|
|
Hot Topics in Contemporary Crystallography
by Croatian Association of Crystallographers
****** Šibenik, Croatia, May, 10th to 15th, 2014 |
|
|
|
Croatian Association of Crystallographers ***** Bijenièka c. 54 HR-10000 Zagreb Croatia
|
Low valent silicon – a carbon copy? Dietmar Stalke, Institut für Anorganische Chemie der Universität Göttingen, Tammannstraße 4, 37077 Göttingen, Germany; dstalke@chemie.uni-goettingen.de Silicon, the heavier congener of carbon, frequently shows different chemistry compared to its sister element. It has been realized that silicon prefers a positive charge as it is most frequently bonded to more electronegative atoms, i. e. carbon of substituents. Additionally, compounds with silicon in lower oxidation states are very important, since they can activate small organic molecules which otherwise are not available using transition metals. In recent papers it was shown experimentally and by DFT calculations that the allene type molecule A can also be rationalized along a structure of type B when the central C(0) atom is bonded by the ligand L, where L perform as donor ligands such as phosphoranes or N-heterocyclic carbenes (NHCs). B with the general formula C(L:)2 which possess two lone pairs have been termed carbones whereas carbenes CR2 accommodate only one lone pair. When L is a carbene ligand in C(L:)2, the compound is termed a carbodicarbene. A related situation may be envisaged for the silicon homologue of allene C.[1] Theoretical studies by Frenking et al.[2] suggested that the trisilaallene that was synthesized by Kira et al.[3] should rather be considered as a silylone Si(L:)2 D which explains why the compound has a rather acute Si-Si-Si angle and why the substituents at the terminal silicon atoms are not orthogonal to each other which is typical for an allene structure.
The Si-C bonds in [(L:)2Si], L= cyclic-alkyl-amino-carbene (cAAC),[4] of 1.841 Å are shorter than typical Si-Caryl single bonds (1.879 Å) but slightly longer than Si-C bonds in some recent Si-Csp3 systems (1.8174(14) Å) and much longer than Si=C double bonds (1.702 - 1.775 Å). Similarly they are marginally shorter than the calculated value for the silylone Si(NHC)2 (1.869 Å). This leaves [(L:)2Si] a particularly interesting candidate for charge density investigations. The same is valid for the germylone [(L:)2Ge].[5] [1] S. Ishida, T. Iwamoto, C. Kabuto, M. A. Kira, Nature 2008, 421, 725 – 727. [2] N. Takagi, R. Tonner, G. Frenking, Chem.-Eur. J. 2012, 18, 1772 – 1780. [3] M. Kira, Chem. Comunm. 2010, 46, 2893 – 2903. [4] K. C. Mondal, H. W. Roesky, F. Klinke, M. C. Schwarzer, G. Frenking, B. Niepötter, H. Wolf, R. Herbst-Irmer, D. Stalke Angew. Chem. Int. Ed. 2013, 52, 1801-1805. [5] Y. Li, K. C. Mondal, H. W. Roesky, H. Zhu, P. Stollberg, R. Herbst-Irmer, D. Stalke, D. M. Andrada, J. Am. Chem. Soc. 2013, 135, 12422−12428. |
The workshop is generously supported by:
Ministry of Science, Education and Sports of the Republic of Croatia
International Union of Crystallography
European Crystallographic Association
|
