Comparison of the Flexibility of Eight-Membered Tetrasiloxane and Stannasiloxane Rings:  A Crystallographic and Computational Study

Abstract

The syntheses and crystal structures of the eight-membered cyclo-stannasiloxanes cyclo-[t-Bu(OH)Si(OSnt-Bu2O)2Si(OH)t-Bu] (1) and cyclo-{t-Bu2Si[OSn(CH2SiMe3)2O]2Sit-Bu2} (2) as well as the synthesis of the six-membered cyclo-stannasiloxane cyclo-{t-Bu2Si[OSn(CH2SiMe3)2]2O} (3) are reported. Compound 1 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)2O (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.

Triorganotin Fluoride Structures: A Ligand Close‐Packing Model with Predominantly Ionic Sn−F Bonds

“…[81] The average internal angle of a n-membered ring is required by three-dimensional geometry to be less than (for puckered rings) or equal to (for flat rings) [(n Ϫ 2)/n] ϫ 180°. [82] Constraining the FϪSnϪF angles to be linear, would therefore necessitate large values of n before the SnϪFϪSn linkages are able to adopt favorable angles (Ͼ 150°). Natural Population Analysis (NPA) [83] atomic charges calculated for 3 and the staggered conformers of 4 and 5 clearly show a significant localization of negative charge on the fluorine atoms in the model compounds, hence providing further evidence of the high ionic character of the SnϪF bond ( Table 2).…”

Transformations of the polycyclic Alumosiloxane Al2(OSiPh2OSiPh2O)3 into new Polycycles and Co(II) and In(III) derivatives of (Ph2SiO)8[Al(O)OH]4

“…It should be recalled that the flexibility of cyclic siloxane and metallo-siloxane rings is high due to small energy differences between conformers (also compare discussion on 3 b and 4 below). [21] In Figure 3 the molecular structure of 2) is shown, which has been obtained as a minor byproduct of Scheme 2. Like compound 1 it crystallizes with a toluene molecule which behaves as space filler in the lattice.…”

Eight membered cyclic-borasiloxanes: synthesis, structural, photophysical, steric strain and DFT calculations

“…The transition from strained cyclic ring to ring opening polymerization would directly account for the entropy term of the Gibbs-Helmholtz equation (DG ¼ DH À TDS). [19][20][21] The entropic gain is attained from the conversion of the inexible ring to a more exible linear chain. The ring strain energy also depends on the ring member count, which atoms comprise the ring and the nature of the peripheral substituents.…”

“…Beckmann et al described six and eight-membered stannosiloxane ring strain and their puckered conformation. 19 Khudobin et al investigated cis and trans cyclosiloxane (RR 0 SiO) 3 ring strain energies. 15a To the best of our knowledge, there is no report in the literature about computational investigations of steric strain energies in cyclic borasiloxanes.…”