The synthesis of the new alcohol-pendant macrocycle 4-(2-hydroxyethyl)-1,4,7,16,19,22-hexaaza-10,13,25,28-tetraoxacyclotriacontane (L2) is reported. This ligand contains two different triamine moieties, one of them bearing an ethanolic sidearm. L2 binds two Zn(II) ions in aqueous solution. The stability constants of the L2 complexes have been determined at 298.1 and 308.1 K by means of potentiometric measurements. Besides a [Zn2 L2]4+ species, a deprotonated [Zn2(L2-H)]3+ complex and a hydroxo [Zn2(L2-H)(OH)]2+ complex are formed in aqueous solution. Zn(II)-assisted deprotonation of the alcoholic group takes place at neutral pH, giving the [Zn2(L2-H)]3+ complex. In [Zn2(L2-H)]3+, the deprotonated R−O- function bridges the two metals, as shown by the crystal structure of [Zn2(L2-H)Br2]BPh4·MeOH. The hydroxo species [Zn2(L2-H)(OH)]2+ is formed at slightly alkaline pH's. This complex contains both a Zn(II)-bound alkoxide and a Zn(II)−OH nucleophilic function. Therefore, it may provide a simple model system for alkaline phosphatases, where both a deprotonated serine and a Zn−OH function are involved in phosphate ester hydrolysis. Indeed, this complex promotes the hydrolysis of the carboxy ester p-nitrophenyl acetate (NA) as well as the cleavage of phosphate ester bis(p-nitrophenyl) phosphate (BNP). The kinetics of promoted hydrolysis of NA and BNP were studied by means of UV and 1H and 31P NMR measurements. In NA hydrolysis, the R−O-−Zn(II) function acts as nucleophile in the first step of the hydrolytic mechanism, to give an acetyl derivative, which is subsequently hydrolyzed to acetate by a Zn−OH group. Similarly, in BNP cleavage, the nucleophilic attack of alkoxide on phosphorus gives a pendant-alcohol phosphorylated intermediate, which undergoes subsequent intramolecular nucleophilic attack of a Zn(II)-bound hydroxide to yield a phosphomonoester product.

The ligand [30]aneN(6)O(4) (L1) binds two Zn(II) in aqueous solution. The stability constants of the L1 complexes have been determined at 308.1 K by means of potentiometric measurements. Dinuclear monohydroxo [Zn(2)L1OH](3+) and dihydroxo [Zn(2)L1(OH)(2)](2+) complexes are formed in aqueous solution from neutral to alkaline pH. The kinetics of promoted hydrolysis of p-nitrophenyl acetate (NA) was studied. Both hydroxo species promote p-nitrophenyl acetate (NA) hydrolysis at 298.1 K with second-order kinetics. The activity of these species in NA hydrolysis is similar to that found for the mononuclear L2-Zn-OH(+) complex (L2 = [15]aneN(3)O(2)), indicating that the hydrolytic process takes place via a simple bimolecular mechanism. The hydrolysis rate of bis(p-nitrophenyl) phosphate (BNP) was measured in aqueous solution at 308.1 K in the presence of the L1and L2 zinc complexes. The hydrolysis rate of BNP is increased almost 10-fold by the dinuclear [Zn(2)L1(OH)(2)](2+) complex with respect to the mononuclear L2-Zn-OH(+) one. This result indicates a cooperative role of the two metals in the hydrolytic mechanism. A bridging coordination of the phosphate ester to the two Zn(II) ions can be suggested. The crystal structure of [Zn(2)L1(&mgr;-PP)(2)(MeOH)(2)](ClO(4))(2) (PP(-) = diphenyl phosphate) (space group P&onemacr;, a = 10.681(5) Å, b = 12.042(1) Å, c = 13.191(3) Å, alpha = 74.63(2) degrees, beta = 71.74(3) degrees, gamma = 68.41(2) degrees, V = 1476.4(8) Å(3), Z = 1, R = 0.0472, R(w)(2) = 0.1166) strongly supports this hypothesis, since in the [Zn(2)L1(&mgr;-PP)(2)(MeOH)(2)](2+) cation the diphosphate anions bridge the two metals. The dinuclear Zn(II) complexes of L1 provide a simple model system for hydrolytic dizinc enzymes.

