SYNTHESIS AND 1ST X-RAY STRUCTURE-ANALYSIS OF A STABILIZED CHIRAL CHLOROBISMUTHINE - FIXATION OF MOLECULAR-GEOMETRY INDUCED BY THE INTRAMOLECULAR COORDINATION OF A SULFONYL GROUP

A sulfonyl-stabilized chiral chlorobismuthine la was synthesized by th e selective fluorodearylation of compound 3a with boron trifluoride-di ethyl ether, followed by halogen exchange of the resulting fluorobismu thine 4a with brine. The H-1 NMR spectrum of compound 1a showed an unu sually large downfield shift of a proton signal (delta 9.16), and X-ra y structure analysis revealed that this unique shift was due to the an isotropic deshielding of the proton adjacent to the bismuth atom by th e chlorine atom in close proximity. The bismuth centre of compound 1a was found to adopt a distorted pseudotrigonal bipyramidal geometry, pr obably through the formation of a hypervalent 3-centre 4-electron bond with the oxygen and chlorine atoms at apical positions. This is in ma rked contrast to the bismuthine 3a which has a pyramidal configuration characteristic of trivalent organobismuth compounds. A quite short Bi -O distance [2.592(5) angstrom] of compound 1a, compared with that [2. 914(6) angstrom] of the bismuthine 3a, indicates that the introduction of an electronegative chlorine atom onto the bismuth atom enhances th e Lewis acidity of the metal centre, leading to the pronounced intramo lecular Bi-O interaction.