Synthesis, Redox, and Magnetic Properties of a Neutral, Mixed-Valent Heptanuclear Manganese Wheel with S = 27/2 High-Spin Ground State
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- 作者：Rolf W. Saalfrank ; Andreas Scheurer ; Raju Prakash ; Frank W. Heinemann ; Takayuki Nakajima ; Frank Hampel ; Roland Leppin ; Bernd Pilawa ; Holger Rupp ; Paul Mü ; ller
- 刊名：Inorganic Chemistry
- 出版年：2007
- 出版时间：March 5, 2007
- 年：2007
- 卷：46
- 期：5
- 页码：1586 - 1592
- 全文大小：290K
- 年卷期：v.46,no.5(March 5, 2007)
- ISSN：1520-510X
文摘
Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads tothe formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {MnII
[MnII2MnIII4Cl6(L3)6]} (4). Themanganese wheel crystallizes in the triclinic space group P
as 4·2CHCl3 or 4·3THF when either diethyl ether orn-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of eachmanganese ion in 4·2CHCl3 or 4·3THF were assigned on the basis of detailed symmetry, bond length, and chargeconsiderations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and werefurther supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4·2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagneticexchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higherthan 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due tothe Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements onsingle crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and byhigh-frequency electron spin resonance measurements on a polycrystalline powder of 4·2CHCl3 (D = -0.068 K,E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U 
10 K. Finally, 2DEG Hallmagnetization measurements revealed that 4·2CHCl3 shows single-molecule magnet behavior up to the blockingtemperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of themagnetization.
[MnII2MnIII4Cl6(L3)6]} (4). Themanganese wheel crystallizes in the triclinic space group P as 4·2CHCl3 or 4·3THF when either diethyl ether orn-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of eachmanganese ion in 4·2CHCl3 or 4·3THF were assigned on the basis of detailed symmetry, bond length, and chargeconsiderations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and werefurther supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4·2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagneticexchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higherthan 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due tothe Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements onsingle crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and byhigh-frequency electron spin resonance measurements on a polycrystalline powder of 4·2CHCl3 (D = -0.068 K,E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U 10 K. Finally, 2DEG Hallmagnetization measurements revealed that 4·2CHCl3 shows single-molecule magnet behavior up to the blockingtemperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of themagnetization.