Kinetic assembly of coordination networks

Magnetic and thermodynamic control of coordination network crystallization using a hexaazaphenalene-based ligand

Assembly of coordination networks from Cd(II) and a multi-interactive hexaazaphenalene-based ligand was successfully modulated using magnetic fields and thermodynamic control. A relatively weak field of only 320 mT was able to perturb the orientational distribution of the ligand in solution nudging the reaction down a different path. The underlying mechanism involved alignment of the ligands along the field lines, which was supported by DFT calculations. This crystallization technique could be extended to the synthesis of other networks and facilitate a deeper exploration of the reaction landscapes.

Polycatenanes Formed of Self-Assembled Metal-Organic Cages

Poly-[n]-catenanes (PCs) self-assembled of three-dimensional (3D) metal organic cages (MOCs) (hereafter referred to as PCs-MOCs) are a relatively new class of mechanically interlocked molecules (MIMs) that combine the properties of MOCs and polymers. The synthesis of PCs-MOCs is challenging because of the difficulties associated with interlocking MOCs, the occurrence of multiple weak supramolecular electrostatic interactions between cages, and the importance of solvent templating effects. The high density of mechanical bonds interlocking the MOCs endows the MOCs with mechanical and physical properties such as enhanced stability, responsive dynamic behavior and low solubility, which can unlock new functional properties. In this Minireview, we highlight the benefit of interlocking MOCs in the formation of PCs-MOCs structures as well as the synthetic approaches exploited in their preparation, from thermodynamic to kinetic methods, both in the solution and solid-states. Examples of PCs-MOCs self-assembled from various types of nanosized cages (i.e., tetrahedral, trigonal prismatic, octahedral and icosahedral) are described in this article, providing an overview of the research carried out in this area. The focus is on the structure-property relationship with examples of functional applications such as electron conductivity, X-ray attenuation, gas adsorption and molecular sensing. We believe that the structural and functional aspects of the reviewed PCs-MOCs will attract chemists in this research field with great potential as new functional materials in nanotechnological disciplines such as gas adsorption, sensing and photophysical properties such as X-ray attenuation or electron conductivity.

Water Content-Controlled Formation and Transformation of Concomitant Pseudopolymorph Coordination Polymers

Two concomitant pseudopolymorph coordination polymers {[Cd₂L₂(OAc)₄]·2DMSO} _n (1) and {[CdL(OAc)₂]·2.75H₂O} _n (2) were synthesized by self-assembly of 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (L) and cadmium acetate in DMSO. Single-crystal X-ray diffraction confirmed that 1D ladder structural motifs exist for pseudopolymorphs 1 and 2 which contain DMSO and water guest molecules, respectively. Our study illustrated the active role of solvent water content in obtaining compound 2. We find that the presence of water as an impurity in the DMSO solvent creates the possibility of formation of concomitant pseudopolymorph coordination polymers which is a unique event. Furthermore, our analyses showed the effect of environmental humidity on the transformation of unstable compound 1. 1D ladder pseudopolymorphic compound 1 could be transformed to guest-free 1D linear compound [CdL(OAc)₂(H₂O)] _n (3') (the powder form of single crystals of 3) through a scarce case of water absorption from air. Also, the crystalline material of coordination polymer 3 was transformed to coordination polymer 2 through the dissolution-recrystallization structural transformation process in DMF or DMSO. Our study clarified that the amount of water in the reaction container can control the formation of one of the compounds 2 or 3. In the presence of a significant amount of water, compound 3 (coordinated water) will be produced, whereas if a small amount of water is present, compound 2 (uncoordinated water) is prepared as an exclusive product.

Experimental X-ray and DFT Structural Analyses of M12L8 Poly-[n]-catenanes Using exo-Tridentate Ligands

