenanthrene adduct (PEt3)2Ni(C14H10) and found that it is indeed reactive. Contrary to previously published predictions based on DFT calculations, which predicted that nickel complexes should only be capable of C-F bond activation, this source of a bis(phosphine)nickel moiety is itself capable of C-H bond activation! The reaction of (PEt3)2Ni(C14H10) with 1,2,4,5-tetrafluorobenzene was found to yield a rapid equilibrium amount of the C-H bond activation product (PEt3)2NiH(2,3,5,6-C6F4H). This surprising result has led our research efforts into the generation of increasingly reactive nickel(0) complexes capable of C-H bond activation in less activated substrates, and produced unexpected insight into some of the difficulties of using nickel complexes in related inert C-F bond activation processes. It also raises interesting questions regarding whether the C-H bond activation processes involving Ni(I) complexes proceed via initial disproportionation to produce a Ni(0) complex that performs the C-H bond activation. Further research with regards to the potential of the reactions in catalytic conversions is underway.