The Chemistry of the Platinum Group metals
The Chemistry of the Platinum Group metals
A REPORT of the EIGHTH INTERNATIONAL CONFERENCE
The themes of the Eighth International Conference of the Chemistry of the Platinum Group metals (PGM8), held at the University of Southampton, from 7th to 12th July 2002, covered a broad spectrum from the chemistry of these fascinating elements, ranging through:
Coordination and supramolecular chemistry
Biological and medicinal chemistry
Surfaces, materials and crystal engineering
Photochemistry and electrochemistry
Catalysis and organic syntheses, to
Theoretical chemistry and physical methods.
The attendees also found time to cruise down Southampton Water (in mist and rain), visit Stonehenge and Sallsbury (only a little rain), walk through in the New Forest (totally dryl), and dine under King Arthur's Round Table in the Great Hall in Winchester.
But the open and challenging atmosphere was the most apparent hallmark of PGM8. Scientists with a breadth of approaches shared their differing experience and targets around common chemical foci, and these can be exemplified by an overview of the reports of the invited speakers.
The well-established antitumour activity of cis-pktin and carboplatin, and the onset of tumour resistance to them, was discussed by Lloyd Kelland (St. George's Hospital Medical School, London, U.K.); and here there still remain many important targets. Phase I trials of a rutheruum(III) complex were reported by Gianni Sava (University of Trieste, Italy), and these show promise for a selective effect on lung metastases. Indeed, ruthenium complexes occupied a significantly important position in the biological and medicinal chemistry theme, with Jackie Barton (Caltech, U.S.A.) using them to monitor electron transfer ranges and identify the effect of oxidative damage on the conductivity of DNA Peter Sadler (University of Edinburgh, U.K.) described how his work on coordination spheres interacted with DNA bases is being extended to organometallic centres.
Control and exploitation of coordination spheres was preeminent in the programme. Many examples were elegant, such as the helicate complexes of open chain tetra- and hexa-dentate phosphines (Bruce Wild, Australian National University, Canberra) and osmabenzenes and fused osma-aromatics (Warren Roper, University of Auckknd, New Zealand), While others challenged conventional thinking, such as the careful design of complexes with monodentate phosphines acting as bridging ligands (Helmut Werner, University of Würzburg, Germany). Probably the ‘simplest’ ligand sets were presented by Gary Schrobilgen (McMaster University, Canada) who compared the high oxidation states of osmium and xenon (these elements have the widest +8 oxidation state chemistry). The simplicity of the formulae belied the technical challenges of unrav-elling this chemical frontier.
For the most part, ligand sets were chosen to engender attractive Physicochemical properties. These included the luminescent properties of ter-pyridyl complexes of iridium and ruthenium (Gareth Williams, University of Durham, U.K.), and the non-linear optical materials based upon dendrimeric oligomers of ruthenium(II) bipyridyls (Hubert Le Bozec, Université de Rennes 1, France). Dendrimers and other polymeric architectures (rings, chains and helices) have been synthesised with impressive control by Shigetoshi Takahashi (Osaka University, Japan), and Ian Manners (University of Toronto, Canada) described his control over the synthesis of different types of ferrocenyl polymers.
Catalysis was one of the recurring reasons for ligand design, with Duncan Bruce (University of Exeter, U.K.) demonstrating that metallorganic liquid crystals could act as templates for the synthesis of heterogeneous metal catalysts based on mesoporous silicas. Jan Bäckvall (University of Stockholm, Sweden) demonstrated the use of allenes as nucleophiles in Palladium-catalysed coupling reactions, and the emphasis of Palladium mediated C–C coupling reactions was continued by Hans de Vries (DSM Research, Geleen, The Netherlands) who presented thoughtful developments of Heck reactions. An alternative approach to C–C coupling, namely hydroformlyation, was also stressed, with Kyoko Nozaki (University of Tokyo, Japan) describing very effective asymmetric hydroformylation catalyst systems, and Eric Hope (University of Leicester, U.K.) showing how fluo-roorganic groups can be exploited in green chemistry: to enhance the solubility of rhodium and ruthenium complexes in supercritical CO2, and also utilising fluorous phases themselves as supercritical solvents.
