Unusual conformations of isoelectronic (SiO)4, (SiOSiN)2 and (SiN)4 rings

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The crystal structures of three compounds containing eight-membered Si4(N,O)4 rings have been determined: [t-Bu2SiOSiMe2O]2 (I), [t-Bu2SiOSiMe2NH]2 (II) and [i-Pr2SiNH]4 (III). I forms monoclinic crystals, space group C2/c, with a 14.124(4), b 13.191(4), c 16.156(6) Å, β 108.09(2)°, Z = 4. II also crystallizes in C2/c, with a 14.234(5), b 13.372(6), c) 16.142(8) Å, β 108.87(3)°, Z = 4. The structure determinations show that I and II are essentially isostructural, with effectively planar eight-membered rings lying on crystallographic inversion centres. However the SiOSi angle in II is almost linear (178.0(1)°) and the SiNSi angle is 145.2(2)°, whereas intermediate values are observed for the SiOSi angles in I (156.0(1), 164.2(1)°). III is tetragonal, space group I4, with a, b 13.722(1), c 8.911(2) Å, Z = 2. The ring adopts a tub conformation with SiNSi 139.1(2)°, and the molecule possesses crystallographic S4(4) symmetry. The structures were refined to R = 0.039, 0.053, and 0.040 for 2285, 2081, 696 unique observed reflections, respectively.