In recent years, it has become clear that to meet the global energy demands of the future and to address global warming, a drastic shift in the world's energy production strategy away from fossil fuels is needed. While there are many possible renewable energy sources, including hydroelectric, wind, nuclear, and biomass, none of these sources alone is capable of satisfying the world's growing energy demands. Solar energy presents a clean and essentially unlimited energy supply, but since variations in the sun's intensity (day-night, sunny-cloudy, summer-winter) do not correlate well with global energy demands, the primary difficulty with solar energy is its storage. One possible strategy that has received recent attention is the storage of solar energy in chemical bonds, that is, the use of solar energy to convert abundant natural resources into clean fuels. While the activation of small molecules such as the splitting of water into hydrogen and oxygen (2H2O --> 2H2 + O2) and the oxidation of methane to methanol (2CH4 + O2 --> 2CH3OH) seem like elementary reactions, thus far a catalyst capable of promoting these processes efficiently has not been discovered. The research in our group focuses on utilizing creative new strategies for the design of catalysts that have the potential to promote the multi-electron, multi-proton conversion of abundant small molecules (CO2, CH4, H2, N2, etc) into useful fuels. The catalysts we are designing all involve the cooperation between different components of bifunctional catalysts. Specifically, I am interested in examining the cooperation between (1) two metal centers in bimetallic frameworks, (2) metal centers and a non-innocent ligands, and (3) metal centers and their secondary coordination spheres, 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 my research is synthesis, the research pursued in my group also entails a large variety of characterization techniques including multinuclear NMR, IR, UV-Vis and EPR spectroscopies, as well as magnetic and electrochemical measurements. In addition, theoretical investigations into the electronic structure and reactivity of particularly interesting transition metal complexes synthesized in my research group are studied using density functional theory (DFT) calculations. Some specific research projects are discussed below:
Heterobimetallic Complexes Supported by Phosphinoamide Ligands. top
In recent years, dinuclear complexes containing two different transition metal centers have received increasing interest owing to the presumption that their reactivity should vary substantially from that of monometallic complexes or homobimetallic complexes. In particular, early-late heterobimetallic complexes featuring interactions between a hard, Lewis acidic early transition metal center and a soft, Lewis basic late transition metal center have potential unique applications towards small molecule activation and homogenous catalysis. While a number of early/late heterobimetallics have been synthesized and characterized, comparatively little is known about their reactivity or the nature of metal-metal interactions in these complexes. We were particularly curious to ascertain the effect that coordination of a Lewis acidic early transition metal would have on a potentially redox-active late transition metal, and if such dative interactions could facilitate small molecule activation at milder potentials. Our group has utilized phosphinoamide ligands to enforce a dative interaction between Zr and Co via reaction of a family of metalloligands, ClZr(R'NPR2)3, with CoI2 accompanied by simultaneous reduction from CoII to CoI. The dative Co-->Zr interaction in the resulting heterobimetallic complex, ClZr(R'NPR2)3CoI, facilitates multielectron redox activity that can not be accessed in the absence of such a metal-metal interaction. In fact, in the absence of a proximal Lewis acidic Zr center, the reduction from CoII to CoI does not occur in the presence of phosphinoamines R2PNHR' unless a strong reductant such as Zn(0) is added. Moreover, we have found that the Co/Zr complexes can bind and activate dinitrogen under reductive conditions at relatively mild potentials (-1.65 V) to form dinitrogen adducts, [(THF)5Na][ClZr(R'NPR2)3CoN2]. In these complexes, Co exerts such a trans effect on Zr that the coordinated Na halide salts dissociate readily upon benzene extraction, leading to neutral two-electron reduced coordinatively unsaturated complexes. The Zr-Co bond distances in these complexes shorten by 0.2-0.5 A, indicative of an increase in Co-Zr bond order. In fact, in the absence of bound dinitrogen, the Co-Zr bond can attain full triple bond character, with a Co-Zr distance of 2.14 A (the shortest M-M distance in an early/late heterobimetallic complex reported to date). DFT calculations have been utilized to examine the electronic nature of the metal-metal bonds in these complexes. Ongoing studies focus on the utility of these heterobimetallic complexes for catalysis and the activation of sigma bonds and small molecules.
Tridentate Pincer-type Ligands Featuring a Central Cationic Phosphenium Donor. top
While N-heterocylic carbene ligands (NHCs) have become ubiquitous in many areas of coordination chemistry and catalysis, their isolobal analogues, N-heterocyclic phosphenium cations, are far less studied in the realm of transition metal chemistry and catalysis. The electrophilic, pi-acceptor nature of phosphenium cations contrasts the donor properties of NHCs and may, therefore, lead to different catalytic activities and applications. Although previous studies of transition metal complexes of phosphenium cations have been plagued by the susceptibility of the cationic phosphorus atom to nucleophilic attack, the incorporation of a phosphenium cation unit into a rigid pincer ligand framework should impart stability and allow for more careful study. Cationic PPNHPP+ ligands will be synthesized and a variety of transition metal complexes will be prepared and compared with analogous NHC pincer ligands. With more information about the fundamental donor properties of these new ligands, the ability of these new ligands to support different types of catalytic transformations will be explored. One particularly interesting goal of this project is to examine whether the electrophilic nature of these ligands can lead to new applications in ligand-assisted heterolytic sigma bond activation processes such as the activation of H2.
Redox-Active Ligands Featuring N-Heterocyclic Carbenes. top
Redox-active ligands have recently emerged as a strategy for facilitating multi-electron redox transformations without affecting transition metal oxidation state. Ongoing research in this area entails the synthesis, characterization and investigation of the coordination chemistry of a new class of redox-active pincer ligands featuring a central N-heterocylic carbene and two arylthiolate donors. As a result of the delocalization of electrons throughout their aromatic pi-systems, preliminary computational studies suggest that these [NHC-S2]2- ligands will be capable of undergoing one- and two-electron uptake upon coordination to transition metal centers. A synthetic route to several derivatives of these ligands has been designed and their coordination to both relatively redox-inert (ZnII, PtII, etc.) and redox-active (CoII, FeII, NiII, etc.) transition metals is being explored. To assess the electronic structure of these complexes and the non-innocent behavior of the ligand, this project involves the utilization of a range of structural, spectroscopic, and computational techniques including X-ray crystallography, EPR spectroscopy, UV-visible absorption spectroscopy, and calculations using density functional theory (DFT). In addition, the reactivity of complexes featuring these new redox active ligands towards multi-electron transformations (e.g. oxidative addition, reductive elimination, oxidative group transfer, and small molecule activation) will be investigated in anticipation of uncovering new accessible reaction pathways using a combination of metal and ligand redox activity.