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On the Magnetic and Spectroscopic Properties of High-Valent Mn3CaO4 Cubanes as Structural Units of Natural and Artificial Water-Oxidizing Catalysts
journal contribution
posted on 2013-04-17, 00:00 authored by Vera Krewald, Frank Neese, Dimitrios A. PantazisThe
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–property 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 “switch off” specific spin sites
in order to examine the magnetic orbitals along individual Mn–Mn
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-μ-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·mol–1, 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.