Magnetic coupling detected by EPR in a paddle-wheel copper(II) complex of the amino acid guanidinoacetic acid

Interesting magnetic properties of a paddle-wheel carboxylate bridged copper(II) complex of guanidinoacetic acid (GAA), Cu2(GAA)4(NO3)2 (tetrakis(μ-guanidinoacetic acid-κ2O:O′) bis[(nitrato-κO) copper(II)]) (I) were detected by electron paramagnetic resonance (EPR). The complex I contains two copper ions coordinated to four GAA ligands (d Cu–Cu 2.65 Å) through the oxygen atoms from the carboxylate group forming a paddle-wheel complex and a supramolecular assembly. The EPR data (40–300 K) revealed two distinct groups of lines. One of them is related to the intra-dimer coupling, an anti-ferromagnetic interaction between the copper ions. The other group of lines, which was only observed above 180 K, can be attributed to the inter-dimer coupling or pair formation.