Despite their potential applications in host-guest chemistry, there are only five reported structures of poly-[n]-catenanes self-assembled by elusive M₁₂L₈ icosahedral nanocages. This small number of structures of M₁₂L₈ poly-[n]-catenanes is because self-assembly of large metal-organic cages (MOCs) with large windows allowing catenation by means of mechanical bonds is very challenging. Structural reports of M₁₂L₈ poly-[n]-catenanes are needed to increase our knowledge about the self-assembly and genesis of such materials. Poly-[n]-catenane (1·p-CT) self-assembly of interlocked M₁₂L₈ icosahedral cages (M = Zn(II) and L = 2,4,6-tris-(4-pyridyl)benzene (TPB)) including a new aromatic guest (p-chlorotoluene (p-CT)) is reported by single-crystal XRD. Despite the huge internal M₁₂L₈ voids (> 2500 ų), p-CT is ordered, allowing a clear visualization of the relative host-guest positions. DFT calculations have been used to compute the electrostatic potential of the TPB ligand, and various aromatic guests (i.e., o-dichlorobenzene (o-DCB), p-chloroanisole (p-CA), and nitrobenzene (NBz)) included (ordered) within the M₁₂L₈ cages were determined by single-crystal XRD. The computed maps of electrostatic potential (MEPs) allow for the rationalization of the guest's inclusion seen in the 3D X-ray structures. Although more crystallographic X-ray structures and DFT analysis are needed to gain insights of guest inclusion in the large voids of M₁₂L₈ poly-[n]-catenanes, the reported combined experimental/DFT structural analyses approach can be exploited to use isostructural M₁₂L₈ poly-[n]-catenanes as hosts for molecular separation and could find applications in the crystalline sponge method developed by Fujita and co-workers. We also demonstrate, exploiting the instant synthesis method, in solution (i.e., o-DCB), and in the solid-state by neat grinding (i.e., without solvent), that the isostructural M₁₂L₈ poly-[n]-catenane self-assembled with 2,4,6-tris-(4-pyridyl)pyridine (TPP) ligand and ZnX₂ (where X = Cl, Br, and I) can be kinetically synthesized as crystalline (yields ≈ 60%) and amorphous phases (yields ≈ 70%) in short time and large quantities. Despite the change in the aromatic nature at the center of the rigid exo-tridentate pyridine-based ligand (TPP vs TPB), the kinetic control gives the poly-[n]-catenanes selectively. The dynamic behavior of the TPP amorphous phases upon the uptake of aromatic guest molecules can be used in molecular separation applications like benzene derivatives.

Syntheses, Structures, and Properties of Coordination Polymers with 2,5-Dihydroxy-1,4-Benzoquinone and 4,4'-Bipyridyl Synthesized by In Situ Hydrolysis Method

The coordination polymers (CPs) with binary ligands, including 2,5-dihydroxy-1,4-benzoquinone (H2DHBQ) and 4,4'-bipyridyl (bpy), were synthesized using in situ hydrolysis of 2,5-dimethoxy-1,4-benzoquinone (DMBQ). Three kinds of CPs were obtained depending on the metal ions. For M = Mn and Zn, a 1D zigzag chain structure with cis conformation ( cis-1D-M) was obtained, whereas Co, Ni, and Cu compounds afforded a 2D net structure with trans conformation (trans -2D-M) with a 1D pore. A linear chain structure was also obtained for M = Cu. Magnetic susceptibility (χ_M T) at 300 K in cis -1D-Mn and trans -2D-Co was evaluated to be 4.421 and 2.950 cm³ K mol^-1, respectively, indicating that both compounds are in the high-spin state. According to the N₂ adsorption isotherms at 77 K, trans -2D-Ni showed microporosity with the BET surface area of 177 m² g^-1, whereas the isomorphic trans -2D-Co rarely adsorbed N₂ at 77 K. This phenomenon was explained by the difference of diffusion kinetics of the adsorbent molecules, which was supported by the CO₂ adsorption isotherms at 195 K. The optical band gaps of cis -1D-Mn, cis -1D-Zn, trans -2D-Co, and trans -2D-Ni were estimated to be 1.6, 1.8, 1.0, and 1.1 eV, respectively, by using UV-vis-NIR spectroscopy.

Metal Ion-Directed Functional Metal-Phenolic Materials

Metal ions are ubiquitous in nature and play significant roles in assembling functional materials in fields spanning chemistry, biology, and materials science. Metal-phenolic materials are assembled from phenolic components in the presence of metal ions through the formation of metal-organic complexes. Alkali, alkali-earth, transition, and noble metal ions as well as metalloids interacting with phenolic building blocks have been widely exploited to generate diverse hybrid materials. Despite extensive studies on the synthesis of metal-phenolic materials, a comprehensive summary of how metal ions guide the assembly of phenolic compounds is lacking. A fundamental understanding of the roles of metal ions in metal-phenolic materials engineering will facilitate the assembly of materials with specific and functional properties. In this review, we focus on the diversity and function of metal ions in metal-phenolic material engineering and emerging applications. Specifically, we discuss the range of underlying interactions, including (i) cation-π, (ii) coordination, (iii) redox, and (iv) dynamic covalent interactions, and highlight the wide range of material properties resulting from these interactions. Applications (e.g., biological, catalytic, and environmental) and perspectives of metal-phenolic materials are also highlighted.

