First-generation chiral metallodendrimers: Stereoselective synthesis of rigid D-3-symmetric tetranuclear ruthenium complexes

Condensation reactions between the enantiopure, substitutionally inert building blocks Delta-[Ru(1,10-phenanthroline- 5,6-dione)(3)][PF6](2) (2) and Delta-[Ru(phen)(2)(1,10-phenanthroline-5,6-diamine)][PF6](2) (3) gave the D-3-symmetric tetramer [(Delta-(phen)(2)Ru(tpphz))(3)-Delta-Ru](8+) (Delta Delta(3-)1) in 68% isolated yield as the PF6- salt (where tpphz is tetrapyrido[3,2-a:2',3'-c:3 '',2 ''-h:2 '',3 ''-j]phenazine). Reactions between appropriate enantiomers of 2 and 3 also yielded the remaining D-3 isomers Lambda Delta(3)-1, Delta Lambda(3)-1, and Lambda Lambda(3)-1 which collectively represent the highest nuclearity Ru oligomers based on bidentate type ligands prepared in a stereospecific fashion to give diasteromerically and enantiomerically pure products. The complexes were purified via cation-exchange HPLC. H-1 and C-13 NMR, COSY and HMQC NMR, and W-visible spectroscopy were employed to characterize these supramolecular assemblies. MALDI-TOF mass spectrometry gave parent molecular ion peaks corresponding to the species m/z 2638 [1 - 8PF(6)](+), 2783 [1 - 7PF(6)](+), 2928 [1 - 6PF(6)](+), 3073 [1 - 5PF(6)](+), 3218 [1 - 4PF(6)](+), and 3363 [M - 3PF(6)](+), proving the macromolecular structure. Circular dichroism spectroscopy was used to determine the absolute stereochemistry and optical purity of each of these stereoisomers.