Single-molecule magnets: A family of MnIII/CeIV complexes with a [Mn8CeO8]12+ core

Abstract

Four heterometallic, enneanuclear Mn8Ce clusters [Mn 8CeO8(O2CMe)12(H2O) 4] (4), [Mn8CeO8(O2CMe) 12(py)4] (5), [Mn8CeO8(O 2CPh)12(MeCN)4] [Mn8CeO 8(O2CPh)12(dioxane)4] (6), and [Mn8CeO8(O2CCHPh2) 12(H2O)4] (7) have been prepared by various methods. Their cores are essentially isostructural and comprise a nonplanar, saddlelike [MnIII8O8]8+ loop containing a central CeIV ion attached to the eight μ3-O2- ions. Peripheral ligation around the [Mn 8CeO8]12+ core is provided by eight μ- and four μ3-O2CR- groups. Terminal ligation on four MnIII atoms is provided by H2O in 4 and 7, pyridine in 5, and MeCN/dioxane in 6. Solid-state magnetic susceptibility studies, fits of dc magnetization vs field and temperature data, and in-phase ac susceptibility studies in a zero dc field have established that complexes 4, 5, and 7 possess S = 16, S = 4 or 5, and S = 6 ± 1 spin ground states, respectively, but in all cases there are very low-lying excited states. The large variation in the ground-state spins for this isostructural family is rationalized as due to a combination of weak exchange interactions between the constituent MnIII atoms, and the presence of both nearest-neighbor and next-nearest-interactions of comparable magnitudes. Magnetization vs applied dc field sweeps on single crystals of 4·4H2O and 7·4H2O·3MeCN·2CH2Cl2 down to 0.04 K have established that these two complexes are new single-molecule magnets (SMMs). The former also shows an exchange-bias, a perturbation of its single-molecule properties from very weak intermolecular interactions mediated by hydrogen-bonding interactions with lattice-water molecules of crystallization. © 2008 American Chemical Society.