Rita Mazzoni

A new tetraarylcyclopentadienone based low molecular weight gelator: synthesis, self-assembly properties and anion recognition

A new class of tetraarylcyclopentadienones bearing 3-hydroxy-1-propynyl substituents has been synthesized. One of them, 3,4-bis (4-(3-hydroxy-3-methylbut-1-ynyl) phenyl)-2,5-diphenylcyclopenta-2,4-dienone, exhibits pronounced aggregation properties in various organic solvents responding to thermal and ultrasound stimuli and represents the first example of a tetraarylcyclopentadienone based low molecular weight organogelator. The hydroxydimethyl group on the ethynyl substituent proved to be essential to perform the gelation process. The 1H NMR analysis and FT-IR spectroscopy suggested that the intermolecular π–π and hydrogen bonding interactions of the gelator with the solvent are the main driving forces for the supramolecular assembly. The SEM images of xerogels show the characteristic gelation morphologies of 3D fibrous network structures. Fluorescence and UV/Vis absorption studies provided more information to define the molecular packing model in the gelation state. In addition the obtained gels show selective response to the fluoride anion.

CC Bond Formation in Diiron Complexes

The growing effort to design new sustainable synthetic methodologies, based on readily available and environmentally friendly transition metals, has boosted research on iron complexes. This review article focuses on C-C-bond-forming reactions occurring at bridging ligands in diiron complexes, aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres. A number of diiron-mediated C-C-bond-forming reactions reported in the literature, including nucleophilic and electrophilic additions and insertion and cycloaddition reactions, have been accumulated over the years, which, together with more recent developments, indicate that diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted C-C bond formation.

Dopamine amperometric detection at a ferrocene clicked PEDOT:PSS coated electrode

Chemically modified electrodes are widely employed in electroanalytical chemistry and an important goal is to strongly anchor redox mediators on the electrode surface. In this work, indium tin oxide (ITO) electrodes have been coated with PEDOT:PSS that has been ferrocene-functionalized, by a two-step procedure consisting of the electrodeposition of PEDOT-N3 followed by copper-catalyzed azide-alkyne cycloaddition of ethynylferrocene. The coated electrodes have been characterized by XPS, showing successful ferrocene immobilization, by AFM, and by cyclic voltammetry (CV), which is dominated by the stable and highly reversible response of ferrocene. The electrocatalytical performance of the device is assessed by analyzing 3,4-dihydroxyphenyl ethylamine, also commonly known as dopamine (DA). The sensor presents a linear range between 0.01 and 0.9 mM, a mean sensitivity of 196 mA M-1 cm-2 and a limit of detection (LoD) of 1 µM.

N-Heterocyclic carbene rhodium(I) complexes containing an axis of chirality: dynamics and catalysis

The novel rhodium(I) complexes [RhCl(NBD)(NHC)] [NBD = norbornadiene, NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me (3a), Bz (3b), Tr (3c), tBu (3d)], containing on one nitrogen the benzyl substituent and on the other increasing bulky alkyl substituents were prepared. All the complexes display restricted rotation around the metal–carbene bond and yield conformational enantiomers. The stereodynamics and racemization barriers about the Rh–carbene have been determined by means of NMR spectroscopy for 3a–c, whereas for the bulkiest 3d only the lower limit (91 kJ mol−1) could be calculated. Whilst the racemization barriers obtained by DFT calculations for 3a,b and 3d matched the experimental values, in the case of 3c the latter (62.3 kJ mol−1) was much smaller with respect to the calculated one (101.7 kJ mol−1). The lower experimental barrier has been attributed to a dissociative pathway that produces a solvated ionic pair in the transition state. The catalytic activity of the neutral rhodium(I) complexes 3a and 3d in the hydrosilylation with HSiMe2Ph of the terminal alkynes PhC≡CH, TolC≡CH, nBuC≡CH, Et3SiC≡CH, and (CPh2OH)C≡CH has been investigated, and compared with the amide-functionalized [RhCl(NBD){1-(2-NHBoc-ethyl)-3-Me-imidazolin-2-ylidene}] (4) and with [RhCl(NBD){1-butyl-3-Me-imidazolin-2-ylidene}] (5).

Iron( ii ) catalyzed dehydrative etherification of alcohols : a convenient route to ferrocenylmethanol-ethers

The iron complex [Fp][OTf] (Fp+[Fe(CO)2(Cp)]+, OTf−SO3CF3−) was found to act as efficient catalyst for the dehydrative etherification of ferrocenylmethanol [HOCH2–Fc] with a variety of alcohols (ROH), providing a valuable route for the formation of ferrocenylmethanol-ethers [ROCH2–Fc]. The complex [Fp][OTf] also catalyzes etherification of propargyl alcohols with other alcohols. The advantages of the method are associated with the use of a non toxic and easily available transition metal as catalyst, and the dehydrative synthetic approach, which produces water as the only by-product.

