Octahedral complexes with predetermined helical chirality: Xylene-bridged bis([4,5]-pineno-2,2'-bipyridine) ligands (chiragen[o-, m-, p-xyl]) with ruthenium(II)

Tetradentate ligands are obtained by joining two optically active [4,5]-pineno-2,2'-bipyridine molecules in a stereoselective reaction, where two new stereogenic centers are created. These ligands are new members of the chiragen family that form OC-6 complexes with predetermined helical chirality. Ru(II) complexes with 4,4'-dimethyl-2,2'-bipyridine occupying the remaining coordination sites have been synthesized with all three new ligands. Characterization of the ruthenium complexes by NMR spectroscopy confirm C-2-symmetric structures in solution. CD spectra show that the complexes are composed of only one helical diastereomer with the expected absolute configurations. In addition, a strong chiral amplification is observed, if precursors of low enantiomeric purity are used. This is due to the inability of ligands that are heterochiral in the two bpy moieties to coordinate to one center. X-ray structural data were obtained for the complex Delta-[RuCG[o-xyl](4,4'-DMbpy)](PF6)(2). Crystal data (Mo K alpha, 298 K): trigonal, space group R3, a = 52.986(4) Angstrom, c = 10.545(1) Angstrom, V = 25639(4) Angstrom(3), Z = 18, R1 = 0.087, and wR2 = 0.0986 for 2609 observed reflections.