Insight on the mechanism of molecular catalysis of CO₂ reduction with Fe(II)-polypyridine complexes

Graphical abstract: Electrochemical behavior of three Fe(II) polypyridinc complexes was explored, to understand the molecular catalysis of carbon dioxide using catalysts with no available coordination sites and inert to substitution. Spectro-electrochemistry and DFT calculations suggest an outer sphere homogenous electron transfer mechanisms between the catalyst and CO₂, where high negative redox potential values favored the molecular catalysis. Display Omitted -- Highlights: •Reversibility of the complex [Fe^ɪ^ɪ(bipy)₃]²⁺ and [Fe^ɪ^ɪ(terpy)₂]²⁺ has been demonstrated. •The metal complexes with more negative redox potential present a better performance in molecular catalysis of CO₂. •Regeneration of the complex [Fe^ɪ^ɪ(bipy)₃]²⁺, as a result of the homogeneous electron transfer between the reduced species [Fe^ɪ^ɪ(bipy⁻)₃]⁻ and CO₂. •LUMO orbitals are degenerate and mainly localized in the polypyridinic rings, suggesting the reduction takes place in the rings. -- Abstract: In this work the electrochemical behavior, of three Fe(II) polypyridinc complexes [Fe(bipy)₃]²⁺, [Fe(phen)₃]²⁺, and [Fe(terpy)₂]²⁺ in acetonitrile solution was explored, to understand the mechanism of the molecular catalysis of carbon dioxide reduction, for metal complexes with no available coordination sites and inert to substitution. Cyclic voltammetry experiments using as working electrodes glassy carbon and platinum in the presence and in the absence of CO₂ were carried out. All the compounds present the oxidation Fe(II)L_n + e⁻ ⇄ Fe(III)L_n. For the tris chelate systems [Fe(phen)₃]²⁺ and [Fe(bipy)₃]²⁺, three reduction processes over the coordinated ligands are observed. On the other hand, the bis chelate complex [Fe(terpy)₂] ²⁺ shows two reductions processes center in ligands. For experiments with a glassy carbon electrode, a third reduction process that yield the species [Fe⁰(terpy ⁻)₂] is detected. DFT calculations support the idea for the electrochemical reduction in the coordinated ligands. For compound [Fe(bipy)₃]²⁺, the catalytic rate constant (k) and the turnover frequency (TOF) relationships were calculated. Spectroelectrochemical experiments in the absence and in the presence of CO₂ were also carried out, demonstrating the regeneration of catalysts. An outer sphere homogenous electron transfer mechanism between the catalyst and CO₂ is proposed, where high negative redox potential values favored the molecular catalysis.