Inorganic Chemistry, Vol.37, No.9, 2308-2316, 1998
(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)], lends 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)(NO3)] 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) Angstrom. The dihedral angle between the plane of the nitrate ion and the closest N-p-Fe-N-p plane is 10.0 degrees. 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) Angstrom (154.84(9)degrees) and 2.083(3) Angstrom (146.14(9)degrees), respectively. Crystal data for [Fe(TpivPP)(NO3)] : a = 23.530(2) Angstrom, b = 10.0822(5) Angstrom, c = 48.748(3) Angstrom, beta = 92.145(5)degrees, monoclinic, space group I2/a, V = 11556.4(14) Angstrom(3), Z = 8, FeN9O7C64H64, 8798 observed data, R-1 = 0.0606, wR(2) = 0.1313, all observations at 127(2) K.
Keywords:NITRITE REDUCTASE;CRYSTAL-STRUCTURE;L-ARGININE;MOLECULAR-STRUCTURE;NITROSYL COMPLEXES;MOSSBAUER-SPECTRA;PROSTHETIC GROUP;OXIDE;OXIDATION;MODEL