RESUMO
We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3IV O4 and YMn3IV O4 complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn3IV O4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C3 -symmetric Ca(Y)Mn3IV O4 core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn3IV O3 (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S2 state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes.
Assuntos
Materiais Biomiméticos/química , Hidrocarbonetos Aromáticos com Pontes/química , Complexos de Coordenação/química , Prótons , Materiais Biomiméticos/síntese química , Hidrocarbonetos Aromáticos com Pontes/síntese química , Cálcio/química , Complexos de Coordenação/síntese química , Espectroscopia de Ressonância de Spin Eletrônica , Manganês/química , Estrutura Molecular , Complexo de Proteína do Fotossistema II/química , Ítrio/químicaRESUMO
Incorporation of Mn into an established water oxidation catalyst based on a Co(III)4O4 cubane was achieved by a simple and efficient assembly of permanganate, cobalt(II) acetate, and pyridine to form the cubane oxo cluster MnCo3O4(OAc)5py3 (OAc = acetate, py = pyridine) (1-OAc) in good yield. This allows characterization of electronic and chemical properties for a manganese center in a cobalt oxide environment, and provides a molecular model for Mn-doped cobalt oxides. The electronic properties of the cubane are readily tuned by exchange of the OAc- ligand for Cl- (1-Cl), NO3- (1-NO3), and pyridine ([1-py]+). EPR spectroscopy, SQUID magnetometry, and DFT calculations thoroughly characterized the valence assignment of the cubane as [MnIVCoIII3]. These cubanes are redox-active, and calculations reveal that the Co ions behave as the reservoir for electrons, but their redox potentials are tuned by the choice of ligand at Mn. This MnCo3O4 cubane system represents a new class of easily prepared, versatile, and redox-active oxido clusters that should contribute to an understanding of mixed-metal, Mn-containing oxides.