Magnetic molecular rectangles constructed from functionalized nitronyl-nitroxide ligands and lanthanide(III) ions

Abstract

Five binuclear complexes with rectangular topology have been synthesized using a nitronyl-nitroxide ligand (L) functionalized with a pyrazole coordinating group: [Ln2(hfac)2L2], with Ln=Eu (1), Gd (2 a, 2 b), Tb (3), Dy (4), Tm (5). The EuIII and TmIII complexes crystallize in the P21/n space group, while the GdIII, TbIII, and DyIII derivatives crystalize in P-1 space group. The crystal structures for compounds 2 a, 3, 4, and 5 have been fully solved, while for compounds 1 and 2 b, the unit cell parameters have been measured and compared with crystals 3 and 5. Crystals 2 a (P21/n) and 2 b (P-1) are polymorphic forms. In these complexes the nitronyl-nitroxide ligand bridges two metal ions through one aminoxyl group and the pyrazole fragment. In the five complexes the LnIII ions show a coordination number of eight with a triangular dodecahedron geometry. The magnetic properties of all five complexes have been investigated. For compounds 2, 3, and 4 the LnIII-Rad interaction was found to be ferromagnetic: JGdRad=2.09(1) cm−1 [H=-2JGdRad(SGd1SRad1+SGd2SRad2], JTbRad=1.93(14) cm−1, JDyRad=1.72(14) cm−1. The TmIII-Rad coupling is antiferromagnetic (JTmRad=−0.53(1) cm−1). For the TbIII, DyIII, and TmIII complexes the effects of the crystal field have been taken into account in the fitting procedure.