| Abstract
| - The following dinuclear exchange-coupled manganese complexes are investigated: [dtneMnIIIMnIV(μ-O)2μ-OAc](BPh4)2 (dtne = 1,2-bis(1,4,7-triazacyclonon-1-yl)ethane), [(CH3)4dtneMnIIIMnIV(μ-O)2μ-OAc](BPh4)2 ((CH3)4dtne = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), [(CH3)4dtneMnIVMnIV(μ-O)2μ-OAc](ClO4)3, [(tacn)2MnIIIMnIV(μ-O)2μ-OAc](BPh4)2 (tacn = 1,4,7-triazacyclononane), [bpy4MnIIIMnIV(μ-O)2](ClO4)3 (bpy = 2,2‘-bipyridyl), and [phen4MnIIIMnIV(μ-O)2](ClO4)3 (phen = 1,10-phenanthroline). Forthree of these complexes, X-ray structural data obtained on single crystals are reported here. All complexesare strongly antiferromagnetically coupled, with exchange coupling constants ranging from J = −110 cm-1(bis-μ-oxo-μ-acetato-bridged) to −150 cm-1 (bis-μ-oxo-bridged). EPR investigations at X- and Q-bandfrequencies are reported for all five mixed-valence MnIIIMnIV complexes. G tensors and 55Mn hyperfine couplingconstants (hfc's) were obtained by simultaneous simulation of the EPR spectra at both frequency bands. Byusing the vector model of exchange-coupled systems, tensor axes could be related to the molecular structureof the complexes. Hyperfine coupling constants from 55Mn cw-electron−nuclear double-resonance (ENDOR)spectra were in agreement with those obtained from the simulation of the EPR spectra. Ligand hyperfinecouplings (1H and 14N) were also measured using cw-ENDOR spectroscopy. Electron spin−echo envelopemodulation spectroscopy (ESEEM) spectra yielded information about small 14N hyperfine and quadrupolecoupling constants that could not be resolved in the ENDOR spectra. On the basis of specifically deuteratedcomplexes and results from orientation-selection ENDOR spectra, some proton hfc's could be assigned topositions within the complexes. Using an extended point-dipole model and the coordinates provided by theX-ray structure analysis, all dipolar hfc's of the complexes were calculated. Comparison of these hfc's withexperimentally obtained values led to a consistent assignment of most hf tensors to molecular positions. Theelectronic structures of the investigated complexes are compared with each other, and the relevance of theresults for metalloenzymes containing at least a dinuclear manganese core is discussed.
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