Decanuclear Ln(10) Wheels and Vertex-Shared Spirocyclic Ln(5) Cores: Synthesis, Structure, SMM Behavior, and MCE Properties

Abstract

The reaction of a Schiff base ligand (LH3) with lanthanide salts, pivalic acid and triethylamine in 1:1:1:3 and 4:5:8:20 stoichiometric ratios results in the formation of decanuclear Ln10 (Ln=Dy(1), Tb(2), and Gd (3)) and pentanuclear Ln5 complexes (Ln=Gd (4), Tb (5), and Dy (6)), respectively. The formation of Ln10 and Ln5 complexes are fully governed by the stoichiometry of the reagents used. Detailed magnetic studies on these complexes (1-6) have been carried out. Complex 1 shows a SMM behavior with an effective energy barrier for the reversal of the magnetization (Ueff)=16.12(8) K and relaxation time (τo)=3.3×10-5 s under 4000 Oe direct current (dc) field. Complex 6 shows the frequency dependent maxima in the out-of-phase signal under zero dc field, without achieving maxima above 2 K. Complexes 3 and 4 show a large magnetocaloric effect with the following characteristic values: -ΔSm=26.6 J kg-1 K-1 at T=2.2 K for 3 and -ΔSm=27.1 J kg-1 K-1 at T=2.4 K for 4, both for an applied field change of 7 T. Homometallic complexes: The reaction of a multidentate flexible Schiff base ligand (LH3; see figure) with [LnCl3]6 H2O affords homometallic decanuclear complexes, [Ln10(LH)10(κ2-Piv)10] (Ln=Dy, Tb, and Gd), and homometallic pentanuclear complexes, [Ln5(LH)4(μ2-η1η1Piv)4(η1Piv)(S)] (Ln=Dy, Tb, and Gd). The Dy3+ analogues exhibit single-molecule magnet (SMM) behavior, whereas the Gd3+ complexes show a significant magnetocaloric effect (MCE). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.