Formation, Spectroscopic Characterization, and Solution Stability of an [Fe₄S₄]²⁺ Cluster Derived from β‑Cyclodextrin Dithiolate

The formation and solution properties, including stability in mixed aqueous–Me₂SO media, have been investigated for an [Fe₄S₄]²⁺ cluster derived from β-cyclodextrin (CD) dithiolate. Clusters of the type [Fe₄S₄(SAr)₄]^2– (Ar = Ph, C₆H₄-3-F) are generated in Me₂SO by redox reactions of [Fe₄S₄(SEt)₄]^2– with 2 equiv of ArSSAr. An analogous reaction with the intramolecular disulfide of 6^A,6^D-(3-NHCOC₆H₄-1-SH)₂-6^A,6^D-dideoxy-β-cyclodextrin (<b>14</b>), whose synthesis is described, affords a completely substituted cluster formulated as [Fe₄S₄{β-CD-(1,3-NHCOC₆H₄S)₂}₂]^2– (<b>15</b>). Ligand binding is indicated by a circular dichroism spectrum and also by UV–visible and isotropically shifted ¹H NMR spectra and redox behavior convincingly similar to [Fe₄S₄(SPh)₄]^2–. One formulation of <b>15</b> is a single cluster to which two dithiolates are bound, each in bidentate coordination. With there being no proven precedent for this binding mode, we show that the cluster [Fe₄S₄(S₂-<i>m</i>-xyl)₂]^2– is a single cubane whose <i>m</i>-xylyldithiolate ligands are bound in a bidentate arrangement. This same structure type was proposed for a cluster formulated as [Fe₄S₄{β-CD-(1,3-SC₆H₄S)₂}₂]^2– (<b>16</b>; Kuroda et al. <i>J. Am. Chem. Soc.</i> <b>1988</b>, <i>110</i>, 4049–4050) and reported to be water-stable. Clusters <b>15</b> and <b>16</b> are derived from similar ligands differing only in the spacer group between the thiolate binding site and the CD platform. In our search for clusters stable in aqueous or organic–aqueous mixed solvents that are potential candidates for the reconstitution of scaffold proteins implicated in cluster biogenesis, <b>15</b> is the most stable cluster that we have thus far encountered under anaerobic conditions in the absence of added ligand