Reports: ND355078-ND3: One-Electron Oxidation of Alkylsulfinic Acids
printer friendlyIn this first year of funding we have investigated the oxidations of two sulfinic acids: CH3SO2H (MSA) and HO2CH(NH2)CH2SO2H (CSA). We investigated the oxidation of MSA by [IrCl6]2– and by [Ni(tacn)2]3+ (tacn = 1,4,7-triazacyclononane); in the case of CSA we investigated the oxidations by [Ni(tacn)2]3+ and [Mo(CN)8]3–; we published previously on the oxidation by [IrCl6]2–.
Our studies on the reaction of CSA with [Ni(tacn)2]3+ were conducted by Ms. Pradeepa Rajakaruna, a first-year graduate student. The major reaction is
2[Ni(tacn)2]3+ + RSO2– + H2O = 2[Ni(tacn)2]2+ + RSO3– + 2H+ (1)
Pradeepa has shown that the kinetics are significantly affected by the presence of dissolved O2, and she has determined the full rate law for the reaction in the absence of O2:
–d[Ni(tacn)23+]/dt = [CSA]tot[Ni(tacn)23+]Ka1(k1Ka2 + k2[H+])/([H+]2 + [H+]Ka1 + Ka1Ka2) (2)
The pH dependence arises because CSA exists in various states of protonation: HCSA+, CSA, CSA-H–, and CSA-2H2–. Here, k1 is 32 M–1 s–1 and k2 is 8.2 M–1 s–1, which correspond to reactions of CSA-H– and CSA respectively. Pradeepa investigated the effect of [NiII(tacn)2]2+ on this reaction at pH 5 where the k1 path is dominant and obtained conclusive evidence that Ni(III) has no significant effect on the kinetics. This result is in stunning contrast with the rate law for oxidation of CSA by [IrCl6]2– and provides strong motivation for investigating the corresponding reactions with MSA.
Our studies on the reaction of CSA with [Mo(CN)8]3– were conducted as a one-semester undergraduate research project by Ms. Lekeia Taylor. Although the results were rather inconclusive, they showed that the reaction is too slow to be convenient for study.
Mr. David Drinnon, a remarkably talented undergraduate, spent the summer after his Freshman year investigating the reaction of MSA with [Ni(tacn)2]3+, working under the guidance of Pradeepa Rajakaruna. David didn't have time to investigate the Ni(II) effect. Nevertheless, the excellent pseudo-first-order kinetics obtained in the absence of added Ni(II) implies that Ni(II) has no significant effect. He obtained a simple rate law for the reaction:
–d[Ni(tacn)23+]/dt = [MSA]tot[Ni(tacn)23+]k1/(1 + [H+]/Ka) (3)
Here k1 is 140 M–1 s–1 and it corresponds to oxidation of CH3SO2–; the rate of direct oxidation of CH3SO2H is indistinguishable from zero. These results are analogous to those obtained for the reaction of Ni(III) with CSA except that the neutral CSA species is also reactive. We attribute this to a zwitterionic form of CSA having a RSO2– moiety.
Preliminary studies of the reaction of MSA with [IrCl6]2– were performed by Mr. Richard Hagen, a graduate student. These studies suggest that the reaction rate law is analogous to those for the reactions of CSA and MSA with [Ni(tacn)2]3+ (described above). This outcome is quite a surprise, since the reaction of CSA with [IrCl6]2– has a remarkably distinct rate law. Mr. Hagen has now left the research group, so study of this reaction will be continued by Pradeepa Rajakaruna.
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 a study of the oxidation of H2S by [IrCl6]2–, and it has now been published; this paper presents the first report of a meaningful rate law for the one-electron oxidation of H2S by a conventional oxidant. It shows how to prevent catalysis by trace levels of copper ions and how to prevent sulfur precipitation by use of phosphines as scavengers, and it shows that the rate-limiting step is oxidation of HS– to HS•. The rate of oxidation of H2S is undetectably slow, in conformity with the general observation that thiols undergo 1-e– oxidation via their thiolate forms. An important difference, however, between the 1-e– oxidations of HS– and RS– is that the former produces polysulfides while the latter produces RSSR, RSO2–, and RSO3–.
Dr. Hu's 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. These species thus represent compounds with terminal sulfur atoms, thioethers, and disulfides. 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. Overall 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, although the H2O2 reactions are much slower. Aside from the insight gained about the reactivity of HCl with sulfur compounds, the faster of these 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 is being prepared for publication.
Impact: This research will have immediate impact on the career of Dr. Hu Ying, since she has two publications supported by this grant and she is now entering the workforce. 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 will base her Ph.D. dissertation 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.
