Effects of paramagnetic [Fe(C(5)Me5)(2)](+) cation on the anionic single-molecule magnet, [Mn12O12(O2CC6H4F(-o))(16)(H2O)(4)](-)

The preparation and physical characterization are reported for the single-m olecule magnet salts A[Mn12O12(O2CC6H4F-(-o))(16)(H2O)(4)] (A(+) = PPh4+ (2 a), Fe(C5Me5)(2)(+) (2b), and Co(C5Me5)(2)(+) (2c)), The effects of the mag netic cation on the magnetization relaxation behavior of the [Mn-12](-) ani ons are investigated. All complexes exhibit out-of-phase ac magnetic suscep tibility (chi " (M)) signals in the 4.8-5.1 K range at 1 kHz ac frequency. The temperature of the chi " (M) peaks is frequency dependent, as expected for a single-molecule magnet. From Arrhenius plots of the frequency depende nce of the temperature of the chi " (M) peaks, the effective energy barrier s U-eff for changing the magnetization direction from spin "up" to spin "do wn" were estimated to be in the 52-57 K range. Magnetization hysteresis loo ps were observed for all the complexes studied. They show clear hysteresis loops with steps, indicating the effect of the magnetic cation on the magne tization relaxation of the anionic [Mn-12](-) complex is rather small. The least-squares fittings of variable-field magnetization data show the ground state of complex 2a is best described as S = 21/2 with g = 1.96 and D = -0 .56 K, while complexes 2b and 2c have S = 19/2 ground states. The fitting p arameters are g= 1.96 and D = -0.54 K for complex 2b and g = 1.95 and D = - 0.57 K for complex 2c. These analyses show the magnetic cation has essentia lly no effect on the ground state spin or on the parameters g and D for the [Mn-12](-)anion. (C) 2001 Elsevier Science Ltd. All rights reserved.