Half-sandwich molybdenum compounds with phosphine–alkylthiolate and phosphine–thioether ligands. Crystal structure of [CpMo(SCH2CH2PPh2)2][BPh4]

Abstract

The reaction of CpMoCl₂ with Ph₂PCH₂CH₂SR (R = H, CH₃) yields the corresponding addition products CpMoCl₂(Ph₂PCH₂CH₂SR), but only the derivative with R = CH₃ (compound 5) is sufficiently stable to be isolated as a crystalline solid. The derivative with R = H evolves rapidly to afford a mixture of compounds [CpMo(SCH₂CH₂PPh₂)₂]⁺Cl^– 1, and [CpMoCl(SCH₂CH₂PPh₂)]₂ 2, the former being favored by a larger ligand∶Mo ratio. Compound 1 undergoes metathesis with NaBPh₄ to afford [CpMo(SCH₂CH₂PPh₂)₂]⁺BPh₄^– 3, which has been characterized by X-ray crystallography. The reaction of CpMoCl₂ with 2 equivalents of Ph₂PCH₂CH₂S^–Li⁺ affords the paramagnetic complex CpMo(SCH₂CH₂PPh₂)₂ 4, which is readily oxidized by Cp₂Fe⁺ or by H⁺ to the corresponding cation. The salts 1 and 3, in turn, may be reduced by Na amalgam, MeLi, or Bu^tOK to compound 4. The reversible redox process interconverting 4 and its cation occurs at E_½ = –1.23 V relative to the ferrocene standard, while compound 5 shows a reversible oxidation process at E_½ = 0.12 V by cyclic voltammetry. The comparison between these potentials and that previously reported for CpMoCl₂(dppe) indicates relative donor abilities in the order Ph₂P > MeS and RS^– > Cl^–. Compound 5 can also be synthesized by Na amalgam or Zn reduction of CpMoCl₄(Ph₂PCH₂CH₂SCH₃) 6, which is obtained by addition of the ligand to CpMoCl₄.