Anion-π and lone pair-π interactions with s-tetrazine-based ligands

Abstract

Most of traditional and contemporary interest in s-tetrazine derivatives focuses onto their redox properties, reactivity and energy density. In recent times, however, an increasing number of reports highlighted the possible usefulness of the s-tetrazine moiety as a binding site for anionic and electron rich species, according to the high and positive quadrupolar moment of this heterocycle and the consequent strength of anion-π and lone pair-π interactions. Herein, after giving a quick perspective on s-tetrazine properties and on how they foster these types of π interactions, we present statistical and critical examination of the available structural data, doing justice to the debated topic of the existence and directionality of anion-and lone pair-π interactions. Finally, available literature material concerning the usage of s-tetrazine as supramolecular binding site in solution, i.e. paving the way to applications such as molecular recognition and sensing, is presented and discussed. Contents 1. Introduction 2. s-Tetrazine Synthesis & Properties 2.1. Synthesis of s-tetrazine derivatives 2.2. General perspective on the properties and applications s-tetrazines 2.3. Anion-π and lone pair-π: nature of the interactions and relevance of the s-tetrazine ring 3. Anion-π and lone pair-π interactions with s-tetrazines in the solid-state: CSD Survey 3.1. Methodology and rationale of the survey 3.2. Structural data analysis 4. Selected evidences of anion-π and lone pair-π interactions with s-tetrazines in the solid state 5. Anion-π and lone pair-π interactions with s-tetrazines in solution 6. Conclusions Appendix A. Supplementary data Referencesthe aromatic s-tetrazine ring enables compact molecular packings which are essential to realize substances with a high (volumetric) energy density [116,117]. This is a field that continues very active in seeking for new s-tetrazine derivatives of interest [118][119][120].Other important applications of s-tetrazines derive from their particular reactivity, that serves as the base of the usefulness that these structures are proving in molecular biology studies, as reacting partners in bioorthogonal chemistry [121] methods (a field that has boosted the research and literature devoted to stetrazines from the first 2000s) [84,122]. These applications take advantage of two properties of stetrazines: i) the propensity of the s-tetrazine nucleus to act as 2,3-diazadiene in fast inverse-electrondemand Diels-Alder (iedDA) reactions with alkenes and alkynes that affords, in a first term, a bicyclic adduct which immediately suffers a retro-Diels-Alder (rDA) reaction, evolving N 2 , to give rise to dihydropyridazines or aromatic pyridazines, respectively (Scheme 2) [123,124]; ii) the inertness of s-tetrazine derivatives towards natural substances under biological conditions. Scheme 2 to be inserted about here.But the property that focused our interest in s-tetrazines, and constitutes the main subject of this review, is their ability to establish non-covalent bonds to negatively charged species (a...

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