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.