Research in our group is at the intersection of the traditional disciplines of organic and inorganic chemistry. A central theme that unifies our projects is the discovery of energy-efficient chemical transformations that minimize byproducts, separation of waste and eliminate precious metals. This motivation is one of the advantages of homogenous catalysis and small molecule activation promoted by soluble, well-defined transition metal complexes. With these goals in mind, we study the chemistry of the transition elements from across the periodic table and use state-of-the-art multinuclear NMR experiments, X-ray diffraction techniques, isotopic labeling and computational studies to elucidate the mechanisms and electronic structures of the molecules we prepare. Specific recent projects are described below.
Catalytic Bond-Forming Reactions with Iron Complexes with Reduced Environmental Impact. Transition metal catalyzed-reactions are now an indispensable component in the synthesis of complex molecular targets. Transformations such as olefin hydrogenation, hydrosilation and metathesis as well as numerous palladium couplings are powerful methods due to the ability to control chemo-, regio- and even enantioselectivity. Research in our laboratory is focused on replacing the expensive and toxic precious metals typically used as catalysts with more environmentally compatible iron compounds. One challenge in accomplishing this objective is controlling the one electron redox chemistry often encountered with the first row ions. To this end, we have employed "redox active", terdentate bis(imino)pyridine chelates to stablize catalytically competent iron compounds.
Using this approach, we have synthesized a family of iron compounds that promotes olefin hydrogenation and hydrosilation reactions with activities comparable to traditional precious metal protocols. In chemistry more unique to iron, [2+2] olefin cycloadditions have also been discovered and provide a powerful method for the construction of substituted cyclobutanes.
A key to elucidating how these catalysts operate is in understanding their electronic structure. In conjunction with our synthetic work, we have also conducted Mössbauer spectroscopic, magnetic and computational studies to demonstrate the involvement of the ligand. Thus, compounds that appear to have reduced iron centers actually have reduced bis(imino)pyridine chelates.
Dinitrogen Functionalization with Early Transition Metal Complexes. The synthesis of ammonia, NH3, from its elements, N2 and H2, is a challenge that has confronted chemists for the past century. While the venerable Haber-Bosch reaction is a key driver of modern society (this single reaction supports 40 % of the world's population) the energy inputs required for an economical industrial process renders crop fermentation products such as ethanol too energetically expensive to serve as replacements for fossil fuels. Research in our laboratory is focused on elaborating the typically inert dinitrogen molecule into more value added products using reduced group 4 transition metal compounds. By judicious choice of cyclopentadienyl substituents, side-on bound dinitrogen complexes have been discovered that undergo hydrogenation at ambient temperature and pressure. Kinetic, mechanistic and computational studies support N2 hydrogenation through a 1,2-addition pathway whereby H-H bond scission accompanies N-H and Zr-H bond formation.
More recent work has focused on nitrogen-carbon bond forming reactions. By switching to more reducing hafnium compounds, N2 functionalization has been accomplished by addition of heterocummulenes such as aryl isocyanates and carbon dioxide. This exciting reactivity is most likely a consequence of higher barrier associated with isomerization of the N2 ligand from side-on to end-on. In related studies with titanium, unusual monomeric bis- and mono(dinitrogen) and carbonyl compounds have been synthesized. These molecules were computationally predicted over three decades ago but until recently eluded isolation. Taken together this work highlights the impact of cyclopentadienyl substituents on the outcome of the chemistry.
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Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. “Bis(imino)pyridine iron alkyls containing β-hydrogens: Synthesis, evaluation of kinetic stability, and decomposition pathways involving chelate participation.” J. Am. Chem. Soc. 2008, 130, 11631-11640.
Pun, D.; Lobkovsky, E.; Chirik, P. J. “Indenyl dinitrogen chemistry: N2 coordination to an isolated zirconium sandwich and synthesis of side-on, end-on dinitrogen compounds.” J. Am. Chem. Soc. 2008, 130, 6047-6054.
Knobloch, D. J.; Toomey, H. E.; Chirik, P. J. “Carboxylation of an ansa-zirconocene dinitrogen complex: Regiospecific hydrazine synthesis from N2 and CO2.” J. Am. Chem. Soc. 2008, 130, 4248-4249.
Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. “Iron diazoalkane chemistry: N-N bond hydrogenation and intramolecular C-H activation.” J. Am. Chem. Soc. 2007, 129, 7212-7213.
Bart, S. C.; Chlopek, Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt, K.; Chirik, P. J. “Electronic structure of bis(imino)pyridine iron dichloride, monochloride and neutral ligand complexes: A combined, structural, spectroscopic and computational study.” J. Am. Chem. Soc. 2006, 128, 13901-13912.
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