Christian Beugholt

“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.

Giant Ring-Shaped Building Blocks Linked to Form a Layered Cluster Network with Nanosized Channels: [Mo124VIMo28VO429(μ3-O)28H14(H2O)66.5]16−

A novel type of electron-rich (basic!) nanosized channels can be built up by linking giant ring-shaped building blocks, which occur in the layered compound of composition [Mo124VIMo28VO429(μ3-O)28H14(H2O)66.5]16− (see figure). The corresponding sodium salt is obtained by reduction of an acidified aqueous molybdate solution under special reaction conditions.

Unusual Stepwise Assembly and Molecular Growth: [H14Mo37O112]14− and [H3Mo57V6(NO)6O189(H2O)12(MoO)6]21−

The understanding of the formation of complex molecular systems from simple building blocks by conservative self-assembly processes is still a challenge. We report the synthesis and structural characterization of the large reduced polyoxometallate compounds (NH4)(14)[H14Mo37O112]. 35 H2O (1), (NH4)(21)[H3Mo57V6(NO)(6)O-183(H2O)(18)]. 55H(2)O (2) (by an improved synthesis) and Na-3(NH4)(18)[H3Mo63V6(NO)(6)O-195(H2O)(12)]. 41H(2)O (3). The cluster systems are formed by a stepwise growth process. This implies the appearance, during the cluster formation, of ephemeral polyoxometallate intermediates (some of which we were able to isolate). The negative charge and therefore the nucleophilicity of the intermediate cluster fragments increase when they are reduced, resulting in further attraction of electrophiles and thus in growth of molecular systems. IN the case of the cluster anion of 3 we observed, correspondingly, the loss of an [MoO](4+) oxometallate fragment by air oxidation; this implies that aits uptake and release are controlled by th degree of reduction of the cluster. correspondingly, intermediates between the anions of 2 and 3 of the {Mo57+xV6} type could be isolated. The unusual anion of 1 is formed by symmetry breaking processed.

Synthese und Struktur des ringförmigen, reduzierten „Metalloxids” [(MoO3)176(H2O)80H32]

Der bislang groste anorganische Cluster [(MoO3)176(H2O)80H32](2±2)−1 (im Bild rechts sind die Mo-O-Polyeder gezeigt), der jetzt rontgenstrukturanalytisch charakterisiert wurde, weist neben einem Hohlraum mit einem Durchmesser von 2.3 nm bemerkenswerterweise die Stochiometrie eines reduzierten, protonierten und hydratisierten „molekularen Molybdantrioxids” auf. Er bildet sich bei der Reduktion wasriger Lithiummolybdatlosung mit Zinn(II)-chlorid bei sehr hohen H+-Konzentrationen.

Facile and Optimized Syntheses and Structures of Crystalline Molybdenum Blue Compounds Including one with an Interesting High Degree of Defects: Na26[Mo142O432(H2O)58H14] · ca. 300 H2O and Na16[(MoO3)176(H2O)63(CH3OH)17H16] · ca. 600 H2O · ca. 6 CH3OH

The syntheses and structures of the two mixedvalence crystalline molybdenum blue compounds Na26[Mo142O432(H2O)58H14] · ca. 300 H2O (1) (containing the maximal number of well defined defects which influence the overall structure and the reactivity of the anionic cluster) and Na16[(MoO3)176(H2O)63(CH3OH)17H16] · ca. 600 H2O · ca. 6 CH3OH (2) (obtained in an optimized high-yield synthesis) are reported with reference to the critical conditions required for the isolation of corresponding crystalline materials.

Towards the construction of mesoscopic species with emergent and functional properties via the derivatisation of molybdenum-oxide ‘Giant-Wheel’ clusters

Polyoxometalate chemistry has proved to be an extraordinary source of a large variety of compounds, clusters and solid‐state structures—these are built using very simple metal oxygen fragments linked together in an enormous variety of modes. Herein we focus on the ‘Giant-Wheel’ clusters which contain 154 and 176 Mo atoms and examine the variety of structural modifications that has been achieved to date with these systems. The generation of structural vacancies (here referred to as defects), substitution of ligands, and incorporation of hetero-metallic centres offers many fascinating possibilities with respect to the targeted functionalisation of these species for a pre-determined function and application. Furthermore, evidence is presented which suggests the identity of the most labile and, therefore, most easily derivatised units present in these systems. The consequences of this assignment are then discussed in the context of inter- (formation of layers and chains) and intra- (assembly inside the cavity) molecular growth. q 2000 Elsevier Science B.V. All rights reserved.

Assembling nanosized ring-shaped synthons to an anionic layer structure based on the synergetically induced functional complementarity of their surface-sites: Na21[MoVI126MoV28O462H14(H2O)54(H2PO2)7]·xH2O (x ≈ 300)

The reaction of an aqueous solution of sodium molybdate with hypophosphorous (phosphinic) acid acting both as reducing agent and ligand at low pH values (≈1) results in the formation of nanosized ring-shaped cluster units which assemble to form layers of the compound Na21[MoVI126- MoV28O462H14(H2O)54(H2PO2)7]·xH2O 1 (x ≈ 300); the assemblage is based on the synergetically induced functional complementarity of amphiphilic OMoL (L = H2O, H2PO2–) groups and corresponds to the replacement of H2O ligands of rings by related terminal MoO groups of other rings (and vice versa) the nucleophilicity of which is induced by coordinated H2PO2– ligands.

Exchanged ligands on the surface of a giant cluster: [(MoO3)176(H2O)63(CH3OH)17Hn](32 – n)–

The synthesis of 1a·(32 – n)Na+·ca. 600 H2O·ca. 30 CH3OH 1 containing the ring-shaped, mixed-valence (MoV/MoVI) cluster [(MoO3)176(H2O)63(CH3OH)17Hn](32 – n)– 1a as a discrete unit in the crystal lattice is reported, which for the first time yields a compound of this type via a facile synthetic method and without amorphous reaction products; remarkably, H2O ligands can be replaced by CH3OH on the surface of a giant metal-oxide based cluster which has a nanometer sized cavity and, in contrast to zeolites, reducing properties.

Kristallstruktur von Na3(NH4)12[Mo57Fe6(NO)6O174(OH)3(H2O)24] · 76H2O, einer Verbindung mit einem ungewöhnlichen Clusteranion

SummaryThe X-ray crystal structure of the compound Na3(NH4)12[Mo57Fe6(NO)6O174(OH)3(H2O)24]·76H2O (3) [P63/mmc;a=2380.6(5),c=2763.4(7),Z=2], the giant cluster anion of which has the shape of a doughnut, shows remarkable details: The cluster [{Fe(H2O)2}6{Mo(μ-H2O)2(μ-OH)Mo}3{ Mo15(MoNO)23+O58(H2O)2}3]15− can be described as being composed of three transferable {Mo17} ligands bridged by cationic centers and contains a novel nanodimensional central cavity.