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 Mossbauer 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 distancesdFe-Mn=0.35 nm anddFe-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=JFe-Mn(SFe1+SFe2)SMn +JFe-Fe(SFe1SFe2) +gΜBStotalB. The spinsSFe1=SFe2 =SMn=5/2 of the complex are antiferromagnetically coupled, yielding a total spin ofStotal=5/2 with exchange coupling constantsFFe-Mn=13.4 cm−1 andJFe-Fe= 4.5 cm−1. Magnetically split Mossbauer spectra were recorded at 1.5 K under various applied fields (20 mT, 170 mT, 4T). The spin-Hamiltonian analysis of these spectra yields isotropic magnetic hyperfine coupling withAtotal/(gNΜN)=−18.5 T. The corresponding local componentAFe is related toAtotal via spin-projection:Atotal=(6/7)AFe. The resultingAFe/(gNΜN)=−21.6 T is in agreement with standard values of ferric high-spin complexes. Spin-Hamiltonian parameters as obtained from Mossbauer studies and exchange coupling constants as derived from susceptibility measurements are corroborated by temperature-dependent EPR studies.