Mössbauer and susceptibility study of inter-heme spin coupling in [Fe (OEP) (2-MeHIm)]+ dimers

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Abstract
Mössbauer spectra of polycrystalline [Fe(OEP)(2-MeHIm)]ClO4 ⋅ CHCl3 (OEP=octaethylporphyrinate; 2-meHIm=2-methylimidazole) were recorded over a range of temperatures (1.54 to 195 K) and applied magnetic fields (0–6 T). Magnetic susceptibility was measured at both 0.2 and 1 T in the temperature range 6–300 K. Mössbauer parameters δ=0.40 mm/s and ΔE=1.39 mm/s at 4.2 K, and the high temperature susceptibility Neff=6 at 300 K indicated high spin iron, but no simple spin Hamiltonian would reproduce the rapid decrease of Neff below 50 K. We interpret the data by assuming a zero field splitting D=12 cm−1 and invoking an antiferromagnetic exchange interaction J=−0.8 cm−1 between the five-coordinate iron atoms of closely spaced face-to-face heme pairs in the crystal. Slow spin relaxation at low temperature permits observation of level crossing and confirms the interpretation. It also permits the detection, through low field Mössbauer measurements, of a small rhombic distortion with E=0.024 cm−1. Mössbauer spectra of the same compound but as a dibromomethane solvate, [Fe(OEP)(2-MeHIm)]ClO4 ⋅ CH2Br2, yield generally similar parameter values, but suggest a slightly larger coupling, J=−0.85 cm−1, consistent with the fact that the paired iron atoms are known to be closer in this material.