Dimer species in aqueous solutions of m–phenylenediamine-N,N,N′,N′-tetra-acetic acid (m-H4pdta) with copper(II) and of pyridine-2,6-diamine-N,N,N′,N′-tetra-acetic acid (2,6-H4pydta) with nickel(II). X-Ray crystal structures of Na4[Cu2(m-pdta)2]·18H2O, Na4[Co2(m-pdta)2]·10H2O, and Na4[Ni2(2,6-pydta)2]·8H2O

Abstract

Potentiometric investigations in aqueous solution at 25 °C and ionic strength 0.1 mol dm^–3 KCl have shown that the dimer complex species [Cu₂L₂]^4–(ligand H₄L, m-phenylenediamine-N,N,N,′N′-tetra-acetic acid, m-H₄pdta) and [Ni₂L₂]^4–(ligand H₄L, pyridine-2,6-diamine-N,N,N′,N′-tetra-acetic acid, 2,6-H₄pydta) are present in significant amounts at C_M > 2 × 10^–3 mol dm^–3. The formation constants of the monomer and dimer have been determined. The formation of the dimer [M₂L₂]^4– from the monomer [ML]^2– or [M(HL)]^– is affected by the Jahn–Teller effect (Cu^II–m-H₄pdta) or by the effect of the withdrawal of electronic density by the pyridine nitrogen of the ring (Ni^II–2,6-H₄pydta system). From a concentrated solution with a ligand: metal ratio of 1 : 1 at pH 6, single crystals of the complexes Na₄[Cu₂(m-pdta)₂]·18H₂O, Na₄[Co₂(m-pdta)₂]·10H₂O, and Na₄[Ni₂(2,6-pydta)₂]·8H₂O, respectively, were obtained. X-Ray diffraction structural analysis revealed that the dimer molecules are centrosymmetrical (C_i), with each metal atom surrounded by four carboxylic oxygens and two amine nitrogens in a distorted octahedron, the copper(II) complex being the most distorted (Jahn–Teller effect). In the nickel(II) complex the pyridine nitrogen is not bonded. Each sodium is co-ordinated to water molecules and carboxyl groups, being six-co-ordinated in the copper(II) complex and five-co-ordinated in the complexes of CO^II and Ni^II. The complexes were also characterized by i.r., electronic, and mass spectra, magnetic measurements, and thermogravimetric analysis.