X-ray diffraction investigation of siloxanes. I. Effects of organic substituents at the silicon atoms on the structure of acyclic trisiloxane-1,5-diols and on formation of different hydrogen bond systems

The molecular and crystal structure of four acyclic trisiloxane compounds, which differ in substituents at the silicon atoms (Ph-phenyl, mPh-methoxyphenyl, 2mPh-dimethoxyphenyl), was investigated by X-ray diffraction analysis. Due to intermolecular hydrogen bonding between the oxygen atoms of the diol fragments, the crystal structure of 1,1,5,5-tetramethyl-3,3-diphenyl-1,3,5-trisiloxane-1,5-diol (C₁₆H₂₄O₄Si₃) (I) is a double chain architecture with hydrogen-bonded dimeric motifs of C(8)R ₄ ⁴ (12) type in graph set representation. In 1,1,5,5-tetramethyl-3,3-(2-methoxybenzo)-1,3,5-trisiloxane-1,5-diol (C₁₈H₂₈O₆Si₃) (II) and 1,1,5,5-tetramethyl-3,3-(2,6-dimethoxybenzo)-1,3,5-trisiloxane-1,5-diol (C₂₀H₃₂O₈Si₃) (III), a double chain structure with a graph set R ₃ ³ (8)D ₃ ³ (10) is formed. In contrast to I–III, 1,1,3,3,5,5-hexaphenyl-1,3,5-trisiloxane-1,5-diol (C₃₆H₃₂O₄Si₃) (IV) has an intramolecular hydrogen bond S(8). The independent molecules are joined by O-H...O intermolecular hydrogen bonds into centrosymmetrical dimers; the system of hydrogen bonds in general may be described as S(8)R ₄ ⁴ (8).