Synthesis, structural features, magnetic properties and thermal decomposition of a new series of polymeric Ln(iii)–Cr(iii) cyclopropane-1,1-dicarboxylates

Abstract

The reactions of chromium(III) nitrate with M₂(cpdc) (cpdc²⁻ – dianion of cyclopropane-1,1-dicarboxylic acid, M = K, Na) and rare-earth metal nitrates taken in a molar ratio of 1 : 3 : 1 yielded heterometallic coordination polymers {[Ln₂Cr₂(cpdc)₆(H₂O)₉]·8.5H₂O}_n (1_Ln, Ln = Eu, Gd, Tb), crystallizing in tetragonal space group P4₃2₁2 (P4₁2₁2). A single crystal X-ray diffraction (XRD) study showed that the crystal structures of these compounds are formed by tris-chelate fragments {Cr(cpdc)₃} linked into {Ln–Cr} layers and further into a 3D framework structure due to the bridging coordination of carboxylate groups to Ln atoms. When passing to Ln = Dy, Ho, Y, Er, Yb, the crystals of compounds 2_Ln form, where the structure of {Ln–Cr} layers is retained, but the disorder of the interlayer Ln atoms, a decrease in the size of a crystal cell, and a change in the space group to I₄ are observed. The use of K₂(cpdc) in the syntheses of 1_Ln and 2_Ln is found to lead to the isolation of a side product, namely, mononuclear complex [Cr(cpdc)(Hcpdc)(H₂O)₂]·2H₂O (3), with the reactions with Na₂(cpdc) enabling obtaining only the crystals of heterometallic Ln^III–Cr^III compounds. Alternating current magnetic susceptibility measurements for the series 1_Ln and 2_Ln revealed the presence of slow relaxation of magnetization only for 1_Eu and 2_Y, which is due to the magnetic anisotropy of chromium ions in the presence of magnetic field and their magnetic isolation in a crystal lattice. For 1_Eu, 1_Gd, 1_Tb, 2_Dy, and 2_Ho, thermal decomposition in an Ar atmosphere was studied. The end product of solid-state thermolysis of 2_Dy in air at 1000 °C is shown to be submicron DyCrO₃ particles (300–400 nm).