Characterization of the Bis(azido)(meso-tetraphenylporphinato)ferrate(III) Anion. An Unusual Spin-Equilibrium System

The complex anion bis(azido)(tetraphenylporphinato)ferrate(III) has been synthesized and characterized by variable-temperature X-ray structure determinations, powder and single-crystal EPR, IR, and Mössbauer spectroscopy, and magnetic susceptibility measurements. The synthesis utilizes 18-crown-6 to solubilize sodium azide in the synthetic procedure. All physical data for [Na(18C6)(H2O)2][Fe(TPP)(N3)2]·2C6H5Cl are consistent with a thermal spin-equilibrium system: low spin (S = 1/2) ⇌ high spin (S = 5/2). Structure determinations at 130 and 293 K show equatorial and axial Fe−N bond elongation at 293 K. Fe−Np = 1.9991(11) Å, Fe−Naz = 1.9734(14) Å at 130 K, and Fe−Np = 2.010(4) Å, Fe−Naz = 1.998(2) Å at 293 K. The EPR g values for a powder sample at 4.2 K are 1.81, 2.18, and 2.70. A fit of the powder EPR spectrum at 4.2 K with a crystal field model that allows quartet and sextet admixtures suggests that the first sextet state is ∼655 cm-1 above the ground doublet. Single-crystal EPR data indicate that the largest g value occurs at an angle of 56° from the porphyrin normal and at 35 and 81° from the Fe−Np vectors. The asymmetric azide IR absorption bands at 2014 and 2036 cm-1 can be assigned to low- and high-spin species, respectively, and display temperature-dependent intensities. The Mössbauer experiments reveal a gradual decrease in the quadrupole splitting as the temperature increases from 140 to 300 K. The magnetic susceptibility measurements show a gradual increase of μeff with temperature. Crystal data for [Na(18C6)(H2O)2][Fe(TPP)(N3)2]·2C6H5Cl (130 K): a = 11.417(2) Å, b = 12.371(4) Å, c = 12.628(2) Å, α = 64.30(2)°, β = 77.18(3)°, γ = 77.67(2)°, triclinic, space group P1̄, Z = 1. Crystal data (293 K): a = 11.7652(12) Å, b = 12.6488(6) Å, c = 12.8608(13) Å, α = 62.02(2)°, β = 75.996(7)°, γ = 75.465(9)°.