Hierarchical self-assembly of heteronuclear co-ordination networks

Abstract

Two ligands L¹ and L² have been prepared which contain a chelating pyrazolyl-pyridine group with a pendant aromatic nitrile (in L¹, a benzonitrile; in L², a naphthonitrile). These ligands react with Ag(I) salts to give a range of infinite coordination networks or dimeric ‘boxes’ in which the pyrazolyl-pyridine chelates and the aromatic nitrile groups both participate in coordination to Ag(I) ions. In contrast, L¹ and L² form simple mononuclear tris-chelates [ML₃]²⁺ with first-row transition metal dications (M = Co, Ni, Zn) in which the aromatic nitrile groups are pendant such that the complexes can be used as ‘complex ligands’. The crystal structures of [M(L²)₃](BF₄)₂ are based on solely the mer tris-chelate geometry although in solution ¹H NMR spectroscopy reveals a mixture of both fac and mer isomers of the tris-chelates. Reaction of these with Ag(I) ions allows the interaction of the pendant nitrile groups with Ag(I) ions to generate coordination networks based on [ML₃]²⁺ cations being crosslinked by Ag(I) ions. In these networks the [ML₃]²⁺ cations have solely the fac geometry and lie on threefold rotation axes with all three pendant nitrile groups coordinated to Ag(I) ions which are three-coordinate. {[AgM(L²)₃][BF₄]₃}_∞ (M = Co, Ni) consist of two interpenetrated (10,3)a nets which have opposite chirality at the [M(L²)₃]²⁺ centres but are not strictly enantiomorphic as the two nets are not crystallographically equivalent. {[AgNi(L¹)₃](BF₄)₃}_∞ in contrast contains two-dimensional sheets which have a (6,3) net structure of hexagonal rings of alternative Ni(II) and Ag(I) centres; although not interpenetrating, two such adjacent (and enantiomorphic) sheets interact with each other via numerous CH⋯π interactions between aromatic ligands. Formation of these structures shows that the differential reactivity of the two binding sites in L¹ and L² (pyrazolyl-pyridine, and nitrile) can be used to generate mixed-metal coordination networks in a hierarchical, stepwise manner.