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 Cu₂L⁴⁺ 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 Cu²⁺ 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 Cu²⁺. 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 Cu₂L⁴⁺ complex shows a significant inertness toward dissociation in acidic solutions. Also Zn²⁺ forms mono- and binuclear complexes with L, although the Zn₂L⁴⁺ complex is much weaker than the Cu₂L⁴⁺ helicate and gives rise to fast dissociation reactions in acidic media. Experimental evidence allows neither to say that also the Zn²⁺ complex has a helicate structure nor to exclude it

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

Ligands <b>L1</b> and <b>L2</b>, 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 H₂L²⁺ (L = <b>L1</b> and <b>L2</b>) forms, bind anions such as F^–, NO₃^–, PF₆^–, ClO₄^–, and SO₄^2– to form stable complexes in water. The crystal structures of H₂<b>L1</b>(PF₆)₂·2H₂O, H₂<b>L1</b>(ClO₄)₂·2H₂O, H₂<b>L2</b>(NO₃)₂, H₂<b>L2</b>(PF₆)₂·H₂O, and H₂<b>L2</b>(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 (Δ<i>G</i>°) into its enthalpic (Δ<i>H</i>°) and entropic (TΔ<i>S</i>°) 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 (−Δ<i>G</i>°, 11.1–17.5 kJ/mol; Δ<i>H</i>°, −2.3 to −0.5 kJ/mol; <i>T</i>Δ<i>S</i>°, 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 <b>L2</b>, 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

Thermodynamics of Anion−π Interactions in Aqueous Solution

Thermodynamic parameters (Δ<i>G</i>°, Δ<i>H</i>°, <i>T</i>Δ<i>S</i>°), obtained by means of potentiometric and isothermal titration calorimetry (ITC) methods, for the binding equilibria involving anions of high negative charge, like SO₄^2–, SeO₄^2–, S₂O₃^2– 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 {H₄L­[Co­(CN)₆]}·2H₂O complex showed strong anion−π interactions between Co­(CN)₆^3– and the protonated H₄L³⁺ 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

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