Matteo Savastano

Thermodynamics of Anion−π Interactions in Aqueous Solution

Thermodynamic parameters (ΔG°, ΔH°, TΔS°), obtained by means of potentiometric and isothermal titration calorimetry (ITC) methods, for the binding equilibria involving anions of high negative charge, like SO(4)(2-), SeO(4)(2-), S(2)O(3)(2-) and Co(CN)(6)(3-), and nitroso-amino-pyrimidine receptors in water suggested that anion-π interactions furnish a stabilization of about -10 kJ/mol to the free energy of association. These anion-π interactions are almost athermic and favored by large entropic contributions which are likely due to the reduced hydrophobic pyrimidine surface exposed to water after anion aggregation, and the consequent reduced disruptive effect on the dynamic water structure. The crystal structure of the {H(4)L[Co(CN)(6)]}·2H(2)O complex showed strong anion-π interactions between Co(CN)(6)(3-) and the protonated H(4)L(3+) receptor. The CN···centroid distance (2.786(3) Å), occurring with a cyanide N atom located almost above the centroid of the pyrimidine ring, is the shortest distance till now reported for the interaction between CN(-) ions and heteroaromatic rings.

Anion Complexes with Tetrazine-Based Ligands: Formation of Strong Anion−π Interactions in Solution and in the Solid State

Ligands L1 and L2, consisting of a tetrazine ring decorated with two morpholine pendants of different lengths, show peculiar anion-binding behaviors. In several cases, even the neutral ligands, in addition to their protonated HL(+) and H2L(2+) (L = L1 and L2) forms, bind anions such as F(-), NO3(-), PF6(-), ClO4(-), and SO4(2-) to form stable complexes in water. The crystal structures of H2L1(PF6)2·2H2O, H2L1(ClO4)2·2H2O, H2L2(NO3)2, H2L2(PF6)2·H2O, and H2L2(ClO4)2·H2O show that anion-π interactions are pivotal for the formation of these complexes, although other weak forces may contribute to their stability. Complex stability constants were determined by means of potentiometric titration in aqueous solution at 298.1 K, while dissection of the free-energy change of association (ΔG°) into its enthalpic (ΔH°) and entropic (TΔS°) components was accomplished by means of isothermal titration calorimetry measurements. Stability constants are poorly regulated by anion-ligand charge-charge attraction. Thermodynamic data show that the formation of complexes with neutral ligands, which are principally stabilized by anion-π interactions, is enthalpically favorable (-ΔG°, 11.1-17.5 kJ/mol; ΔH°, -2.3 to -0.5 kJ/mol; TΔS°, 9.0-17.0 kJ/mol), while for charged ligands, enthalpy changes are mostly unfavorable. Complexation reactions are invariably promoted by large and favorable entropic contributions. The importance of desolvation phenomena manifested by such thermodynamic data was confirmed by the hydrodynamic results obtained by means of diffusion NMR spectroscopy. In the case of L2, complexation equilibria were also studied in a 80:20 (v/v) water/ethanol mixture. In this mixed solvent of lower dielectric constant than water, the stability of anion complexes decreases, relative to water. Solvation effects, mostly involving the ligand, are thought to be responsible for this peculiar behavior.

Binding and removal of octahedral, tetrahedral, square planar and linear anions in water by means of activated carbon functionalized with a pyrimidine-based anion receptor†

Binding of S2O32−, SeO42−, Pt(CN)42−, Co(CN)63−, Au(S2O3)23− and Fe(CN)64− anions by the protonated (positively charged) forms of tren (tris(2-aminoethyl)amine) and of the tren-derivative (HL) containing a pyrimidine residue was studied by means of potentiometric measurements in 0.1 M NMe4Cl solutions at 298.1 ± 0.1 K. Both ligands form stable complexes with these anions which appear to be mostly stabilized by electrostatic forces. In the case of HL, an anion–π interaction with the pyrimidine residue of the ligand also affords a significant contribution to complex stability. Some shape preference for tetrahedral and octahedral anions over square planar ones is observed. A hybrid AC/HL material obtained by adsorption of HL on commercial activated carbon (AC) was used to study the extraction of these anions from water. AC/HL shows enhanced adsorption capacity toward all the anions studied with respect to AC. This behavior is ascribed to the stronger interaction of anions with the HL function of AC/HL than with the Cπ-H3O+ sites of unfunctionalized AC. Of special interest is the enhancement of the adsorption capacities found for Au(S2O3)23− and Pt(CN)42−, two anions of great relevance for the extraction of platinum and gold from ores and from metallic wastes.

