Pentanuclear Heterometallic {Ni₂Ln₃} (Ln = Gd, Dy, Tb, Ho) Assemblies. Single-Molecule Magnet Behavior and Multistep Relaxation in the Dysprosium Derivative

The reaction between Ln­(III) chloride and NiCl₂·4H₂O salts in presence of a multidentate sterically unencumbered ligand, (<i>E</i>)-2,2′-(2-hydroxy-3-((2-hydroxy­phenyl­imino)­methyl)-5-methyl­benzyl­azanediyl)­diethanol (<b>LH</b>_<b>4</b>) leads to the synthesis of four isostructural pentanuclear hetereometallic complexes [Ni₂Dy₃(LH)₄]Cl (<b>1</b>), [Ni₂Gd₃(LH)₄]Cl (<b>2</b>), [Ni₂Tb₃(LH)₃(LH₂)]­Cl₂ (<b>3</b>), [Ni₂ Ho₃ (LH)₃ (LH₂)]­Cl₂ (<b>4</b>) with unprecedented topology. Here the two compounds <b>1</b> are <b>2</b> are monocationic and crystallize in chiral space group, <i>P</i>2₁2₁2₁ whereas compounds <b>3</b> and <b>4</b> are dicationic and crystallize in achiral space group <i>P</i>2₁/<i>n</i>. The total metal framework, {Ni₂Ln₃} unit is held by four triply deprotonated ligands [LH]^3– in <b>1</b> and <b>2</b> whereas in case of <b>3</b> and <b>4</b> three triply deprotonated [LH]^3– and one doubly deprotonated [LH₂]^2– ligands are involved. In these complexes both the lanthanide ions and the nickel­(II) ions are doubly bridged and the bridging is composed of oxygen atoms derived from either phenolate or ethoxide groups. The analysis of SQUID measurements reveal a high magnetic ground state and a slow relaxation of the magnetization with two relaxation regimes for <b>1</b>. For the thermally activated regime we found an effective energy barrier of <i>U</i>_eff = 85 K. Micro Hall probe loop measurements directly proof the single-molecule magnet (SMM) nature of <b>1</b> with a blocking temperature of <i>T</i>_B = 3 K and an open hysteresis for sweep rates faster than 50 mT/s