Cobalt and nickel diimine-dioxime complexes as molecular electrocatalysts for hydrogen evolution with low overvoltages

Abstract

Hydrogen production through the reduction of water appears to be a convenient solution for the long-run storage of renewable energies. However, economically viable hydrogen production requests platinum-free catalysts, because this expensive and scarce (only 37 ppb in the Earth's crust) metal is not a sustainable resource [Gordon RB, Bertram M, Graedel TE (2006) Proc Natl Acad Sci USA 103:1209-1214]. Here, we report on a new family of cobalt and nickel diimine-dioxime complexes as efficient and stable electrocatalysts for hydrogen evolution from acidic nonaqueous solutions with slightly lower overvoltages and much larger stabilities towards hydrolysis as compared to previously reported cobaloxime catalysts. A mechanistic study allowed us to determine that hydrogen evolution likely proceeds through a bimetallic homolytic pathway. The presence of a proton-exchanging site in the ligand, furthermore, provides an exquisite mechanism for tuning the electrocatalytic potential for hydrogen evolution of these compounds in response to variations of the acidity of the solution, a feature only reported for native hydrogenase enzymes so far.

Scheme 1.

Molecular structures for the various Co and Ni diimine-dioxime complexes considered in this study. The structure of the cobaloxime complex [Co(dmgBF₂)₂L₂] is represented in the Upper Right.

Fig. 1.

X-ray structure of [Co(MO)(MOH)pnCl₂] (50% thermal ellipsoids). Selected bond lengths (Å) and angles (°): Co-N(4) 1.8902(19), Co-N(1) 1.8952(18), Co-N(3) 1.9140(17), Co-N(2) 1.9247(17), Co-Cl(1) 2.2373(6), Co-Cl(2) 2.2400(6), O(1)-N(1) 1.331(2), O(2)-N(4) 1.324(2), O(1)-H(1O) 1.03(3), O(2)-H(1O) 1.43(3), O(1)…O(2) 2.443(2), N(4)-Co-N(1) 96.79(8), N(4)-Co-N(3) 82.00(8), N(1)-Co-N(3) 178.77(8), N(4)-Co-N(2) 177.91(8), N(1)-Co-N(2) 82.04(7), N(3)-Co-N(2) 99.17(8), N(4)-Co-Cl(1) 90.78(6), N(1)-Co-Cl(1) 90.12(6), N(3)-Co-Cl(1) 89.65(6), N(2)-Co-Cl(1) 90.95(5), N(4)-Co-Cl(2) 88.64(6), N(1)-Co-Cl(2) 89.58(6), N(3)-Co-Cl(2) 90.64(6), N(2)-Co-Cl(2) 89.62(5), Cl(1)-Co-Cl(2) 179.32(3), O(1)-H(1O)…O(2) 169(3).

Fig. 2.

Cyclic voltammograms of [Co((DO)₂BF₂)pnBr₂] and [Co(DO)(DOH)pnBr₂]. (Top) Cyclic voltammograms of [Co((DO)₂BF₂)pnBr₂] (1 mM) in the presence of p-cyano-anilinium tetrafluoroborate (0, 1, 3, 5, 10 equiv). (Middle) Cyclic voltammograms of [Co(DO)(DOH)pnBr₂] (1 mM) in the presence of anilinium tetrafluoroborate. (Inset) Zoom on the Co^III/Co^II process. (Bottom) Same catalyst, but p-cyanoanilinium tetrafluoroborate; glassy carbon electrode–CH₃CN solution containing 0.1 mol·L⁻¹ [ⁿBu₄N][BF₄]. Scan rate: 100 mV·s⁻¹.

Scheme 2.

Possible mechanistic pathways for H₂ evolution.

Scheme 3.

Stoichiometric hydrogen formation from a reduced Co^I complex.

Scheme 4.

Possible intramolecular proton transfer steps along the catalytic mechanism of H-linked dioxime H₂-evolving catalysts.