Protonated N,N′-Dioxide-4,4′-bipyridine, an Interesting Synthon for the Building of Polar H-Bonded Networks?

A slow liquid–gas diffusion method in strong acid conditions has allowed stabilizing for the first time in the solid state the diprotonated form of the 4,4′-bipyridine-N,N′-dioxide (bp4do), H2bp4do2+ as two polymophs of the chloride salt (H2bp4do)Cl2, while in the presence of a metallic salt M(III)Br3 (M = Bi, Sb), the monoprotonated form Hbp4do+ is mainly stabilized as (Hbp4do)2[Sb2Br8] and (H2bp4do)(Hbp4do)4[Bi4Br18]·2H2O. The dehydration of this last phase leads to (H2bp4do)(Hbp4do)4[Bi4Br18]. In the structures of (H2bp4do)Cl2 salts, the hydroxyl H atoms point toward the chloride anions leading to close (O)H···Cl contacts in the range 1.831–1.839 Å. A one-dimensional network of H-bonded Hbp4do+ cations is found in the structure of (Hbp4do)2[Sb2Br8], whereas H-bonded supramolecular cations of five entities are found in the structures of bromobismuthate salts. In all the halometallate structures, X-ray analyses (293 K and 120 K data) did not allow us to locate the acid H atoms of protonated bp4do molecules certainly due to a disorder phenomenon but also revealing an asymmetrical H-bonding situation. The bromobismuthate hybrids are nonlinear optical active with a powder second harmonic generation efficiency being half that of urea.