One underlying theme of research in my group is catalysis. Catalysis involves getting molecules to react to form products with less energy input; thus catalysis is energy efficient but it is often also atom economical. A second theme of research in my group involves the activation of small molecules, since molecules like oxygen, hydrogen, and carbon dioxide are readily available but difficult to use for chemical reactions. In attempting to do chemistry with these molecules and others, we look to metalloenzymes (nature’s catalysts) for inspiration since nature performs chemical reactions under mild conditions and at extremely fast rates. A third theme of my research is designing water-soluble inorganic compounds. The use of water (or alcohol) soluble organic molecules (ligands) that can bind to metals should allow us to create catalysts that do not require the use of organic solvents, thus decreasing how much hazardous waste is generated in chemical reactions. Water solubility also opens up the possibility of new types of reactivity, since in particular hydrolysis reactions should be more efficient in water.
Our approach is to make ligands that are water-soluble and capable of providing hydrogen bonding interactions, thus we hope to mimic hydrophilic enzyme active sites where hydrogen bonds influence the function of the enzymes. The enzymes for which we aim to make structural and functional mimics play important biological roles including the activation oxygen and carbon dioxide and the hydrolysis (break down) of peptides and phosphotriesters (found in pesticides and certain chemical weapons). This bioinorganic project is described further below.
More recently, my group has begun an organometallic research project which is described further below. This project involves designing catalysts that can convert ketones and aldehydes to alcohols; this important chemical transformation is frequently used by the pharmaceutical industry since many drugs contain alcohol groups. Current methods to do this chemical transformation are inefficient. We hope to design more efficient methods that use hydrogen gas for this reaction and ideally use water or alcohols as a solvent (alternatively, an alcohol like isopropanol can be both the solvent and the hydrogen source in a process known as transfer hydrogenation). We are also interested in energy related applications of organometallic chemistry, including CO2 hydrogenation for energy storage, and water oxidation catalyzed by homogeneous organometallic catalysts. We have a provisional patent related to some of these applications.
New projects: We are always coming up with new ideas and new uses for our ligands and complexes. If you are a student interested in our group, stop by and ask what new projects we have recently started.
Our research efforts are all linked by the use of catalysis and water-soluble inorganic complexes for greener chemistry. In general, we are using new chelates capable of providing N, O and C donor atoms to transition metals and we take our inspiration from both natural catalysts (enzymes) and man-made catalysts. I will now describe these research areas in more detail for a technical audience.
Bioinorganic Chemistry. We are interested in making new ligands and complexes to mimic the structure and function of important metalloenzymes. Bioinorganic chemists have gotten very good at modeling the primary coordination sphere of metalloenzymes, but few examples mimic the secondary coordination sphere and thus water-soluble ligands and fast catalysis of hydrolysis reactions have been rare. This gap in the knowledge base is a problem because it prevents a full understanding of the role that the secondary coordinaton sphere plays in metalloenzyme function. We are interested in modeling phosphotriesterase (PTE) since it catalyzes the hydrolysis of both pesticides and nerve gases. Our approach is novel, in that we use triazole based ligands where nitrogen atoms enhance the water solubility through hydrogen bonding, rather than charged groups. Triazole based ligands are hydrophilic and have a propensity for hydrogen bonding interactions that make them well suited for modeling non covalent interactions in metalloenzymes. We are the first group to use sterically demanding tris(triazolyl)borate ligands for transition metal complexes. Furthermore we plan to explore the effect of hydrogen bonding in the vicinity of the metal. It is anticipated that this could stabilize unusual structures. This may lead to novel reactivity since hydrogen bonding, improved water-solubility, and sterically bulky ligands have not yet been combined to form low coordinate complexes with biologically relevant metals. These ligands, complexes, and any resulting catalysts could have a big impact due to their relevance to metalloenzymes and the potential for green chemistry applications. The scope of potential research applications is large and includes modeling various esterases, carbonic anhydrase and oxygenases. Tris(triazolyl)borate complexes are structurally very similar to the corresponding tris(pyrazolyl)borate complexes, but should contain a more electrophilic metal center that may be ideal for Lewis acid catalysis in environmentally friendly solvents. This research is supported by NSF CAREER.
