Anion–π interactions with the ligand give rise to infinite 2D networks established through short interhalogen contacts between I₃⁻and I⁻ions.

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.

How do different supramolecular forces contribute to the stabilization of complexes of organic anions in water? Oftentimes, when debating such a theme, we refer to broad concepts like positive or negative cooperative effects; the focus of the present work is rather on their interplay, i.e. on the way different kinds of stabilizing interactions (salt bridges, H-bonds, anion–π interactions, π–π stacking, solvent effects, etc.) dialogue among themselves. What happens if we tune the strengths of salt bridges by altering the basicity of the anion? What if we change the geometry of the charged group? How does shifting towards more hydrophilic or hydrophobic anions impact the stability of complexes in water? What happens in the solid state? Will aromatic anions go for a π–π stacking or an anion–π interaction mode and do they all behave in the same manner? Does the host/guest size make any difference? What if we play with regiochemistry: will one of the isomers be selectively recognized? Here we present a case study featuring the tetrazine-based ligands L1 and L2 and a series of selected organic anions; potentiometric, NMR, and XRD data and in silico simulations are employed to render such a complex picture.

Four molecules (L1–L4) constituted by an s-tetrazine ring appended with two identical aliphatic chains of increasing length bearing terminal morpholine groups were studied as anion receptors in water. The basicity properties of these molecules were also investigated. Speciation of the anion complexes formed in solution and determination of their stability constants were performed by means of potentiometric (pH-metric) titrations, while further information was obtained by NMR and isothermal titration calorimetry (ITC) measurements. The crystal structures of two neutral ligands (L3, L4) and of their H₂L3(ClO₄)₂∙2H₂O, H₂L4(ClO₄)₂∙2H₂O, H₂L3(PF₆)₂, and H₂L3(PF₆)₂∙2H₂O anion complexes were determined by single crystal X-ray diffraction. The formation of anion–π interactions is the leitmotiv of these complexes, both in solution and in the solid state, although hydrogen bonding and/or formation of salt-bridges can contribute to their stability. Evidence of the ability of these ligands to form anion–π interactions is given by the observation that even the neutral (not-protonated) molecules bind anions in water to form complexes of significant stability, including elusive OH⁻ anions.

This paper reports copper-catalyzed direct functionalization of multi-walled carbon nanotubes (MWCNTs) with polyamines. Notably, and contrary to previous reports on direct amination of carbon nanotubes, the yield of this catalytic procedure is independent of the degree of defects on the MWCNT graphenic surface, which results in the efficient functionalization of both very defective and highly regular MWCNTs. This methodology allows us to obtain very high degrees of surface functionalization (much higher than those previously reported) under mild conditions (refluxing toluene). Hybrid materials containing up to 30.9 wt% hyperbranched polyethyleneimine (HBPEI, M_n = 1800) were prepared following this procedure. The most efficient catalysts are Cu(I) salts, in particular the iodide, while Cu(II) and Ni(II) salts result in poorer functionalization degrees. The method proposed is useful to functionalize MWCNTs with polyamines of diverse molecular weight: from ethylenediamine, diethylenetriamine or tris(2-aminoethyl)amine to HBPEIs of M_n = 600, 1800 and 10 000. The structural analysis of the amine-functionalized products (hybrids) supports the covalent grafting of amine nitrogen to the MWCNT walls. The characteristics of these hybrids open many possibilities for potential uses as supports for a wide range of catalytic metal/metal oxide nanoparticles. Moreover, they show improved dispersibility in water, their electronic behavior exhibits a semiconductor character and they also present a good ability to trap carbon dioxide.