Electrocatalytic CO₂Reduction with a Binuclear Bis‐Terpyridine Pyrazole‐Bridged Cobalt Complex**

Abstract

A pyrazole‐based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO2) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV‐vis spectro‐electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro‐electrochemical studies under CO2 and CO atmosphere are consistent with a reduced CO‐containing dicobalt complex which results from the electroreduction of CO2. In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single‐site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at −1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO‐containing species formed during the electrocatalytic process, which inhibit the reduction of CO2.

Bimetallic Molecular Catalyst Design for Carbon Dioxide Reduction

“…Recently, a dicobalt catalyst with terpyridine ligands linked by a pyrazole unit was reported by the same group (catalyst 23). [62] The catalyst showed selective electrocatalytic reduction of CO 2 to CO in DMF with 1.5 M trifluoroethanol at À 1.1 V vs NHE. However, it suffered from low FE of 39 %, deviating from the better performance of monometallic Co quaterpyridines, possibly indicating non-innocent role of the pyrazole linker.…”

“…Recently, a dicobalt catalyst with terpyridine ligands linked by a pyrazole unit was reported by the same group (catalyst 23 ) [62] . The catalyst showed selective electrocatalytic reduction of CO 2 to CO in DMF with 1.5 M trifluoroethanol at −1.1 V vs NHE.…”

Kinetic isotope effect offers selectivity in CO2 reduction

“…Similar carbonyl species were previously reported in a binuclear cobalt complex bridged by pyrazole. 11 In the presence of saturated CO 2 and 5% (V/V) H 2 O, a strong reduction current is observed at À2.2 V vs. Fc + /Fc 0 with a precatalytic feature at À1.65 V vs. Fc + /Fc 0 which indicates CO 2 binding to the reduced Ni I (Fig. 1A).…”

“…Substantial progress has been made in the last few years and several molecular and material catalysts have been developed for this purpose and several of these show considerable reactivity. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Reduction of CO 2 selectively to any of the several possible products poses a challenge. The 2e À /2H + reduction of CO 2 can produce either HCOOH or CO and competes with the reduction of protons to form H 2 .…”

“…Substantial progress has been made in the last few years and several molecular and material catalysts have been developed for this purpose and several of these show considerable reactivity. 1–14…”