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

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Abstract
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.