Steric and Electronic Properties of Phosphinimide-Based SilylenesThe Influence of the Phosphine Moiety

In the present study, we investigated the steric and
electronic
properties of differently substituted phosphinimide-based silylenes.
PMe2Ph, PPh3, and P(tBu)3 were chosen as starting materials and the corresponding siliranes
were synthesized as “masked” silylenes. In order to
compare the silylenes, iron–carbonyl complexes of these of
the type Fe(CO)4L were analyzed regarding their CO vibrations
via IR spectroscopy. Furthermore, kinetic experiments of the activation
reaction of NH3 were conducted with the siliranes and density
functional theory calculations of the HOMO and LUMO energy levels
were performed for the free silylenes. It could be shown that the
steric and electronic properties of phosphinimide-substituted silylenes
can be effectively tuned through the modification of the phosphine
moiety of the ligand. In addition, the trends observed for the silylenes
are in good agreement with the trends shown in the “steric
and electronic map” from C. A. Tolman for the utilized phosphines.
This implies that the properties of further phosphinimide-based silylenes
can be estimated with the help of Tolman’s map which could
serve as a guide for future ligand design.