Multi N-Heterocyclic Carbene Complexes of Ruthenium for the Coordination and Reactivity of Small Molecules

The utilisation of small molecules such as O2, H2 and N2 in chemical transformations relies on them being able to first coordinate to reactive transition metal complexes. We have recently reported (J. Am. Chem. Soc. 2009, 131, 9618; Chem. Eur. J. 2009, 15, 10912) a new ruthenium complex containing four N-heterocyclic carbene (NHC) ligands that shows selectivity in binding these gases as a function of the NHC substituents. We now wish to build upon these initial observations to see whether this Ru multi-NHC 'scaffold' can be used to incorporate the bound O2, H2 and N2 molecules into catalytic reactions (oxidation, hydrogenation etc) and, on the basis or additional preliminary results, whether we can induce more exotic types of molecules such as white phosphorus (P4) and ammonia boranes (R2HNBH3) to take part in both novel stoichiometric and catalytic transformations.Efforts will be directed at preparing new Ru(NHC)4 fragments that contain abnormally coordinated NHCs, afford reactive heteroatom hydride products and precursors to long sought after naked 16e RuL4 species for intermolecular C-H activation. We will build upon more preliminary results with a Ru(NHC)3 fragment in an effort to bring about catalytic H-X addition to alkenes. It is important to establish whether the rich chemistry of these multi-NHC complexes hinted at by our published work and preliminary data can be extended on from Ru to other transition metals, both in the same group (specifically Fe) and surrounding groups (Mo, W, Re).

The research outlined in this proposal will be of immediate interest to researchers in the coordination of small molecules, activation of inert bonds, catalysis and ligand design. The methodology outlined herein will appeal to the general synthetic community, although it would be premature to talk up the immediate commercial aspects of the work - two aspects of the proposed chemistry, namely the general reactivity associated with N-heterocyclic carbene ligands and C-H activation immediately help to demonstrate the outside impact that the work will have. The value of NHC ligands is probably best exemplified by the work of Grubbs in Ru catalysed alkene metathesis, as recognised by the award of the 2005 Nobel Prize in Chemistry. Moreover, the use of molecules containing unreactive or inert bonds continues to be a magnet for chemists working in both academia and industry. A case in point is the Centre for Enabling New Technologies through Catalysis (based at the University of Washington, USA) which is a conglomeration of university research groups and industry funded by the NSF for $15M over five years to tackle issues relating to the activation of strong chemical bonds.