From coordination complexes to coordination polymers through self-assembly

Metal ion coordination in metallo-supramolecular assemblies offers the opportunity to fabricate and study devices and materials that are equally important for fundamental research and new technologies. Metal ions embedded in a specific ligand field offer diverse thermodynamic, kinetic, chemical, physical and structural properties that make these systems promising candidates for active components in functional materials. In particular, dynamic coordination polymers offer exciting opportunities to provide materials with responsive properties. In addition, this approach allows to incorporate the well known properties of metal complexes in polymeric architectures. This review highlights the improvements and the possible applications based on metallo-supramolecular systems with an emphasis on materials science. Examples for new materials such as molecular magnets, coordination polymers as carrier package as well as molecular electronics are featured in this article.     

Triazoles and tetrazoles: Prime ligands to generate remarkable coordination materials

The current great interest in preparing functional metal-organic materials is inevitably associated with tremendous research efforts dedicated to the design and synthesis of new families of sophisticated multi-nucleating ligands. In this context, the N-donor triazole and tetrazole rings represent two categories of ligands that are increasingly used, most likely as the result of the recent dramatic development of “click chemistry” and Zeolitic Imidazolate Frameworks (ZIFs). Thus, azole-based complexes have found numerous applications in coordination chemistry.In the present review, we focus on the utilization of 1,2,3-triazole, 1,2,4-triazole and tetrazole ligands to create coordination polymers, metal complexes and spin-crossover compounds, reported to the end of 2009. In the first instance, we present a compendium of all the relevant ligands that have been employed to generate coordination polymers and Metal-Organic Frameworks (MOFs). Due to the huge amount of reported MOFs and coordination polymers bearing these azole rings, three representative examples for each category (therefore nine in total) are described in detail. The second section is devoted to the use of the bridging abilities of these azole ligands to prepare metal complexes (containing at least two metal centers). Given the large number and the great structural diversity of the polynuclear compounds found in the literature, these have been grouped according to their nuclearity. Finally, in the last section, the triazole- and tetrazole-containing coordination compounds exhibiting spin-crossover properties are presented.     

Heteroleptic copper dipyrromethene complexes: synthesis, structure, and coordination polymers

The synthesis of neutral [Cu(dpm)2] and [Cu(dpm)(acac)] (dpm = dipyrromethene, acac = acetylacetonato) complexes is presented. The formation of the asymmetric metal complexes was monitored by electronic absorption and infrared spectroscopy. Two of the complexes investigated, containing pyrdpm ligands (pyrdpm = pyridyldipyrromethene), form 1-dimensional coordination polymers. The coordination polymers formed by these complexes have been characterized by single-crystal X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The complexes possess square pyramidal coordination geometries with the apical position occupied by the meso-pyridyl donor of a neighboring complex in the crystal lattice. The features of these coordination complexes that facilitate formation of extended solids have been probed. Symmetric [Cu(pyrdpm)2] complexes are unable to form coordination solids due to steric hindrance at the metal center. Use of cyano donors in complexes such as [Cu(cydpm)(acac)] (cydpm = cyanodipyrromethene) in lieu of pyridyl donors also fail to form network solids. Through these systematic studies, both the basic coordination chemistry of these complexes and the fundamental design requirements for synthesizing this novel class of coordination polymers have been defined.     

In situ hydrothermal synthesis of tetrazole coordination polymers with interesting physical properties

Tetrazole compounds have been studied for more than one hundred years and applied in various areas. Several years ago Sharpless and his co-workers reported an environmentally friendly process for the preparation of 5-substituted 1H-tetrazoles in water with zinc salt as catalysts. To reveal the exact role of the zinc salt in this reaction, a series of hydrothermal reactions aimed at trapping and characterizing the solid intermediates were investigated. This study allowed us to obtain a myriad interesting metal-organic coordination polymers that not only partially showed the role of the metal species in the synthesis of tetrazole compounds but also provided a class of complexes displaying interesting chemical and physical properties such as second harmonic generation (SHG), fluorescence, ferroelectric and dielectric behaviors. In this tutorial review, we will mainly focus on tetrazole coordination compounds synthesized by in situ hydrothermal methods. First, we will discuss the synthesis and crystal structures of these compounds. Their various properties will be mentioned and we will show the applications of tetrazole coordination compounds in organic synthesis. Finally, we will outline some expectations in this area of chemistry. The direct coordination chemistry of tetrazoles to metal ions and in situ synthesis of tetrazole through cycloaddition between organotin azide and organic cyano group will be not discussed in this review.     

