Molecular Hosts for the Sensing and Separation of 99TcO4.

In recent years, European Union member states have hastened energy policy deliberations to address supply and sustainability concerns, placing a significant emphasis on nuclear energy as a means to achieve decarbonization goals. However, despite its significant role in power generation, nuclear energy faces significant challenges linked to fuel reprocessing and waste disposal, that hinder its broader expansion. In this context, the separation of technetium represents a concerning issue. Indeed, technetium's catalytic activity can impede the extraction of uranium, neptunium, and plutonium, affecting waste reprocessing efficiency. Additionally, the stable form of technetium in aerobic conditions, pertechnetate (TcO4 -), poses risks of groundwater contamination due to its mobility and solubility. Hence, sensing and separation of TcO4 - is imperative for both nuclear fuel processing and minimising radioactive contamination in the environment. However, the binding of TcO4 - and its separation from contaminated solutions present challenges due to the acidic (or basic) waste components and the high ionic strength in real matrices. Supramolecular chemists have addressed these issues by designing receptors inspired by molecular recognition principles. This article explores recent advancements and future directions in TcO4 - sensing and separation (using extraction and sorption) with a focus on molecular hosts. Metal-organic receptors will also be discussed.

A Supramolecular Approach to Antimicrobial Surfaces.

In this paper, we report on the preparation of Imidazole-functionalized glass surfaces, demonstrating the ability of a dinuclear Cu(II) complex of a macrocyclic ligand to give a "cascade" interaction with the deprotonated forms of grafted imidazole moieties. In this way, we realized a prototypal example of an antimicrobial surface based on a supramolecular approach, obtaining a neat microbicidal effect using low amounts of the described copper complex.

Towards Building Blocks for Supramolecular Architectures Based on Azacryptates.

In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with triphenyl spacers. The binding properties of its dicopper complex for aromatic dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramolecular structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu2L]4+ in water/acetonitrile mixture (pH = 7) showed a high affinity for biphenyl-4,4'-dicarboxylate (dfc2-), with a binding constant of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage's cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu2L]4+ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc2-) and short anions (e.g., terephthalate). The good affinity of [Cu2L]4+ for these dicarboxylates, together with hydrophobic interactions within the cage's cavity, may promote the self-assembly of a stable 1:1 complex in water mixture. These results represent a good starting point for the application of these molecular systems as building units for the design of new supramolecular architectures based on non-covalent interactions, which could be of interest in all fields related to supramolecular devices.

Anion Binding by Dimetallic Nickel(II) and Nickel(III) Complexes of a Face-to-Face Bicyclam: Looking for a Bimacrocyclic Effect.

The dinickel(II) complex of the face-to-face bicyclam ligand α,α'-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene (L∩L) in a dimethyl sulfoxide solution exists as a mixture of high- and low-spin forms and uptakes up to three halide and pseudohalide ions (X(-)), according to stepwise equilibria, whose constants were determined through spectrophotometric titration experiments. In the case of halides, the first anion goes into the intermetallic cavity, whereas pseudohalides first coordinate the metal center from outside. Comparison with equilibrium data for the complex with the macrocycle 5,7-dimethyl-6-benzyl-1,4,8,11-tetraazacyclotetradecane (L) shows that the dinuclear complex [Ni(II)2(L∩L)](4+) displays an affinity for the first halide distinctly higher than the mononuclear complex [Ni(II)(L)](2+), thus disclosing the existence of a bimacrocyclic effect for anion binding. Differential pulse voltammetry studies typically showed a three-peak profile: the most anodic pertaining to the [Ni(II)2(L∩L)](4+) → Ni(III)2(L∩L)](6+) two-electron process, then one originating from the [Ni(II)2(L∩L)X](3+) → Ni(III)2(L∩L)X](5+) two-electron process, and one deriving from the two two-electron half reactions [Ni(II)2(L∩L)X2](2+) → Ni(III)2(L∩L)X2](4+) and [Ni(II)2(L∩L)X3](+) → Ni(III)2(L∩L)X3](3+), taking place at nearly the same potential. The crystal structure of the [Ni(II)2(L∩L)(µ-NCO)(NCO)2]ClO4·2.5H2O complex salt showed a caterpillar arrangement of the three metal-bound cyanate ions.

Copper(II) Complexes of Cyclams Containing Nitrophenyl Substituents: Push-Pull Behavior and Scorpionate Coordination of the Nitro Group.

