Single-molecule magnets: Jahn-Teller isomerism and the origin of two magnetization relaxation processes in Mn12 complexes.
Single-molecule magnets: Jahn-Teller isomerism and the origin of two magnetization relaxation processes in Mn12 complexes.
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单分子磁体：Jahn-Teller 异构现象以及 Mn12 配合物中两个磁化弛豫过程的起源。
DOI：
10.1021/ic000911
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发表时间：
2001
影响因子：
4.6
通讯作者：
D. N. Hendrickson
中科院分区：
文献类型：
--
作者：
S. Aubin;Ziming Sun;H. Eppley;E. Rumberger;I. Guzei;K. Folting;P. Gantzel;A. Rheingold;G. Christou;D. N. Hendrickson
几种组成为[Mn12O12(O2CR)16(H2O)x] （x = 3或4）的单分子磁体表现出两个非相交流磁化率信号，一个在4- 7k区，另一个在2- 3k区。制备了新的Mn12配合物，并对其结构进行了表征，系统地考察了两种磁化弛豫过程的起源。给定R取代基的Mn12配合物存在不同的结晶形式，其中两种形式具有不同的结晶溶剂分子组成，这导致具有相同R取代基的两种不同结晶形式的结合水和羧酸配体的两种不同排列。[Mn12O12(O2CEt)16(H2O)3]立方晶体的x射线结构4H2O（空间群P1）（配合物2a）之前已经报道过。更常见的针状[Mn12O12(O2CEt)16(H2O)3]（配合物2b）在单斜空间群P2(1)/c中结晶，在-170℃时，a = 16.462(7) a, b = 22.401(9) a, c = 20.766(9) a, β = 103.85(2)度，Z = 4。在两种晶体形式中，Mn12分子上的H2O和羧酸配体的排列是不同的。配合物[Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4]。8CH2Cl2(5)在单斜空间群C2/c中结晶，在-172℃时，a = 29.697(9) a, b = 17.708(4) a, c = 30.204(8) a, β = 102.12(2)度，Z = 4。配合物5的交流磁化率数据表明，它在2 ~ 3 K和4 ~ 7 K范围内都有异相信号。对苯甲酸甲酯络合物的两种异构体的x射线结构也被报道。[Mn12O12 (O2CC6H4-p-Me) 16 (H2O) 4)。（HO2CC6H4-p-Me）(6)在单斜空间群C2/c中结晶，在193 K时，a = 40.4589(5) a, b = 18.2288(2) a, c = 26.5882(4) a, β = 125.8359(2)度，Z = 4。[Mn12O12 (O2CC6H4-p-Me) 16 (H2O) 4)。3(H2O)(7)在单斜空间群I2/a中结晶，在223 K时a = 29.2794(4) a, b = 32.2371(4) a, c = 29.8738(6) a, β = 99.2650（10）度，Z = 8。配合物6和7中的Mn12分子在四个结合水配体的排列上不同。配合物6在2 ~ 3 K区有一个chi(M)′′信号，而水合物配合物7在4 ~ 7 K区有chi(M)′′信号。此外，然而，在配合物6中，一个Mn（III）离子具有指向一个氧化离子的异常Jahn-Teller畸变轴，因此6和7是Jahn-Teller异构体。与复合体7相比，这降低了复合体6核心的对称性。因此，配合物6可能具有更大的隧穿矩阵元素，这解释了为什么该配合物在2-3 K区域显示chi(M)‘ ’信号，而配合物7在4-7 K区域具有chi(M)‘ ’峰值，即配合物6的磁化隧穿率大于配合物7。详细的1H NMR实验（2-D COSY和TOCSY）确定了苯甲酸盐和对甲基苯甲酸Mn12配合物的所有质子共振，并证实了（Mn12O12）配合物在溶解后的结构完整性。溶液中有快速的配体交换，没有证据表明在固态中有不同的同分异构体形式的Mn12配合物。
Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4-7 K region and the other in the 2-3 K region. New Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]. 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degrees C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(9) A, beta = 103.85(2) degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4].8CH2Cl2 (5) crystallizes in the monoclinic space group C2/c, which at -172 degrees C has a = 29.697(9) A, b = 17.708(4) A, c = 30.204(8) A, beta = 102.12(2) degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)16(H2O)4]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) A, b = 18.2288(2) A, c = 26.5882(4) A, beta = 125.8359(2) degrees, and Z = 4. [Mn12O12(O2CC6H4-p-Me)16(H2O)4].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) A, b = 32.2371(4) A, c = 29.8738(6) A, beta = 99.2650(10) degrees, and Z = 8. The Mn12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi(M)' ') in the 2-3 K region, whereas the hydrate complex 7 has a chi(M)' ' signal in the 4-7 K region. In addition, however, in complex 6, one Mn(III) ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus, complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi(M)' ' signal in the 2-3 K region, whereas complex 7 has its chi(M)' ' peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed 1H NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p-methyl-benzoate Mn12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapid ligand exchange and no evidence for the different isomeric forms of Mn12 complexes seen in the solid state.