A dinuclear Mn^III-Cu^II single-molecule magnet

The reaction of 1/3 equivalent of CuCl₂·2H₂O with MnCl₂·4H₂O and 5-bromo-2-salicylideneamino-1- propanol (H₂5-Br-sap) in methanol gave dark brown crystals of [Mn^III-Cu^IICl(5-Br-sap)₂(MeOH)] (1). Complex 1 has an alkoxo-bridged dinuclear core of Mn^III and Cu^II ions, which have elongated octahedral and square-planar coordination geometries, respectively. In dc magnetic susceptibility measurements, χ_mT values increased as the temperature was lowered, followed by a sudden decrease below 20 K. This behavior is indicative of the occurrence of intramolecular ferromagnetic interactions, and fitting gave an S = 5/2 spin ground state with an exchange coupling constant J_MnCu of + 78 cm^-1. Magnetization data collected as a function of temperature and applied magnetic field were analyzed by using a spin Hamiltonian with isotropic Zeeman and axial zero-field splitting (ZFS) terms, and a negative D_5/2 value (-1.86 cm^-1) was obtained. A high-field EPR (HFEPR) spectrum (342.0 GHz) at 4.2 K was composed of four peaks, and two additional peaks at higher magnetic field appeared as the temperature was increased. The temperature dependences in the HFEPR spectra are indicative of a negative D_5/2 value, and fitting of the data gave D_5/2 = -1.81 cm^-1. In the ac magnetic susceptibility measurements, frequency dependent in-phase (χ_m′) and out-of-phase (χ_m″) signals with peak maxima at 0.7-1.5 K were observed and small peaks below 0.7 K appeared. The ac susceptibility data supports that 1 is a single-molecule magnet (SMM). Arrhenius plots for the χ_m″ peaks from 0.7-1.5 K gave the re-orientation energy barrier (ΔE) of 10.5 K with a pre-exponential factor of 8.2 × 10-8 s.