Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties.

The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe2S2] complex containing a 5,5'-diisocyanide-2,2'-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe2S2] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe2S2] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe2S2] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe2S2] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF4 salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental-computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

Synthesis of oxaspiranic compounds through [3 + 2] annulation of cyclopropenones and donor­acceptor cyclopropanes.

The Sc(OTf)3-catalyzed [3 + 2]-annulation reaction between cyclopropenones and donor­acceptor cyclopropanes is described. The process leads directly to the formation of 4-oxaspiro[2.4]hept-1-ene derivatives in good to excellent reaction yields. Density functional theory calculations suggest that the [3 + 2]-annulation pathway is strongly preferred over the possible [3 + 3]-process.

Competitive gold-activation modes in terminal alkynes: an experimental and mechanistic study.

The competition between π- and dual σ,π-gold-activation modes is revealed in the gold(I)-catalyzed heterocyclization of 1-(o-ethynylaryl)urea. A noticeable effect of various ligands in gold complexes on the choice of these activation modes is described. The cationic [Au(IPr)](+) (IPr=2,6-bis(diisopropylphenyl)imidazol-2-ylidene) complex cleanly promotes the π activation of terminal alkynes, whereas [Au(PtBu3 )](+) favors intermediate σ,π species. In this experimental and mechanistic study, which includes kinetic and cross-over experiments, several σ-gold, σ,π-gold, and other gold polynuclear reaction intermediates have been isolated and identified by NMR spectroscopy, X-ray diffraction, or MALDI spectrometry. The ligand control in the simultaneous or alternative π- and σ,π-activation modes is also supported by deuterium-labeling experiments.

Structure and photoreactivity of stable zwitterionic group 6 metal allenyls.

The synthesis and mechanistic study of the unprecedented reactivity of a series of zwitterionic η(1)-metal allenyls are reported.

A new family of "clicked" estradiol-based low-molecular-weight gelators having highly symmetry-dependent gelation ability.

Reported herein is the discovery of a novel family of "clicked" estradiol-based LMWGs whose gelation ability highly depends on the gelator symmetry. These LMWGs that gel different organic solvents in the presence of H(2)O even at concentrations as low as 0.04 wt% are readily accessible using "click" chemistry.

NHC-stabilized gold(I) complexes: suitable catalysts for 6-exo-dig heterocyclization of 1-(o-ethynylaryl)ureas.

3-Substituted 1-(o-ethynylaryl)ureas 1 selectively undergo either 6-exo-dig or 5-endo-dig cyclization (to give 4-methylene-3,4-quinazolin-2-ones 2 or indoles 3, respectively) depending on the choice of the metal, ligand, and reaction conditions. The best results (up to 96% yield) in the preparation of the hydroamination products 2 are achieved with the highly bulky NHC-stabilized cationic gold(I) complex [Au(IPr)](+). Conversely, ureas bearing an internal alkyne lead to the 5-endo-dig cyclization mode regardless of the gold(I) complex employed. Whereas the nature of the substituent at N-3 does not have any influence on the regiochemistry observed, it does, in some cases, affect the efficiency of these transformations.

[3.3]Ferrocenophanes with guanidine bridging units as multisignalling receptor molecules for selective recognition of anions, cations, and amino acids.

The synthesis, electrochemical, and optical properties of a new [3.3]ferrocenophane framework in which two ferrocene subunits, with similar electronic environments, are linked through two substituted guanidine moieties, are reported. The receptors 4-7 have been prepared in good yields by the reaction of bis(carbodiimide) 3 with primary amines. This architecture is exceptionally "tunable" because a variety of "legs" may be appended to the basic [3.3]ferrocenophane scaffold to give a wide range of signaling units. These receptors show remarkable ion-sensing properties, due to the presence of a redox active unit (ferrocene), and an amphoteric binding site (guanidine). In this nitrogen-rich structural motif the guanidine bridges act as multipoint binding sites for anions, cations, and amino acids. Sensing of anions takes place both by unprecedented redox-ratiometric measurements (F-, Cl-, AcO-, NO3-, HSO4-, H2PO4-, and HP2O7(3-)), and colorimetric change (F-, AcO-, H2PO4-, and HP2O7(3-)). Sensing and discrimination of amino acids takes place by redox-ratiometric measurements, whereas the recognition of metal cations (Zn2+, Ni2+, and Cd2+) is achieved either by electrochemical or fluorescence measurements. Moreover, the reported receptors display splitting of the oxidation wave of the Fe(II)/Fe(III) redox couple, and form the mixed-valence species 4+* -7+* by electrochemical partial oxidation which, interestingly, show intervalence charge-transfer transitions associated to the appearance of absorption bands in the near infrared spectral region.