Unusual MnIII/IV4 Cubane and MnIII16M4 (M = Ca, Sr) Looplike Clusters from the Use of Dimethylarsinic Acid

Three complexes are reported from the initial use of dimethylarsinic acid (Me2AsO2H) in MnIII/IV cluster chemistry, [Mn4O4­(O2AsMe2)6] (3; 2MnIII, 2MnIV), and [Mn16X4O8(O2CPh)16­(Me2AsO2)24] (X = Ca2+ (4) or Sr2+ (5); 16MnIII). They were obtained from reactions with [Mn12O12(O2CR)16(H2O)4] (R = Me, Ph) either without (3) or with (4 and 5) the addition of X2+ salts. Complex 3 contains a [Mn4O4]6+ cubane, whereas isostructural 4 and 5 contain a planar loop structure comprising four Mn4 asymmetric “butterfly” units linked by alternating anti,anti μ-O2AsMe2 and {X2(O2AsMe2)­(O2CPh)2} units. Variable-temperature magnetic susceptibility (χM) data were collected on dried microcrystalline samples of 3–5 in the 5.0–300 K range in a 0.1 T (1000 G) direct-current (dc) magnetic field. Data for 3 were fit to the appropriate Van Vleck equation (using the H = −2JŜi·Ŝj convention) for a cubane of virtual C2v symmetry, giving J33 = 0.0(1) cm–1, J34 = −3.4(4) cm–1, J44 = −9.8(2) cm–1, and g = 1.99(1), where the Jij subscripts refer to the oxidation states of the interacting Mn atoms. The ground state thus consists of two coupled MnIV and two essentially noninteracting MnIII. For 4 and 5, low-lying excited states from the high nuclearity and weak couplings prevent fits of dc magnetization data, but in-phase alternating current susceptibility χ′MT data down to 1.8 K indicate them to possess S = 4 ground states, if considered single Mn16 units. If instead they are treated as tetramers of weakly coupled Mn4 units, then each of the latter has an S = 2 ground state. Complexes 4 and 5 also exhibit very weak out-of-phase χ″M signals characteristic of slow relaxation, and magnetization versus dc field scans on a single crystal of 4·15MeCN at T ≥ 0.04 K showed hysteresis loops but with unusual features suggesting the magnetization relaxation barrier consists of more than one contribution.本文首次将二甲基次砷酸（dimethylarsinic acid，缩写为Me₂AsO₂H）应用于三价锰（MnIII）/四价锰（MnIV）簇合物化学，报道了三种配合物：[Mn₄O₄(O₂AsMe₂)₆]（记为3；含2个MnIII、2个MnIV），以及[Mn₁₆X₄O₈(O₂CPh)₁₆(Me₂AsO₂)₂₄]（X=Ca²⁺时为4，X=Sr²⁺时为5；含16个MnIII）。上述配合物均由[Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄]（R为甲基（Me）、苯基（Ph））与X²⁺盐反应制得：无X²⁺盐时得到配合物3，加入X²⁺盐则得到配合物4和5。配合物3具有[Mn₄O₄]⁶⁺立方烷结构；而同构的配合物4和5则拥有平面环状结构，该结构由4个Mn₄不对称“蝶式”单元通过交替的anti,anti μ-O₂AsMe₂桥联配体与{X₂(O₂AsMe₂)(O₂CPh)₂}单元连接而成。在0.1 T（即1000 G）的直流（dc）磁场中，于5.0~300 K温度范围内对干燥后的3~5的微晶样品进行变温磁化率（χ_M）测试。针对具有近似C₂ᵥ对称性的立方烷结构，采用H = −2JŜ_i·Ŝ_j的哈密顿约定，对配合物3的测试数据进行范弗莱克（Van Vleck）方程拟合，得到耦合参数J₃₃=0.0(1) cm⁻¹、J₃₄=−3.4(4) cm⁻¹、J₄₄=−9.8(2) cm⁻¹，以及g因子g=1.99(1)；其中J_ij的下标代表相互作用的锰原子的氧化态。因此，配合物3的基态由两个耦合的MnIV与两个基本无相互作用的MnIII组成。对于配合物4和5，由于其高核性与弱耦合作用产生了低能激发态，因此无法通过直流磁化数据进行拟合；但在1.8 K以上温度下测得的同相交流（alternating current, ac）磁化率χ′_M T数据表明，若将其视为单一的Mn₁₆单元，则其基态自旋S=4。若将其视为4个弱耦合Mn₄单元组成的四聚体，则每个Mn₄单元的基态自旋均为S=2。配合物4和5还展现出极弱的异相交流磁化率χ″_M信号，这是慢弛豫行为的典型特征；在T≥0.04 K条件下对4·15MeCN（MeCN为乙腈）的单晶进行磁化强度随直流磁场变化的扫描测试，结果显示其存在磁滞回线，但该回线具有反常特征，表明磁化弛豫势垒由多种贡献共同构成。