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-degrees-C [GRAPHICS] liberates butadiene and free 2,5-DHT (relative ratio 1:4). Uncoordinated 2,5-DHT itself does not decompose at 120-degrees-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-degrees-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 (mu-2-H)Ru3(CO)9(mu-3-1-4-eta-4-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.