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Solutions containing Zn(II) and Cu(II) complexes with [15]aneN(3)O(2) rapidly adsorb atmospheric CO(2) to give {[ZnL](3)(&mgr;(3)-CO(3))}.(ClO(4))(4) (2) and {[CuL](3)(&mgr;(3)-CO(3))}.(ClO(4))(4) (4) complexes. The crystal structures of both complexes have been solved (for 2, space group R3c, a, b = 22.300(5) Å, c = 17.980(8) Å, V = 7743(4) Å(3), Z = 6, R = 0.0666, R(w)(2) = 0.1719; for 4, space group R3c, a, b = 22.292(7) Å, c = 10.096(8) Å, V = 7788(5) Å(3), Z = 6, R = 0.0598, R(w)(2) = 0.1611), and the spectromagnetic behavior of 4 has been studied. In both compounds a carbonate anion triply bridges three metal cations. Each metal is coordinated by one oxygen of the carbonate, three nitrogens, and an oxygen of the macrocycle; the latter donor weakly interacts with the metals. Although the two compounds are isomorphous, they are not isostructural, because the coordination geometries of Zn(II) in 2 and Cu(II) in 4 are different. The mixed complex {[CuZn(2)L(3)](&mgr;(3)-CO(3))}.(ClO(4))(4) has been synthesized. X-ray analysis (space group R3c, a, b = 22.323(7) Å, c = 17.989(9) Å, V = 7763(5) Å(3), Z = 6, R = 0.0477, R(w)(2) = 0.1371) and EPR measurements are in accord with a &mgr;(3)-carbonate bridging one Cu(II) and two Zn(II) ions in {[CuZn(2)L(3)](&mgr;(3)-CO(3))}(4+). Both the Zn(II) and Cu(II) cations exhibit the same coordination sphere, almost equal to that found in the trinuclear Zn(II) complex 2. The systems Zn(II)/L and Cu(II)/Lhave been studied by means of potentiometric measurements in 0.15 mol dm(-)(1) NaCl and in 0.1 mol dm(-)(3) NaClO(4) aqueous solutions; the species present in solution and their stability constants have been determined. In both systems [ML](2+) species and hydroxo complexes [M(II)LOH](+) (M = Zn, Cu) are present in solution. In the case of Cu(II), a [CuL(OH)(2)] complex is also found. The process of CO(2) fixation is due to the presence of such hydroxo-species, which can act as nucleophiles toward CO(2). In order to test the nucleophilic ability of the Zn(II) complexes, the kinetics of the promoted hydrolysis of p-nitrophenyl acetate has been studied. The [ZnLOH](+) complex promotes such a reaction, where the Zn(II)-bound OH(-) acts as a nucleophile to the carbonyl carbon. The equilibrium constants for the addition of HCO(3)(-) and CO(3)(2)(-) to the [ZnL](2+) complex have been potentiometrically determined. Only [ML(HCO(3))](+) and [ML(CO(3))] species are found in aqueous solution. A mechanism for the formation of {[ML](3)(&mgr;(3)-CO(3))}.(ClO(4))(4) is suggested.

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...

Ligand 2,5,8-triaza[9]-10,23-phenanthrolinophane (L) contains a triamine chain connecting the 2,9 positions of a phenanthroline unit. Protonation of L has been studied by means of potentiometric and 1H and 13C NMR techniques, allowing the determination of the basicity constants and of the stepwise protonation sites. Protonation strongly affects the fluorescence emission properties of the chemosensor L. The two benzylic amine groups, namely, the two aliphatic amine groups adjacent to phenanthroline, are the most efficient nitrogens in fluorescence emission quenching. In the diprotonated receptor [H2 L]2+ both of these nitrogens are protonated, and therefore this species is the most emissive. In the [H3 L]3+ species the three acidic protons are located on the amine groups of the polyamine chain. This species is still emissive, but less so than [H2 L]2+, due to formation of a hydrogen bond network involving the phenanthroline nitrogens, as shown by the crystal structure of the [H3 L]Br3·H2O salt. A potentiometric investigation of Zn(II) binding in aqueous solution suggests that some nitrogen donors are not involved, or weakly involved in metal coordination. Actually, the crystal structure of the [ZnL(H2O)](ClO4)2 complex shows that both of the benzylic amine groups are weakly bound to the metal. This Zn(II) complex does not show any fluorescence emission. This rather unusual feature can be explained considering an electron transfer process involving the benzylic nitrogens.

Two new polyamine macrocycles 2,5,8-triaza[9]- [9](2,9)[1,10]-phenanthrolinophane (L 1 ) and 2,5,8,11-tetraaza[12]- [12](2,9)[1,10]-phenanthrolinophane (L 2 ) have been synthesized and characterized. They contain a triamine (L 1 ) or a tetraamine (L 2 ) chain linking the 2,9 positions of a phenanthroline moiety. Like L 3 , which contains a pentaamine chain connecting the 2,9-phenanthroline positions, they form stable 1 : 1 lead() complexes in aqueous solutions. These complexes can readily be extracted in non-aqueous solvents, such as CHCl 3 or CH 2 Cl 2 . The thermodynamic parameters for lead() complexation have been determined by means of potentiometric and microcalorimetric measurements in aqueous solutions. The stability of the complexes decreases from L 1 to L 3 mainly due to a marked decrease of the enthalpy changes related to the formation of the complexes, indicating that the overall metal-ligand interaction decreases from L 1 to L 3 . Most likely, in the [PbL 1 ] 2ϩ complex all donors are strongly involved in metal co-ordination, while in [PbL 2 ] 2ϩ and [PbL 3 ] 2ϩ some of the amine groups are weakly bound, or not bound, to the metal. These considerations are supported by the crystal structures of [(PbL 1 Br) 2 (µ-Br)][PbL 1 Br 2 ]Brؒ5H 2 O. In the latter complex the macrocycle wraps around the metal. On the other hand, three nitrogen donors interact at remarkably longer distance than the others. This observation may justify the lower stability of the [PbL 3 ] 2ϩ complex with respect to that of [PbL 1 ] 2ϩ .

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