Issue 14, 2004

Tren centered tris-macrocycles as polytopic ligands for Cu(ii) and Ni(ii)

Abstract

Two novel symmetric polytopic ligands L1 and L2 have been synthesized. They are composed of three 1,4,8,11-tetraazacyclotetradecane macrocycles which are connected to a central tren moiety via an ethylene and a trimethylene bridge, respectively. The complexation potential and the speciation diagrams of L1 and L2 towards Cu2+ and Ni2+ were determined by spectrophotometric and potentiometric titrations. Insight into the geometry of the Cu2+ complexes is provided by UV-VIS and EPR spectroscopy. The simplified ligands L3 and L4 are utilized as references for an aminoethyl- and a tren-substituted tetraaza macrocycle to help assign the EPR spectra of the polytopic ligands L1 and L2. At a metal-to-ligand ratio of 3 : 1, the metal cations are preferentially bound to the tetraaza macrocycles of L1 and L2 in a square planar geometry. At high pH values, a nitrogen atom of the tren moiety in L1 serves as an additional ligand in an axial position leading to a square pyramidal coordination around Cu2+, whereas in L2 no such geometry change is observed. At a metal-to-ligand ratio of 4 : 1, the additional metal cation resides in the central tren moiety of L1 and L2. However, in contrast to the typical trigonal bipyramidal geometry found in the [Cutren]2+ complex, the fourth Cu2+ has a square pyramidal coordination caused by the interaction with the Cu2+ cations in the macrocycles (as evidenced by EPR spectra). Since the sequence of metal complexation is such that the first three metal ions always bind to the three macrocycles of L1 and L2 and the fourth to the tren unit, it is possible to prepare heteronuclear complexes such as [Cu3NiL]8+ or [Ni3CuL]8+, which can be unambiguously identified by their spectral properties.

Graphical abstract: Tren centered tris-macrocycles as polytopic ligands for Cu(ii) and Ni(ii)

Article information

Article type
Paper
Submitted
10 Feb 2004
Accepted
13 May 2004
First published
28 May 2004

Dalton Trans., 2004, 2115-2124

Tren centered tris-macrocycles as polytopic ligands for Cu(II) and Ni(II)

L. Siegfried, C. N. McMahon, T. A. Kaden, C. Palivan and G. Gescheidt, Dalton Trans., 2004, 2115 DOI: 10.1039/B402089H

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