Supramolecular charge transfer adducts of rare earth 3,5-dinitrobenzoates and diaminodurene: a new approach to increasing spin density in lanthanide complexes

Seven isostructural supramolecular adducts, [Ln₂(O₂CC₆H₃(NO₂)₂)₆(DMSO)₄]·4(1,4-(H₂N)₂C₆Me₄) (Ln = Sm (1), Gd (2), Tb (3), Dy (4), Ho (5), Er (6), Y (7)), were synthesized by reacting LnCl₃·6H₂O with potassium 3,5-dinitrobenzoate in acetonitrile in the presence of 2,3,5,6-tetramethyl-1,4-phenylenediamine (DAD) and DMSO, and characterized by X-ray diffraction analysis. The charge transfer (CT) between DAD molecules and binuclear 3,5-dinitrobenzoate fragments gives rise to stacking interactions, which determine the supramolecular structures of complexes 1-7. Optical spectroscopy of complexes 1-7 corroborates the occurrence of significant CT, whereas magnetic studies substantiate the presence of a paramagnetic ion-radical structure which contributes to the magnetic moment of all the complexes and determines the paramagnetism of the yttrium compound 7. In the case of the latter complex, the value of the paramagnetic contribution resulting from CT was determined directly by magnetic measurement. It was demonstrated that this contribution decreases with the lowering of temperature, reflecting the depopulation of the triplet state of the CT complex, the ion-radical pair. A comprehensive EPR study of complex 7 was carried out by means of both continuous-wave (CW) and pulsed EPR spectroscopy in X- and Q-bands. The magnetic properties of complexes 2-6 indicate the prevalence of weak antiferromagnetic interactions within the binuclear fragments. The Dy complex exhibits field-induced single-molecule magnet (SMM) behaviour. The CT in the complex structures was modelled using DFT calculations.