Comparison of the flexibility of eight-membered tetrasiloxane and stannasiloxane rings: a crystallographic and computational study

The syntheses and crystal structures of the eight-membered <i>cyclo</i>-stannasiloxanes <i>cyclo</i>-[<i>t</i>-Bu(OH)Si(OSnt-Bu₂O)₂Si(OH)<i>t</i>-Bu] (<b>1</b>) and <i>cyclo</i>-{<i>t</i>-Bu₂Si[OSn(CH₂SiMe₃)₂O]₂Si<i>t</i>-Bu₂} (<b>2</b>) as well as the synthesis of the six-membered <i>cyclo</i>-stannasiloxane <i>cyclo</i>-{<i>t</i>-Bu₂Si[OSn(CH₂SiMe₃)₂]₂O} (<b>3</b>) are reported. Compound <b>1</b> crystallizes as its <i>trans</i> isomer, but the <i>cis</i> isomer dominates in solution. In agreement with the experimentally obtained results, ab initio and DFT calculations on the model compounds <i>cyclo</i>-(H₂SiO)₄ (<b>4</b>), <i>cyclo</i>-[H₂Si(OSnH₂)OSiH₂] (<b>5</b>), <i>cyclo</i>-O(H₂SiOSnH₂)₂O (<b>6</b>), and <i>cyclo</i>-[H₂Si(OSiH₂)OSnH₂] (<b>7</b>) indicate that the energetic preference to adopt puckered structures increases and the ring flexibility decreases with an increasing number of tin atoms in the ring. The rich diversity of puckered conformations is attributed to the steric demand of the different organic substituents. <br />