Structural diversity of metal–organic frameworks via employment of azamacrocycles as a building block

Organo-macrocycle-containing hierarchical metal-organic frameworks and cages: design, structures, and applications

Developing hierarchical ordered systems is challenging. Using organo-macrocycles to construct metal-organic frameworks (MOFs) and porous coordination cages (PCCs) provides an efficient way to obtain hierarchical assemblies. Macrocycles, such as crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, can be incorporated within MOFs/PCCs and they also endow the resultant composites with enhanced properties and functionalities. This review summarizes recent developments of organo-macrocycle-containing hierarchical MOFs/PCCs, emphasizing applications and structure-property relationships of these hierarchically porous materials. This review provides insights for future research on hierarchical self-assembly using macrocycles as building blocks and functional ligands to extend the applications of the composites.

Synthesis and crystal structure of trans-diaqua(1,4,8,11-tetraazaundecane)copper(II) isophthalate monohydrate

The complex cation of the title compound contains a tetra­gonally distorted trans-CuN₄O₂ octa­hedron. In the crystal, the components are linked by numerous N—H⋯O and O—H⋯O hydrogen bonds, forming electroneutral sheets oriented parallel to the ac plane, which are further consolidated into bilayers due to hydrogen-bonding with the participation of the water mol­ecule of crystallization.

In the title hydrated mol­ecular salt, [Cu(C₇H₂₀N₄)(H₂O)₂](C₈H₄O₄)·H₂O, the metal ion is coordinated by the two primary and two secondary N atoms of the amine ligand and the mutually trans O atoms of the water mol­ecules in a tetra­gonally distorted octa­hedral geometry. The average equatorial Cu—N bond lengths (2.013 and 2.026 Å for Cu—N_prim and Cu—N_sec, respectively) are substanti­ally shorter than the average axial Cu—O bond length (2.518 Å). The tetra­amine ligand adopts its energetically favored conformation with its five- and six-membered chelate rings in gauche and chair conformations, respectively. In the crystal, the N—H donor groups of the tetra­amine, the acceptor carboxyl­ate groups of the isophthalate dianion and both the coordinated water mol­ecules and the water mol­ecule of crystallization are involved in numerous N—H⋯O and O—H⋯O hydrogen bonds, resulting in the formation of electroneutral layers oriented parallel to the ac plane.

Fluorogenic Detection of Sulfite in Water by Using Copper(II) Azacyclam Complexes

Copper(II) azacyclam complexes (azacyclam = 1,3,5,8,12-pentaazacyclotetradecane) containing naphthyl or dansyl subunits can be prepared by template synthesis involving proper sulfonamide derivatives as locking fragments. The macrocyclic complexes are very poorly emissive due to the fluorescence-quenching behavior displayed by Cu²⁺ ions. However, the fluorescence can be recovered as a result of the decomposition of the complexes, which induces the release of free light-emitting subunits to the solution. This reaction takes place very slowly in neutral water but its rate is increased by the presence of sulfite. Therefore, [Cu(azacyclam)]²⁺ derivatives have been investigated as simple chemical probes for the fluorogenic detection of sulfite both on laboratory and real samples. Preliminary tests performed on samples of white wine provided sulfite concentration values that are in agreement with those obtained by a standard analytical method.

Crystal structure of trans-di-aqua-(1,4,8,11-tetra-aza-undeca-ne)nickel(II) bis-(pyridine-2,6-di-carboxyl-ato)nickel(II)

The coordination polyhedra of the nickel(II) ions of the title compound in the complex cation and the anion, viz., trans-NiN₄O₂ and trans-NiO₄N₂, are distorted octa­hedra. In the crystal, the donor groups of the tetra­amine and the coordinated water mol­ecules and the carboxyl­ate groups of the pyridine-2,6-di­carboxyl­ate anions are involved in numerous N—H⋯O and O—H⋯O hydrogen bonds, thereby forming sheets of ions lying parallel to the (001) plane.

