Examples of our research in popular media:
- NJN Interview with Alan
- BBC Radio Interview with Alan
- Video Presentation on Alkane Metathesis
- Renewable and Sustainable Fuels - IGERT video presentation
Catalytic Functionalization of C-H Bonds
The catalytic functionalization of alkanes and other molecules with normally inert C-H bonds is a scientifically challenging problem that presents great opportunities in terms of economics, environmental benefits, and energy self-sufficiency. Catalysts for both the dehydrogenation and the carbonylation of alkanes have been developed in our group; these were among the first efficient organometallic alkane functionalization catalysts.
Pincer catalysts for alkane dehydrogenation
We reported the first efficient solution-phase catalysts for alkane dehydrogenation that require neither the use of photochemical irradiation nor a sacrificial hydrogen acceptor. These “pincer catalysts” have also been found to catalyze reactions as industrially significant as dehydrogenation of n-alkanes, to give the important alpha-olefin products, or dehydrogenation of polymers to allow entry into a diverse manifold of functionalized polymers. Concomitantly, applications in organic synthesis are being investigated.
Alkane Metathesis and other Tandem Systems for Catalytic Hydrocarbon Transformations
Olefins are ubiquitous as intermediates in the petrochemical, commodity chemical, and pharmaceutical industries. Tandem systems that could effect dehydrogenation of alkanes or alkyl groups, followed by a useful secondary reaction of the resulting olefin, offer potentially powerful routes to various products, while avoiding undesirable secondary reactions that can occur in simple alkane dehydrogenation systems. Under the auspices of “CENTC” (see below) and in collaboration with the group of Maurice Brookhart at UNC, we have developed one such system that effects the metathesis of alkanes. A potential application of this system is in the upgrading of Fischer-Tropsch alkane product mixtures to afford greater yields of C9-C19 n-alkanes, ultimately obtained from feedstocks such as coal or biomass. Known as “FT diesel”, this comprises a transportation fuel that burns cleanly and gives ca. 35% greater mileage per ton CO2 emitted than gasoline.
Another tandem reaction we have discovered is alkane aromatization. Remarkably this system converts n-alkanes to aromatics, under relatively mild conditions. This is the first homogeneous system reported for dehydroaromatization, and the first catalytic system of any type that converts higher n-alkanes to aromatics of the same carbon number (e.g. n-dodecane gives C12 n-alkyl aromatics).
C-H bond activation toward reactivity at sites other than the C-H bond
Oxidative addition and its reverse, reductive elimination, comprise perhaps the most important and distinctive class of reactions of transition metal based catalysts and reagents. While this class of reactions is critical for the catalytic transformations of many types of molecules, until now it appeared inaccessible with sp3-C-F and C-O bonds. Such bonds are of great interest in many contexts, ranging from pharmaceuticals to the conversion of biomass to fuels and chemicals.
Our lab has recently discovered the first example of oxidative addition of sp3-C-F bonds, an outgrowth of recent work on the novel addition of sp3-C-O bonds. Perhaps most interestingly, the reactions are found to proceed via an unprecedented pathway, in which the metal atom initially inserts into a carbon-hydrogen (C-H) bond in the molecule. This unusual pathway must also be operative for the reverse reaction, in which C-F or C-O bonds are formed.
In another surprising reaction we have found that C-H addition of an aromatic (tropone) results in nucleophilic activity at a remote site on that molecule.
Oxidative addition of C-H bonds has been assumed to have great potential for the purpose of catalyzing functionalization in which there is an overall cleavage of the C-H bond. The discovery of these reactions highlights the possible applicability of C-H bond addition toward functionalization of various substrates, not necessarily at the site of the C-H bond cleavage.
Hydrocarbylation of Olefins
A new target of our research is the “hydrocarbylation” of olefins. Like dehydrogenation, this reaction has an unlimited number of potential applications ranging from natural gas liquefaction and petrochemical conversion to complex organic syntheses.
The fixation of nitrogen is possibly the single most important reaction practiced by chemists. It supports approximately half of the human population through the production of fertilizer from NH3 from N2 plus H2. Currently, however, it is also responsible for about 2% of global consumption of fossil fuel and the commensurate emission of CO2. The development of a sustainable method of N2 fixation is thus a critical challenge for scientists.
Sustainable energy (solar, wind, geothermal, nuclear) is generally available as electric power and thus electrochemical nitrogen fixation, i.e. electrochemical nitrogen reduction (ENR) is perhaps the most obvious answer to this challenge. The reaction of N2 with protons and electrons (which must be obtained from H2O for to be economically and environmentally practical) to yield NH3 is thermodynamically very feasible, requiring less driving force than simple hydrolysis. The key to such a reaction is development of an active robust catalyst.
Catalysts for ENR have been a dream of chemists for many decades. Two general pathways have been proposed, the “Distal” and “Alternating”. Our alternative approach has focused on the the use of metal complexes that can effect bimetallic N2 cleavage. This leads to metal nitrides and precludes the intermediacy of partially reduced species like NNH2 or HN=NH which tend to be very high in energy.
This project is conducted in collaboration with researchers at University of North Carolina (Alex Miller), Yale (Pat Holland, Robert Crabtree) and American University Beirut (Faraj Hasanayn). This affords students extensive opportunity to interact with fellow graduate students and faculty at these institutions who bring diverse perspectives and expertise.
Computational Organometallic Catalysis
In addition to experimental approaches, ab initio molecular orbital calculations are conducted in collaboration with Prof. Karsten Krogh-Jespersen. This work has yielded new perspectives on the most fundamental aspects of organometallic chemistry such as the nature of the metal-CO bond or the process of C-H addition. We now believe that the power of computational chemistry has reached the point where the modification or even the de novo design of catalysts using MO calculations is entirely feasible; efforts in this direction are currently underway.