Research in the Gilliard Laboratory is multidisciplinary and combines various aspects of organic, inorganic, main-group, and materials chemistry. We develop novel synthetic methods to access molecules that are relevant to a wide range of energy-related problems. Researchers work in a collaborative environment, taking on challenging scientific problems with leading researchers in the United States and abroad. Trainees gain extensive experience in chemical synthesis with a focus on the s- and p-block elements. Researchers also become experts in various characterization methods including but not limited to multinuclear NMR, EPR, UV-vis, photoluminescence, and single crystal X-ray diffraction. The Gilliard Group is equipped with standard tools for chemical synthesis and a variety of specialty equipment for handling reactive molecules. Current activities involve three broad research thrusts:
Main-Group Element-Doped Materials for Energy
The materials subgroup of our lab is working to explore the synthesis, reactivity, properties, and applications of group 13 element-doped polycyclic aromatic hydrocarbons (PAHs). PAH materials are well known as molecular vehicles for energy conversion and storage (e.g. energy efficient lighting sources, sensors, molecular electronics, solar energy processes), however, our group is interested in understanding the impact main-group heteroatoms have on traditional organic materials. Recently, we synthesized boron-doped heterocycles such as borafluorene (i.e., the boron-doped analogue of fluorene−a five-membered heterocycle sandwiched by two benzene rings) and borepin (i.e. a flexible, but planar, 6π-electron 7-membered aromatic ring). This synthesis-enabled doping process results in molecules with vastly different photophysical and electronic properties compared to their all-carbon analogues. While it is common to synthesize borafluorene and borepin complexes as neutral molecules, we recently prepared cationic and radical materials based on these heterocyclic platforms. This has resulted in new thermochromic and thermoluminescent materials, as well as compounds with unusual excited states. We are also interested in luminescent materials that result from the fusion of main-group heterocycles, such as our work on pyrene- and benzene-fused N-heterocyclic boranes. Recent articles from these efforts include: Chem.−Eur. J. 2019, 25, 12512-12516; Angew. Chem. Int. Ed. 2020, 59, 3850-3852; Angew. Chem. Int. Ed. 2020, 59, 3971-3975; Chem.−Eur. J. 2020, 26, 10072-10082.
Subvalent and Cationic Organometallics for Bond Activation
Our group is interested in the development of low-coordinate, subvalent (e.g. a formal oxidation state lower than normal), hydridic, and cationic complexes of the s and p-block elements (i.e., groups 1-2 and 13-18 of the periodic table). Many of these main-group elements have the benefits of being earth-abundant and relatively non-toxic. The types of molecules we have synthesized are typically reactive, and thus, have the potential to interact with bonds that are relatively inert, including those of energy-relevant small molecules [e.g. carbon dioxide (CO2), dihydrogen (H2), and ammonia (NH3)]. These molecules also serve as chemical synthons to produce substances of higher value, and their bond activation chemistries play an important role in the development of new catalysts. Despite these advantages, these molecules are very challenging to prepare, which require our laboratory to employ specialized synthetic methods and techniques that protect these compounds from air and moisture. These include the utilization of inert atmosphere gloveboxes, Schlenk manifolds, and other custom-made glassware. Recently, we have studied the coordination chemistry of beryllium, magnesium, and bismuth, which has led to unusual structures that advance our understanding of chemical bonding−critical for predicting new reactivity trends for these elements. Recent articles from these efforts include: Chem.−Eur. J. 2019, 25, 4335-4339; Chem. Commun. 2019, 55, 1967-1970; J. Am. Chem. Soc. 2020, 142, 4560-4564; Angew. Chem. Int. Ed. 2021, DOI: 10.1002/anie.202016027. Our work in this area has been highlighted by ChemistryViews and Chemical and Engineering News (C&EN).
Carbene-Mediated Strategies for CO2 Reduction
Carbon dioxide (CO2), a rather notorious greenhouse gas, is becoming increasingly abundant in Earth’s atmosphere. Thus, there has been substantial effort directed toward negative emissions, and the removal of carbon dioxide from the atmosphere. The synthetic difficulties associated with carbon dioxide reduction is rooted in the strength of the relatively inert C−O bonds, which generally require catalysts or additional reagents to promote reactivity. Our laboratory is exploring CO2 conversion chemistry which is mediated by carbene (i.e., a molecule containing a neutral carbon atom with two valence electrons). Carbenes react with CO2 to form zwitterionic adducts (i.e. N,N’-disubstituted imidazolium-2-carboxylates or, more generally, carbene-carboxylates) which make CO2 more reactive. In a collaboration with the Machan lab, we have discovered that carbene-carboxylates are redox-active, and can be electrochemically reduced to carbon monoxide and carbonate. These studies led us to investigate the chemical reduction of carbene-carboxylates with earth-abundant main-group reductants in their elemental form, such as alkali metals (e.g. Li, Na, K). When reacted with carbene-carboxylates, these reductants form electronically diverse alkali clusters of reduced CO2 – a fundamentally important step in CO2 conversion chemistry–and a reaction that does not proceed in the absence of the carbene ligand. Recent articles from these efforts include: Chem.−Eur. J. 2019, 25, 6098-6101; Chem. Sci. 2021, DOI: 10.1039/D0SC06851A. Our work in the area has been highlighted by Chemical and Engineering News (C&EN).