X-ray structures and fluorescence spectra of binuclear Zn2+ and Cd2+ complexes of an amide-based naphthalenophane

The formation of binuclear Zn2+ and Cd2+ complexes with an amide-based chelating naphthalenophane has been confirmed by X-ray crystal analyses: the complexes are formulated as [(H2O)MLM(OH2)]0 (M=Zn or Cd), and the naphthalenophane, LH4, is 2,9,22,29-tetraoxo-4,7,24,27-tetrakis(carboxymethyl)-1,4,7,10,21,24,27,30-octaaza[10.10](1,5)naphthalenophane. The Zn2+ complex has a six-coordination geometry and the Cd2+ complex has a seven-coordination geometry. The naphthyl groups are deformed from the planar structure by metal complexation in both complexes. Fluorescence from the uncoordinated ligand is weakened by protonation. The coordination of Zn2+ enhances the fluorescence whereas the coordination of Cd2+ weakens the emission: the intensity ratio, F(L):F(Zn2L):F(Cd2L)=1:12:0.2 at pH 10. The Zn2+ complex exhibits a large change in the fluorescence intensity with pH, as a result of interconversion between [(H2O)ZnLZn(OH2)]0 and [(HO)ZnLZn(OH)]2−; the emission of the latter formed at higher pH is 20 times stronger than that of the former formed at lower pH. Structural changes that occur in the chelating units upon protonation or metal complexation propagate to the fluorescent units through the amide groups that link the two functional units. This propagation results in the fluorescence properties characteristic of the amide-based naphthalenophane.