Here is a summary of our research interests.
 

Our main research focus is on the use of late transition metal complexes bearing N-heterocyclic carbene (NHC) ligands for the activation and functionalization of inert bonds, including C-H and C-F bonds. NHCs are a class of compounds that have received a great deal of interest by transition metal chemists in the last ten years, principally because they can act as replacements for phosphine ligands in catalytic reactions. Our interest lies in investigating the fundamental reactivity of NHCs and developing an understanding of reaction mechanisms. In this way, we hope over time to bring about the rational development of new catalysts.

 

N-heterocyclic carbene complexes of ruthenium

 

      (a) Intramolecular C-C and C-H bond activation

We showed some time ago that thermolysis of ruthenium phosphine dihydride complexes with the bulky N-aryl substituted carbene IMes {= bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene)} resulted in substitution of one or two phosphine ligands. Extended thermolysis of the bis-carbene complex afforded an unprecedented C-C bond activation of an sp2sp3 aryl-CH3 linkage in the IMes ligand. In contrast, the mono-carbene complex reacted by intramolecular C-H bond activation upon addition of alkene.

The reversibility of the C-H activation process upon addition of either H2 or an alcohol at room temperature led to collaborative work with Professor Jon Williams in Bath on the use of C-H activated NHC complexes as catalysts in the ‘borrowing hydrogen methodology’, exemplified by the indirect Witting reaction below shown with a C-H activated IiPr2Me2 based complex.

 

 

      (b)    C-N bond activation and tautomerism

Simple changes to ruthenium precursors bring about dramatic changes to reactivity. Upon reacting IiPr2Me2 with Ru(PPh3)3(CO)HCl, we found that C-H activation was accompanied by C-N activation to afford an NH substituted carbene complex. Ultimately this could be converted to the N-bound tautomer shown in the box below, the first demonstration of such behaviour in an NHC.

 

 

Ring-expanded N-heterocyclic carbene complexes of nickel

 

Very recently, we have started to probe the ability of ring-expanded NHCs (i.e. those based on >5-membered rings) to stabilise low coordination numbers/unusual oxidation states. For this work we have turned to nickel and found that 6-membered NHCs such as 6-Mes will undergo C-H activation with Ni(0) to give Ni(II) products, but afford three-coordinate Ni(I) species upon comproportionation of Ni(0)/Ni(II).