Hierarchical self-assembly of heteronuclear co-ordination networks.

Two ligands L(1) and L(2) have been prepared which contain a chelating pyrazolyl-pyridine group with a pendant aromatic nitrile (in L(1), a benzonitrile; in L(2), 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, L(1) and L(2) form simple mononuclear tris-chelates [ML(3)](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(L(2))(3)](BF(4))(2) are based on solely the mer tris-chelate geometry although in solution (1)H 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 [ML(3)](2+) cations being crosslinked by Ag(I) ions. In these networks the [ML(3)](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(L(2))(3)][BF(4)](3)}(infinity) (M = Co, Ni) consist of two interpenetrated (10,3)a nets which have opposite chirality at the [M(L(2))(3)](2+) centres but are not strictly enantiomorphic as the two nets are not crystallographically equivalent. {[AgNi(L(1))(3)](BF(4))(3)}(infinity) 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...pi interactions between aromatic ligands. Formation of these structures shows that the differential reactivity of the two binding sites in L(1) and L(2) (pyrazolyl-pyridine, and nitrile) can be used to generate mixed-metal coordination networks in a hierarchical, stepwise manner.