Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics, electron distributions, and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processes

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](o) (P = porphyrin; X = F, Cl, PF6) and (b) high-valent manganese-ore species. For [(P)Mn(PF6)(2)](o), 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)](o) (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl2](o) and [(P)MnF2](o) 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(TV)-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 Mn-IV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the Mn-IV = O bond is significantly weaker than the Fe-IV = O bond in an analogous molecule. Thus, the optimized metal-oxygen bond distances are 1.654 and 1.674 Angstrom for (P)Fe-IV(O)(Py) and (P)Mn-IV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that Mn-IV = O stretching frequencies are over 10% lower than Fe-IV = O stretching frequencies for analogous compounds. For [(p)Mn(O)(PF6)](o), [(P)Mn(O)(Py)](+), and [(P)Mn(O)(F)](o), the ground states clearly correspond to a (d(xy))(2) Mn(V) configuration and the short Mn-O distances of 1.541, 1.546, and 1.561 Angstrom 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.