High-Nuclearity 3d-4f Clusters as Enhanced Magnetic Coolers and MolecularMagnets. -The Co II /Co III (9:1) mixed compounds (III) and the Ni II compounds (V) are isostructural and crystallize in the monoclinic space group P21/m with Z = 2 (single crystal XRD). (IIIa) and (Va) exhibit the largest magnetocaloric effects among any known 3d-4f complexes, which is significant for their potential applications in magnetic cooling technology in the ultralow temperature range. Compounds (IIIb) and (Vb) display slow relaxation of the magnetization.

The hydrolysis of Ln(ClO4)3 in the presence of acetate leads to the assembly of the three largest known lanthanide-exclusive cluster complexes, [Nd104(ClO4)6(CH3COO)60(μ3-OH)168(μ4-O)30(H2O)112]·(ClO4)18·(CH3CH2OH)8·xH2O (1, x ≈ 158) and [Ln104(ClO4)6(CH3COO)56(μ3-OH)168(μ4-O)30(H2O)112]·(ClO4)22·(CH3CH2OH)2·xH2O (2, Ln = Nd; 3, Ln = Gd; x ≈ 140). The structure of the common 104-lanthanide core, abbreviated as Ln8@Ln48@Ln24@Ln24, features a four-shell arrangement of the metal atoms contained in an innermost cube (a Platonic solid) and, moving outward, three Archimedean solids: a truncated cuboctahedron, a truncated octahedron, and a rhombicuboctahedron. The magnetic entropy change of ΔS(m) = 46.9 J kg(-1) K(-1) at 2 K for ΔH = 7 T in the case of the Gd104 cluster is the largest among previously known lanthanide-exclusive cluster compounds.

High-nuclearity cluster-type metal complexes are a unique class of compounds, many of which have aesthetically pleasing molecular structures. Their interesting physical and chemical properties arise primarily from the electronic and/or magnetic interplay between the component metal ions. Among the extensive studies in the past two decades, those on lanthanide-containing clusters, lanthanide-exclusive or heterometallic with transition metal elements, are most notable. The research was driven by both the synthetic challenges for these generally elusive species and their intriguing magnetic properties, which are useful for the development of energy-efficient and environmentally friendly magnetic cooling technologies. Our efforts in this vein have been concentrated on developing rational synthetic methods for high-nuclearity lanthanide-containing clusters. By means of the now widely adopted approach of "ligand-controlled hydrolysis" of lanthanide ions, a great variety of cluster-type lanthanide hydroxide complexes had been prepared in the first half of this developing period (1999-2006). In this Account, our efforts since 2007 are summarized. These include (1) further development of synthetic strategies in order to expand the ligand scope and/or to increase the nuclearity (>25) of the cluster species and (2) magnetic studies pertinent to the pursuit of materials with a large magnetocaloric effect (MCE). Specifically, with the hope of expanding the family of ligands and producing clusters of previously unknown structures, we tested under hydrothermal or solvothermal conditions the use of readily available yet not commonly used ligands for controlling lanthanide hydrolysis; such ligands, carboxylates as mundane examples, tend to form insoluble complexes prior to any possible hydrolysis. We have also validated the use of preformed transition metal complexes as metalloligands for subsequent control of lanthanide hydrolysis toward heterometallic 3d-4f clusters. Furthermore, we demonstrated using ample examples that the presence of small anions as templates is essential to the assembly of high-nuclearity lanthanide-containing clusters and that maintaining a low concentration of the anion template(s) is a key to such success. It has been found that slow production/release of such anion templates by in situ ligand decomposition or absorption of atmospheric CO is effective in preventing precipitation of their lanthanide salts, allowing not only controllable lanthanide hydrolysis but also gradual and modular assembly of the giant cluster species. Magnetic studies targeting potential applications of such clusters as molecular magnetic coolers have also been conducted. The results are summarized in the second portion of this Account in an effort to establish a certain magneto-structure relationship. Of particular relevance is the possible correlation between MCE (evaluated using the isothermal magnetic entropy change, -ΔS) and magnetic density, and the intracluster antiferromagnetic exchange coupling. We have also made some prel...

NNSFC[20825103, 20901064, 90922031, 2007CB815304, 21021061]; Fundamental Research Funds for the Central Universities[2010121016