The asymmetric unit of the title compound, trans-di­aqua­(1,4,8,11-tetra­aza­undecane-κ⁴N¹,N⁴,N⁸,N¹¹)nickel(II) bis­(pyridine-2,6-di­carboxyl­ato-κ³O²,N,O⁶)nickel(II) {[Ni(L)(H₂O)₂][Ni(pdc)₂] where L = 1,4,8,11-tetra­aza­undecane (C₇H₂₀N₄) and pdc = the dianion of pyridine-2,6-di­carb­oxy­lic acid (C₇H₃NO₄²⁻)} consists of an [Ni(L)(H₂O)₂]²⁺ complex cation and a [Ni(pdc)₂]^2– anion. The metal ion in the cation is coordinated by the four N atoms of the tetra­amine ligand and the mutually trans O atoms of the water mol­ecules in a tetra­gonally elongated octa­hedral geometry with the average equatorial Ni—N bond length slightly shorter than the average axial Ni—O bond [2.087 (4) versus 2.128 (4) Å]. The ligand L adopts its energetically favored conformation with five-membered and six-membered chelate rings in gauche and chair conformations, respectively. In the complex anion, the Ni^II ion is coordinated by the two tridentate pdc^2– ligands via their carboxyl­ate and nitro­gen atom donors in a distorted octa­hedral trans-NiO₄N₂ geometry with nearly orthogonal orientation of the planes defining the carboxyl­ate rings and the average Ni—N bond length [1.965 (4) Å] shorter than the average Ni—O bond distance [2.113 (7) Å]. In the crystal, the NH donor groups of the tetra­amine, the carb­oxy­lic groups of the pdc^2– anion and the coordinated water mol­ecules are involved in numerous N—H⋯O and O—H⋯O hydrogen bonds, leading to electroneutral sheets oriented parallel to the (001) plane.

Theoretical modeling of the singlet-triplet spin transition in different Ni(II)-diketo-pyrphyrin-based metal-ligand octahedral complexes

The structural stability, charge transfer effects and strength of the spin-orbit couplings in different Ni(ii)-ligand complexes have been studied at the DFT (B3LYP and CAM-B3LYP) and coupled cluster (DLPNO-CCSD(T)) levels of theory. Accordingly, two different, porphyrin- and diketo-pyrphyrin-based four-coordination macrocycles as planar ligands as well as pyridine (or pyrrole) and mesylate anion molecular groups as vertical ligands were considered in order to build metal-organic complexes with octahedral coordination configurations. For each molecular system, the identification of equilibrium geometries and the intersystem crossing (the minimum energy crossing) points between the potential energy surfaces of the singlet and triplet spin states is followed by computing the spin-orbit couplings between the two spin states. Structures, based on the diketo-pyrphyrin macrocycle as the planar ligand, show stronger six-coordination metal-organic complexes due to the extra electrostatic interaction between the positively charged central metal cation and the negatively charged vertical ligands. The results also show that the magnitude of the spin-orbit coupling is influenced by the atomic positions of deprotonations of the ligands, and implicitly the direction of the charge transfer between the ligand and the central metal ion.

Crystal structure of [3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne]nickel(II) diperchlorate

The square-planar nickel(II) title complex, [Ni(C24H36F2N6)](ClO4)2 or [NiL](ClO4)2 (L = 3,10-bis-(4-fluoro-pheneth-yl)-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne) was synthesized by a one-pot reaction of template condensation and its X-ray crystal structure was determined. The nickel(II) ion lies close by a twofold axis and the complex displays whole-mol-ecule disorder. Ligand L, a hexa-aza-cyclo-tetra-decane ring having 4-fluoro-phenethyl side chains attached to uncoordinated nitro-gen atoms, adopts a trans III (R,R,S,S) configuration. The average Ni-N bond distance is 1.934 (9) Å, which is quite similar to those of other nickel(II) complexes with similar ligands. The nickel(II) ion is located 0.051 (7) Å above the least-squares plane through the four coordinated N atoms. The average C-N bond distance and C-N-C angle involving uncoordinated nitro-gen atoms are 1.425 (12) Å and 118.0 (9)°, respectively, indicating a significant contribution of sp 2 hybridization for these N atoms. The inter-molecular N-H⋯O, C-H⋯O/F hydrogen bonds of the complex form a network structure, which looks like a seamless floral lace pattern.

