Fixation of carbon dioxide by macrocyclic lanthanide(iii) complexes under neutral conditions producing self-assembled trimeric carbonato-bridged compounds with μ3-η2:η2:η2 bonding

Abstract

A series of mononuclear lanthanide(III) complexes [Ln(LH₂)(H₂O)₃Cl](ClO₄)₂ (Ln = La, Nd, Sm, Eu, Gd, Tb, Lu) of the tetraiminodiphenolate macrocyclic ligand (LH₂) in 95 : 5 (v/v) methanol–water solution fix atmospheric carbon dioxide to produce the carbonato-bridged trinuclear complexes [{Ln(LH₂)(H₂O)Cl}₃(μ₃-CO₃)](ClO₄)₄·nH₂O. Under similar conditions, the mononuclear Y^III complex forms the dimeric compound [{Y(LH₂)(H₂O)Cl}(μ₂-CO₃){Y(LH₂)(H₂O)₂}](ClO₄)₃·4H₂O. These complexes have been characterized by their IR and NMR (¹H, ¹³C) spectra. The X-ray crystal structures have been determined for the trinuclear carbonato-bridged compounds of Nd^III, Gd^III and Tb^III and the dinuclear compound of Y^III. In all cases, each of the metal centers are 8-coordinate involving two imine nitrogens and two phenolate oxygens of the macrocyclic ligand (LH₂) whose two other imines are protonated and intramolecularly hydrogen-bonded with the phenolate oxygens. The oxygen atoms of the carbonate anion in the trinuclear complexes are bonded to the metal ions in tris-bidentate μ₃-η²:η²:η² fashion, while they are in bis-bidentate μ₂-η²:η² mode in the Y^III complex. The magnetic properties of the Gd^III complex have been studied over the temperature range 2 to 300 K and the magnetic susceptibility data indicate a very weak antiferromagnetic exchange interaction (J = −0.042 cm⁻¹) between the Gd^III centers (S = 7/2) in the metal triangle through the carbonate bridge. The luminescence spectral behaviors of the complexes of Sm^III, Eu^III, and Tb^III have been studied. The ligand LH₂ acts as a sensitizer for the metal ions in an acetonitrile–toluene glassy matrix (at 77 K) and luminescence intensities of the complexes decrease in the order Eu³⁺ > Sm³⁺ > Tb³⁺.