Magnetic and infrared properties of the azide complex of (2,7,12,17-tetrapropylporphycenato)iron(III): A novel admixing mechanism of the S = 5/2 and S = 3/2 states

The five-coordinate ferric azide complex of 2,7,12,17- tetrapropylporphyrcene [Fe(TPrPc)N₃] was characterized with proton NMR, electron paramagnetic resonance (EPR), Mossbauer, and IR techniques. The paramagnetic NMR showed the pyrrole-H signal that exhibits anomalous anti-Curie type of 1/T-dependence over a 310-190 K range. The EPR of the solid sample resolved a signal with g = 5.42, 5.40, and 2.00 at 5 K. The quadrupole splitting parameter of the Mössbauer spectrum was 2.20 mms^-1 at 290 K, and the magnetic moment was 4.70 μ_B at 290 K for the microcrystalline sample. These results suggest that [Fe(TPrPc)N₃] is not pure high-spin (S = 5/2) but a spin mixture of the S = 5/2, 3/2 states. The azide in [Fe(TPrPc)N₃] exhibited split IR bands ascribed to the two spin isomers at 2066 and 2049 cm^-1. The 2049-cm^_1 band, assigned to the S = 3/2 species, was reversibly intensified with decreasing temperature. The IR results demonstrate that the S = 3/2, 5/2 states are not quantum-mechanically admixed but in thermal equilibrium. The thermodynamic parameters associated with the spin equilibrium, ΔH = -3 990 cal mol ^-1 and ΔS = -16.5 cal mol^-1 K^-1, were obtained from the NMR shift analysis, and the population of the S = 3/2 isomer was calculated to be 0.19 at 293 K. A minimum lifetime of the two spin isomers was estimated to be 0.3 ps with IR.