[M(CO)₄(2,2′‐bipyridine)] (M=Cr, Mo, W) Complexes as Efficient Catalysts for Electrochemical Reduction of CO₂ at a Gold Electrode

Abstract

Zum Ende des Sommers nimmt die Zahl der Insekten in Wald und Flur deutlich ab. Nur noch wenige Schmetterlinge gaukeln über blütenarme Wiesen. Doch einige häufige Nachtfalter‐Raupen sind gerade im September und Oktober kaum zu übersehen und ermöglichen somit interessante Beobachtungen im Herbst.

Electrochemical Reduction of CO2 by M(CO)4(diimine) Complexes (M=Mo, W): Catalytic Activity Improved by 2,2′‐Dipyridylamine

“…In the region from 1600 to 1700 cm À1 absorptions related to the formation of bicarbonate and formate species appear. [9] Thus, IR-SEC data suggestst hat in 1 the ability of the phenylg roupst od elocalize the double negative chargep lays an important role in stabilizingt he protonated reduced form, rather than affecting the kinetics of the CO-loss process.T he net result is that there is no remarkable difference in the catalytic activity of W(CO) 4 (L1)i nc omparison with the reference compound [W(CO) 4 (bpy)],a so bserved in the voltammetric data.…”

“…[9] However,a ts lightly higher n CO absorption values than those of the radical anion,w eo bserved at ransient reduced species (the role of which in the catalytic cycle under CO 2 is presently unknown), whichw ea ssigned [9] to the protonated two-electron reduced…”

“…The reductionp rocesses of all the studied compoundsw ere centered at negative potentials, where the direct reduction of acids to H 2 becomes relevant.T he electrochemical behavior of 1 and 2 under an inert atmosphere is similart ot hat reported for other tetracarbonyl diimine complexes of Group VI transition metals,s howingt wo successive one-electronr eduction steps, both mainly localized on the bipyridyl moiety ( Figure 2). [8][9][10] The first reversible reduction (at E 1/2 = À1.42 and À1.48 Vv s. SCE for 1 and 2,r espectively) that gave the corresponding radical anions was centered on the diimine. [12] Moreover,f or both compounds as econd irreversible reduction took place at more negative potentials (E p = À2.04 Vand À2.25 Vv s. SCE for 1 and 2,r espectively).…”

“…[8,9] 2) For M(CO) 4 (dpa) (M = Mo, W) catalysts, the presence of atetracarbonyl catalytically active species suggeststhat the dpa ligand is noninnocent, and that the catalytic process of CO formation is mainly localized on the bidentate ligand, ratherthan on the electron-rich metal atom. In particular,f or 3 and 4,t he reduction is ligand-centered,s ot hat the increased nucleophilicity of the bridging Na tom could favor interaction with CO 2 .I th as been postulated that CO 2 may undergo insertion into ap yridinium ion [3c, 17] ,b ut the energy barriero fi ts insertion into the NÀHb ond of ar adical PyH group is too high.…”

“…However, the main drawback in using these complexes is the negative reduction potentials required for the two-electron catalytic process, resulting in considerably lower catalytic activity compared to that of Group VII metal complexes.T he use of gold as aw orkinge lectrode decreases the potentialr equired for the catalytic conversion of CO 2 to CO by [M(CO) 4 (bpy)] (M = Mo, W, Cr), via at wo-electron pathway involving adoubly reduced tricarbonyl species. [9] As reported in the literature, [10] the introductiono fp henyl groups shifts the reduction potentials to less negative values, whereas bulky substituents may have ap ositive effect on the stabilitya nd activity of the metal-complex catalysts.…”