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Conformational studies of some ditertiary arsine chelate complexes Ward, John Edward Henry

Abstract

Several new ditertiary arsines, (CH₃)₂AsCR₁R₂CH₂As(CH₃)₂(R₁ = Si(CH₃)₃, SiCl₃, F, CN, CI; R₂ = H and R₁ = R₂ = F) and (CH₃)₂AsCFHCF₂As(CH₃)₂ were prepared by the addition of tetramethyl-diarsine to an appropriately substituted olefin. When these ligands were reacted with Group VI hexacarbonyls or pentacarbonylmanganese halides, complexes were frequently obtained which contained a five-membered chelate ring. The nuclear magnetic resonance parameters derived from high resolution spectra of the ditertiary arsines and their chelate complexes were qualitatively interpreted in terms of their rotational and conformational behavior. In the complexes, a trimethylsilyl group was found to adopt an "equatorial" position on the two-carbon bridge of the chelate ring, while a fluorine substituent favored an "axial" orientation. Such extreme preferences were not indicated for cyano or chloro substituents. The chelate rings in the complexes of 1,2-bis(dimethylarsino)-1,1-difluoroethane probably undergo rapid conformational inversion. Changes in coupling constants due to transition metal variations in the chelate complexes were rationalized by a qualitative geometrical approach. However, the effects of substituting the arsenic atoms by phosphorus donor atoms could not be predicted by similar arguments. Nevertheless, variations in solvent, temperature, donor atoms, metal atoms, and their substituents did not drastically alter the conformations of the chelate rings. Apparently, the rotational behavior of the neat ditertiary arsines is partially dictated by the "bulky" dimethylarsino substituents.

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