The iono- and urothermal methods, both involving solvents based on e-urea (2-imidazolidinone), the 1:2 choline chloride:e-urea deep eutectic solvent and e-urea hemihydrate respectively, have been explored as synthetic approaches for the preparation of strontium-based metal-organic frameworks (Sr-MOFs). Using these media and seven different di-carboxylic acid ligands, a series of ten Sr-MOFs has been obtained and characterized by X-ray diffraction as well as by thermogravimetric and elemental analyses, infra-red spectroscopy and their optical properties have been investigated. While both preparation methods led to the same unique crystalline phase in some instances, different architectures were obtained for the majority of the ligands explored herein, depending on the synthetic approach employed. This highlights the key role of choline chloride in the ionothermal method and of water in the urothermal strategy, that can act as ligands/templates and thereby impact the formation of intermediate phases and the stability of the materials.

The reactivity of tetra(pyridin-4-ylthio)tetrathiafulvalene (L1) toward dihalogens (Br2, I2)/interhalogens (ICl, IBr) was investigated by crystallographic, microanalytical, spectroscopic, and hybrid-DFT approaches. A divergence in reactivity was observed depending on the halogen reagent. The reactions of L1 with I2 yielded a 1:4 adduct L1·4I2 (1), featuring four independent N···I─I interactions within a single polytopic scaffold. Reactions of L1 with Br2, ICl, and IBr afforded ionic species (2-6), containing the protonated donor counterbalanced by discrete tri(inter)halide anions such as [Br3]-, [ICl2]-, and [IBr2]-. These compounds showed diverse supramolecular cage-like architectures, often involving hydrogen-bonded networks. [H4L1](L1)[IBr2]4·2CH3CN·2.4CHCl3 (4) contains a tetracationic capsule [(H4L1)(L1)]4+ stabilized by N-H···N HBs and templated at a [IBr2]- guest. Both the N···I─I fragment in compound 1 and the tri(inter)halides in compounds 2-6 follow a common structural correlation between the normalized bond elongations typical of three-body systems. Hybrid-DFT QTAIM, NBO, Hirshfeld, and Bader charges, along with NCI index calculations allowed to quantify the CT interactions and the extent of ionic/covalent contributions to the HaBs in compound 1, analysis of the anion-cation interactions in the inclusion complex [(H4L1)(L1)(IBr2)]3+, and reproduction of the observed structural trends, highlighting the continuous nature of covalent-to-electrostatic interactions in apparently different three-body systems.

The urothermal approach in 1,3-dimethylimidazolidin-2-one (e-murea) was employed for the successful preparation of a series of nine alkaline earth (AE) metal-organic frameworks (AE-MOFs) with two different dihydroxy terephthalic acid ligands (2,5-dobdcH4 and 2,3-dobdcH4; dobdcH4 = dihydroxyterephthalic acid). The compounds were characterized by single-crystal and powder X-ray diffraction as well as elemental analysis and infra-red spectroscopy to confirm their purity. With the 2,5-dobdcH2 2- ligand, two isostructural 3D MOFs were obtained with Mg(II) and Ca(II), whereas 2D architectures featuring coordination of the hydroxyl groups were isolated with Sr(II) and Ba(II). Binding of the hydroxyl groups in a chelate fashion is consistently observed in AE-MOFs incorporating the 2,3-dobdcH2 2- ligand. These AE-MOFs present strong luminescence in the solid state with quantum yields ranging between 0.45 and 0.84, remarkable for this type of compounds. Interestingly for the AE-MOFs based on the 2,5-dobdcH2 2- ligand, a dependence on the nature of the alkaline earth metal cation was observed, as expressed for the heavier Sr(II) and Ba(II) systems in the presence of two different emission processes in thermal equilibrium as confirmed by investigation of the solid-state luminescence at 77K. For the AE-MOFs based on the 2,3-dobdcH2 2- ligand, such metal- and temperature-dependence is not observed.

