Two Cyclotetrasiloxanes at 143K

The crystal structures of octaphenylcyclotetrasiloxane [C₄₈H₄₀O₄Si₄, (I)] and 2,4,6,8-tetrakis(tetramethylene)cyclotetrasiloxane [a tetraspiro compound: cyclotetrasiloxane-2,4,6,8-tetraspiro-tetrakis(cyclopentane), C₁₆H₃₂O₄Si₄, (II)] were determined at 143 K. (I) is compared with the room-temperature structure and the structure of the monoclinic polymorph, both reported previously. Whereas the siloxane ring of (I) is nearly planar, the siloxane ring of (II) exhibits a chair conformation. The angles between the siloxane ring and the various phenyl substituents of (I) vary considerably and are on average smaller in the triclinic form than in the monoclinic form. Greatly differing Si-O-Si angles, as found in the monoclinic form (153 and 167°), appear only in one of the molecules in the asymmetric unit of (I) [153.4 (1) and 163.7 (1)°]; the Si-O-Si angles of the second molecule differ only by 3.0 (2)°. The Si-O-Si angles of (II) display a difference of 5.2 (4)°. The silacyclopentane rings of (II) exhibit different conformations.

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