Nanocomposite synthesis strategies based on the transformation of well-tailored metal-organic frameworks

Increasing the complexity of nanomaterials in terms of their structure and chemical composition has attracted significant attention, because it can yield unique scientific outcomes and considerable improvements for practical applications. Various approaches are being developed for the synthesis of nanostructured composites. Coordination polymers (CPs) emerged as new precursors in solid-state reactions for nanomaterials nearly two decades ago; the repetitively arranged inorganic and organic units can facilitate the production of nanoscale particles and porous carbon upon thermal decomposition. Metal-organic frameworks (MOFs), a subgroup of CPs featuring crystalline and porous structures, have subsequently become primary objects of interest in this field, as can be seen by the rapidly increasing number of reports on this topic. However, unique composite materials with increasingly complex nanostructures, which cannot be achieved via conventional methods, have been rarely realised, even though conventional MOF research has enabled the delicate control of structures at the molecular level and extensive applications as templates. In this regard, a comprehensive review of the fabrication strategies of MOF-based precursors and the thermal transformation into functional nanomaterials is provided herein, with a particular emphasis on the recent developments in nanocomposite research. We briefly introduce the roles and capabilities of MOFs in the synthesis of nanomaterials and subsequently discuss diverse synthetic routes for obtaining morphologically or compositionally advanced composite nanomaterials, based on our understanding of the MOF conversion mechanism.

A 2D coordination polymer assembled from a nickel(II) tetraazamacrocyclic cation and 4,4'-(dimethylsilanediyl)diphthalate(3-) linker

The preformed nickel(II) complex of the 14-membered macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane (cyclam, L), when treated with 4,4'-(dimethylsilanediyl)diphthalic acid (H₄A) in a DMF/H₂O mixture (4:1 v/v) under heating, leads to [Ni(L)]₃(HA)₂·3DMF (I·DMF). Redissolution of this compound in a DMF/H₂O/MeOH mixture (4:1:30 v/v/v) with mild acidification under gentle heating results in the formation of a similar compound but containing water and methanol molecules of crystallization, [Ni(L)]₃(HA)₂·5H₂O·2MeOH (II·H₂O). At lower temperature and concentration of reactants and longer reaction time, single crystals of composition {[{Ni(L)}₃(HA)₂]·4CH₃OH}_n (II·MeOH) were isolated. Single-crystal X-ray diffraction analysis of this compound, which, according to PXRD is isostructural with II·H₂O but different from I·DMF, revealed its two-dimensional (2D) polymeric structure, i.e. poly[[bis{μ₃-4-[(4-carboxy-3-carboxylatophenyl)dimethylsilyl]benzene-1,2-dicarboxylato-κ³O¹:O²:O^3'}tris(1,4,8,11-tetraazacyclotetradecane-κ⁴N)trinickel(II)] methanol tetrasolvate], {[Ni₃(C₁₈H₁₃O₈Si)₂(C₁₀H₂₄N₄)₃]·4CH₃OH}_n. It is built up of the monoprotonated tricarboxylate HA^3- ligand coordinated in a monodentate manner in the axial positions of two crystallographically independent Ni^II cations, one of which is located on a crystallographic inversion centre. Both metal ions adopt a slightly tetragonally elongated trans-N₄O₂ octahedral geometry. The compound has a lamellar structure with polymeric layers oriented parallel to the (10-2) plane, which are in turn linked via hydrogen bonds involving protonated carboxylic acid groups of the ligand. Bulk compounds I·DMF and II·H₂O were characterized by FT-IR and diffuse reflectance spectroscopy and thermogravimetry, which provide evidence of their structural differences.

Solvent-triggered single-crystal-to-single-crystal transformation from a monomeric to polymeric copper(II) complex based on an aza macrocyclic ligand

Reversible solvent-triggered single-crystal-to-single-crystal (SCSC) transformations are observed between two copper(II) azamacrocyclic complexes: [Cu(C₁₆H₃₈N₆)(H₂O)₂](C₁₂H₆O₄) (1) and [Cu(C₁₆H₃₈N₆)(C₁₂H₆O₄)] (2). Complex (1) was prepared via self-assembly of a copper(II) azamacrocyclic complex containing butyl pendant groups, [Cu(C₁₆H₃₈N₆)(ClO₄)₂], with 2,7-naphthalenedicarboxylic acid. When monomeric compound (1) was immersed in CH₃OH, coordination polymer (2) was obtained, indicating a solvent-triggered SCSC transformation. Furthermore, when (2) was immersed in water, an reverse SCSC transformation from (2) to (1) occurred. Complex (1) presents a 3D supramolecular structure formed via intermolecular hydrogen-bonding interactions, whereas complex (2) features a 1D zigzag coordination polymer. The reversible SCSC transformation of (1) and (2) was characterized using single-crystal X-ray diffraction and in situ powder X-ray diffraction techniques. Despite its poor porosity, complex (2) displayed interesting CO₂ adsorption behaviour under CO₂ gas.