The 1 : 2 choline chloride : 2-imidazolidinone (e-urea) deep eutectic solvent (DES) was employed for the ionothermal synthesis of strontium-based metal-organic frameworks (Sr-MOFs) using eight dicarboxylic acid-based ligands with varying relative orientations of the two coordinating moieties. With the four linear ligands based on the 2,5-difunctionalized terephthalic acid backbone, the nature of the atom/group at these sites (hydrogen, bromine, trifluoromethyl or hydroxyl) was shown to impact the nature of the secondary building unit (SBU) of the three-dimensional MOFs isolated and characterized by single-crystal X-ray diffraction. Modifying the exocyclic bond angle of the dicarboxylate linker was found to affect both the SBU and dimensionality of the coordination polymer. While the thiophene-based ligand afforded a two-dimensional arrangement, a three-dimensional organization was observed with the furan-based system and a one-dimensional coordination polymer with the isophthalic derivative. Reticulation of the latter system into a three-dimensional MOF was successfully undertaken by the use of the 3,3 ',5,5 '-azobenzenetetracarboxylic acid ligand comprising two bridged isophthalic acid units, highlighting the robustness of the SBU, in this case. The eight Sr-MOFs obtained feature e-urea molecules bound to the metal cation via coordination of the carbonyl unit assisted by hydrogen bonding of the neighbouring NH groups. These coordinated solvent molecules were found to occupy the pores and could unfortunately not be removed via thermal activation towards exploitation of the potential porosity. The optical properties of the Sr-MOFs were characterized by absorption and emission spectroscopy. The Sr-MOF based on the 2,5-dihydroxyterephthalic acid ligand was determined to be luminescent (lambda em = 575 nm) with a 26% quantum yield in the crystalline state, upon excitation at 360 nm.

A series of gold bis(dithiolate) compounds were prepared by direct reaction of KAuCl4 with 2-oxobenzo[d][1,3] dithiole-4-carbonitrile in the presence of a base, followed by precipitation with different cations. These cations range from organic discrete species such as tetraphenylphosphonium (Ph4P+), tetrabutylammonium (n-Bu4N+), tetraethylammonium (Et4N+), and tetraethylammonium (Me4N+), in decreasing size, to alkali metals such as sodium and potassium. For the alkali metal-based complexes, additional solvent molecules are required to stabilize the cation coordination sphere, leading to structural instability and loss of crystallinity. The introduction of crown ethers for complexation of alkali metal cations afforded more stable compounds. The alkali metal compounds yielded coordination polymers, demonstrating the potential of gold bis(dithiolate) complexes to form coordination polymers and metal-organic frameworks (MOFs). The monoanionic gold bis(dithiolate) complex, reported here for the first time, exhibits in cyclic voltammetry one set of redox processes observed at 0.03 and 0.53 mV vs. Ag/AgNO3. However, crystallization of the neutral form of this compound has not yet been achieved.

Biocompatible cyclodextrin-based metal-organic frameworks (CD-MOFs) form porous suspensions when dispersed in ionic liquids. We present two porous ionic liquids prepared using different CD-MOFs, which exhibit up to 56% higher carbon dioxide absorption compared to the pure ionic liquid at 303 K. These porous suspensions, when prepared under moisture-free conditions, demonstrate stability for several weeks provided smaller CD-MOF crystals are used. The suspension structure was characterized using X-ray scattering, while light scattering measurements were employed to monitor their stability over time.

A recurring secondary building unit is observed in Ca-MOFs prepared in deep eutectic solvents based on 1 : 2 combinations of choline chloride and urea derivatives.

