Computational Insights into Mn and Fe Azoaromatic Pincer Complexes: Modulating Ligands for Enhanced CO2-to-Formic Acid Conversion

Abstract Formic acid, produced by hydrogenation of CO2, is an attractive liquid organic hydrogen carrier. It offers a promising route for sustainable energy storage; however, its practical application remains constrained by the need for strong basic conditions. Here, we used DFT to rationally design eight Mn(I) and Fe(II) azoaromatic pincer complexes and assess their catalytic performance in converting CO2 to formic acid under base-free and aqueous conditions. At 25 °C, the iron-based L1 and L2 ligands exhibited extraordinary catalytic activity with computed TOFs of 3.9 × 106 h–1 and 2.9 × 106 h–1, respectively, compared to L3 (3.3 × 10–3 h–1) and L4 (4.5 × 105 h–1) ligands. Among manganese pincer complexes, L4 showed an excellent computed TOF of 1.0 × 106 h–1 as compared to L1 (2.2 × 102 h–1), L2 (1.0 × 103 h–1), and L3 (6.0 × 10–4 h–1) ligands. This structure–activity relationship was further confirmed using electronic descriptors, such as condensed Fukui functions, which highlight electrophilicity at the metal center as a predictive descriptor that correlates with the computed TOF. This work weaves electrophilicity-driven catalyst design as a guiding principle for developing environmentally benign and cost-effective CO2 hydrogenation methods under ambient conditions.