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

Article abstract of DOI:10.1021/ic970855l

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 [Fe^III(TpivPP)(NO₃)] and [Fe^II(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)J 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) A?. The dihedral angle between the plane of the nitrate ion and the closest N_p-Fe-N_p plane is 10.0°. The Fe-N_p bonds (and trans N_p-Fe-N_p angles) perpendicular and parallel to the plane of the axial ligand average to 2.060(5) A? (154.84(9)°) and 2.083(3) A? (146.14(9)°), respectively. Crystal data for [Fe(TpivPP)(NO₃)]: a = 23.530(2) A?, b = 10.0822(5) A?, c = 48.748-(3) A?, β = 92.145(5)°. monoclinic, space group 121 a, V= 11556.4(14) A?³, Z= 8, FeN₉O₇C₆₄H₆₄, 8798 observed data, R₁ = 0.0606, wR₂ = 0.1313, all observations at 127(2) K.