Structural chemistry of organo-siloxanes: Composition and structure of Si _n (O,C) _m (n = 2–21) clusters with Si-O-Si bridging bonds

Abstract

Geometrical and topological analysis of 884 crystalline structures of organosiloxanes is carried out (TOPOS software package, Cambridge structural database CSD). Fifty-eight topological types of Si _n (O,C) _m cluster-polymers with Si-O-Si bridging bonds were established for the first time. The largest number of crystalline structures contains 2T-chains Si₂(O,C)₇ (571 compounds), twinned 4T-rings Si₈(O,C)₂₀ (89 compounds), monocyclic 4T-rings Si₄(O,C)₁₂ (77 compounds), 3T-rings Si₃(O,C)₉ (50 compounds), and 3T-chains Si₃(O,C)₁₀ (41 compounds). Huge families of clusters-isomers with n = 6 (9 types), 8 (10 types), and10 (7 types) are allocated. Simple, unbranched L _n -chains are formed at n = 3 (41 compounds), 4 (12 compounds), and 5 (2 compounds). Simple monocyclic R _n -rings are formed at n = 3 (50 compounds), 4 (77 compounds), 5 (3 compounds), 6 (12 compounds) and 8 (3 compounds). Polyhedral P _n -clusters are established at n = 6 (9 compounds), 8 (89 compounds), 10 (6 compounds), and 12 (6 compounds). The upper limit of the structure’s complexity of Si _n (O,C) _m clusters by value of polymerization degree n = 21. Polymers with high values of n = 12, 16, 18, 20, and 21 are tubular supraclusters with hierarchic structure. Supraclusters in organosiloxanes C18 H54 O15 Si12, C32 H72 O20 Si16 and C20 H70 O25 Si20 are assembled from three, four, or five equivalent 4T-chains. Supraclusters in C36 H108 O18 Si18 and C42 H126 O21 Si21 are assembled from six and seven equivalent 3T-chains. the maximum axial symmetry of these supraclusters is described by point groups C _nv (n = 3–7).