Electronic Structure of Antiferromagnetically Coupled Dinuclear Manganese (Mn^IIIMn^IV) Complexes Studied by Magnetic Resonance Techniques

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). For three of these complexes, X-ray structural data obtained on single crystals are reported here. All complexes are 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-band frequencies are reported for all five mixed-valence MnIIIMnIV complexes. G tensors and 55Mn hyperfine coupling constants (hfc's) were obtained by simultaneous simulation of the EPR spectra at both frequency bands. By using the vector model of exchange-coupled systems, tensor axes could be related to the molecular structure of 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 hyperfine couplings (1H and 14N) were also measured using cw-ENDOR spectroscopy. Electron spin−echo envelope modulation spectroscopy (ESEEM) spectra yielded information about small 14N hyperfine and quadrupole coupling constants that could not be resolved in the ENDOR spectra. On the basis of specifically deuterated complexes and results from orientation-selection ENDOR spectra, some proton hfc's could be assigned to positions within the complexes. Using an extended point-dipole model and the coordinates provided by the X-ray structure analysis, all dipolar hfc's of the complexes were calculated. Comparison of these hfc's with experimentally obtained values led to a consistent assignment of most hf tensors to molecular positions. The electronic structures of the investigated complexes are compared with each other, and the relevance of the results for metalloenzymes containing at least a dinuclear manganese core is discussed.

Prediction of Exchange Coupling Constant for Mn12 Molecular Magnet Using Dft+U

“…al. [20] a Experimental data is taken from [28] b Experimental Data is taken from [29] c BSDFT calculation from ref [21] d BSDFT calculation from ref [16] Our calculated data agree with the experimental value to within 15%, for both molecule with and without Acetate Bridge, compare to 65% deviation given by broken symmetry Density Functional theory. …”

Carboxylate Ligands Drastically Enhance the Rates of Oxo Exchange and Hydrogen Peroxide Disproportionation by Oxo Manganese Compounds of Potential Biological Significance

“…The hyperfine couplings with the 55 Mn nuclear spin result in a characteristic 16-line signal. [42] On varying the power of the microwave input, we noted a very fast signal saturation, even for low powers (P > 6 mW approximately). Experiments at various temperatures (4.5 to 40 K) did not indicate any change in the shape of the recorded spectrum, apart from a decrease in the intensity of the signal when the temperature was increased, consistently with the depopulation of the S = 1 = 2 ground state.…”

“…[27] As a result of the observation that the bisA C H T U N G T R E N N U N G (m-oxo)-Mn III Mn IV complexes are rather easily obtained, we decided to use them as precursors in a reductive procedure. We anticipated that the reaction should be easily monitored through EPR spectroscopy as the starting compounds possess the well known characteristic 16-line spectrum, [42] while the expected product would be EPR silent and the Mn II products potentially obtained by "over-reduction" or disproportionation should be easily detectable through their characteristic six-line spectra. [43] As reducing agent we chose the benzil radical, because benzil is highly soluble in the reaction medium.…”

“…The parameters thus evaluated are of the same order of magnitude as those given in the literature for various bisA C H T U N G T R E N N U N G (m-oxo)Mn III Mn IV complexes. [42] The parameter B, which corresponds to the half-width at middle height of the EPR lines, decreases by approximately 0.1-0.2 mT from 1 to 3 and, more importantly (ca. 0.35 mT), to 4.…”

Redox Chemistry of a Dimanganese(II,III) Complex with an Unsymmetric Ligand: Water Binding, Deprotonation and Accumulative Light‐Induced Oxidation

“…It bears the typical signatures of a S = 1/2 ground state of a strongly coupled Mn 2 III,IV complex, being about 2300 gauss wide and displaying 16 hyperfine lines with a spacing of 75-80 gauss. [34,42] The hyperfine splitting arises because of strong antiferromagnetic coupling typical of di-µ-oxo-or di-µ-hydroxo-bridged Mn 2 III,IV dimers. For [Mn 2 (bpmp)(µ-OAc) 2 ] + we could demonstrate that the aquo and hydroxo complexes formed by ligand exchange in the lower oxidation states were ultimately transformed into the di-µ-oxo complex upon oxidation to the Mn 2 III,IV level.…”