Protonated N-oxide-4,4 '-bipyridine: from luminescent Bi-III complexes to hybrids based on H-bonded dimers or H-bonded open 2D square supramolecular networks

The N-oxide-4,4'-bipyridine (bp4mo) ligand has been rarely used in coordination or supramolecular chemistry. Depending on experimental conditions in the bp4mo/BiX3\ (X = Cl, Br) system, several compounds, in which the degree of protonation of bp4mo (tau = H/bp4mo) varies, have been stabilized. In acidic conditions, either a complete N-protonation and a partial O-protonation (tau = 1.5) or a complete N-protonation (tau = 1.0) can occur, leading to either a hybrid salt based on H-bonded dimers [H(Hbp4mo)(2)](3+), or to O(Hbp4mo)-Bi photoluminescent complexes. In neutral conditions, a partial N-protonation (tau = 0.5) occurs in [H(bp4mo)(2)][BiCl4]. Although the N-oxide parts are free to bind Bi3+ cations, the organic entities prefer self-organizing through H-bonding (NH center dot center dot center dot N and CH center dot center dot center dot O) to give an impressive 2D supramolecular square network which accommodates the 1D BiCl4- polymeric anion.