THE CAVITAND CONCEPT IN THE SYNTHESIS OF SUBSITE-DIFFERENTIATED ANALOGS OF BIOLOGICAL [4FE-4S/SE]2+ CLUSTERS - CLUSTER CAPTURE REACTIONS, LIGAND CONFORMATIONAL-ANALYSIS, AND THE STRUCTURE OF A TRIGONAL [4FE-4SE]2+ ANALOG

The previously reported clusters [Fe4S4(Me2LS3)L′]2−(L′ = RS−, Cl−) contain the cubane-type [Fe4S4]2+core and are derived from the trithiol Me2L(SH)3(1, l,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2, 4, 6-tris(p-tolylthio)benzene), which as the trianion functions as a semirigid tridentate ligand. Iron sites are differentiated in the ratio 3:1. These clusters are analogous to certain biological clusters that undergo regiospecific reactions at the differentiated Fe site. In this work, the factors that render this unique ligand effective in cluster capture are explored. The reaction [Fe4Se4(SEt)4]2−+ 1 yields [Fe4Se4(Me2LS3)(SEt)]2−; treatment of the latter cluster with 1 equiv of t-BuCOCl affords [Fe4Se4(Me2LS3)Cl]2−(5). (Ph4P)2[5]·2DMF crystallizes in the trigonal space group P3 with a = 24.733 (3) Å, c = 14.595 (3) Å, and Z = 3. The crystal structure consists of the three crystallographically independent clusters with imposed trigonal symmetry; these occur in a Δ:Λ ratio of 2:1 as refined crystallographically. The crystal is inversion-twinned, with only a slight preference for one enantiomer. The [Fe4Se4]2+core structures of the three clusters are nearly constant, but ligand conformations, described by tilt and cant angles of the coordinating arms of the ligand, are different. Formation of 5 reveals the flexibility of the ligand, inasmuch as the volume of the [Fe4Se4]2+core is 10% larger on the basis of atom coordinates and about 25% larger on the basis of van der Waals radii than that of the [Fe4S4]2+core. Cluster structures are analyzed in terms of ligand flexibility and conformation, cavity occupancy, and core structure. The clusters [Fe4S4(Me2LS3)Cl)]2−and [Fe4S4(t-BuLS3)Cl]2−were isolated in high yield from the ligand-substitution reactions of 1 and t-BuL(SH)3(2, l,3,5-tris((3-mercapto-5-tert-butylphenyl)thio)-2,4,6-tris(p-tolylthio)benzene), respectively, with [Fe4S4(SEt)4]2−followed by reaction with pivaloyl chloride. In clear contrast, the related ligand L(SH)3(3), lacking the 4,6-dimethyl or 5-t-Bu substituents but otherwise identical, forms a mixture of soluble and polymeric clusters under the same conditions.1H NMR evidence is presented that the ligands exist in a solution conformation in which the three thiol-containing substituents (arms) are on one side of the central ring and the three p-tolylthio groups (legs) are on the other side. Because of the steric directing effects of the ring substituents on the arms of 1 and 2, these molecules adopt dominant configurations with the three thiol groups turned inward over the central benzene ring. These configurations are recognized by the shielding of protons on the arms near the central ring. They are less populated with 3 because the molecule lacks the orienting effects of ring substituents. This situation is consistent with the results of a molecular dynamics analysis of 1 and 3. Consequently, 1 and 2 are predisposed to capture a cubane-type cluster in ligand-substitution reactions and to suppress polymer formation. In this regard, these ligands may be considered to possess the principal attribute of cavitand molecules. © 1990, American Chemical Society. All rights reserved.