On the Magnetic and Spectroscopic Properties of High-Valent Mn3CaO4 Cubanes as Structural Units of Natural and Artificial Water-Oxidizing Catalysts
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- 作者：Vera Krewald ; Frank Neese ; Dimitrios A. Pantazis
- 刊名：The Journal of the American Chemical Society
- 出版年：2013
- 出版时间：April 17, 2013
- 年：2013
- 卷：135
- 期：15
- 页码：5726-5739
- 全文大小：671K
- 年卷期：v.135,no.15(April 17, 2013)
- ISSN：1520-5126
文摘
The Mn(IV)3CaO4 cubane is a structural motif present in the oxygen-evolving complex (OEC) of photosystem II and in water-oxidizing Mn/Ca layered oxides. This work investigates the magnetic and spectroscopic properties of two recently synthesized complexes and a series of idealized models that incorporate this structural unit. Magnetic interactions, accessible spin states, and 55Mn isotropic hyperfine couplings are computed with quantum chemical methods and form the basis for structure鈥損roperty correlations. Additionally, the effects of oxo-bridge protonation and one-electron reduction are examined. The calculated properties are found to be in excellent agreement with available experimental data. It is established that all synthetic and model Mn(IV)3CaO4 cubane complexes have the same high-spin S = 9/2 ground state. The magnetic coupling conditions under which different ground spin states can be accessed are determined. Substitution of Mn(IV) magnetic centers by diamagnetic ions [e.g., Ge(IV)] allows one to 鈥渟witch off鈥?specific spin sites in order to examine the magnetic orbitals along individual Mn鈥揗n exchange pathways, which confirms the predominance of ferromagnetic interactions within the cubane framework. The span of the Heisenberg spin ladder is found to correlate inversely with the number of protonated oxo bridges. Energetic comparisons for protonated models show that the tris-渭-oxo bridge connecting only Mn ions in the cubane has the lowest proton affinity and that the average relaxation energy per additional proton is on the order of 18 kcal路mol鈥?, thus making access to ground states other than the high-spin S = 9/2 state in these cubanes unlikely. The relevance of these cubanes for the OEC and synthetic oxides is discussed.