Subsite-Differentiated Analogues of Biological [4Fe-4S] Clusters Effected by Binding of a Macrocyclic Polyether Trithiol

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The macrocyclic polyether trithiol 7 (c-L(SH)3) was synthesized in five steps from resorcinol. The cyclic structures of the tris (1,3-phenylene) crown ether (5) and protected trithiol (6) intermediates were demonstrated by X-ray methods. Compound 5 (C26H28O6) crystallizes in the monoclinic space group P21/c, with a = 10.318 (5) Å, b = 17.333 (5) Å, c = 13.653 (5) Å, β = 111.31 (3)°, and Z = 4. Compound 6·Et2O (C51H56O7S3) crystallizes in the monoclinic space group P21/n, with a = 14.315 (9) Å, b = 12.823 (8) Å, c = 25.56 (1) Å, β = 90.94 (5)°, and Z = 4. Reaction of 7 with [Fe4S4(SEt)4]2- in acetonitrile results in quantitative cluster capture and the formation of [Fe4S4(c-LS3)(SEt)]2- (8), in which the Fe subsites are differentiated in a 3:1 ratio. Treatment of 8 with pivaloyl chloride affords [Fe4S4(c-LS3)Cl]2- (9). This cluster reacts with a variety of ligands to generate the site-differentiated clusters [Fe4S4(c-LS3)L′]2-, with L′ = p-MeC6H4S-, CN-, tacn, (t-BuNC)3, and a probable sulfide-bridged double cubane cluster (L′ = ½S2-). Reactions were monitored by 1H NMR spectroscopy, which detected cluster substitution because of the sensitivity of isotropic shifts to variations in the ligands L′ at the unique site. In all cases, the spectra were consistent with mirror symmetry, indicating trithiolate ligand binding to a single [Fe4S4]2+ core unit. Isotropic shifts and [Fe4S4]2+/+ redox potentials are tabulated. Deprotonated trithiol 7 is the second trithiolate ligand shown to form subsite-differentiated cubane-type clusters and to support subsite-specific substitution reactions. Properties of the clusters [Fe4S4(c-LS3)L′]z- and the previously reported [Fe4S4(LS3)L′]z- species, derived from a trithiol of different structure and conformational rigidity, are similar at constant L′. This suggests that neither tridentate ligand imposes a highly specific environment on the Fe4S4 core and thus indicates that either ligand may be utilized in the chemistry of subsite-differentiated clusters with comparable results.



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