Structure of cyclooctatetraene at 129 K

Abstract

CsH s, Mr= 104.2, orthorhombic, Aba2, a = 7.664 (6), b = 7.650 (3), c = 10.688 (6) A, V= 627 (1)/~3, T= 129 K, Z = 4, D x = 1.10 Mg m -3, F(000) = 224, 2(Mo Ka) = 0.71069 A, /~= 0.06 mm -1, final R = 0.036 (wR = 0.043) for 529 observed reflections (I> 2o(/)) (4525 (+h, +k, +l) measured, 597 averaged reflections (Rint 0.05) and 52 refined parameters). The molecular structure of COT in the solid state confirms earlier electron-diffraction results for the title compound suggesting a boat conformation. This is in accordance with recent structural investigations of COT derivatives. Introduction. Cyclooctatetraene (COT) ring systems are capable of three structural changes, which involve ring inversion, bond shifting and valence isomerization (Paquette, 1975). Whereas the first process requires a planar transition state only, all bond-shifting mechan- isms involve additionally an equalization of bond lengths, and consequently slightly higher energy barriers are found for the latter. As a ligand to transi- tion metals, COT usually adopts a boat form or a twisted boat conformation (Brauer & Krfiger, 1976); frequently the ligand shows fluxionality (Mann, 1982). As a dianionic ligand the COT skeleton is planar (Streitwieser et al., 1973; R6sch & Streitwieser, 1978). For free, non-substituted COT a non-planar ring shape was found by electron diffraction (Bastiansen, Hedberg & Hedberg, 1957; Tr~etteberg, 1966); preliminary results of a crystal structure analysis of COT have been quoted in the literature (Bordener, Parker & Stanford, 1972). A low-temperature X-ray diffraction study of COT was undertaken to reveal basic structural data for this important compound in the solid state.