Predetermined chirality in mono- and dinuclear cyclometalated rhodium(III)complexes

Dinuclear aid polynuclear metal complexes with octahedral centers coordinat ed to di- or polydentate ligands are often obtained as complicated mixtures of various stereoisomers. Stereospecific synthesis of such species is ther efore of high current interest. Chiral derivatives of pyridine can be used for this purpose. Dinuclear mu-chloro-bridged Rh-III complexes with two did entate, cyclometalated thienylpyridine-type ligands at each metal center ar e formed stereoselectively when pinene groups are fused to the pyridine rin gs. The two octahedral Rh-III centers have homochiral configurations, Delta Delta and Lambda Lambda. The heterochiral diastereomer Delta Lambda is not observed. With (8R,10R)-2-(2'-thienyl)-4,5-pinenopyridine [Hth4,5-(R,R)ppy ] the Delta Delta to Lambda Lambda ratio is 9:1 when the separation eluent contains NaCl. Modeling the Lambda Lambda and the Delta Lambda isomers of t he dinuclear species shows crowding of the pinene groups in both cases; how ever, the strain can be released by relatively small distortions only in th e case of the Lambda Lambda isomer. NO3- cleaves the dichloro bridge, yield ing the mononuclear species Delta[Rh(L-2)(2)(NO3)] (2) in a completely ster eoselective manner when NaCl is replaced by KNO3 in the eluent mixture. The molecular structure has been determined by X-ray structure analysis for bo th the Delta Delta and the mononuclear complex Delta[Rh(L-2)(2)(NO3)] (2) i n order to confirm the configuration at the metal center. H-1-NMR, C-13-NMR and CD spectra were measured and the latter shows that the CD activity is solely due to the chirality at the metal center.