Electronic Structure and Reactivity of Isomeric Oxo-Mn(V) Porphyrins:  Effects of Spin-State Crossing and pK_a Modulation

Abstract

The reactivity of the isomeric oxo-Mn(V)-2-tetra-N-methylpyridyl (2-TMPyP) and oxo-Mn(V)-4-tetra-N-methylpyridyl (4-TMPyP) porphyrins has been investigated by a combined experimental and theoretical approach based on density functional theory. The unusual higher reactivity of the more electron-rich 4-TMPyP species appears to be related to both the higher basicity of its oxo ligand, compared to that of the 2-TMPyP isomer, and the smaller low-spin-high-spin promotion energy of 4-TMPyP, compared to that of 2-TMPyP, because of the stabilization of the A2u orbital in the latter isomer. Therefore, in a two-state energy profile involving crossing of the initial singlet and final quintet potential energy surfaces, the 4-TMPyP isomer should be kinetically favored. The calculated differences in the singlet-quintet gaps for the 2-TMPyP and 4-TMPyP systems compare well with the measured differences in the activation energies for two isomeric porphyrins. Both effects, proton affinity and electron-promotion energy, contribute to reduce the reactivity of the more electrophilic oxidant when electron-withdrawing groups are closer to the active site, contrary to the usual expectations based on simple chemical reactivity correlations. These theoretical results are in accord with new experimental data showing O=Mn(V)-O-H pK(a)s of 7.5 and 8.6 for the isomeric 2-TMPyP and 4-TMPyP systems, respectively.

Modeling the haloperoxidases: Reversible oxygen atom transfer between bromide ion and an oxo-Mn(V) porphyrin

“…The fast, reversible oxygen transfer reaction between Mn(III)[TDMImP] (1)/OBr À and oxo-Mn(V) [TDMImP] (2)/Br À (Scheme 1) reported here and comparisons to similar reactions of the 2-and 4-N-methylpyridyl porphyrins we have previously described [33,34] have revealed much about the mechanism of oxidative catalysis. We have shown that the rate of formation of the high-valent oxoMn(V) complex 2 increases under basic conditions whereas the reverse process, the oxygen transfer reaction to substrates, is accelerated at low pH values.…”

“…Meunier et al have described the chlorination of dimedone by the water-soluble porphyrin Mn(III)TMPS supported on a polyvinylpyridinium polymer [32]. The direct oxidation of bromide by oxygen atom transfer from oxo-Mn(V) complexes has been observed in our laboratory as well as the reverse reaction, i.e., oxidation of the Mn(III) porphyrin with HOBr/ OBr À [33,34]. Recently, Naruta et al [35] reported the formation and characterization of a dinuclear oxo-Mn(V) porphyrin complex.…”

“…Few cases involving hypobromite, a slightly weaker oxidant than hypochlorite, have been reported [52,53]. The porphyrin complexes 3 and 4 were also reacted with this oxidant in aqueous solutions and an oxo-Mn(V) species was detected spectroscopically and kinetically characterized in both cases [33,34].…”

The Groves–Spiro Dioxomanganese(V) Story

“…Experimental data that would provide information about the manganese-oxo bond strength only existed for oxoaqua species, such as 3, and it was consistent with the expected triple bond between manganese and oxygen (a bond order of 3). [8] The uniqueness of the most recent contribution by Groves, Spiro, and co-workers is that they have now obtained physical (spectroscopic) data for species 1 (Scheme 1) and demonstrated the relationship of the structural and electronic features with chemical reactivity. [5] In doing so, they turned their attention to the manganese complexes of the non watersoluble 5,10,15,20-tetramesitylporphyrinato dianion (TMP), and worked in acetonitrile/water solutions.…”

Porphyrins in complex with DNA: Modes of interaction and oxidation reactions

“…At pH 8 the activated porphyrin, Mn V =O, produces mainly oxygen atom transfer products while at pH 6 the products of reaction originate from electron transfer [173]. The shift in reactivity as a function of pH may be explained by the protonation state of the axial water molecule trans to the metal-oxo whose pKa value was thus estimated between 7 and 8 ( Figure 19) (for pKa2 value see [179]) …”