Thermolysis of Cp^*Rh{(η⁴:η¹-C₄Me₄S)Fe(CO)₄}: A Case Study in Thiophene Desulfurization

Cp*Rh(η⁴-C₄Me₄S) (1) and Fe₃(CO)₁₂ react to give the ferrole Cp*Rh{η⁵-C₄Me₄Fe(CO)₃} (3) together with trace amounts of Cp*Rh{(η⁴:η¹-C₄Me ₄S)Fe(CO)₄} (2). Compound 2 was prepared in high yield by the reaction of 1 with Fe(CO)₅ in the presence of Me₃NO. Thermolysis of 2 in refluxing toluene gave 3, free C₄Me₄S (tetramethylthiophene, TMT), and (Cp*Rh)₂(μ-CO)(μ₃-S)Fe(CO)₃ (4) in ca. 2:2:1 molar ratio. The efficiency of the conversion of 2 to 3 increased with added Fe₃(CO)₁₂ concomitant with the diminution of the yield for 4. Control experiments showed that 3 and 4 are stable to Fe₃(CO)₁₂ in refluxing toluene. Furthermore, by labeling both the rhodium, with C₅Me₄Et, and the thiophene, as C₄Me₄S-3,4-d₂, we showed that the Rh-C₄Me₄S moiety remains intact during its desulfurization. The structures of 2-4 were determined by single-crystal X-ray diffraction. The desulfurization process illustrates the following mechanistic points: (i) transition metals play a dual role in thiophene desulfurization by separately stabilizing the desulfurized hydrocarbon and accepting the sulfur, (ii) the hydrocarbon is stabilized in the form of a metallacycle that structurally resembles thiophene, and (iii) the hydrocarbon and sulfide are stabilized in heterometallic environments.