The detailed structure of catalytic center of water oxidation, Mn4Ca-cluster, in photosystem ⅡI (PSII) has been reported recently. However, due to the radiation damage induced by X-ray and the complexity of the Mn4Ca-cluster, the assignment of the μ4-O5 atom coordinated by three Mn and one Ca2+ ions is still lack of essential evidences. In this article, we synthesized one Mn complex containing two μ4-O atoms. It is found that the lengths of all μ4-O-Mn bonds in this Mn complex are in the range of 1.89-2.10 , which are significantly shorter than 2.40-2.61 distance of μ4-O5-Mn bonds in Mn4Ca-cluster observed in the crystal structure of PSII. In addition, DFT calculations have been carried out on the Mn4Ca-cluster. It is found that the O atom of μ4-O or μ4-OH always trends to deviate from the center position of four metal ions, resulting in unequal bond lengths of four μ4-4-M (M=Mn or Ca), which is obviously different with larger and nearly equal distances between μ4-O and four metal ions observed in the crystal structure. Based on these results, we suggest that the μ4-atom in Mn4Ca-cluster of PSII is unlikely to be a μ4-O, μ4-OH or μ4-OH2 , and its assignment is still an open question.
The understanding of the structure-function relationship of the oxygen-evolving center(OEC), a Mn_4 Cacluster, in photosystem II is impeded mainly due to the complexity of the protein environment and lack of rational chemical models as a reference. In this study, two novel Mn_4-oxido complexes have been synthesized and characterized, in which the peripheral ligands of the [Mn_4~Ⅲ] core are provided by eight μ_2-carboxylate groups and two neutral terminal ligands(pyridine or isoquinoline). This type of peripheral ligation is very similar to the Mn_4Ca-oxide model complexes recently reported to mimic the OEC. The new Mn_4-oxide complex can catalyze the oxygen-evolving reaction in the presence of Bu^tOOH as an oxidant. The structure and redox properties comparison of the Mn_4-oxido and Mn_4Ca-oxido complexes provide important clues to understanding the functional role of Ca in the OEC in natural photosynthesis, and develop more efficient artificial catalysts for the water-splitting reaction in the future.
A new ligand, 2-(2-hydroxyphenyl)-5,6-dichlorobenzimidazole, H2pbmCl2(1), and a novel MnIII complex, [MnIII(HpbmCl2)(pbmCl2)(DMF)2](2),(DMF = N,N-dimethylformamide), have been synthesized and characterized. The crystal of compound 1(C13H8Cl2N2O, Mr = 279.12) belongs to the monoclinic system, space group P21 with a = 3.770(5), b = 25.20(3), c = 5.865(7) , = 92.727(17)o, V = 556.6(12) 3, Z = 2, Dc = 1.665 g/cm3, S = 1.137, μ= 0.568 mm-1, F(000) = 284, the final R = 0.0876 and wR = 0.2334 for 1848 independent reflections. The molecule is planar due to the presence of a strong intramolecular hydrogen bond between O–H group of phenol and N atom of imidazole. H2pbmCl2(1) molecules are arranged into a one-dimensional linear chain through intermolecular hydrogen bonds(N–H···O and C–H···Cl). The crystal of complex 2(C32H27Cl4MnN6O4, Mr = 756.34) belongs to the monoclinic system, space group P21/c with a = 19.043(10), b = 10.808(5), c = 18.704(11), = 115.540(6)o, V = 3473(3) 3, Z = 4, Dc = 1.446 g/cm3, S = 1.3, μ = 0.733 mm-1, F(000) = 1544, the final R = 0.1219 and wR = 0.2681 for 7811 independent reflections. The Mn ion adopts a distorted octahedral geometry coordinated by two deprotonated H2pbmCl2 ligands and two DMF molecules. The [MnIII(HpbmCl2)(pbmCl2)(DMF)2] molecules are arranged into a three-dimensional structure through hydrogen bonds(N–H···N, C–H···N and C–H···Cl) and weak π···πinteractions. The activity measurements suggest that complex 2 is able to serve as a catalyst for H2O2 disproportionation reaction to form O2 in neutral water solution.
