Coordination of PPh₃ with a [Mo₂O₂(μ-S)₂]²⁺ Phosphinoyldithioformate Complex and Its Influence on Episulfide Sulfur Abstraction

Phosphinoyldithioformate ligands in binuclear molybdenum-sulfur complexes offer a synergistic environment for sulfur transfer (SAT) reactions. Relevant complex, 3, and its SCN^- derivative, 4, as two salts 4a and 4b were formed. Reactions of 3, and 4b with cyclohexene sulfide (CS), styrene sulfide (SS), and propylene sulfide (PS) were studied. Different stabilities of 3 and 4b, as 4b > 3, allowed comparisons of reactivity based on the ability of the ligands to accommodate the episulfides. A moderate reactivity of CS, SS, and PS with 3 was arranged as CS > SS > PS, while its reactivity with 4b was negligible. The addition of PPh₃ was found to increase the rate of reaction of 3 with CS to form Ph₃PS. PPh₃ reacted with SS and PS, to form Ph₃PS and polythioether as SS < PS. PPh₃ reacts with 3 to abstract a sulfur atom. The mechanism of the SAT reaction is proposed to take place in two steps to form a sulfurated complex followed by its sulfur expulsion, while the addition of PPh₃ replaces the expulsion step with a faster Ph₃PS formation. 3 was ascertained as an efficient catalyst and the nonrigid ligands accommodated the episulfides and PPh₃ in their reactions without evident catalyst deactivation.