Heteronuclear Triple Bonds to Silicon

The unusual structural and electronic flexibility of multiple bonded main group units has attracted great interest in the recent past. An increasing number of inorganic pi-bonds is incorporated as chromophore or dopant into extended pi-systems aiming at electronic devices or sensors. Especially the element silicon might play a key role in combining features useful for classical solid state semi-conductors as well as for molecular electronics. The objective of the underlying project is to investigate the possibility to prepare a novel class of compounds featuring triple bonds involving the element silicon. Unlike the very recently discovered homonuclear -Si=Si- units in disilynes, we are aiming at heteronuclear triple bonds of the type R-Si=E (E: =P, =N, =C-R`; R, R`: organic substituent). According to quantum chemical calculations, the polar nature of such triple bonds is compatible with the pi-donating properties of aryl based bulky substituents such as terphenyl or Tbt which in turn are unsuitable for unpolar -Si=Si- units. While the primary goal is to develop a synthetic access to molecules containing heteronuclear R-Si=E units, other important aspects include: a) the investigation and improvement of the thermal stability of such unsaturated units with respect to ring or clusterlike oligomers, depending on the nature of the protecting substituent(s); b) investigation of the bond situation in this class of compounds based on experimental (spectroscopic and structural data) and quantum chemical results; c) exploring the general chemical reactivity of such functionalities, e.g. their behavior in complexation, addition and cycloaddition reactions. The preparation of multiple bonded silicon compounds is synthetically challenging and often results in materials with unique properties. Therefore, the development of methods to prepare such a novel class of compounds is of fundamental interest, and may ultimately lead to materials with novel desirable properties.

During the project the possibility to access chemical compounds with a high bond order between the elements silicon and carbon was explored. Owing to their unique bonding situation such compounds provide unusual electronic properties which are interesting for molecular electronics. In the course of the project we discovered that the envisaged compounds are intrinsically unstable against rearrangement into a compound with lower energy content having the same chemical composition but different connectivity of the atoms involved. Several examples of the low energy variety (so called silaallenes) could be prepared and for the first time silaallenes with substituents other than hydrocarbon could be obtained which allow the chemical connection into larger entities. The electronic properties have been explored experimentally and with quantum chemical methods. As by-products of the project also phosphorus compounds with unusual photophysical properties have been obtained and moreover derivatives of the investigated silicon compounds have been identified and were explored regarding their properties and suitability for material related purposes.