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 chel ating naphthalenophane has been confirmed by X-ray crystal analyses: the co mplexes are formulated as [(H2O)MLM(OH2)](0) (M = Zn or Cd), and the naphth alenophane, 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 ha s a six-coordination geometry and the Cd2+ complex has a seven-coordination geometry. The naphthyl groups are deformed from the planar structure by me tal complexation in both complexes. Fluorescence from the uncoordinated lig and is weakened by protonation. The coordination of Zn2+ enhances the fluor escence whereas the coordination of Cd2+ weakens the emission: the intensit y ratio, F(L):F(Zn2L):F(Cd2L) = 1:12:0.2 at pH 10. The Zn2+ complex exhibit s a large change in the fluorescence intensity with pH, as a result of inte rconversion between [(H2O)ZnLZn(OH2)](0) and [(HO)ZnLZn(OH)](2-); the emiss ion 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 u nits through the amide groups that link the two functional units. This prop agation results in the fluorescence properties characteristic of the amide- based naphthalenophane. (C) 2000 Elsevier Science S.A. All rights reserved.