Ferromagnetic Coupling in a 1D Coordination Polymer Containing a Symmetric [Cu(μ_1,1-N₃)₂Cu(μ_1,1-N₃)₂Cu]²⁺ Core and Based on an Organic Ligand Obtained from the Solid State

Addition of rctt-tetrakis(2-pyridyl)cyclobutane (2,2‘-tpcb) in a CuII/N3- solution afforded the 1D coordination polymer [Cu3(N3)6(2,2‘-tpcb)(DMF)2]n (1). The ligand 2,2‘-tpcb serves as a tetradentate bis-chelating ligand by linking linear [(DMF)Cu(μ1,1-N3)2Cu(N3)2(μ1,1-N3)2Cu(DMF)] trinuclear units to produce a zigzag chain. Within each centrosymmetric trinuclear unit there exist two irregularly asymmetric end-on double azido-bridged [Cu(μ1,1-N3)2Cu]2+ cores, while one of the largest Cu−Nazide−Cu angles is observed. Magnetic susceptibility data, measured from 2 to 300 K, show bulk moderate ferromagnetic coupling within the magnetically isolated trinuclear units. These data were fitted to the appropriate equation derived from the Hamiltonian H = −J1(SA1SB + SA2SB) − J2SA1SA2, giving the parameters J1 = +70(3) cm-1, J2 = −3(2) cm-1, g = 2.12(1), with an intertrimer interaction parameter θ = −0.74(2) K. The coupling constants were correlated with the structural parameters. Density functional calculations reproduce very well the experimental J values and show that ferromagnetism for this complex is mainly due to the topology of the magnetic orbitals and the different coordination spheres of two neighboring CuII atoms, resulting in a small overlap of the orbitals possessing the unpaired electrons.