6-COORDINATE QUANTUM-MECHANICALLY WEAKLY SPIN-MIXED (S = 5/2, 3/2) (TRIFLATO)AQUOIRON(III) PICKET-FENCE PORPHYRIN COMPLEX - SYNTHESIS AND STRUCTURAL, MOSSBAUER, EPR, AND MAGNETIC CHARACTERIZATION

The iron(III) “picket-fence” porphyrin complex [FeIII(TPpivP)(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(TPpivP)(OSO2CF3)(H2O)] (C65H66N8O8F3SFe); monoclinic; a = 13.161 (3), b = 19.196 (6), c = 26.212 (6) Å; β = 103.34 (2)°; Z = 4; dcalc = 1.270 g cm-3; space group P21/c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-Np = 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 μB 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, ΔEQ≈ 2.2 mm s-1, which lies between the ΔEq 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. © 1990, American Chemical Society. All rights reserved.