Carboxylic Acid Functionalized Clathrochelate Complexes: Large, Robust, and Easy-to-Access Metalloligands

Synthetic and analytical considerations for the preparation of amorphous metal-organic frameworks

Metal-organic frameworks (MOFs) are hybrid porous materials presenting several tuneable properties, allowing them to be utilised for a wide range of applications. To date, focus has been on the preparation of novel crystalline MOFs for specific applications. Recently, interest in amorphous MOFs (aMOFs), defined by their lack of correlated long-range order, is growing. This is due to their potential favourable properties compared to their crystalline equivalents, including increased defect concentration, improved processability and gas separation ability. Direct synthesis of these disordered materials presents an alternative method of preparation to post-synthetic amorphisation of a crystalline framework, potentially allowing for the preparation of aMOFs with varying compositions and structures, and very different properties to crystalline MOFs. This perspective summarises current literature on directly synthesised aMOFs, and proposes methods that could be utilised to modify existing syntheses for crystalline MOFs to form their amorphous counterparts. It outlines parameters that could discourage the ordering of crystalline MOFs, before examining the potential properties that could emerge. Methodologies of structural characterisation are discussed, in addition to the necessary analyses required to define a topologically amorphous structure.

Preparation, X-ray Characterization, and Reactivity of the Rod-like and Angular Germanium- and Titanium(IV)-Capped Iron(II) Bis-Clathrochelates and Their Mono- and Bis-Capped (Semi)clathrochelate Precursors

Transmetalation of the bis{triethylantimony(V)}-capped iron(II) tris-α-dioximate with n-butylboronic acid afforded the mixed antimony, boron cross-linked clathrochelate with single reactive antimony(V)-based apical fragment. This macrobicyclic precursor easily underwent the transmetalation reactions with germanium and titanium(IV) alkoxides to give the rod-like and angular FeII2MIV-trinuclear bis-clathrochelates. Those of the aforementioned diantimony(V)-capped complex with 3- and 4-carboxyphenylboronic acids afforded the monoboron-capped iron(II) semiclathrochelates, undergoing a double-cyclization (macrobicyclization) with germanium- and titanium(IV)-based capping agents. The reactions in the low-temperature range unexpectedly gave the stable 2:1 associates, formed by the bridging of two carboxyl-terminated macrobicyclic molecules of the mixed carboxylboron, triethylantimony-capped iron(II) clathrochelate with a triethylantimony(V)-based linker fragment. The obtained complexes were characterized using elemental analysis, MALDI-TOF, 1H and 13C{1H} NMR and UV-vis spectra, and single-crystal XRD experiments. The encapsulated iron(II) ion in their 3D-molecules is situated almost in the center of its FeN6-coordination polyhedron possessing a truncated trigonal-pyramidal geometry. Fe-N distances fall in the range 1.887(7)-1.945(4) Å characteristic of the low-spin iron(II) complexes. The cross-linking titanium and germanium(IV) ions in the corresponding bis-clathrochelate molecules form the octahedral MIVO6-coordination polyhedra, the MIV-O distances of which vary from 1.946(2) to 1.964(2) Å and from 1.879(7) to 1.907(6) Å, respectively.

Multistep synthesis and X-ray structures of carboxyl-terminated hybrid iron(II) phthalocyaninatoclathrochelates and their postsynthetic transformation into polytopic carboranyl-containing derivatives

A multistep general synthetic strategy towards polytopic carboranyl-containing (semi)clathrochelate metal complexes, based on the template synthesis, transmetallation, amide condensation and 1,3-dipolar cycloaddition reactions, is developed. Their mono(semi)clathrochelate precursors with a single reactive group were obtained using a transmetallation of the triethylantimony-capped macrobicyclic precursor. The thus obtained carboxyl-terminated iron(II) semiclathrochelate underwent a macrobicyclization with zirconium(IV) phthalocyaninate to form the corresponding phthalocyaninatoclathrochelate. The direct one-pot template condensation of the suitable chelating and cross-linking ligand synthons on the Fe²⁺ ion as a matrix was also used for its preparation. Further amide condensation of the aforementioned semiclathrochelate and hybrid complexes with propargylamine in the presence of carbonyldiimidazole gave the (pseudo)cage derivatives with a terminal CC bond. Their "click" reaction with an appropriate carboranylmethyl azide afforded the ditopic carboranosemiclathrochelates and the tritopic carboranyl-containing phthalocyaninatoclathrochelates with a flexible spacer fragment between their polyhedral entities. The obtained new complexes were characterized using elemental analysis, MALDI-TOF mass spectrometry, multinuclear NMR, and UV-vis spectroscopy, and by single crystal X-ray diffraction experiments. Their FeN₆-coordination polyhedra show a truncated trigonal-pyramidal geometry, while the cross-linking heptacoordinate Zr⁴⁺ or Hf⁴⁺ cations in the hybrid compounds form the M^IVN₄O₃-coordination polyhedra with the geometry of a capped trigonal prism.

