Effects of Paramagnetic Ferrocenium Cations on the Magnetic Properties of the Anionic Single-Molecule Magnet [Mn12O12(O2CC6F5)16(H2O)4]-

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Abstract
The preparation and physical characterization are reported for the single-molecule magnet salts [M(Cp‘)2]n[Mn12O12(O2CC6F5)16(H2O)4] (M = Fe, n = 1, Cp‘ = C5Me5 (2a), C5H5 (2b); M = Co, n = 1, Cp‘ = C5Me5 (2c), C5H5 (2d); M = Fe, n = 2, Cp‘ = C5Me5 (2e), C5H5 (2f)) to investigate the effects of paramagnetic cations on the magnetization relaxation behavior of [Mn12]- anionic single-molecule magnets. Complex 2a·2H2O crystallizes in the orthorhombic space group Aba2, with cell dimensions at 173 K of a = 25.6292(2) Å, b = 25.4201(3) Å, c = 29.1915(2) Å, and Z = 4. Complex 2c·2CH2Cl2·C6H14 crystallizes in the monoclinic space group P21/c, with cell dimensions at 173 K of a = 17.8332(6) Å, b = 26.2661(9) Å, c = 36.0781(11) Å, β = 92.8907(3)°, and Z = 4. These two salts consist of either paramagnetic [Fe(C5Me5)2]+ cations or diamagnetic [Co(C5Me5)2]+ cations, and [Mn12O12(O2CC6F5)16(H2O)4]- anions. The structures of the anions in the two salts are similar, consisting of a central Mn4O4 cubane moiety, surrounded by a nonplanar ring of eight Mn atoms that are bridged by and connected to the cube via μ3-O2- ions. The oxidation states of four Mn sites out of eight outer Mn ions in complex 2a were assigned to be +2.75 from the valence bond sum analysis although the disordering of bridging carboxylates prevents more precise determination. On the other hand in complex 2c, one Mn site out of eight outer Mn ions was identified as a MnII ion, accommodating the “extra” electron; this was deduced by a valence bond sum analysis. Thus, the anion in complex 2c has a MnII1MnIII7MnIV4 oxidation state description. The Jahn−Teller axes of the MnIII ions in both anions are roughly aligned in one direction. All complexes studied exhibit a single out-of-phase ac magnetic susceptibility (χ‘ ‘M) signal in the 4.6−4.8 K range for complexes 2a−2d and in the 2.8−2.9 K range for complexes 2e and 2f at 1 kHz ac frequency. The temperature of the χ‘ ‘M peaks is frequency dependent, as expected for single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the χ‘ ‘M maxima, the effective energy barriers Ueff for changing spin from “up” to spin “down” were estimated to be 50−54 K for complexes 2a−2d and 27−28 K for complexes 2e and 2f. The least-squares fits of the reduced magnetization data indicate that both complexes 2a and 2d have ground states of S = 21/2. High-frequency EPR spectra were recorded for complex 2a at frequencies of 217, 327, and 434 GHz in the 4.5−30 K range. The observed transition fields were least-squares fit to give g = 1.91, D = −0.35 cm-1, and B40 = −3.6 × 10-7 cm-1 for the S = 21/2 ground state. The effective energy barrier Ueff is slightly lower than U estimated from D, which is consistent with the thermally assisted tunneling model. Magnetization hysteresis loops were observed for complexes 2a and 2c. Although 2a was oriented in a different manner as expected by strong magnetic field, both complexes show clear hysteresis loops with some steps on them, indicating that the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn12]- complex is rather small. An 11% 57Fe enriched complex 2b was studied by means of Mössbauer spectroscopy down to as low as 1.7 K. Slow paramagnetic relaxation broadening and magnetic hyperfine splitting were evident in the low-temperature spectra, indicating that the iron atoms feel a growing magnetic field owing to slow magnetization reversal in the [Mn12]- anions.