Effects of paramagnetic ferrocenium cations on the magnetic properties of the anionic single-molecule magnet [Mn12O12(O2CC6F5)(16)(H2O)4](-)

The preparation and physical characterization are reported for the single-m olecule mag et salts [M(CP ')(2)](n)- [Mn12O12(O2CC6F5)(16)(H2O)(4)] (M = F e, n = 1, Cp ' = C5Me5 (2a), C5H5 (2b); M = Co, n = 1, Cp ' = C5Me5 (2c), C 5H5 (2d); M = Fe, n = 2, Cp ' = C5Me5 (2e), C5H5 (2f)) to investigate the e ffects of paramagnetic cations on the magnetization relaxation behavior of [Mn-12](-) anionic single-molecule magnets. Complex 2a . 2H(2)O crystallize s in the orthorhombic space group Aba2, with cell dimensions at 173 K of a = 25.6292(2) Angstrom, b = 25.4201(3) Angstrom, c = 29.1915(2) Angstrom, an d Z = 4. Complex 2c . 2CH(2)Cl(2).C6H14 crystallizes in the monoclinic spac e group P2(1)/c, with cell dimensions at 173 K of a = 17.8332(6) Angstrom, b = 26.2661(9) Angstrom, c = 36.078 1 (11) Angstrom, beta = 92.8907(3)degre es, 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 Mn404 cubane moiety, surrounded by a nonp lanar ring of eight Mn atoms that are bridged by and connected to the cube via mu (3)-O2- ions. The oxidation states of four Mn sites out of eight out er Mn ions in complex 2a were assigned to be +2.75 from the valence bond su m 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 Mull ion, accommodating the "extra" electron; this was deduced by a valence bond sum analysis. Thus, the anion in complex 2c has a (Mn1Mn7Mn4IV)-Mn-II-Mn-III oxidation state description . The Jahn-Teller axes of the Mn-III ions in both anions are roughly aligne d in one direction. All complexes studied exhibit a single out-of-phase ac magnetic susceptibility (chi "M) signal in the 4.6-4.8 K range for complexe s 2a-2d and in the 2.8-2.9 K range for complexes 2e and 2f at I kHz ac freq uency. The temperature of the chi "M peaks is frequency dependent, as expec ted for single-molecule magnets. From Arrhenius plots of the frequency depe ndence of the temperature of the chi "M maxima, the effective energy barrie rs U-eff 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-sq uares fits of the reduced magnetization data indicate that both complexes 2 a and 2d have ground states of S = 21/2. High-frequency EPR spectra were re corded 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 B-4(0) -3.6 x 10(-7) cm(-1) for the S = 21/2 g round state. The effective energy barrier U-eff is slightly lower than U es timated from D, which is consistent with the thermally assisted tunneling m odel. Magnetization hysteresis loops were observed for complexes 2a and 2c. Although 2a was oriented in a different manner as expected by strong magnet ic field, both complexes show clear hysteresis loops with some steps on the m, indicating that the effect of the magnetic cation on the magnetization r elaxation of the anionic [Mn-12](-) complex is rather small. An 11% Fe-57 e nriched complex 2b was studied by means of Mossbauer spectroscopy down to a s low as 1.7 K. Slow paramagnetic relaxation broadening and magnetic hyperf ine splitting were evident in the low-temperature spectra, indicating that the iron atoms feel a growing magnetic field owing to slow magnetization re versal in the [Mn-12](-) anions.