Synthesis, X-ray structure and magnetic properties of the apically functionalized monocapped cobalt(II) tris-pyridineoximates possessing SMM behaviour

The title cobalt(II) pseudoclathrochelate complexes possess an intermediate trigonal prismatic-trigonal antiprismatic geometry. As follows from PPMS data, they exhibit an SMM behaviour with Orbach relaxation barriers of approximately 90 K. Paramagnetic NMR experiments confirmed a persistence of these magnetic characteristics in solution. Therefore, a straightforward apical functionalization of this 3D molecular platform for its targeted delivery to a given biosystem can be performed without substantial changes.

Synthesis, crystal polymorphism and spin crossover behavior of adamantylboron-capped cobalt(II) hexachloroclathrochelate and its transformation into the CoIIICoIICoIII-bis-macrobicyclic derivative

Fast crystallization of the monoclathrochelate cobalt(II) intracomplex [Co(Cl₂Gm)₃(BAd)₂] (where Cl₂Gm^2- is a dichloroglyoxime dianion and BAd is an adamantylboron capping group, 1), initially obtained by the direct template condensation of the corresponding chelating α-dioximate and cross-linking ligand synthons on the Co²⁺ ion as a matrix, from benzene or dichloromethane afforded its structural triclinic and hexagonal polymorphs. Its prolonged recrystallization from dichloromethane under air atmosphere and sunlight irradiation unexpectedly gave the crystals of the Co^IIICo^IICo^III-trinuclear dodecachloro-bis-clathrochelate intracomplex [[Co^III(Cl₂Gm)₃(BAd)]₂Co^II] (2), the molecule of which consists of two macrobicyclic frameworks with encapsulated low-spin (LS) Co³⁺ ions, which are cross-linked by a μ₃-bridging Co²⁺ ion as a bifunctional Lewis-acidic center. The most plausible pathway of such a 1 → 2 transformation is based on the photoinitiated radical oxidation of dichloromethane with air oxygen giving the reactive species. Cobalt(II) monoclathrochelate 1 was found to undergo a temperature-induced spin crossover (SCO) both in its solutions and in the solid state. In spite of the conformational rigidity of the corresponding quasiaromatic diboron-capped tris-α-dioximate framework, the main parameters of this SCO transition (i.e., its completeness and gradual character) are strongly affected by the nature of the used solvent (in the case of its solutions) and by the structural polymorphism of its crystals (in the solid state). In the latter case, the LS state (S = 1/2) of this complex is more thermally stable and, therefore, the cobalt(II)-centered 1/2 → 3/2 SCO is more gradual than that in solutions.

Creating Optimal Pockets in a Clathrochelate-Based Metal-Organic Framework for Gas Adsorption and Separation: Experimental and Computational Studies

The rational design and synthesis of robust metal-organic frameworks (MOFs) based on novel organic building blocks are fundamental aspects of reticular chemistry. Beyond simply fabricating new organic linkers, however, it is important to elucidate structure-property relationships at the molecular level to develop high-performing materials. In this work, we successfully targeted a highly porous and robust cage-type MOF (NU-200) with an nbo-derived fof topology through the deliberate assembly of a cyclohexane-functionalized iron(II)-clathrochelate-based meta-benzenedicarboxylate linker with a Cu₂(CO₂)₄ secondary building unit (SBU). NU-200 exhibited an outstanding adsorption capacity of xenon and a high ideal adsorbed solution theory (IAST) predicted selectivity for a 20/80 v/v mixture of xenon (Xe)/krypton (Kr) at 298 K and 1.0 bar. Our extensive computational simulations with grand canonical Monte Carlo (GCMC) and density functional theory (DFT) on NU-200 indicated that the MOF's hierarchical bowl-shaped nanopockets surrounded by custom-designed cyclohexyl groups─instead of the conventionally believed open metal sites (OMSs)─played a crucial role in reinforcing Xe-binding affinity. The optimally sized pockets firmly trapped Xe through numerous supramolecular interactions including Xe···H, Xe···O, and Xe···π. Additionally, we validated the unique pocket confinement effect by experimentally and computationally employing the similarly sized probe, sulfur dioxide (SO₂), which provided significant insights into the molecular underpinnings of the high uptake of SO₂ (11.7 mmol g^-1), especially at a low pressure of 0.1 bar (8.5 mmol g^-1). This work therefore can facilitate the judicious design of organic building blocks, producing MOFs featuring tailor-made pockets to boost gas adsorption and separation performances.

