Advances in the coordination chemistry of nitrogen ligand complexes of coinage metals

Coinage metals nitrogen chemistry has not been studied extensively until recently. The focus of this review is the base- and halide-free complexes of the monoanionic nitrogen ligands. This review describes how minor ligand modifications can result in a drastic change in the metal–metal interactions in multinuclear compounds. Crystal structures of these complexes show individual complexes, dimers, supramolecular columnar packing or more complex supramolecular aggregates. Bulky substituents on the ligands can prevent intermolecular metal–metal interactions or the formation of supramolecular architectures. The nuclearity and metal–metal interactions in these complexes are controlled by ligand steric and electronic factors and solvent of crystallization. Many classes of nitrogen ligand coordination compounds have given rise to advances in several fundamental and applied research aspects. Recent potential applications of nitrogen ligand complexes are highlighted particularly for those complexes included in this review.     

Si?H and C?H Activation by Transition Metal Complexes: A Step Towards Isolable Alkane Complexes?

The recognition of the fundamental contributions by G. A. Olah on the elucidation of the structure of nonclassical carbocations, in the form of the award of the Nobel prize for chemistry, has recently emphasized the importance of electron-deficient bonds in the understanding of chemical bonding in organic chemistry. In the field of coordination chemistry, the formulation of electron-deficient bonds has been used for some time to describe nonclassical interactions between atoms. Traditional ligands in coordination chemistry such as amines and phosphanes bond to metal centers through their lone pair of electrons. Synergistic bonding effects dominate in the coordination of π-bonded ligands such as alkenes. In the mid-1980s the discovery of dihydrogen complexes having side-on coordination of H₂ gave fresh impetus to transition metal chemistry as well as to the understanding of the interaction of σ-coordinating ligands with transition metals. In the meantime, transiton metal complexes can be obtained with a variety of σ-coordinated X-H fragments, and their mode of bonding can be understood by a common and quite general model. The chemistry of σ-bound silane ligands is particularly varied and well-investigated. These silane ligands enable the investigation of a large range of σ-coordinated metal complex fragments up to complete oxidative addition with cleavage of the Si? H bond and formation of silyl(hydrido) complexes, which has thus also widened our general understanding of the bonding of other σ-bound ligands. Whilst there is a large range of isolable and stable H₂ and SiR₄ complexes available, there are no such alkane analogues known at present. Only when the C? H bond is part of a ligand that is already directly bonded to the transition metal center will the resulting chelate effect stabilize this agostic C-H-M interaction. The complexation of SiH₄, the simplest heavier homologue of CH₄, was achieved recently. This is a further step towards the understanding of the factors which govern σ-complexation of ligands at transition metal centers.     

Diol-type ligands as central 'players' in the chemistry of high-spin molecules and single-molecule magnets

The combination of diol-type ligands with paramagnetic transition metal ions has led to the isolation of a host of new homometallic and heterometallic clusters, high-spin molecules and single molecule magnets ranging in nuclearity from two to forty four and with spin ground states as large as S = 61/2. The ligands, whose cluster coordination chemistry is discussed in this article, are 1,3-propanediol and its derivatives, diethanolamine and its derivatives, pyridine-2,6-dimethanol and the gem-diol form of di-2-pyridyl ketone. The structural diversity of the complexes stems from the ability of the ligands to adopt a variety of bridging coordination modes depending on the positions of the two hydroxyl groups in the molecule, the presence/absence of extra donor groups and on the reaction conditions. Examples of 'true' reactivity chemistry involving clusters of diol-type ligands are also given. The activation of pyridine-2,6-dimethanol and di-2-pyridyl ketone by 3d-metal centres towards further reactions seems to be an emergent area of synthetic chemistry.     

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.     

Chiral Auxiliaries as Emerging Tools for the Asymmetric Synthesis of Octahedral Metal Complexes

New methods for the stereocontrolled synthesis of octahedral metal complexes are needed in order to fully exploit the stereochemical richness of the octahedron in the fields of catalysis, materials sciences, and life sciences. Whereas a large body of work exists regarding the diastereoselective coordination chemistry with chiral ligands, such efforts are restricted to certain carefully designed chiral ligands which remain in the coordination sphere. The emerging strategy of chiral‐auxiliary‐mediated asymmetric synthesis holds promise to solve the problem of controlling relative and absolute configuration in octahedral metal complexes in a general fashion, thus hopefully in the future providing access to any desired optical active octahedral metal complex without the need for chiral separations. This short review will summarize reported examples of chiral auxiliaries applied to the asymmetric synthesis of octahedral metal complexes.     

