We have recently developed new methods for the direct C-H functionalisation of pyridine and its derivatives. Using half-sandwich ruthenium complexes, such as 1 we have demonstrated that it is possible to formally insert an alkyne into the C-H bond of pydridine (Scheme 1).1 The key mechanistic step in this process revolves around the formation of a remarkable class of ligands, pyridylidenes, which are an interesting class of metal carbene complexes. We have been able to isolate complexes containing these ligands (2) and (in collaboration with Dr John Slattery) used density functional theory to show that pyridylidene ligands are indeed key intermediates in the C-H bond activation and C-C bond formation steps.
Furthermore, this study has allowed us to develop a one-pot protocol for this reaction2 which uses a commercially availabe ruthenium precursor and has allowed for facile evaluation of the factor affecting the efficeny of this reaction.
A further area of research is focused around the organometallic chemistry of the transition metal elements3 and in particular, the understanding the chemistry of alkynes within the coordination sphere of metals. The overall aim of this research is to utilise the reactivity of transition metal complexes to perform complex organic transformation in a selective manner. One example of this research is shown in Scheme 2. We have examined the conversion of the rhodium alkyne complex 3 into the resulting alkynyl hydride 4 and ultimately the vinylidene complex 5. Importantly, we were able to perform a kinetic analysis on the data (Figure 2) and demonstrate that 3 and 4 are in equilibrium and the formation of 5 is essentially irreversible.4 Central to this approach is a colloboration with Dr John Slattery in which the synergic relationship between experimental and theoretical studies may be used to gain valuble mechanistic insight.
We have also investigated the role which a coordinate acetate ligand may affect the chemistry of transition metal alkyne and vinylidene ligand. A study of the system based on [Ru(κ2-OAc)2(PPh3)2] demonstrated that the reaction with alkynes proceeds far more rapidly than the corresponding chloride derivatives.5 The princple reason for the acceleration is the coordinated acetate ligand acting to both deprotonate and repronate the coordinated alkyne. This Ligand-Assisted Proton Shuttle (LAPS) process signficnaly lowers the barrier to the formation of the vinylidene complex (Figure 3). The [Ru(κ2-OAc)2(PPh3)2] fragment is also an excellent reporter for the electronic properties of a number of ligands and we have employed the changes in the spectroscopic and metric properties of this complex to evaluate the donor/acceptor properties of a range of ligands.6
In collaboration with Dr Natalie Fey (Univeristy of Bristol) we have also demonstrated7 how it is possible to use computational methods to evaluate the key factors involved in the stabilisation of transition metal vinylidene compelxes when compared to their alkyne tautomers. By calucalting the strucutres and energies of a wide number of different metal complexes, alkynes and different ligand combiations it is possible to extrapolate the key factors needed to stabilise each tautomer.
There has been considerable recent interest in the beneficial effects of CO in biological systems. In particular, at low concentrations, CO has been shown to be a vasorelaxant and it also exhibits anti-inflammatory properties. CO gas does not, however, provide a long-term therapeutic solution to exploiting these beneficial properties. Therefore, in collaboration with Professor Ian Fairlamb we are investigating a range of metal carbonyl compounds designed to liberate CO under biological conditions. These have primarily focused on preparation of new metal carbonyls which contain bio-compatible ligands and on attempting to find the factors which affect CO-release from metal complexes in biological systems.8