Zinc(ii) and cadmium(ii) amorphous metal-organic frameworks (aMOFs): study of activation process and high-pressure adsorption of greenhouse gases

Two novel amorphous metal-organic frameworks (aMOFs) with chemical composition {[Zn2(MTA)]·4H2O·3DMF} n (UPJS-13) and {[Cd2(MTA)]·5H2O·4DMF} n (UPJS-14) built from Zn(ii) and Cd(ii) ions and extended tetrahedral tetraazo-tetracarboxylic acid (H4MTA) as a linker were prepared and characterised. Nitrogen adsorption measurements were performed on as-synthesized (AS), ethanol exchanged (EX) and freeze-dried (FD) materials at different activation temperatures of 60, 80, 100, 120, 150 and 200 °C to obtain the best textural properties. The largest surface areas of 830 m2 g-1 for UPJS-13 (FD) and 1057 m2 g-1 for UPJS-14 (FD) were calculated from the nitrogen adsorption isotherms for freeze-dried materials activated at mild activation temperature (80 °C). Subsequently, the prepared compounds were tested as adsorbents of greenhouse gases, carbon dioxide and methane, measured at high pressures. The maximal adsorption capacities were 30.01 wt% CO2 and 4.84 wt% CH4 for UPJS-13 (FD) and 24.56 wt% CO2 and 6.38 wt% CH4 for UPJS-14 (FD) at 20 bar and 30 °C.

Architectural and catalytic aspects of designer materials built using metalloligands of pyridine-2,6-dicarboxamide based ligands

This perspective presents the design, synthesis and crystal structures of a large number of architectures constructed using assorted metalloligands of pyridine-2,6-dicarboxamide based ligands. The metalloligands offered various appended functional groups, whereas design strategies included varying both their position and number. A combination of these parameters resulted in the development of assorted architectures including discrete trinuclear and tetranuclear complexes as well as 1D/2D/3D coordination polymers. The metalloligand strategy not only assisted in the construction of ordered crystalline materials with varied dimensionalities but also judiciously allowed the incorporation of Lewis acidic and redox-active secondary metals in the resultant architectures. As a result, such designer architectures illustrated their noteworthy role both as homogenous and heterogeneous catalysts in different organic transformation reactions.

Two-Step Synthesis of Linear and Bent Dicarboxylic Acid Metalloligands with Lengths of up to 3 nm

Nanometer-sized polycarboxylate ligands are interesting building blocks for metallasupramolecular chemistry, but access to these compounds is often limited by complicated synthetic pathways. Here, we describe a simple two-step protocol, which allows preparing linear and bent dicarboxylate ligands with lengths of up to 3 nm from commercially available compounds. The ligands are prepared by iron-templated polycondensation reactions involving arylboronic acids and nioxime. The final products contain two iron clathrochelate complexes and two terminal carboxyphenylene groups. To demonstrate that the new ligands are suitable for the construction of more complex molecular nanostructures, we have prepared a Cu-based metal-organic polyhedron, which represents the largest M₄L₄ cage described so far.

