Michael K. Chan

Pulsed 1H and 55Mn ENDOR studies of dinuclear Mn(III)Mn(IV) model complexes

The high resolution pulsed EPR technique of ESE-ENDOR is applied to bis-μ-oxo dinuclear Mn(III)Mn(IV) model complexes ligated by either 2,2′-bipyridine (bipy) or 1,10-phenanthro-line (phen). Such complexes, which model a building block of which the Mn4 cluster of the oxygen evolving complex of photosystem II is thought to be contracted, are strongly anti-ferromagnetically coupled, class II mixed valence compounds. The 1H hyperfine couplings between ligand protons and the dinuclear spin centre reflect significant couplings to each Mn ion: a situation which requires a more complex analysis than that used in the case of a simple single point-dipole. 55Mn ENDOR spectra of phen recorded at field positions across the EPR envelope reveal its Mn hyperfine and quadrupole coupling constants. These ENDOR spectra are discussed in relation to a general analysis of 55Mn ENDOR spectra, including the importance of angle and m I selection, in addition to obtaining ENDOR spectra across the EPR envelope in cases where 55Mn ...