Silicon such as its lighter conger carbon exhibits four valence electrons and thus usually has four bonding partners. Compounds with a silicon atom connected to less than four bonding partners are called low valent. These silicon compounds usually show a very high reactivity.
However, using the concept of kinetic stabilisation several compounds containing low valent silicon have been reported to date.[1-8] The two most prominent classes of compounds containing low valent silicon are probably the disilenes and the silylenes, in which silicon has the oxidation state +II (Figure 1). Especially the silylenes have gained attention in the recent past because of their strong donor ability, which renders them as potential ligands in various catalytic reactions.[9-12]
(a) (b)
Figure 1: Annual number hits for scifinder® search with keyword: silene (a) and silyene (b).
In contrast to low valent silicon of oxidation state +II, reports on silicon of oxidation state zero are extremely rare. In 2003 Kira et al [13] synthesised a trisilaallene containing a silicon in formal oxidation state zero. In 2008 Robison et al.[14] reported on a disilicon stabilised by two N-heterocyclic carbenes (NHCs). Another adduct of two cyclic alkyl amino carbenes (cAACs) with disilicon was reported recently.[15] This work will focus on the structural analysis on an even more interesting class of silicon(0) compounds, the silylones. This class of divalent silicon compounds showing two non-bonding lone pairs was fist synthesised by Roesky et al.[16] in 2013. To date only one further example has been published (Figure 2).[17]
Figure 2: Lewis diagrams of the silylones by Roesky et al. and Driess et al.
Introduction
The bonding situation in silylones usually is described by two donor-acceptor bonds between ligand molecules and a central silicon atom. In the following an arrow will be used in order to indicate this donor-acepptor interaction. However, in contrast to a noraml dash in the lewis diagramm this arrow does not inclued two electrons, which therfore will be drawn seperatly. However, the applicability of this bonding model, originating from the field of ‘coordination chemistry’, at low valent main group elements is debated vigorously.[18-21]
Therefore, it is essential to investigate the structure of these low valent silicon compounds experimentally. Consequently, this work will investigate the electron density distribution (EDD) of a silylone via experimental charge density study based on high-resolution X-ray data. Doubtlessly the EDD is one of the most information rich observables in natural science, allowing deep insights into a compound’s structure, which is the key for a deeper understanding of the fundamental rules of chemistry.[22-23] As X-rays are mainly scattered by electrons, single crystal X-ray diffraction is a powerful tool to investigate a compound’s structure experimentally. The field of X-ray crystallography has developed tremendously since the first publication about the interference of X-rays with a crystal in 1912 by Friedirch, Knipping and von Laue[24] and the first structures obtained from X-ray crystal-lography by father and son Bragg[25-26]. The development in the field of detection devices, more brilliant X-ray tubes as well as synchrotrons, the easy and cheap access to computa-tional time and improvements of the software enabled X-ray structure determination nowa-days to be completed within hours. By this X-ray structure determination has become a standard analytical method, because it is the easiest way to obtain a three dimensional structure.
However, the information about the bonding drawn from these standard X-ray structure determinations is limited. Moreover, since a direct correlation between bond length and bond strength is not given[27] more precise studies, such as experimental charge density studies, are needed to extract information from the EDD. Yet, these studies are far from being routine and especially an investigation of low valent silicon compounds pushes experimental charge density investigations to their limit.[28] In these investigations the model becomes very complex, because the extremely reactive compounds need bulky sub-stituents in order to be stabilised kinetically. The complexity of the model raises questions about the reliability and validity of the derived model. In routine crystal structure analysis, a rule of thumb is that the data to parameter ratio should be larger than ten to give a reason-able model. However, for charge density studies a simple limit for the data to parameter ratio cannot be given and new methods have to be developed in order to avoid overfitting.[29-31] Another challenge when investigating such complex compounds, such as
Introduction
to extract the subtle bonding features systematic errors, which distort the outcome of the study, should be minimized or at least corrected for. Certainly one of most important neglected systematic errors in experimental charge density studies is thermal diffuse scattering (TDS). This resolution- and temperature-dependent inelastic scattering adds intensity to the Bragg maxima. Thus the modelled EDD is distorted as well, which may lead to false interpretations.
Therefore, the following points are essential in order to perform a reasonable analysis of a silylone’s bonding situation; reduction of or correction for systematic errors,[32] develop-ment of refinedevelop-ment procedures as well as tools for the analysis of the model quality.[29]
Consequently this work will not only concentrate on the topological analysis of the EDD, but also will focus on the correction of TDS induced errors (Section 4) and on the use of cross-validation in charge density refinements (Section 7). In this way a charge density investiga-tion can give a much deeper insight into the silylone’s structure and answer quesinvestiga-tions about the bonding type from an experimental study.[33-45] The results of this investigation will be given in Section 5. However, first a short introduction into the most important principals of the structure investigation using X-ray diffraction data (Section 2) and of the topological analysis of EDDs will be given (Section 3).
Single Crystal X-ray Diffraction