Iron-Sulfur Clusters with SiMe(2)-Bridged Cyclopentadienyl Ligands: [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(5)S(12), [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6), and [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6)(CO)

The synthesis and characterization of iron-sulfur clusters stabilized by dimethylsilyl-bridged cyclopentadienyl groups are reported. The thermal reaction of Me(2)Si(eta(5)-C(5)H(4))(2)Fe(2)(CO)(4) (1) with S(8) yields the tetranuclear cubane-type cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6) (4) and the pentanuclear cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(5)S(12) (3) in high yields. The photochemical reaction of 1 with S(8) yields the tetranuclear cluster compound [Me(2)Si(eta(5)-C(5)H(4))(2)](2)Fe(4)S(6)(CO) (5), which contains one residual terminal carbonyl. The crystal structures of 3 and 4 have been determined. Crystal data: 3.CH(2)Cl(2), monoclinic, C2/c, a = 23.480(13) Å, b = 11.192 (4) Å, c = 17.84 (3) Å, beta = 118.58(9) degrees, V = 4118(7) Å(3), Z = 4, R = 0.078; 4, triclinic, P , a = 8.4787(7) Å, b = 12.9648(9) Å, c = 13.4990(9) Å, alpha = 79.857(8) degrees, beta = 75.293(8) degrees, gamma = 74.041(11) degrees, V = 1370.9(2) Å(3), Z = 2, R = 0.0447. The Fe(5)S(12) core of 3 has a bowtie structure in which a central iron atom is octahedrally coordinated by six sulfur atoms from one tetrasulfido and four disulfido groups. The structure of 4 resembles the structure of the known iron-sulfur cluster Cp(4)Fe(4)S(6). However, 4 shows a markedly enhanced thermal stability compared to Cp(4)Fe(4)S(6). In their cyclic voltammograms, 4 and 5 exhibit electrochemical behavior typical of cubane-type Cp-iron-sulfur clusters, whereas the cyclic voltammogram of 3 is quite different. The nu(CO) mode of 5 has been measured for four different oxidation states of the cluster by means of IR spectroelectrochemical methods. The Mössbauer spectra of 3 and 3(+) are in accordance with their pentanuclear structure.