More detailed fundamental studies relating to homogeneous catalytic processes were a feature of the programme. The elegant and penetrating studies of Bob Bergman (University of California, U.S.A.) provided great insight into C-H bond activation processes, and Zhenyang Lin (Hong Kong University of Science and Technology) showed how theotetical studies can add to the insight in an incisive way. Sylviane Sabo-Etienne (Laboratoire de Chimie de Coordination, Toulouse, France) described the activation of boranes and silanes, demonstrating the main group elements to hydrogen bonds as η2-ligands. Richard Eisenberg (University of Rochester, U.S.A.) reported the power of parahydrogen-induced polarisation to track through the mechanistic pathways of H2 through a catalytic cycle, while Jon Iggo (University of Liverpool, U.K.) reported on impressive technical developments with a flow Cell to allow in situ NMR under high pressures, without the attendant problems of slow gas dissolution due to poor mixing. This direct observation of homogeneous catalysis, such as hydroformylation and carbonylation, can be achieved under representative conditions.
Although many of the complexes and materials describes above were oligomeric and polymeric, few had direct metal—metal interactions. However, these were evident in the heterogeneous catalysts described by Stan Golunski (Johnson Matthey, U.K.). He emphasised the ability of metals, espe-cially Palladium, to mediate the transfer of oxide ions from an oxide surface to a catalysis substrate.
Two talks, though, demonstrated differing but fascinating properties of nanoscopic metal structures; Phil Bardett (University of Southampton, U.K.) described how liquid crystals and Solid microspheres (of polystyrene and silica) could be used as templates for the chemical and electrochemical formation on mesoporous metals. These materials provide large surface areas, like those of the nanoparticles in heterogeneous catalysts, but the area within is a concave, rather than a convex, surface and generates different types of metal surface sites. It might also be expected that the large arrays that comprise mesoporous metals may be less prone to sintering than the Clusters within a high dispersion metal catalyst. So novel chemical applications of these materials in catalysis, electro-catalysis and sensors can be anticipated.
The other approach was that of Günter Schmid (University of Essen, Germany). His ligand-sta-biUsed Clusters lie at the boundary of molecular complexes and colloids. The Au55 type of duster with PPh3 and the predominant protecting ligand was reported to form 2-dimensional monolayers at a water-CH2Cl2 boundary, and l-dimensional structures with different templates. A supramolec-ular chemistry was established between these high nuclearity cluster materials. The electrical conductivity across a single cluster molecule was also measured. In the junction to the nanoelectrodes, the ligand sheath acted as an insulating layer. The metal core itself behaved as a coulomb well with properties attributable to quantum size effects, rather than being merely a segment of an extended metal array.
It is unfair to the excellent contributed papers and to the poster presenters that I have concentrated on the contributions of the invited speakers. In many ways they accentuated the perception that Platinum metals chemistry is a mature, but still youthful science, with new vistas opening. Indeed, that view was expressed by Helmut Werner in his thanks to Günter Schmid. The boundaries of Platinum metals chemistry are still there to be probed in a fundamental way, aided by the great array of structural, spectroscopic, imaging and analytical techniques now available to us.
Indeed, the platinum group metals themselves are now part of the array of analytical techniques, used for example in understanding the effects of damage within DNA, and the capability for effective functioning is continually being extended. Ligand design and synthesis are developing apace, and can be used to construct clefts at single metal atoms, helicate grooves in oligomers, and complex surfaces in dendrimers and polymers. As yet we do not understand these new structures well enough to predict the applications in molecular electronics, optoelectronics and catalysis. However, we can see extended arrays of metal nanostructures that have a totally untapped potential. Perhaps even less do we understand how such complexes interact with living tissue. That they can do so to therapeutic benefit is a major impetus to research. The conference demonstrated that the range of complexes and materials that could be tested comprise a vast array of types. And, as always, development of the underlying theory of all of these interactions is essential to orient further synthetic developments.
We are grateful for the organisation provided by the Royal Society of Chemistry, and also to our sponsors: Johnson Matthey, Synetix, BP Chemicals and Nycomed Amersham. On behalf of the National and Local Organising Committees, we would like to thanks all of the attendees for a memorable scientific meeting.
John Evans is a Professor of Chemistry at the University of Southampton. His main interests are in surface organometallic chemistry and heterogeneous catalysis, mechanisms of homogeneous catalysis reactions and X-ray absorption spectroscopy. Professor Evans was awarded the Royal Society of Chemistry Tilden Medal in 1994.