New Polynuclear Manganese Clusters from the Use of the Hydrophobic Carboxylate Ligand 2,2-Dimethylbutyrate

The syntheses, structures, and magnetic properties are reported of [Mn12O12(O2CPet)16(MeOH)4] (4), [Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)] (5), [Mn9O6(OH)(CO3)(O2CPet)12(H2O)2] (6), and [Mn4O2(O2CPet)6(bpy)2] (7,
bpy = 2,2‘-bipyridine), where Pet = tert-pentyl (PetCO2H = 2,2-dimethylbutyric acid). These complexes were all
prepared from reactions of [Mn12O12(O2CPet)16(H2O)4] (3) in CH2Cl2. Complex 4·2MeCN crystallizes in the triclinic
space group P1̄ and contains a central [MnIV4O4] cubane core that is surrounded by a nonplanar ring of eight
alternating MnIII and eight μ3-O2- ions. This is only the third Mn12 complex in which the four bound water molecules
have been replaced by other ligands, in this case MeOH. Complex 5·1/2CH2Cl2 crystallizes in the monoclinic space
group P21/c and contains two [Mn3(μ3-O)]7+ units linked at two of their apexes by two PetCO2- ligands and one
μ4-CH2O22- bridge. The complex is a new structural type in Mn chemistry, and also contains only the third example
of a gem-diolate unit bridging four metal ions. Complex 6·H2O·PetCO2H crystallizes in the orthorhombic space
group Cmc21 and possesses a [MnIII9(μ3-O)6(μ-OH)(μ3-CO3)]12+ core. The molecule contains a μ3-CO32- ion, the
first example in a discrete Mn complex. Complex 7·2H2O crystallizes in the monoclinic space group P21/c and
contains a known [MnIII2MnII2(μ3-O)2]6+ core that can be considered as two edge-sharing, triangular [Mn3O] units.
Additionally, the synthesis and magnetic properties of a new enneanuclear cluster of formula [Mn9O7(O2CCH2But)13(THF)2] (8, THF = tetrahydrofuran) are reported. The molecule was obtained by the reaction of [Mn12O12(O2CCH2But)16(H2O)4] (2) with THF. Complexes 2 and 4 display quasireversible redox couples when examined by
cyclic voltammetry in CH2Cl2; oxidations are observed at −0.07 V (2) and −0.21 V (4) vs ferrocene. The magnetic
properties of complexes 4−8 have been studied by direct current (DC) and alternating current (AC) magnetic
susceptibility techniques. The ground-state spin of 4 was established by magnetization measurements in the 1.80−4.00 K and 0.5−7 T ranges. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating
a full powder average and including only axial anisotropy, gave S = 10, g = 2.0(1), and D = −0.39(10) cm-1. The
complex exhibits two frequency-dependent out-of-phase AC susceptibility signals (χM‘ ‘) indicative of slow magnetization
relaxation. An Arrhenius plot obtained from χM‘ ‘ vs T data gave an effective energy barrier to relaxation (Ueff) of
62 and 35 K for the slower and faster relaxing species, respectively. These studies suggest that complex 4 is a
single-molecule magnet (SMM). DC susceptibility studies on complexes 5−8 display overall antiferromagnetic behavior
and indicate ground-state spin values of S ≤ 2. AC susceptibility studies at <10 K confirm these small values and
indicate the population of low-lying excited states even at these low temperatures. This supports the small ground-state spin values to be due to spin frustration effects.

本文报道了[Mn12O12(O2CPet)16(MeOH)4]（4）、[Mn6O2(O2CH2)(O2CPet)11(HO2CPet)2(O2CMe)]（5）、[Mn9O6(OH)(CO3)(O2CPet)12(H2O)2]（6）及[Mn4O2(O2CPet)6(bpy)2]（7，bpy=2,2'-联吡啶）的合成、结构与磁性质，其中Pet代表叔戊基（tert-pentyl），对应羧酸为2,2-二甲基丁酸（PetCO2H=2,2-二甲基丁酸）。上述配合物均以[Mn12O12(O2CPet)16(H2O)4]（3）在二氯甲烷（CH2Cl2）中的反应制得。
配合物4·2MeCN结晶于三斜晶系空间群P1̄，其结构包含一个中心[MnIV4O4]立方烷型核，周围环绕着由8个交替的MnIII离子与8个μ3-氧离子构成的非平面环。这是第三例通过其他配体取代四个配位水分子的Mn12配合物，本研究中取代配体为甲醇（MeOH）。
配合物5·1/2CH2Cl2结晶于单斜晶系空间群P21/c，包含两个[Mn3(μ3-O)]7+单元，二者通过两个PetCO2-配体与一个μ4-偕二醇二阴离子（μ4-CH2O22-）桥连于其中两个顶点处。该配合物属于锰化学中的新型结构类型，同时也是第三例以偕二醇盐配体桥连四个金属离子的实例。
配合物6·H2O·PetCO2H结晶于正交晶系空间群Cmc21，具有[MnIII9(μ3-O)6(μ-OH)(μ3-CO3)]12+型核。该分子包含一个μ3-碳酸根离子，为离散锰配合物中首例此类结构。
配合物7·2H2O结晶于单斜晶系空间群P21/c，包含一个已知的[MnIII2MnII2(μ3-O)2]6+型核，该核可视为两个共边的三角形[Mn3O]单元。
此外，本文还报道了一种新型九核簇合物[Mn9O7(O2CCH2But)13(THF)2]（8，THF=四氢呋喃）的合成与磁性质。该分子通过[Mn12O12(O2CCH2But)16(H2O)4]（2）与四氢呋喃（THF）反应制得。
通过在二氯甲烷中进行循环伏安法测试发现，配合物2与4呈现准可逆氧化还原对，相对于二茂铁的氧化电位分别为-0.07 V（2）与-0.21 V（4）。
对配合物4~8的磁性质采用直流（DC）与交流（AC）磁化率技术进行了研究。通过在1.80~4.00 K与0.5~7 T范围内的磁化强度测量，确定了配合物4的基态自旋。通过全矩阵对角化对约化磁化强度数据进行拟合，该拟合包含完整的粉末平均效应且仅考虑轴向各向异性，得到参数S=10，g=2.0(1)，D=-0.39(10) cm-1。该配合物表现出两个与频率相关的交流磁化率虚部信号（χM''），表明其具有缓慢的磁化弛豫行为。根据χM''与温度（T）的数据绘制的阿伦尼乌斯图，得到慢速与快速弛豫物种的有效弛豫能垒（Ueff）分别为62 K与35 K。上述研究表明，配合物4属于单分子磁体（single-molecule magnet, SMM）。
对配合物5~8的直流磁化率研究显示其整体呈现反铁磁行为，且基态自旋值S≤2。低于10 K下的交流磁化率测试证实了这一较小的自旋值，并表明即便在如此低的温度下，仍存在低能级激发态的布居，这支持了小基态自旋值源于自旋阻挫效应的结论。