Mixed-Valence Tetranuclear Manganese Single-Molecule Magnets
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- 作者：Jae Yoo ; Akira Yamaguchi ; Motohiro Nakano ; J. Krzystek ; William E. Streib ; Louis-Claude Brunel ; Hidehiko Ishimoto ; George Christou ; and David N. Hendrickson
- 刊名：Inorganic Chemistry
- 出版年：2001
- 出版时间：August 27, 2001
- 年：2001
- 卷：40
- 期：18
- 页码：4604 - 4616
- 全文大小：178K
- 年卷期：v.40,no.18(August 27, 2001)
- ISSN：1520-510X
文摘
The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn4(hmp)6Br2(H2O)2]Br2·4H2O (2) and [Mn4(6-me-hmp)6Cl4]·4H2O (3), where hmp- is the anion of 2-hydroxymethylpyridineand 6-me-hmp- is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2·4H2O crystallizes in the spacegroup P21/c, with cell dimensions at -160 
C of a = 10.907(0) Å, b = 15.788(0) Å, c = 13.941(0) Å, 
=101.21(0), and Z = 2. The cation lies on an inversion center and consists of a planar Mn4 rhombus that ismixed-valence, Mn2IIIMn2II. The hmp- ligands function as bidentate ligands and as the only bridging ligands in2·4H2O. Complex 3·4H2O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 C ofa = 17.0852(4) Å, b = 20.8781(5) Å, c = 14.835(3) Å, = 90.5485(8), and Z = 4. This neutral complex alsohas a mixed-valence Mn2IIIMn2II composition and is best described as having four manganese ions arranged in abent chain. An 
2-oxygen atom of the 6-me-hmp- anion bridges between the manganese ions; the Cl- ligands areterminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex2·4H2O has a S = 9 ground state whereas complex 3·4H2O has S = 0 ground state. Fine structure patterns areseen in the HFEPR spectra, and in the case of 2·4H2O it was possible to simulate the fine structure assuming S= 9 with the parameters g = 1.999, axial zero-field splitting of D/kB = -0.498 K, quartic longitudinal zero-fieldsplitting of B4
/kB = 1.72 × 10-5 K, and rhombic zero-field splitting of E/kB = 0.124 K. Complex 2·4H2Oexhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complexfunctions as a SMM. The AC susceptibility data for complex 2·4H2O were measured in the 0.05-4.0 K rangeand when fit to the Arrhenius law, gave an activation energy of 
E = 15.8 K for the reversal of magnetization.This E value is to be compared to the potential-energy barrier height of U/kB = 
D
= 40.3 K calculated for2·4H2O.
C of a = 10.907(0) Å, b = 15.788(0) Å, c = 13.941(0) Å, =101.21(0), and Z = 2. The cation lies on an inversion center and consists of a planar Mn4 rhombus that ismixed-valence, Mn2IIIMn2II. The hmp- ligands function as bidentate ligands and as the only bridging ligands in2·4H2O. Complex 3·4H2O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 C ofa = 17.0852(4) Å, b = 20.8781(5) Å, c = 14.835(3) Å, = 90.5485(8), and Z = 4. This neutral complex alsohas a mixed-valence Mn2IIIMn2II composition and is best described as having four manganese ions arranged in abent chain. An 2-oxygen atom of the 6-me-hmp- anion bridges between the manganese ions; the Cl- ligands areterminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex2·4H2O has a S = 9 ground state whereas complex 3·4H2O has S = 0 ground state. Fine structure patterns areseen in the HFEPR spectra, and in the case of 2·4H2O it was possible to simulate the fine structure assuming S= 9 with the parameters g = 1.999, axial zero-field splitting of D/kB = -0.498 K, quartic longitudinal zero-fieldsplitting of B4/kB = 1.72 × 10-5 K, and rhombic zero-field splitting of E/kB = 0.124 K. Complex 2·4H2Oexhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complexfunctions as a SMM. The AC susceptibility data for complex 2·4H2O were measured in the 0.05-4.0 K rangeand when fit to the Arrhenius law, gave an activation energy of E = 15.8 K for the reversal of magnetization.This E value is to be compared to the potential-energy barrier height of U/kB = D = 40.3 K calculated for2·4H2O.