Magnetic anisotropy of Co^II-W^V ferromagnet: Single crystal and ab initio study

The novel layered {Co^II₃(2,2′-bpdo) ₄(H₂O)₄[W^V(CN)₈] ₂}·8H₂O (1) network (2,2′-bpdo = 2,2′-bipyridine-1,1′-dioxide) is built of 2-D corrugated cyanide-bridged brick-wall skeletons parallel to the bc crystallographic plane. The coordination of chelating 2,2′-bpdo to Co(ii) gives 7-membered coordination rings. Magnetic and heat capacity measurements performed on monocrystalline samples reveal ferromagnetic ordering below T_C of 6 K. Magnetic anisotropy of the ordered phase is represented by the easy axis oriented in the ac crystallographic plane. Bulk magnetic anisotropy is confronted with a local anisotropy of Co complexes: a strong axial anisotropy of trans,cis,cis-[Co^II(μ-NC)₂(H₂O) ₂(2,2′-bpdo)] moieties represented by g_x ⁽²⁾ = 2.06, g_y⁽²⁾ = 2.95 and g_z ⁽²⁾ = 7.30, and a planar anisotropy of trans,cis,cis-[Co ^II(μ-NC)₂(2,2′-bpdo)₂] represented by g_x⁽³⁾ = 5.40, g_y⁽³⁾ = 4.82 and g_z⁽³⁾ = 2.28, determined by CASSCF/RASSI/SINGLE-ANISO ab initio calculations. The experimental data are explained by the molecular field simulation of the magnetic structure using the above theoretical data.