Crystal structures of trans-di-aqua-(3-R-1,3,5,8,12-penta-aza-cyclo-tetra-deca-ne)copper(II) isophthalate hydrates (R = benzyl or pyridin-3-ylmethyl)

The asymmetric units of the title compounds, trans-di-aqua-(3-benzyl-1,3,5,8,12-penta-aza-cyclo-tetra-decane-κ4 N 1,N 5,N 8,N 12)copper(II) isophthalate monohydrate, [Cu(C16H29N5)(H2O)2](C8H4O4)·H2O, (I), and trans-di-aqua-[3-(pyridin-3-ylmeth-yl)-1,3,5,8,12-penta-aza-cyclo-tetra-decane-κ4 N 1,N 5,N 8,N 12]copper(II) iso-phthalate 0.9-hydrate, [Cu(C15H28N6)(H2O)2](C8H4O4)·0.9H2O, (II) consist of one di-aqua macrocyclic cation, one di-carboxyl-ate anion and uncoordinated water mol-ecule(s). In each compound, the metal ion is coordinated by the four secondary N atoms of the macrocyclic ligand and the mutually trans O atoms of the water mol-ecules in a tetra-gonally distorted octa-hedral geometry. The average equatorial Cu-N bond lengths are significantly shorter than the average axial Cu-O bond lengths [2.020 (9) versus 2.495 (12) Å and 2.015 (4) versus 2.507 (7) Å for (I) and (II), respectively]. The coordinated macrocyclic ligand in the cations of both compounds adopts the most energetically favorable trans-III conformation. In the crystals, the complex cations and counter-anions are connected via hydrogen-bonding inter-actions between the N-H groups of the macrocycles and the O-H groups of coordinated water mol-ecules as the proton donors and the O atoms of the carboxyl-ate as the proton acceptors. Additionally, as a result of O-H⋯O hydrogen bonding with the coordinated and water mol-ecules of crystallization, the isophthalate dianions form layers lying parallel to the (01) and (100) planes in (I) and (II), respectively.

Crystal structure of trans-di-aqua-(3,10-dimethyl-1,3,5,8,10,12-hexa-aza-cyclo-tetra-deca-ne)copper(II) pamoate

The asymmetric unit of the title compound, trans-di-aqua-(3,10-dimethyl-1,3,5,8,10,12-hexa-aza-cyclo-tetra-decane-κ4 N 1,N 5,N 8,N 12)copper(II) 4,4'-methyl-ene-bis(3-hy-droxy-naphthalene-2-carboxyl-ate), [Cu(C10H26N6)(H2O)2](C23H14O6) {[Cu(L)(H2O)2](pam), where L = 3,10-dimethyl-1,3,5,8,10,12-hexa-aza-cyclo-tetra-decane and pam = dianion of pamoic acid} consists of two independent halves of the [Cu(L)(H2O)2]2+ cation and one di-carboxyl-ate anion. The CuII atoms, lying on inversion centres, are coordinated by the four secondary N atoms of the macrocyclic ligands and the mutually trans O atoms of the water mol-ecules in a tetra-gonally elongated octa-hedral geometry. The average equatorial Cu-N bond length is significantly shorter than the average axial Cu-O bond length [2.007 (10) and 2.486 (18) Å, respectively]. The macrocyclic ligand in the complex cations adopts the most energetically stable trans-III conformation. The complex cations and anions are connected via hydrogen-bonding inter-actions between the N-H groups of the macrocycles and the O-H groups of coordinated water mol-ecules as the proton donors and the O atoms of the carboxyl-ate as the proton acceptors into layers lying parallel to the (11) plane.

Supramolecular interactions induced distortion of BTB ligands: breaking convention to reproduce an unusual (3,4,4)-connected MOF topology

An unusual (3,4,4)-connected MOF topology has been reproduced by a 4,4',4''-benzene-1,3,5-triyl-tribenzoate (BTB) ligand which was previously judged to be not feasible to form this network. The outcome demonstrates that supramolecular interactions play a key role in the formation of this highly distorted network. Moreover, this triple interpenetrated framework exhibits high BET surface area.

Tuning of the flexibility in metal–organic frameworks based on pendant arm macrocycles

An isostructural series of flexible MOFs based on pendant arm macrocycles was developed to tune flexibility depending on functional groups.