One of the most intriguing nitrogen based aromatic heterocycles is 1,2,4,5-tetrazine or s-tetrazine (TTZ) thanks to its electron acceptor character and fluorescence properties and the possibilities of functionalization in the 3 and 6 positions allowing access to various ligands. In this review we focus on the two main families of TTZ based ligands, i.e. ditopic symmetric and monotopic non-symmetric, together with their metal complexes, with a special emphasis on their solid state structures and physical properties. After a description of the most representative complexes containing unsubstituted TTZ as a ligand, symmetric TTZ ligands and complexes derived thereof are discussed in the order: 3,6-bis(2-pyridyl)-tetrazine, 3,6-bis(3-pyridyl)-tetrazine, 3,6-bis(4-pyridyl)-tetrazine, 3,6-bis(2-pyrimidyl)-tetrazine, 3,6-bis(2-pyrazinyl)-tetrazine, 3,6-bis(monopicolylamine)-tetrazine, 3,6-bis(vanillin-hydrazinyl)-tetrazine and TTZ containing carboxylic acids. Remarkable results have been obtained in recent years for metal–organic frameworks and magnetic compounds in which magnetic coupling is enhanced when the tetrazine bridge is reduced to radical anions. Non-symmetric ligands, such as dipicolylamine-TTZ and monopicolylamine-TTZ, are comparatively more recent than the symmetric ones. They allow in principle the preparation of mononuclear complexes in a controlled manner, although binuclear complexes have been isolated as well. Moreover, in the monopicolylamine-TTZ-Cl ligand, deprotonation of the amine, thanks to the electron acceptor character of TTZ, afforded a negatively charged ligand equivalent of a guanidinate.

Five new highly photochromic hybrid organic–inorganic materials were successfully prepared by supramolecular assembly of normal (1⁺) and mixed (2⁺) cationic dithienylethenes (DTEs) and polyoxometalates (POMs) units. Single-crystal X-ray diffraction studies reveal that (1)₃[PM₁₂O₄₀]·5ACN (M = W, Mo) (1-PW₁₂ and 1-PMo₁₂) and (2)₃[PM₁₂O₄₀]·5ACN (M = W, Mo) (2-PW₁₂ and 2-PMo₁₂) are isostructural, while (2)₄[Mo₈O₂₆]·4DMF (2-Mo₈) is an isotype of (1)₄[Mo₈O₂₆]·4DMF (1-Mo₈) i.e., the first POM–DTE ionic assembly, recently reported. The solid-state photochromic properties of these materials at room temperature are highly tunable with the nature of the DTEs and POMs used, which has been interpreted taking into account steric and electronic factors. Especially, while 1-PMo₁₂ and 2-PMo₁₂ exhibit negligible photoresponses which have not been considered herein, thorough investigations of the photocoloration and fading kinetics in ambient conditions have highlighted the precise impact of both organic and inorganic components on the photoswitching abilities of 1-Mo₈, 2-Mo₈, 1-PW₁₂ and 2-PW₁₂. Upon UV-light irradiation, the absorption of the normal and mixed DTEs in their closed-ring form exhibits a hypsochromic shift which is more pronounced in hybrid systems integrating β-[Mo₈O₂₆]⁴⁻ rather than α-[PW₁₂O₄₀]³⁻, due to the higher negative charge density of the octamolybdate unit. The photocyclization rates of DTEs are also systematically increased when combined with the β-[Mo₈O₂₆]⁴⁻ unit, while in contrast, their back cycloreversion rates are not affected whatever the nature of the POMs used. In addition, within isostructural series, hybrids integrating mixed DTEs exhibit faster fading processes than their normal DTE counterparts when exposed to visible light. Finally, among this new series, 2-Mo₈ shows the best photochromic performances with a fast photocoloration rate associated with a high coloration contrast, a fast and complete fading process and a high cyclability.

We describe herein the synthesis and structural characterization of a tetrazine‐based bis(picolylamine) ligand, namely, N³,N⁶‐bis(pyridin‐2‐ylmethyl)‐1,2,4,5‐tetrazine‐3,6‐diamine (L, 1), which was prepared by the direct reaction between 2 equiv. of picolylamine and 3,6‐dichloro‐1,2,4,5‐tetrazine. Its reaction with cobalt(II) hexafluoroacetylacetonate hydrate afforded the mononuclear complex [Co(hfac)₂L]·CH₃CN (2, Hhfac = hexafluoroacetylacetone) and the chain compound {[Co(hfac)₂(µ‐L)][Co(hfac)₂(CH₃OH)₂]}_n (3). The crystal structures of 1–3 were determined by X‐ray structure analysis. The metal centre in 2 is trischelated by two hfac ligands and one picolylamine unit. The two crystallographically independent cobalt(II) ions in 3 (Co1 and Co2) are six‐coordinate. Two bidentate hfac groups in equatorial positions and two trans‐positioned pyridine nitrogen atoms build the coordination polyhedron around Co1 to form a neutral zigzag chain compound, which cocrystallizes with the mononuclear species trans‐[Co₂(hfac)₂(CH₃OH)₂]. The magnetic properties of 2 and 3 were investigated in the temperature range 1.9–300 K. The decrease of the χ_MT values of 2 and 3 upon cooling is mainly due to the depopulation of the higher energy Kramers doublets of the magnetically isolated six‐coordinate cobalt(II) ions. Alternating‐current (ac) magnetic susceptibility measurements of 2 and 3 revealed the occurrence of frequency‐dependent out‐of‐phase signals under applied direct‐current (dc) fields at very low temperatures, and this result is indicative of single‐ion magnet (SIM) behaviour.

The 3-Cl-6-amino-(2′-picolyl)-1,2,4,5-tetrazine ligand HL¹ has been synthesized and structurally characterized. Its versatile coordination behavior has been evidenced through reactions with Cu(Hfac)₂ and Cu(triflate)₂ precursors, which provided mixed-valence bimetallic Cu^1.5Cu^1.5 complexes [Cu₂(μ-Cl)(L¹)₂] 2 and [Cu₂(μ-triflate)(L¹)₂] 5. Changing the Cu(II) precursor and the solvent leads to the formation of mononuclear octahedral Cu(II) complexes [CuCl₂(HL¹)₂]·2CH₃CN 3 and [Cu(Hfac)₂(HL¹)] 4, in which only the amino-pyridine unit is involved in the coordination of the metal center. In contrast, in complexes 2 and 5, the ligands are deprotonated and bridge the metal centers as pyridine-amido-tetrazine fragments, while a bridging chloride or triflate ligand completes the coordination sphere of the metal ions. The Cu–Cu distances of 2.4313(4) Å in 2 and 2.5198(10) Å in 5 lie among the shorter values within mixed-valence bimetallic Cu complexes. Mixed-valence character is strongly supported by DFT calculations, showing the equal repartition of the unpaired electron between the two metal centers.