Novel iron(II)-N-heterocyclic carbene catalysts for efficient transfer hydrogenations under mild condition

Abstract

Four novel N-heterocyclic carbene (NHC) based Fe(II) complexes, [Fe(L1)Cp(CO)]I (1), [L1 = 3-methyl-1-(pyridin-2-ylmethyl)-benzimidazol-2-ylidene], [Fe(L2)Cp(CO)]Br (2) [L2 = 3-benzyl-1-(pyridin-2-ylmethyl)-benzimidazol-2-ylidene], [Fe(L3)Cp-(CO)]Br (3), [L3 = 3-(4-tert-butyl-benzyl)-1-(pyridin-2-ylmethyl)-benzimidazol-2-ylidene] and [Fe(L4)Cp(CO)]I (4), L4 = 1,3-di-(2- methylenepyridyl methyl)-benzimidazol-2-ylidene] have been synthesized and characterized as efficient catalysts for transfer hydrogenation of carbonyl compounds. The disappearance of 1H NMR signals at 10.09–11.78 ppm of NHC ligand confirms the complex formation via carbene coordination to low spin Fe(II) center and it is further supported by the two distinct doublets for two diastereotopic protons of the methylene group connects pyridine and benzimidazole rings. The IR spectra show νstr, 1939–1958 cm−1 for the coordinated CO, which are virtually independent of the wingtip substituents and possibly this limits the influence of those groups on the electronic properties of iron(II) center. The molecular structure of the complexes 1, 2 and 4 reveals the coordination of NHC ligand, cyclopentadiene (Cp) and CO to low spin FeII center (S = 0). The bonds Fe-Ccarbene (1.907–1.934 Å), Fe-NPy (1.995–2.019 Å), Fe−CO (1.743–1.762 Å) and Fe-Cp (2.058–2.097 Å) are almost identical in all the complexes. The bite angles CO-Fe-Ccarbene (90.9°– 113.8°) are higher than Cp-Fe-Ccarbene (92.6°–93.2°) but the bite angle CO-Fe-Npy are merely indentical in all complexes (89.7°). All the complexes showed two redox couples at 1.050 - 1.080 V and 0.560 – 0.585 V corresponds to FeII/FeIII and ligand-based redox couples respectively. The complexes exhibited the excellent transfer hydrogenation for aldehydes up to yield of 99% with the turnover frequency (TOF) of 13,680 h−1 at room temperature. The ketone substrates showed conversion of alcohols with the yield and TOF of 98% and 2264 h−1 respectively at 60 °C.