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

Fugel, Malte; Hesse, Maxie F.; Pal, Rumpa; Beckmann, Jens; Jayatilaka, Dylan; Turner, Michael J.; Karton, Amir; Bultinck, Patrick; Chandler, Graham S.; Grabowsky, Simon (2018). Covalency and Ionicity Do Not Oppose Each Other-Relationship Between Si−O Bond Character and Basicity of Siloxanes. Chemistry - a European journal, 24(57), pp. 15275-15286. Wiley-VCH 10.1002/chem.201802197

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Covalency and ionicity are orthogonal rather than antipodal concepts. We demonstrate for the case of siloxane systems [R3Si−(O−SiR2)n−O−SiR3] 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 H3SiOSiH3 with varying Si‐O‐Si angles, and n‐membered cyclic siloxane systems Si2H4O(CH2)n−3. 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 φSiOSi=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−1 for the siloxane–water complex and 18.40 kJ mol−1 for the siloxane–silanol complex.

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