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

摘要

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.