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

This paper investigates the problem of adaptive neural tracking control via output-feedback for a class of switched uncertain nonlinear systems without the measurements of the system states. The unknown control signals are approximated directly by neural networks. A novel adaptive neural control technique for the problem studied is set up by exploiting the average dwell time method and backstepping. A switched filter and different update laws are designed to reduce the conservativeness caused by adoption of a common observer and a common update law for all subsystems. The proposed controllers of subsystems guarantee that all closed-loop signals remain bounded under a class of switching signals with average dwell time, while the output tracking error converges to a small neighborhood of the origin. As an application of the proposed design method, adaptive output feedback neural tracking controllers for a mass-spring-damper system are constructed.

A chiral, cagelike, high-nuclearity lanthanide hydroxide cluster containing 60 Er(III) ions is reported. The cluster core possesses a fascinating sodalite-like structure with 24 vertex-sharing cubane-like [Er(4)(mu(3)-OH)(4)](8+) units. The hexagonal face of the sodalite cage features a templating mu(6)-CO(3)(2-) ion. Magnetic studies revealed weak antiferromagnetic interactions.

High-nuclearity metal complexes are a unique class of molecules. [1][2][3][4][5][6] Often in the nanoscopic size regime, they display fascinating structural diversity and possess properties that are potentially useful for developing novel catalysts, [1] materials for adsorption and storage, [2] molecular electronics, [3] optics, [4] and magnetism. [5,6] Particular interest in this field has been directed towards heterometallic complexes that feature both d-and f-block elements, and the distinct coordination behaviors of different metal ions have been observed in a large number of stunningly beautiful complexes. [7][8][9][10][11][12][13][14][15][16][17] The unique arrangement of the multiple metal centers within the complex framework, coupled with their inherently disparate electronic structures, often leads to attractive properties, of which novel magnetic phenomena such as single-molecule [14,15] and single-chain magnetism [16,17] are arguably the most notable. Our own efforts along this line of research have resulted in a number of giant 3d-4f clusters containing up to 50 metal ions. [18,19] These clusters display both stunningly beautiful structures and magnetic behaviors ranging from ferromagnetic to antiferromagnetic couplings. Herein we report the synthesis, structure, and magnetic studies of a giant heterometallic cluster containing 108 metal ions. [20] was obtained under hydrothermal conditions from a mixture of Ni(NO 3 ) 2 ·6 H 2 O, Gd(NO 3 ) 3 ·6 H 2 O, and iminodiacetic acid in deionized water and its composition was verified by satisfactory microanalysis.The four-shell, nesting doll-like structure of the cationic cluster is shown in Figure 1 a. Moving outward, the innermost shell (shell 1) contains six Ni II and two Gd III ions and is followed by shell 2 with 20 Gd III ions, shell 3 with 32 Gd III ions, and the outermost shell (shell 4) with 48 Ni II ions (Figure 1 b). The geometry of the shells approximates that of a cube. Inter-shell connections are provided primarily by triply bridging hydroxo groups, which afford a highly compact, brucite-like core structure. Similar structural motifs have been observed in both transition metal [21] and lanthanide clusters.[22] The appearance of the multi-shell structure is comparable to those of multi-shell Pd/Pt clusters, [23,24] nanocapsules of polyoxometalates, [25] and fullerene-like structures built from interpenetrating reciprocal polyhedra. [26,27] Six Ni II and two Gd III ions occupy the vertices of the cube in shell 1 (Figure 2 a). The Gd III ions, which are disposed diagonally, are bridged by an aqua ligand. Triply bridging OH groups, each of which bridges two neighboring metal ions (Gd or Ni) within the cube and a Gd III ion at the edge center of shell 2, form the 12 cube sides (Figure 2 b). The coordination sphere of each of the eight metal ions is completed by three additional m 3 -OH groups. All Ni II ions are therefore hexa-

The discovery of fullerenes in 1985 opened a new chapter in the chemistry of highly symmetric molecules. Fullerene-like metal clusters, characterized by (multi)shell-like structures, are one rapidly developing class of molecules that share this shape. In addition to creating aesthetically pleasing molecular structures, the ordered arrangement of metal atoms within such frameworks provides the opportunity to develop materials with properties not readily achieved in corresponding mononuclear or lower-nuclearity complexes. In this Account, we survey the great variety of fullerene-like metal-containing clusters with an emphasis on their synthetic and structural chemistry, a first step in the discussion of this fascinating field of cluster chemistry. We group the compounds of interest into three categories based on the atomic composition of the cluster core: those with formal metal-metal bonding, those characterized by ligand participation, and those supported by polyoxometalate building blocks. The number of clusters in the first group, containing metal-metal bonds, is relatively small. However, because of the unique and complex bonding scenarios observed for some of these species, these metalloid clusters present a number of research questions with significant ramifications. Because these cores contain molecular clusters of precious metals at the nanoscale, they offer an opportunity to study chemical properties at size ranges from the molecular to nanoscale and to gain insights into the electronic structures and properties of nanomaterials of similar chemical compositions. Clusters of the second type, whose core structures are facilitated by ligand participation, could aid in the development of functional materials. Of particular interest are the magnetic clusters containing both transition and lanthanide elements. A series of such heterometallic clusters that we prepared demonstrates diverse magnetic properties including antiferromagnetism, ferrimagnetism, and ferromagnetism. Considering the diversity of their composition, their distinct electronic structures, and the disparate coordination behaviors of the different metal elements, these materials suggest abundant opportunities for designing multifunctional materials with varied structures. The third type of clusters that we discuss are based on polyoxometalates, in particular those containing pentagonal units. However, unlike in fullerene chemistry, which does not allow the use of discrete pentagonal building blocks, the metal oxide-based pentagonal units can be used as fundamental building blocks for constructing various Keplerate structures. These structures also have a variety of functions, including intriguing magnetic properties in some cases. Coupled with different linking groups, such pentagonal units can be used for the assembly of a large number of spherical molecules whose properties can be tuned and optimized. Although this Account focuses on the topological aspects of fullerene-like metal clusters, we hope that this topical review will stimulate more ...