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)-dmg(3)Mn(II)-Fe(III)-L](ClO4)(2) have been characterized by magnetic susceptibility, EPR, and Mossbaue r studies. L represents 1,4,7-trimethyl-,1,4,7-triazacyclononane and d mg represents dimethylglyoxime. X-ray diffraction measurements yield t hat the arrangement of the three metal centers is strictly linear with atomic distances d(Fe-Mn) = 0.35nm and d(Fe-Fe) = 0.7nm. Magnetic sus ceptibility measurements (3-295 K) were analyzed in the framework of t he spin-Hamiltonian formalism considering Heisenberg exchange and Zeem an interaction: H = J(Fe-Mn) (S-Fe1 + S-Fe2)S-Mn + J(Fe-Fe)S(Fe1)S(Fe2 ) + g mu(B)S(total)B. The spins S-Fe1 = S-Fe2 = S-Mn = 5/2 of the comp lex are antiferromagnetically coupled, yielding a total spin of S-tota l = 5/2 with exchange coupling constants F-Fe-Mn = 13.4 cm(-1) and J(F e-Fe)= 4.5 cm(-1). Magnetically split Mossbauer spectra were recorded at 1.5 K under various applied fields (20 mT, 170 mT, 4 T). The spin-H amiltonian analysis of these spectra yields isotropic magnetic hyperfi ne coupling with A(total)/(g(N) mu(N)) = -18.5 T. The corresponding lo cal component A(Fe) is related to A(total) via spin-projection: A(tota l) = (6/7)A(Fe). The resulting A(Fe)/(g(N) mu(N)) = -21.6 T is in agre ement with standard values of ferric high-spin complexes. Spin-Hamilto nian parameters as obtained from Mossbauer studies and exchange coupli ng constants as derived from susceptibility measurements are corrobora ted by temperature-dependent EPR studies.