Pierre et al (2001) New J Chem 4 - First synthesis and crystal structure of a Mn3+ complex derived from the Ogawa porphyrin-like ligand (original) (raw)
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Israel Journal of Chemistry, 2000
We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX2]0 (P = porphyrin; X = F, Cl, PF6) and (b) high-valent manganese-oxo species. For [(P)Mn(PF6)2]0, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO4)2]0 (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl2]0 and [(P)MnF2]0 have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for MnIV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the MnIV = O bond is significantly weaker than the FeIV = O bond in an analogous molecule. Thus, the optimized metal—oxygen bond distances are 1.654 and 1.674 Å for (P)FeIV(O)(Py) and (P)MnIV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that MnIV = O stretching frequencies are over 10% lower than FeIV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF6)]0, [(P)Mn(O)(Py)]+, and [(P)Mn(O)(F)]0, the ground states clearly correspond to a (dxy)2 Mn(V) configuration and the short Mn–O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn–O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species.
Journal of Inorganic Biochemistry, 2003
Manganese(III) meso-tetrakis(4-carboxypheny)porphyrin (MnTBAP) is a readily available and widely used agent to scavenge reactive oxygen species. A major limitation of MnTBAP is its relatively weak potency due to its low metal centered redox potential. The goal of these studies was to prepare a more potent analog of MnTBAP by increasing its redox potential through b-substitution on the porphyrin ring by bromination. Manganese(III) b-octabromo-meso-tetrakis(4-carboxyphenyl)porphyrin (MnBr TBAP) was prepared in three steps 8 starting from the methyl ester of the free ligand meso-tetrakis(4-carboxyphenyl)porphyrin, with an overall yield of 50%. The superoxide dismutase (SOD)-like activity of MnBr TBAP (IC 50.7 mM) was the same as manganese(III) meso-tetrakis(N-methylpyridinium-4-8 5 0 51 yl)porphyrin (MnTM-4-PyP ), while the metal-centered redox potential of the first was considerably higher than the second (E 51128 and 0 mV vs. normal hydrogen electrode, respectively). However, a number of these cationic Mn-porphyrins (such as 1 / 2 51
Journal of Molecular Catalysis A: Chemical, 2007
A new Mn-porphyrin bearing four pyridinium substituents bound through their nitrogen atoms on the meso-positions of the tetrapyrrole ring was synthesized in three steps from Zn--octaethylporphyrin. It was characterized by elemental analysis, electrospray mass spectrometry, UV-vis and dual polarization mode EPR spectroscopy, and electrochemistry. Electrochemical and EPR studies showed that the Mn-porphyrin prepared by this method existed as a 80/20 Mn(II)/Mn(III) mixture, the redox potential of the Mn(III)/Mn(II) couple being +345 mV (versus SCE, in CH 3 CN). It catalyzed alkene epoxidation and alkane hydroxylation by PhIO with characteristics comparable to those of Mn[TDCPP = meso-tetra-(2,6dichlorophenyl)porphyrin]Cl. It also catalyzed the hydroxylation of anisole, naphthalene and ethylbenzene by H 2 O 2 in CH 2 Cl 2 /CH 3 CN, as well as the hydroxylation of the drug diclofenac by oxone in water. It is a new biomimetic catalyst exhibiting two distinctive characteristics: a good solubility in both hydrophobic aprotic solvents and water for pH > 5, and an unusual structure with four positive charges very close to the metallic centre.
The manganese complex of 2,3,7,8,12,13,17,18-octaphenylporphyrin as epoxidation catalyst
Journal of Porphyrins and Phthalocyanines (JPP), 2001
chloride was prepared and its behaviour as epoxidation catalyst was studied. The role of the phenyl substituents in the ß-pyrrolic positions concerning the stability of the catalyst to the reaction conditions and the efficiency and selectivity of the oxidations was accessed in a comparative study with MnTPP and MnEtio II.
Journal of Porphyrins and Phthalocyanines, 2001
Manganese complexes of 5,15-diaryl-b-substituted-porphyrins were prepared and their behaviour as oxidation catalysts was studied. The role of the pyrrolic and meso-substituents on the activity and selectivity of these catalysts was studied to reveal new structure-activity relationships in these porphyrin-based epoxidation catalysts. The beneficial effect of the halogen atoms at the meso-phenyls is still observed with these catalysts but, for the first time, a strong dependence on the selectivity of the epoxide production was found to be dependent on the nature of the non-halogen substituents at the b-pyrrolic positions of the porphyrin.
Biomimetic Oxidation Reactions of a Naked Manganese(V)-Oxo Porphyrin Complex
The intrinsic reactivity of a manganese(V)-oxo porphyrin complex, a typically fleeting intermediate in catalytic oxidation reactions in solution, has been elucidated in a study focused on its gas-phase ion-chemistry. The naked high-valent MnV-oxo porphyrin intermediate 1 ([(tpfpp)MnVO]+; tpfpp=meso-tetrakis(pentafluorophenyl) porphinato dianion), has been obtained by controlled treatment of [(tpfpp)MnIIICl]&&square brackets have been used throughout also for the neutral species, ok?&& (2-Cl) with iodosylbenzene in methanol, delivered& &“generated”, better?&& in the gas phase by electrospray ionization and assayed by FT-ICR mass spectrometry. A direct kinetic study of the reaction with selected substrates, each containing a heteroatom X (X=S, N, P) including amines, sulfides, and phosphites, was thus performed. Ionic products arising from electron transfer (ET), hydride transfer (HT), oxygenatom transfer (OAT), and formal addition (Add) may be observed, with a predominance of two-electron processes, whereas the product of hydrogenatom transfer (HAT), [(tpfpp)MnIVOH]+, is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electrontransfer ability of a MnV-oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)MnIVO]. Linear free-energy analyses of the reactions of para-substituted N,N-dimethylanilines and thioanisoles indicate that a considerable amount of positive charge is developed on the heteroatom in the oxidation transition state. Substrates endowed with different heteroatoms, but similar ionization energy display a comparable reaction efficiency, consistent with a mechanism initiated by ET. For the first time, the kinetic acidity of putative hydroxo intermediates playing a role in catalytic oxidations, [(tpfpp)FeIVOH]+ and [(tpfpp)MnIVOH]+, has been investigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)FeIVO], with respect to the manganyl, [(tpfpp)MnIVO], unit. Finally, the neat association reaction of 2 has been studied with various ligands showing that harder ligands are more strongly bound.