Predetermined chirality at metal centers of various coordination geometries: A chiral cleft ligand for tetrahedral (T-4), square-planar (SP-4), trigonal-bipyramidal (TB-5), square-pyramidal (SPY-5), and octahedral (OC-6) complexes

Two tetradentate bispinene-bipyridine type ligands, each with six stereogen ic carbon centers, were synthesized from (-)-alpha-pinene. Their ability to predetermine chiral configurations at metal centers was studied. The two d iastereoisomers, L1 and L2, differ in their absolute configuration at the b ridgehead position. These ligands form metal complexes with Ag-I, Pd-II, Zn -II, Cu-II, and Cd-II, with coordination numbers four, five, and six and wi th complete control of chirality at the metal centers. Using L1 rather than L2 leads to complexes of inverted absolute configuration at the metal cent ers. These diastereomeric coordination species can be obtained either as se parate compounds or, in some cases, as solids containing them in a 1:1 rati o. Ligands L1 and L2 thus show that the pinene-bipyridines are versatile mo lecule for the formation of metal complexes with predetermined chirality. I n all cases, absolute configurations were determined in the solid state by X-ray diffraction methods and in solution by CD spectroscopy. The sign of e xciton couplers from the pi-pi* transitions always agrees with the expectat ions for a given local configuration at the metal center. The five-coordina te, inherently chiral species of Zn-II and Cu-II described in this article are the first examples of trigonal-bipyramidal metal complexes with predete rmined; absolute configuration containing topologically linear ligands.