“True” Iron(V) and Iron(VI) Porphyrins: A First Theoretical Exploration

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Abstract
We present here a first theoretical characterization of iron(V) (S = 3/2) and iron(VI) (S = 0) porphyrin intermediates. The Fe(V) calculations exhibit exceptionally narrow convergence radii and we believe that for this reason they have long eluded researchers working on high-valent iron intermediates. The FeV−Nnitrido bond distance in the DFT(PW91/TZP) optimized geometry of FeV(P)(N) is 1.722 Å, comparable to and slightly longer than the FeIV−O bond distance of 1.684 Å in FeIV(P)(O) and the FeIV−Nimido bond distance of 1.698 Å in FeIV(P)(NH). In contrast, the FeVI−Nnitrido bond distances in [FeVI(P)(N)]+ (S = 0) and FeVI(P)(N)(F) (S = 0) are dramatically shorter, 1.508 and 1.533 Å, respectively, consistent with the formal triple bond character of the FeVI−Nnitrido bond. The nitrido ligand appears to be uniquely capable of stabilizing a “true” Fe(V) center, in the sense defined in the paper. All three unpaired electrons in FeV(P)(N) are completely localized on the FeV−Nnitrido axis, with the Fe and N gross atomic spin populations being 1.579 and 1.550, respectively. In contrast, an axial ligand set consisting of an oxide and a fluoride do not stabilize an Fe(V) ground state but favor an electronic structure best described as an FeIV-oxo porphyrin π-cation radical.