The Cobalt Hydride that Never Was: Revisiting Schrauzer’s “Hydridocobaloxime”

Abstract

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.

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“…52–54 It is particularly unlikely that a CoBF 2 catalyst bound to a protein will diffusionally interact with another Co( iii )–H to proceed through the homo-coupling mechanism. 55,56 Protein aggregates can be excluded as intact complexes can be observed by ICP-AES after photocatalysis or transient optical experiments with similar metal binding before and after catalysis. Thus, our experimental results favour the Co( iii )–H scheme for catalysis, which is supported by our results with PSI–Co biohybrids 57 as well as recent electrochemical studies on cobaloxime complexes.…”

Development of a Bio‐Inspired Dual Catalytic System for Alkane Dehydrogenation

“…Building from the pioneering report of Espenson, [62] the groups of Peters, [63,64] Gray, [65][66][67] Artero, [68,69] and others have established the cobaloxime to be a privileged architecture for electrocatalytic hydrogen evolution. Mechanistic studies of this process have been challenging due to the elusive nature of the Co(III) hydride equivalent, [70][71][72] a species which spontaneously evolves hydrogen through the aforementioned bimolecular combination; however, careful studies by Gray and coworkers have provided a high degree of support for a complementary, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 monometallic hydrogen-evolution cycle accessed under electrochemical conditions (Figure 13). [66] While the specific means of hydrogen evolution for our desired dehydrogenation project is somewhat inconsequential (so long as the cycle can be turned over), the existence of several possible routes for this outcome was heartening as it suggests that this terminating process might be possible regardless of the identity of the preceding steps.…”

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