Hartmut Bögge

Molecular growth from a Mo176 to a Mo248 cluster

In polyoxometalate chemistry a large variety of compounds, clusters and solid-state structures can be formed by the linking together of well-defined metal–oxygen building blocks, . These species exhibit unusual topological and electronic properties, andfind applications ranging from medicine to industrial processes. The recently reported ring-shaped mixed-valence polyoxomolybdates of the type {Mo154} (refs 5, 6) and {Mo176} (refs 7, 8) represent a new class of giant clusters with nanometre-sized cavities and interesting properties for host–guest chemistry. Here we describe the formation of related clusters of the type {Mo248} formed by addition of further units to the inner surface of the {Mo176 } ‘wheel’. The additional units arrange themselves into two {Mo36} ‘hub-caps’ on the initial wheel—clusters that are not stable in isolation. These findings reveal a new pathway to the development of complex coordination clusters.

Archimedean Synthesis and Magic Numbers: “Sizing” Giant Molybdenum-Oxide-Based Molecular Spheres of the Keplerate Type

Pythagorean harmony can be found in the spherical polyoxometalate clusters described here (see illustration for an example of a structure), since there are interesting relationships between the so-called magic numbers (12, 32, 42, 72, 132) relevant for spherical viruses and the number of the building blocks in the cluster. The size of these Keplerate clusters can be tailored by varying the type of connections between the pentagons by means of different spacers.

Organizational Forms of Matter: An Inorganic Super Fullerene and Keplerate Based on Molybdenum Oxide

Plato and Kepler would have been pleased. Despite the large number of atoms present the cluster anion 1 resembles an icosahedral-type structure. This represents definitively an unprecedented event in chemistry! The structure is made up of 12 {Mo11 } fragments such that the fivefold symmetry axes are retained in the resulting spherical object. As an inscribed icosahedron can be recognized in the spherical shell of 1 (see picture), similarities with Keplers famous shell model of the cosmos can be seen.

Unveiling the Transient Template in the Self-Assembly of a Molecular Oxide Nanowheel

A Hidden Template The entropic challenge inherent in forming a ring-shaped molecule generally increases considerably with the size of the ring. Assuming that a linear precursor must bind its ends together, extending its length diminishes the likelihood of the opposite ends approaching one another. In the absence of an external force, how then can a family of molybdenum oxide rings, several nanometers in diameter (quite large at the molecular scale), self-assemble? Miras et al. (p. 72, see the cover; see the Perspective by Whitmire) have now uncovered an internal template guiding the process. By carefully controlling conditions in a flow reactor, they were able to halt the assembly process partway through and characterize a smaller molybdenum oxide core cluster, around which the larger ring was forming. Ejection of this template then yielded the hollow finished product. Use of a flow reactor reveals a key intermediate in the formation of a molybdenum oxide nanostructure. Self-assembly has proven a powerful means of preparing structurally intricate nanomaterials, but the mechanism is often masked by the common one-pot mixing procedure. We employed a flow system to study the steps underlying assembly of a previously characterized molybdenum oxide wheel 3.6 nanometers in diameter. We observed crystallization of an intermediate structure in which a central {Mo36} cluster appears to template the assembly of the surrounding {Mo150} wheel. The transient nature of the template is demonstrated by its ejection after the wheel is reduced to its final electronic state. The template’s role in the self-assembly mechanism is further confirmed by the deliberate addition of the template to the reaction mixture, which greatly accelerates the assembly time of the {Mo150} wheel and increases the yield.

“Molecular Symmetry Breakers” Generating Metal‐Oxide‐Based Nanoobject Fragments as Synthons for Complex Structures: [{Mo128Eu4O388H10(H2O)81}2]20−, a Giant‐Cluster Dimer