Formation of double-strand dimetallic helicates with a terpyridine-based macrocycle

The macrocyclic ligand (L), containing two terpyridine (terpy) and two ethylenediamine (en) groups arranged in a cyclic terpy-en-terpy-en sequence, forms a double-strand helicate Cu2L(4+) complex made especially stable by the formation of interstrand π-π stacking interactions involving opposite pyridine rings. The crystal structure of this complex shows the Cu(2+) cations in square pyramidal coordination environments defined by the donor atoms of half ligand chain composed, in sequence, by one pyridine ring, the connected ethylenediamine moiety and the two adjacent pyridine rings of the successive terpyridine. In aqueous solution, L forms both mono- and binuclear complexes with Cu(2+). The stability constants determined for these complexes evidence the combined action of the two metal ions in the assembly of the very stable helicate species, the binding of the first metal ion favoring the entrance of the second one. UV adsorption and emission spectra corroborate these equilibrium results. Furthermore, the Cu2L(4+) complex shows a significant inertness toward dissociation in acidic solutions. Also Zn(2+) forms mono- and binuclear complexes with L, although the Zn2L(4+) complex is much weaker than the Cu2L(4+) helicate and gives rise to fast dissociation reactions in acidic media. Experimental evidence allows neither to say that also the Zn(2+) complex has a helicate structure nor to exclude it.

Inorganic Mercury Sequestration by a Poly(ethylene imine) Dendrimer in Aqueous Solution

The interaction of the G-2 poly(ethylene imine) dendrimer L, derived from ammonia as initiating core, with Hg(II) and HgCl42− was studied in aqueous solution by means of potentiometric (pH-metric) measurements. Speciation of these complex systems showed that L is able to form a wide variety of complexes including 1:1, 2:1, 3:1 and 3:2 metal-to-ligand species, of different protonation states, as well as the anion complexes [(H7L)HgCl4]5+ and [(H8L)HgCl4]6+. The stability of the metal complexes is very high, making L an excellent sequestering agent for Hg(II), over a large pH range, and a promising ligand for the preparation of functionalized activated carbons to be employed in the remediation and the prevention of environmental problems.

Halide and hydroxide anion binding in water

The formation of halide and hydroxide anion complexes with two ligands L1 (3,6-bis(morpholin-4-ylmethyl)-1,2,4,5-tetrazine) and L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) was studied in aqueous solution, by means of potentiometric and ITC procedures. In the solid state, HF2-, Cl- and Br- complexes of H2L22+ were analysed by single crystal XRD measurements. Further information on the latter was obtained with the use of density functional theory (DFT) calculations in combination with the polarizable continuum model (PCM). The presence of two halide or bifluoride HF2- (F-H-F-) anions forming anion-π interactions, respectively above and below the ligand tetrazine ring, is the leitmotiv of the [(H2L2)X2] (X = HF2, Cl, Br, I) complexes in the solid state, while hydrogen bonding between the anions and protonated morpholine ligand groups contributes to strengthen the anion-ligand interaction, in particular in the case of Cl- and Br-. In contrast to the solid state, only the anion : ligand complexes of 1 : 1 stoichiometry were found in solution. The stability of these complexes displays the peculiar trend I- > F- > Br- > Cl- which was rationalized in terms of electrostatic, hydrogen bond, anion-π interactions and solvent effects. DFT calculations performed on [(H2L2)X]+ (X = F, Cl, Br, I) in PCM water suggested that the ligand assumes a U-shaped conformation to form one anion-π and two salt bridge interactions with the included anions and furnished structural information to interpret the solvation effects affecting complex formation. The formation of hydroxide anion complexes with neutral (not protonated) L1 and L2 molecules represents an unprecedented case in water. The stability of the [L(OH)]- (L = L1, L2) complexes is comparable to or higher than the stability of halide complexes with protonated ligand molecules, their formation being promoted by largely favourable enthalpic contributions that prevail over unfavourable entropic changes.