Triazine 2D Nanosheets as a New Class of Nanomaterials: Crystallinity, Properties and Applications

Based on the recent (2015–2021) literature data, the authors analyze the mutual dependence of crystallinity/amorphism and specific surface area and porosity in covalent triazine frameworks (CTFs), taking into account thermodynamic and kinetic control in the synthesis of these 2D nanosheets. CTFs have now become a promising new class of high-performance porous organic materials. They can be recycled and reused easily, and thus have great potential as sustainable materials. For 2D CTFs, numerous examples are given to support the known rule that the structure and properties of any material with a given composition depend on the conditions of its synthesis. The review may be useful for elder students, postgraduate students, engineers and research fellows dealing with chemical synthesis and modern nanotechnologies based on 2D covalent triazine frameworks.

Mechanochemical synthesis of mechanical bonds in M12L8 poly-[n]-catenanes

Using mechanochemistry by grinding TPB and ZnBr₂, an amorphous poly-[n]-catenane of interlocked M12L8 nanocages is obtained in good yields (∼80%) and within 15 minutes. The mechanical bond among the icosahedral M12L8 cages in the amorphous phase has been demonstrated by single crystal XRD, powder XRD and FT-IR spectroscopy following an amorphous-to-crystalline transformation by guest uptake of the amorphous phase. High-resolution solid-state ¹³C NMR spectroscopy gives insights into the local structure of the amorphous catenane focusing on TPB aromatic-aromatic interactions.

Stepwise Observation of Iodine Diffusion in a Flexible Coordination Network Having Dual Interactive Sites

We created dual interactive sites in a porous coordination network using a CuI cluster and a rotation-restricted ligand, tetra(3-pyridyl)phenylmethane (3-TPPM). The dual interactive sites of iodide and Cu ions can adsorb I₂ via four-step processes including two chemisorption processes. Initially, one I₂ molecule was physisorbed in a pore and successively chemisorbed on iodide sites of the pore surface, and then the next I₂ molecule was physisorbed and chemisorbed on Cu ions to form a cross-linked network. We revealed the four-step I₂ diffusion process by single-crystal X-ray structure determination and spectroscopic kinetic analysis.

Through-Space Charge Transfer in Copper Coordination Networks with Copper-Halide Guest Anions

We prepared coordination networks that show relatively strong emission with through-space charge-transfer (TSCT) transitions. Thermolysis of a kinetically assembled network with Cu₂Br₂ dimer connectors, which was assembled from a CuBr cluster and the T_d ligand 4-4-tetrapyridyltetraphenylmethane (4-TPPM), generated a highly luminescent network composed of Cu⁺ connectors and 4-TPPM linkers with CuBr₂^- guests. We clarified that the electronic transitions in this network include TSCT in addition to the typical metal-ligand charge transfer (MLCT) observed in conventional Cu complexes.

Thermodynamically metastable chain and stable layered Co(NCS)2 coordination polymers: thermodynamic relations and magnetic properties

Reaction of Co(NCS)2 with 4-bromopyridine leads to the formation of discrete complexes with the composition Co(NCS)2(4-bromopyridine)4·(CH3CN)0.67 (1), Co(NCS)2(4-bromopyridine)2(H2O)2 (2), Co(NCS)2(4-bromopyridine)2(CH3OH)2 (3) and Co(NCS)2(4-bromopyridine)2(CH3CN)2 (4). Upon heating compounds 2 and 4 transform into a crystalline product with the composition Co(NCS)2(4-bromopyridine)2 (5-I) that also can easily be obtained from solution. In this compound, the Co cations are linked by single μ-1,3-bridging thiocyanate anions into layers. Thermal decomposition of 3 leads to a second isomer (5-II), which is thermodynamically metastable and can also be synthesized from solution under kinetic control. In contrast to 5-I, the Co cations are linked by pairs of anionic ligands into linear chains. The magnetic exchange is very weak in 5-I, but much stronger and ferromagnetic along the linear chains in 5-II. AF ordering in 5-II is reached at 3.05 K, and magnetic relaxation is observed at the metamagnetic transition with an Arrhenius barrier of 17.1(3) cm-1. Ab initio computational studies reveal a different type of magnetic anisotropy to be present in the two crystallographically - independent Co centers in 5-II.

Structural elucidation of microcrystalline MOFs from powder X-ray diffraction

Ab initio powder XRD structure solution and MOFs.

Thermodynamically stable and metastable coordination polymers synthesized from solution and the solid state

Different Fe(ii) and Cd(ii) thiocyanate coordination compounds with unusual topology including isomeric modifications were synthesized with 4-picoline and investigated in regard to their thermodynamic relations.

New isomeric Ni(NCS)2 coordination compounds: crystal structures, magnetic properties as well as ex situ and in situ investigations on their synthesis and transition behaviour

Some stable and metastable isomeric Ni(NCS)₂ chain compounds were synthesized and structurally characterized and investigated for their transformation behavior and their magnetic properties using a variety of methods.

