Multifaceted Role of the Noninnocent Mabiq Ligand in Promoting Selective Reduction of CO₂ to CO

Abstract

We have investigated the ability of Co– and Fe–Mabiq complexes (Mabiq = 2–4:6–8-bis­(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2′-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6) to act as electrocatalysts for CO2 reduction. We observed marked differences in activity when switching the metal center, as the Fe complex outperforms its Co-containing analogue, both in terms of overpotential (η) and faradaic efficiency (FE). [Fe­(Mabiq)2(MeCN)2]­PF6 ([2] + ) selectively reduces CO2 to CO with an overpotential requirement of 500 mV. We have synthesized and fully characterized the two-electron reduced Na­(OEt2)­[Fe­(Mabiq)] ([2] – ), which consists of an intermediate spin FeII center coupled to a ligand biradical and exhibits a unique S = 1 spin state. Both electrochemical and reactivity studies with [2] – point toward a protonated precatalytic intermediate (I PhOH ). The molecular structure of I PhOH indicates the diketiminate carbon as the site of protonation and the ability of the Mabiq ligand to engage in hydrogen bonding interactions. The noninnocent Mabiq ligand, therefore, acts not only as an electron reservoir but also as a proton storage site. Our ligand system uniquely combines two beneficial features, a redox-active unit and a proton donor site, that in combination with the metal ion reduces overpotentials and facilitates selective CO2 conversion.

Influence of a neighbouring Cu centre on electro- and photocatalytic CO2 reduction by Fe-Mabiq

“…The first two reductions are each shifted anodically by 150 mV, while the final reductive event occurs at 400 mV more positive for 2 (Table S4, ESI †). As the first two reductions of our Mabiq complexes are ligand-centred, 17,18 coordination of the outer metal ion likely modulates the electron density on the ligand, and is consistent with the differences observed in the NMR spectra (vide supra and see Fig. S9, ESI †).…”

“…We assigned this behaviour to the formation of an intermediate species, I PhOH , in which the Mabiq ligand becomes protonated at the diketiminate carbon. 17 However, a similar proton-dependent shift of the formal Fe I/0 couple is not observed for 2 (Fig. 1b and Fig.…”

“…Our macrocyclic Fe-Mabiq complex [Fe(Mabiq)-(MeCN) 2 ] + (1, Chart 1) selectively reduces CO 2 to CO electrocatalytically with a good overpotential requirement of only 500 mV. 17 We showed that electron and proton uptake by the Mabiq ligand is a unique feature of our catalyst and contributes to selective CO formation. The Mabiq ligand offers a valuable platform to probe the influence of a neighbouring metal centre on catalysis due to the second metal binding site.…”

“…1c and d). 17 Under our standard photocatalysis conditions (2 mM catalyst, 200 mM Ru-PS, 170 mM PhOH, 100 mM BIH, 1 h irradiation at 455 nm) both complexes produced CO as the only product in excellent yields, leading to TONs of up to 16 Â 10 3 for 1 and 24 Â 10 3 for 2 (Table S5, ESI † and Fig. 2a) and TOFs of 4.4 s À1 and 6.7 s À1 , respectively.…”

“…This is in accord with the previously observed, [PhOH]-dependent electrocatalytic behaviour of 1. 17 A substantial amount of CO was produced in the absence of PhOH (Table S7 and Fig. S20, ESI †), presumably caused by trace water.…”