Synthesis and Desulfurization of 2,5-Dihydrothiophene Transition-Metal Complexes: Models for the Hydrodesulfurization (HDS) of Thiophene

Several S-coordinated 2,5-dihydrothiophene (2,5-DHT) transition-metal complexes were synthesized in order to determine whether or not this type of coordination promotes butadiene elimination from the 2,5-DHT ligand, a step proposed in a mechanism for the hydrodesulfurization (HDS) of thiophene. Thermal decomposition of W(CO)₅(2,5-DHT) and Re₂(CO)₉(2,5-DHT) at 110 [formula omited] °C liberates butadiene and free 2,5-DHT (relative ratio 1:4). Uncoordinated 2,5-DHT itself does not decompose at 120 °C after 3 days. Thus, S-coordination of 2,5-DHT to these metal centers does promote the liberation of butadiene. Upon being heated at 180 °C, MCl₂(2,5-DHT)₂ (M = Pd, Pt) gives off mainly thiophene and free 2,5-DHT (1:1) with only a small amount of butadiene. Thus, depending on the complex either butadiene or thiophene may be evolved. The reaction of Ru₃(CO)₁₂ with 2,5-DHT forms the trinuclear (μ₂-H)Ru₃(CO)₉(μ₃-1-4-η⁴-DHT), whose X-ray structure determination shows that the sulfur and olefin of the 2,5-DHT coordinate to two different ruthenium atoms but also C-H cleavage occurs at C(2) forming a C-Ru bond. The structure of an S-coordinated DHT complex [Cp(PMe₃)₂Ru(2,5-DHT)](PF₆) is also reported.