How to Make the Ionic Si−O Bond More Covalent and the Si−O−Si Linkage a Better Acceptor for Hydrogen Bonding

Abstract

Variation of a bond angle can tune the reactivity of a chemical compound. To exemplify this concept, the nature of the siloxane linkage (Si-O-Si), the most abundant chemical bond in the earth's crust, was examined using theoretical calculations on the molecular model compounds H(3)SiOSiH(3), (H(3)Si)(2)OHOH, and (H(3)Si)(2)OHOSiH(3) and high-resolution synchrotron X-ray diffraction experiments on 5-dimethylhydroxysilyl-1,3-dihydro-1,1,3,3-tetramethyl-2,1,3-benzoxadisilole (1), a molecular compound that gives rise to the formation of very rare intermolecular hydrogen bonds between the silanol groups and the siloxane linkages. For theoretical calculations and experiment, electronic descriptors were derived from a topological analysis of the electron density (ED) distribution and the electron localization function (ELF). The topological analysis of an experimentally obtained ELF is a newly developed methodology. These descriptors reveal that the Si-O bond character and the basicity of the siloxane linkage strongly depend on the Si-O-Si angle. While the ionic bond character is dominant for Si-O bonds, covalent bond contributions become more significant and the basicity increases when the Si-O-Si angle is reduced from linearity to values near the tetrahedral angle. Thus, the existence of the exceptional intermolecular hydrogen bond observed for 1 can be explained by its very small strained Si-O-Si angle that adopts nearly a tetrahedral angle.

Morphological behavior of thin polyhedral oligomeric silsesquioxane films at the molecular scale

“…Moreover, variation of this bond angle can slightly tune the reactivity of POSS molecules with respect to hydrogen bonding between the silanol groups and the siloxane linkages. Covalent bond contributions become more significant and the basicity increases when the SiAOASi angle is reduced from linearity to values near the tetrahedral angle, and the siloxane linkages are able to act as hydrogen bond acceptors [51]. As already mentioned, the different morphological behavior of deposited OG and OC films on hydrophobic and hydrophilic surfaces can be related to the different flexibility of the organic arms.…”

Structural Study of Mismatched Disila-Crown Ether Complexes

“…This approach was later discarded in favor of hyperconjugation interactions between p(O)→σ*(Si-C) [6][7][8]. Alternatively, in an opposed model based on calculations of the electron density function, the Si-O bond was described as essentially ionic due to the high difference in electronegativity between Si and O [9,10]. Careful theoretical studies on the basicity of O(SiH 2 Me 2 ) 2 and OEt 2 revealed that the lower electrostatic attraction in siloxanes results from the repulsion between the positively charged Si atoms and Lewis acids [11].…”

Hybrid Disila-Crown Ethers as Hosts for Ammonium Cations: The O–Si–Si–O Linkage as an Acceptor for Hydrogen Bonding

“…Taking all observations into account, the hydrogen bonding of siloxanes continues to be an uncommon motif and is declared as an unusual phenomenon [21]. However, it is possible to increase the ability of siloxanes to form hydrogen bonds by decreasing the ϕ-angle, which could be shown in several publications and was also supported by experimental data provided by the group of Beckmann [5,17,[21][22][23]. The relatively small pool of experimental data motivated us to extend the coordination chemistry of hybrid disilacrown ethers to ammonium cations.…”