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.