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-Me(2)SO media, have been investigated for an Fe(4)S(4) cluster derived from β-cyclodextrin (CD) dithiolate. Clusters of the type Fe(4)S(4)(SAr)(4) (Ar = Ph, C(6)H(4)-3-F) are generated in Me(2)SO by redox reactions of Fe(4)S(4)(SEt)(4) with 2 equiv of ArSSAr. An analogous reaction with the intramolecular disulfide of 6(A),6(D)-(3-NHCOC(6)H(4)-1-SH)(2)-6(A),6(D)-dideoxy-β-cyclodextrin (14), whose synthesis is described, affords a completely substituted cluster formulated as Fe(4)S(4){β-CD-(1,3-NHCOC(6)H(4)S)(2)}(2) (15). Ligand binding is indicated by a circular dichroism spectrum and also by UV-visible and isotropically shifted (1)H NMR spectra and redox behavior convincingly similar to Fe(4)S(4)(SPh)(4). One formulation of 15 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 Fe(4)S(4)(S(2)-m-xyl)(2) is a single cubane whose m-xylyldithiolate ligands are bound in a bidentate arrangement. This same structure type was proposed for a cluster formulated as Fe(4)S(4){β-CD-(1,3-SC(6)H(4)S)(2)}(2) (16; Kuroda et al. J. Am. Chem. Soc.1988, 110, 4049-4050) and reported to be water-stable. Clusters 15 and 16 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, 15 is the most stable cluster that we have thus far encountered under anaerobic conditions in the absence of added ligand.