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

Abstract

Cubane-type clusters [Fe₄S₄(SR*)₄]^2– containing chiral thiolate ligands with R* = CH(Me)Ph (<b>1</b>), CH₂CH(Me)Et (<b>2</b>), and CH₂CH(OH)CH₂OH (<b>3</b>) have been prepared by ligand substitution in the reaction systems [Fe₄S₄(SEt)₄]/R*SH (<b>1</b>–<b>3</b>, acetonitrile) and [Fe₄S₄Cl₄]^2–/NaSR*(<b>3</b>, Me₂SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe₄S₄L_4–<i>n</i>(SR*)_<i>n</i>]^2– (L = SEt, Cl) with <i>n</i> = 1–4. Clusters <b>1</b> and <b>2</b> were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (<i>n</i> = 1) and seven diastereomers and their enantiomers (<i>n =</i> 2–4). Reactions were monitored by isotropically shifted ¹H NMR spectra in acetonitrile or Me₂SO. In systems affording <b>1</b> and <b>2</b> as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et₄N)₂[<b>1</b>] afforded <b>1</b>-[<i>SS</i>(<i>RS)(RS)</i>] in which two sites are disordered because of occupancy of <i>R</i> and <i>S</i> ligands. Similarly, (Et₄N)₂[<b>2</b>] led to <b>2</b>-[<i>SSSS</i>], a consequence of spontaneous resolution upon crystallization. The clusters <b>3</b>-[<i>RRRR</i>] and <b>3</b>-[<i>SSSS</i>] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with <i>n =</i> 1–4 by appearance of four pairs of diastereotopic SC<i>H</i>₂ signals in both acetonitrile and Me₂SO reaction systems. Identical spectra were obtained with racemic, <i>R-</i>(−), and <i>S</i>-(+) thiols, indicating that ligand–ligand interactions are too weak to allow detection of diastereomers (e.g., [<i>SSSS</i>] vs [<i>SSRR</i>]). The stability of <b>3</b> in Me₂SO/H₂O media is described