Antiferromagnetic exchange and long-range magnetic ordering in supramolecular networks constructed of hexacyanido-bridged LnIII(3-pyridone)–CrIII (Ln = Gd, Tb) chains

Abstract

The complexes of paramagnetic lanthanide(3+) ions (Gd³⁺,Tb³⁺) with 3-pyridone molecules combined in an aqueous solution with paramagnetic hexacyanidochromate(III) metalloligands led to the formation of two novel crystalline {[Ln^III(3-pyridone)₂(H₂O)₄][Cr^III(CN)₆]}·H₂O (Ln = Gd, 1; Ln = Tb, 2) materials. They are composed of bimetallic Ln–Cr cyanido-bridged zig-zag chains which are further tightly connected by the rich system of hydrogen bonds involving terminal cyanides, water molecules and 3-pyridone ligands. Both 1 and 2 have a very weak yellowish colour as their strong absorption, related to the d–d transitions of the cyanide Cr(III) complex and 3-pyridone ligands, is moved to the UV range. 1 and 2 exhibit Ln–Cr antiferromagnetic exchange mediated within the coordination chains by molecular cyanide bridges. The magnetic susceptibility for the Gd^III–Cr^III 7/2–3/2 spin alternating chains of 1 was successfully analysed using quantum Monte Carlo simulations, giving a magnetic coupling constant, J, of −0.82 K. The additional interchain magnetic interactions ensured by the hydrogen bonding network result in the low temperature phase transition to the magnetically ordered state. Magnetic heat capacity measurements showed that 1, which has isotropic Gd(III) spin centers, becomes a molecule-based magnet below 1.06 K, while 2, bearing the highly anisotropic Tb(III), shows a higher critical temperature of 1.61 K. Thus, we show and discuss the crucial role of the non-covalent interactions in the induction of magnetic ordering for low dimensional cyanido-bridged coordination systems, and the positive impact of magnetic anisotropy on the strengthening of magnetic interactions.