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 synthesis 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 sp ² hybridised oxygen donors, but the π-interaction between the oxygen p _z 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, $<{\rm M} \cdots {\rm O}\hbox{--}{\rm N}$ angles increase with decreasing perpendicular distance of the metal from the plane of the pyridine-N-oxide. Finally, ${\rm M} \cdots {\rm M}$ (M = d-block metal) separations in complexes containing anti- and syn-conformation bridging ligands fall in similar ranges. However, those with syn-conformation ligands show an increase in ${\rm M} \cdots {\rm M}$ separation with increasing $<{\rm M} \cdots {\rm O} \cdots {\rm O} \cdots {\rm M}$ torsion angle, while those anti-conformation ligands show an increase in ${\rm M} \cdots {\rm M}$ separation with increasing $<{\rm M} \cdots {\rm O}\hbox{--}{\rm N}$ angle.