The projects in our group are diverse but often share the same philosophy. In general, the ideal-case research strategy is to: 1) conceive of or discover a new reaction or a ligand environment; 2) demonstrate unusual reactivity, structural, or electronic novelty; 3) apply the findings to develop or discover a new catalytic process. We strive to use the diversity of the pursued goals to our advantage through synergy and exposure of students to a breadth of ideas.
Powering the Planet: A Chemical Bonding Center in the Direct Conversion of Sunlight into Chemical Fuel
Powering the Planet (PtP) is a multi-investigator Center (18 PI’s from 10 institutions) sponsored by the National Science Foundation. Its objective is the development of efficient means of transforming the energy of the sunlight into usable chemical fuels. One of the major thrusts of the Center is the conversion of water into hydrogen and oxygen using electricity produced by sunlight capture. The research can be defined along three directions: light capture and conversion, electrocatalytic reduction of water to hydrogen, and electrocatalytic oxidation of water to oxygen.
Our group is part of “Team Oxygen”. Oxidation of H2O to O2 looks deceptively simple on paper but in truth is a rather complex process. Two O-H bonds have to be broken, a new O-O bond has to be formed, and four electrons have to be removed. Moreover, these transformations have to take place with minimal loss of energy in order to fulfill the goal of storing solar energy in chemical bonds. The chemistry of oxygen-oxygen bond formation is particularly underdeveloped. Consider the level of sophistication that exists in the science of carbon-carbon or carbon-heteroatom bond formation; such knowledge and understanding is completely absent where oxygen-oxygen bond formation is concerned. Our group’s goal is to discover new ways of forming O-O bonds using lessons learned in organotransition metal chemistry. A pair of potential examples are given below. We work in concert and in collaboration with the other PtP groups. PtP conducts monthly videoconference group meetings and annual retreats for the participants.
Other Current Projects
Ligand design. Ligand design is an important part of our research, yet it is merely a tool and not a goal in and of itself. We are interested in tri- and tetradentate ligands that possess a rigid structure and impose certain geometry at the metal center. Besides increasing the stability of transition metal complexes, rigid ligand construction, to borrow synthetic organic terminology, decreases the complexity of what may happen at the metal center. We are developing a series of ligands with systematically varied properties. The variation is not only in the common sense of using substituent effects to control sterics and electronics, but also in modifying trans-influence, charge, π-donor/ π-acid properties in the context of the analogous, enforced geometries. Our approaches also lend themselves to adapting the ligands for recyclability and introduction of chirality.
New catalysts for coupling reactions. We have recently discovered that the (PNP)Rh and (PNP)Ir fragments readily undergo a variety of oxidative addition and reductive elimination reactions while maintaining the integrity of the (PNP)M fragment (e.g., JACS 2005, 127, 16772; JACS 2006, 128, 2808). We are targeting developing new catalytic reactions, in particular for coupling of aryl halides and aryl organosulfonates with carbon- and heteroatom-based nucleophiles. Other processes include, for instance, highly regioselective alkyne dimerization (Chem. Commun. 2006, 197). This project should offer new avenues for catalysis, different from the classical Pd0/PdII chemistry. In addition, elaboration of the pincer ligands should allow recyclability through either supporting the catalysts on solids or enabling fluorous solubility. This work is currently supported by the NSF.
Catalysis with manganese? Much of the classical organometallic chemistry involves the heavier transition elements, those of the 5th and 6th period (“4d” and “5d” metals). Utilization of 3d metals as catalysts offers obvious advantages of lower cost, greater biocompatibility, and lower environmental hazard. Catalysis with Ti, Cr, Fe, Co, Ni, Co, and Zn is either well-known or at least heavily pursued. We note that manganese remains underutilized and would like to develop practical Mn catalysis chemistry. Mn is a challenging metal to work with in part because of the prevalence of the high-spin d5 Mn(II) oxidation state with the reactivity akin to Ca2+ or Zn2+ rather than a typical mid-transition series metal. We are working to “tame” Mn by using designer rigid ligands that would stabilize “organometallically-relevant” electronic states. We target developing Mn catalysts for simpler processes, such as olefin hydrogenation, at first. This is a relatively high-risk project that will eye enantioselective transformations and NIH funding if substantial progress is made.
Hypercoordinate main group compounds. The rigidity of PNP ligands offers interesting possibilities for synthesis of hypercoordinate main group compounds. The key idea is that the rigid backbone of the pincer ligand may help enforce binding of both phosphine arms to the central main group element and create bonding situations that are otherwise unattainable. Some examples of potential and accomplished targets are shown (e.g., Mend. Comm. 2007, 17, 63). This work is currently funded by ACS-PRF.
Early metal-carbon multiple bonds. We have been using the PNP ligands to stabilize unusual examples of early metal-carbon multiple bonds. The enforcement of phosphine binding (even to hard, early metals!) leads to the increase in steric congestion at the metal which in the case of polyalkyl complexes often leads to a-abstraction reactions and formation of metal alkylidenes and alkylidynes. We have so far shown the viability of this approach through preparation of Zr alkylidenes (OM 2004, 23, 4700) and a Ta bis(methylidene) complex (OM 2007, 26, 4866), the first known M(=CH2)2 complex for any metal. The Mindiola group at Indiana U is also pursuing similar goals; we collaborate and divide the spheres of attention, as necessary. In my group, this project has been mainly moved by undergraduates in the recent years. We are also collaborating with the Kiplinger group at LANL on the studies of PNP complexes of lanthanides. Lanthanide alkylidenes, as of yet unknown, are an exciting target!
Lewis acid catalysis. We are interested in developing a family of mid- to late-metal, pincer-supported Lewis acid catalysts for organic reactions. We have recently shown that (PNP)NiOTf is an efficient catalyst for coupling of nitriles and aldehydes (Chem. Commun. 2005, 4450). (P2C=)Ru(H)(OTf) is also an active catalyst. Work is underway to include coupling reactions of aldehydes, ketones, and imines on one hand with nitriles, nitroalkanes, and esters on the other. There is much potential here for developing new enantioselective reactions through application of chiral pincer ligands. This is part of the research currently supported by an NSF grant.
Electrophilic C-F bond activation. Activation of carbon-fluorine bonds is both a formidable fundamental challenge and a problem of relevance to environmental problems. Perfluoroalkanes are extremely potent and long-lived greenhouse gases while chlorofluoroalkanes (freons) are a threat to the ozone layer. Our group has achieved a breakthrough in catalytic C-F activation by using highly electrophilic silylium cation-like catalysts (JACS 2005, 127, 2852). The choice of the anion to supported the highly electrophilic species is critical. We have recently embraced the extremely enduring carborane anions ([HCB11X11]-, X = H or halogen) and that now allows us to activate even the least reactive C-F bonds in perfluoroalkyl moieties with turnover numbers in the thousands at room temperature. The simplest catalytic process is the replacement of F by H (hydrodefluorination or HDF), but we are making progress towards replacement of F by hydrocarbyl groups, as well. This work is supported by a DOE grant.
Hydrogenation of biomass molecules. Hydrogenation of biomass is an attractive way of producing usable fuels from renewable resources and may also be viewed as a part of the hydrogen storage solution. Interactions of biomass molecules (fats, sugars, peptides) with transition metal reagents remain poorly understood at best. Biomolecules may present special challenges, different from small organic analogs, in part having to do with the high density of organic functionalities. We are interested in discovering new homogeneous catalysts for hydrogenation of biomass molecules as well as in investigating the fundamental aspects of the interaction of common biomolecules with reactive transition metal fragments.
We are grateful for the financial support from the agencies highlighted below
