In this project, funded by the Chemistry Division's Chemical Structure, Dynamics and Mechanisms Program, Professor Tierney of the University of New Mexico will systematically investigate a large library of high-spin (hs) Co(II) coordination compounds with mixed N/O/S donor sets, encompassing 4-, 5- and 6-coordination. The work is founded on an integrated approach to paramagnetic resonance, simultaneously applying frozen-solution multi-frequency electron paramagnetic resonance (EPR) and fluid-solution nuclear magnetic resonance (NMR), to map the field- and temperature-dependence of electron-nuclear hyperfine interactions. EPR-based techniques are used to map ground state electron-nuclear hyperfine interactions onto three-dimensional structure. Excited state effects are examined in detail by UV-vis-NIR spectroscopy, by field- and temperature-dependent NMR relaxation and by NMRD. The resulting information will define the factors controlling electron dynamics in these electronically complex systems. Pulsed EPR techniques (ENDOR and ESEEM at both X- and Q-bands) and NMR of heteronuclei, including 11B in pyrazolylborate complexes and 19F incorporated into ligands, will be utilized to aid in solution speciation. The goal is to develop a deeper understanding of how excited state mixing determines ground state electronic properties and solution dynamics. Target molecules include a series of spin-crossover complexes, examining the role of structural dynamics in the magnetic hysteresis required for translation into functional materials.
Complexes of high-spin cobalt(II) are important to the study of metals in biochemistry, as well as to modern magnetic materials and catalysts. For example, these complexes can be used as spectroscopic probes of the mechanisms of enzymatic reactions. Also, they can used to construct switchable magnets on the molecular or nano-scale. Such materials are at the frontier of research in modern computer miniaturization. A wide range of magnetic resonance techniques will be employed for determining the structural features of these interesting molecules. This research provides training opportunites for early-career scientists, as well as graduate and undergraduate researchers. Many students involved in this research have presented their research locally, and several have made presentations at regional and national meetings. Results from this research program have been incorporated into the undergraduate chemistry curriculum, including experiments that utilize low-temperature EPR, paramagnetic NMR, and X-ray structure determination.
This project involves a systematic investigation of cobalt-containing compounds relevant to magnetic materials and biologically important systems. Our goal is to dissect the electron-nuclear magnetic interactions in these materials, which tells us about the local structure around the metal atom. We employ a wide range of techniques, and this approach allows us to probe the metal’s environment with unprecedented detail. During the course of this award, we were able to directly connect magnetic behavior of a particular set of cobalt compounds in solution to a phenomenon thought only to occur in metals and extended solids, and we bring to light a dynamic process that happens on a picosecond (10-12 s) timescale. To our knowledge, this is the first time that electron-spin relaxation has been directly connected to a dynamic structural process. The importance of these observations comes in the need for thermally stable molecular magnetic materials, and for understanding the processes that lead to their instability. We have also begun to unravel some long-standing discrepancies in the electron magnetic resonance of cobalt-containing systems, by combining electron and nuclear magnetic resonance techniques. The work associated with this project has afforded a unique training environment for more than 40 student researchers. The majority of these are undergraduate students, getting their first research experience, and nearly all go on to graduate (Ph. D.) or professional (M. D., Pharm. D.) schools. The students gain invaluable hands-on expertise in a wide array of modern techniques, as well as training in critical thinking, that positions them well to go on to pursue higher goals.
