A molecular gyroscope which has a phenylene rotator encased in three long siloxaalkane spokes was synthesized. The phenylene ring is observed by X-ray crystallography at three rotational positions around the 1,4-axis between 223–303 K. The area in which the phenylene ring is found is significantly reduced at 173 K owing to the deformation of a siloxaalkane spoke; the temperature-dependent phenylene disorder is reversible.
) 2,3-Halophenylene-bridged polysilaalkane macrocages 1b and 1c were synthesized as novel molecular gyroscopes. X-ray crystallographic data showed that the structures of the macrocages were deformed to avoid contact between the halogens and the silaalkane cage. The 2,3-dichlorophenylene derivative 1c shows restricted rotation of the phenylene in solution as observed by variable temperature 1 H NMR spectroscopy.Macrocyclic molecules having bridged ³-electron systems have attracted much attention in terms of their unique structures, dynamics, and functions.15 Especially phenylene-bridged macrocages are expected to have functions of gyroscopes and compasses as first proposed by Garcia-Garibay et al. 3 Recently, such cage molecules were reported as molecular gyroscopes. 35 We have applied polysilaalkane chains of novel cyclophanes.2a,5 These chains are easily constructed, chemically stable, and exhibit good crystallinity. Recently we reported the synthesis and structure of a molecular gyroscope having a bridged phenylene encased in a large silaalkane cage.5 Cage compound 1a was found by X-ray crystallography to show remarkable disorder at 30°C. The phenylene ring was observed at three positions around the threefold molecular axis, suggesting the phenylene rotates around the axis even in crystals. We report herein the synthesis and structure of 2,3-halophenylenebridged polysilaalkane macrocages 1b and 1c and reveal steric effects on the structure and dynamics of the corresponding macrocages.The 2,3-halophenylene-bridged molecular gyroscopes were synthesized by hydrolysis reactions of hydrosilanes 2a2d (Scheme 1).6,7 Statistically, the ratio of the formation of a cage structure to its isomer is calculated to be 1:3; therefore, the yield of the cage structure is lower than that of the isomer. In this reaction, a remarkable dependence of the yields on the bulkiness of the internal rotator was observed. The yields of the cage compounds decreased with increasing atomic size of the halogens on the rotator. Indeed, there was no observation of a 1,2-dibromophenylene-bridged molecular gyroscope 1d due to the steric hindrance in the reaction. Compounds 1b and 1c were identified by 1 H, 29 Si, and 13 C NMR spectroscopy, and mass spectrometry.The molecular structures of the molecular gyroscopes 1b and 1c were confirmed by X-ray crystallography (Figure 1). 8 In the 2,3-difluorophenylene-bridged molecular gyroscope 1b, the shape of the frame was slightly deformed from that of the phenylene-bridged derivative, and the molecule 1b has C 2 symmetry. This deformation was most likely due to steric hindrance of the fluorines on the phenylene ring. The phenylene ring was observed at two positions around the molecular axis. Figure 1. Molecular structure of (a) compound 1b at 0°C and (b) compound 1c at ¹120°C. Left: equatorial view with 30% thermal probability ellipsoid; Right: axial view. Hydrogen atoms and structural disorder on the side chains are omitted for clarity. Structural disorder of phenylene ring in 1b is shown, and ra...
Macrocage molecules with a bridged rotor have been synthesized as molecular gyroscopes. The kinetics of the oxidation reaction of the thiophene-bridged molecular gyroscope, whose thiophene ring was bridged inside a silaalkane cage, was investigated. A remarkable kinetic stabilization against the oxidation of the thiophene moiety induced by the molecular cage framework was observed.
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