Stereochemically non-rigid silanes, germanes, and stannanes. Part 6. Poly(cyclopentadienyl)silanes. A comparison of thermodynamic parameters derived from approximate methods with those obtained using full lineshape analysis of carbon-13 nuclear magnetic resonance data

Abstract

Reaction of dichloro- or trichloro-silane with the cyclopentadienide anion yields bis(cyclopenta-1,3-dien-5-yl)-silane, (1), chlorobis(cyclopenta-1,3-dien-5-yl)silane, (2), and tris(cyclopenta-1,3-dien-5-yl)silane, (3), for which i.r. and mass spectral data are reported. The activation parameters for degenerate metallotropic rearrangement in compounds (1)—(3) and in Si(C₅H₅)H₃, Si(C₅H₅)ClH₂, and Si(C₅H₅)Cl₂H have been calculated from (i) the temperature dependence of coalesced linewidth in variable-temperature ¹H n.m.r. spectra using an established approximation, and (ii) full lineshape analysis of corresponding proton-noise decoupled ¹³C n.m.r. data. Both methods give ΔG‡ of 55—60 kJ mol^–1 as the range for the compounds investigated with no evidence for a dependence on chlorine substitution or the number of C₅ rings attached at the Si. Derived ΔS‡ and ΔH‡ values are subject to large errors, the implications of which in relation to previously reported data are discussed. Compound (2) exhibits anisochronous character between two ring carbon atoms, and the temperature dependence of the diastereotopically shifted ¹³C resonances is interpreted as further evidence for a 1,2 metallotropic migration.