This award by the Inorganic, Bioinorganic and Organometallic Chemistry Program to Professor Andreja Bakac of Iowa State University is to support research on the development of new methods for generating aqueous chromium(V) ions, to examine the reactivity of oxygen atoms in solution, and to carry out studies on the formation and reactivity of intermediates generated from chromium(II) complexes and molecular oxygen. In a general sense, all of these topics deal with various aspects of oxygen activation and mechanistic oxidative chemistry. The work is expected to produce crucial kinetic data and insight into the reaction mechanisms. The intellectual merit of the proposed research thus lies in the expected contribution to the fundamental understanding of oxygen activation by thermal and photochemical means. The broader impacts of this proposal include the education and training of undergraduate and graduate students and postdoctoral associates. Additionally, mechanistic studies on oxygen activation provide a basis for the development of catalytic processes utilizing molecular oxygen in more environmentally benign reactions. Such chemistry also plays a major role in biological systems owing to the constant and complex interactions between molecular oxygen, metal complexes, and organic matter.
Intellectual merit. The focus of this research was on activation of molecular oxygen and nitrogen monoxide with transition metal complexes. Also examined were related reactions of partly reduced and oxidized forms of these molecules including hydrogen peroxide, nitrous acid, and coordinated nitrate. The motivation for this work comes from the known interaction between transition metal complexes (TMC) with small diatomic molecules such as molecular oxygen and nitrogen monoxide in a myriad of processes that are essential for life on Earth. Activation of oxygen with TMCs is also an important step in many laboratory and industrial oxidations, both of which share a number of mechanistic features and types of intermediates with biological oxidations. For example, oxygen binding to metals, partial reduction to hydroperoxo/peroxo forms, and involvement of high-valent metal-oxo species are essential in both natural and human-designed oxidations with dioxygen. In this project, studies were carried out to: (1) Obtain kinetic and mechanistic information about reactions of oxygen and nitrogen monoxide with TMCs as models for biological and catalytic reactions; (2) Utilize thermal and photochemical means to generate and study novel intermediates believed or known to be involved in such reactions; (3) Prepare new molecules as sources of nitrogen monoxide. The key findings can be summarized as follows: (a) Oxidation and reduction of superoxo and nitrosyl metal complexes (metal = chromium, rhodium) leads to rapid release of oxygen or nitrogen monoxide. In the case of macrocyclic rhodium nitrosyls, it was possible to observe and characterize the short-lived initial oxidation product prior to the release of nitrogen monoxide. (b) Thermally stable nitrosyl rhodium complexes readily release nitrogen monoxide upon irradiation with visible light. One of the rhodium complexes prepared as part of this project utilizes light more efficiently than any of the nitrosyl complexes ever reported, and is a potential candidate as a photochemical source of nitrogen monoxide for medicinal and laboratory purposes. (c) Complexes of chromium(III) that are related to those found in standard vitamin supplements may be transformed to chromate (carcinogenic form of chromium) under physiological conditions. These results urge that additional studies be carried out, especially with biological molecules, to check the safety of food supplements containing chromium(III), an oxidation state that has been traditionally considered safe for human consumption. Our results suggest that this supposition may not be true. Broader impacts. Education and training of undergraduate and graduate students and postdoctoral associates are an integral part of this project. The synthesis and kinetic and mechanistic studies will allow students to develop laboratory skills, good judgment and scientific thinking, as well as to master the use of modern techniques and instrumentation. Every effort will be made to recruit female and minority students and postdoctorals for this project. Undergraduate and graduate students and postdoctoral associates will be encouraged to present results of their research at the local, regional, and national meetings. The synthesis and photochemical studies of metal-nitrosyl complexes will result in better understanding of the factors responsible for efficient photochemical cleavage of metal-nitrosyl bonds with visible and near-infrared light. This work has a potential to lead to new compounds for use in photodynamic therapy which utilizes light as trigger for the release and targeted delivery of nitric oxide to biological tissues for medicinal purposes.
