Synthesis, Structures and Single-Molecule Magnet Behaviour of Octanuclear and Decanuclear Dysprosium Clusters Based on [Dy4(μ4-O)] Tetrahedral Subunits

Two novel [Dy-4(mu(4)-O)] based dysprosium(III) clusters, namely, [Dy-8(bpt)(8)(mu(4)-O)(2)(mu-OMe)(8)(mu(1,3,3)-N-3)(mu(1,3)-N-3)(N-3)(2)]center dot 11H(2)O center dot 9MeOH (1) and [Dy-10(bpt)(6)(mu(4)-O)(4)(mu(3)-OMe)(4)(mu-OMe)(8)(mu-OAc)(2)(OAc)(2)].40H(2)0 (2) (Hbpt=3,5-bis(pyridin-2-y1)-1,2,4-trizole), are synthesized successfully. To obtain complex 1, a mixture of DyCl3 (0.1 mmol), 2,2'-Hbpt (0.1 mmol) and NaN3 (0.15 mmol) was sealed in a 25 mL vessel and then heated at 160 degrees C in methanol condition (8 mL) for 72 h. The reaction of Dy(0Ac)(3) (0.17 mmol) and 2,2'-Hbpt (0.1 mmol) under the same condition as complex 1 yielded complex 2. Single-crystal X-ray diffraction reveals that complex 1 consists of a pair of [Dy-4(mu(4)-O)] tetrahedral units bridged by two azido groups in mu(1,1,3,3) and mu(1,3) modes as well as two bpt ligands, while complex 2 possesses four edge-sharing [Dy-4(mu(4)-O)] tetrahedral units. Magnetic susceptibility of polycrystalline samples were carried out using a SQUID magnetometer in the temperature range 2 similar to 300 K at 500 Oe dc field. The chi T-M value are 107.8 cm(3).K.mol-(1) for complex 1 and 133.6 cm(3).K.mol-(1) for complex 2 at 300 K, which are close to the expected values. The maximum values of magnetization at 1.8 K are 40.81 N beta and 51.62 N beta for 1 and 2, slightly smaller than the expected saturation values. For further investigation of the dynamic behaviour, ac susceptibility measurements were undertaken under zero-dc field between 1 similar to 1500 Hz, they both show temperature dependence and have maximum values in the chi(M) vs v plots above 1.8 K, indicating the presence of slow relaxation of the magnetization. After linear fitting the first four points of 1n(tau/s) vs 1/T plot, we get energy barrier Ueff=9.83(9) K, pre-exponential factor 1.63(2)X 10(-5) s for 1, and U-eff=12.05(2) K, tau(0)=6.75(8) X 10(-7) s for 2 respectively. The bridging mode of azido in complex 1 is novel and complex 2 holds the record for highest nuclearity among the reported pure lanthanide single-molecule magnets (SMMs) under zero dc field.