Crystal and Molecular Structures of (η4-Cyclohexa-1,3-Diene) (2-Tert-Butyl-1,1,4,4-Tetrafluoro-1,4-Disilabutene) Metal Carbonyls. MFe, W

Abstract

Molecular structures of the two title compounds were investigated to reveal the relationship between the structure of the reaction intermediates and the reaction pathways in the transition metal-mediated cycloaddition reaction between tetrafluorodisilacyclobutene and conjugated dienes. The structures were determined by X-ray diffraction. Both cyrstals are orthorhombic. Compound I. C14H18O2Si2F4Fe. is of space group P212121 with Z=4; a=6.862(7) A. b=10.9560(10) A. c=23.597(25) A. and Dc=1.562 gcm−1. Compound II, C15H18O3Si2F4W, is of space group Pbca with Z=8; a=16.999(8) A, b=15.948(6), c=14.045(7) A, and Dc=1.962 gcm−1. Both structures were solved by heavy-atom methods and refined by full-matrix least-squares using anisotropic temperature factors for all non-hydrogen atoms to R values of 0.092 for 1544 observed reflections and 0.078 for 2751 observed reflections, respectively, for compound I and compound II. The butadiene segment of the cyclohexadiene ligand is essentially planar in compound I while it is twisted with a torsional angle of 22° (2) in compound II. The five-membered disilametallacycle ring is planar in compound II while ii is in an envelope comformation in compound I. The coordination geometry of the iron atom may be considered as a distorted tetragonal-pyramidal with the diene unit occupying the apical site, That of tungsten is a distorted seven-coordinated pentagonal-bipyramidal with the two axial carbonyl groups displaced toward the disilabutene ligand. The disilabutene of compound I lies almost parallel and cis to the plane of the butadiene segment while that of compound II orients almost perpendicular to the diene group. The differences in the structural features of the two complexes explain clearly their product patterns and the reactions are apparently stereo-controlled.