A cubic Fe4Mo4 oxo framework and its reversible four-electron redox chemistry.

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron-molybdenum oxo compounds. In this context, [(Me3TACN)Fe](OTf)2 was reacted with (nBu4N)2[MoO4], which led to an oxo cluster, [[(Me3TACN)Fe][µ-(MoO4-κ(3)O,O',O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)Fe(2+) and [Mo═O](4+) units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive Fe(II) → Fe(III) transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe(II)2Fe(III)2Mo(VI)4 constellation, [Fe4Mo4](TCNQ)2 (2), could be isolated after treatment with TCNQ. The Fe(II)Fe(III)3Mo(VI)4 stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF6), giving [Fe4Mo4](DDQ)3 (4) or [Fe4Mo4](PF6)3 (5), respectively. The fully oxidized Fe(III)4Mo(VI)4 state was generated through oxidation with 4 equiv of ThPF6, leading to [Fe4Mo4](PF6)4, which showed a unique behavior: upon storage, one of the [Mo═O](4+) corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me3TACN)Fe]4[µ-([MoO4]3[MoO4(MeCN-κN)])-κ(3)O,O',O″)](PF6)4 (7). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for 1, 3, and 5 from the oxidized side; no discrete redox events were observed. Compounds 1-3 and 7 were characterized structurally and 1, 3, and 7 additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. (16)O/(18)O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-(18)O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO3 or Fe2O3.

Deposition of ordered layers of tetralactam macrocycles and ether rotaxanes on pyridine-terminated self-assembled monolayers on gold.

The deposition of tetralactam macrocycles and the corresponding benzyl ether rotaxanes on gold substrates is investigated for the first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated self-assembled monolayers (SAMs) are developed that are used as well-ordered template layers. The two SAMs differ with respect to the rigidity of the terminal pyridines as shown by angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The template layers are then used for the metal-mediated self-assembly of macrocylces and rotaxanes on solid supports. The SAM with the more rigid terminal pyridine shows a higher coverage with the macrocycles and is therefore preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited supramolecules to be oriented preferentially upright. This order is only achieved for the macrocycles through the deposition on the more rigid SAM template, whereas rotaxanes form oriented layers on both SAMs. Time-of-flight secondary-ion mass spectrometry analysis was used to determine the deposition time required for the self-assembly process.

Mass Spectrometry and Gas-Phase Chemistry of Bismuth-Oxido Clusters.

The mass spectrometric characterisation of bismuth-oxido clusters is described together with tandem mass spectrometric experiments aimed at investigating their gas-phase chemistry. Clusters with different ligand shells (nitrate, salicylates, and methacrylate) and different sizes ({Bi6 O8 }, {Bi22 O26 }, and {Bi38 O45 }) were selected for this study. The following aspects were addressed by (tandem) electrospray ionisation Fourier-transform ion-cyclotron-resonance (ESI-FTICR) mass spectrometry: 1) Electrospray ionisation was used successfully for the generation of intact cluster ions, but led to rather complicated distributions of charge states and solvent adducts. The cluster core remained intact. 2) Mass spectrometry was used to follow the cluster growth from {Bi6 O8 } to {Bi22 O27 } and finally {Bi38 O45 } clusters. These experiments revealed that careful tuning of the conditions is required to avoid precipitates forming that hamper ESI ionisation. The stages of growth strongly depend on the experimental conditions. 3) Fragmentation experiments in the gas phase performed with the {Bi22 O27 } clusters obtained in the growth experiments resulted in the preferential formation of {Bi6 O8 } fragments and point to the particular stability of this cluster and its role in cluster growth. 4) Solution-phase ligand-exchange reactions on {Bi22 O26 } salicylate clusters were qualitatively monitored by mass spectrometry, but were limited by the complicated ion series formed that led to overlapping isotope patterns. 5) A number of fragmentation reactions occurred within the ligand shell when the clusters surrounded by an organic ligand shell were subjected to infrared multiphoton dissociation experiments. 6) For the nitrate clusters, fragmentation within the core was observed yielding {Bi6 O8 } fragments together with the corresponding clusters of reduced size.

[4]Pseudorotaxanes with remarkable self-sorting selectivities.

The synthesis and characterization of several self-assembled [4]pseudorotaxanes is reported, some of which form in a programmed way based on two similar yet orthogonal crown ether/secondary ammonium ion binding motifs. A preference for the formation of a [4]pseudorotaxane with an antiparallel rather than parallel alignment of crown ether building blocks is observed even in the absence of such orthogonal binding sites, when a homodivalent axle is used.