Hierarchical self-assembly of heteronuclear co-ordination networks.

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.