Hierarchical Self-assembly and Controlled Disassembly of a Cavitand-based Host-Guest Supramolecular Polymer

There is a considerable interest in dynamic materials featuring modular components with nano-scale dimensions and controlled responsiveness to external stimuli. Supramolecular polymers are a class of materials that fulfill nicely all these conditions. Here, we present a family of host-guest supramolecular polymers that combine the outstanding complexing properties of tetraphosphonate cavitands toward N-methylpyridinium guests with molecular switching. The designed monomer is a cavitand featuring four inward facing P=O groups at the upper rim and a single N-methylpyridinium unit at the lower rim, forming instantaneously a polymeric species in solution thanks to the high complexation constants measured for these host-guest interactions. This system has been analyzed by NMR spectroscopic and electrochemical techniques. In order to interpret the results of diffusion-sensitive experiments, we took advantage of the X-ray crystal structure obtained for the polymeric species and developed an original treatment of the PGSE data by non-linear fitting. The analysis of the experimental data identified an isodesmic polymerization model at monomer concentration below 20 mM, driven by intrachain host-guest interactions, and an additional level of tetrameric bundle aggregation above 20 mM, due to interchain dipolar and quadrupolar interactions. Two orthogonal disassembly procedures have been implemented: electrochemical reduction for the linear chains and solvent-driven dissolution for the bundles

Abatement of the ecotoxicological risk of landfill leachate by heterogeneous Fenton-like oxidation

Landfill leachates are highly contaminated liquid waste, and their treatment and detoxification are a challenging task. The current system of ecotoxicological risk assessment is complex and time-consuming. It is of fundamental importance to develop simpler and faster tools for the evaluation of the treated liquid waste and for an easier preliminary screening of the most active catalytic formulation/reaction conditions of the Fenton-like process. Here, several analytical techniques have been used for the assessment of the reduction of toxicity of the landfill leachate after Fenton process over copper-zirconia catalyst (ZrCu). Ultraviolet-visible (UV-vis) spectroscopy and absorbable organic halogens (AOX) analysis have been coupled to achieve further insight into the degradation of contaminants. In addition, for the first time, the qualitative abatement of organic compounds is monitored through proton nuclear magnetic resonance (H-1 NMR) analysis, providing a new method for evaluating the effectiveness of the treatment. Spectroscopic techniques reveal that the Fenton process induces a significant abatement of the aromatic and halogen compounds (51%) in the landfill leachate with a reduction of the toxicity that has been confirmed by ecotoxicological test with algae. These results validate the investigated tool for a simple rapid preliminary evaluation of the detoxification efficacy

Sustainable solvent-free selective oxidation of benzyl alcohol using Ru(0) supported on alumina

The selective oxidation of primary alcohols into their corresponding carbonyl compounds is challenging because of the easy over oxidization to acids and esters. The traditional reaction requires large amounts of solvent and oxidant, causing serious environmental issues. Recently, several efforts have been made to transform the reaction into a more sustainable process. Here, we investigated the solvent-free oxidation of benzyl alcohol using air as a green oxidant in the presence of ruthenium supported on alumina and zirconia, thereby meeting atom economy and environmental requirements. The materials were extensively characterized and, in addition to their activity, selectivity, and reusability, the environmental sustainability of the process was assessed according to green chemistry metrics. XRD, TEM, and XPS analyses suggest that the formation of metallic Ru on the support plays a key role in the catalytic activity. Ru supported on alumina, after a reduction treatment, achieves good activity (62% conversion) and a complete selectivity in a very sustainable process (without a solvent and with air as oxidant), as indicated by the very low E-factor value. The formulation is very stable and maintains high activity after recycling.Peer ReviewedPostprint (published version

Selectively measuring π back-donation in gold(I) complexes by NMR spectroscopy

Even though the Dewar-Chatt-Duncanson model has been successfully used by chemists since the 1950s, no experimental methodology is yet known to unambiguously estimate the constituents (donation and back-donation) of a metal-ligand interaction. It is demonstrated here that one of these components, the metal-to-ligand π back-donation, can be effectively probed by NMR measurements aimed at determining the rotational barrier of a C-N bond (ΔHr (≠) ) of a nitrogen acyclic carbene ligand. A large series of gold(I) complexes have been synthesized and analyzed, and it was found that the above experimental observables show an accurate correlation with back-donation, as defined theoretically by the appropriate charge displacement originated upon bond formation. The proposed method is potentially of wide applicability for analyzing the ligand effect in metal catalysts and guiding their design

Diffusion Ordered NMR Spectroscopy (DOSY)

