Magnetic and ⁵⁷Fe Mössbauer Study of the Single Molecule Magnet Behavior of a Dy₃Fe₇ Coordination Cluster

The reaction between N-methydiethanolamine (mdeaH2), benzoic acid, FeCl3, and DyCl3 yields a decanuclear coordination cluster, [Dy3Fe7(µ4-O) 2(µ3-OH)2(mdea)7(µ-benzoate) 4(N3)6] · 2H2O · 7CH3OH (1) whose single crystal structure exhibits three and seven crystallog raphically distinct Dy(III) and Fe(III) Ions; six of the Fe(III) Ions arepseudo octahedrally coordinated, whereas the seventh has a trigonal-bipyramldal coordination geometry. Both direct current (dc) and alternating current (ac) magnetic susceptibility studies Indicate that, upon cooling, intracluster antlferromagnetlc interactions are dominant In 1, yielding a ferrlmagnetic spin arrangement. The out-of-phase (X ) ac susceptibility reveals that 1 undergoes a slow relaxation of its magnetization mainly resulting from the anlsotropy of the Dy(III) Ions. This slow relaxation has been confirmed both by magnetization measurements on an oriented single crystal of 1 and by the observation of hysteresis loops below 1.9 K. The macroscopic magnetic studies yield an effective energy barrier, Ueff, of 33.4 K for this relaxation, a barrier that is the highest yet reported for a lanthanlde(III)Fe(III) single molecule magnet. The 57Fe Mössbauer spectra of 1 obtained between 3 and 35 K are consistent with the presence of Fe(III) intracluster antiferromagnetic coupling with slow magnetic relaxation relative to the Larmor precession time, thus confirming, on a microscopic scale, the presence of a barrier to the magnetic relaxation below 35 K. Between 55 and 295 K the Mössbauer spectra reveal paramagnetic behavior with six partially resolved quadrupole doublets, one for the trigonal-bipyramidal Fe(III) site and five for the six pseudo-octahedral Fe(III) sites