Bimetallic Cooperativity in Proton Reduction with an Amido‐Bridged Cobalt Catalyst

Abstract

The bimetallic catalyst [Co (L )(bpy) ]ClO (1), in which L is an [NN' O ] fused ligand, efficiently reduced H to H in CH CN in the presence of 100 equiv of HOAc with a turnover number of 18 and a Faradaic efficiency of 94 % after 3 h of bulk electrolysis at -1.6 V (vs. Ag/AgCl). This observation allowed the proposal that this bimetallic cooperativity is associated with distance, angle, and orbital alignment of the two Co centers, as promoted by the unique Co-N -Co environment offered by L . Experimental results revealed that the parent [Co Co ] complex undergoes two successive metal-based 1 e reductions to generate the catalytically active species [Co Co ], and DFT calculations suggested that addition of a proton to one Co triggers a cooperative 1 e transfer by each of these Co centers. This 2 e transfer is an alternative route to generate a more reactive [Co (Co -H )] hydride, thus avoiding the Co -H required in monometallic species. This [Co (Co -H )] species then accepts another H to release H .

Electrocatalytic proton reduction by dinuclear cobalt complexes in a nonaqueous electrolyte

“…The formation of [Co( ii )(Co( ii )–H − )] is responsible for the hydrogen evolution. 20 The Orio group suggested the ECEC pathway for a dicobalt complex [Co 2 L 2 (NCS) 2 ] (where H 2 L = {bis[4-( p -methoxyphenyl)thiosemicarbazone]}-2,3-butane) as the electrocatalyst for the hydrogen evolution, which was inferred based on electrochemical and DFT studies. 21 Miras and co-workers reported on a set of two electrocatalysts [Co II 2 (μ 2 -L) 2 ] (where H 2 L = N 2 , N 6 -di(quinolin-8-yl)pyridine-2,6-dicarboxamide and [Co II 2 (μ 2 -L Bu ) 2 ] (H 2 L Bu = 4-( tert -butyl)- N 2 , N 6 -di(quinolin-8-yl)pyridine-2,6-dicarboxamide), which catalyzes the reduction event with AcOH as an acid source in DMF/H 2 O medium, resulting in a TOF value of 10 and 34.6 h −1 mol cat −1 , respectively.…”

“…The formation of [Co(II)(Co(II)-H À )] is responsible for the hydrogen evolution. 20 The Orio group suggested the ECEC pathway for a dicobalt complex [Co 2 L 2 (NCS) 2 ] (where H 2 L = {bis [4-(p-methoxyphenyl)thiosemicarbazone]}-2,3butane) as the electrocatalyst for the hydrogen evolution, which was inferred based on electrochemical and DFT studies. 21 , respectively.…”

A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution

“…Gray and co‐workers reported a dinuclear cobalt(III) complex consisting of two cobaloxime units covalently linked by a long alkyl chain, which performed electrocatalytic HER through a heterolytic pathway, but the efficiency was no greater than that of its mononuclear analogue . Recently, Verani and co‐workers reported an arylamido‐bridged dinuclear Co II complex (Figure ), in which favourable Co orbital orientations in the reduced [Co I Co I ] species facilitated a cooperative two‐electron‐transfer process to give a more reactive [Co II (Co II −H − )] intermediate, which, in turn, underwent heterolytic attack from a proton to generate H 2 —the only dicobalt complex proposed to perform the HER through an intramolecular bimetallic mechanism …”

Electrocatalytic hydrogen production by dinuclear cobalt(ii) compounds containing redox-active diamidate ligands: a combined experimental and theoretical study

“…This value is similar to the ostensible overpotentials of 440 and 560 mV for [Cu II (HL)] + and [Ni II (HL)], respectively, both monometallic complexes with one quinoline-protonated ligand that operate with 92% faradaic efficiency. 20,21 The overpotential is comparable to related Co-based electrocatalysts, such as [Co II 2 (bpy) 2 (L 1 )] + (η = 660 mV; L 1 is a pentadenate diarylamide-bridged Schiff base), 13 [Co II (R 3 tpy) 2 ] 2+ (η = 690 mV; R 3 tpy are chelating trispara-substituted-terpyridine ligands), 16 [Co III Cl(L 1CvO )] and [Co II Cl(L 2 )] (η = 690 and 700 mV, respectively; L 1CvO and L 2 are pentadenate [N 2 N py 3 ] polypyridyl ligands), 40 [Co II 2 (L N6O2 )] + (η = 600 mV; L N6O2 is a bis( phenolate) tetrakis-Schiff base macrocyle), 41 and well below the performance of archetypal cobaloximes at 90 mV. 9 Theoretical calculations Density functional theoretical (DFT) calculations were undertaken to explore a possible mechanism by which 1 catalyzes the production of H 2 .…”

“…11 Although not unheard of, dinuclear cobalt complexes are comparatively rare. 12,13 The presence of multiple metal centers can aid a homolytic reaction pathway in which adjacent Co III -H units dissociate to give hydrogen radicals H • which subsequently combine to produce H 2 . 14 This reaction pathway requires two metal centers in close proximity, which is possible for monomeric units in homogenous solution, but in a fully realized device in which the catalyst is grafted onto a solid substrate this reaction pathway would be available only in dinuclear catalysts.…”