Donor-stabilised fluorido cations of groups 15 and 16

Abstract

The fluoride-ion donor properties of the strong Lewis-acids SbF5 and AsF5 were investigated. In the presence of the bidentate nitrogen base 1,10-phenanthroline (phen) SbF5 and AsF5 were found to autoionise generating [PnF4(phen)][PnF6] (Pn = As or Sb) salts; this is the first time that SbF5 has demonstrated the ability to act as a fluoride-ion donor. In the presence of the more flexible nitrogen base 2,2′-bipyridine (bipy), SbF5 was again found to autoionise yielding [SbF4(bipy)][SbF6], whereas AsF5 forms the neutral adduct (AsF5)2·bipy. In the presence of the tridentate base 2,2′:6′,2′′-terpyridine (terpy) SbF5 autoionises to form [SbF4(terpy)][SbF6]; the [SbF4(terpy)]+ cation is the first structurally characterised fluorido Sb(V) complex with a coordination number of seven and adopts a heavily distorted pentagonal bipyramidal geometry. The structure and bonding in these unique systems was studied using a combination of spectroscopic, crystallographic and computational techniques. The [SF3]+ cation forms 1:1 adducts with pyridine and 4-dimethylaminopyridine (DMAP) which adopt different structures. Whereas pyridine coordinates in the axial position of the seesaw-geometry, DMAP is located in the equatorial position. The latter structure leads to facile dismutation to the [SF2(dmap)2]2+ cation and SF4, while [SF3(py)]+ is stable towards dismutation. Further dismutation afforded the square-pyramidal [SF(dmap)4]3+ cation. Attempts to form adducts of [SF3]+ cation with triphenylphosphine oxide, resulted in deoxofluorination.