Assembly of discrete and oligomeric structures of organotin double-decker silsesquioxanes: inherent stability studies

Abstract

Double-decker silsesquioxane (DDSQ), a type of incompletely condensed silsesquioxane, has been used as a molecular precursor for synthesizing new organotin discrete and oligomeric compounds. The equimolar reaction between DDSQ tetrasilanol (DDSQ-4OH) and Ph₂SnCl₂ in the presence of triethylamine leads to obtaining discrete [Ph₄Sn₂O₄(DDSQ)(THF)₂] (1). The change of sterically bulky aryl Ph₂SnCl₂ precursor to linear alkyl ⁿBu₂SnCl₂ led to the isolation of oligomeric [ⁿBu₄Sn₂O₄(DDSQ)] (2). The structures of compounds 1 and 2 have been demonstrated using single-crystal X-ray diffraction measurements. Indeed, the formation of oligomeric organotin DDSQ compound (2) was determined using GPC and MALDI-TOF mass spectroscopy. In compound 1, the geometry of the tin atom is five-coordinated trigonal bipyramidal by two phenyl groups, two Si–O from DDSQ and one tetrahydrofuran. Compound 2 contains four coordinated two peripheral tin atoms and two five-coordinated central tin atoms, in which, the fifth coordinating oxo groups in the central tin atoms create the bridge between two different DDSQ units that leads to the formation of oligomeric structure. Density functional theory calculations on organotin DDSQs infer that the obtained lattice energy for compound 1 is far higher than for the case of compound 2, which indicates that the crystal of compound 1 is better stabilized in its crystal lattice with stronger close packing via intermolecular interactions between discrete molecules with coordinated THF compared to the crystal of compound 2. The greater stability arises mainly due to the sterically bulkier phenyl groups attached to the tin centers in compound 1, which provide accessibility for accommodating the THF molecule per tin via Sn–THF bonding, while contrarily the smaller n-butyl groups aid the polymerization of the four repeating units of [SnSi₄O₇] or two Sn₂O₄(DDSQ) through μ-oxo groups. Both compounds 1 and 2 were chosen to be promising precursors for the synthesis of ceramic tin silicates. The thermolysis of 2 at 1000 °C afforded the mixture of crystalline SnSiO₄ and SiO₂ but the same mixture was only formed by thermolysis of 1 at relatively higher temperature (1500 °C), which infers that compound 1 is more stable than compound 2 that is in good synergy with theoretical lattice energy.