Zhang Zhan, Coats Katherine L, Chen Zhuqi, Hubin Timothy J, Yin Guochuan
Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology , Wuhan 430074, P. R. China.
Inorg Chem. 2014 Nov 17;53(22):11937-47. doi: 10.1021/ic501342c. Epub 2014 Nov 6.
Available data from different laboratories have confirmed that both Ca(2+) and Cl(-) are crucial for water oxidation in Photosystem II. However, their roles are still elusive. Using a manganese(II) complex having a cross-bridged cyclen ligand as a model, the influence of Ca(2+) on the oxidative reactivity of the manganese(II) complex and its corresponding manganese(IV) analogue were investigated. It has been found that adding Ca(2+) can significantly improve the oxygenation efficiency of the manganese(II) complex in sulfide oxidation and further accelerate the oxidation of sulfoxide to sulfone. Similar improvements have also been observed for Mg(2+), Sr(2+), and Ba(2+). A new monomeric manganese(IV) complex having two cis-hydroxide ligands has also been isolated through oxidation of the corresponding manganese(II) complex with H2O2 in the presence of NH4PF6. This rare cis-dihydroxomanganese(IV) species has been well characterized by X-ray crystallography, electrochemistry, electron paramagnetic resonance, and UV-vis spectroscopy. Notably, using the manganese(IV) complex as a catalyst demonstrates higher activity than the corresponding manganese(II) complex, and adding Ca(2+) further improves its catalytic efficiency. However, adding Cl(-) decreases its catalytic activity. In electrochemical studies of manganese(IV) complexes with no chloride ligand present, adding Ca(2+) positively shifted the redox potential of the Mn(IV)/Mn(III) couple but negatively shifted its Mn(V)/Mn(IV) couple. In the manganese(II) complex having a chloride ligand, adding Ca(2+) shifted both the Mn(IV)/Mn(III) and Mn(V)/Mn(IV) couples in the negative direction. The revealed oxidative reactivity and redox properties of the manganese species affected by Ca(2+) and Cl(-) may provide new clues to understanding their roles in the water oxidation process of Photosystem II.
来自不同实验室的现有数据证实，Ca(2+)和Cl(-)对于光系统II中的水氧化都至关重要。然而，它们的作用仍然难以捉摸。使用具有交叉桥连环糊精配体的锰(II)配合物作为模型，研究了Ca(2+)对锰(II)配合物及其相应的锰(IV)类似物氧化反应活性的影响。研究发现，添加Ca(2+)可以显著提高锰(II)配合物在硫化物氧化中的氧合效率，并进一步加速亚砜氧化为砜的反应。对于Mg(2+)、Sr(2+)和Ba(2+)也观察到了类似的改善。通过在NH4PF6存在下用H2O2氧化相应的锰(II)配合物，还分离出了一种具有两个顺式氢氧根配体的新型单体锰(IV)配合物。这种罕见的顺式二羟基锰(IV)物种已通过X射线晶体学、电化学、电子顺磁共振和紫外可见光谱进行了充分表征。值得注意的是，使用锰(IV)配合物作为催化剂显示出比相应的锰(II)配合物更高的活性，并且添加Ca(2+)进一步提高了其催化效率。然而，添加Cl(-)会降低其催化活性。在不存在氯配体的锰(IV)配合物的电化学研究中，添加Ca(2+)使Mn(IV)/Mn(III)电对的氧化还原电位正向移动，但使其Mn(V)/Mn(IV)电对负向移动。在具有氯配体的锰(II)配合物中，添加Ca(2+)使Mn(IV)/Mn(III)和Mn(V)/Mn(IV)电对都向负方向移动。所揭示的受Ca(2+)和Cl(-)影响的锰物种的氧化反应活性和氧化还原性质可能为理解它们在光系统II水氧化过程中的作用提供新的线索。
