43917-AC3
Reactivity Studies of Metal Coordinated Thiyl Radicals
Carbon-sulfur bond formation between organic thiyl radicals and unsaturated hydrocarbons is well known. Recently, we have developed new routes to extend this reactivity to metal coordinated thiyl radicals. Previously, we reported the addition of alkenes and selected ketones to Ru(DPPBT)3 (DPPBT = 2-diphenylphosphinobenzenethiolate) upon oxidation. Exchanging rhenium for ruthenium yields a system with reversible C-S bond formation/cleavage between ethylene and [Re(DPPBT)3] (1) and its oxidized derivatives in a redox-regulated process.
Complex 1 is unreactive with ethylene even after prolonged exposure to saturated solutions. However, oxidation of the formal Re(III) complex 1 by one electron yields [1ox1]+ as a dark blue complex that reacts rapidly with ethylene. The product of this addition has been confirmed by spectroscopy and x-ray crystallography as the dithioether complex [2ox1]+, which results from addition of ethylene across cis-sulfur donors via the formation of two new C-S bonds. By analysis of the electronic spectrum, it is evident that the reaction is not complete and equilibrium ratios of [2ox1]+: [1ox1]+ were estimated at 2.5:1. Consistent with this observation, purging of the solution with nitrogen initiates C-S bond cleavage, which releases ethylene and restores [1ox1]+. Numerous C-S bond formation/cleavage cycles can be accessed with no significant changes in efficiency.
The voltammogram of the dithioether complex [2ox1]+ displays two redox events assigned as a one electron reduction to 2 and a one electron oxidation to [2ox1]+. Reduction to 2 results in rapid C-S bond cleavage and loss of ethylene. This result is consistent with the apparent lack of reactivity of 1 with ethylene. Oxidation of [2ox1]+ by one electron yields [2ox2]2+, which will not release ethylene even under vacuum or reflux. These results underscore the role of oxidation state in regulate ethylene coordination.
By employing a variety of voltammetric techniques, the kinetics and thermodynamics of ethylene binding in three distinct oxidation states has been evaluated. The one electron oxidized complex [1ox1]+ reacts with ethylene with a first order rate constant of (1.2 ± 0.2) x 10-1 M-1 s-1. The corresponding rate constant for C-S bond cleavage from [2ox1]+ was determined as (3.0 ± 0.4) x 10-2. From these, an equilibrium constant for the monocation derivatives of 1 and 2 was determined as 4.0 ± 0.8, in excellent agreement with predictions from the electronic spectra. Using this value of K and measured redox potentials, the equilibrium constant for the neutral parents, 1 and 2, was determined as (1.9 ± 0.4) x 10-11 consistent with observations of an unstable C-S bond. Similarly, the equilibrium constant for the dication derivatives [1ox2]2+ and [2ox2]2+ was determined to be (2.5 ± 0.9) x 109, which is large and consistent with the observed stability of [2ox2]2+.
Overall, the system displays oxidation state dependent C-S bond formation/cleavage with access to “locked on”, “locked out”, or concentration dependent ethylene addition. The kinetics of ethylene C-S bond formation/cleavage allow facile trap and release of ethylene over the period of several minutes. Our currently system modulates complete and rapid reversibility even at 258 K when purged with N2 upon reduction to 2 representing a significant improvement over related systems. Further studies to determine the mechanism of addition and substrate specificity are underway.
