Molecular magnetism of a linear Fe(III)-Mn(II)-Fe(III) complex. Influence of long-range exchange interaction

The magnetic properties of [L-Fe(III)-dmg₃Mn(II)-Fe(III)-L] (ClO₄)₂ have been characterized by magnetic susceptibility, EPR, and Mössbauer studies. L represents 1,4,7-trimethyl-, 1,4,7-triazacyclononane and dmg represents dimethylglyoxime. X-ray diffraction measurements yield that the arrangement of the three metal centers is strictly linear with atomic distances d_Fe-Mn=0.35 nm and d_Fe-Fe=0.7 nm. Magnetic susceptibility measurements (3-295 K) were analyzed in the framework of the spin-Hamiltonian formalism considering Heisenberg exchange and Zeeman interaction:Ĥ=J_Fe-Mn(S_Fe1+S_Fe2)S_Mn +J_Fe-Fe(S_Fe1S_Fe2) +gΜ_BS_totalB. The spins S_Fe1=S_Fe2 =S_Mn=5/2 of the complex are antiferromagnetically coupled, yielding a total spin of S_total=5/2 with exchange coupling constants F_Fe-Mn=13.4 cm^-1 and J_Fe-Fe= 4.5 cm^-1. Magnetically split Mössbauer spectra were recorded at 1.5 K under various applied fields (20 m T, 170 mT, 4T). The spin-Hamiltonian analysis of these spectra yields isotropic magnetic hyperfine coupling with A_total/(g_NΜ_N)=-18.5 T. The corresponding local component A_Fe is related to A_total via spin-projection:A_total=(6/7)A_Fe. The resulting A_Fe/(g_NΜ_N)=-21.6 T is in agreement with standard values of ferric high-spin complexes. Spin-Hamiltonian parameters as obtained from Mössbauer studies and exchange coupling constants as derived from susceptibility measurements are corroborated by temperature-dependent EPR studies.