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43917-AC3
Reactivity Studies of Metal Coordinated Thiyl Radicals
The oxidation of metallothiolates is complicated by the "non-innocence" or potential redox activity of sulfur to yield thiyl (RS•) radicals. Recently, much effort has been directed towards the stabilization and characterization of metal-coordinated thiyl radicals. In contrast to metal-coordinated phenoxyl radicals, metal-coordinated phenylthiyl radicals tend to delocalize the unpaired electron density over the metal-sulfur bond. As such, metal-coordinated thiyl radicals can be stabilized by sufficiently bulky groups that prevent disulfide formation thereby opening alternate reaction pathways. While oxidation of metallothiolates to disulfides is well-known, C-S bond formation via trapping of the radical intermediate with alkenes is less well studied. The addition of organic thiyl radicals to unsaturated hydrocarbons in C-S bond forming reactions is well known, with applications for cis/trans isomerization, sulfide synthesis, and polymerization. Alkene addition to oxidized metal-sulfur complexes has been previously reported, although a metal-coordinated thiyl radical was not invoked.
Preliminary efforts in our laboratory indicated that oxidation of the ruthenium tri-thiolate [Ru(DPPBT)3]- (DPPBT = 2-diphenylphosphinobenzenethiolate) produces a metal-coordinated thiyl radical with potential utility for reversible alkene addition. The oxidation of [Ru(DPPBT)3]- to an observable metal-coordinated thiyl radical intermediate has been investigated via combined theoretical and reactivity studies. Initial experimental observation revealed that the oxidation of the anionic complex [Ru(DPPBT)3]- by electrochemical methods proceeds in two successive one-electron steps. The first oxidation is reversible and yields the isolable and stable neutral complex, [Ru(DPPBT)3], which formally contains a Ru(III) ion. Density functional theory (DFT) investigations performed by using Jaguar and the LACVP(Fe)/6-31G(d)(rest) basis set confirm the doublet ground state of [Ru(DPPBT)3]. Interestingly, analysis of the spin density suggested the formal Ru(III)-thiolate assignment was too rigid and contributions from a Ru(II)-thiyl ground state needed to be considered.
Further oxidation of [Ru(DPPBT)3] yield a short-lived, but spectroscopically observable intermediate, [Ru(DPPBT)3]+. Based on DFT analysis, the ground state of the intermediate is best described as a singlet diradical. Although this ground state contains contributions best described as Ru(IV)-thiolate and Ru(III)-thiyl radical, the most significant contributions have Ru(II)-dithiyl radical character. As such, the two unpaired electrons sit in nearly orthogonal sulfur-centered orbitals. It is specifically this arrangement that retards disulfide formation and stabilizes the intermediate for reactivity with other substrates. In particular, the orbital arrangement favors addition of substrates across the cis-sulfur sites.
Initial efforts showed that in the presence of methyl ketones, the intermediate reacts to generate Ru(II)-thioether complexes. The reaction is presumed to proceed via reaction with the enol-tautomer of the ketone. As such, the intermediate should display rapid reactivity with alkenes. In our studies, oxidation of [Ru(DPPBT)3] to [Ru(DPPBT)3]+ in the presence of various alkenes including ethylene, 1-hexene, cyclohexene, styrene, and norbornene generates a series of Ru(II)-dithioether complexes by addition of the alkene across the cis-sulfur sites. These results provide definitive evidence for the addition of alkenes to a metal-coordinated thiyl radical. Interestingly, no evidence of H-atom abstraction has been observed, even when styrene was added to the reaction mixture.
The reversible redox regulated addition of alkenes across cis-sulfur has been proposed for olefin purification. However, earlier systems were limited by complex reduction and intra- versus inter-ligand addition of alkenes. While our complexes display the requisite inter-ligand addition, they cannot be reduced. As such, although the oxidized intermediate shows great affinity for alkenes, there is no mechanism for redox regulated reversible addition of alkenes. Current efforts are focused on derivatives to promote reversible alkene addition.
