Ligand Redox Effects in the Synthesis, Electronic Structure, and Reactivity of an Alkyl−Alkyl Cross-Coupling Catalyst

The ability of the terpyridine ligand to stabilize alkyl complexes of nickel has been central in
obtaining a fundamental understanding of the key processes involved in alkyl−alkyl cross-coupling reactions.
Here, mechanistic studies using isotopically labeled (TMEDA)NiMe2 (TMEDA = N,N,N‘,N‘-tetramethylethylenediamine) have shown that an important catalyst in alkyl−alkyl cross-coupling reactions, (tpy‘)NiMe
(2b, tpy‘ = 4,4‘,4‘ ‘-tri-tert-butylterpyridine), is not produced via a mechanism that involves the formation of
methyl radicals. Instead, it is proposed that (terpyridine)NiMe complexes arise via a comproportionation
reaction between a Ni(II)−dimethyl species and a Ni(0) fragment in solution upon addition of a terpyridine
ligand to (TMEDA)NiMe2. EPR and DFT studies on the paramagnetic (terpyridine)NiMe (2a) both suggest
that the unpaired electron resides heavily on the terpyridine ligand and that the proper electronic description
of this nickel complex is a Ni(II)−methyl cation bound to a reduced terpyridine ligand. Thus, an important
consequence of these results is that alkyl halide reduction by (terpyridine)NiRalkyl complexes appears to be
substantially ligand based. A comprehensive survey investigating the catalytic reactivity of related ligand
derivatives suggests that electronic factors only moderately influence reactivity in the terpyridine-based
catalysis and that the most dramatic effects arise from steric and solubility factors.