The cobalt hydride that never was: revisiting Schrauzer's "hydridocobaloxime"

Molecular cobalt-dmg (dmg = dimethylglyoxime) complexes are an important class of electrocatalysts used heavily in mechanistic model studies of the hydrogen evolution reaction (HER). Schrauzer's early isolation of a phosphine-stabilized "[H-Co(III)(dmgH)2P(nBu)3]" complex has long provided circumstantial support for the plausible intermediacy of Co(III)-H species in HER by cobaloximes in solution. Our investigation of this complex has led to a reassignment of its structure as [Co(II)(dmgH)2P(nBu)3], a complex that contains no hydride ligand and dimerizes to form an unsupported Co-Co bond in the solid state. A paramagnetic S = 3/2 impurity that forms during the synthesis of [Co(II)(dmgH)2P(nBu)3] when exposed to adventitious oxygen has also been characterized. This impurity features a (1)H NMR resonance at -5.06 ppm that was recently but erroneously attributed to the hydride resonance of "[H-Co(III)(dmgH)2P(nBu)3]". We draw attention to this reassignment because of its relevance to cobaloxime hydrides and HER catalysis and because Schrauzer's "hydridocobaloxime" is often cited as the primary example of a bona fide hydride that can be isolated and characterized on this widely studied HER platform.