Magnetic Circular Dichroism Studies of the Low-Energy Absorption Bands of Mn(III) Porphyrin Complexes

Linder, Robert E.; Rowlands, J. R.

doi:10.1080/00387017108064643

Cited by 11 publications

(9 citation statements)

References 4 publications

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“…This approach has been shown to be quite powerful in the treatment of high-spin Mn III centers. 1e,13e,14a,14c–f,15a,15b As demonstrated here, MCD spectroscopy is equally adept at treating low-spin Mn III systems. This technique, particularly with the use of variable fields and temperatures (VTVH-MCD) allowed elucidation of the range of relevant electronic excited states for 1 and 2 .…”

Section: Discussionmentioning

confidence: 78%

“…Thus, a careful analysis can yield transition polarizations for randomly-oriented samples, which can be a great aid in developing spectral assignments. While this approach has been used with success in analyzing the electronic structure of a limited number of mononuclear Mn III centers in Mn-dependent enzymes 14 and corresponding model complexes, 1e,13e,15 all of these previous examples contained S = 2 Mn III centers. To the best of our knowledge, the MCD method has not been applied to Mn III centers with S = 1 ground states.…”

Section: Introductionmentioning

confidence: 99%

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Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

et al. 2015

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Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp− and Tp*− (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]−, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SBF6), [Tp*2Mn]SbF6 (2SBF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the “conventional” high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10%, < 2 cm−1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures.

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Section: Discussionmentioning

confidence: 78%

Section: Introductionmentioning

confidence: 99%

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

et al. 2015

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“…While band 4 is hard to detect in the UV–vis spectrum, it becomes apparent as a weak signal (−17 M –1 cm –1 T –1 ) in the MCD spectrum. An analogous feature at 17200 cm –1 in [Mn III (TPP)(Cl)(H 2 O)] has also been assigned to a π → π* (0) transition, with the possibility of it also being CT (3) . ,, According to the TD-DFT calculations, bands 4 and 5 (just like bands 2 and 3) also show an admixture of a symmetry-forbidden CT transition, Cl(p x / y )_d π → d x 2 – y 2 , with 25 and 8% contributions, respectively.…”

Section: Results and Analysismentioning

confidence: 96%

“…To start, a weak and rather broad feature just above the baseline of the UV–vis spectrum is observed at 14935 cm –1 (ε = 1163 M –1 cm –1 ) that has no corresponding feature in the MCD spectrum (band 1). We consider this feature because previous studies on Mn III porphyrin chloro complexes display near-IR bands within this energy region, at 14500 and 14472 cm –1 . , Although the experimental polarization for this band is not available, relative energies and oscillator strengths may be used to assign this feature in comparison with theoretical data. This band is possibly identified with two calculated features at 15186 and 15569 cm –1 , which are xy - and z -polarized, respectively.…”

Section: Results and Analysismentioning

confidence: 99%

“…The splitting of the Soret band (which will be referred to as the “split Soret band” from here on) is attributed to the mixing of the π → π* (0) transitions with other porphyrin → metal CT transitions as discussed in detail below, and not the two components of the E u symmetric Soret state. The unique spectral features in this Mn III complex may be attributed to significant π interactions, involving the metal (d π ) and porphyrin and chloride donor orbitals, respectively. , Although tentative assignments of the UV–visible absorption spectra of Mn III porphyrins have been reported, ,, a definitive assignment of the absorption features of [Mn III (TPP)Cl] has yet to be accomplished. In this work, we present a complete description of the different electronic transitions associated with the complicated UV–visible spectrum of hs [Mn III (TPP)Cl].…”

Section: Introductionmentioning

confidence: 98%

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Elucidating the Electronic Structure of High-Spin [Mn^III(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy

et al. 2020

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Manganese porphyrins are used as catalysts in the oxidation of olefins and nonactivated hydrocarbons. Key to these reactions are high-valent Mn−(di)oxo species, for which [Mn(Porph)(X)] serve as precursors. To elucidate their properties, it is crucial to understand the interaction of the Mn center with the porphyrin ligand. Our study focuses on simple high-spin [Mn III (TPP)X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [Mn III (TPP)Cl], using variabletemperature variable-field magnetic circular dichroism spectroscopy and time-dependent density functional theory to help with band assignments. The optical properties of [Mn III (TPP)Cl] are complicated and unusual, with a Soret band showing a high-intensity feature at 21050 cm −1 and a broad band that spans 23200−31700 cm −1 . The 15000−18500 cm −1 region shows the Cl(p x/y ) → d π (CT (Cl,π) ), Q band, and overlap-forbidden Cl(p x/y )_d π → d x 2 −y 2 transitions that gain intensity from the strongly allowed π → π* (0) transition. The 20000−21000 cm −1 region displays the prominent pseudo A-type signal of the Soret band. The strongly absorbing features at 22500−28000 cm −1 exhibit A 1u ⟨79⟩/A 2u ⟨81⟩ → d π , CT (Cl,π/σ) , and symmetry-forbidden CT character, mixed with the π → π* (0) transition. The strong d x 2 −y 2 _B 1g ⟨80⟩ orbital interaction drives the ground-state MO mixing. Importantly, the splitting of the Soret band is explained by strong mixing of the porphyrin A 2u (π)⟨81⟩ and the Cl(p z )_d z 2 orbitals. Through this direct orbital pathway, the π → π* (0) transition acquires intrinsic metal-d → porphyrin CT character, where the π → π* (0) intensity is then transferred into the high-energy CT region of the optical spectrum. The heavier halide complexes support this conclusion and show enhanced orbital mixing and drastically increased Soret band splittings, where the 21050 cm −1 band shifts to lower energy and the high-energy features in the 23200−31700 cm −1 range increase further in intensity, compared to the chloro complex.

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σ‐Pyridine Coordination Compounds with Transition Metals

1985

Chemistry of Heterocyclic Compounds: A Series of Monographs

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Magnetic Circular Dichroism Studies of the Low-Energy Absorption Bands of Mn(III) Porphyrin Complexes

Cited by 11 publications

References 4 publications

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

Elucidating the Electronic Structure of High-Spin [Mn^III(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy

σ‐Pyridine Coordination Compounds with Transition Metals

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Magnetic Circular Dichroism Studies of the Low-Energy Absorption Bands of Mn(III) Porphyrin Complexes

Cited by 11 publications

References 4 publications

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

Spectroscopic and Computational Investigation of Low‐Spin MnIII Bis(scorpionate) Complexes

Elucidating the Electronic Structure of High-Spin [MnIII(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy

σ‐Pyridine Coordination Compounds with Transition Metals

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Elucidating the Electronic Structure of High-Spin [Mn^III(TPP)Cl] Using Magnetic Circular Dichroism Spectroscopy