Structural conversion of three copper(ii) complexes with snapshot observations based on the different crystal colours and morphology

Three novel Cu(ii) complexes [Cu₂(L)₂(MeOH)₂] (1), [Cu₂(L)₂(H₂O)₂] (2) and [CuL(H₂O)] (3) (L = (E)-2-((2-hydroxy-4-methoxybenzylidene)amino)acetic acid) have been obtained in different time scales of reaction processing. Complexes 1 and 2 are kinetically controlled products and 3 is a thermodynamically stable product. Single crystal X-ray diffraction analyses revealed that 1 and 2 are binuclear complexes except for different coordination solvents. 3 is a mononuclear complex. Complex 1 is mainly obtained in methanol solution, while 2 and 3 are stable in aqueous solvents. Based on the understanding of crystal structures of the three complexes, reversibly transforming crystal 2 to crystal 1 at room temperature has been realised, which has been confirmed by the change of colours and morphology measured by SEM. The research work is very important for controllable synthesis of coordination complexes.

Structural conversion of three novel Cu(ii) complexes [Cu₂(L)₂(MeOH)₂] (1), [Cu₂(L)₂(H₂O)₂] (2) and [CuL(H₂O)] (3) (L = (E)-2-((2-hydroxy-4-methoxybenzylidene)amino)acetic acid) in different time scales of reaction processing.

Solid-Gas Phase Synthesis of Coordination Networks by Using Redox-Active Ligands and Elucidation of Their Oxidation Reaction

Formation of coordination networks is a complex process affected by a multitude of factors. Many synthetic strategies have been developed that attempt to control these factors and direct the structure of the final product. Coordination bond formation and structural assembly processes, however, typically take place either in the solution or solid states. In comparison, gas-phase network synthesis remains largely unexplored. Herein, two new two-dimensional coordination networks are obtained from the solid-gas phase reaction between ZnX₂ (X=I, Br) and the redox-active 2,5,8-tri(4-pyridyl)1,3-diazaphenalene (HTPDAP) ligand. Their structures were solved by ab initio powder X-ray diffraction analysis and feature a novel Zn halide trimeric cluster. This strategy is contrasted with a conventional solvothermal synthesis, which led to a one-dimensional coordination polymer instead. The intrinsic electroactive properties of these materials were probed by solid-state cyclic voltammetry measurements, which revealed the presence of HTPDAP and halide-based processes. Chemical oxidation of the two-dimensional networks by using NOPF₆ agent, unexpectedly, led to the formation of a nitrated analog of HTPDAP, the PF₆ ^- salt of diprotonated 4,6,7,9-tetranitro-2,5,8-tris(4-pyridyl)diazaphenalene cation (denoted N-TPDAP), which was isolated and characterized. These results provide deeper insights into the oxidation process of HTPDAP-containing networks and uncover unique redox-induced chemical transformations.

Structures, Thermodynamic Relations, and Magnetism of Stable and Metastable Ni(NCS)2 Coordination Polymers

Reaction of Ni(NCS)₂ with 4-aminopyridine in different solvents leads to the formation of compounds with the compositions Ni(NCS)₂(4-aminopyridine)₄ (1), Ni(NCS)₂(4-aminopyridine)₂(H₂O)₂ (2), [Ni(NCS)₂(4-aminopyridine)₃(MeCN)]·MeCN (3), and [Ni(NCS)₂(4-aminopyridine)₂] _n (5-LT). Compounds 1, 2, and 3 form discrete complexes, with octahedral metal coordination. In 5-LT the Ni cations are linked by single thiocyanate anions into chains, which are further connected into layers by half of the 4-aminopyridine coligands. Upon heating, 1 transforms into an isomer of 5-LT with a 1D structure (5-HT), that on further heating forms a more condensed chain compound [Ni(NCS)₂(4-aminopyridine)] _n (6) that shows a very unusual chain topology. If 3 is heated, a further compound with the composition Ni(NCS)₂(4-aminopyridine)₃ (4) is formed, which presumably is a dimer and which on further heating transforms into 6 via 5-HT as intermediate. Further investigations reveal that 5-LT and 5-HT are related by enantiotropism, with 5-LT being the thermodynamic stable form at room-temperature. Magnetic and specific heat measurements reveal ferromagnetic exchange through thiocyanate bridges and magnetic ordering due to antiferromagnetic interchain interactions at 5.30(5) K and 8.2(2) K for 5-LT and 6, respectively. Consecutive metamagnetic transitions in the spin ladder compound 6 are due to dipolar interchain interactions. A convenient formula for susceptibility of the ferromagnetic Heisenberg chain of isotropic spins S = 1 is proposed, based on numerical DMRG calculations, and used to determine exchange constants.