Tuning of Magnetic Properties of Polynuclear Lanthanide(III)−Octacyanotungstate(V) Systems: Determination of Ligand-Field Parameters and Exchange Interaction

Abstract

The self-assembly reaction between trivalent lanthanide ions, 2,2‘:6‘,2‘ ‘-terpyridine (terpy) ligand, and octacyanotungstate(V) leads to the formation of two series of isomorphous cyano-bridged compounds: (i) one-dimensional (1-D) chains [Ln(terpy)(DMF)4][W(CN)8]·6H2O·C2H5OH (Ln = Ce−Dy) and (ii) dinuclear molecules [Ln(terpy)(DMF)2(H2O)2][W(CN)8]·3H2O (Ln = Ho, Er, Yb) and the ionic [TmIII(terpy)(DMF)2(H2O)3][WV(CN)8]·4H2O·DMF (DMF = N,N-dimethylformamide) system. The crystal structures of 1-D chains consist of alternating {[W(CN)8]} and {[Ln(terpy)]} building blocks. The neighboring chains are weakly linked through the π−π stacking interactions of the aromatic rings, leading to two-dimensional supramolecular layers. The dinuclear species are weakly linked through the hydrogen bonds between H2O molecules and terminal cyano ligands resulting in a columnlike arrangement of dimers. Taking into account the ligand-field splitting and the exchange interaction, we have estimated the magnetic couplings between the LnIII and WV centers in a series of polycrystalline 1-D chains and in dimeric systems. The corresponding exchange constants have been shown to change the sign along the series of chains. The coupling is antiferromagnetic for 1 (J = −0.24 cm-1) and 2 (J = −0.07 cm-1), whereas 3 (J = +0.47 cm-1), 7 (J = +0.28 cm-1), and 8 (J = +0.23 cm-1) have ferromagnetic character. In the case of dimeric systems, the coupling constants seem to be independent of the lanthanide center. The splitting structures of the ground-state multiplets of the LnIII centers have been shown to explain the temperature dependences of the magnetic susceptibilities.