[FE6S6(PET3)6]+ - EXTENSION OF STABILIZATION OF THE BASKET CORE TOPOLOGY TO THE [FE6S6]+ OXIDATION LEVEL

The influence of terminal ligands on Fe-S cluster structures formed in self-assembly systems is evident. In recent work it has been shown that systems containing Fe(II), Et3P, and L = halide or thiolate assemble the clusters Fe6S6(PEt3)4L2(1, 2), whose [Fe6S6]2+cores possess a basket-type stereochemistry with the bridging modalities Fe6(µ2-S)(µ3-S)4(µ4-S). In order to determine the nature of the cluster formed when the only possible terminal ligand is a tertiary phosphine, the assembly system 1:4:1 [Fe(OH2)6](BF4)/Et3P/Li2S in THF was investigated. The compound [Fe6S6(PEt3)6](BF4) was isolated in 14% purified yield. It crystallizes in monoclinic space group P21/m with a = 12.370 (5) ⇋,b= 16.745 (6) ⇋, c = 15.151 (4) ⇋, β = 93.76 (3)°, and Z = 2. The crystal structure contains the cluster [Fe6S6(PEt3)6]+(4), which has imposed C, symmetry but closely approaches C2vsymmetry. Cluster 4 has the basket stereochemistry of 1 and 2 with only relatively minor dimensional differences. As in the latter two clusters, the six Fe sites of 4 divide into two that have tetrahedral stereochemistry and four that exhibit distorted trigonal-planar coordination. The tetrahedral sites are those that bind halide and thiolate in 1 and 2. Retention of this geometry in 4 indicates that it is intrinsic to the basket core topology, which has now been shown to stabilize the (reduced) [Fe6S6]+oxidation level. Thus far, the basket structure has been observed only in the oxidation levels [Fe6S6]2+°+and only with four or six Et3P ligands. Cluster 4 does not show clean electrochemical reactions but does react with chloride or chlorinated solvents to give Fe6S6(PEt3)4Cl2, and with dioxygen and elemental sulfur to afford the known species [Fe6S8(PEt3)6]2+and [Fe6Ss(PEt3)6]+, respectively. The two oxidation reactions are further examples of core conversions of basket clusters, in this case affording a stellated octahedral cluster product. The characterization of 4 raises to 5 the number of characterized hexanuclear Fe-S clusters. Those of the type [Fe6S6L6]2−3−(L = RO−, RS−, halide), with more oxidized cores than in 1, 2, and 4, have been shown earlier to have the prismane stereochemistry. © 1990, American Chemical Society. All rights reserved.