Computational Study of CO2 Reduction Catalyzed by Iron(I) Complex at Different Spin States: Cooperativity of Hydrogen Bonding and Auxiliary Group Effect

To explore alternative approaches to the CO2 reduction to formate and provide an insight into the spin state effect on the CO2 reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as Fe-2(I), Fe-4(I), and Fe-6(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO2 reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO2 molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO2. For the iron(I) compound, it was found that the lowest spin state Fe-2(I) is the most favorable for the CO2 reduction as the reactions barriers involving Fe-2(I), Fe-4(I), and Fe-6(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high-low spin shift from Fe-4(I) and Fe-6(I) to Fe-2(I) is likely through the reverse intersystem crossing (RIC) because the Fe-4(I) and Fe-6(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO2 and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the -COOH moiety from CO2 and the Fe-O bond. With the addition of H-2, one H-2 molecule is split by the Fe-O bond and thus serves as H atom sources for both the CO2 reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO2 reduction, which calls for further experimental verifications.