Cation, Anion and Ion-Pair Complexes with a G-3 Poly(ethylene imine) Dendrimer in Aqueous Solution

The G-3 poly(ethylene imine) ligand L2 shows a multifaceted coordination ability, being able to bind metal cations, anions and ion-pairs. The equilibrium constants for the formation of metal (Cu2+, Zn2+), anion (SO42−) and ion-pair (Cu2+/SO42−) complexes were determined in 0.1 M Me4NCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric titrations. Thanks to its dendrimeric nature, L2 can form highly nucleated metal complexes, such as Cu5L210+ and Zn4L28+, in successive and well-defined complexation steps. Protonated forms of L2 give rise to relatively weak anion complexes with SO42−, but the addition of Cu2+ significantly enhances the binding ability of the ligand toward this anion below pH 9. In more alkaline solutions, an opposite trend is observed. The coordination properties of L2 are discussed with the support of modelling calculations. According to results, L2 is a promising molecule for the preparation of solid supported materials for the recovery of cations and anions from aqueous media and/or for applications in heterogeneous catalysis.

Polyfunctional Tetraaza-Macrocyclic Ligands: Zn(II), Cu(II) Binding and Formation of Hybrid Materials with Multiwalled Carbon Nanotubes

The binding properties of HL1, HL2, and HL3 ligands toward Cu(II) and Zn(II) ions, constituted by tetraaza-macrocyclic rings decorated with pyrimidine pendants, were investigated by means of potentiometric and UV spectrophotometric measurements in aqueous solution, with the objective of using the related HL-M(II) (HL = HL1–HL3; M = Cu, Zn) complexes for the preparation of hybrid MWCNT-HL-M(II) materials based on multiwalled carbon nanotubes (MWCNTs), through an environmentally friendly noncovalent procedure. As shown by the crystal structure of [Cu(HL1)](ClO4)2, metal coordination takes place in the macrocyclic ring, whereas the pyrimidine residue remains available for attachment onto the surface of the MWCNTs via π–π stacking interactions. On the basis of equilibrium data showing the formation of highly stable Cu(II) complexes, the MWCNT-HL1-Cu(II) material was prepared and characterized. This compound proved very stable toward lixiviation processes (release of HL1 and/or Cu(II)); thus, it was used for the preparation of its reduced MWCNT-HL1-Cu(0) derivatives. X-ray photoelectron spectroscopy and transmission electron microscopy images showed that MWCNT-HL1-Cu(0) contains Cu(0) nanoparticles, of very small (less than 5 nm) and regular size, uniformly distributed over the surface of the MWCNTs. Also, the MWCNT-HL1-Cu(0) material proved very resistant to detachment of its components. Accordingly, both MWCNT-HL1-Cu(II) and MWCNT-HL1-Cu(0) are promising candidates for applications in heterogeneous catalysis.

Network Formation via Anion Coordination: Crystal Structures Based on the Interplay of Non-Covalent Interactions

We describe the synthesis and the structural characterization of new H2L(CF3CO2)2 (1) and H2L(Ph2PO4)2 (2) compounds containing the diprotonated form (H2L2+) of the tetrazine-based molecule 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine. X-ray diffraction (XRD) analysis of single crystals of these compounds showed that H2L2+ displays similar binding properties toward both anions when salt bridge interactions are taken into account. Nevertheless, the different shapes, sizes and functionalities of trifluoroacetate and diphenyl phosphate anions define quite different organization patterns leading to the peculiar crystal lattices of 1 and 2. These three-dimensional (3D) architectures are self-assembled by a variety of non-covalent forces, among which prominent roles are played by fluorine–π (in 1) and anion–π (in 2) interactions.

MWCNTs-Supported Pd(II) Complexes with High Catalytic Efficiency in Oxygen Reduction Reaction in Alkaline Media

We report here the remarkable catalytic efficiency observed for two Pd(II) azamacrocyclic complexes supported on multiwalled carbon nanotubes (MWCNTs) toward oxygen reduction reactions. Beyond a main (>90%) 4e- process and an onset potential close to or better than those of commercial Pt electrodes, the MWCNTs functionalization strategy, aimed at chemically defined Pd(II)-based catalytic centers, allowed the half-cell to exceed the proton-exchange-membrane fuel-cell reference/target mass activity efficiency set by the U.S. Department of Energy for 2020 (440 mA/mgPGM at 0.9 V vs reversible hydrogen electrode).