Diffusion NMR spectroscopy has become an essential tool for investigating the supramolecular assembling processes that from molecular “bricks” lead to the construction of functional nanomaterials and nano-sized catalysts. This is probably due to the implementation and commercialization of new NMR instrumentations with the default capability of generating pulsed-field gradients (PFGs) along the direction of the magnetic field. Furthermore, while a robust package of analytical techniques is available to investigate molecules and extended materials or large biomolecules, which are the two-dimensional extremes, the characterization of the chemical mesoscale (several nanometers) is particularly challenging. It is just in this context, that is, the characterization of objects with an intermediate dimen- sion ranging from dozens of angstroms to hundreds of nanometers, that diffusion NMR spectroscopy shows all its potentialities. The aim of this chapter is not to discuss in detail the underlying NMR pulse sequences of diffusion experiments. The basic methodology is longstanding and excellent reviews have already been published. Here, we want to discuss diffusion NMR experiments from a pragmatic point of view in order to show what information can be obtained and how reliable it is, focusing attention on supramolecular objects of “intermediate” dimensions. In particular, after recalling the principles underlying diffusion NMR spectroscopy and the measure- ment of the translational self-diffusion coefficient (Dt) (Section 2), we show how accurate hydrodynamic dimensions can be derived from Dt once the shape and size of the diffusing particles have been correctly taken into account (Section 3). Later on, the application of diffusion NMR to the study of supramolecular systems is described (Section 4) in terms of determination of the average hydrodynamic dimensions and thermodynamic parameters of the self-assembly processe

NMR Techniques for Investigating the Supramolecular Structur of Coordination Compounds in Solution

The text explores important spectroscopic approaches. It first describes intermolecular nuclear Overhauser effect (NOE) NMR experiments and diffusion experiments, offering examples that demonstrate theoretical aspects of the methodolog

Combining diffusion NMR and conductometric measurements to evaluate the hydrodynamic volume of ions and ion pairs

A simple methodology for rapidly and accurately determining the hydrodynamic volume of single ions and ion pairs, based on the combination of information derived from diffusion NMR spectroscopy and conductometry, is proposed. © 2007 American Chemical Society

Metallic palladium, PdO, and palladium supported on metal oxides for the Suzuki-Miyaura cross-coupling reaction: A unified view of the process of formation of the catalytically active species in solution

The Pd-catalysed Suzuki\u2013Miyaura reaction is a powerful and widely used method for the synthesis of asymmetric biaryls, which has found increasing industrial application for the production of pharmaceuticals, fine chemicals and materials. Despite its widespread popularity, there is still a need to better understand some features of the whole process. Although the catalytic cycle is well established in all its key steps (aryl halide oxidative addition to Pd(0), transmetalation, C\u2013C bond formation/reductive elimination of the coupling product), the effective nature of catalysis when a Pd-containing solid material is employed as the catalyst is still debated. More specifically, the question is whether the oxidative addition of Ar\u2013X occurs on the surface of the catalyst (heterogeneous catalysis) or on leached metal atoms (homogeneous catalysis). This critical review is an attempt to answer this question, drawing upon findings from recent research based on the application of different forms of metallic palladium, PdO, and metal oxide-supported Pd catalysts to the Suzuki\u2013Miyaura coupling. On the basis of the results of studies conducted so far, there is convergence towards a unique scenario, namely, the solid (pre)catalyst acts as a \u201creservoir\u201d of soluble catalytically active palladium species. Furthermore, there is strong evidence, at least for catalysts of the type Pd/MxOy, that the noble metal is released in the form of Pd2+ ions, generally from amorphous PdO present on the surface

NMR investigation of non-covalent aggregation of coordination compounds ranging from dimers and ion pairs up to nano-aggregates

This review summarizes the results recently obtained by our research group investigating the non-covalent aggregation of coordination compounds in solution through NMR spectroscopy. First, systems that can undergo only weak non-covalent interactions, such as dispersive and dipole-dipole ones, are considered; successively, coordination compounds that are capable to establish more energetic non-covalent interactions, such as hydrogen bonding and/or extended π-π stacking interactions, are taken into account. The parallelism between the energy of non-covalent interactions and the level of aggregation is highlighted. The results concerning the latter are mainly obtained through diffusion NMR experiments. In some cases, information about the structure of non-covalent aggregation in solution, obtained through intermolecular NOE studies, is discussed and contrasted with that observed in the solid state (by means of X-ray single crystal investigations, mainly carried out by our group) and/or derived from theoretical calculations. © 2007 Elsevier B.V. All rights reserved

Self-Aggregation Tendency of All Species Involved in the Catalytic Cycle of Bifunctional Transfer Hydrogenation

The self-aggregation tendency of [RuX(N,N)(eta(6)-p-cymene)] [N,N = amino amidate, X = Cl (1) and H (2)] and [Ru(N,N)(eta(6)-p-cymene)] [N,N = amido amidate, 3] in various solvents was investigated by diffusion NMR spectroscopy. The proper evaluation of the molecular hydrodynamic volume of the 1-3 monomeric species allowed understanding that 1 and 2 are mainly present as monomers in isopropanol-d(8) at concentrations below the millimolar level. Dimers start to become relevant at concentrations over ca. 10 mM [Delta G(0)(aggregation) = -2.2 kcal mol(-1)]. The self-aggregation tendency of I and 2 in CDCl(3) is marked [Delta G(0)(aggregation) = -3.4 kcal mol(-1)] and much higher than that of 3. In toluene-d(8), 3 and I readily form dimers and higher aggregates, respectively, even at millimolar concentrations. The structures and energetics of 1-1, 2-2, and 3-3 dimers were investigated by ONIOM(B3PW91/HF) calculations. It was found that the main interaction at the origin of the dimerization process is the establishment of an intermolecular H-bond between one N-H on one monomer and the oxygen of the SO(2) moiety of the other. In 3, the amido group is less acidic and less spatially available for H-bonding than in 1 and 2, which explains the reduced tendency to form dimers