Thermal Bimolecular Homodimerization Initiates Nickel(I) Catalysis from 2,6-Aryl-Unsubstituted Bipyridyl Nickel(II) Oxidative Addition Complexes

A procedure for the general synthesis of nickel(II)–aryl bromide oxidative addition complexes without aryl substitution at the 2,6-positions was achieved by inverse addition of the requisite bipyridyl nickel(0)-1,5-cyclooctadiene (COD) complex to the aryl bromide, yielding 2,2′-bipyridyl (bpy) and 4,4′-di- tert -butyl-2,2′-bipyridyl (dtbbpy) supported nickel(II)–aryl bromide compounds in 38–93% yield. During reductive C(sp 2 )–C(sp 3 ) cross-electrophile cross-coupling in DMA at 60 °C, UV–vis absorbance spectroscopy determined (dtbbpy)nickel(II)–aryl bromide complexes to be the predominant catalyst resting states. The observed catalyst resting states were implicated in forming the organic products of C(sp 2 )–C(sp 3 ) cross-coupling and of C(sp 2 )–C(sp 2 ) homocoupling, the homocoupling of which was more predominant with electron-donating substituents on the C(sp 2 ) fragment. Dissolution of (dtbbpy)nickel(II)–aryl bromide compounds in 0.5 mM DMA in the absence of excess aryl bromide established decay of the compounds within minutes to form aryl homodimer and dimeric (dtbbpy)nickel(I) bromide as the exclusive products, in which compounds with the fastest rates of decay were those bearing electron-donating substituents (OMe, t -Bu), correlated with more nucleophilic character of the aryl and weaker Ni–aryl bond strength. Determination of concentration-dependence for homodimerization of (dtbbpy)NiBr(4-F-C 6 H 4 ) and (bpy)NiBr(4-F-C 6 H 4 ) compounds measured by UV–visible absorbance spectroscopy exhibited 1.5-order and 1.3-order dependence, respectively, rationalized as exhibiting concurrent bimolecular and unimolecular mechanisms, namely, bimolecular transmetalation/reductive elimination and unimolecular autoreduction, respectively, the latter of which was informed by rapid comproportionation of nickel(II)–aryl with nickel(0), yielding nickel(I)–aryl intermediates detected by X-band EPR spectroscopy at 77 K. These studies informed kinetically relevant aryl homodimerization as the predominant mechanism for low-valent nickel formation, initiating the nickel(I/II/III) alkyl radical chain mechanism by homogeneous turnover of the (dtbbpy)nickel(II)–aryl bromide catalyst.