Research AK Prof. Dr. Andreas Schnepf


Synthesis with outer space molecules:

Direct insight into the nanoscaled borderland between molecules and the solid state of metals and semi-metals.


The majority of elements on the periodic table are metals and their chemistry, especially their formation and dissolution, belong to the oldest technical chemical processes. Thereby, the synthesis of metals plays a central role in the evolution of mankind and thus complete periods are named according to the metals used there for the first time (Copper Age: 4300 – 2200 B.C.; Bronze Age: 2200 – 1000 B.C.; Iron Age: 1000 – 40 B.C.). However, to date awareness of metals beyond bulk metals and their stable compounds (e.g. salts, oxides, sulphides, in solution or in bulk) is limited. Basic knowledge of the intermediates in the formation and breaking of metal-metal bonds are mostly unknown even though this process plays a vital role in the evolution of mankind.

 

Ideal model compounds to shed light on the borderland between molecules and the solid state of metals at a molecular level are metalloid clusters of the general formula MnRm (n > m; M = metal like Al, Au, Sn etc.; R = ligand like S-C6H4-COOH, N(SiMe3)2 etc.). As the size of such metalloid cluster compounds is within the nanometer range, research in this field gains a technological aspect in the field of nanotechnology where metal nanoparticles move more and more into the focus of actual research. Such metal nanoparticles normally exhibit a certain size distribution and are thus a mixture of metalloid clusters of different size. Consequently, metalloid clusters are also ideal model compound to establish structure property relations for metal nanoparticles. However, the synthesis of metalloid clusters is quite complex as they are metastable intermediates on the way to the bulk phase as outlined in scheme 1.

 

Consequently, the dearth of understanding the intermediate states of metals can be attributed to the lack of useful starting materials and accessible synthetic routes to synthesize metalloid cluster compounds localized in this intermediate range. Additionally, when such compounds are identified the high reactivity and their metastable character partly hinder isolation for further investigations.

 

 

Scheme 1: Development of the energy during the synthesis of a metal from oxidized starting materials.

 

One possible synthetic route to metalloid clusters applies the disproportionation reaction of a metastable precursor like monohalides of group 13 and group 14 (see synthetic concept). Such metastable solutions are accessible via a preparative co condensation technique which is a central issue of our research (see cocondensation plant) and which gives access to novel reagents based on molecules that are only stable under drastic reaction conditions. Therefore: „The chemists can now devise experiments taking advantage of starting materials that might be regarded as esoteric or even unattainable from a synthetic point of view; M. Moskovitz, G. A. Ozin in Cryochemistry, 1976, Wiley, New York.”

 

Due to the high intrinsic reactivity of the metastable subhalides we could establish a synthetic route to metalloid cluster compounds of group 14. The properties of these metalloid clusters give a direct insight into the grey area between molecules and the solid state, showing that novel bonding situations and structures are realized in this nanoscaled molecular regime. Additionally, the disproportionation reaction gives access to the element at low temperature, opening the door to further investigations like template based synthesis of elemental germanium and tin.

 

Beside this fundamental insight into the formation of the element on an atomic scale, subsequent reactions of metalloid clusters open the way to novel materials:
Starting from the metalloid germanium cluster {Ge9[Si(SiMe3)3]3}- subsequent reactions with transition metals give access to compounds which can be seen as molecular cables. Photophysical- and gas phase investigations give further insight into the physical and chemical properties of these cluster compounds.

 

 

Another possible route to metalloid clusters is the reduction of a suitable precursor which is best possible for precious metals where the formation of the element is highly favored. Thereby gold is the most used element nowadays and recently we could establish an approach to multishell metalloid gold clusters like Au108S24(PPh3)16 using thiolated silyl ligands as sources of sulfur atoms during the synthesis. Within the metalloid gold cluster Au108S24(PPh3)16 novel structural motives are realized like a Au4S4 ring in the ligand shell. The arrangement of the gold atoms in the Au44 core of naked gold atoms resemble a cut-out of the fcc packing of elemental gold getting more molecular on approaching the outer shell as it frequently observed within metalloid clusters in general.

 

Main research themes: