Self-assembled triple helicates with preferential helicity

Abstract

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 ΔΔ-configuration of each metal centre (P helicate) and the L^SS ligand a ΛΛ-configuration (M helicate). ¹H 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.