Tailor-made copper(ii) coordination polymers based on the C3 symmetric 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.