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-CoIII(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 [CoII(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 [CoII(dmgH)2P­(nBu)3] when exposed
to adventitious oxygen has also been characterized. This impurity
features a 1H NMR resonance at −5.06 ppm that was
recently but erroneously attributed to the hydride resonance of “[H-CoIII(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.