Coordination and supramolecular chemistry multinucleating ligands containing two or more pyrazolyl-pyridine 'arms'

A wide variety of new ligands has been prepared in which two or three biden tate or terdentate chelating units, based on pyrazolyl-pyridine (bidentate) or pyrazolyl-bipyridine (terdentate) fragments, have been attached to a ce ntral spacer. This may be a borohydride unit to give bis- or tris(pyrazolyl )borate derivatives, or an aromatic (phenyl or biphenyl) unit. A general ch aracteristic of these ligands is that they can coordinate all of their bind ing 'arms' to a single metal ion, giving mononuclear complexes, or they can coordinate each separate,arm' to a different metal ion to give assembly of polynuclear species of occasionally quite unexpected structural complexity . We have observed self-assembly of helicates, helical rings, and tetrahedr al cages, in some cases with non-coordinated anions playing a crucial templ ating role in the assembly process. The tetrahedral cages in particular, wh ich have been prepared with both M-4(mu (3)-L)(4) and M-4(mu (2)-L)(6) stoi chiometries, provide elegant examples of how high-symmetry structures can a ssemble from simple components. (C) 2001 Elsevier Science B.V. All rights r eserved.