Slow Magnetic Relaxation Induced by a Large Transverse Zero-Field Splitting in a Mn^IIRe^IV(CN)₂ Single-Chain Magnet

The model compounds (NBu₄)₂[ReCl₄(CN)₂] (1), (DMF)₄ZnReCl₄(CN)₂ (2), and [(PY5Me₂)₂Mn₂ReCl₄(CN)₂]­(PF₆)₂ (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)₄MnReCl₄(CN)₂ (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1–3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the Re^IV centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.