Dr Jason Lynam

01904 322534
Email: jason.lynam@york.ac.uk

Transition Metal and Main Group Chemistry

Transition Metal Complexes of Nucleobases

We have considerable interest in the synthesis of transition metal complexes containing nucleobases with a view to utilising the hydrogen bonding properties of these species to direct the self assembly of supramolecular organometallic architectures. In order to achieve this goal we have designed a new strategy for the preparation of nucleobase complexes that leaves the Lewis basic nitrogen and oxygen centres in the nucleobase free from metal coordination so that hydrogen bonding may be maximised. For example, reaction of the uracil substituted alkyne 1 with CpRu(PPh3)2Cl in the presence of NH4PF6 results in the formation of the vinylidene complex 2 (Scheme 1).1 In the solid state 2 self assembles into remarkable hexameric arrays (Figure 1) mediated by hydrogen bonds between the uracil groups. This structure is propagated to the macroscopic level as hexagonal crystals are obtained. Work is currently underway to expand this methodology and to assess how the steric and electronic environments of the metal centres affect the hydrogen bonded structures that exist.

scheme 1
Scheme 1

figure 1
Figure 1

Transition Metal Alkyne Chemistry

A further area of research is focused around the organometallic chemistry of the transition metal elements 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.2

scheme 2
Scheme 2

figure 2
Figure 2

CO-Releasing Molecules (CO-RMs)

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 Dr Ian Fairlamb and Anne Duhme Klair (York) and Dr Roberto Motterlini (Northwick Park Hospital), we have investigated a range of metal carbonyl compounds designed to liberate CO under biological conditions. These have primarily focused on preparation of new metal carbonyls (such as 6) which contain bio-compatible ligands and on attempting to find the factors which affect CO-release from metal complexes in biological systems.3

Main Group Chemistry

We also have considerable interest in the synthesis and study of phosphorus-rich cage compounds. For example, we have shown that the triphosphorus tricyclic cage compound ClP3(CBut)2, 7, may be employed as a versatile synthon for the preparation of the planar anionic ring [P3(CBut)2]–, 8, and also to the cationic cluster [P3C2But 2]+ 9 (Scheme 3). If one considers that the P and CR groups are isolobal and the diagonal (P/C) relationship then these compounds may be thought of a analogues to C5H5 – and C5H5 + respectively.

In more recent studies we have investigated the nature of the degenerate exchange reaction between free and coordinated chloride within the cage 7. By using a combination of NMR spectroscopy and DFT theory we were able to show that this exchange occurs via two competing mechanisms (Figure 3). The first is a “classical” SN2 process proceeding by nucleophilic attack at the chloride-substituted phosphorus atom. The second process occurs via nucleophilic attack at a remote phosphorus atom and, crucial, proceeds via an intermediate (as opposed to a transition state in SN2 chemistry). For these reasons we have described this second process as an AE` mechanism.5

scheme 3
Scheme 3

figure 3
Figure 3

All of these projects involve the synthesis of a range of air-sensitive compounds and characterisation of materials using a range of techniques, but principally multinuclear NMR spectroscopy and X-ray crystallography.

Selected References

  1. Transition Metal Vinylidene Complexes as Supramolecular Building Blocks: Nucleobase-Mediated Self-Assembly of Crystals with Hexagonal Symmetry.
    M J Cowley, J M Lynam and A C Whitwood, Dalton Trans., 2007, 4427.
  2. A Mechanistic Study into the Interconversion of Rhodium Alkyne, Alkynyl-Hydride and Vinylidene Complexes.
    M J Cowley, J M Lynam and J M Slattery, Dalton Trans., 2008, DOI: 10.1039/b806358c.
  3. η1-2-Pyrone Metal Carbonyl Complexes as CO-Releasing Molecules (CO-RMs): A Delicate Balance Between Stability and CO Liberation.
    I J S Fairlamb, J M Lynam, B E Moulton, I E Taylor, A K Duhme-Klair, P Sawle and R Motterlini, Dalton Trans., 2007, 3603.
  4. Selective Preparation of the [C2 tBu2P3]– Ion and Synthesis and the Structure of the Cationic Species nido-[C2 tBu2P3]+ Isoelectronic with [C5Me5]+.
    J M Lynam, M C Copsey, M Green, J C Jeffery, J E McGrady, C A Russell and A C Swain, Angew. Chem., Int. Ed. Engl., 2003, 42, 2778.
  5. A New Reaction Pathway in Organophosphorus Chemistry: Competing SN2- and AE'-Pathways for Nucleophilic Attack at a Phosphorus-Cage Compound.
    C Fish, M Green, J C Jeffery, R J Kilby, J M Lynam, J E McGrady, D A Pantazis, C A Russell, A C Whitwood and C E Willans, Angew. Chem., Int. Ed. Engl., 2006, 45, 3628.
  6. Nucleophilic Substitution Reactions of the Tricyclic Triphosphorus Cage P3(CBut)2: a Novel Route to Polyphosphorus Phosphenium Complexes.
    C E Willans, C Fish, M Green, R J Kilby, J M Lynam, C A Russell and A C Whitwood, Dalton Trans., 2008, 3442.