Platinum Metals Review - Volume 45, Issue 1, 2001

To construct molecular devices it is necessary to use mixed-valence metal complexes which have a large metal-metal separation distance and which exhibit strong coupling between the metals, so that errors which might arise from electrostatic interaction between the metal ions are prevented. Bridges, or spacers, are needed between two metal terminal sites to operate as effective molecular wires when one metal terminal site is in the excited state, and/or when both the terminal components are in the ground state. Binuclear ruthenium complexes, consisting of tris(β-diketonato)ruthenium(III) units, which are suitable as the terminal redox sites, can be used to evaluate how well the bridges function as molecular wires in the ground state. This is because their Ru(III)-Ru(II) and Ru(IV)-Ru(III) mixed-valence states are accessible for experimental use. In this article, a polyyne system and an ethynylated aromatic system are evaluated as molecular wires, using the binuclear (β-diketonato)ruthenium(III) complexes containing these systems as the bridges. In the Ru(IV)-Ru(III) mixed-valence state, the ruthenium complexes show relatively strong electronic interactions between the metal centres. This is interpreted by a superexchange (through-bond) hole transfer mechanism via the highest occupied molecular orbitals of the bridge. Molecular orbital calculations provide a guide to the molecular design of bridging ligands for long-range electronic coupling.

The combination of RuCl2(PPh3)3 and 2,2′,6,6′-tetramethylpiperidine N-oxyl (TEMPO) affords an efficient catalytic system for the aerobic oxidation of a variety of primary and secondary alcohols, giving the corresponding aldehydes and ketones, in > 99 per cent selectivity in all cases. This interesting catalytic system is probably based on a hydridometal mechanism, involving a ‘RuH2(PPh3)3’-species as the active catalyst. TEMPO acts as a hydrogen transfer mediator and is regenerated by oxygen.

The start-up operation in a nitric acid plant is one of the most important stages, from the viewpoint of safety and platinum loss, in the entire ammonia oxidation process. Usually, flame burners using hydrogen or hydrogen-containing gas preheat the platinum alloy gauze catalyst to the operating temperature. However, there are disadvantages and peculiarities in this method of initiating the start-up reaction. These are discussed here and a new electrical heating device and a new technique are described for preheating the platinum alloy gauze catalyst. The device and technique are free of these disadvantages, and together they reduce the explosive hazard and platinum losses during start-up. Results from their commercial utilisation over a 12-year period at various nitric acid plants are described.