Non-covalent aggregation of discrete metallo-supramolecular helicates into higher assemblies by aromatic pathways: Structural and chemical studies of new aniline-based neutral metal(II) dihelicates

Neutral manganese(II), iron(II), cobalt(II), nickel(II), zinc(II) and cadmium(II) complexes with an N-tosyl-substituted N-4-donor Schiff base containing a 4,4'-methylenedianiline residue as spacer [H2La: N,N'-bis(2-tosylaminobenzylidene)-4,4'-methylenedianiline], and the zinc(II) complex with an analogous ligand [H2Lb : N,N'-bis(2-tosylaminobenzylidene)-4,4'-oxodianiline] have been prepared by an electrochemical procedure. FAB and ESI mass spectra of the complexes show peaks due to species corresponding to a general formula [M-2(L-a,L-b)(2) + H](+), thereby suggesting their dinuclear nature. A detailed study of the crystal packing in the unit cell of the zinc(II) complex with H2La shows that the helicates aggregate to form discrete prismatic moieties containing three molecules held together by pi-pi and sigma-pi interactions. Moreover, the Zn-II neutral dihelicate with H2Lb forms a 3D network in the solid state due to intermolecular pi-stacking interactions. H-1 NMR studies of the diamagnetic compounds reported herein have been performed. Finally, the ligand H2La and its Zn-II and Cd-II complexes have been studied by spectrophotometric and spectrofluorimetric techniques in order to get a better understanding of the formation mechanisms of the complexes and of the nature of their fluorescence emission. Emission studies show that the Zn-II and Cd-II dihelicates with H2La display a green fluorescence in acetonitrile solution (lambda = 473 nm, Phi = 0.03 and lambda = 476 nm, Phi = 0.01, respectively).