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Reports: ND355078-ND3: One-Electron Oxidation of Alkylsulfinic Acids

David M. Stanbury, PhD, Auburn University

            The objective of this project is to gain insight into the kinetics and mechanisms of one-electron oxidation of alkylsulfinic acids in aqueous solution. The results will have intrinsic merit and will also inform on broader issues relating to the oxidation of thiols. The latter can generate both sulfinic (RSO2H) and sulfonic (RSO3H) acids, so the current project will help in understanding the final product distributions.

            In the second year of funding we have continued investigation of the oxidations of two sulfinic acids: CH3SO2H (MSA) and HO2CH(NH2)CH2SO2H (CSA).

            Our studies on the reaction of CSA with [Ni(tacn)2]3+ (Ni(III)) were conducted by Ms. Pradeepa Rajakaruna, a second-year graduate student. Previously she showed that the reaction is strongly affected by dissolved O2, so all subsequent work has been conducted with exclusion of O2. The major reaction is

2[Ni(tacn)2]3+ + RSO2– + H2O  = 2[Ni(tacn)2]2+ + RSO3– + 2H+                                          (1)

This year Pradeepa discovered that solutions of [Ni(tacn)2]3+ are remarkably sensitive to room light. Her current results show that the reaction rate is first-order in [Ni(III)] and first order in [CSA], independent of pH between pH 3 and 6, and inhibited by the reaction product, [Ni(tacn)2]2+. This Ni(II) inhibition is relatively weak, making it possible to collect pseudo-first-order results even without Ni(II) present initially. These results are in stunning contrast with the rate law for oxidation of CSA by [IrCl6]2– where the product inhibition is much stronger, meaningful pseudo-order data can be obtained only with a large excess of the [IrCl6]3– product, and the rates are second-order in [IrCl62–].ADDIN BEC{Bhattarai and Stanbury, 2014, J. Phys. Chem. B, 118, 1097-1101}1

            Pradeepa is now investigating the reaction of MSA with [Ni(tacn)2]3+. Her work builds on the preliminary results obtained last year by David Drinnon. She has shown that the final products are as in eq 1. The kinetics are first-order in [Ni(III)] and only mildly inhibited by Ni(II). However, the dependence on [MSA] shows first-order and second-order terms. She is now determining the relationship between the Ni(II) inhibition and the MSA dependence. The existence of a term in the rate law that is second-order in [MSA] comes as a great surprise since analogous terms have not been detected in any other studies of RSO2– oxidation.

            Preliminary studies of the reaction of MSA with [IrCl6]2– were performed by Mr. Richard Hagen, a graduate student, during the first year of this grant. Mr. Hagen has now left the research group, so study of this reaction is being continued by Ms. Yixuan Yang, a new graduate student in the group. She has shown that the reaction rates are first-order in [MSA] and in [IrCl6]2–. They are independent of pH between pH 3 and 6. She is now determining the dependence on [IrCl6]3–, and the initial results indicate only a weak dependence. This weak dependence stands in strong contrast with our prior results on the CSA/[IrCl6]2– reaction.

            Given the puzzling set of rate laws described above, Pradeepa is investigating the reaction of [Os(phen)3]3+ with CSA. The reaction is analogous to eq 1, but the rate law differs significantly. It is first order in [Os(III)] and [CSA], and it is inhibited by Os(II). The exact dependence on Os(II) is still being worked out.

            As the above paragraphs detail, we are not yet at a stage where we can make general statements about the mechanisms of one-electron oxidations of alkylsulfinic acids. It is our hope that we can resolve the various issues described above in the forthcoming year, when we will be continuing work under this PRF grant with a no-cost one-year extension. Both Pradeepa and Yixuan are developing fine lab skills, so this objective seems within reach.

            The research of Dr. Ying Hu, a postdoc supported by a stipend from China, received support from this PRF grant in the form of materials and supplies. Her first project was published during the first year of this grant. Her second project was a study of the kinetics of oxidation of several sulfur compounds by HOCl in alkaline media. The sulfur compounds include S4O62–, S2O32–, thiourea, thioglycolic acid, (methylthio)acetate, dithiodiglycolic acid, and dithiodipropionic acid. With S4O62– and dithiodiglycolic acid the rate-limiting step is base hydrolysis of the S–S bond, but for the other species the rate-limiting step is Cl+ transfer from HOCl to the sulfur center. These S-Cl species then undergo hydrolysis to form S-O products. The rate constants for Cl+ transfer to sulfur span an astounding range of more than seven orders of magnitude. There is a parallel trend between these rate constants and those for the corresponding H2O2 reactions. Many of the reactions also show terms in their rate laws corresponding to direct reaction between OCl– and the sulfur substrate. There is a fine LFER relating the log OCl– rate constants to those of H2O2, having a slope of 1.2; this is an indication of oxygen-atom transfer as opposed to Cl+-transfer assigned to the HOCl reactions. Further support comes from the LFER obtained for reactions of OCl– in comparison to those of [Pt(CN)4Cl2]2–. Here the slope is 2.7, indicating that the Pt(IV) reactions go by Cl+ transfer. Some of these HOCl reactions provide routes to unstable oxy-sulfur derivatives that are otherwise poorly characterized, such as sulfenic acids (RSOH, which are believed to be precursors to sulfinic acids), OSSO32–, and (NH2)2CSO. This work has now been published.

            Impact: This research has had an immediate impact on the career of Dr. Hu Ying, since she has two publications supported by this grant and she is now working as a Professor in China. Mr. Drinnon is still in the early phases of his college education, so his PRF-supported research may have a strong influence on his ultimate career path. Ms. Rajkaruna and Ms. Yang will base their Ph.D. dissertations largely on this PRF-supported project. This PRF grant is enhancing the PI's career by enabling him to continue to provide exciting research experiences to his students and to solve vexing problems in mechanistic redox chemistry.