Synthesis, characterization, electrochemical properties and catalytic reactivity of N-heterocyclic carbene-containing diiron complexes

Abstract

(μ-dmedt)[Fe(CO)₃]₂ (I, dmedt = 2,3-butanedithiol) was chosen as the parent complex. A series of new model complexes, N-heterocyclic carbene (NHC) substituted (μ-dmedt)[Fe–Fe]–NHC (II, (μ-dmedt)[Fe(CO)₂]₂[I_Me(CH₂)₂I_Me], I_Me = 1-methylimidazol-2-ylidene; III, {(μ-dmedt)[Fe₂(CO)₅]}₂[I_Me(CH₂)₂I_Me]; IV, (μ-dmedt)[Fe₂(CO)₅]IMes, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; V, (μ-dmedt)[Fe₂(CO)₅]IMe, IMe = 1,3-dimethylimidazol-2-ylidene) as mimics of the [Fe–Fe]–H₂ase active site were synthesized from I and characterized using solution IR spectroscopy, NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. The electrochemical properties of complexes I–V, with and without the addition of HOAc, were investigated by cyclic voltammetry in the coordinating solvent CH₃CN to evaluate the effects of different NHC ligands on the redox properties of the iron atoms of the series of complexes. It was concluded that all the new complexes are electrochemical catalysts for proton reduction to hydrogen. The symmetrically substituted cisoid basal/basal coordination complex II displays the most negative reduction potential owing to the stronger δ-donating ability of the NHC and the orientation of the NHC donor carbon as a result of the constraints of the bridging bidentate ligands. A new application for the [Fe–Fe]–NHC model complexes in the direct catalytic hydroxylation of benzene to phenol was also studied. Under the optimized experimental conditions (II, 0.01 mmol; benzene, 0.1 mL; CH₃CN, 2.0 mL; H₂O₂, 6.0 mmol; 60 °C, 3 h), the maximal phenol yield was 26.7%.