“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.