New iron and cobalt bis(dithiolene) complexes [M(3cbdt)2] (3cbdt = 3-cyanobenzene-1,2-dithiolate) were prepared as tetraphenylphosphonium (Ph4P+) salts for Fe in the monoanionic state and for Co in both the dianionic and monoanionic states: (Ph4P)2[Fe(III)(3cbdt)2]2 (1); (Ph4P)2[Co(III)(3cbdt)2]2 (2); (Ph4P)2[Co(II)(3cbdt)2] (3). These compounds were characterized by single-crystal X-ray diffraction, cyclic voltammetry, EPR, and static magnetic susceptibility. Their properties are discussed in comparison with the corresponding complexes based on the isomer ligand 4-cyanobenzene-1,2-dithiolate (4cbdt) and 4,5-cyanobenzene-1,2-dithiolate (dcbdt), previously described by us. The Fe(III) and the Co(III) compounds (1 and 2) are isostructural, crystallizing in the triclinic P1¯ space group, with cis [M(III)(3cbdt)2] complexes dimerized in a trans fashion, and the transition metal (M = Fe, Co) has a distorted 4+1 square pyramidal coordination geometry. The Co(II) compound (3) crystallizes in the triclinic P1¯ space group, with the unit cell containing one cis and three trans inequivalent [Co(II)(3cbdt)2] complexes with the transition metal (Co) and having a square planar coordination geometry. The Fe(III) complex (1) is EPR-silent, and the static magnetic susceptibility shows a temperature dependence typical of dimers of antiferromagnetically coupled S = 3/2 spins with −J/kB = 233.6 K and g = 1.8. Static magnetic susceptibility measurements of compound (3) show that this Co(II) complex is paramagnetic, corresponding to an S = ½ state with g = 2, in agreement with EPR spectra showing in solid state a hyperfine structure typical of the I(59Co) = 7/2. Static susceptibility measurements of Co(III) complex (2) showed an increase in the paramagnetic susceptibility upon warming above 100 K, which is consistent with strong AFM coupling between dimerized S = 1 units with a constant −J/kB ~1286 K.

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Nanosized chiral octahedral M32 coordination cages were prepared via self-assembly of sulfonylcalix[4]arene tetranuclear M(ii) clusters (M = Co or Ni) with enantiomerically enriched linkers based on tris(dipyrrinato)cobalt(iii) complexes, appended with peripheral carboxylic groups. Two pairs of enantiomers of cages were obtained and unambiguously characterized from a structural point of view, using single crystal X-ray diffraction. Chiral-HPLC was used to evidence the enantiomers. In the solid state, the compounds present intrinsic and extrinsic porosity: the intrinsic porosity is linked with the size of the cages, which present an inner diameter of ca. 19 & Aring;. The obtained solid-state supramolecular architectures demonstrated good performances as adsorbents for water and 2-butanol guest molecules.

To deeply investigate the interaction between a tetrathiafulvalene (TTF) unit and a Ti(IV) center, a monomeric heteroleptic octahedral Ti(IV) complex containing a diimine ligand composed of a 1,10-phenanthroline core fused with a TTF fragment (ligand 2a) was prepared. The stable complex formulated as Ti(1)2(2a), where 1 is a 2,2'-biphenolato derivative, was efficiently synthesized by following a one-step approach. This complex and its model species [Ti(1)2(2b)] were fully characterized in solution, and their solid-state structures were established by single-crystal X-ray diffraction analysis. Density functional theory calculations allowed the assignment of the frontier orbitals involved in the electronic transitions characterized by ultraviolet-visible absorption spectroscopy. Electrochemical and spectroelectrochemical studies revealed that the TTF unit within Ti(1)2(2a) can undergo two reversible one-electron oxidation processes; a reversible one-electron reduction of the Ti(IV) atom was highlighted. The photophysical measurements performed for this donor-acceptor molecular system indicated that an electron transfer process upon light excitation occurred within Ti(1)2(2a).

The use of urea derivatives as solvents for the preparation of metal–organic frameworks is reviewed.

The enantioenrichment of the MIL-103 metal–organic framework using the natural deep eutectic solvents (DES) proline : thymol (1 : 7) and menthol : thymol (1 : 1) in their enantiopure form has been demonstrated by circular dichroism.

