Chemical Modeling of the Oxygen-Evolving Center in Plants. Synthesis, Structure, and Electronic and Redox Properties of a New Mixed Valence Mn-Oxo Cluster: [Mn2III,IVO2
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- 作者：Yves-Michel Frapart ; Alain Boussac ; Rolf Albach ; Elodie Anxolabé ; hè ; re-Mallart ; Michel Delroisse ; Jean-Baptiste Verlhac ; Geneviè ; ve Blondin ; Jean-Jacques Girerd ; Jean Guilhem ; Mich&e
- 刊名：Journal of the American Chemical Society
- 出版年：1996
- 出版时间：March 20, 1996
- 年：1996
- 卷：118
- 期：11
- 页码：2669 - 2678
- 全文大小：502K
- 年卷期：v.118,no.11(March 20, 1996)
- ISSN：1520-5126
文摘
The compound[Mn2III,IVO2(bisimMe2en)2](ClO4)3·H2O(bisimMe2en =N,N'-dimethyl-N,N'-bis(imidazol-4-ylmethyl)ethane-1,2-diamine) was synthesized. Itcrystallizes in the monoclinic space group C2/cwith a = 18.139(8) Å, b = 12.694(5) Å, c =17.694(8) Å, 
= 107.5(6)
, V =3885(6) Å3, and Z = 4. Thecation[Mn2III,IVO2(bisimMe2en)2]3+contains a di-manganese di-
-oxo unit. The Mn-Mn axis is aC2 axis. The Mn-imidazole distances for MnIII or MnIV aredistinct: respectively, 2.208(9) and 2.004(8) Å. Theground state is a
SA=2,SB=3/2,S=1/2>state separated by 420 cm-1 from the S =3/2 state. The EPR spectrum has beensimulatedwith A1x,y =160.10-4 cm-1,A1z = 136.10-4cm-1,A2x,y =72.10-4 cm-1,A2z = 71.10-4cm-1, gxy = 1.997,gz= 1.993. The ESEEM spectrum is reported. The 2Hnuclei corresponding to the exchangeable hydrogen atoms ofimidazole groups in D2O have been detected at 2.4 MHz at3480 G. In cyclic voltammetry,[Mn2III,IVO2(bisimMe2en)2]3+ presents in oxidation two reversiblewaves at 1.04 and 1.40 V (all potentials versus NHE) in strongcontrastwith what has been always observed on otherMn2III,IVO2 units, for which onlyone wave in oxidation ([III,IV] 
[IV,IV]) is observed. EPR spectroelectrochemistry revealsthe disappearance of the 16 line spectrum of the[III,IV]species on oxidation at 1.24 V. UV-vis spectroelectrochemistry atthis potential confirms the formation of the [IV,IV] dimer. The existence of two waves was related to the presenceof slow equilibrium between two different[III,IV] forms. UV-irradiation of the starting[Mn2III,IVO2(bisimMe2en)2]3+complex in an aqueous borate buffer atpH = 10 at 77 K resulted in the formation of an EPR signal that isattributed to an imidazole radical coupled to theMnIII-MnIV pair. This signal is unsplitand is understandable in a simple model of two spins S =1/2 in magneticinteraction. It has some analogy with that observed by Boussac etal. (Nature 1990, 347, 303-306) onthe S3 stateof the Ca2+ and Cl--depletedoxygen-evolving center, although in that case the signal was split.We relate thisdifference to small differences in the magnetic interaction of the twospins S = 1/2. ESEEM spectraof the irradiated[Mn2III,IVO2(bisimMe2en)2]3+complex are reported for magnetic fields corresponding to the resonanceof the radicalor the metal cluster. These results suggest that the formedimidazole radical stays linked to the metal core despitethe weak magnetic interaction detected. This rises the possibilitythat the S3 radical in the natural system, is adirectligand to Mn cluster.
= 107.5(6), V =3885(6) Å3, and Z = 4. Thecation[Mn2III,IVO2(bisimMe2en)2]3+contains a di-manganese di--oxo unit. The Mn-Mn axis is aC2 axis. The Mn-imidazole distances for MnIII or MnIV aredistinct: respectively, 2.208(9) and 2.004(8) Å. Theground state is aSA=2,SB=3/2,S=1/2>state separated by 420 cm-1 from the S =3/2 state. The EPR spectrum has beensimulatedwith A1x,y =160.10-4 cm-1,A1z = 136.10-4cm-1,A2x,y =72.10-4 cm-1,A2z = 71.10-4cm-1, gxy = 1.997,gz= 1.993. The ESEEM spectrum is reported. The 2Hnuclei corresponding to the exchangeable hydrogen atoms ofimidazole groups in D2O have been detected at 2.4 MHz at3480 G. In cyclic voltammetry,[Mn2III,IVO2(bisimMe2en)2]3+ presents in oxidation two reversiblewaves at 1.04 and 1.40 V (all potentials versus NHE) in strongcontrastwith what has been always observed on otherMn2III,IVO2 units, for which onlyone wave in oxidation ([III,IV] [IV,IV]) is observed. EPR spectroelectrochemistry revealsthe disappearance of the 16 line spectrum of the[III,IV]species on oxidation at 1.24 V. UV-vis spectroelectrochemistry atthis potential confirms the formation of the [IV,IV] dimer. The existence of two waves was related to the presenceof slow equilibrium between two different[III,IV] forms. UV-irradiation of the starting[Mn2III,IVO2(bisimMe2en)2]3+complex in an aqueous borate buffer atpH = 10 at 77 K resulted in the formation of an EPR signal that isattributed to an imidazole radical coupled to theMnIII-MnIV pair. This signal is unsplitand is understandable in a simple model of two spins S =1/2 in magneticinteraction. It has some analogy with that observed by Boussac etal. (Nature 1990, 347, 303-306) onthe S3 stateof the Ca2+ and Cl--depletedoxygen-evolving center, although in that case the signal was split.We relate thisdifference to small differences in the magnetic interaction of the twospins S = 1/2. ESEEM spectraof the irradiated[Mn2III,IVO2(bisimMe2en)2]3+complex are reported for magnetic fields corresponding to the resonanceof the radicalor the metal cluster. These results suggest that the formedimidazole radical stays linked to the metal core despitethe weak magnetic interaction detected. This rises the possibilitythat the S3 radical in the natural system, is adirectligand to Mn cluster.