The three nitrophenyl-cyclam derivatives (nitrocyclams): 1-(4-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (2), 1-(2-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (3), and 1-(2,4-dinitrophenyl)-1,4,8,11-tetraazacyclotetradecane (4), in an MeCN solution, specifically incorporate the Cu(II) ion according to an irreversible process signaled by disappearance of the yellow color for a concentration c < 1 × 10(-4) M and by a yellow-to-red color change for c ≥ 1 × 10(-3), and must be considered efficient and specific dosimeters of copper(II) salts. When present in the ortho position of the nitrophenyl substituent, the -NO2 group coordinates the Cu(II) according to a scorpionate mode, while the metallocyclam system exhibits a trans-I configuration. In an MeCN solution the red trans-I-[Cu(II)(3)](2+) and trans-I-[Cu(II)(4)](2+) scorpionate complexes slowly convert into the violet trans-III scorpionate complexes. Kinetic aspects of the trans-I-to-trans-III configurational rearrangement were investigated in detail for the [Cu(II)(4)](2+) system. In particular, the conversion is spectacularly accelerated by catalytic amounts of Cl(-), NCO(-), and F(-). While for Cl(-) and NCO(-) the effect can be associated with the capability of the anion to stabilize through coordination a possible dissociative intermediate, the amazingly powerful effect of F(-) must be related to the preliminary deprotonation of one N-H fragment of the macrocycle, driven by the formation of the HF2(-) ion. Most of the metal complex species studied in solution were isolated in a crystalline form, and their molecular structures were elucidated through X-ray diffraction studies. This study documents the first examples of effective metal coordination by the nitro group.

Oxo-anion recognition by mono- and bisurea pendant-arm macrocyclic complexes.

The novel macrocyclic copper(II) complexes [2](2+) and [3](2+), carrying one or two (nitrophenyl)urea fragments appended to an azacyclam or diazacyclam framework, exploit the hydrogen-bond-forming abilities of the urea subunits, along with the metal-ligand interaction, in the recognition of anionic species. Equilibrium studies in acetonitrile performed on [2](2+) and [3](2+) show that (nitrophenyl)urea pendant arms strongly interact with anionic species such as carboxylates and phosphates, which display both coordinating tendencies toward copper(II) and good affinity toward urea subunits. Stability constants of the adducts are considerably higher than those determined for the interaction of the same anions with a "plain urea" reference compound, confirming the synergistic action of metallomacrocyclic and urea subunits. Complex [2](2+) forms 1:1 adducts with acetate, benzoate, hydrogendiphosphate, and dihydrogen phosphate, while complex [3](2+) interacts with the same anions according to both 1:1 and 1:2 stoichiometries, with the exception of hydrogendiphosphate, which forms only the 1:1 adduct with a distinctly high association constant (log K > 7). Spectrophotometric investigations suggest that oxoanionic species interact with the complexes according to a "bridged" mode, inducing the macrocyclic systems to adopt a scorpionate-like conformation, as confirmed by crystallographic studies on the [3](2+)/succinate adduct.

Mixing the spacers in azacryptands: effects on halide recognition.

In this work, we report on two new asymmetric dicopper cryptates, characterized by alternate furanyl and p-xylyl spacers. The results of the potentiometric, UV-vis and X-ray diffraction studies are discussed. In particular, for one of the cages in the octaprotonated form, the crystal structure of the complex with nitrate is described. From the point of view of the anion binding in water, the new dicopper cryptates display stronger similarities to each other than to the symmetric analogues (i.e. p-xylyl and furan cryptates). The substitution of even only one spacer modifies the distance between the metal ions. This prevents the development of the CT band, typically accompanying the binding of halides by the furan cryptate, and favors the formation of complexes of different stoichiometry with the linear azide anion, not observed for the symmetric cryptates. The obtained results may be very helpful in the design of new molecular receptors based on dimetallic cryptates, in particular for the tuning of the sensitivity towards specific anionic substrates.

Anion recognition by coordinative interactions: metal-amine complexes as receptors.

Alfred Werner's complexes of formula [M(III)(NH(3))(6-n)X(n)]X(3-n) involved inert metal centres (M = Cr, Co), and anions X(-)'frozen' in the coordination sphere, a circumstance which allowed the isolation of a variety of isomers. Amine complexes of labile transition metal ions, studied later, do not form isomers, yet they allow the investigation of the fast and reversible interaction of the anion X(-) with the metal-amine core. On these bases, anion receptors of varying degrees of sophistication have been synthesised, which consist of coordinatively unsaturated polyamine metal complexes and whose vacant coordination sites can be occupied by anion donor atoms. A thoughtful design of the polyamine framework may introduce geometrical selectivity, resulting from the matching between anion shape and size and the geometrical features of receptor's cavity. Compared to their purely organic counterparts, metal containing receptors display several advantages: (i) metal-anion interactions are strong enough to more than compensate anion dehydration energy, which allows recognition studies to be carried out in water; (ii) transition metal ions of different electronic configurations exhibit different geometrical preferences, which addresses anion binding and introduces a further element of selectivity. Chosen examples of polyamine metal complexes, including macrocycles and cages, displaying selective binding tendencies towards anions will be illustrated in this tutorial review.

