Hypervalent interactions in anthraquinone-based Group 15 ’onium salts – fact or fiction?

Metal ion-catalysed replacement of bromine in 1-amino-2-methyl-4-bromoanthraquinone by triphenylphosphine and triphenylstibine occurs readily under mild conditions to give the respective phosphonio- and stibonio-anthraquinone salts, the carbonyl group acting as a coordination template for the metal ion, facilitating replacement of the halogen. The extent to which the Group 15 element may be involved in hypercoordination from the adjacent carbonyl oxygen atom in the ’onium salts has been investigated by X-ray crystallography. Both salts involve considerable distortion of bond angles about the Group 15 atom in the direction of trigonal bipyramidal geometry, with phosphorus–oxygen (2.661 Å) and antimony–oxygen (2.497 Å) distances which are well within the sum of the van der Waals radii. In both structures, the Group 15 element and the adjacent carbonyl oxygen atom are bent out of the plane of the anthraquinone system. However, the extent of out of plane deformation is smaller in the case of the larger antimony atom, suggesting that there is a genuine hypercoordinative interaction which increases as the Group is descended.