No abstract

Combining analytical and theoretical methods, we present a detailed study of a heteropolytungstate cluster encapsulated in a shell of dendritically branching surfactants, namely (C(52)H(60)NO(12))(12)[(Mn(H(2)O))(3)(SbW(9)O(33))(2)], 3. This novel surfactant-encapsulated cluster (SEC) self-assembles spontaneously from polyoxometalate-containing solutions treated with a stoichiometric amount of dendrons. Compound 3 exhibits a discrete supramolecular architecture in which a single polyoxometalate anion resides in a compact shell of dendrons. Our approach attempts to combine the catalytic activity of polyoxometalates with the steric properties of tailored dendritic surfactants into size-selective catalytic systems. The structural characterization of the SEC is based on analytical ultracentrifugation (AUC) and small-angle neutron scattering (SANS). The packing arrangement of dendrons at the cluster surface is gleaned from molecular dynamics (MD) simulations, which suggests a highly porous shell structure due to the dynamic formation of internal clefts and cavities. From analysis of the MD trajectory of 3, a theoretical neutron-scattering function is derived that is in good agreement with experimental SANS data. Force field parameters used in MD simulations are partially derived from a quantum mechanical geometry optimization of [(Zn(H(2)O))(3)(SbW(9)O(33))(2)](12)(-), 2b, at the density functional theory (DFT) level. DFT calculations are corroborated by X-ray structure analysis of Na(6)K(6)[(Zn(H(2)O))(3)(SbW(9)O(33))(2)].23H(2)O, which is isostructural with the catalytically active Mn derivative 2a. The combined use of theoretical and analytical methods aims at rapidly prototyping smart catalysts ("dendrizymes"), which are structurally related to naturally occurring metalloproteins.

A metal‐organic framework (MOF) containing redox active CoII atoms, [CoII(BPB)]·3DMF (1), has been prepared from the solvothermal reaction of CoII nitrate and 1,4‐bis(4′‐pyrazolyl)benzene (H2‐BPB) in dimethylformamide (DMF). Compound 1 constitutes a porous coordination framework that is built up by interconnecting 1D CoII polymer chains with (BPB)2− ligands. The thermal stability of 1 was investigated by thermogravimetric (TG) analysis and variable‐temperature X‐ray powder diffraction (VTXRPD), which indicate that the framework of 1 is stable upon removal of solvent molecules. The argon adsorption isotherm of 1 at 77 K indicates a porous structure with a BET surface area of 1207 m2/g. As a test reaction for catalytic activity of 1, the oxidation of cyclohexene was examined employing tert‐butyl hydroperoxide as oxidant. The maximum substrate conversion achieved after 12 h was 62 % with an estimated turn‐over number (TON) of 44 based on the number of converted substrate molecules (cyclohexene) per CoII atom.

The formation of supramolecular host-guest complexes of fullerene (C(60)) and two novel tribenzotriquinacene based hosts (5 a and 5 b) was investigated in solution and in the solid state. Stability constants for 1:1 and 2:1 complexes were obtained from spectroscopic (UV/Vis, (1)H NMR) titration experiments. Association constants of K(1)=(2908+/-360) L mol(-1) and K(2)=(2076+/-300) L mol(-1) for C(60)/5 a, and K(1)=(5608+/-220) L mol(-1) and K(2)=(673+/-160) L mol(-1) for C(60)/5 b were obtained. Single crystal X-ray structural analysis of compound C(60) subset5 b3 toluene revealed that a molecule of C(60) was located at short van der Waals contact distances in the open pre-organised cavity of the rigid host. The supramolecular complex created resembles an engineered nanosized ball joint and represents the first member for a future nanomechanics construction kit.

The synthesis of a stereochemically pure concave tribenzotriquinacene receptor (7) for C60 fullerene, possessing C3 point group symmetry, by threefold condensation of C2 -symmetric 1,2-diketone synthons (5) and a hexaaminotribenzotriquinacene core (6) is described. The chiral diketone was synthesized in a five-step reaction sequence starting from C2h -symmetric 2,6-di-tert-butylanthracene. The highly diastereo-discriminating Diels-Alder reaction of 2,6-di-tert-butylanthracene with fumaric acid di(-)menthyl ester, catalyzed by aluminium chloride, is the relevant stereochemistry introducing step. The structure of the fullerene receptor was verified by (1)H and (13)C NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. VCD and ECD spectra were recorded, which were corroborated by ab initio DFT calculations, establishing the chiral nature of 7 with about 99.7 % ee, based on the ee (99.9 %) of the chiral synthon (1). The absolute configuration of 7 could thus be established as all-S [(2S,7S,16S,21S,30S,35S)-(7)]. Spectroscopic titration experiments reveal that the host forms 1:1 complexes with either pure fullerene (C60) or fullerene derivatives, such as rotor 1'-(4-nitrophenyl)-3'-(4-N,N-dimethylaminophenyl)-pyrazolino[4',5':1,2][60]fullerene (R). The complex stability constants of the complexes dissolved in CHCl3/CS2 (1:1 vol. %) are K([C60 ⊂7]) = 319(±156) M(-1) and K([R⊂7]) = 110(±50) M(-1). With molecular dynamics simulations using a first-principles parameterized force field the asymmetry of the rotational potential for [R⊂7] was shown, demonstrating the potential suitability of receptor 7 to act as a stator in a unidirectionally operating nanoratchet.