The rationalized pathway from field-induced slow magnetic relaxation in CoII–WIV chains to single-chain magnetism in isotopological CoII–WV analogues†
Abstract
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 {[CoII(bpp)(H2O)]2[WIV(CN)8]}·6H2O (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 {[CoII(bpp)X0.5(H2O)0.5]2[WV(CN)8]}·2H2O (X = Cl (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 antiferromagnetic interchain correlation which results in the metamagnetism of 2 and 3 showing the field-induced transition from an antiferromagnetic phase with TN 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 CoII 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.