Cubane-Type Fe4S4 Clusters with Chiral Thiolate Ligation: Formation by Ligand Substitution, Detection of Intermediates by 1H NMR, and Solid State Structures Including Spontaneous Resolution Upon Crystallization

Cubane-type clusters [Fe4S4(SR*)4]2– containing chiral thiolate ligands with R* = CH(Me)Ph (1), CH2CH(Me)Et (2), and CH2CH(OH)CH2OH (3) have been prepared by ligand substitution in the reaction systems [Fe4S4(SEt)4]/R*SH (1–3, acetonitrile) and [Fe4S4Cl4]2–/NaSR*(3, Me2SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe4S4L4–n(SR*)n]2– (L = SEt, Cl) with n = 1–4. Clusters 1 and 2 were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (n = 1) and seven diastereomers and their enantiomers (n = 2–4). Reactions were monitored by isotropically shifted 1H NMR spectra in acetonitrile or Me2SO. In systems affording 1 and 2 as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et4N)2[1] afforded 1-[SS(RS)(RS)] in which two sites are disordered because of occupancy of R and S ligands. Similarly, (Et4N)2[2] led to 2-[SSSS], a consequence of spontaneous resolution upon crystallization. The clusters 3-[RRRR] and 3-[SSSS] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with n = 1–4 by appearance of four pairs of diastereotopic SCH2 signals in both acetonitrile and Me2SO reaction systems. Identical spectra were obtained with racemic, R-(−), and S-(+) thiols, indicating that ligand–ligand interactions are too weak to allow detection of diastereomers (e.g., [SSSS] vs [SSRR]). The stability of 3 in Me2SO/H2O media is described.