A team of researchers recently published a paper in the journal Coordination Chemistry Reviews that reviewed one-dimensional (1D) coordination polymers based on nitrogen-donor (N-donor) ligands.
Study: One-dimensional coordination polymers based on metal–nitrogen linkages. Image Credit: ibreakstock/Shutterstock.com
1D coordination polymers (CPs) have been studied extensively in the last decade owing to their versatile applications, unique architectures, and fascinating properties. In 1D CPs, the ligand plays the most crucial role in determining their properties and structures.
In this study, researchers reviewed 1D CPs based on N-donor ligands developed in 2015. Researchers also discussed the outlook and challenges of N-donor ligand-based 1D CPs.
The N-donor ligands were classified depending on the N-donor functional groups, including azide, dipyrrin, phenanthroline, terpyridine, pyrrole, Schiff base, amino, pyrimidine, tetrazole, triazole, pyrazole, imidazole, and pyridine.
1D CPs with Structural Motifs based on N-donor Ligands
Pyridinic N-donor Ligands
Bis(pyridyl) compounds containing two pyridine groups connected to the two ends of a spacer are the most common type of ligands used to prepare 1D CPs through pyridinic N coordination with different metal centers.
1D CPs with linear structures can be prepared easily using linear spacing ligands to coordinate with the metal ions. Typically, 4,4′ -bipyridine (L1) has been utilized to prepare several linear CPs.
For instance, a valence tautomer chiral linear CP was synthesized using cobalt (II) perchlorate hexahydrate, 2,3-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene, and L1 ligand through the slow diffusion method. The cobalt center of the complex displayed a slightly distorted octahedral coordination environment, and the complex crystallized in the P3221 space group.
Ribbon-like polymers are typically obtained from the interconnection of metal centers and flexible ligands in intertwist/bend structures to form macrocycles. For instance, a ribbon-chain complex with catalytic properties was prepared using the nitrate salt of cadmium and N, N′ -bis-(3-pyridyl) terephthalamide (L21) ligand.
The complex crystallized in the monoclinic P21/n space group and asymmetric unit cell contained a coordinated DMF molecule, a nitrate ion, a half cadmium ion, and an L21 ligand. The cadmium center demonstrated a twisted octahedral geometry.
Zigzag structures are also common in 1D CPs and are typically prepared from flexible exoditopic ligands and cis or linear-coordinated tetrahedral or octahedral metal ions. Moreover, polynuclear metal clusters can be utilized to link ligands to synthesize 1D CPs. Thermoschromism and solid-state luminescence are the major properties of these CPs.
Ladder CPs are composed of spacer ligands as rungs and rails and metal ions in the ratio of 1.5:1 to form ‘‘T-shaped” building blocks. For instance, a ladder CP was synthesized through the solvothermal reaction of sodium hydroxide, benzenesulfonate (BS) ligand, and 1,2-bi(4-pyridyl)ethane (L3) ligand in water and methanol. The orientation, shape, and length of spacer ligands determined the cavity formed by the ladder.
Helical CPs are prepared from chiral or flexible ligands. A helical CP was prepared through the solvothermal reaction of the tetrakis(4-pyridyloxymethyl)methane (L24), sodium azide, and manganese salt in a mixture of water and methanol. The prepared CP was crystallized in a P3121 trigonal crystal system and displayed a 1D helical structure.
Additionally, several CPs were developed with unique structural motifs. For instance, a tubular 1D CP was synthesized through the self-assembly of the cadmium iodide, Y-type tridentate ligand, and 2,6-bis[(2-isonicotinoyloxy-5-methylphenyl)methyl]-p-tolylisonicotinate (L27) ligand in a diiodomethane-containing mixed solvent system.
Photoluminescence, photocyclopropanation, fluorescence, and thermal stability are the major properties of pyridinic N-ligand donor CPs with ladder, helical, and other unique structures.
Although the 1D CP structural motifs are primarily dependent on the linking ligand architecture, the use of auxiliary ligands affected the 1D CP conformation. Additionally, anions also affect the 1D CP conformation as common inorganic anions such as halides, thiocyanate, and nitrate interact with the metal center owing to their high charge densities.
Imidazole N-donor Ligands
Imidazole compounds can be coordinated with metal ions to prepare zeolitic imidazolate frameworks. Imidazole-capped ligands containing functional spacers can be utilized to prepare 1D CPs through the regulation of their reaction conditions and molecular structure.
Most 1D CPs based on imidazole N-donor ligands demonstrate a zigzag chain structure. For instance, a CP with a zigzag structure and catalytic properties was synthesized using an auxiliary aromatic 1,2-benzenedicarboxylic acid ligand and a rigid 3,6-bis(imidazole-1-yl)pyridazine (L32) ligand. The zigzag type CP showed a crystalline structure consisting of a Pbca space group in the orthorhombic system.
