Record Ferromagnetic Exchange through Cyanide and Elucidation of the Magnetic Phase Diagram for a CuIIReIV(CN)2 Chain Compound

Reaction of the high-magnetic anisotropy building unit [ReCl4(CN)2]2− with [Cu(MeCN)6]2+ and hydrotris(pyrazol-1-yl)borate (Tp−) affords the zigzag chain compound (Bu4N)[TpCuReCl4(CN)2]. Dc magnetic susceptibility measurements reveal the presence of ferromagnetic exchange coupling between ReIV and CuII centers along each chain and a fit to the data gives an exchange constant of J/kB = +41 K (+29 cm−1), representing the strongest ferromagnetic coupling yet observed through cyanide. Below 11.4 K and at applied fields of less than 3600 Oe, the compound undergoes a phase transition to an antiferromagnetic ground state, stemming from weak π−π interchain interactions of strength J⊥/kB = −1.7 K (−1.2 cm−1). This metamagnetic behavior is fully elucidated using both experimental and theoretical methods. In addition, theoretical modeling provides a detailed determination of the local anisotropy tensors corresponding to the [ReCl4(CN)2]2− units and demonstrates that the zigzag arrangement of the ReIV centers significantly reduces the effective anisotropy of the chain. These results demonstrate the utility of the ReIV−CN−CuII linkage and the importance of anisotropic spin orientation in designing strongly coupled systems, which will aid in both the realization of single-chain magnets with higher relaxation barriers and in the construction of high-dimensional cyano-bridged materials exhibiting higher ordering temperatures.