Electronic structure of antiferromagnetically coupled dinuclear manganese ((MnMnIV)-Mn-III) complexes studied by magnetic resonance techniques

The following dinuclear exchange-coupled manganese complexes are investigat ed: [dtneMn(III)Mn(IV)-(mu O-)(2)mu-OAc](BPh4)(2) (dtne 1,2-bis(1,4,7-triaz acyclonon-1-yl)ethane), [(CH3)(4)dtneMn(III)Mn(IV)(mu-O)(2)mu-OAc]-(BPh4)(2 ) ((CH3)(4)dtne 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), [(C H3)(4)dtneMn(III)Mn(IV)(mu-O)(2)mu- OAc](ClO4)(3), [(tacn)(2)(MnMnIV)-Mn-II I(mu-O)(2)mu-OAc](BPh4)(2) (tacn = 1,4,7-triazasyclononane), [bpy(4)Mn(III) Mn(IV)(mu-O)(2)](ClO4)(3) (bpy = 2,2'-bipyridyl), and [phen(4)Mn(III)Mn(IV) (mu-O)(2)](ClO4)(3) (phen 1,10-phenanthroline). For three of these complexe s, X-ray structural data obtained on single crystals are reported here. All complexes are strongly antiferromagnetically coupled, with exchange coupli ng constants ranging from J = -110 cm(-1) (bis-mu-oxo-mu-acetato-bridged) t o -150 cm(-1) (bis-mu-oxo-bridged). EPR investigations at X- and Q-band fre quencies are reported for all five mixed-valence (MnMnIV)-Mn-III complexes. G tensors and Mn-55 hyperfine coupling constants (hfc's) were obtained by simultaneous simulation of the EPR spectra at both frequency bands. By usin g the vector model of exchange-coupled systems, tensor axes could be relate d to the molecular structure of the complexes. Hyperfine coupling constants from Mn-55 cw-electron-nuclear double-resonance (ENDOR) spectra were in ag reement with those obtained from the simulation of the EPR spectra. Ligand hyperfine couplings (H-1 and N-14) were also measured using cw-ENDOR spectr oscopy. Electron spin-echo envelope modulation spectroscopy (ESEEM) spectra yielded information about small N-14 hyperfine and quadrupole coupling con stants that could not be resolved in the ENDOR spectra. On the basis of spe cifically deuterated complexes and results from orientation-selection ENDOR spectra, some proton hfc's could be assigned to positions within the compl exes. Using an extended point-dipole model and the coordinates provided by the X-ray structure analysis, all dipolar hfc's of the complexes were calcu lated. Comparison of these hfc's with experimentally obtained values led to a consistent assignment of most hf tensors to molecular positions. The ele ctronic structures of the investigated complexes are compared with each oth er, and the relevance of the results for metalloenzymes containing at least a dinuclear manganese core is discussed.