Ferromagnetic Exchange in Bichloride Bridged Cu(II) Chains: Magnetostructural Correlations between Ordered and Disordered Systems

The synthesis, structure,
magnetic properties, and theoretical
analysis of a new phase of dichloro­(2-chloro-3-methylpyridine)­copper­(II)
(2) and its isomorphous analogue dichloro­(2-bromo-3-methylpyridine)­copper­(II)
(3) are reported. Both complexes crystallize in the orthorhombic
space group Pbca and present square pyramidal Cu­(II)
ions bridged into chains by chloride ions with each copper­(II) bearing
a single pyridine ligand. Variable temperature magnetic susceptibility
measurements were well fit by a uniform one-dimensional ferromagnetic
chain model with 2, J = 69.0(7) K, C = 0.487 emu-K/mol-Oe; 3, J = 73.9(4) K, C = 0.463 emu-K/mol-Oe (H = −JΣSi·Sj Hamiltonian). The experimental J-values were confirmed
via theoretical calculations. Comparison to a known disordered polymorph
of dichloro­(2-chloro-3-methylpyridine)­copper­(II), 1,
shows marked differences as there are significant antiferromagnetic
next-nearest neighbor interactions in 1 in addition to
randomness induced by the disorder which provide a distinctly different
magnetic response. The differences in magnetic behavior are attributed
principally to the structural difference in the Cu­(II) coordination
sphere, 1 being significantly closer to trigonal–bipyramidal,
whose difference changes both the nearest and next-nearest neighbor
interactions.