Heterometallic clusters based on an uncommon asymmetric V-shaped [Fe3+(μ-OR)Ln3+(μ-OR)2Fe3+]6+ (Ln = Gd, Tb, Dy, Ho) structural core and the investigation of the slow relaxation of the magnetization behaviour of the [Fe2Dy] analogue

The synthesis, crystal structures, Mossbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe3+/Ln(3+) complexes, [Fe(2)Ln(PhCO2)(3)((py)(2)CO2)((py)(2)C(OMe)O)(2)(NO3)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)(2)CO22- and (py)(2)C(OMe)O- are the anions of the gem-diol and hemiketal derivatives of di-2-pyridyl ketone, are reported. Compounds 1/Ln are based on an asymmetric V-shaped [Fe3+(mu-OR)Ln(mu-OR)(2)Fe3+](6+) structural core formed from the connection of the two terminal Fe3+ centers to the central Ln(3+) ion either through one or two alkoxide groups originating from the alkoxide-type bridging ligands. Direct current magnetic susceptibility studies reveal the presence of weak antiferromagnetic interactions between the Fe3+ ions. Alternating current magnetic studies indicate the presence of a slow-magnetic relaxation process in 1/Dy with an energy barrier U-eff = 6.7 (& PLUSMN;0.3) K and a pre-exponential factor, tau(0) = 2.2 (& PLUSMN;0.4) x 10(-7) s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature Fe-57 Mossbauer spectroscopy. At T > 80 K the spectra from the complexes comprise two quadrupole doublets the hyperfine parameters of which reflect the distinct coordination environment of the two Fe3+ terminal sites. At T < 20 K, the Mossbauer spectra for 1/Dy are affected by magnetic relaxation effects. At 1.5 K, the spectrum of 1/Dy comprises well defined magnetic sextets indicating relaxation times slower than the characteristic time of the Mossbauer technique (10(-7) s) in agreement with the dynamic magnetic measurements. 1/Gd exhibits broad unresolved magnetic sextets at 1.5 K indicating that the spin relaxation time is of the order of the Mossbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mossbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10(-7) s.

Automated summary

The synthesis, crystal structures, Mossbauer spectra, and magnetic susceptibility studies of a new family of trinuclear heterometallic Fe3+/Ln(3+) complexes are reported. The complexes exhibit weak antiferromagnetic interactions between the Fe3+ ions and slow-magnetic relaxation processes in 1/Dy. The Mossbauer spectroscopy results show distinct magnetic relaxation effects for the different complexes.