Spin topology and ferromagnetic coupling: Synthesis and magnetic properties of [Cu₃^IICr^III] tetranuclear complexes of [14]N₄ macrocyclic oxamides

Three novel tetranuclear macrocyclic complexes, [(CuL¹)₃Cr](ClO₄)₃ · 3H₂O (1), [(CuL²)₃Cr](ClO₄)₃ · 3H₂O (2) and [(CuL³)₃Cr](ClO₄)₃ · 3H₂O (3) [L¹, L² and L³ are the dianions of three [14]N₄ and [15]N₄ macrocyclic oxamides, namely 2,3-dioxo-5,6:13,14-dibenzo-7,12-bis(ethoxycarbonyl)-1,4,8,11-tetraazacyclotetra deca-7,11-diene, 2,3-dioxo-5,6:13,14-dibenzo-9-methyl-7,12-bis(ethoxycarbonyl)-1,4,8,11-tetraazac yclotetradeca-7,11-diene and 2,3-dioxo-5,6:14,15-dibenzo-7,13-bis(ethoxycarbonyl)-1,4,8,12-tetraazacyclotetra deca-7,12-diene, respectively], have been prepared and characterized. These complexes are the first examples of Cu₃Cr species exhibiting spin topology with all the spins parallel in the ground state. Cryomagnetic studies on (1) (2-300 K) and (3) (77-300 K) revealed that the Cu^II and Cr^III ions interact ferromagnetically through the oxamido bridge, with the exchange integral J = 6.7 cm^-1 for (1) and J = 6.3 cm^-1 for (3) based on Ĥ = -2J ∑ Ŝ_iŜ_j. The ferromagnetic interaction has been rationalized in terms of strict orthogonality of the magnetic orbitals. The oxamido bridge is also more efficient than the oxalato bridge at propagating the ferromagnetic interaction.