Supramolecular Chemistry of 4,4′-bipyridine-N,N′-dioxide in transition metal complexes: A rich diversity of co-ordinate, hydrogen-bond and aromatic stacking interactions

Abstract

4,4′-Bipyridine-N,N′-dioxide (L1) has enormous flexibility as a supramolecular linker since it can be involved not only in co-ordinate and hydrogen bonds via its N,N′-dioxide oxygen centres, but the pyridine-N-oxide rings can also form aromatic π-π stacking interactions. Thus, L1 can bridge between, or act as a pendant ligand to metal centres and can support hydrogen-bonds within a lattice in a site remote from the metal centre. Of the structurally characterised transition metal complexes abstracted from the literature for this review, 26 form molecular compounds, 14 form 1D chains, 9 form 2D sheets of either 36, 44 or 63 topology, while 5 form 3D networks with either 41263 (α-Po type) or 48668 topology. To target multidimensional architectures it has been found to be necessary to avoid aqueous solutions and strongly co-ordinating anions, and consequently the syn-thesis of multidimensional L1-bridged transition metal co-ordination polymers has usually involved reaction of L1 with metal salts of weakly co-ordinating anions in low molecular weight alcohols. Of the 98 distinct molecules of L1 reported for complexes in the literature, 42 are bridging, 36 pendant and 20 are non-co-ordinated hydrogen-bonded molecules. Approximately 75% of the bridging L1 molecules adopt an anti-conformation, while the remainder adopt a syn-conformation. This prevalence of the anti-conformation contrasts markedly with the situation observed for lanthanide compounds, for which approximately 75% adopt a syn-conformation. A number of trends in the co-ordination behaviour of L1 with transition metals can be identified. Co-ordination to metal centres is based on sp2 hybridised oxygen donors, but the π-interaction between the oxygen pz orbital and the aromatic ring is sufficiently weak that the oxygen lone pairs are normally twisted out of the plane of the pyridine-N-oxide by a steric clash between the metal centre and the α-hydrogen of the pyridine ring. As a result of this steric hindrance,