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 31 P 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 n Ch → p Si interaction.

We present the preparation of tert-butyldiphenylsilanes differing in one functional group. The molecular structures of the phenyl (3), methoxy (4), and amino derivatives (5) were elucidated by single-crystal X-ray diffraction analysis and their crystal packing investigated by Hirshfeld surface analysis along with 2D fingerprint plots. In the all-C derivative 3, the high symmetry dependence of the crystal packing enables a multitude of directional C(methyl)À H•••C(π) interactions between the tert-butyl and phenyl groups. The methoxy derivative 4 is characterized by considerably short H•••H contacts possibly resulting from pre-orienting C(aryl)À H•••O and C(aryl)À H•••C(π) hydrogen bonds. In the amino derivative 5, the nitrogen atom is not involved in intermolecular interactions, instead dispersive H•••H contacts might become more important for the crystal cohesion. These findings once again underline the pronounced lone electron pair density transfer from the nitrogen atom towards the silicon atom.

Hydrogen and halogen bonds are important anisotropic attractive interactions in the molecular crystalline state. 2,6dibromophenol (1) was analyzed by single-crystal X-ray diffraction for the first time. The intermolecular interaction pattern was studied by Hirshfeld surface analysis along with 2D fingerprint diagrams. The characteristic interactions that dominate the crystal packing are electrostatic type-II Br⋅⋅⋅Br interactions, OÀ H⋅⋅⋅O hydrogen bonds, and an offset parallel πstacking arrangement. Compound 1 was compared with 2,6difluoro-(2) and 2-bromo-6-chlorophenol (3) in terms of their interplay between hydrogen and halogen bonding. Whereas the OÀ H⋅⋅⋅O hydrogen bond is more pronounced in the lighter homologues, the halogen⋅⋅⋅halogen interaction becomes a particularly important directional, attractive interaction in the crystal structure of 2,6-dibromophenol.

The controlled design of functional oligosiloxanes is an important topic in current research. A consecutive SiÀ OÀ Si bond cleavage/formation using siloxanes that are substituted with 1,2-diaminobenzene derivatives acting as molecular scissors is presented. The method allows to cut at certain positions of a siloxane scaffold forming a cyclic diaminosilane or -siloxane intermediate and then to introduce new functional siloxy units. The procedure could be extended to a direct one-step cleavage of chlorooligosiloxanes. Both siloxane formation and cleavage proceed with good to excellent yields, high regioselectivity, and great variability of the siloxy units. Control of the selectivity is achieved by the choice of the amino substituent. Insight into the mechanism was provided by low temperature NMR studies and the isolation of a lithiated intermediate.