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

Abstract

The synthesis, crystal structures, Mössbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe3+/Ln3+ complexes, [Fe2Ln(PhCO2)3((py)2CO2)((py)2C(OMe)O)2(NO3)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)2CO22− and (py)2C(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+(μ-OR)Ln(μ-OR)2Fe3+]6+ structural core formed from the connection of the two terminal Fe3+ centers to the central Ln3+ 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 Ueff = 6.7 (±0.3) K and a pre-exponential factor, τ0 = 2.2 (±0.4) × 10−7 s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature 57Fe Mössbauer 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 Mössbauer 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 Mössbauer 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 Mössbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mössbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10−7 s.