Structural and Electronic Effects on Phosphine Chalcogenide Stabilized Silicon Centers in Four-Membered Heterocyclic Cations

Understanding the interplay of structural and electronic
parameters
in the stabilization of Lewis acidic silicon centers is crucial for
stereochemical questions and applications in bond activation and catalytic
transformations. Phosphine chalcogenide functionalized (Ch = O, S,
and Se) hydrosilanes having tert-butyl and 2,4,6-trimethoxyphenyl
(TMP) substituents on the silicon atom were synthesized, and the ring-closing
reactions to afford the heterocyclic four-membered CPChSi cations
were investigated. Synthetic access was only achieved for the sulfur-
and selenium-based cations. A thorough study by means of single-crystal
X-ray structure determination, NMR spectroscopic data, and density
functional theory (DFT) calculations provided insight into important
electronic and structural parameters affecting the stability of the
intramolecularly stabilized cations. Detailed structural considerations
were made on the contributions to the ring strain (angular strain
and steric repulsion). Thermochemical investigations showed that the
substituents on the silicon and phosphorus atoms play an important
role for the stability of the cationic heterocycles. In the absence
of large steric repulsions through bulky substituents (methyl groups
on silicon and tert-butyl groups on phosphorus),
an intrinsic stability sequence of the intramolecular Ch–Si
coordination depending on the chalcogen atom in the direction Se ≤
S < O can be observed. However, the order is reversed (O < S
< Se) in the case of strong repulsions between sterically demanding
substituents (tert-butyl groups on both silicon and
phosphorus atoms). Natural bond orbital (NBO) analysis supported the
explanations for the observed deshielding trends in 31P
NMR spectroscopy and revealed that the O–Si bond is more ionic
in nature compared to the S–Si and Se–Si bonds, with
the latter exhibiting higher covalent character due to a more efficient
charge transfer through a σ-type nCh → pSi interaction.