A Dinuclear Fe-NHC/Pyrazolato Complex as Catalyst in Olefin Epoxidation and Electroreduction of CO2

Abstract: Calix[4]imidazolium[2]pyrazole represents a suitable platform to ligate two metal ions upon the removal of the protons of the tetradentate cavities, leading to robust bimetallic complexes. Within the framework of this project, the synthesis of a bisiron(II) complex based on a methylene–bridged calix[4]imidazolylidene[2]pyrazolate macrocyclic ligand was carried out. The latter was characterized via NMR spectroscopy, HR-MS (ESI+, ESI-), elemental analysis and cyclic voltammetry. The catalytic properties of the complex were tested towards the epoxidation of cis–cyclooctene and the electrochemical reduction of carbon dioxide. As epoxidation catalyst, the iron complex showed a scarce stability under ambient conditions at different temperatures, in the presence of hydrogen peroxide as oxidizing agent. Nevertheless, the addition of additives such as Sc(OTf)3 enabled the achievement of higher performances in terms of conversion, TOF and TON, maintaining the selectivity towards the epoxide (>99%). Preliminary experiments regarding the reactivity with molecular oxygen were performed via HR-MS (ESI+) highlighting the formation of adducts with oxygen and dioxygen. The redox behavior of the complex was studied through cyclic voltammetry. The two reversible couples (E1/2 = –0.2 V and +0.1 V vs Fc+/Fc) indicate the elevate tendency of the complex to be oxidized, enhanced by the strong sigma–donor NHC ligands. Differently, when the potential was scanned to more negative potentials (E< –2 V vs Fc+/Fc) the complex showed irreversibility, and side phenomena such as deposition onto the electrode surface affected the measurements. The activity towards CO2 reduction was proved by the appearance of a sharp cathodic peak starting from –2 V (vs Fc+/Fc) in the absence of proton sources. The addition of traces of water and phenol, contrary to the expectations, did not improve the catalytic performance of the iron complex, undermining instead the stability of the catalyst.