From Phosphinimines to Four‐Membered SiCPN Cations: Insights Into N–Si Bonding and Ring Strain

A systematic study of four-membered cationic SiCPN cycles derived from sterically tuned hydrosilyl-functionalized phosphinimines is presented. Hydride abstraction of the methyl- and iso-propyl-substituted precursors successfully affords the corresponding cyclic cations, while the tert-butyl analogue resists cyclization, yielding a protonated intermediate instead. Multinuclear NMR spectroscopy reveals significant downfield shifts in both 31P and 29Si NMR signals, reflecting enhanced cationic character at silicon and pronounced modulation of hyperconjugative nN → σ*(P–C) interactions. Correlated shifts at phosphorus and silicon highlight efficient electronic communication across the R3P–N–SiMe3 framework, consistent with its isoelectronic analogy to disiloxane linkages. Structural data from single-crystal X-ray diffraction analysis, thermochemical density functional theory investigations, and natural bond orbital analyses show that increasing steric bulk at silicon weakens intramolecular N–Si bonding, in line with systematically reduced ring-opening Gibbs energies. These findings provide a clear picture of the interplay between steric and electronic effects in SiCPN cations and offer design principles for strained donor–acceptor silicon heterocycles.