Reduced anionic Mn12 molecules with half-integer ground states as single-molecule magnets
Reduced anionic Mn12 molecules with half-integer ground states as single-molecule magnets
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DOI：
10.1021/ic990613g
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发表时间：
1999-11-15
影响因子：
4.6
通讯作者：
Hendrickson, DN
中科院分区：
文献类型：
--
作者：
Aubin, SMJ;Sun, ZM;Hendrickson, DN
报道了单分子磁体(PPh4)-[Mn12O12(O2CPh)(16)(H2O)(4)]的制备、表征和X射线结构。8(CH2Cl2)(2)配合物2的晶型为三斜空间群P(1)，在213K时，a=17.2329(2)，b=17.8347(2)，c=26.8052(2)埃，α=90.515(2)，β=94.242(2)，γ=90.515(2)度，Z=2。阴离子的(Mn12O12)(15+)核心是由八个Mn原子组成的外环，由Mu(3)-O2-离子桥联到由四个Mn原子组成的内部四面体。由于苯环和溶剂分子的无序性，很难用键价和值来确定每个锰原子的氧化状态。分子内部含有一个四氧化三锰立方单元，这些锰原子都是Mn-IV离子。对于八个“外部”的锰原子，键价和值不能很好地定义它们的氧化状态。对于这八个锰原子，不可能确定是否适合使用捕获价态的Mn(II)Mn(III)7或电子离域描述。收集了(Mn4Mn7MnII)-[Mn12O12(O2CEt)(16)(H2O)(4)](1)结构表征的(Mn4Mn7MnII)-[Mn12O12(O2CEt)(16)(H2O)(4)](1)在328.2和437.69 GHz的高频电子顺磁共振数据。在强磁场作用下，晶体定向生长，HFEPR谱为类假单晶，而不是粉末状。光谱特征归因于S=19/2络合物在帽(Z)(2)上经历轴向零场分裂D(S)的精细结构，其中D=-0.62 cm(-1)。D的符号是由光谱的温度依赖性确定的。配合物2在3-6K范围内表现出一个异相交流磁化率(X_1“(M))信号，与单分子磁体一样，X_1”(M)峰的温度与频率有关。根据在不同频率(1-1512赫兹)的交流磁场振荡下收集的X_1(M)数据，估算了磁化方向从“上”到“下”的反转速度。这给出了配合物2的磁化驰豫速率为2.86-4.51K，配合物1的磁化驰豫速率为3.2-7.2K。通过磁化衰减实验确定了配合物1的磁化驰豫速率为1.80-2.50K。在后一种情况下，多晶样品在较大的德磁场中是磁饱和的。在磁场迅速降低到零之后，监测磁化到零的衰减。根据异相交流信号的频率依赖性和络合物1的去弛豫衰变实验估算的反应速率符合Arrhenius图，其活化能为U-ef=57K，指前速率为1/tau(0)=7.2×10(7)S(-1)。从高频顺磁共振数据来看，配合物1的基态为S=19/2，D=-0.62 cm(-1)。这给出了双势垒图的势垒U=79K。U-ef的值小于势垒高度U，这是因为当单个[Mn-12(-)]阴离子从自旋“向上”转变为“向下”时，它们不仅可以被热激活越过U=79K势垒，而且还可以量子力学地穿过m(S)=-n和m(S)=n能级之间的势垒。在低于10K的低温下，分子吸收声子能量并被从一个m(S)能级逐渐激发到另一个能级的多声子奥尔巴赫过程很可能参与其中。
The preparation, characterization, and X-ray structure are reported for the single-molecule magnet (PPh4)-[Mn12O12(O2CPh)(16)(H2O)(4)]. 8(CH2Cl2) (2) Complex 2 crystallizes in the triclinic space group P (1) over bar, which at 213 K has a = 17.2329(2), b = 17.8347(2), c = 26.8052(2) Angstrom, alpha = 90.515(2), beta = 94.242(2), gamma = 101.437(2)degrees, and Z = 2. The salt consists of PPh4+ cations and [Mn12O12(O2CPh)(16)(H2O)(4)](-) anions. The (Mn12O12)(15+) core of the anion is formed by an external ring of eight Mn atoms bridged by mu(3)-O2- ions to an internal tetrahedron of four Mn atoms. Because of disorder in both phenyl rings and solvate molecules, it was difficult to use bond valence sum values to determine definitively the oxidation state of each Mn atom. There is a Mn4O4 cubane unit in the internal part of the molecule and these Mn atoms are all Mn-IV ions. For the eight "external" Mn atoms the bond valence sum values did not define well their oxidation states. For these eight Mn atoms, it was not possible to determine whether a trapped-valence Mn(II)Mn(III)7 or an electronically delocalized description is appropriate. High-frequency EPR (HFEPR) data were collected for the previously structurally characterized (Mn4Mn7MnII)-Mn-IV-Mn-III valence-trapped salt (PPh4)[Mn12O12(O2CEt)(16)(H2O)(4)] (1) at 328.2 and 437.69 GHz. In the high magnetic field the crystallites orient and the HFEPR spectra are pseudo-single-crystal like, not powder patterns. The spectral features are attributed to the fine structure expected for a S = 19/2 complex experiencing axial zero-field splitting D (S) over cap(z)(2), where D = -0.62 cm(-1). The sign of D was definitively determined by the temperature dependence of the spectrum. Complex 2 exhibits one out-of-phase ac magnetic susceptibility (chi "(M)) signal in the 3-6 K range. The temperature of the chi "(M) peak is frequency dependent, as expected for a single-molecule magnet. The rate at which the direction of magnetization reverses from "up" to "down" was evaluated from chi "(M) data collected at various frequencies (1-1512 Hz) of oscillation of the ac magnetic field. This gives magnetization relaxation rates in the 2.86-4.51 K range for complex 2 and in the 3.2-7.2 K range for complex 1. Rates were also determined in the 1.80-2.50 K range for complex 1 via magnetization decay experiments. In this latter case; the polycrystalline sample is magnetically saturated in a large de field. After the magnetic field is rapidly decreased to zero, the decay of the magnetization to zero is monitored. The rates evaluated by both the frequency dependence of the out-of-phase ac signal and de relaxation decay experiments for complex 1 fit on an Arrhenius plot to give an activation energy of U-eff = 57 K and a preexponential rate of 1/tau(0) = 7.2 x 10(7) s(-1). From the HFEPR data, complex 1 has a S = 19/2 ground state with D = -0.62 cm(-1). This gives a potential-energy barrier of U = 79 K for the double-well potential-energy diagram.The value of U-eff is less than the barrier height U, because when individual [Mn-12(-)] anions convert from spin "up" to spin "down", they can not only be thermally activated to go over the U = 79 K barrier, they can also quantum mechanically tunnel through the barrier between m(s) = -n and m(s) = n levels. A multiphonon Orbach process involving molecules absorbing phonon energies and being excited incrementally from one m(s) level to another is likely involved at these low temperatures below 10 K.