Magnetic properties of carboxylate-bridged ferromagnetic copper(II) chains coupled by cation-π interactions

The exchange interactions between copper ions in (L-tryptophyl-glycinato)copper(II), C13H13CuN3O3 (named Cu(II)Trp-Gly) are determined by ac magnetic susceptibility and de magnetization measurements in powder samples, and by EPR measurements in single-crystal samples. The copper ions in this compound are arranged in two symmetry-related types of chains along the b axis. Neighbor coppers in a chain at 5.14 Angstrom are connected by equatorial syn-anti carboxylate bridges, which transmit the intrachain exchange interactions. Indole rings of neighbor tryptophan residues at apical positions of the copper ions provide cation-pi contacts that give rise to pathways supporting exchange interactions between the chains. The susceptibility and magnetization data obtained in the temperature range 1.8 < T < 100 K, with applied dc magnetic fields (B) up to 9 T are explained assuming one-dimensional spin chains with ferromagnetic intrachain coupling 2J/k = 3.9 +/- 0.1 K, calculated from the zero-field susceptibility. The magnetization and ac susceptibility with applied magnetic field calculated with this value agree qualitatively with the experimental results. Quantitative discrepancies between experimental and calculated ac susceptibility, observed at low rand intermediate B are attributed to anisotropies of the intra- and interchain interactions. The g factor and line width of the single EPR line observed were measured in single crystals at 9.5 and 35 GHz. The angular variation of the g factor is analyzed in terms of the electronic properties. The angular variation of the line width allows us to evaluate the interchain exchange interactions, \ 2J ' \ /k = 0.061 +/- 0.002 K. We discuss the values of J and J ' in terms of the structures of the carboxylate bridges (J) and the cation-pi interactions (J '). This is the first time that a small magnetic interaction transmitted through a cation-pi interaction between a metal ion and an aromatic ring can be identified and evaluated.