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article
New Helical Zinc Complexes with Schiff Base Derivatives of beta-Diketonates or beta-Keto Esters and Ethylenediamine
Abstract
<p>The tetradentate Schiff bases H(2)L = H(2)AC [N,N'-ethylenebis(4-iminopentan-2-one)], H(2)MAL1 [N,N'-ethylene-bis(methyl-3-iminobutanoate)], and H(2)MAL(2) [N,N'-ethylenebis(tert-butyl-3-iminobutanoate)] were found to possess sufficient flexibility and were tailored to self assemble with zinc ions to form neutral bimetallic helicates with the overall composition Zn(2)L(2). In solution for all ligands, an equilibrium between the tautomeric forms exists, which was confirmed by IR and NMR spectroscopy. X-ray single-crystal analysis performed for H(2)MAL1 (1) and H(2)MAL2 (2) reveals that they have similar geometrical parameters and crystallize in a trans conformation. In the crystal structures of the zinc complexes [Zn(2)(AC)(2)]center dot C(6)H(6) (3), [Zn(2)(MAL1)(2)]center dot 0.5C(7)H(8) (4), and [Zn(2)(MAL(2))(2)]center dot C(7)H(8) (5), the Schiff base ligands are helically wrapped around the two four-coordinate metal ions, which leads to an idealized D(2) symmetry. The coordination polyhedron around the Zn ions in 3-5 can be described as a tetrahedral geometry comprising two O- and two N-donor atoms of the Schiff base ligands. The intramolecular separation Zn center dot center dot center dot Zn varies from 3.465 to 3.628 angstrom. The total length of the helix is 5.85, 5.84, and 6.01 angstrom for structures 3, 4, and 5, respectively, while the helical pitch values are in the range 7.8-9.2 angstrom. Both the ligands H(2)L and the zinc helicates are thermally stable up to similar to 150-160 degrees C, as shown by thermogravimetric analysis under a nitrogen atmosphere. The formation of the zinc complexes in solution is confirmed by spectrophotometric titrations, along with ESI-MS, and has almost no effect on the absorption spectra of H(2)L. The Schiff bases and the Zn(2)L(2) complexes display broad-band, blue emission in the range 350-600 nm under a 337-nm excitation at 77 K. Finally, the impact of electronic effects of the Schiff base substituents on the structure of the Zn complexes, especially on bond lengths within the chelate rings, and the absorption and photoluminescence spectra, are discussed. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)</p>