Homolytic C−S Bond Scission in the Desulfurization of Aromatic and Aliphatic Thiols Mediated by a Mo/Co/S Cluster:  Mechanistic Aspects Relevant to HDS Catalysis

The kinetics of the reaction of a series of aromatic and aliphatic thiols with cluster 1 were determined. These reactions form cluster 2 and the arene or alkane corresponding to the thiol: Cp‘ 2 Mo 2 Co 2 S 3 (CO) 4 ( 1 ) + RSH → Cp‘ 2 Mo 2 Co 2 S 4 (CO) 2 ( 2 ) + RH + 2CO. These reactions are first order in thiol and first order in cluster 1 with appreciable negative entropies of activation. These data suggest that the rate determining step of the desulfurization reaction is the initial association of the thiol to the cluster. The more nucleophilic thiolate anions react with 1 at −40 °C to form an adduct in which the thiolate anion is bound η 1 to the Co atom. At −25 °C, the initial adduct rearranges to a fluxional μ 2, η 1 -bound thiolate. The fluxional process is proposed to involve a concerted “walking” of the thiolate and a μ 2 -bound sulfide ligand on the surface of the cluster. Near 35 °C, the thiolate−cluster adduct undergoes C−S bond homolysis to give the paramagnetic anion of cluster 1 and the phenyl or alkyl radical. The radical nature of the C−S bond cleavage was confirmed by the desulfurization of the radical clock reagents, cyclopropylmethanethiol and -thiolate anion, that form the cyclopropylmethyl radical which rearranged to the butenyl radical. The possible similarity in the C−S bond cleavage mechanism in these desulfurization reactions to those occurring in hydrodesulfurization (HDS) over Co/Mo/S catalysts is discussed.