Iron(II) Complexes with Amide-Containing Macrocycles as Non-Heme Porphyrin Analogues

Iron(II) complexes of macrocyclic pentadendate ligands 3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pydioneN5) and 16-chloro-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pyCldioneN5) were synthesized and fully characterized. Complexes with one or two deprotonated amide
groups of H2pydione were both isolated. In the former case the metal ion has a distorted octahedral coordination
sphere; in the latter case the complex adopts a pentagonal−bipyramidal geometry. NMR experiments show that
the protonation state of the ligand is preserved in a dimethyl sulfoxide (DMSO) solution. The complexes maintain
a high-spin state even at low temperatures. Detailed kinetic studies of oxygenation of the iron(II) complexes showed
that the deprotonation state of the complex has a profound effect on the reactivity with dioxygen. Oxygenation of
the dideprotonated complex of iron(II), Fe(pydioneN5), in aprotic solvents proceeds via a path that is analogous to
that of iron(II) porphyrins:  via iron(III) superoxo and diiron(III) peroxo species, as evidenced by the spectral changes
during the reaction, which is second-order in the concentration of the iron(II) complex, and with an inverse dependence
of the reaction rate on the concentration of dioxygen. The final products of oxygenation are crystallographically
characterized iron(III) μ-oxo dimers. We have also found that the presence of 1-methylimidazole stabilizes the
diiron peroxo intermediate. The reaction of Fe(pydioneN5) with dioxygen in methanol is distinctly different under the
same conditions. The reaction is first-order in both iron(II) complex and dioxygen, and no intermediate is
spectroscopically observed. Similar behavior was observed for the monodeprotonated complex Fe(HpydioneN5)(Cl).
The presence of an accessible proton either from the solvent (reactions in methanol) or from the complex itself (in
Fe(HpydioneN5)(Cl)) proves sufficient to alter the oxygenation pathway in these macrocyclic systems, which is
reminiscent of the properties of iron(II) porphyrin complexes. The new amidopyridine macrocycles can be considered
as new members of the “expanded porphyrin analogue” family. The expansion of the cavity provides control over
the spin state and availability of protons. These macrocyclic systems also allow for easy synthetic modifications,
paving the way to new, versatile metal complexes.