Single-Step Insertion of Organic Sulfur Into a Fe6C Carbide Carbonyl Cluster, Including the Natural Amino Acid L-Cysteine: Vibrational Circular Dichroism and Chirality Transfer.

We present a straightforward and versatile synthetic route to Fe6C carbide carbonyl clusters containing organosulfur ligands, including the chiral amino acid L- and D-cysteine. Reaction of the reduced cluster [NEt4]4[Fe6C(CO)15] (1) with thiols (RSH) or disulfides (RSSR) affords novel functionalized hexa-iron carbide clusters [NEt4]3[Fe6C(CO)14(SR)] (R = CH3, 2; C6H5, 3; p-C6H4CH3, 4; L-cysteine, L-5; D-cysteine, D-5). Compounds 2-5 have been characterized by Fourier transform infrared (FT-IR), 1H and 13C{1H} nuclear magnetic resonance (NMR) spectroscopy, and the molecular structures of 2-4 have been determined by single-crystal X-ray diffraction (SC-XRD). The chirality of the L-5 and D-5 enantiomers has been investigated by vibrational circular dichroism (VCD), and this represents the first VCD study of chiral metal carbonyl clusters. Combined analyses of VCD and calculated density functional theory (DFT) spectra clearly point out the occurrence of chirality transfer from the chiral organosulfur ligand to the CO ligands, as indicated by the presence of VCD bands in the νCO region of both L-5 and D-5. This unprecedented transfer of chirality among different ligands arises from close interactions between the CH2 and CH hydrogens of coordinated cysteine and the CO shell of the clusters.