At the heart of sustainable, renewable, and "green" fuel production lies the activation of naturally abundant small molecules and their transformation to useful chemical feedstocks. Whether it be the splitting of H2O into H2 and O2, the conversion of CO2 into a useful C1 feedstock, or the conversion of CH4 and other saturated hydrocarbons into useful energy-rich value-added products, the future of the global energy economy is dependent upon the development of new technologies to maximize efficiency and drive the world towards a more sustainable future. The science behind such technological developments is reliant on the fundamental design of homogeneous and/or heterogeneous catalysts (or those that lie at the interface of the two) capable of activating the s and p bonds in small molecule substrates such as CO2, H2, carbonyl compounds and hydrocarbons. In many cases these transformations are thermodynamically unfavorable, and typically involve multielectron redox processes. The so-called "noble" or "precious" metals of the late 2nd and 3rd row transition metal series (e.g. Pt, Pd, Rh, Ir) are the most well-studied for such multielectron redox processes, as they are known to commonly undergo Mn/Mn+2 redox cycling. Unfortunately, these metals are also among the least abundant elements on the periodic table, and therefore cost prohibitive. The most economical catalysts, however, would involve far less expensive and more Earth-abundant metals such as those in the first row transition series (e.g. Cr, Mn, Fe, Co, and Ni) or early transition metals such as Ti, Zr, Nb, or Mo. These metals, however, are often relatively redox inert and/or favor one-electron transformations, which are often difficult to control or predict. Research within the Thomas laboratory focuses on addressing these challenges by exploring cooperation between different components of bifunctional catalysts. Specifically, the Thomas group is interested in fundamental catalysts design principles involving (1) two metal centers in bimetallic frameworks and (2) metal centers and non-innocent ligands, and the unique effects that such cooperation can have on the reactivity of these species. All projects in the Thomas group entail the synthesis of new ligands and transition metal complexes. Although the primary focus of our research is synthesis, the research pursued the Thomas lab also entails a large variety of characterization techniques including multinuclear NMR, IR, UV-Vis, Mossbauer and EPR spectroscopies, as well as magnetic and electrochemical measurements using cyclic voltammetry and SQUID magnetometry. Many of these techniques are done in collaboration with groups at universities throughout the Boston area. In addition, theoretical investigations into the electronic structure and reactivity of particularly interesting transition metal complexes synthesized in the lab are studied using density functional theory (DFT) calculations using computational software including Gaussian09 and Orca. Some specific research projects are discussed below:
Heterobimetallic Complexes Supported by Phosphinoamide Ligands. top
As a method to address the fundamental challenge of designing catalysts for multielectron redox transformations, our group has been investigating well-defined bimetallic systems to learn more about the fundamental interactions between these two metals and how such interactions might be used to promote multielectron redox chemistry and determine the mechanism by which small molecules might interact with the two metal sites to facilitate the cleavage of s and p bonds. While we are primarily interested in investigating homogeneous catalyst design strategies, our ultimate goal is to provide insight into some of the interactions and mechanistic pathways that might also be at play in heterogeneous catalysis where multiple metal sites are certainly present and involved in catalysis. Ongoing research involves the synthesis of new homo- and heterobimetallic complexes featuring metal-metal interactions, spectroscopic and computational studies into the electronic structure and metal-metal bonding in these complexes, and the exploration of the reactivity of these new heterobimetallic platforms towards unusual bond activation processes and catalytic transformations.
Tridentate Pincer-type Ligands Featuring a Central Cationic Phosphenium Donor. top
Research in the Thomas lab also focuses on the synthesis and coordination chemistry of a new class of tridentate pincer ligand incorporating a central N-heterocyclic phosphenium (NHP+) unit. While catalytic applications of N-heterocyclic carbene (NHC) ligands have been explored extensively, their phosphorus analogues NHP+s have not been evaluated in transition metal catalysis, and far less is known about their coordination chemistry. Prof. Thomas and coworkers have designed an NHP+-containing diphosphine pincer ligand as a method to impart stability and enforce transition metal coordination to allow more extensive studies into the transition metal chemistry of NHP+ ligands. The Thomas group has shown that NHP ligands can adopt two coordination modes: planar, indicative of an NHP+ phosphenium description, and pyramidal, indicative of an NHP- phosphido description. Similar to nitrosyl ligands, the two NHP coordination geometries correspond to metal formal oxidation states that differ by two electrons. Thus, NHP ligands have the potential to serve as non-innocent ligands, participating in redox transformations via switching coordination geometry. Researchers in the Thomas lab are investigating the coordination chemistry of the new NHP-diphosphine ligands with a variety of transition metals to evaluate the factors that dictate the preferences for planar vs. pyramidal geometries. DFT and NBO calculations are used to better understand the metal-ligand interactions in these complexes. In addition, we are interested in addressing the redox activity of transition metal NHP complexes using cyclic voltammetry to explore the possibility of ligand-based redox activity. The group is also designing and investigating new generations of NHP pincer ligands in which backbone and side-arm donor substituents are varied. Owing to the importance of multi-electron redox transformations in catalysis, the ultimate goal of the proposed research project is to utilize these analogues of both nitrosyls and NHCs to facilitate catalytic transformations such as C-C or C-X bond forming or breaking reactions, transfer hydrogenation, cross-coupling and other ?-bond activation processes.