IRON-SULFUR CLUSTERS WITH SIME2-BRIDGED CYCLOPENTADIENYL LIGANDS - [ME2SI(ETA(5)-C5H4)(2)](2)FE5S12, [ME2SI(ETA(5)-C5H4)(2)](2)FE4S6, AND [ME2SI(ETA(5)-C5H4)(2)](2)FE4S6(CO)

The synthesis and characterization of iron-sulfur clusters stabilized by dimethylsilyl-bridged cyclopentadienyl groups are reported. The the rmal reaction of Me2Si(eta(5)-C5H4)(2)Fe-2(CO)(4) (1) with S-8 yields the tetranuclear cubane-type cluster compound [Me2Si(eta(5)-C5H4)(2)]( 2)Fe4S6 (4) and the pentanuclear cluster compound [Me2Si(eta(5)-C5H4)( 2)](2)-Fe5S12 (3) in high yields. The photochemical reaction of 1 with S-8 yields the tetranuclear cluster compound [Me2Si(eta(5)-C5H4)(2)]( 2)Fe4S6(CO) (5), which contains one residual terminal carbonyl. The cr ystal structures of 3 and 4 have been determined. Crystal data: 3 . CH 2Cl2, monoclinic, C2/c, a = 23.480(13) Angstrom, b = 11.192 (4) Angstr om, c = 17.84(3) Angstrom, beta = 118.58(9)degrees, V = 4118(7) Angstr om(3), Z = 4, R = 0.078; 4, triclinic, <P(1)over bar>, a = 8.4787(7) A ngstrom, b = 12.9648(9) Angstrom, c = 13.4990(9) Angstrom, alpha = 79. 857(8)degrees, beta = 75.293(8)degrees, gamma = 74.041(11)degrees, V = 1370.9(2) Angstrom(3), Z = 2, R = 0.0447. The F5S12 core of 3 has a b owtie Structure in which a central iron atom is octahedrally coordinat ed by six sulfur atoms from one tetrasulfido and four disulfido groups . The structure of 4 resembles the structure of the known iron-sulfur cluster Cp4Fe4S6. However, 4 shows a markedly enhanced thermal stabili ty compared to Cp4Fe4S6. In their cyclic voltammograms, 4 and 5 exhibi t electrochemical behavior typical of cubane-type Cp-iron-sulfur clust ers, whereas the cyclic voltammogram of 3 is quite different. The v(CO ) mode of 5 has been measured for four different oxidation states of t he cluster by means of IR spectroelectrochemical methods. The Mossbaue r spectra of 3 and 3(+) are in accordance with their pentanuclear stru cture.