The rationalized pathway from field-induced slow magnetic relaxation in Co-II-W-IV chains to single-chain magnetism in isotopological Co-II-W-V analogues

Single-chain magnets (SCMs) and single-molecule magnets (SMMs), and their sub-class of single-ion magnets (SIMs) exhibit slow relaxation of magnetization leading to the magnetic hysteresis loop applicable for high-density data storage. We report an efficient method to generate the slow magnetic relaxation in bimetallic {[Co-II(bpp)(H2O)](2)[W-IV(CN)(8)])center dot 6H(2)O (1) (bpp = 2,6-di(1-pyrazolyl)pyridine) coordination chains and to modify them into SCMs by replacing diamagnetic W(IV) with paramagnetic W(V) in the isotopological ([Co-II(bpp)X-0.5(H2O)(0.5)](2)(W-V(CN)(8)])center dot 2H(2)O (X = CI (2), Br (3)) systems. 1-3 are the chains of vertex-sharing squares differing in the oxidation state of W centres that lead to the partial exchange of coordinated water with halogeno ligands in 2-3. All of them incorporate octahedral high-spin Co(II) complexes with tridentate bpp ligands producing easy-axis-type magnetic anisotropy. In 1, it results in the field-induced slow magnetic relaxation below 3 K related to paramagnetic Co(II) centres separated by diamagnetic W(IV) complexes. In 2 and 3, ferromagnetic Co(II)-W(V) coupling leads to the SCM effect and the magnetic hysteresis loops below 7 K, with the coercive fields of 700 and 450 Oe at T = 1.8 K, for 2 and 3, respectively. The intrachain magnetic coupling is accompanied by non-negligible anti-ferromagnetic interchain correlation which results in the metamagnetism of 2 and 3 showing the field-induced transition from an antiferromagnetic phase with T-N of 9.5 K (2) or 9.4 K (3) to a ferromagnetic phase observed at 2180 Oe (2) or 2050 Oe (3) at 2 K. The appearance of the SCM effect upon transition from Co(II)-W(IV) to Co(II) W(V) chains was rationalized by the results of the ab initio calculations of the Co-II crystal field which were employed in modelling the experimental magnetic properties. For 1, the magneto-structural models of a 12-membered zig-zag chain or ring well reproduce the dc data indicating the weak antiferromagnetic coupling (J . = -0.6 cm(-1)) between Co(II) centres giving the exchange states that precludes the Orbach relaxation. In 2 and 3, the model of a 6-membered chain shows the best fit to the dc data giving the ferromagnetic Co(II)-W(V) exchange of J = 25.1 cm(-1). The calculated exchange states indicate the SCM energy barriers of 54.56 and 43.67 cm(-1) for 2 and 3, respectively, which agrees with the experimental trend of 45.4(7) cm(-1) (2) and 41.5(4) cm(-1) (3), confirming stronger magnetic anisotropy in 2.