SELF-ASSEMBLY OF A HETERODUPLEX HELICATE FROM 2 DIFFERENT-LIGAND STRANDS AND CU(II) CATIONS

Cu(II) ions have been reacted with a 1/1 mixture of two linear ligands , one containing three 2,2'-bipyridine groups and the other three 2,2' :6',2 ''-terpyridine groups. Absorption spectroscopy and fast atom bom bardment mass spectrometry indicate the formation of a trinuclear comp lex containing one ligand of each kind. Determination of the crystal s tructure of this compound has confirmed that it is indeed a linear tri nuclear complex in which two different ligands are wrapped in a helica l fashion around the pentacoordinated metal ions. The central coordina tion geometry is trigonal bipyramidal; the two lateral Cu(II) ions are in a square pyramidal environment. Thus, a heteroduplex helicate is f ormed by the self-assembly of two different ligand strands and three s pecific metal ions induced by the coordination number and geometry of the latter. The self-assembly process may be considered to result from the reading of the steric and binding information present in the two ligands by Cu(II) ions through a pentacoordination algorithm. The same ligands have been shown earlier to yield homoduplex helicates from io ns of tetrahedral and octahedral coordination geometry and strands of bidentate bipyridines and tridentate terpyridines, respectively. These two types of artificial double helical species may be related on one hand to the natural homoduplex nucleic acids and on the other hand to the DNA:RNA heteroduplex.