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

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.

Genesis of Complex Polyiodide Networks: Insights on the Blue Box/I−/I2 Ternary System

“…Among our recent contributions, we demonstrated the use of tetrazine-based ligands for the coordination of inorganic [15,16] and organic anions [17], emphasizing coordination of halide [18] and pseudohalide anions [19]. Stabilization of anion coordination networks [20], including polyiodide-dense ones [21], has been achieved with s-tetrazine-based ligands through anion-π interactions [22], building on previous evidences of the usefulness of anion-π forces for polyiodide stabilization in perfluoroarene-based systems [23].…”

Words in supramolecular chemistry: the ineffable advances of polyiodide chemistry

“…35,76 In solution, the joining of anion and ring surfaces requires their desolvation: entropic effects connected with the release of coordinated solvent molecules back into the bulk oftentimes act as a driving force for association in solution. 33,77,81,82 In a sense, recognition of the relevance of ESP, and of in silico calculations in order to understand it, somewhat connects anion-π forces with σ-hole interactions (and thus to the whole discussion) in terms of the underlying basic concepts and required tools. Yet again, we see the "classic" controversial "anion-π" and "lone pair-π" vocabulary shifting towards the new wave "π-hole" interactions, calling for their synthesis and official collocation in a comprehensive view of supramolecular forces.…”

“…Englert, Espinosa et al 139 for a further report of layered cation-polyiodide assembly and I⋯I bond investigation). This peculiar organization was made possible by the s-tetrazine-based ligand L2, exploiting the strong anion-π interactions of this family of ligands 35,81,82,140 to stabilize polyiodide sheets.…”

Stabilisation of Exotic Tribromide (Br3−) Anions via Supramolecular Interaction with a Tosylated Macrocyclic Pyridinophane. A Serendipitous Case

“…It is known that azines may act as π-acid ligands, which can bind anions above their centres according to their positive electrostatic potentials (ESPs) and their molecular quadrupole moments ( Figure 2) [35]. For example, we have recently shown that s-tetrazine-based ligands can form anion complexes [36] with both inorganic [37][38][39][40][41][42] and organic [43] anions featuring clear anion-π interactions. Spontaneous oxidation of iodide in solution and stabilisation of triiodide (I 3 − ) anions in the solid state were observed in the presence of these s-tetrazine based ligands [38], while bromide was found to interact with the rings of these π-acids, but no oxidation to Br 3 − was detected [39].…”