Alkali metal organocyclotrisiloxanolates [RSi(O)OM]3 with vinyl and alkyl substituents at the silicon center

A series of crystalline sodium and potassium organo substituted cyclotrisiloxanolates (I–IV) has been synthesized from the corresponding organotrialkoxysilanes and alkali bases in aqueous alcohol solvent media. Sodium ethyl- and n-propylcyclotrisiloxanolates (I, II) are obtained in high yields in a wide range of reaction conditions, while for the reaction of methyl- and vinyltrialkoxysilanes the use of potassium hydroxide under strictly controlled conditions for the formation of crystalline products (III, IV) is required. The four differently substituted alkali metal cation based cyclotrisiloxanolates were characterized by single crystal X-ray analyses that proved the six-membered siloxanolate cycles to adopt a distorted sofa or boat conformation, most probably resulting from intermolecular interactions. Regarding the crystal packing the corresponding cations and coordinated water molecules form layers that are coated with organotrisiloxanolate anions and ethanol molecules. This supramolecular organization is best described by hydrophilic layers with hydrophobic coatings regardless the nature of alkali metal cations. Relevant quantum chemical calculations show that the contribution of O–H⋯O bonding for the stabilization of the hydrophilic layers is predominant over that of the coordination K–O and Na–O bonds. The analysis of the bonding situation between the hydrophobic parts of the organotrisiloxanolate anions proves that the sum of energies of the H⋯H and C⋯H interactions might be characteristic for bonding within the interlayer space and this is much less for sodium cyclotrisiloxanolates compared to their potassium cation analogs (6.89/7.32 kcal mol−1 vs. 10.1/12.7 kcal mol−1). From these calculations important insight into the energies of non-covalent interactions, which play a crucial role for the stabilization of the molecular structures of sodium and potassium organocyclotrisiloxanolates, is obtained.