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

The high resolution pulsed EPR technique of ESE-ENDOR is applied to bis-mu- oxo dinuclear Mn(III)Mn(IV) model complexes ligated by either 2,2'-bipyridi ne (bipy) or I,10-phenanthroline (phen). Such complexes, which model a buil ding block of which the Mn-4 cluster of the oxygen evolving complex of phot osystem II is thought to be contructed, are strongly antiferromagnetically coupled, class II mixed valence compounds. The H-1 hyperfine couplings betw een ligand protons and the dinuclear spin centre reflect significant coupli ngs to each Mn ion: a situation which requires a more complex analysis than that used in the case of a simple single point-dipole. Mn-55 ENDOR spectra of phen recorded at field positions across the EPR envelope reveal its Mn hyperfine and quadrupole coupling constants. These ENDOR spectra are discus sed in relation to a general analysis of Mn-55 ENDOR spectra, including the importance of angle and m(I) selection, in addition to obtaining ENDOR spe ctra across the EPR envelope in cases where Mn-55 m(I) splitting is resolve d in the EPR spectra.