Electrocatalytic Reduction of CO₂ to Formate by an Iron Schiff Base Complex

Abstract

The synthesis, structural characterization, and reactivity of an iron(III) chloride compound of 6,6'-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(dhbpy)Cl), under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO) under anhydrous conditions in N,N-dimethylformamide (DMF), this complex mediates the 2e reductive disproportionation of two equivalents of CO to carbon monoxide (CO) and carbonate (CO). Upon addition of phenol (PhOH) as a proton source under CO saturation, catalytic current is observed; product analysis from controlled potential electrolysis experiments shows the majority product is formate (68 ± 4% Faradaic efficiency), with H as a minor product (30 ± 10% Faradaic efficiency) and minimal CO (1.1 ± 0.3% Faradaic efficiency). On the basis of data obtained from cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC), the release of CO from intermediate Fe carbonyl species is extremely slow and undergoes competitive degradation, limiting the activity and lifetime of this catalyst. Mechanistic studies also indicate the phenolate moieties coordinated to Fe are sensitive to protonation in the reduced state, suggesting the possibility of cooperative pendent proton interactions being involved in CO reduction.

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

“…Regarding the catalysts employed, molecular complexes have been widely investigated for the carbon dioxide reduction reaction (CO 2 RR) due to the possibility of fine tuning the ligand structure (steric and electronic effects, as well as second coordination sphere effects) . Earth abundant metal‐based catalysts have been shown to efficiently catalyze the production of CO and formate in organic solvents and in water, even if examples in pure aqueous solutions are less numerous. In the latter case, the catalysts are typically dispersed into thin conductive films usually made of carbon nanomaterials, such as carbon black or carbon nanotubes .…”

Unexpected Effect of Intramolecular Phenolic Group on Electrocatalytic CO2 Reduction

“…Fe porphyrins have received considerable attention among these catalysts . They show high activity and stability for the selective CO 2 ‐to‐CO conversion, and they provide an ideal platform to investigate various structural effects on CO 2 RR, because the meso ‐substituents of the porphyrin ring can be systematically modified to introduce different acid/base groups and positively/negatively charged moieties .…”

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

“…The Fe I catalyst containing 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline [Fe I (dophen)] − yields HCOOH at 74% FE, as well as oxalate (7%) and CO (13%) as minor byproducts, upon electrolysis in dimethyl sulfoxide (DMSO) at −1.76 V vs. SCE on a carbon electrode (Scheme 9) [74]. An iron(III) complex of 6,6′-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2′-bipyridine (Fe(III)Cl( tbu dhbpy)) was also reported to catalyze the reduction of CO2 to formate in the presence of an added proton source (PhOH; FE = 68%, TON = 2.7, t = 10 h), whereas, in the absence of acid, only CO is formed (FE = 1.1%, TON = 3, t = 15 h) [75]. electroreduction, yielding CO at an optimal FE of 90% (overpotential = 840 mV) on a glassy carbon electrode; however, the catalytic efficiency was found to decrease significantly upon prolonged electrolysis [72].…”

“…The Fe I catalyst containing 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline [Fe I (dophen)] − yields HCOOH at 74% FE, as well as oxalate (7%) and CO (13%) as minor byproducts, upon electrolysis in dimethyl sulfoxide (DMSO) at −1.76 V vs. SCE on a carbon electrode (Scheme 9) [74]. An iron(III) complex of 6,6′-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2′-bipyridine (Fe(III)Cl( tbu dhbpy)) was also reported to catalyze the reduction of CO2 to formate in the presence of an added proton source (PhOH; FE = 68%, TON = 2.7, t = 10 h), whereas, in the absence of acid, only CO is formed (FE = 1.1%, TON = 3, t = 15 h) [75]. Recently, Lau and Robert reported Co and Fe quaterpyridine complexes (Scheme 10), [M(qpy)(OH2)2] 2+ (qpy = 2,2′:6′,2":6",2‴-quaterpyridine), as catalysts for CO2-to-CO electroreduction in acetonitrile with a selectivity of >95% in the presence of phenol at low overpotentials of 140 and 240 mV, respectively, and an impressive turnover frequency (TOF) of 3.3 × 10 4 s −1 was reported for the Fe catalyst [76].…”

“…Structures of [Fe I (dophen)] − and (Fe(III)Cl( tbu dhbpy)[74,75]; dophen-2,9-bis(2hydroxyphenyl)-1,10-phenanthroline;tbu dhbpy-,6′-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2′bipyridine.Recently, Lau and Robert reported Co and Fe quaterpyridine complexes (Scheme 10), [M(qpy)(OH2)2] 2+ (qpy = 2,2′:6′,2":6",2‴-quaterpyridine), as catalysts for CO2-to-CO electroreduction in acetonitrile with a selectivity of >95% in the presence of phenol at low overpotentials of 140 and 240 mV, respectively, and an impressive turnover frequency (TOF) of 3.3 × 10 4 s −1 was reported for the Fe catalyst[76].…”