This award in the Chemical Synthesis (SYN) program within the CHE Division supports work by Professor John F. Berry at the University of Wisconsin ? Madison to carry out fundamental studies of the redox properties of sulfur and selenium bound to transition metals. These studies will test the hypothesis that a new oxidation state of sulfur, subsulfide, can be stabilized by coordination to transition metals. This investigation involves examination of the electronic structure of previously reported sulfur/transition metal compounds, as well as newly designed compounds, using a variety of physical methods. Electrochemical and chemical reactivity of coordinated sulfur species will be investigated so that the electronic and chemical properties of the subsulfide ion may be elucidated.
The oxidation and reduction chemistry of sulfur and the nature of sulfur-sulfur bonding are critical to many important biological problems and new technologies. The development of subsulfide chemistry can have a long-term impact on the understanding of disulfide bond formation in proteins, and can potentially lead to new technologies such as new catalytic processes and dyes for photovoltaic devices.
Bond formation and cleavage are fundamental to the study of chemistry because bonds are what hold molecules together. Work in the Berry group on proposed half-bonds in a new class of compounds called "subsulfide" and "subselenide" compounds has been funded for the last two years by the US National Science Foundation’s Chemical Synthesis program. In common sulfur-containing materials, the sulfur atoms are most typically found in the doubly-reduced ‘sulfide’ form, S2–, or a binuclear dianion, ‘disulfide’, with a sulfur-sulfur bond, S22–. The aim of this work has been to test the hypothesis that a new reduced state of sulfur, i.e. S23– or subsulfide, can actually be stabilized through coordination to transition metals. This work is of particular relevance to the field of biology, since many proteins use sulfur-sulfur bonds to support their structure. It may also be a key step in the development of innovative technologies, such as new lithium-sulfur batteries. The presence of the new reduced subsulfide state of sulfur suggests that sulfur-sulfur bonds are created and broken one electron at a time, contrary to the conventional two-electron theory. The subsulfide state also suggests the existence of a formal half-bond between the two sulfur atoms. These conclusions would also apply to the heavier elements that are related to sulfur in the chalcogen family, namely, selenium (Se) and tellurium (Te). Using cutting-edge spectroscopic techniques and X-ray crystallography, the Berry group has provided a wealth of data suggesting that the new reduced subsulfide level does indeed exist, and several groups of compounds in the literature that have previously been poorly understood or were mis-assigned are now much better understood as subsulfide complexes. The Berry group has also been the first to explore the chemistry of the subsulfide ion: it is readily oxidized by one electron to the disulfide ion. This oxidation is concommittant with a significant strengthening and shortening of the sulfur-sulfur bond.
