Tailor-made copper(ii) coordination polymers based on the C3symmetric methanetriacetate as a ligand

Abstract

The construction of coordination polymers based on flexible ligands in a pre-designed manner is a difficult task which needs deep knowledge of the ligands involved. With this objective in mind, we have investigated the complex formation between methanetriacetic acid (H3mta) and copper(II) in the presence of 2,2′-bipyridine (bpy) as a blocking coligand to limit the number of coordination sites available at the metal ion. In this primary target, five complexes of formulae [Cu3(bpy)6(mta)](ClO4)3·2H2O (1), [Cu4(bpy)6(mta)2(H2O)2](ClO4)2·20H2O (2), [Cu(bpy)(Hmta)]n (3), {[Cu4(bpy)4(mta)2](ClO4)2·3H2O}n (4) and {[Cu3(mta)2(bpy)3(H2O)]3·3dmf·13H2O}n (5) were synthesized and their crystal structures were solved by single crystal X-ray diffraction. Compounds 1 and 2 are tri- and tetranuclear species, respectively, whereas 3 and 4 are one-dimensional compounds and 5 is a three-dimensional coordination polymer. This series of complexes reveals that methanetriacetate behaves as a tritopic ligand which keeps its C3 symmetry despite its flexibility. On the basis of the structural knowledge of 1–5, a (6,3)-honeycomb network of formula {[Cu12(tppz)6(mta)4(H2O)24](NO3)12·66H2O}n (6) [tppz = tetrakis(2-pyridyl)pyrazine] was prepared by design where each methanetriacetate ligand acts as a three-connected node. Magnetic susceptibility measurements in the temperature range 2.0–300 K for 1–6 show the occurrence of very weak antiferro- (1 and 2) and ferromagnetic (3–5) interactions, whereas an intermediate antiferromagnetic coupling (J = −43 cm−1, the spin Hamiltonian being defined as H = −JS1·S2) is observed for 6. The different size and nature of the magnetic interactions in 1–6 are analyzed and discussed in the light of the respective exchange pathways involved.