New nickel and copper bisdithiolene complexes were prepared as tetraphenylphosphonium (Ph4P+) salts, (Ph4P)n[M(3cbdt)2], [n = 1, 2; M = Ni, Cu; 3cbdt = 3-cyanobenzene-1,2-dithiolate]: (Ph4P)2[Ni(3cbdt)2] (1); Ph4P[Ni(3cbdt)2] (2); (Ph4P)2[Cu(3cbdt)2] (3); Ph4P[Cu(3cbdt)2] (4), and characterised by single crystal X-ray diffraction, cyclic voltammetry, EPR and static magnetic susceptibility for the paramagnetic species. These Cu and Ni complexes obtained as stable monoanionic and dianionic anions are the first members of this family of cyano benzene functionalized bisdithiolene transition metal complexes, opening the way to the preparation of analogous compounds with other transition metals and their future use as building blocks for the preparation of molecular materials. Their properties are discussed in comparison with the corresponding complexes based on the 4-cyanobenzene-1,2-dithiolate (4cbdt) ligand previously described by us. The salts (Ph4P)n[M(3cbdt)2], [n = 1, 2; M = Ni, Cu] present the metal complexes always in a square-planar coordination geometry with a ligand trans configuration, but revealing a rich polymorphism. The Ni and Cu compounds are isostructural and occur as two polymorphs (& alpha;, & beta;) with the exception of the salt of the monoanionic Ni complex, that was found only as the & alpha;-polymorph. The salt of the dianionic Ni complex was found to crystallise also with one acetonitrile molecule, when recrystallised from this solvent. The salts of monoanionic [Ni(3cbdt)2] and dianionic [Cu(3cbdt)2] complexes have been characterised by EPR and static magnetic susceptibility as paramagnetic S = 1/2 species. New nickel and copper cyanobenzene-functionalised bisdithiolene transition metal complexes reveal a rich polymorphism and pave the way for analogous compounds with other transition metals as building blocks for molecular materials.

Giant octahedral M32 coordination cages were prepared via self-assembly of sulfonylcalix[4]arene-supported tetranuclear M(II) clusters (M = Co, Ni) with hybrid linker based on tris(dipyrrinato)cobalt(III) complexes appended with peripherical carboxylic groups. Due to intrinsic and extrinsic porosity, the obtained solid-state supramolecular architectures demonstrated good performance as adsorbents for the separation of industrially important gases mixtures.

Deep eutectic solvents (DES) based on different urea derivatives have been demonstrated to be efficient green alternatives for the ionothermal synthesis of the prototypical Mg-MOF-74 with a strong impact on the morphology and sorption properties of the material. While the synthesis of the material is rather straightforward in the reline (choline chloride:urea 1:2) DES, higher temperatures and longer reaction times are necessary in the e-urea (2-imidazolidinone, ethylene-urea) based analogous system. Interestingly, in the latter, a variety of intermediate crystalline phases could be observed and characterized by single-crystal X-ray diffraction. In these compounds, coordination of the DES components – the chloride anion and e-urea derivative – to the Mg(II) cation was found to compete with the carboxylate linker. It was rationalized that the difference in the synthesis conditions and in the isolation of intermediate systems originate from the varying decomposition kinetics of the DES and hence from the basicity of the solvent. Although the same material is obtained as ascertained by powder X-ray diffraction and elemental analysis, the final morphology characterized by SEM and TEM is dependent on the nature of the solvent. Whereas the classical rod-like shape is observed in reline, an unusual morphology showing slices perpendicular to the main growth axis is present in the e-urea based DES. For the material featuring this unusual morphology, a higher specific surface area and CO2 uptake were found, which were associated with a higher degree of microporosity.

The ionothermal synthesis of Ca-MOFs has been performed using the 1 : 2 choline chloride : e-urea deep eutectic solvent, allowing the preparation of water sensitive materials.

The use of deep eutectic solvents (DES) as media for the preparation of metal- and covalent organic frameworks (MOFs and COFs) and their post-synthetic modification towards composites is reviewed.

The fate of HKUST-1 (Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylate) in the green Deep Eutectic Solvent (DES) reline (choline chloride/urea 1 : 2) was investigated, highlighting that not only reline can be used to make this MOF but also to transform it into another crystalline material. The synthesis of HKUST-1(reline) showing good textural properties and a particular rose morphology was indeed successfully achieved in this solvent. However, upon optimizing the reaction conditions such as concentration and metal/ligand ratio, it was found that another structure Cu2(BTC)Cl also forms. It was unequivocally demonstrated that, upon heating in reline, HKUST-1 converts to the non-porous chloride-incorporating material. Hence, a novel feature of DES in MOF synthesis is uncovered: its role as a structure-directing agent, triggering the transformation between two different MOF structures.