Magnetic and <sup>57</sup>Fe Mössbauer Study of the Single Molecule Magnet Behavior of a Dy<sub>3</sub>Fe<sub>7</sub> Coordination Cluster

The reaction between <i>N</i>-methydiethanolamine (mdeaH<sub>2</sub>), benzoic acid, FeCl<sub>3</sub>, and DyCl<sub>3</sub> yields a decanuclear coordination cluster, [Dy<sub>3</sub>Fe<sub>7</sub>(μ<sub>4</sub>-O)<sub>2</sub>(μ<sub>3</sub>-OH)<sub>2</sub>(mdea)<sub>7</sub>(μ-benzoate)<sub>4</sub>(N<sub>3</sub>)<sub>6</sub>]·2H<sub>2</sub>O·7CH<sub>3</sub>OH (<b>1</b>) whose single crystal structure exhibits three and seven crystallographically distinct Dy(III) and Fe(III) ions; six of the Fe(III) ions are <i>pseudo</i>-octahedrally coordinated, whereas the seventh has a trigonal-bipyramidal coordination geometry. Both direct current (<i>dc</i>) and alternating current (<i>ac</i>) magnetic susceptibility studies indicate that, upon cooling, intracluster antiferromagnetic interactions are dominant in <b>1</b>, yielding a ferrimagnetic spin arrangement. The out-of-phase (χ′′) <i>ac</i> susceptibility reveals that <b>1</b> undergoes a slow relaxation of its magnetization mainly resulting from the anisotropy of the Dy(III) ions. This slow relaxation has been confirmed both by magnetization measurements on an oriented single crystal of <b>1</b> and by the observation of hysteresis loops below 1.9 K. The macroscopic magnetic studies yield an effective energy barrier, <i>U</i><sub>eff</sub>, of 33.4 K for this relaxation, a barrier that is the highest yet reported for a lanthanide(III)-Fe(III) single molecule magnet. The <sup>57</sup>Fe Mössbauer spectra of <b>1</b> 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 <i>pseudo</i>-octahedral Fe(III) sites.