Molecular Design of Cyclopentadienyl Tantalum Sulfide Complexes

Use of (Me₃Si)₂S and [Ta(η⁵-C₅Me₅)Cl₄] (1) in a 4:3 ratio afforded the trimetallic sulfide cluster [Ta₃(η⁵-C₅Me₅)₃Cl₃(μ₃-Cl)(μ-S)₃(μ₃-S)] (2) with loss of SiClMe₃. A similar reaction between 1, TaCl₅, and (Me₃Si)₂S in a 2:1:4 ratio resulted in the analogous complex [Ta₃(η⁵-C₅Me₅)₂Cl₄(μ₃-Cl)(μ-S)₃(μ₃-S)] (3). Single-crystal X-ray diffraction analyses of 2 and 3 showed in all cases trinuclear tantalum sulfide clusters. On the other hand, thermal treatment of 2 with SiH₃Ph generated very cleanly the dinuclear tantalum(IV) sulfide complex [Ta₂(η⁵-C₅Me₅)₂Cl₂(μ-S)₂] (4) in a quantitative way. Likewise, we found that 4 was synthesized more easily by a one-pot reaction of 1, (Me₃Si)₂S, and SiH₃Ph in toluene. Reactions of 4 with a series of alkylating reagents rendered the dinuclear peralkylated sulfide complexes [Ta₂(η⁵-C₅Me₅)₂R₂(μ-S)₂] (R = Me 5, Et 6, CH₂SiMe₃ 7, C₃H₅ 8, Ph 9). Single-crystal X-ray diffraction analyses of 4, 5, and 9 showed in all cases a trans disposition of the chloro or alkyl substituents. The short Ta-Ta distances (2.918(1)-2.951(1) Å) along with DFT calculations indicate a σ-Ta-Ta interaction. Complexes 5, 6, and 8 undergo trans- cis isomerization, and mechanistic proposals are discussed based on DFT calculations.