2,6-萘二甲酸构筑配合物的研究进展

2,6-萘二甲酸是一个具有多个配位原子的线性刚性配体,迄今,已报道的2,6-萘二甲酸构筑的配合物有100多个,它们展现了丰富的拓扑结构和潜在的应用价值。本文总结了2,6-萘二甲酸配体的结构特点以及配位模式,并按照金属离子的种类对2,6-萘二甲酸配合物进行分类归纳总结。着重介绍了2,6-萘二甲酸配合物在气体吸附分离、光致发光、磁性及多相催化方面的应用性质,对该类配合物的发展前景进行了展望。     

Bis(oxalato)chromium(III) complexes: Versatile tectons in designing heterometallic coordination compounds

The mononuclear oxalato-containing chromium(III) complexes of general formula [Cr(AA)(C₂O₄)₂]^? (AA = α-diimine type ligand) are able to produce a large variety of heterometallic complexes by acting as ligands towards either fully solvated metal ions or preformed cationic complexes with available coordination sites. This review focuses on the structural diversity of the polynuclear complexes (oligonuclear and coordination polymers) which are generated by the bis(oxalato)chromate(III) species, with a special emphasis to their magnetic properties.     

Design and synthesis of coordination networks containing amide,pyridine and carboxylate functionalities

The reactions of bis(pyridinecarboxamido)alkane with copper(II) in the presence of mono or dicarboxylic acids resulted in discrete species, one-dimensional and two-dimensional networks. The carboxylates considered for this study include m-nitrobenzoic acid, isophthalic acid and succinic acid. In the presence of m-nitrobenzoic acid the ligands with Cu(II) form 1D coordination networks which include m-nitrobenzoic acid and water molecules as guests. The use of isophthalic acid resulted in discrete species while the use of succinic acid resulted in a two-dimensional layer containing rectangular grids of dimension 9.7 × 16.5 Å². The 2D layers in this complex exhibit inclined 2-fold interpenetration. Further, all these coordination networks are assembled via hydrogen bonding interactions between the amides and water molecules. The Cu(II) centre exhibits a unique octahedral coordination geometry, for the complexes reported here, as it coordinates with two each of pyridine moieties, water molecules and carboxylates.     

Functional porous coordination polymers

The chemistry of the coordination polymers has in recent years advanced extensively, affording various architectures, which are constructed from a variety of molecular building blocks with different interactions between them. The next challenge is the chemical and physical functionalization of these architectures, through the porous properties of the frameworks. This review concentrates on three aspects of coordination polymers: 1). the use of crystal engineering to construct porous frameworks from connectors and linkers ("nanospace engineering"), 2). characterizing and cataloging the porous properties by functions for storage, exchange, separation, etc., and 3). the next generation of porous functions based on dynamic crystal transformations caused by guest molecules or physical stimuli. Our aim is to present the state of the art chemistry and physics of and in the micropores of porous coordination polymers.     

Binding sites on the outside of metallo-supramolecular architectures; engineering coordination polymers from discrete architectures

Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based approach. Silver(I) complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first, the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array. In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves not part of the architectures, and these separate metal centres link the units to form the macromolecular array. The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than those observed in the discrete pyridylimine architectures.     

Soft functional polynuclear coordination compounds containing pyrimidine bridges

In this account, we describe the use of simple pyrimidine derivatives in combination with metal ions to build highly structured molecular architectures containing functional nanoenvironments, cavities and surfaces that can interact with additional species. The supramolecular structure of these systems can be rationally controlled by metal fragment geometry, reaction conditions and presence of templating agents. Thus, the use of transition metals with low coordination numbers or blocked bonding positions in combination with pyrimidines (e.g. 2-hydroxypyrimidine, 4-hydroxypyrimidine, 2,4-dihydroxypyrimidine, 2-aminopyrimidine) leads to the formation of either discrete assemblies, 1D polymers or helixes. When metal ions with higher coordination possibilities are applied instead, 2D and 3D networks are generated. Some of the assemblies built in this way possess functional cavities, pores and surfaces that can interact with additional species by means of hydrophobic, electrostatic, H-bonding interactions and coordinative bonds to give rise to recognition processes. The latter range from molecular recognition in homogeneous phase as well as clathrate formation, to heterogeneous solid-gas and solid-liquid adsorption phenomena. It should be noted that these materials are not rigid but able to undergo guest-induced reorganisation processes even in the solid state. Finally, some of these materials also combine additional interesting magneto-optical properties. Thus, dual systems can be envisaged in which two or more of these properties are present in the same material.     

Recent advances in biosensory and medicinal therapeutic applications of zinc(II) and copper(II) coordination complexes

Historically, Lewis acidic metal coordination complexes have played a pivotal and defining role in the broad field of molecular recognition and more specifically in the sensing and sequestration of biologically relevant anionic species. More recently, with the expanding interest in chemical biology, there has been a resurgence in the use of coordination complexes, specifically, through their application as medicinal therapeutics and chemo/biosensors. From the disruption of oncogenic protein–protein interactions to the fluorescent sensing of PTP1B phosphatase enzyme activity, the powerful binding potency of coordination complexes has been harnessed to great effect. The ingenuity of the rationally designed coordinating ligands has facilitated the diversity of roles played by Lewis metal complexes. Herein, we will review the recent advances in the application of coordination complexes in medicinal and chemo/biosensory roles over the last decade. In particular, this review will focus on Cu(II) and Zn(II) coordination complexes.     

