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 cyclo-stannasiloxanes cyclo-[t-Bu(OH)Si(OSnt-BU2O)(2)Si(OH)t-Bu] (1) and cyclo-{t-Bu2Si [OSn(CH2SiMe3)(2)O](2)Sit-Bu-2} (2) as well as the synthesis of the six-membered cyclo-stannasiloxane cyclo-{t-Bu2Si[OSn(CH2-SiMe3)(2)](2)O} (3) are reported. Compound I crystallizes as its trans isomer, but the cis isomer dominates in solution. In agreement with the experimentally obtained results, ab initio and DFT calculations on the model compounds cyclo-(H2SiO)(4) (4), cyclo-[H2Si(OSnH2)OSiH2] (5), cyclo-O(H2SiOSnH2)(2)O (6), and cyclo-[H2Si(OSiH2)OSnH2] (7) 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.