Studies of C-S bond cleavage reactions of Re(V) dithiolates: Synthesis, reactivity, and mechanism

A series of rhenium(V) complexes, [(X)(ReO)(dt)(PPh3)] and [(o-SC6H4PPh2)(ReO)(mtp)], were prepared to explore electronic effects on the C-S cleavage reaction that occurs upon reaction with PAr3 at ambient temperature [where X = S(C6H4-P-Z) (Z = OMe, Me, H, F, Cl), OPh, Cl, and SC2H5, and dt is the chelating dithiolate ligand derived from 2-(mercaptomethyl)thiophenol, 1,2-ethanedithiol, 1,3-propanedithiol, 1,3-butanedithiol, and 2,4-pentanedithiol]. The scope and selectivity of the C-S activation were examined. The C-S bond cleavage to form metallacyclic Re(V) complexes with a Re equivalent to S core occurs only for the complexes with mtp and pdt frameworks and X = SAr and SC2H5. The difference in reactivity is due to the different donating abilities of ancillary and dithiolate ligands, especially their,pi-donating ability, which plays a critical role in C-S activation. The kinetics of the C-S activation process was determined; nucleophilic attack of PPh3 on the oxo group of the (ReO)-O-V core appears to be the rate-controlling step. The reaction is accelerated by electron-poor ArS ligands, but is unaffected by the substituents on phosphines. A detailed mechanistic study is presented. The results represent a rare example of migration of alkanethiolate leading to the formation of alkylthiolato complexes.