Inorganic Chemistry, Vol.48, No.24, 11871-11881, 2009
Cubes, Squares, and Books: A Simple Transition Metal/Bridging Ligand Combination Can Lead to a Surprising Range of Structural Types with the Same Metal/Ligand Proportions
Reaction of two structurally related bridging ligands L-26Py and L-13Ph, in which two bidentate chelating pyrazolyl-pyridine units are connected to either a 2,6-pyridine-diyl or 1,3-benzene-diyl central group via methylene spacers, with first-row transition metal dications, results in a surprising variety of structures. The commonest is that of an octanuclear coordination cage [M8L12]X-16 [M = Co(II) or Zn(II); X = perchlorate or tetrafluoroborate] in which a metal ion is located at each of the eight vertices of an approximate cube, and one bis-bidentate bridging ligand spans each edge. The arrangement of fac and mer tris-chelate metal centers around the inversion center results in approximate (non-crystallographic) S-6 symmetry. Another structural type observed in the solid state is a hexanuclear complex [Co-6(L-13Ph)(9)](ClO4)(12) in which the six metal ions are in a rectangular array (two rows of three), folded about the central Co-Co vector like a partially open book, with each metal-metal edge containing one bridging ligand apart from the two outermost metal-metal edges which are spanned by a pair of bridging ligands in a double helical array. The final structural type we observed is a tetranuclear square [Ni-4(L-26Py)(6)](BF4)(8), with the four Ni-Ni edges spanned alternately by one and two bridging ligand such that it effectively consists of two dinuclear double helicates cross-linked by additional bridging ligands. A balance between the "cube" and "book" forms, which varied from compound to compound, was observed in solution in many cases by H-1 NMR and ES mass spectrometry studies.