The Rybak-Akimova Group

Kinetics & Mechanisms of Small Molecule Binding/Activation by Transition Metal Complexes

Much of our work regarding the investigation of reaction mechanisms requires the ability to study very rapid reactions over a broad temperature range under anaerobic conditions. The stopped-flow method is specifically designed for this purpose. Detailed low-temperature single- and multi-mixing stopped-flow kinetic studies provide information on the rates of formation/decay of reactive intermediates and the rates of their interactions with substrates. Identification of kinetically competent intermediates is critical for designing functional models of metalloenzymes that promote efficient and selective oxidative transformations.
Our stopped-flow instrument (TgK Scientific, UK)

Kinetic Studies of Oxygen Binding & Cleavage by a Manganese(II) Complex

In collaboration with the Kovacs group, University of Washington

Two fundamental processes of life, that of dioxygen-induced DNA repair, as well as photosynthetic O2 evolution, are proposed to involve a common metastable binuclear manganese intermediate, which has yet to be characterized. We report kinetic evidence for the involvement of two metastable intermediates in the reaction between O2 and a model Mn(II) complex, studied using the stopped-flow method in conjunction with UV-visible spectroscopy. Two colored intermediate species were detected during the reaction; formation rates for each intermediate were determined at various temperatures and oxygen concentrations. From the kinetic data, the following mechanism was proposed.

Kinetics of Oxygen Binding to V(III) tris-Anilide Complex

In collaboration with the Hoff group, University of Miami and the Cummins group, MIT

Early transition metal peroxo complexes are useful natural and industrial catalysts. Although they can often be generated using peroxides as oxidants, there are a limited number of cases where structurally characterized peroxo complexes have been prepared directly from reactions with O2. Activation of O2 at a metal center can yield end-on (η1) and side-on (η2) complexes. Formation of η2-O2 complexes may occur through a concerted mechanism or proceed through an η1-O2 intermediate which subsequently rearranges. Although the latter is generally proposed, there are few examples in which stepwise formation of η2-O2 complexes has been observed or quantified. We were able to observe the two-step activation of O2 by the sterically shielded complex V(N[tBu]Ar)3 using stopped-flow. Oxygen binding to V(N[tBu]Ar)3 is very rapid at low temperatures (-80 °C to -53 °C) and time resolved visible spectra clearly revealed a two-step process. The rate of formation of (η1-O2)V(N[tBu]Ar)3 and isomerization to (η2-O2)V(N[tBu]Ar)3 were directly measured.

Kinetics and Mechanism of Oxygen Atom Transfer (OAT) from N-Oxides to V(III) tris-Anilide Complex

In collaboration with the Hoff group, University of Miami and the Cummins group, MIT

We have also studied the mechanism of OAT from a variety of N-oxides to the sterically hindered V(N[tBu]Ar)3 complex via stopped-flow methodology. The range of oxygen atom transfer reagents studied, except for N2O, which undergoes dinuclear OAT, all follow a general kinetic scheme in which the ligand binds to metal center and cleaves to form the V(V) oxo product. Despite sharing a common mechanism, stopped-flow kinetic experiments revealed a wide range of kinetic behavior where the rates of OAT are influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. The nature of the intermediate formed can undergo significant change depending on the identity of X, and factors such as spin state, bond strength, charge transfer, and adduct geometry contribute to the magnitudes of the various rate constants. An interesting finding was that the rates of OAT from either N2O or MesCNO towards V(N[tBu]Ar)3 could be greatly enhanced by forming adducts of the XNO fragments with organic leaving groups: N-heterocyclic carbene adducts of MesCNO & N2O yield the more reactive SiPr/MesCNO and iPr/N2O, respectively, and a dihydroanthracene adduct of N2O yields the more reactive dbabhNO substrate.