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

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-N-6) 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 (eta) 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 Fe-II 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.