Introducing Novel Redox-Active Bis(phenolate) N‑Heterocyclic Carbene Proligands: Investigation of Their Coordination to Fe(II)/Fe(III) and Their Catalytic Activity in Transfer Hydrogenation of Carbonyl Compounds

A simple and efficient procedure for synthesizing novel pincer-type tridentate N-heterocyclic carbene bisphenolate ligands is reported. The synthesis of pincer proligands with N,N′-disubstituted imidazoline core, 5 and 6, was carried out via triethylorthoformate-promoted cyclization of either N,N′-bis­(2-hydroxy-3,5-di-tert-butylphenyl)­cyclohexanediamine, 3, or N,N′-bis­(2-hydroxyphenyl)­cyclohexanediamine, 4, in the presence of concentrated hydrochloric acid. Cyclic voltammograms of the ligands revealed ligand-centered redox activity, indicating the noninnocent nature of the ligands. The voltammograms of the ligands exhibit two successive one-electron oxidations and two consecutive one-electron reductions. In contrast to previous reports, the redox-active ligands in this study exhibit one-electron oxidation and reduction processes. All products were thoroughly characterized by using 1H and 13C NMR spectroscopy. The base-promoted deprotonation of the proligands and subsequent reaction with iron­(II) and iron­(III) chlorides yielded compounds 7 and 8. These compounds are binuclear and tetranuclear iron­(III) complexes that do not contain carbene functional groups. Complexes 7 and 8 were characterized by using elemental analysis and single-crystal X-ray crystallography. At low catalyst loadings, both 7 and 8 exhibited high catalytic activity in the transfer hydrogenation of selected aldehydes and ketones.