An electrochemically controlled release of NHCs using iron bis(dithiolene) N-heterocyclic carbene complexes

A series of five coordinated iron bis(dithiolene) complexes [Fe(NHC)(S2C2R2)2] (R = C6H5 or C6H4-p-OCH3) containing N-heterocyclic carbene (NHC) (NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene or 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) were isolated in high yield (84-92%). The iron complexes were characterized by NMR spectroscopy and confirmed by single crystal X-ray diffraction studies. The combination of cyclic voltammetry and spectroelectrochemical analysis revealed that iron complexes undergo Fe-CNHC bond cleavage and release NHC upon subjection to electrochemical reduction. The electrochemically released NHC was trapped using 1-naphthylisothiocyanate and the adduct was isolated in nearly quantitative yield (∼99%). As a proof of concept, the electrochemically released NHC was subsequently used as a catalyst for synthesis of γ-butyrolactones from commercially available cinnamaldehydes.