Solid State and Solution Study on the Formation of Inorganic Anion Complexes with a Series of Tetrazine-Based Ligands.

Matteo Savastano,Carla Bazzicalupi,Paola Gratteri,Maria Dolores López De La Torre,Antonio Bianchi,Manuel Melguizo,Claudia Giorgi,Celeste García-Gallarín

doi:10.3390/molecules24122247

Matteo Savastano, Carla Bazzicalupi + Show 6 more

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https://doi.org/10.3390/molecules24122247

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Abstract

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.

Highlights

On the experimental evidence evidence that anion–s-tetrazine ring interactions play an active role in determining the architecture that anion–s-tetrazine ring interactions play an active role in determining the architecture of some of some metalla-macrocycles
[8,9,10,11,12], the anion–s-tetrazine interaction emerging as a promising tool promising tool for the construction anion receptors as well as of foranion the self-assembling of anion the construction of anion receptors as of well as for the self-assembling coordination frameworks coordination frameworks (ACF)
In previous studies dealing with the formation of anion complexes with a polyamine ligand containing an electron-deficient nitroso-amino-pyrimidine group, we showed that linear correlations containing an electron-deficient nitroso-amino-pyrimidine group, we showed that linear correlations between the binding free energies and the receptor charge for systems combining salt-bridge and between the binding free energies and the receptor charge for systems combining salt-bridge and anion–π interactions may offer the opportunity to determine the individual contribution of each anion–π interactions may offer the opportunity to determine the individual contribution of each type type of interaction

Summary

Introduction

On the experimental evidence evidence that anion–s-tetrazine ring interactions play an active role in determining the architecture that anion–s-tetrazine ring interactions play an active role in determining the architecture of some of some metalla-macrocycles [6,7]. More it recently, it was shown that s-tetrazine rings as doabehave a metalla-macrocycles [6,7]. Was shown that s-tetrazine rings do behave bindingassite binding siteeven for in anions even in solution [8,9,10,11,12], the anion–s-tetrazine interaction emerging asfor a for anions solution [8,9,10,11,12], the anion–s-tetrazine interaction emerging as a promising tool promising tool for the construction anion receptors as well as of foranion the self-assembling of anion the construction of anion receptors as of well as for the self-assembling coordination frameworks coordination frameworks (ACF)

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