From activation of dioxygen to formation of high-valent oxo species: Ab initio DFT studies (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.
ChemPhysChem, 2004
The 3d-transition-metal dioxo-, peroxo-, and superoxoclusters with the general composition MO 2 , M(O 2), and MOO (M Mn, Fe, Co, and Ni) were studied by DFT by the B1LYP functional. The dioxides in their ground states represent the global minima for the M O 2 system. Both ground-state dioxides and the lowest-energy peroxides are in their (d-only) highest spin states. The 6 A 1 state of Co(O 2) exceeds the d-only spin-multiplicity value (quartet), being nearly isoenergetic with the 4 A 1 state of Co(O 2). The energy gain on transforming the peroxides to the corresponding dioxides decreases in the order Mn(O 2) > Fe(O 2) > Co(O 2) > Ni(O 2) and varies in the range 0.27 ± 1.8 eV. The dissociation energy to M O 2 for all studied peroxides is less than 1 eV being the lowest (0.47 eV) for Mn(O 2). The Mn and Fe peroxides need less than 0.3 eV to rupture one of the MO bonds to form the corresponding superoxide. Mn and Fe superoxides are less stable than the corresponding peroxides; the superoxide of Co is more stable than its peroxide, while Ni superoxide is unstable-its energy is above the limit of dissociation to Ni O 2. According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophilic than those in the dioxides and superoxides, in which the terminal oxygen atom is more nucleophilic than the M-bonded oxygen atom. This result differs from the expectations based on charge-distribution analysis.
Nature Chemistry, 2013
Manganese porphyrins have been extensively investigated as model systems for the natural enzyme cytochrome P450 and as synthetic oxidation catalysts. Here, we report single-molecule studies of the multistep reaction of manganese porphyrins with molecular oxygen at a solid/liquid interface, using a scanning tunnelling microscope (STM) under environmental control. The high lateral resolution of the STM, in combination with its sensitivity to subtle differences in the electronic properties of molecules, allowed the detection of at least four distinct reaction species. Real-space and realtime imaging of reaction dynamics enabled the observation of active sites, immobile on the experimental timescale. Conversions between the different species could be tuned by the composition of the atmosphere (argon, air or oxygen) and the surface bias voltage. By means of extensive comparison of the results to those obtained by analogous solutionbased chemistry, we assigned the observed species to the starting compound, reaction intermediates and products. C ytochrome P450, an abundant enzyme in the animal, plant and microbial kingdoms, acts as a mono-oxygenase in various detoxification and biosynthesis pathways 1 . Its active site contains an Fe(III)-haem complex, which can activate molecular oxygen for the oxidation of a variety of organic substrates. Taking this efficient natural catalyst as a blueprint, many artificial systems containing Mn(III) or Fe(III) porphyrins have been developed 2 and used in catalytic oxidation reactions 3 . Numerous studies have been devoted to elucidating the role of manganese porphyrins in the complex oxygen-transfer mechanisms between oxidant and substrate 4-7 . The majority of these studies use ensemble techniques in solution, such as electrochemistry and absorption, electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The scanning tunnelling microscope (STM) has proven to be a promising tool for studying reactions at the molecular level 8-12 , and initial experiments have been reported in which the reactive properties of metallated porphyrins were studied on a surface at the single-molecule level by STM in ultrahigh vacuum (UHV) and under ambient conditions .
Dioxygen Activation and O−O Bond Formation Reactions by Manganese Corroles
Activation of dioxygen (O 2) in enzymatic and biomi-metic reactions has been intensively investigated over the past several decades. More recently, O−O bond formation, which is the reverse of the O 2-activation reaction, has been the focus of current research. Herein, we report the O 2-activation and O−O bond formation reactions by manganese corrole complexes. In the O 2-activation reaction, Mn(V)-oxo and Mn(IV)-peroxo intermediates were formed when Mn(III) corroles were exposed to O 2 in the presence of base (e.g., OH −) and hydrogen atom (H atom) donor (e.g., THF or cyclic olefins); the O 2-activation reaction did not occur in the absence of base and H atom donor. Moreover, formation of the Mn(V)-oxo and Mn(IV)-peroxo species was dependent on the amounts of base present in the reaction solution. The role of the base was proposed to lower the oxidation potential of the Mn(III) corroles, thereby facilitating the binding of O 2 and forming a Mn(IV)-superoxo species. The putative Mn(IV)-superoxo species was then converted to the corresponding Mn(IV)-hydroperoxo species by abstracting a H atom from H atom donor, followed by the O−O bond cleavage of the putative Mn(IV)-hydroperoxo species to form a Mn(V)-oxo species. We have also shown that addition of hydroxide ion to the Mn(V)-oxo species afforded the Mn(IV)-peroxo species via O−O bond formation and the resulting Mn(IV)-peroxo species reverted to the Mn(V)-oxo species upon addition of proton, indicating that the O−O bond formation and cleavage reactions between the Mn(V)-oxo and Mn(IV)-peroxo complexes are reversible. The present study reports the first example of using the same manganese complex in both O 2-activation and O−O bond formation reactions.
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.
Ab initio molecular dynamics study of manganese porphine hydration and interaction with nitric oxide
The Journal of chemical physics, 2007
We use ab initio molecular dynamics (AIMD) and the DFT+U method to compute the hydration environment of the manganese ion in manganese (II) and manganese (III) porphines (MnP) dispersed in liquid water. These are intended as simple models for more complex water soluble porphyrins, which have important physiological and electrochemical applications. The manganese ion in Mn(II)P exhibits significant out-of-porphine plane displacement, and binds strongly to a single H2O molecule in liquid water. The Mn in Mn(III)P is on average coplanar with the porphine plane, and forms a stable complex with two H2O molecules. The residence times of these water molecules exceed 15 ps. The DFT+U method correctly predicts that water displaces NO from Mn(III)P-NO, but yields an ambiguous spin state for the MnP(II)-NO complex.
The Journal of Physical Chemistry, 1994
Local density functional (LDF) calculations, including geometry optimization, have been carried out on oxo-(porphyrinato)iron(IV), PFeO, and the corresponding cation, [PFeO]+, which have been chosen as simple models of peroxidase compounds I1 and I, respectively. In the optimized structure of PFeO, the Fe-0 distance was 1.622 A and the iron atom was positioned 0.215 A above the plane of the four porphyrin nitrogens. The harmonic Fe-0 stretching frequencies of PFeO and [PFeO]+ were 934 and 964 cm-I, respectively. A threebody (P-Fe-0) vibrational analysis revealed negligible coupling between the Fe-0 stretch and the displacement of the iron atom out of the porphyrin plane. In both PFeO and [PFeO]+, two unpaired spins, corresponding to a (IT*)* configuration, were cleanly localized on the ferryl moiety, being divided among Fe and 0 in the ratio 1.2:O.g. The third unpaired spin of [PFeO]+ was distributed over the porphyrin ring as a n "Az,"-type cation radical. Overall, these L D F results represent good agreement between first-principles theory and experiment. Spin-restricted Hartree-Fock theory is known to provide a poor description of both the porphyrin ligand and the ferryl group. The C A S S C F method provides a good description of the ferryl group but is computationally unwieldy for large molecules such as hemes. Density functional theory appears to provide a n expedient solution to the problem of several configurations with significant contributions to the wave functions of ferryl intermediates and is a practical theoretical tool for studying ferryl species.