The synthesis and manipulation of a huge variety of nanoscaled species of similar chemical nature under one-pot reaction conditions requires access to a potential TMdynamic library∫ of appropriate building blocks.[1a] For instance, by exploiting a detailed knowledge of polyoxometalate chemistry, a variety of discrete clusters (see ref. [1b ± g]) and related extended structures[2] can be formed by the linking of welldefined metal ± oxygen building blocks. These types of compounds have been shown to exhibit unusual topological as well as electronic properties and, furthermore, are interesting for materials science.[3±5] A couple of years ago, we reported wheel-shaped mixed-valence molybdenum clusters of the type {Mo154}, {Mo176}, 6, 7] and {Mo248}; of these, the first two parent species–exhibiting nanometer-sized cavities and therefore presenting fascinating perspectives for a new type of host ± guest chemistry–can now be obtained in high yields in facile syntheses.[8] Herein, we describe for the first time a dimer of two giant clusters, that is, of structurally well-defined covalently linked nanoobjects with a rather high degree of complexity. The dimer contains two elliptical molybdenum oxide based units, linked together by two Eu-O-Mo bonds, each unit incorporates 128 MoVI/V and 4 EuIII centers and includes large fragments of the above-mentioned parent clusters. The interpretation would be that these dimers are formed by EuIII centers acting as symmetry breakers which prevent the corresponding highly symmetrical parent-ring closure.[1b, 6] Of general importance is that in systems showing growth, potential (abundant) agents, such as EuIII centers, can act as TMsymmetry breakers∫ which results in the generation of structural complexity. In any case, it is important to realize that large nanoobject fragments can, in principle, be used as synthons. The ability to connect or assemble clusters in a predefined manner may allow the design of nanoscopic devices using the TMbottom up∫ method (that is, generating large objects from small units). While the TMclassical∫ reduction of an acidified aqueous molybdate solution leads to the blue, wheel-shaped tetraand hexadecameric parent-cluster anions mentioned above,[6] the generation of smaller species requires the presence of electrophiles, such as PrIII ions which increase the curvature by replacing the larger electrophilic {Mo2} -type building units (see below). In the presence of smaller EuIII ions, even ring closure to the parent clusters does not take place, which allows the isolation of compound 1 containing a novel cluster collective. Compound 1 was characterized by single-crystal X-ray structure analysis[9] (including bond valence sum (BVS) calculation to aid in the determination of the (formal) number of MoV centers and protonation sites),[10] elemental analyses ((K), Eu, Mo; see details in ref. [12]), thermogravimetric analysis, redox titration (to aid in the determination of the (formal) number of MoV centers), IR, and EXAFS spectroscopy (Eu-LIII edge,[11] with the option to distinguish in principle between the different Eu centers in the lattice and cluster sites) as well as magnetic susceptibility measurements with a SQUID magnetometer.

Nucleation Process in the Cavity of a 48‐Tungstophosphate Wheel Resulting in a 16‐Metal‐Centre Iron Oxide Nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.

Materielle Organisationsformen: ein anorganisches Superfulleren und Keplerat auf Molybdänsauerstoffbasis

Da hatten wohl auch Platon und Kepler ihre Freude gehabt! Trotz der grosen Zahl an Atomen weist das Cluster-Anion 1 eine ikosaedrische Struktur auf – sicherlich ein Novum in der Geschichte der Chemie! Es setzt sich derart aus 12 {Mo11}-Fragmenten zusammen, das im resultierenden kugelformigen Gebilde die funfzahligen Drehachsen erhalten bleiben. Da man in 1 ein in eine Kugel einbeschriebenes Ikosaeder erkennen kann, ahnelt dieses Anion strukturell dem beruhmten Keplerschen Schalenmodell des Kosmos (siehe Bild).

Unprecedented and Differently Applicable Pentagonal Units in a Dynamic Library: A Keplerate of the Type {(W)W5}12{Mo2}30

