Six-Coordinate Quantum-Mechanically Weakly Spin-Mixed (S = 5/2, 3/2) (Triflato)aquoiron(III) “Picket-Fence” Porphyrin Complex: Synthesis and Structural, Mössbauer, EPR, and Magnetic Characterization

The iron(III) “picket-fence” porphyrin complex [Fe^III(TP_pivP)(OSO₂CF₃)(H₂O)] (1) was synthesized and characterized by its UV-visible, ¹H NMR, EPR, magnetic, and Mössbauer properties. The X-ray structure of 1 was determined at -100 °C. Crystal data: [Fe(TP_pivP)(OSO₂CF₃)(H₂O)] (C₆₅H₆₆N₈O₈F₃SFe); monoclinic; a = 13.161 (3), b = 19.196 (6), c = 26.212 (6) Å; β = 103.34 (2)°; Z = 4; d_calc = 1.270 g cm^-3; space group P2₁/c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-N_p = 2.021 (16) Å), to an oxygen atom of the triflate ion (Fe-O(triflate) = 2.188 (5) Å), placed inside the molecular cavity of the picket-fence porphyrin, and to a water molecule (Fe-O(water) = 2.133 (5) Å). The susceptibility measurements in an external field of 1.5 T show that the effective magnetic moment varies from 4.0 to 5.7 μ_B in the temperature range 2-300 K. The EPR data yield g_⊥ = 5.7, which corresponds to a mixture of 85% spin sextet ⁶A₁ and 15% spin quartet ⁴A₂ in the lowest Kramers doublet. This mixture is somewhat stronger than in typical high-spin iron porphyrins. Mössbauer spectra were recorded at temperatures varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature-independent quadrupole splitting, ΔE_Q≈ 2.2 mm s^-1, which lies between the ΔE_q values characteristic for high-spin (S = 5/2) and intermediate-spin (S = 3/2) porphyrins. The magnetic hyperfine patterns of the measured Mössbauer spectra, in the slow and fast relaxation limit, are successfully simulated within the 10-state model for the spin mixture between ⁶A₁ and ⁴A₂ by using parameters that have been derived from susceptibility and EPR data within the same model. The applicability of the usual spin (S = 5/2) Hamiltonian analysis and its relation to the 10-state model are discussed. Spin-spin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime within the framework of the (S = 5/2) spin Hamiltonian. The degree of spin mixture together with the axial and porphyrinato-nitrogen coordination of iron in 1 and related complexes is discussed on the basis of a putative spin-state/stereo-chemical relationship.