Single-molecule magnetism behavior of [Mn12O12(O2CR)16(H2O)4]2- salts

(PPh4)(2)[Mn12O12(O2CCHCl2)(16)(H2O)(4)] (3) has been prepared by the two-electron reduction of [Mn12O12(O2CCHCl2)(16)(H2O)(4)] (2) using iodide. Crystallization from CH2Cl2/hexanes yields a mixture of two crystal forms, 3.4CH(2)Cl(2) . H2O (3a) and 36CH(2)Cl(2) (3b), which are triclinic and monoclinic, respectively. They are both trapped valence 2Mn(II), 6Mn(III), 4Mn(IV). DC magnetization data for dried, unsolvated 3 in 1.80-4.00 K and 10-70 kG ranges were fit to give S = 10, D = -0.28 cm(-1), g = 2.00 Frequency-dependent out-of-phase (chi(M)'') signals in AC susceptibility studies on crystalline sample of 3a and 3b combined with DC relaxation decay data were fit to the Arrhenius equation to give an effective energy barrier of U-eff = 18.5 and 30.3 K, respectively. Magnetization vs. DC field sweeps on single crystals of 3a and 3b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded \D\/g values of 0.087 and 0.14 cm(-1), and consequently U = 20 and 39 K (for g = 2) for 3a and 3b, respectively, suggesting that the differences in U-eff are primarily caused by changes to D. This work demonstrates the sensitivity of the magnetic properties of [Mn-12](2-) single-molecule magnets to subtle differences in their environment. (C) 2003 Elsevier Science Ltd. All rights reserved.