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Fig. 1 The [Au2] dumbbell in the structure of Au2(SO4)2. The metal-metal bonded gold atoms have a distance of only 249 pm and are surrounded by two chelating and two monodentate sulfate groups

Noble metals, i.e. the elements silver, gold, palladium, platinum, ruthenium, and osmium have outstanding properties making them useful for various applications, for example in catalysis and electronics. Nevertheless the chemistry of these elements is still not fully developed. This is especially true for oxoanionic noble metal compounds that have only rarely been investigated. We have been working on the development of suitable reactions for the preparation of these compounds for a couple of years. It turned out that mineralic acids and their anhydrides are good candidates for both reaction medium and reactant. Astonishingly even the elemental metals can be oxidised in some cases, at least if the reactions are carried out under harsh conditions. The reactions were carried out in sealed thick-walled glass ampoules at elevated temperatures, and have led to a large number of unique compounds with fascinating structural motifs. Very illustrative examples are metal-metal bonded systems like the unprecedented gold sulfate Au₂(SO₄)₂ (Fig. 1). It is one of the very few truly divalent gold compounds and exhibits a [Au₂] dumbbell with a very short bond (249 pm). Metal-metal bonds are also frequently observed in platinum sulfates. In this case the platinum atoms in the [Pt₂] dumbbell are trivalent and the dumbbell is surrounded by four chelating sulfate ions leading to a so-called ‘paddlewheel’ structure. Even if the majority of platinum sulfates display this motif, also the oxidation of the metal into its tetravalent state can be achieved if the reaction is carried out with pure SO₃. In this case the [Pt(S₂O₇)₃]^2- complex forms showing the metal atom co-ordinated by three chelating disulfate groups.

Besides sulfates, our group became interested in nitrates of noble metals because these could be of great interest as precursor materials. Nitrates usually have quite low decomposition temperatures and thus the compounds should be usable for the deposition of structured noble metals. Beautiful examples are the complex gold nitrates (NO)[Au(NO₃)₄] and (NO₂)[Au(NO₃)₄] which can be gained from elemental gold and pure N₂O₅. These nitrates can be decomposed either by thermal treatment or with the help of an electron beam. The latter process is called EBID (electron beam induced decomposition) and plays an important role in the structuring of noble metals. The use of the nitrates as precursors is advantageous because their decomposition leads to very clean deposits. All of the decomposition products are volatile, and problematic contaminants like carbon or chlorine are not part of the precursor. This is the main difference to the precursor materials known so far which often lead to a significant contamination of the deposits.

In order to enlarge our precursor library our current research is strongly devoted to the preparation of other noble metal nitrates. Even mixed metal nitrates are in our focus because they might allow for the deposition of noble metal alloys. The investigation of deposition processes is a strongly interdisciplinary task, and will be conducted in the course of several co-operations. For that purpose there is a common activity within the COST programme of the European Science Foundation. This project is named CELINA (“Chemistry for ELectron-Induced Nanofabrication”).

Professor Mathias S Wickleder
Inorganic Functional Materials
Institute of Chemistry
University of Oldenburg
tel: +49 441 798 3660
[email protected]
Wickleder group: http://www.uni-oldenburg.de/ac-wickleder/
CELINA project: http://celina.uni-bremen.de