Additionally, these 1D CPs also possess helical, ladder, ribbon, and linear structures. For instance, a photoluminescent CP with a ribbon chain structure was synthesized using cadmium salt, bis(3,5-bis((1 H-imidazol-1-yl)methyl)-2,4,6-trimethylphenyl)methane (L45) ligand, and a tetraimidazole ligand.
1D CPs can be assembled easily into three-dimensional (3D) or two-dimensional (2D) frameworks owing to their weak interactions, including van der Waals and electrostatic interactions, π– π interactions, and hydrogen bonding. Coordinated metal centers affect the structural motifs of 1D CPs owing to the differences in the coordination environments of different metal ions.
Triazole N-donor Ligands
Triazole-based ligands, such as 1,2,3-triazole, benzotriazole, and 1,2,4-triazole, have gained significant attention due to their combined coordination configuration of imidazoles and pyrazoles. Several 1D CPs based on bis(triazole) ligands with flexible, semi-rigid, and rigid spacers were developed due to their diverse functional properties and topological structures.
Ribbon structure is the most prevalent conformation in the triazole N-donor ligand-based 1D CPs. For instance, a luminescent complex with a ribbon structure was synthesized through the reaction of 3,5-di(1,2,4-triazol-1-yl)benzoic acid (L55) ligand and zinc acetate under the solvothermal condition. The complex crystallized in the Pccn orthorhombic space group. Tubular, helical, zigzag, and linear are the other prominent structural motifs of triazole N-donor ligand-based 1D CP.
The single-crystal to single-crystal (SC-SC) structural transformation caused by several stimuli, such as metal or ligand replacement, allows the accurate visualization of the coordination assembly crystalline state.
Other N-donor Ligands
Several 1D CPs with zigzag, helical, linear, and ribbon structures, can be synthesized using rare N-donor ligands, such as dicyanamide, tetrazole, and pyrazole. For instance, a dicyanamide-based 1D CP with a square pyramidal architecture and magnetic properties was synthesized by reacting sodium dicyanamide, 1,2-ethanediamine, and copper (II) chloride.
Multiple N-donor Ligands
An efficient ligand can contain several types of N-donor groups, such as azide, pyrimidine, Schiff base, and pyrazole. These N-donor groups, together with functional spacers, can create different N-donor ligands, which can be used to prepare 1D CPs with linear, zigzag, helical, ladder, or ribbon structures by coordinating with metal ions. Multiple N-donor ligand-based 1D CPs possess redox-induced molecular metamorphism, luminescent properties, and electrical properties.
N-donor Ligand-based 1D CPs without Structural Motifs
Several 1D CPs based on N-donor ligands do not possess structural motifs owing to the flexible nature of specific N-donor ligands, including dipyrrin, phenanthroline, and terpyridine.
The 2,2′:6′,2′′ -terpyridyl compounds are a form of tridentate chelating ligand used to prepare bis(terpyridine) metal complexes with a quasi-octahedral structure and iron, cadmium, chromium, and manganese as central metals. Similarly, bis(1,10-phenanthroline) ligands can also be used to synthesize 1D metallosupramolecular polymers by coordinating with metal ions.
Bis(dipyrrin) ligands with functional spacers can coordinate with different metal ions to form 1D bis(dipyrrinato)metal complexes, which can undergo exfoliation to form single-strand wires through sonication. Switchable photoluminescence, visible absorption, electrochromism, cytotoxicity, DNA binding, and ionic conductivity are the major properties of these CPs.
Azide N-donor Ligand-based 1D CPs
The small pseudo-halide azide ion can bind to metal ions through different coordination modes, including end-on (EO) modes comprised of single EO and double EO modes, end-to-end (EE) modes consisting of single EE mode and double EE modes, and several additional complicated modes such as the alternate bridges of EE and EO.
Auxiliary ligands are used extensively to ensure that the synthesized metal azides possess a high degree of thermodynamic stability and crystallinity. Azide N-donor ligand-based 1D CPs containing EO mode possess double chain, ribbon, or zigzag structures, while CPs containing EE mode have ribbon or zigzag structures.
CPs containing both EE- and EO-bridged modes demonstrate double chain and ribbon structures. The magnetic property of azide N-donor ligand-based CPs produced by the magnetic interaction propagation among the π-electron-delocalized N atoms is the most beneficial feature of these complexes.
Challenges and Outlook
Predicting and controlling the crystal architecture of the final 1D CPs is still a significant challenge due to the unregulated self-assembly procedure and complexity of starting materials that result in the production of different kinetic products beyond the thermodynamically stable and kinetically preferential products.
Thus, future studies must focus on regulating the product morphology, identifying the new properties of these 1D CPs beyond the existing magnetic, optical, and electric properties, and exploring new materials with new structural features, including synthesizing and designing new metal cluster nodes, ligands, and reaction conditions.
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Zhu, J., Lu, C., Qiu, F. et al. (2022) One-dimensional coordination polymers based on metal–nitrogen linkages. Coordination Chemistry Reviews. https://doi.org/10.1016/j.ccr.2022.214735
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