First-Generation Chiral Metallodendrimers:  Stereoselective Synthesis of Rigid D 3-Symmetric Tetranuclear Ruthenium Complexes

Condensation reactions between the enantiopure, substitutionally inert building blocks Δ-[Ru(1,10-phenanthroline-5,6-dione)3][PF6]2 (2) and Δ-[Ru(phen)2(1,10-phenanthroline-5,6-diamine)][PF6]2 (3) gave the D 3-symmetric tetramer [(Δ-(phen)2Ru(tpphz))3-Δ-Ru]8+ (ΔΔ3-1) 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 ΛΔ3-1, ΔΛ3-1, and ΛΛ3-1 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. 1H and 13C NMR, COSY and HMQC NMR, and UV−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 − 8PF6]+, 2783 [1 − 7PF6]+, 2928 [1 − 6PF6]+, 3073 [1 − 5PF6]+, 3218 [1 − 4PF6]+, and 3363 [M − 3PF6]+, proving the macromolecular structure. Circular dichroism spectroscopy was used to determine the absolute stereochemistry and optical purity of each of these stereoisomers.