Platinum Group Metals

Materials chemistry relies heavily on the synthesis of new molecular building blocks, mostly organic or organometallic compounds equipped with particular functions. A vast array of such substances was reported during the conference, a large fraction of which required the use of platinum, palladium or rhodium catalysts for the key assemblage steps. Most notably, homogeneous Pd catalysts underpinned the construction of new molecular architectures based on Heck or Suzuki reactions, Negishi or Sonogashira coupling, Stille reactions or Buchwald-Hartwig amination. Cyclisation reactions, catalysed by Pt and utilising new alkyne derivatives, provided easy access to routes to a plethora of novel structures possessing advanced properties. In addition, ruthenium and iridium complexes were used extensively to generate triplet emitters and/or photocatalysts. In sessions covering progress in artificial photosynthesis, Pt- and Ru-based reagents proved to be the most effective catalysts for H₂ and O₂ liberation from water. A common theme running throughout the conference was that, while the properties and functions of the new materials were described in detail, the essential chemistry leading to their synthesis remained in the background. Thus, the platinum group metals (pgms) were less prominent than would be have been the case if the main focus had concerned synthesis.

Even so, the pgms made numerous key contributions to the conference. Among the many excellent presentations, we draw attention to the newly developed Pt- and Ir-containing multimetallic triplet emitters reported by Gareth Williams et al. (Durham University, UK). These compounds are based on cyclometallated Ir(III) and Pt(II) complexes and important differences in the recorded photophysical properties are noted between mononuclear and multinuclear species (Figure 1). Such materials have applications in organic light-emitting diodes (OLED) and bioimaging. Somewhat related multiple chromophoric assemblies were reported by Anthony Harriman and Raymond Ziessel (Newcastle University, UK), with the main emphasis being placed on the development of artificial light-harvesting arrays (Figure 2). The use of Ru(II) complexes as photosensitisers for the visible light induced oxidation of water was reported by Dominic Walsh et al. (University of Bath, UK). In this latter study, the oxygen-evolving catalysts were formed from manganese dioxide (MnO₂) and cobalt oxide (CoO_x ). High rates of water oxidation were described by Richard Walton (University of Warwick, UK) using novel forms of ruthenium(IV) oxide (RuO₂) and iridium(IV) oxide (IrO₂). These catalysts were employed in the form of mixed-metal pyrochlores and have possible applications in water electrolysers. Photochemical water oxidation was reported by Borbala Kiss et al. (University of Liverpool, UK) The active substrate was formed by Rh substitution into the yttrium titanate (Y₂Ti₂O₇) pyrochlore and it was noted that oxygen evolution rivalled that observed from tungsten trioxide (WO₃) but with much improved light harvesting capability. Despite the enormous effort expended on the search for new oxygen-evolving catalysts, it is clear that the optimum performance is derived from RuO₂ and IrO₂ based materials.

Interesting results were reported by Richard Baker et al. (University of St Andrews, UK) regarding the preparation of Pd-based catalysts for methanol reforming. Here, the support was a high surface area, nanostructured ceria while the adsorbate was Pd or PdO. In certain cases, samarium was also doped into the ceria support. These materials appear to exhibit impressive performance for both methane combustion and alcohol reforming under relatively mild conditions. The synthesis of Pt-functionalised tin dioxide (SnO₂) sheets for propan-2-ol gas detection was described by Igor Djerdj (Rudjer Boskovic Institute, Croatia).

Soft Materials

Photoresponsive soft matter materials were described by Yann Molard et al. (University of Rennes 1, France) and by Etienne Borré et al. (University of Strasbourg, France). Such materials, especially when engineered in the form of organogels, could have important applications in luminescence-based technologies. Incorporating the pgms into these soft materials is not a straightforward procedure but could lead to novel matrices with useful optoelectronic applications. Likewise, Pt(II) and Pd(II) ions can be used to assemble metallo-supramolecular structures with interesting optical properties. Used in the form of a gel, these compounds give rise to stimuli-responsive systems with sensing properties. In fact, this is a particularly active research field, fuelled by the recognition that the key features of organogels have not been adequately exploited to date. The presence of certain pgms switches on the possibility for luminescence sensing, such as the in situ detection of molecular oxygen. Extension of Pt(II)-based ensembles into polymeric matrices was considered by Kevin Chan et al. (The University of Hong Kong, China), with particular reference to electronic energy transfer processes. The photophysical properties of such Pt(II) polyynes can be modulated by incorporating an azobenzene spacer into the conjugated ligand, as illustrated by Raya Al-Balushi and Muhammad Khan (Sultan Qaboos University, Oman). When substituted with fullerenes, these materials could provide improved performance of organic photocells. Replacing Pt(II) with Ir(III) leads to marked changes in the luminescence properties, as described by Ruth Daniels et al. (Northumbria University, UK), while mixed Pt(II)-Ir(III) complexes favour intramolecular triplet energy transfer.

Photochemical Dissociation of Water

The possible generation of a useful fuel via the photochemical dissociation of water continues to attract attention. One of the tried-and-tested methodologies for H₂ generation involves the ultraviolet (UV)-photolysis of titanium dioxide (TiO₂) coated with deposits of Pt. This system has been around for more than four decades but new results were presented by Greta Haselmann et al. (University of Münster, Germany). Here, the Pt deposit plays the key role of catalyst for proton reduction (Figure 3). Related work was presented by Michael Nolan, Alexander Cowan and Dave Adams (University of Liverpool) and concerned the replacement of the TiO₂ semiconductor with photoconductive perylene bisimide gelators.

Conclusion

Overall, the meeting was a huge success. The venue was excellent, access to lecture rooms was easy, and the large number of delegates catered for in a relaxed but competent manner. Poster sessions were well organised and disciplined. The plenary lectures were outstanding.