In the field of polyoxometalate chemistry, porous spherical molybdenum oxide-based clusters of the type {(Mo)Mo5}12(linker)30, [1] called Keplerates, are notable not only from an aesthetic point of view but also because they show properties of interest for different areas of science. Some of these clusters can act as artificial cells exhibiting gated pores while interacting specifically with their environments; others are of interest for several aspects of materials science. In detail, of interest are a) solution properties in connection with a new type of assembly process leading to vesicles, including magnetic ones, b) the option to employ the characteristic interactions with amphiphiles for the generation of monolayers and Langmuir–Blodgett films as well as highly ordered honeycomb nanostructures at air– water interfaces, c) the integration into sol–gel-derived silica to obtain unprecedented hybrids, and d) the discovery of novel magnetic properties, which are of interest for the understanding of Kagom lattices spin states. While Keplerates of the mentioned type with binuclear linkers {Mo2O4} 2+ have been synthesized by partly reducing an aqueous molybdate solution in the presence of appropriate bidentate bridging ligands 7] (leading in the case of acetate to {Mo2O4(OOCCH3)} + linkers), Keplerates of the type {(Mo)Mo5}12M30 ( {Mo72M30}) can be obtained directly by the addition of mononuclear linkers M such as Fe, Cr, and VO to a dynamic library of the pentagonal units {(Mo)Mo5O21(H2O)6} 6 , or by reaction of, for example, Fe with {(Mo)Mo5}12{Mo2}30 ( {Mo132}), thereby exchanging binuclear for mononuclear linkers. As the pentagonal {(Mo)Mo5}-type units appear in this context as transferable ligands (parallels have been made between infinite coordination polymers and discrete metallosupramolecular arrays), the mentioned clusters can be called coordination polymers with spherical periodicity. This analogy points to a route for the planned syntheses of other corresponding molecular spheres based on a careful selection of appropriate ligands and metal ions with specific coordination behavior. Herein we report the synthesis of a {(W)W5}12{Mo2}30 ( {W72Mo60})-type species with 30 {Mo2O4(acetate)} + linkers with metal–metal bonds and 12 unprecedented pentagonal {(W)W5O21(H2O)6} 6 ( {(W)W5}) ligands, which are necessary for the formation of the spherical cluster and are generated in a dynamic library of tungstates (see below). Whereas the {(Mo)Mo5}-type building blocks were known to occur in [Mo36O112(H2O)8] 8 present at low pH values in H2O, [11] no corresponding pentagonal unit has been observed to date among the thousands of polyoxotungstates. On the basis of the present work, there is the option for the extension of the molybdate-based Keplerate chemistry to the tungstates, which is possible owing to the use of the transferable pentagonal {(W)W5}-type units now available; this allows, for example, the synthesis of {(W)W5}12M30 clusters, which are of importance for materials science aspects, with the abovementioned magnetic M linkers. Transferable, pentagonal building blocks are extremely rare in chemistry but are, generally speaking, relevant for the construction of curved giant species, such as the often highlighted hedgehog {Mo368} cluster. 5] On the other hand, pentagons in a general sense play a unique role starting from modern solid-state chemistry and dating backwards via many topics to the ideas of the ancient Greek mathematicians. In any case, the present discovery allows us to follow new interesting routes in chemistry and materials science, especially because the surface properties of tungsten oxide based systems are different from those of the molybdates. Whereas the high formation tendency of the spherical shell of the type (pentagon)12(linker)30 based on pentagonal units has been discussed earlier in terms of a general chemical principle, it is also evident that the formation of the present pentagonal units can be considered in the framework of a “constitutional dynamic chemistry” formulated by Lehn, that is, with respect to systems responding to external stimuli (see below). [*] C. Sch ffer, Dr. A. Merca, Dr. H. B gge, Dr. A. M. Todea, Prof. Dr. A. M ller Fakult t f r Chemie, Universit t Bielefeld Postfach 100131, 33501 Bielefeld (Germany) Fax: (+ 49)521-106-6003 E-mail: a.mueller@uni-bielefeld.de Homepage: http://www.uni-bielefeld.de/chemie/ac1/

Open and shut for guests in molybdenum-oxide-based giant spheres, baskets, and rings containing the pentagon as a common structural element

A novel exchange between ligands and/or guest molecules can be accomplished in giant molecular spheres (an example is shown in the picture) which are in equilibrium with the corresponding giant baskets in solution.

Synthetic, spectroscopic, and X-ray crystallographic studies on binuclear copper(II) complexes with a tridentate NNS-bonding 2-formylpyridine thiosemicarbazone ligand. The characterization of both neutral and deprotonated co-ordinated ligand structures

The preparation of the complexes [(CuLX)2](HL = 2-formylpyridine thiosemicarbazone) from the interaction of HL with the appropriate copper salt (X = CH3COO, Cl, NO3, or ClO4) or by reaction of an HL–KX mixture with copper(II) acetate monohydrate (X = Br or I) is described. Dissolution of [{CuL(CH3COO)}2] in H2SO4 or HCl yields the protonated ligand complexes [{Cu(HL)(SO4)}2] and [{Cu(HL)Cl2}2]·2H2O respectively. The complexes have been characterized by a variety of spectroscopic techniques and the crystal and molecular structures of [{CuL(CH3COO)}2] and [{Cu(HL)(SO4)}2] determined by single-crystal X-ray diffraction techniques from diffractometer data. Crystals of [{CuL(CH3COO)}2] are triclinic, space group P, with a= 8.827(3), b= 8.813(3), c= 8.997(3)A, α= 117.49(2), β= 110.96(3), γ= 91.65(3)°, and Z= 1. Crystals of [{Cu(HL)(SO4)}2] are monoclinic, space group C2/c, with a= 14.751(3), b= 9.138(2), c= 17.468(4)A, β= 104.91(2)°, and Z= 4. After full-matrix least-squares refinement the final R value was 0.027 for both complexes (2 216 and 2 206 observed reflections were used respectively). Both complexes consist of discrete centrosymmetric dimers, the monomeric units being bridged by two acetato or sulphato ligands. The copper atoms have a distorted square-pyramidal co-ordination geometry with three donor atoms (NNS) coming from L or HL to form a tricyclic ligating system. The fourth donor atom (oxygen) comes from the bridging CH3COO– or SO42– ion. The fifth co-ordination position is occupied by a less strongly bound oxygen from the second bridging anion.