Synthesis, structural characterization, and chemical properties of pentacoordinate model complexes for the active site of [Fe]-hydrogenase

Abstract

Based on the synthesis of two new hexa-coordinate N-heterocyclic carbene (NHC) substituted precursors FeI₂(CO)₃(NHC) with NHC = IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) (2) and NHC = SIPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylide) (3) and two known complexes FeI₂(CO)₃SIMes, SIMes = (1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylide) (1) and FeI₂(CO)₃IMe IMe = 1,3-bismethylimidazol-2-ylidene (4), four new mononuclear [Fe]-hydrogenase model complexes Fe(CO)₂(NS)(NHC) (NS = aminothiophenol; 5, NHC = SIMes; 6, NHC = IPr; 7, NHC = SIPr; 8, NHC = IMe) were prepared by substitution of two iodine ligands with the help of dipotassium-aminothiophenol salt and subsequently the absence of a CO ligand. New complexes 2, 3 and 5–8 were fully structurally characterized by infrared spectroscopy (IR), elemental analysis, NMR spectroscopy, and X-ray crystallography. IR spectroscopy studies show that complexes 5–8 exhibit similar IR patterns and absorption wavelengths in terms of ν(CO) with the active site of [Fe]-hydrogenase. The facile protonation/deprotonation of the NS ligand of complexes 6 and 7 was disclosed with the assistance of IR spectroscopy. The NS ligand accepts a proton reversibly as an internal base, generating two protonated species, [Fe(CO)₂IPr(H-NS)]⁺ and [Fe(CO)₂SIPr(H-NS)]⁺, which play the same role with the intrinsic cysteine thiolate ligand in [Fe]-hydrogenase. DFT results showed that the N atom of the NS ligand is the thermodynamically active proton acceptor in acetone while the NS ligand is prone to be dually protonated first in the N atom and then the S atom in the gas phase. Complex 5 exhibited simultaneous protonation and the combination of CO, forming a new mer-tricarbonyl species in the presence of CO and HBF₄. It showed easy reversible deprotonation and deprivation of CO with the assistance of t-BuOK or Et₃N. Also, the electrochemical properties of these new pentacoordinate model complexes were explored through cyclic voltammetry, which enabled us to identify the contributions of different NHC ligands to the complexes' redox properties.