Coordination chemistry of 2,4,6-tri(pyridyl)-1,3,5-triazine ligands

This review covers the rich coordination chemistry of 2,4,6-tri(pyridyl)-1,3,5-triazine ligands. These polypyridyl derivatives have been coupled to transition metals and lanthanides, and the complexes obtained have been used in various fields such as luminescent materials, for the preparation of coordination polymers and networks as well as for the synthesis of discrete metalla-assemblies. The synthetic and structural aspects of the different isomers of 2,4,6-tri(pyridyl)-1,3,5-triazine are presented, and a survey of their coordination chemistry is given.     

Luminescent lanthanide coordination polymers with transformative energy transfer processes for physical and chemical sensing applications

The photophysical process of lanthanide(III) ion is based on the 4f-4f transition, which is the Laporte forbidden with narrow emission band and long emission lifetime. The 4f-4f emission process is affected by introducing aromatic organic ligands. In this review, recent progress of one-, two-, and three-dimensional polymer-typed lanthanide complexes, luminescent lanthanide coordination polymers, are focused for physical and chemical sensing applications. Their changeable luminescence depended on the physical and chemical environments come from the energy transfer between lanthanide(III) ions and aromatic organic ligands. The characteristic physical (temperature, pressure, pH and mechanical force) and chemical (adsorption of metal ions and molecules) sensitive luminescence of lanthanide coordination polymers are useful for future sensing applications.     

Design and its limitations in the construction of bi- and poly-nuclear coordination complexes and coordination polymers (aka MOFs): a personal view

This article, presented from a personal point of view, is concerned with the design of ligands intended to give specifically either binuclear or tetranuclear metal complexes or coordination polymers. No attempt is made to provide a comprehensive coverage of these topics, the focus being mainly upon results from our laboratory. Some emphasis is placed upon aspects of the historical development of the deliberate construction of coordination polymers (aka MOFs)--materials promising useful applications, the study of which continues to expand exponentially. Some of our recent research is described in which the carbonate ion and the tetracyanoquinodimethane dianion are used as bridging ligands to generate targeted coordination polymers. It is intended that Dalton Perspectives be easily comprehensible to non-specialists in the field; an average second year university chemistry student should be easily able to understand the present contribution.     

Flexible microporous coordination polymers

In a decade, many porous coordination polymers have been synthesized, providing a variety of properties ranging from storage, separation and exchange of guests in their cavities, magnetism, conductivity and catalysis by their frameworks. Recent advent of flexible porous coordination polymers, which exhibit elastic guest accommodation in contrast to rigid three-dimensional (3-D) frameworks of conventional porous materials, have acquired a position as a new class of porous materials. Such flexible porous properties induce highly selective guest accommodation and magnetic modulation, which could now be a unique class of practical materials. In this review, we introduce recent flexible porous coordination polymers (3-17) and their functional properties, categorizing with the four types of pores with framework deformation.     

Highly stable olefin-Cu(I) coordination oligomers and polymers

Highly stable Cu(I)-olefin coordination oligomers and polymers have been successfully prepared and applied to construct metal-organic frameworks (MOFs) with interesting physical and chemical functions in recent years. In this review, we present the olefin-Cu(I) coordination oligomers and polymers and their novel physical properties. From structure to functions, particular emphasis is placed on the coordination and organometallic chemistry of olefin-Cu(I) coordination oligomers and polymers, their structures and potential applications as solids possessing unusual physical functional properties such as electrochemical, chiral separation, fluorescent sensing and ferroelectricity.     

Anions as templates in coordination and supramolecular chemistry

This review aims to highlight the most important recent advances in the area of anion-templated syntheses in supramolecular and coordination chemistry. We published a comprehensive review on this area in 2003 and hence examples prior to this date will only be discussed when essential for clarity of presentation. The current review has been divided into three main sections: (a) anion-templated synthesis of systems with well-defined molecular weights; this includes macrocycles and cages, interlocked species (such as catenanes and rotaxanes), helical assemblies and other selected examples. (b) Anions as templates in polymeric systems; this includes metal-organic frameworks, molecularly imprinted polymers and other selected examples, such as liquid crystalline materials. (c) Anion templates in dynamic combinatorial libraries.     

Ring-opening polymerisation of coordination rings and cages

Despite the great interest in crystalline coordination polymers (sometimes called metal organic frameworks) surprisingly little is known about how they form. However, there has recently been some attention given to characterising their solution-based precursors. Of particular interest is the formal ring-opening polymerisation (ROP) relationship between the structures of some precursors and polymers, and the actual observation of ring-opened oligomers by solution state NMR spectroscopy. These points are highlighted and discussed along with related aspects of isomerism in discrete and polymeric coordination structures, and polymer-polymer interconversion.     

Lanthanide-transition metal coordination polymers based on multiple N- and O-donor ligands

Lanthanide-transition metal (Ln-M) coordination polymers have attracted extensive interest because they exhibit novel physical properties originating from the interactions between distinct metal ions. This review mainly describes our recent work in the design of Ln-M coordination polymers through the assembly of different metal ions and organic ligands, especially the ligands with multiple N- and O-donor atoms. Many of these crystalline Ln-M materials exhibit intriguing structural motifs and interesting magnetic properties.