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.