SusChEM: Mechanisms of small molecule activation in constrained molecular environments

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In the modern era of scarce resources, developing chemical processes that can eventually generate useful materials and fuels from readily available, cheap, renewable starting materials is of paramount importance. Small molecules (such as oxygen or its close relative, hydrogen peroxide) are ideal sources of oxygen atoms in synthetic applications. New strategies for utilizing oxygen in chemical synthesis and energy production may lead to environmentally clean oxidations that generate water as the only byproduct and rely on readily available reagents. Dr. Rybak-Akimova at Tufts University conducts research to gain a detailed understanding of the pathways in oxygen and peroxide binding and reactivity in selected examples of chemical systems. She aims to identify the "bottlenecks" of these reactions and gain fundamental knowledge important for informing the design of efficient and non-toxic reagents in synthetic applications. This research project provides opportunities for graduate and undergraduate students to be trained in high-level mechanistic studies while also participating in collaborative efforts to design, prepare, characterize, and apply new synthetic systems for small molecule activation. Dr. Rybak-Akimova plans to actively recruit female and minority students for the project through the Tufts Center for STEM Diversity. Outreach to high-school students from public schools in the area includes visits to and presentations at science fairs, field trips by groups of Medford and Somerville high school students to research laboratories at Tufts, and recruitment of high school students as summer researchers. The Macromolecular, Supramolecular and Nanochemistry Program and the Chemical Catalysis Program of the NSF Division of Chemistry jointly support the research group of Dr. Rybak-Akimova to study the role of secondary sphere effects, including hydrophobic interactions with sterically bulky substituents and hydrogen bonding, in selected chemical systems. More specifically, this project investigates: (1) O2 binding and activation at sterically protected metal centers capable of multi-electron oxidations (e.g. Pd(0)/Pd(II) or V(III)/V(V)), and the reactivity of thermally unstable end-on dioxygen adducts (metal-superoxo intermediates) compared to analogous stable side-on metal-peroxo species; (2) encapsulation of peroxide guest inside carboxamide cryptand host and the reactivity of this peroxocryptand with externally added oxidants or reductants; (3) generation and reactivity of metal-bound oxygen intermediates in amide-containing three-dimensional cages and analogous "open" tripodal or two-dimensional macrocyclic hosts; (4) anion recognition of high-valent peroxy anions (e.g. peroxyvanadates) with amide-containing tripods, macrocycles, and cages for selective substrate oxidations. This research aims to establish the mechanisms of stepwise coordination of dioxygen to low-valent metal centers supported by ligands with controlled level of steric bulk and/or the number and location of proton-donating groups at the periphery, measure the reactivity of these species with externally added substrates in direct double-mixing kinetic experiments, identify competent oxidants, examine steric and H-bonding modulation of peroxide reactivity encapsulated in metal-free hosts in reactions with external oxidants or reductants, and determine the mechanisms of electron transfer and atom transfer. Stopped-flow instrumentation and expertise in technically demanding kinetic measurements of rapid reactions developed at Tufts continues to have significant impact on the infrastructure for mechanistic research.