(Nitro)Iron(III) Porphyrins. EPR Detection of a Transient Low-Spin Iron(III) Complex and Structural Characterization of an O Atom Transfer Product

The reaction of BF3·OEt2 with the
bis(nitro) complex of iron(III) picket-fence porphyrin,
[K(18C6)(OH2)][Fe(TpivPP)(NO2)2], leads to the formation of
a transient porphyrin intermediate, assigned on the basis of its
rhombic
low-spin EPR spectrum as the five-coordinate N-bound mono(nitro)
iron(III) derivative, [Fe(TpivPP)(NO2)].
This
species is reactive and readily undergoes oxygen atom transfer to form
[FeIII(TpivPP)(NO3)] and
[FeII(TpivPP)(NO)]. The reactions have been followed by EPR and IR
spectroscopy. [Fe(TpivPP)(NO2)] has a
rhombic EPR
spectrum (g = 2.60, 2.35, and 1.75) in chlorobenzene and
CH2Cl2 and is spectroscopically distinct
from the
bis(nitro) starting material (g = 2.70, 2.50, and
1.57). Oxidation of the nitrosyl species to
[Fe(TpivPP)(NO3)]
proceeds via an intermediate assigned as
[Fe(TpivPP)(NO2)] on the basis of its EPR
spectrum. The crystal structure
of one of the reaction products,
[Fe(TpivPP)(NO3)], has been determined.
The nitrate ion of
[Fe(TpivPP)(NO3)]
is bound to the iron(III) ion in a “symmetric” bidentate
fashion within the ligand-binding pocket of the porphyrin
pickets. Individual Fe−O distances are 2.123(3) and
2.226(3) Å. The dihedral angle between the plane of
the
nitrate ion and the closest Np−Fe−Np plane
is 10.0°. The Fe−Np bonds (and trans
Np−Fe−Np angles)
perpendicular and parallel to the plane of the axial ligand average to
2.060(5) Å (154.84(9)°) and 2.083(3) Å
(146.14(9)°), respectively. Crystal data for
[Fe(TpivPP)(NO3)]:  a =
23.530(2) Å, b = 10.0822(5) Å, c
= 48.748(3) Å, β = 92.145(5)°, monoclinic, space group
I2/a, V = 11556.4(14)
Å3, Z = 8,
FeN9O7C64H64, 8798
observed
data, R1 = 0.0606, wR2
= 0.1313, all observations at 127(2) K.