Siloxane Coordination Revisited: Si−O Bond Character, Reactivity and Magnificent Molecular Shapes

Abstract

Siloxanes have evolved into a multi‐million dollar business due to their manifold of commercial and industrial applications. As siloxanes have high hydrophobicity, low basicity, high flexibility and also high chemical inertness in common, their chemistry differs significantly from that of organic ethers. The discovery of organic crown ethers, for instance, is commonly accepted as the birth of synthetic host‐guest chemistry. Regarding the chemical properties of siloxanes, cyclic siloxanes which formally resemble silicon analogues of crown ethers, have received considerably less interest in terms of their host‐guest chemistry. Hence, only little is known about siloxane coordination chemistry in the chemical community and the number of published works in this field has been very low till lately. In the last few years, the field has significantly advanced and elegant methods were established to enable the Si−O−Si unit for coordination. This review therefore summarizes the recent developments in the field, recapitulates the historical aspects of siloxane coordination chemistry and describes the specific Si−O bond character with regard to different siloxane linkages. Implications on Si−O bond activation are included and the limits of siloxane coordination are redefined.

Stand up for Electrostatics: The Disiloxane Case

“…[27,28] An excellent review has been published recently dealing with some of these issues in more depth. [29] There is some interest, then, also from the experimental point of view, in understanding how to tune the oxygen basicity in SiÀ O groups.…”

“…The basic character of oxygen in siloxane linkages is key to understand the different cation binding capabilities of the silicon counterparts of crown ethers, the so‐called silacrown ethers [27,28] . An excellent review has been published recently dealing with some of these issues in more depth [29] . There is some interest, then, also from the experimental point of view, in understanding how to tune the oxygen basicity in Si−O groups.…”

Influence of Amino Functions on the Coordination Ability of Silyl Ethers and Disiloxanes

“…The low basicity of oxygen atoms in Si–O–Si units has been known for many years but found to be strongly dependent on the Si–O–Si angle . Understanding the interactions of Si–O–Si units with Lewis acids in general and metal centers in particular is of great interest, especially with regard to challenging siloxane bond cleavage reactions and the development of silicon analogues of crown ethers and cryptands . In the past few years, the provision of molecular model systems for studying Si–O–Al and Si–O–Zn , structural motifs has also become a fascinating research area.…”

Offsets between hyperconjugations, p→d donations and Pauli repulsions impact the bonding of E–O–E systems. Case study on elements of Group 14

“…[9][10][11][12][13][14][15] Explaining the features of molecules containing heavier group fourteen elements has thus gained increased interest in the chemical community since more than five decades ago. Special attention has been paid to siloxanes, [16][17][18][19][20][21][22] owing to their ubiquity in nature as constituent parts of silicates, 23 but also for their widespread applications in day-to-day life in fields such as material science, 24,25 catalysis, [25][26][27] cosmetics 28 and so forth. Various theoretical 6,7,[29][30][31][32][33][34][35][36][37][38] and experimental 22,[39][40][41][42][43] studies regarding the nature of the Si-O bond were mainly influenced by the conceptual thoughts of two different bonding schools, those supporting the ionic character of the Si-O-Si linkage 36,37 versus the advocates of the covalent nature of Si-O bonding, 44,45 that often culminated in contrasting opinions.…”

“…Special attention has been paid to siloxanes, [16][17][18][19][20][21][22] owing to their ubiquity in nature as constituent parts of silicates, 23 but also for their widespread applications in day-to-day life in fields such as material science, 24,25 catalysis, [25][26][27] cosmetics 28 and so forth. Various theoretical 6,7,[29][30][31][32][33][34][35][36][37][38] and experimental 22,[39][40][41][42][43] studies regarding the nature of the Si-O bond were mainly influenced by the conceptual thoughts of two different bonding schools, those supporting the ionic character of the Si-O-Si linkage 36,37 versus the advocates of the covalent nature of Si-O bonding, 44,45 that often culminated in contrasting opinions. 37 Yet a recent paper of Grabowsky and co-workers 38 stressed on the basis of a comprehensive bond-index study that these apparently antipodal concepts are rather complementary as both the ionicity and the covalency of Si-O-Si units increase simultaneously with angle widening.…”