Electrocatalytic hydrogen evolution by cobalt difluoroboryl-diglyoximate complexes
Xile Hu, Brandi M. Cossairt, Bruce S. Brunschwig, Nathan S. Lewis, Jonas C. Peters
2005
Chemical Communications
In the presence of moderately strong acids in CH 3 CN, cobalt complexes with BF 2 -bridged diglyoxime ligands are active catalysts for the reduction of protons to H 2 at potentials as positive as 20.28 V vs. SCE. The search for transition metal complexes that are capable of catalyzing the reduction of protons to dihydrogen at low overpotentials presents an exciting challenge for coordination chemists. 1 Much attention has been drawn to structural and functional models of the active sites of
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... ogenases, especially the H-clusters of the Fe-only hydrogenases, which feature dithiolatebridged, bimetallic iron cofactors that are rich in CO and CN 2 auxiliary ligands. 2,3 The Fe-only hydrogenases catalyze the reduction of protons to dihydrogen at the thermodynamic potential for H 2 uptake/production (ca. 20.41 V vs. NHE at pH 5 7 in water, or ca. 20.65 vs. SCE), 4,5 whereas current biomimetic model compounds only catalyze hydrogen evolution at significantly more negative potentials (from ca. 21.1 to 22 V vs. SCE). 3,4,6 Transition metal complexes that are structurally distinct from the hydrogenase H-cluster effect catalytic hydrogen evolution at comparable, and in some cases more positive, potentials than the biomimetic diiron model systems. Cobaltocene, 7 [CpCo(PR 3 ) 2 ] + , 8 metalloporphyrins, 9 and certain macrocyclic complexes of cobalt 10,11 and nickel, 12 effect catalytic hydrogen evolution either in the presence of sacrificial chemical reductants or electrocatalytically. A cobaloxime system, Co II (dmgBF 2 ) 2 (H 2 O) 2 (dmgBF 2 5 (difluoroboryl)dimethylglyoxime), that catalyzes the reduction of protons to hydrogen by chromous ion in acidic aqueous solution was reported by Connolly and Espenson nearly two decades ago. 11 Mechanistic studies suggested that in this system the ratedetermining step for proton reduction involved electron-transfer (ET) from Cr II to Co II via an inner-sphere ET pathway (L n Cr II -Cl-Co II (dmgBF 2 ) 2 L), followed by dissociation of the bridged species to generate a reactive Co(I) complex [Co I (dmgBF 2 ) 2 L] 2 that is rapidly protonated to provide the hydride [Co III (H)(dmgBF 2 ) 2 L]. 11 The slightly unfavorable thermodynamics for the ET process were thought to be responsible for the slow overall rate of the catalysis. The Co(dmgBF 2 ) 2 system might therefore be well-suited to electrocatalytic hydrogen evolution because reduction of the parent Co(II) complex to the active Co(I) species by a solid-state electrode might be rapid, and the process
doi:10.1039/b509188h
pmid:16175305
fatcat:ntrwlp6uorflhhkelpaonr6g2m