A prototype for the chemosensing of Ba2+ based on self-assembling fluorescence enhancement.

Barium ion can be revealed at the micromolar concentration level by the blue-green fluorescence which arises upon the self-assembling process involving the metal ion and a novel bis-15-crown-5-naphthalenediimide derivative.

Dramatically enhanced carbon acidity of the nitrobenzyl fragment in a nickel(II) scorpionate complex.

The -CH(2)- group of the 2-nitrobenzyl pendant arm of the scorpionate complex I deprotonates in basic aqueous solution (pK(a) = 10.6), due to the coordination of the nitronate group to the nickel(II) center. Metal coordination enhances 2-nitrobenzene acidity by 10 orders of magnitude. [reaction: see text]

Does a reinforced kinetic macrocyclic effect exist? the demetallation in strong acid of copper(II) Complexes with open and cyclic tetramines containing a piperazine fragment.

The demetallation in acidic solution of the Cu(II) complexes with open-chain and cyclic tetramines containing a piperazine unit (2 and 3) has been investigated in terms of its kinetic aspects and compared with the behaviour of unsubstituted counterparts (tetramines 1 and 4). The presence of the piperazine fragment slows demetallation of the open-chain-ligand complex owing to the activation barrier associated with the conformational change from boat-to-half-boat; however, it does not affect the demetallation of the macrocyclic complex, which involves the spontaneous boat-to-twist conformational change. Thus, lateral reinforcement of a cyclam-like ligand does not add any further contribution to the typical inertness in demetallation of macrocyclic complexes.

Molecular motions in the solid state: the thermochromic nitro-nitrito Interconversion in nickel(II) bis(diamine) complexes.

The NO(2)(-) ion, in the trans-octahedral [Ni(II)(N,N'-dimethylethylenediamine)(2)(NO(2))(2)](H(2)O) complex, coordinates the metal through the nitrogen atom (nitro form). On heating the solid complex, the anion rotates to give nitrito coordination (oxygen bound), according to a reversible process. The coordination mode of NO(2)(-) to Ni(II) is related to the steric interplay between the anion and the alkyl substituents on the diamine.

Water soluble molecular switches of fluorescence based on the Ni(III)/Ni(II) redox change.

The water soluble Ni(II) complexes of the cyclam derivatives with 1,3-benzodioxole and 1,2,3-trimethoxybenzene display the fluorescent emission typical of the covalently linked fluorophores, which results from a charge transfer excited state. On oxidation to Ni(III), the fluorescence is completely quenched due to the occurrence of an electron transfer (eT) process from the excited fluorogenic fragment Fl to the oxidized metal. Thus, fluorescence can be switched off/on at will, for several cycles, by consecutively oxidizing and reducing the metal center, in controlled potential electrolysis experiments both in acetonitrile and in aqueous 0.1 M HClO4. Occurrence of an eT process from Fl to Ni(III) ultimately depends upon the easy oxidation of Fl to Fl+, whereas failure of the occurrence of an eT process from Ni(II) to Fl has to be ascribed to the particular resistance of Fl fragments to the reduction.

pH-controlled fluorescent emission in the nickel(II) complex of a bifunctional tetramine macrocycle.

In the Ni(II) complex of a reinforced tetramine macrocycle, equipped with both a sulfonamide pendant arm and a naphthalene fragment, light emission of the fluorophore is pH-controlled, as a result of a change of the geometry of the complex.

Metal-induced assembling/disassembling of fluorescent naphthalenediimide derivatives signalled by excimer emission.

The new quadridentate bischelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with Zn(II), Cd(II) and Cu(I) in CHCl3 or MeCN according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H NMR experiments indicate that the complex species formed in the presence of stoichiometric amounts of metal ion are nonhelical [2 + 2] adducts. The metal-induced self-assembling process is signalled by an intense excimer-type emission caused by the intramolecular interaction of two naphthalenediimide subunits that face each other in the [2 + 2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal/ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form because the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum.