Self-assembled triple helicates with preferential helicity

The enantiomerically pure ligands L-RR and L-SS (N,N'-bis(-2,2'-bipyridyl-5-yl)carbonyl-(1S/R,2S/R)-(+/-)-1,2- diaminocyclohexane) have been synthesised by linking two 2,2'-bipyridine units by (R,R)- and (S,S)-1,2- diaminocyclohexane respectively. The crystal structure confirmed that the ligand had a twisted orientation between the two chelating units. The reaction of L-RR and L-SS with Fe(II), Co(III), Cd(II) and Zn(II) afforded dinuclear complexes confirmed by electrospray mass spectroscopy. CD spectroscopy indicated that the chiral diaminocyclohexane conferred helicity to the metal centre giving a dominant triple helicate diastereoisomer, with the L-RR ligand giving a DeltaDelta-configuration of each metal centre (P helicate) and the L-SS ligand LambdaLambda-configuration (M helicate). H-1 NMR spectroscopy confirmed a dominant major diastereoisomer with cadmium. The Zn(II) and Cd(II) complexes however were observed to undergo rapid ligand dissociation in solution.