Record ferromagnetic exchange through cyanide and elucidation of the magnetic phase diagram for a Cu^IIRe^IV(CN)₂ Chain compound

Reaction of the high-magnetic anisotropy building unit [ReCl ₄(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 Re^IV and Cu^II centers along each chain and a fit to the data gives an exchange constant of J/k_B = +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 ₁/k_B = -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 Re ^IV centers significantly reduces the effective anisotropy of the chain. These results demonstrate the utility of the Re^IV-CN-Cu ^II 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.