A PGSE NMR approach to the characterization of single and multi-site halogen-bonded adducts in solution

We demonstrate here that the Pulsed field Gradient Spin Echo (PGSE) NMR diffusion technique can be effectively used as a complementary tool for the characterization of mono- and multi-site intermolecular halogen bonding (XB) in solution. The main advantage of this technique is that it provides the possibility of unambiguously determining the stoichiometry of the supramolecular adduct, information that is particularly important when multi-site molecular systems are studied. As an example, PGSE NMR measurements in chloroform indicate that hexamethylenetetramine (HMTA), a potentially four-site XB acceptor, actually exploits only two sites for the interaction with the XB donor N-bromosuccinimide (NBS), leaving the other two nitrogen sites unoccupied. Charge displacement calculations suggest that this is due also to the anti-cooperativity of the XB interaction between HMTA and NBS

Heterogenized water oxidation catalysts prepared by immobilizing Klaui-type organometallic precursors

An efficient heterogenized water oxidation catalyst (2_TiO2) has been synthesized by immobilizing the Kläui-type organometallic precursor [Cp*Ir{P(O)(OH)2}3]Na (2, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl ligand) onto rutile TiO2. Iridium is homogeneously distributed at the molecular and atomic/small cluster level in 2_TiO2 and 2'_TiO2 (solid catalyst recovered after the first catalytic run), respectively, as indicated by STEM-HAADF (scanning transmission electron microscopy - high angle annular dark field) studies. 2'_TiO2 exhibits TOF values up to 23.7 min-1 in the oxidation of water to O2 driven by NaIO4 at nearly neutral pH, and a TON only limited by the amount of NaIO4 used, as indicated by multiple run experiments. Furthermore, while roughly 40¿% leaching is observed during the first catalytic run, 2'_TiO2 does not undergo any further leaching even when in contact with strongly basic solutions and completely maintains its activity for thousands of cycles. NMR studies, in combination with ICP-OES (inductively coupled plasma optical emission spectrometry), indicate that the activation of 2_TiO2 occurs through the initial oxidative dissociation of PO43-, ultimately leading to active centers in which a 1:1 P/Ir ratio is present (derived from the removal of two PO43- units) likely missing the Cp* ligand.Peer ReviewedPostprint (author's final draft

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

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

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