In our lab we design transition metal complexes where we assert subtle structural changes in order to observe its influence as well as study its capacity to regulate the molecule’s electronic properties. Designing molecular systems which can be conveniently altered for purposes of tuning electronic properties continues to hold considerable interest in the scientific community due to their potential use in applied areas such as molecular electronics, energy conversion, information recording, and catalysis. Many studies in this area have involved metals coordinated to aromatic N-heterocyclic ligands containing various combinations of five and six member rings. Only a few of these studies have systematically varied the ligand design in order to observe its influence on the redox and/or spin state properties on first row transition metals. To some extent this is due to the observed lack of stability encountered in many first row transition metal systems composed of mono and/or bidentating ligands. Hence, we have found it necessary to use tridentating molecules in order to avoid complications that can arise from ligand scrambling.
My research interests have primarily revolved around tuning redox (electron transfer) and spin state (magnetic) properties on first row transition metals such as Fe(II) and Co(II) by way of employing slight modifications on the ligand structure. For instance, we have found that one can methodically and rationally control the metal’s electronic behavior by (1) varying the ligand substituents’ electron withdrawing/donating properties, (2) altering the ratio of pyridine to pyrazoles, (3) manipulating steric factors between neighboring ligands, and (4) incorporating ionizable proton sites that are capable of operating as a switch between different electronic states. In our group, we typically characterize redox properties by using cyclic and/or differential pulse voltammetry. Room temperature spin state behavior is analyzed by measuring solution magnetic susceptibility carried out with standard NMR methodology (based on the Evans Method).
The molecule above illustrates a tridentating ligand, terpyridine, coordinated on a metal center with substituents (represented by R1, R2, and R3) varied in either the 4’ or the 4, 4’, 4” positions. As was noted in the first case, it was discovered that by changing the electron donating/withdrawing character of the substituents, the M(III/II) redox potential (where M represents Fe, Co, and Mn) could be altered in a rational manner. Also, by varying the inductive nature of the substituents we were able to tune between the high and low spin state with Co(II) and are currently characterizing this type of behavior with Mn(II). In contrast, the Fe(II) remained predominately low spin due to its relatively large d-orbital energy splitting which is reflected in its stronger metal-ligand binding strength. This general trend is depicted in following diagram where pseudo-octahedral geometry is assumed.
In the second case, we were able to measure a shift between the high and low ground spin states by varying the ratio of metal-pyridyl coordination sites to that of metal-pyrazole. This work was carried out by using the following series of molecules shown below where L1 is commercially available while L2 and L3 were prepared by literature procedures.
Due to the weaker metal-ligand with pyrazole as compared to pyridine, the spin state of both Fe(II) and Co(II) were observed to have a diminishing d-orbital energy splitting going from L1 to L3 thereby driving high spin behavior. Of particular interest was the behavior of Fe(II) which was low spin when coordinated to L1 or L2 but switched to predominantly high spin behavior with the L3 ligand. Assuming the bis-L2 iron complex was close to the high spin arrangement, we decided to synthetically design small changes in the L2 structure (Case 3) that intentionally incorporated steric factors when coordinated onto the metal center. The results showed that careful use of sterics can be used as an additional tool for tuning both redox and spin state behavior.
Other work in our lab has been focused at using proton transfer coupled to an electron transfer, termed proton coupled electron transfer (PCET), and investigating the ability to use proton transfer to reversibly switch between different spin states. We designed a tridentating molecule which contains a single ionizable proton specifically for these studies.
This ligand was synthesized by essentially derivatizing 2,2’-bipyridine at the 6-position with a benzimidazole followed by its coordination onto a number of different transition metals such as Ru(II), Fe(II), Co(II) and Mn(II). Each metal complex has exhibited strong stability in both the fully protonated and fully deprotonated states. These metal systems have two ionizable protons that can be conveniently removed and replaced in a reversible manner illustrated in the following figure.
Deprotonation of the two ionizable proton sites results in a significant change in the M(III/II) redox potential, and in the case of Co(II), results from our lab indicate a change in the ground spin state arrangement. We are further exploring this phenomena with Mn(II). The PCET behavior is analyzed by measuring the M(III/II) redox couple, via cyclic voltammetry, as a function of solution pH. For each metal complex, both one-proton/one-electron and two-proton/one-electron processes have been observed.
Publications (UWG students represented in blue)
Spencer J. Slattery, Ph.D.
Professor & Department Chair
Office: 2135 TLC, Lab: 2107 TLC
Ph.D. Inorganic Chemistry
Florida State University, 1993
University of West Florida, 1988
B.S. Ed. Biological Education
University of West Florida,1987