Unravelling the Nature of Elusive Transition Metal-Oxyl Complexes

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

Transition-metal complexes in high oxidation states, especially those possessing metal-oxygen multiple bonds (so-called metal-oxo complexes), play key roles in many important chemical processes in Nature, including hormone biosynthesis, drug metabolism, and in photosynthesis. In the chemical and pharmaceutical industry, reactive metal-oxo complexes serve as catalysts for the production of chemical feedstocks and in the synthesis of fine chemicals and pharmaceuticals. Metal-oxo complexes are also relevant reactive species in solar energy conversion, required for supplying humanity with clean energy that is derived from water, solar energy, and CO2. Whereas much effort has been spent in past years to investigate such reactive oxo complexes with iron, manganese and vanadium as the metal centers, scientists lack a detailed understanding of the properties of corresponding late-transition metal-oxo complexes, which show great potential for applications in chemical catalysis, but which are underutilized as scientists cannot control their reactivity well. In this project, Professors Nicolai Lehnert and Jason Shearer will investigate different Cobalt-, Nickel and Copper-based oxo complexes to improve our understanding of the structural and electronic properties of these unstable species, and establish connections that will allow scientists to better control their reactivity. Profesor Lehnert is actively engaged in STEM outreach programs focused on recruiting underrepresented minority (URM) high school students to STEM fields. He directs the D-RISE program, which provides 6-weeks, full-time summer research opportunities to high school students from the greater Detroit area. Professor Shearer’s laboratory works exclusively with undergraduate (UG) students, which are recruited through mechanisms that aim at increasing the participation of URM UG students in the sciences (in particular, Trinity University’s McNair Scholar’s program). With funding from the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professors Lehnert (University of Michigan) and Shearer (Trinity University) are developing an understanding of the structural and electronic properties of a number of high-valent Co-, Nickel- and Copper-oxo complexes. A key open question in our understanding of high-valent transition metal-O chemistry is how the metal-oxo versus metal-oxyl character of these high-valent intermediates effects their stability and reactivity. This becomes particularly important as one moves across the transition metal series to the late transition metals of groups 9 – 11, where bond inversion effects become pronounced. For example, Cu(III)-O intermediates have been proposed in a number of monocopper oxygenases, with computational studies indicating that these species would have “pure” Cu(II)-oxyl character, thus being on the extreme of the metal-oxo vs metal-oxyl continuum. However, the significance of this finding regarding fundamental reactivity is unknown, as all purported high-valent Cu-O species are too unstable to be observed in both enzymes and model systems. In this project, high-valent Co-X and Ni-X complexes (X = oxo and imido groups), using coligands that enforce tetragonal and trigonal-bipyramidal ligand fields, will be investigated. Importantly, these Co- and Ni-O complexes are right at the transition point from dominantly metal-oxo to dominantly metal-oxyl ground states. By studying these intermediates, how an increasing degree of metal-oxyl character fundamentally changes the properties, stabilities, and, most importantly, reactivities of high-valent transition metal-O intermediates could be achieved. An isolable formally Cu(IV)-oxo species contained in a redox-active ligand scaffold will aslo be studied. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.