New Helical Zinc Complexes with Schiff Base Derivatives of β‐Diketonates or β‐Keto Esters and Ethylenediamine

Abstract

AbstractThe tetradentate Schiff bases H2L = H2AC [N,N′‐ethylene‐bis(4‐iminopentan‐2‐one)], H2MAL1 [N,N′‐ethylene‐bis(methyl‐3‐iminobutanoate)], and H2MAL2 [N,N′‐ethylene‐bis(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 Zn2L2. 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 H2MAL1 (1) and H2MAL2 (2) reveals that they have similar geometrical parameters and crystallize in a trans conformation. In the crystal structures of the zinc complexes [Zn2(AC)2]·C6H6 (3), [Zn2(MAL1)2]·0.5C7H8 (4), and [Zn2(MAL2)2]·C7H8 (5), the Schiff base ligands are helically wrapped around the two four‐coordinate metal ions, which leads to an idealized D2 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···Zn varies from 3.465 to 3.628 Å. The total length of the helix is 5.85, 5.84, and 6.01 Å for structures 3, 4, and 5, respectively, while the helical pitch values are in the range 7.8–9.2 Å. Both the ligands H2L and the zinc helicates are thermally stable up to ≈150–160 °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 H2L. The Schiff bases and the Zn2L2 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)