Ruthenium hydroxycyclopentadienyl N-heterocyclic carbene complexes as transfer hydrogenation catalysts

A series of novel cationic hydroxycyclopentadienyl and methoxycyclopentadienyl N-heterocyclic carbene ruthenium(II) complexes have been synthesized from the corresponding neutral ruthenium(0) complexes containing both the non-innocent cyclopentadienone ligand and variously functionalized N-heterocyclic carbenes (NHCs). In particular an NHC derivative containing a pyridine group in the side chain has been designed and developed in order to evaluate the influence of a basic, potentially cooperative substituent in the catalytic activity of these complexes. All the prepared complexes were employed as selective catalysts for transfer hydrogenation reactions employing refluxing iPrOH as a hydrogen source and several ketones and aldehydes as substrates. We found that while the presence of oxidizing additives such as CAN and benzoquinone is mandatory to activate the neutral ruthenium(0) complexes, no activation is needed for the cationic Ru(II) catalysts. The catalytic activity of the latter is also influenced by the coordinating ability of the counterion, and indeed the cationic complexes having a pyridine-functionalized NHC ligand and CF3SO3− as counterion, present the best conversion (>99%) thus demonstrating the fundamental role played by the basic pyridine in the catalytic activity. With regard to the hydrogenation reaction mechanism, the release of the CO ligand was demonstrated to be the key step and the presence of hydride species has been detected at the end of the reaction.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN- and amino-carbyne ligands have been elucidated. After the first reduction, CN- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN- that forms a Fe(CNH) moiety: i.e., the acidic source for the H2 generation. The hydride (formally 2e/H+), necessary to the heterocoupling with H+ is thus provided by the μ-CN(Me)2 ligand and not by Fe centers, as occurs in typical L6Fe2S2 derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN- to CN(Me)2 is highlighted. The incompetence for the HER of the related species [Fe2{μ-CN(Me)2}(μ-CO)(CO)2(Cp)2]+ (2+) has been investigated and attributed to the loss of proton responsiveness caused by CN- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe2 core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe2 scaffold, could be exploited to develop more active and robust catalysts for the HER.

CHAPTER 1:Homogeneous, Heterogeneous and Nanocatalysis

The past decade has seen ever-increasing interest in the catalytic aerobic oxidation of alcohols, which is one of the pivotal functional group transformations in organic chemistry. Nevertheless, most of the current methods for alcohol oxidation are not catalytic, hence the use of catalysts and green oxidants such as O2 or air, instead of stoichiometric quantities of inorganic oxidants, will provide a highly desirable approach to this reaction. This chapter summarizes the latest breakthroughs in the use of homogeneous and heterogeneous catalysts in aerobic alcohol oxidation in the liquid phase; the use of microwaves and photochemistry to assist and promote catalytic activities is also highlighted. Moreover, since nanoparticle systems may be considered an interesting compromise between heterogeneous and homogeneous catalytic systems, the recent development of soluble transition metal colloids as active nanocatalysts for aerobic alcohol oxidation is also presented.

Click-Derived Triazolylidenes as Chelating Ligands: Achievement of a Neutral and Luminescent Iridium(III)–Triazolide Complex

Versatility in the synthesis of triazole derivatives was exploited to obtain convenient mesoionic carbenes working as chelating or cyclometalating ligands for the preparation of cationic or neutral iridium(III) complexes. We present the synthesis and characterization of three new cationic cyclometalating iridium(III) complexes (1-3-BF4) and a neutral one (4), equipped with functionalized triazolylidene ligands. All the complexes are obtained in good yields, present irreversible or quasi-reversible oxidation and reduction processes, and display good photophysical stability. The complexes emit from 3MLCT or 3LC states, depending on the nature of the ancillary ligand. Compounds 1-3-BF4 display very low photoluminescence quantum yields (PLQY ≈ 1% in acetonitrile solution). Density functional theory calculations show that the luminescence of these three complexes is quenched by the presence of low-lying 3MC states, leading to a reversible detachment of the neutral ancillary ligands from the metal coordination sphere. On the contrary, this nonradiative deactivation pathway is not present in the case of the neutral complex 4, which in fact shows PLQYs above 10% and is the best emitter of the series. Moreover, complex 4 represents the first reported example of a photochemically and thermally stable neutral triazolide iridium(III) complex.