Covalency and Ionicity Do Not Oppose Each Other—Relationship Between Si−O Bond Character and Basicity of Siloxanes

Abstract

Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R Si-(O-SiR ) -O-SiR ] that both covalency and ionicity of the Si-O bonds impact on the basicity of the Si-O-Si linkage. The relationship between the siloxane basicity and the Si-O bond character has been under debate since previous studies have presented conflicting explanations. It has been shown with natural bond orbital methods that increased hyperconjugative interactions of LP(O)→σ*(Si-R) type, that is, increased orbital overlap and hence covalency, are responsible for the low siloxane basicity at large Si-O-Si angles. On the other hand, increased ionicity towards larger Si-O-Si angles has been revealed with real-space bonding indicators. To resolve this ostensible contradiction, we perform a complementary bonding analysis, which combines orbital-space, real-space, and bond-index considerations. We analyze the isolated disiloxane molecule H SiOSiH with varying Si-O-Si angles, and n-membered cyclic siloxane systems Si H O(CH ) . All methods from quite different realms show that both covalent and ionic interactions increase simultaneously towards larger Si-O-Si angles. In addition, we present highly accurate absolute hydrogen-bond interaction energies of the investigated siloxane molecules with water and silanol as donors. It is found that intermolecular hydrogen bonding is significant at small Si-O-Si angles and weakens as the Si-O-Si angle increases until no stable hydrogen-bond complexes are obtained beyond φ =168°, angles typically displayed by minerals or polymers. The maximum hydrogen-bond interaction energy, which is obtained at an angle of 105°, is 11.05 kJ mol for the siloxane-water complex and 18.40 kJ mol for the siloxane-silanol complex.

Revisiting a Historical Concept by Using Quantum Crystallography: Are Phosphate, Sulfate and Perchlorate Anions Hypervalent?

“…However, there are many definitions of an atom (such as Bader's formalism or methods based on natural atomic orbitals) and, thus, an atom may be either hypervalent or non‐hypervalent depending on the method used to define the atom. Therefore, it is crucial to perform a complementary bonding analysis that combines different bonding descriptors from real space and orbital space, so that no aspect of bonding is neglected or missed …”

Preise des Royal Australian Chemical Institute 2018

“…Karton beschäftigt sich mit quantenchemischen Rechnungen für die exakte Ermittlung chemischer Eigenschaften und für Fragen von Struktur, Mechanismus und Design. Er ist Koautor eines Beitrags in Chemistry—A European Journal über kovalente Bindung und Ionenbindung in Siloxansystemen …”

Alkaline Earth Metal Template (Cross‐)Coupling Reactions with Hybrid Disila‐Crown Ether Analogues

“…[16][17][18] Both, covalency and ionicity gain simultaneously in importance when the Si-O-Sia ngle increases and basicity lowers. [19][20][21] Given that the understanding of the SiÀOb ond was established in this case, research in the field of coordination chemistry with cyclosiloxanes andr elatedl igand systems showed clearlyt hat these effects have to be taken into account. Nonetheless, the coordination chemistry turned out to be very suitable for early Group 1a nd 2m etal ions, especially within disila-ligands, which providem ore suitable bite angles than cyclosiloxanes of D n type (D = Me 2 SiO, n = 6, 7).…”

“…Concerning the ionic model, the highly polarized Si−O bond features spatially diffuse electron pairs around the O‐atom and repulsive interaction in between Si δ+ and metal n + disturbs silyl ether bonding . Both, covalency and ionicity gain simultaneously in importance when the Si‐O‐Si angle increases and basicity lowers . Given that the understanding of the Si−O bond was established in this case, research in the field of coordination chemistry with cyclosiloxanes and related ligand systems showed clearly that these effects have to be taken into account.…”