Single-Molecule Magnets: A New Class of Tetranuclear Manganese Magnets
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- 作者：Jae Yoo ; Euan K. Brechin ; Akira Yamaguchi ; Motohiro Nakano ; John C. Huffman ; A. L. Maniero ; Louis-Claude Brunel ; Kunio Awaga ; Hidehiko Ishimoto ; George Christou ; and David N. Hendrickson
- 刊名：Inorganic Chemistry
- 出版年：2000
- 出版时间：August 7, 2000
- 年：2000
- 卷：39
- 期：16
- 页码：3615 - 3623
- 全文大小：183K
- 年卷期：v.39,no.16(August 7, 2000)
- ISSN：1520-510X
文摘
The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1·2MeCN·Et2O crystallizes in the triclinic spacegroup P
acr.gif">, with cell dimensions at 130 K of a = 11.914(3) Å, b = 15.347(4) Å, c = 9.660(3) Å, 
= 104.58(1)
,
= 93.42(1), 
= 106.06(1), and Z = 1. The cation lies on an inversion center and consists of a planar Mn4rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function aseither bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonatedoxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile andether solvate molecules to give complex 1, which with time becomes hydrated to give 1·2.5H2O. Direct currentand alternating current magnetic susceptibility data are given for 1 and 1·2.5H2O and indicate that the desolvatedcomplex has a S = 8 ground state, whereas the hydrated 1·2.5H2O has a S = 9 ground state. Ferromagneticinteractions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII andS = 2 MnIII ions. High-frequency EPR spectra were run for complex 1·2.5H2O at frequencies of 218, 328, and436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structureis clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99,D = -0.451 K, and B4 = 2.94 × 10-5 K for the S = 9 ground state of 1·2.5H2O. A molecule with a large-spinground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magneticsusceptibility. Out-of-phase ac signals (
' 'M) were seen for complexes 1 and 1·2.5H2O to show that these complexesare single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the acfield oscillating at frequencies in the 1.1-1000 Hz range. A single peak in ' 'M vs temperature plots was seen foreach frequency; the temperature of the ' 'M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetizationrelaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, asexpected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization directionreversal for an S = 8 complex with D = -0.35 K. The 1·2.5H2O complex with an S = 9 ground state has its ' 'Mpeaks at higher temperatures.
acr.gif">, with cell dimensions at 130 K of a = 11.914(3) Å, b = 15.347(4) Å, c = 9.660(3) Å, = 104.58(1), = 93.42(1), = 106.06(1), and Z = 1. The cation lies on an inversion center and consists of a planar Mn4rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function aseither bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonatedoxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile andether solvate molecules to give complex 1, which with time becomes hydrated to give 1·2.5H2O. Direct currentand alternating current magnetic susceptibility data are given for 1 and 1·2.5H2O and indicate that the desolvatedcomplex has a S = 8 ground state, whereas the hydrated 1·2.5H2O has a S = 9 ground state. Ferromagneticinteractions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII andS = 2 MnIII ions. High-frequency EPR spectra were run for complex 1·2.5H2O at frequencies of 218, 328, and436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structureis clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99,D = -0.451 K, and B4 = 2.94 × 10-5 K for the S = 9 ground state of 1·2.5H2O. A molecule with a large-spinground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magneticsusceptibility. Out-of-phase ac signals (' 'M) were seen for complexes 1 and 1·2.5H2O to show that these complexesare single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the acfield oscillating at frequencies in the 1.1-1000 Hz range. A single peak in ' 'M vs temperature plots was seen foreach frequency; the temperature of the ' 'M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetizationrelaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, asexpected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization directionreversal for an S = 8 complex with D = -0.35 K. The 1·2.5H2O complex with an S = 9 ground state has its ' 'Mpeaks at higher temperatures.