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首页> 外文期刊>Inorganic Chemistry: A Research Journal that Includes Bioinorganic, Catalytic, Organometallic, Solid-State, and Synthetic Chemistry and Reaction Dynamics >Conversion of core oxos to water molecules by 4e(-)/4H(+) reductive dehydration of the Mn4O26+ core in the manganese-oxo cubane complex Mn4O4(Ph2PO2)(6): A partial model for photosynthetic water binding and activation
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Conversion of core oxos to water molecules by 4e(-)/4H(+) reductive dehydration of the Mn4O26+ core in the manganese-oxo cubane complex Mn4O4(Ph2PO2)(6): A partial model for photosynthetic water binding and activation

机译:锰-氧代古巴锰矿配合物Mn4O4(Ph2PO2)(6)中Mn4O26 +核的4e(-)/ 4H(+)还原脱水将核心oxos转化为水分子:光合水结合和活化的部分模型

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摘要

Reaction of the Mn4O46+ "cubane" core complex, Mn4O4L6 (1) (L = diphenylphosphinate, Ph2PO2-), with a hydrogen atom donor, phenothiazine (pzH), forms the dehydrated cluster Mn4O2L6 (2), which has lost two mu-oxo bridges by reduction to water (H2O). The formation of 2 was established by electrospray mass spectrometry, whereas FTIR spectroscopy confirmed the release of water molecules into solution during the reduction of 1. UV-vis and EPR spectroscopies established the stoichiometry and chemical form of the pzH product by showing the production of 4 equiv of the neutral pz(.) radical. By contrast, the irreversible decomposition of 1 to individual Mn(II) ions occurs if the reduction is performed using electrons provided by various proton-lacking reductants, such as cobaltocene or electrochemical reduction. Thus, cubane 1 undergoes coupled four-electron/four-proton reduction with the release of two water molecules, a reaction formally analogous to the reverse sequence of the steps that occur during photosynthetic water oxidation leading to O-2 evolution. H-1 NMR of solutions of 2 reveal that all six of the phosphinate ligands exhibit paramagnetic broadening, due to coordination to Mn ions, and are magnetically equivalent. A symmetrical core structure is thus indicated. We hypothesize that this structure is produced by the dynamic averaging of phosphinato ligand coordination or exchange of mu-oxos between vacant mu-oxo sites. The paramagnetic H-1 NMR of water molecules in solution shows that they are able to freely exchange with water molecules that are bound to the Mn ion(s) in 2, and this exchange can be inhibited by the addition of coordinating anions, such as chloride. Thus, 2 possesses open or labile coordination sites for water and anions, in contrast to solutions of 1, which reveal no evidence for water coordination. Complex 2 exhibits greater paramagnetism than that of 1, as seen by H-1 NMR, and it possesses a broad (440 G wide) EPR absorption, centered at g = 2, that follows a Curie-Weiss temperature dependence (10-40 K) and is visible only at low temperatures, compared to EPR-silent 1. Its comparison to a spin-integration standard reveals that 2 contains 2 equiv of Mn(II), which is in agreement with the formal oxidation state of 2Mn(II)2Mn(III) that was derived from the titration. The EPR and NMR data for 2 are consistent with a loss of two of the intermanganese spin-exchange coupling pathways, versus 1, which results in two "wingtip" Mn(II) S = 5/2 spins that are essentially magnetically uncoupled from the diamagnetic Mn2O2 base. Bond-enthalpy data, which show that O-2 evolution via the reaction 1 --> 2 + O-2, is strongly favored thermodynamically but is not observed in the ground state due to an activation barrier, are included. This activation barrier is hypothesized to arise,in part, from the constraining effect of the facially bridging phosphinate ligands. [References: 47]
机译:Mn4O46 +“古巴”核心配合物Mn4O4L6(1)(L =二苯基次膦酸酯,Ph2PO2-)与氢原子供体吩噻嗪(pzH)的反应形成脱水簇Mn4O2L6(2),失去了两个mu-oxo通过还原成水(H2O)来建立桥梁。 2的形成是通过电喷雾质谱确定的,而FTIR光谱证实了还原1期间水分子向溶液中的释放。紫外可见光谱和EPR光谱通过显示4的生成确定了pzH产物的化学计量和化学形式。中性pz(。)自由基的当量。相反,如果使用由各种缺乏质子的还原剂(例如钴茂或电化学还原)提供的电子进行还原,则1会分解为单个的Mn(II)离子。因此,古巴1经历了两个电子分子的释放而发生的四电子/四质子还原反应,该反应在形式上类似于光合作用的水氧化过程中发生的步骤的相反顺序,从而导致O-2逸出。 2溶液的H-1 NMR显示,由于与Mn离子配位,所有六个次膦酸酯配体均表现出顺磁性加宽,并且在磁性上等效。因此示出了对称的芯结构。我们假设这种结构是由膦酸酯配体的动态平均或空的mu-oxo位点之间的mu-oxos交换动态产生的。溶液中水分子的顺磁性H-1 NMR表明,它们能够与与2中的Mn离子结合的水分子自由交换,并且这种交换可通过添加配位阴离子(例如氯化物。因此,与1的解决方案相反,2具有对水和阴离子的开放或不稳定的配位位点,而对1的解决方案没有揭示出水配位的证据。从H-1 NMR可以看出,配合物2的顺磁性大于1,顺磁性,并且它具有广泛的(440 G宽)EPR吸收,以g = 2为中心,遵循居里-魏斯温度依赖性(10-40 K ),并且仅在低温下才可见(与EPR-silent 1相比)。与自旋积分标准的比较表明,2含有2当量的Mn(II),这与2Mn(II)的形式氧化态相符由滴定法得到的2Mn(III)。 2的EPR和NMR数据与两个锰间自旋交换耦合路径的损失一致,而1则导致两个“翼尖” Mn(II)S = 5/2的自旋,这些自旋基本上与磁性抗磁性Mn2O2碱。其中包括键焓数据,该数据表明通过反应1-> 2 + O-2进行的O-2放出在热力学上得到了大力支持,但由于激活势垒而未在基态下观察到。假定该活化障碍部分是由于面部桥接次膦酸酯配体的约束作用引起的。 [参考:47]

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