Iron−Sulfur Clusters with SiMe₂-Bridged Cyclopentadienyl Ligands:  [Me₂Si(η⁵-C₅H₄)₂]₂Fe₅S₁₂, [Me₂Si(η⁵-C₅H₄)₂]₂Fe₄S₆, and [Me₂Si(η⁵-C₅H₄)₂]₂Fe₄S₆(CO)^†

The synthesis and characterization of iron−sulfur clusters stabilized by dimethylsilyl-bridged cyclopentadienyl groups are reported. The thermal reaction of Me₂Si(η⁵-C₅H₄)₂Fe₂(CO)₄ (1) with S₈ yields the tetranuclear cubane-type cluster compound [Me₂Si(η⁵-C₅H₄)₂]₂Fe₄S₆ (4) and the pentanuclear cluster compound [Me₂Si(η⁵-C₅H₄)₂]₂Fe₅S₁₂ (3) in high yields. The photochemical reaction of 1 with S₈ yields the tetranuclear cluster compound [Me₂Si(η⁵-C₅H₄)₂]₂Fe₄S₆(CO) (5), which contains one residual terminal carbonyl. The crystal structures of 3 and 4 have been determined. Crystal data:  3·CH₂Cl₂, monoclinic, C2/c, a = 23.480(13) Å, b = 11.192 (4) Å, c = 17.84 (3) Å, β = 118.58(9)°, V = 4118(7) ų, Z = 4, R = 0.078; 4, triclinic, P1̄, a = 8.4787(7) Å, b = 12.9648(9) Å, c = 13.4990(9) Å, α = 79.857(8)°, β = 75.293(8)°, γ = 74.041(11)°, V = 1370.9(2) ų, Z = 2, R = 0.0447. The Fe₅S₁₂ 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 Cp₄Fe₄S₆. However, 4 shows a markedly enhanced thermal stability compared to Cp₄Fe₄S₆. 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 ν_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.