EPR and HYSCORE investigation of the electronic structure of the model complex Mn(imidazole)6: Exploring Mn(II)-imidazole binding using single crystals

The electronic structure of the Mn(II)-imidazole binding was studied by EPR spectroscopy using the model complex Mn(Im)6 diluted in a single crystal of Zn(Im)6Cl2·4(H2O). The second rank zero-field splitting (ZFS) tensor (D tensor) of the two sites, a and b, present in the crystal was determined by measuring the orientation patterns of the echo-detected EPR spectra in three different planes at 10 K (Da = −106, Db = −118, Ea = −17, Eb = −22 × 10−4 cm−1. Euler angles with respect to the crystal habitus: αa = 13°, βa = 76°, γa = 108.5°, αb = 14°, βb = 73.5°, γb = 103.5°). The contribution of cubic ZFS terms to the spectrum allowed us to determine the orientation of the N-Mn-N directions of the complex as well (Euler angles in the D tensor reference frame α = 100°, β = 23°, γ = 0°, both centers having the same orientation). The hyperfine interactions with 14N were explored by HYSCORE spectroscopy. The correlation patterns and modulation amplitudes in the 2D experiments were studied for different electron spin transitions and orientations of the crystal. Signals of three different pairs of nitrogens were found. The results were analyzed considering that the N-Mn binding directions are principal directions of the hyperfine and nuclear quadrupole tensor of 14N. All three pairs of nitrogens were found to be almost equivalent with an isotropic contribution of Aiso ≈ 3.2 MHz and an almost axial anisotropic coupling of 2T ≈ 1.1 MHz along the N-Mn bonding direction. The nuclear quadrupole principal values are 1.5 MHz along the bonding direction, −0.6 MHz in the direction perpendicular to the imidazole plane, and −0.9 MHz in the direction perpendicular to both.