Solvent-Induced Encapsulation of Cobalt(II) Ion by a Boron-Capped tris-Pyrazoloximate

Boron-cross-linked cobalt(II) pseudoclathrochelate was obtained by the template reaction of 2-acetylpyrazoloxime, phenylboronic acid, and a new DMF cobalt(II) solvato complex with a decachloro-closo-decaborate dianion. As confirmed by single-crystal X-ray diffraction, this complex crystallizes with two symmetry-independent cobalt(II) pseudoclathrochelate cations, one decachloro-closo-decaborate dianion, one benzene, one dichloromethane solvent molecule, and two molecules of DMF. The latter act as pseudocapping fragments to the monocapped tris-pyrazoloximate ligands by forming N-H···O hydrogen bonds with their pyrazole groups. The Co^IIN₆-coordination polyhedra adopt a nearly ideal TP geometry with distortion angles φ equal to 1.22(16) and 2.58(17)° for two symmetry-independent pseudoclathrochelate cations, both containing the encapsulated cobalt(II) ion in its high-spin state (Co-N 2.115(4)-2.198(3) Å). Magnetic properties of this complex were studied both by dc-magnetometry and by solution-state NMR spectroscopy to reveal a high magnetic anisotropy, thus suggesting a large magnetic susceptibility tensor anisotropy (25.8 × 10^-32 m³ at 298 K) and a large negative zero-field splitting energy (-85 cm^-1). The results of magnetometry studies in the ac magnetic field suggest a single molecule magnet behavior of this TP complex with an effective magnetization reversal barrier of approximately 130 cm^-1. Its pseudocapping DMF molecules that form H-bonds with tris-pyrazoloximate fragments are easy to substitute by strong H-bond acceptors, such as chloride ions and di- and tetramethylureas, thus affecting the magnetic properties of a whole pseudomacrobicyclic paramagnetic system.

New salicylaldoximato-borate ligands resulting from anion hydrolysis and their respective copper and iron complexes

Anion hydrolysis reactions between salicylaldoximato ligands (L'-L''') and copper and iron BF₄^- metal salts, have resulted in the formation of new salicylaldoximato borate containing transition metal complexes: [Fe₂(L' + 2H)₂](BF₄)₂(MeOH)₄ (C1), [Fe₃(L'' + 4H)(OH)₂(Py)₂](BF₄)₂(H₂O)₂(Py)₂ (C2), and [Cu₂(L''' + H)₂Cl₂] (C3). Each of the complexes have been structurally characterised, revealing the indirect role boron plays in the formation of these complexes. For complexes C1 and C2, Mössbauer spectroscopy confirmed the existence of Fe(iii) oxidation states. SQUID magnetometry measurements were performed on complexes C2 and C3, revealing the presence of two competing exchange pathways between the three Fe(iii) centres in C2, with antiferromagnetic exchange dominating. For C3 weak antiferromagnetic exchange dominated between the two Cu(ii) centres.

Porous networks based on iron(ii) clathrochelate complexes

Microporous networks based on boronate ester-capped iron(ii) clathrochelate complexes are described. The networks were obtained by covalent cross-linking of tetrabrominated clathrochelate complexes via Suzuki-Miyaura polycross-coupling reactions with diboronic acids, or by Sonogashira-Hagihara polycross-coupling of clathrochelate complexes with terminal alkyne functions and 1,3,5-tribromobenzene. The networks display permanent porosity with apparent Brunauer-Emmett-Teller surface areas of up to SABET = 593 m2 g-1. A clathrochelate complex based on an enantiopure dioximato ligand was used to prepare chiral networks. One of these networks was shown to preferentially absorb d-tryptophan over l-tryptophan.

