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Mn K-edge X-ray absorption studies of oxo- and hydroxo-manganese(IV) complexes: experimental and theoretical insights into pre-edge properties.

Leto DF, Jackson TA - Inorg Chem (2014)

Bottom Line: Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn-O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity.These results underscore the importance of reporting experimental pre-edge areas rather than peak heights.Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 Mn(V)═O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas , Lawrence, Kansas 66045, United States.

ABSTRACT
Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [Mn(II)(Cl)2(Me2EBC)], [Mn(IV)(OH)2(Me2EBC)](2+), and [Mn(IV)(O)(OH)(Me2EBC)](+), which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [Mn(IV)(O)(OH)(Me2EBC)](+) revealed Mn-O scatterers at 1.71 and 1.84 Å and Mn-N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [Mn(II)(Cl)2(Me2EBC)] and [Mn(IV)(OH)2(Me2EBC)](2+) are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn-O(H) distances and pre-edge peak areas of Mn(IV)═O and Mn(IV)-OH complexes, but this trend was strongly modulated by the Mn(IV) coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn-O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal Mn(IV)═O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn═O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn═O σ* molecular orbital (MO) but also show intense transitions to 3dx(2)-y(2) and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal Mn(IV)═O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 Mn(V)═O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

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Correlationbetween the calculated pre-edge energy (with the +32.6 eV energy correction)and the experimental pre-edge energy for [MnII(Cl2)(Me2EBC)] (1), [MnIV(OH)2(Me2EBC)]2+ (2),[MnIV(O)(OH)(Me2EBC)]+ (3), [MnIV(O)(N4py)]2+, [MnIV(O)(Bn-TPEN)]2+, [MnIV(O)(salen)]and [MnIV(OH)(salen)]+.
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fig9: Correlationbetween the calculated pre-edge energy (with the +32.6 eV energy correction)and the experimental pre-edge energy for [MnII(Cl2)(Me2EBC)] (1), [MnIV(OH)2(Me2EBC)]2+ (2),[MnIV(O)(OH)(Me2EBC)]+ (3), [MnIV(O)(N4py)]2+, [MnIV(O)(Bn-TPEN)]2+, [MnIV(O)(salen)]and [MnIV(OH)(salen)]+.

Mentions: In general, the experimental pre-edge propertiesfor the nine MnIV complexes investigated in this studyare well reproduced by a TD-DFT method initially calibrated usinga large test set of Mn(II) and Mn(III) complexes.34 To better judge the success of this correlation in thepresent case, experimental and calculated pre-edge peak energies andareas are compared in Figures 9 and 10, respectively. A linear correlation, albeit withsome scatter, is observed between the experimental and calculatedpre-edge energies of 1, 2, 3, [MnIV(O)(N4py)]2+, and [MnIV(O)(Bn-TPEN)]2+. The calculated pre-edge energiesfor 2, 3, [MnIV(O)(N4py)]2+, and [MnIV(O)(Bn-TPEN)]2+ aresystematically overestimated by approximately 0.5 eV relative to experiment(Figure 9 and Table 4). The [MnIV(O)(salen)] and [MnIV(OH)(salen)]+ complexes are large outliers to this trend, as the experimentalenergies are nearly 2 eV lower than the theoretical values. The mostlikely explanation for this deviation is the use of a different methodfor energy calibration of the XAS data. The [MnIV(O)(salen)]and [MnIV(OH)(salen)]+ samples were calibratedto Cu foil,14 whereas the other oxomanganese(IV)samples were calibrated either to manganese foil10,14,15 or to KMnO4 powder.16 An alternate explanation would be that the [MnIV(O)(salen)] and [MnIV(OH)(salen)]+ samples underwent photoreduction during X-ray irradiation.


Mn K-edge X-ray absorption studies of oxo- and hydroxo-manganese(IV) complexes: experimental and theoretical insights into pre-edge properties.

Leto DF, Jackson TA - Inorg Chem (2014)

Correlationbetween the calculated pre-edge energy (with the +32.6 eV energy correction)and the experimental pre-edge energy for [MnII(Cl2)(Me2EBC)] (1), [MnIV(OH)2(Me2EBC)]2+ (2),[MnIV(O)(OH)(Me2EBC)]+ (3), [MnIV(O)(N4py)]2+, [MnIV(O)(Bn-TPEN)]2+, [MnIV(O)(salen)]and [MnIV(OH)(salen)]+.
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Related In: Results  -  Collection

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fig9: Correlationbetween the calculated pre-edge energy (with the +32.6 eV energy correction)and the experimental pre-edge energy for [MnII(Cl2)(Me2EBC)] (1), [MnIV(OH)2(Me2EBC)]2+ (2),[MnIV(O)(OH)(Me2EBC)]+ (3), [MnIV(O)(N4py)]2+, [MnIV(O)(Bn-TPEN)]2+, [MnIV(O)(salen)]and [MnIV(OH)(salen)]+.
Mentions: In general, the experimental pre-edge propertiesfor the nine MnIV complexes investigated in this studyare well reproduced by a TD-DFT method initially calibrated usinga large test set of Mn(II) and Mn(III) complexes.34 To better judge the success of this correlation in thepresent case, experimental and calculated pre-edge peak energies andareas are compared in Figures 9 and 10, respectively. A linear correlation, albeit withsome scatter, is observed between the experimental and calculatedpre-edge energies of 1, 2, 3, [MnIV(O)(N4py)]2+, and [MnIV(O)(Bn-TPEN)]2+. The calculated pre-edge energiesfor 2, 3, [MnIV(O)(N4py)]2+, and [MnIV(O)(Bn-TPEN)]2+ aresystematically overestimated by approximately 0.5 eV relative to experiment(Figure 9 and Table 4). The [MnIV(O)(salen)] and [MnIV(OH)(salen)]+ complexes are large outliers to this trend, as the experimentalenergies are nearly 2 eV lower than the theoretical values. The mostlikely explanation for this deviation is the use of a different methodfor energy calibration of the XAS data. The [MnIV(O)(salen)]and [MnIV(OH)(salen)]+ samples were calibratedto Cu foil,14 whereas the other oxomanganese(IV)samples were calibrated either to manganese foil10,14,15 or to KMnO4 powder.16 An alternate explanation would be that the [MnIV(O)(salen)] and [MnIV(OH)(salen)]+ samples underwent photoreduction during X-ray irradiation.

Bottom Line: Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn-O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity.These results underscore the importance of reporting experimental pre-edge areas rather than peak heights.Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 Mn(V)═O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas , Lawrence, Kansas 66045, United States.

ABSTRACT
Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [Mn(II)(Cl)2(Me2EBC)], [Mn(IV)(OH)2(Me2EBC)](2+), and [Mn(IV)(O)(OH)(Me2EBC)](+), which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [Mn(IV)(O)(OH)(Me2EBC)](+) revealed Mn-O scatterers at 1.71 and 1.84 Å and Mn-N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [Mn(II)(Cl)2(Me2EBC)] and [Mn(IV)(OH)2(Me2EBC)](2+) are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn-O(H) distances and pre-edge peak areas of Mn(IV)═O and Mn(IV)-OH complexes, but this trend was strongly modulated by the Mn(IV) coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn-O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal Mn(IV)═O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn═O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn═O σ* molecular orbital (MO) but also show intense transitions to 3dx(2)-y(2) and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal Mn(IV)═O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 Mn(V)═O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

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