Electronic structure and reactivity of isomeric oxo-Mn(V) porphyrins: Effects of spin-state crossing and pK_a modulation

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_as of 7.5 and 8.6 for the isomeric 2-TMPyP and 4-TMPyP systems, respectively.