Clathrochelate Metalloligands in Supramolecular Chemistry and Materials Science

The term "clathrochelate" describes a complex in which a coordinatively saturated metal ion is surrounded by a macropolycyclic ligand. First examples of clathrochelate complexes were reported 50 years ago. Meanwhile, the synthesis and reactivity of clathrochelates have been investigated in detail, and numerous applications have been explored. In this Account, we summarize work on the utilization of transition metal clathrochelates as metalloligands in supramolecular chemistry and materials science, with special focus on results from our group. First, we discuss the chemistry of boron-capped clathrochelates. These complexes are facile to synthesize by metal-templated condensation reactions. The synthesis is modular, and it is straightforward to implement structural variations. Importantly, it is possible to attach functional groups such as amines, pyridines, or carboxylic acids to the ligand periphery. Other noteworthy features of boron-capped clathrochelates are high thermodynamic and kinetic stability, tunable redox potential, and good solubility. Next, we show that clathrochelate-based metalloligands can be used to build molecularly defined metal-ligand assemblies of nanoscale dimensions. Different molecular architectures are described, including coordination cages with unusual gyrobifastigium or square orthobicupola-like structures. Metalloligands containing multiple clathrochelate complexes are particularly well suited to build large metal-ligand assemblies (>3 nm) with minimal synthetic efforts. Boron-capped clathrochelates have also been investigated in the context of materials chemistry. Linear or cross-linked clathrochelate polymers were found to display permanent porosity. Furthermore, such polymers were used to prepare conducting films on electrodes. Clathrochelate metalloligands are well suited to prepare metal-organic frameworks (MOFs). The high stability of clathrochelates ensures compatibility with harsh reaction conditions, and it mitigates potential problems such as exchange reactions. Boron-capped clathrochelates can be decorated with functional groups in lateral and apical position, and it is possible to use these complexes as multiconnected nodes in polymeric structures. Overall, we hope to convey the utility of clathrochelate complexes in supramolecular chemistry and materials science. The work published thus far gives a first glimpse of the potential of these compounds, but there are other directions, which are waiting to be explored. For example, it will be interesting to study the properties of nanostructures based on chiral clathrochelate complexes. Furthermore, the redox and magnetic properties of clathrochelates may give rise to novel functional materials. Given that clathrochelates are straightforward to prepare, we hope that others will join the efforts to explore the supramolecular and materials chemistry of these interesting molecular building blocks.

Metal complexes of a novel heterocyclic benzimidazole ligand formed by rearrangement-cyclization of the corresponding Schiff base. Electrosynthesis, structural characterization and antimicrobial activity

One-pot electrochemical synthesis of metal complexes containing a novel heterocyclic benzimidazole ligand is reported and characterized.

Construction of metal–organic frameworks (MOFs) and highly luminescent Eu(iii)-MOF for the detection of inorganic ions and antibiotics in aqueous medium

A series of Ln(iii) complexes were synthesized. The highly luminescent chemically stable Eu-MOF shows multi-responsive luminescence sensing functions.

The Intricate Structural Chemistry of MII2nLn-Type Assemblies

The reaction of cis-blocked, square-planar M^II complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M_2nL_n. These assemblies typically adopt barrel-like structures, with the ligands paneling the sides of the barrels. However, alternative structures are possible, as demonstrated by the recent discovery of a Pt₈L₄ cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of M^II_2nL_n complexes are not well understood. Herein, we provide a geometric analysis of M_2nL_n complexes, and we discuss how size and geometry of the ligand is expected to influence the self-assembly process. The theoretical analysis is complemented by experimental studies using different cis-blocked Pt^II complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt₈L₄, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt₁₀L₅ complex and a Pt₁₆L₈ complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt₁₆L₈ complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M_2nL_n complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt₁₀L₅ barrels, which are formed in nearly quantitative yields.

PdII2L4-type coordination cages up to three nanometers in size

The utilization of large ligands in coordination-based self-assembly represents an attractive strategy for the construction of supramolecular assemblies more than two nanometers in size. However, the implementation of this strategy is hampered by the fact that the preparation of such ligands often requires substantial synthetic effort. Herein, we describe a simple one-step protocol, which allows large bipyridyl ligands with a bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, we provide evidence that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains. Packing effects of this kind are frequently encountered in protein assemblies, but they are seldom used as a design element in metallasupramolecular chemistry.

A new layer-stacked porous framework showing sorption selectivity for CO2 and luminescence

Luminescent Zn-MOF based stacked layers were assembled, featuring multiple CO₂-binding sites and selective CO₂ adsorption over CH₄.

Surface functionalization of dinuclear clathrochelates via Pd-catalyzed cross-coupling reactions: facile synthesis of polypyridyl metalloligands

Clathrochelates can be decorated with pyridyl groups by cross-coupling reactions. They represent interesting ligands for supramolecular chemistry and materials science.

Large heterometallic coordination cages with gyrobifastigium-like geometry

Large coordination cages with unusual gyrobifastigium-like geometry were obtained by combining cis-blocked Pt^II or Pd^II complexes with clathrochelate-based metalloligands.

Dinuclear clathrochelate complexes with pendent cyano groups as metalloligands

Dinuclear clathrochelate complexes can be decorated with two, three, four, or five cyano groups. These complexes represent versatile metalloligands for the construction of coordination polymers.