We recently started working on using Ttz complexes to mimic the chemistry of Cu nitrite reductase. Nitrite reduction from nitrite to NO(g) is a key step in the global denitrification process, and has implications for health, water purification, and global warming, as the greenhouse gas N2O is an undesirable byproduct in denitrification.
Some highlights from our bioinorganic papers are shown below:
Kumar, M.; Papish, E. T.; Zeller, M.; Hunter, A. D. “Zinc Complexes of TtzR,Me with O and S Donors Reveal Differences Between Tp and Ttz Ligands: Acid Stability and Binding to H or an Additional Metal (TtzR,Me = tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)” Dalton Trans. 2011, 40, 7517-7533. DOI: 10.1039/C1DT10429B
Kumar, M.; Papish, E. T.; Zeller, M.; Hunter, A. D. “New alkylzinc complexes with bulky tris(triazolyl)borate ligands: Surprising water stability and reactivity” Dalton Trans. 2010, 39, 59-61. DOI: 10.1039/b918328k
(TtztBu,Me)CuCO dot(H2O) from Dalton Trans. 2008 p2923-2925.
And our first paper on bulky Ttz ligands, lays out the background: Inorg. Chem 2007 p 360-362. http://pubs.acs.org/doi/abs/10.1021/ic061828a
Organometallic Chemistry. N-Heterocyclic carbene (NHC) ligands have the potential to replace phosphines in many catalytic processes. They could make important organic transformations such as C-H activation, C-C coupling, and ionic and transfer hydrogenations greener, more robust and more efficient. Very few sterically bulky chelating carbene ligands have been synthesized, and thus low coordination numbers have rarely been achieved with chelates. We plan to use bulky imidazole and water-soluble triazole rings to design novel bidentate and tridentate carbene ligands. These ligands will be complexed with late transition metals and the resulting structures will be determined by NMR, IR, and UV-Vis spectroscopies and single crystal X-ray diffraction. Then we will test for stoichiometric or catalytic hydrogenation of polar double bonds and perform kinetic studies to elucidate the mechanism of new reactions. Our NHC ligands will be unique in their ability to 1) create three metal-carbon bonds with a geometry similar to tris(pyrazolyl)borate complexes, 2) allow the formation of coordinatively unsaturated, highly active metal centers, and 3) allow electronic modification through triazole based carbenes. Additionally, many of our carbene ligands will offer advantages in terms of acid stability and charge neutrality that allows the analogy between phosphines and carbenes to be used in catalyst design. These qualities will allow our ligands and complexes to be used in a broad range of organometallic catalysis applications. This research was supported by the American Chemical Society’s Petroleum Research Fund (ACS-PRF) from 2008-2011.
For the organometallic project, our goal is Ionic Hydrogenation or transfer hydrogenation, and thus far we have used NHC ligands, Ttz ligands, and 6,6'-dihydroxybipyridine to support Ru complexes that do transfer hydrogenation. The last ligand class is described further below.
We recently devised a novel synthetic route to 6,6'-dihydroxybipyridine (dhbp) complexes. The new ligand, dhbp, places hydrogen bonding groups near the metal center, and ruthenium complexes with this ligand allow for “green” transfer hydrogenations run in water as the solvent. This article was a team effort and included postdoctoral, graduate student, and undergraduate co-authors. This work was funded by ACS-PRF and NSF CAREER.
Transfer Hydrogenation in Water via a Ruthenium Catalyst with OH Groups near
the Metal Center on a bipy Scaffold
Ismael Nieto, Michelle S. Livings, John B. Sacci, Lauren E. Reuther, Matthias Zeller and Elizabeth T. Papish
Organometallics, Article ASAP
Publication Date (Web): November 11, 2011
Copyright © 2011, American Chemical Society
Student Learning Outcomes. Over the past eight years, I have had many students (over 20 undergraduates and 4 graduate students) and two postdocs working with me on these projects. Students in my group have gained hands on experience in organic synthesis, inorganic synthesis, handling air sensitive materials, various forms of spectroscopy, and reactivity studies. But more importantly, they learned how to design control experiments and ask the sorts of questions that would allow new chemical information to be discovered. I try to help students learn for themselves how to think critically and solve problems.