The chemistry of tri- and high-nuclearity palladium(II) and platinum(II) complexes

This review gives an overview of the progress on tri- and high-nuclearity palladium(II) platinum(II) complexes and discusses recent developments in the chemistry of these complexes. Three or more square-planar metal atoms can be assembled in several ways resulting into myriad geometric forms of the resulting complexes. These square planes may be sharing a corner, an edge and two edges or even separated by ligands having their donor atoms incapable of forming chelates yielding dendrimers and self-assembled molecules. A variety of ligands have been used to stabilize these complexes. The chemistry of these complexes has been dealt based on nuclearity of the complexes. Synthetic, spectroscopic, structural aspects and applications of these complexes are described in this review.     

Metal coordination complexes in nonaqueous redox flow batteries

The ongoing search for new electroactive materials for next-generation redox flow batteries has within the last decade encompassed metal–ligand coordination chemistry. Here, we review the handful of metal coordination complexes proposed as redox flow battery electrolytes. We highlight examples with careful ligand design, driving research towards higher energy density storage materials. Metal coordination complexes designed to be highly soluble not only in the initial redox state but also in all charged states accessed during the battery cycle give particularly impressive performances. Optimisation of flow cell conditions for metal coordination complexes remains largely unexplored, with most complexes screened in nonideal H-cell experiments with little investigation into membrane and electrode materials.     

Synthesis,Structure and Reactivity of Sulfur-Rich Cyclopentadienyl-Transition Metal Complexes: Sulfur Chemistry from an Organometallic Point of View

Metal-sulfur centers play an important role in the activity of metalloproteins in enzymatic catalysis and the activity of metal sulfides as heterogeneous catalysts. The systematic search for M? S model compounds led to the discovery of an interesting and novel structural chemistry, which stems from the numerous coordination possibilities of sulfur ligands. The intention of this review article is to present and outline new approaches to sulfur chemistry from the organometallic point of view. Reactive cyclopentadienyl-transition metal fragments incorporate elemental sulfur to give polynuclear sulfur-rich complexes, which can contain either mono-, di- or polysulfido ligands or several such ligands in combined form. The versatile structural chemistry of the complexes formed and their reactivity towards organic, inorganic and organometallic compounds are discussed, and examples of some simple and rational procedures for their synthesis starting from cyclopentadienylcarbonyl- and cyclopentadienylhydrido-complexes are outlined. Their reactivity is manifested in numerous metal- and ligandcentered reactions. Finally the, albeit far less extensive, complex chemistry of the other chalcogens (O, Se, Te) is also considered for comparison, thus providing a more detailed survey of particular aspects of this area of chemistry.     

Metal complexes with new types of chiral ligands based on natural terpenes

Some problems of the chemistry of coordination metal compounds with chiral ligands based on natural organic compounds, especially terpenes, are generalized. The results on the synthesis and study of the structure and properties of the chiral transition metal complexes with several groups of new “hybrid” ligands based on monoterpenes pinene, (+)-3-carene, and limonene and sesquiterpene caryophyllene are considered.     

四氮唑-1-乙酸及其衍生物金属构筑配合物的研究进展

四氮唑-1-乙酸(Htza)类有机配体具有良好的配位能力和多样的配位形式,在配位化学领域得到了迅速且广泛的研究。本文总结了基于四氮唑-1-乙酸配体构筑的金属配合物的研究进展和这些金属配合物的结构特征,讨论了基于四氮唑-1-乙酸构筑的金属配合物在光学、磁性、催化方面的应用,并对这类配体的发展前景进行了展望。     

Methods for the Synthesis of μ-Hydrocarbon Transition Metal Complexes without Metal–Metal Bonds

Transition metal complexes in which hydrocarbons serve as σ,σ-, σ,π- or π,π-bound bridging ligands are currently of great interest. This review presents efficient and directed syntheses for such compounds, which often have very aesthetic structures. These reactions are among the most important reaction types in modern organometallic chemistry. They can be a useful aid for the synthesis of tailor-made compounds, for example, for models of catalytic processes and, specifically, for the construction of heterometallic compounds. We will discuss reactions of electrophilic complexes with nucleophilic ones, numerous transformations of (functionalized) hydrocarbons with metal complexes, the currently very topical complexes with bridging acetylide and carbide ligands, and organometallic polymers, which can be expected to have interesting and novel materials properties. Chisholm

1 M. H. Chisholm, Polyhedron 1988 , 7, 757–1077.

has described the importance of these complexes as follows: “Central to the development of polynuclear and cluster chemistry are bridging ligands and central to organometallic chemistry are metal–carbon bonds. Thus bridging ligands hold a pivotal role ins the development of Binuclear and polynuclear organometallic chemistry”.     

Metal nuclearity modulated four-, six-, and eight-connected entangled frameworks based on mono-, bi-, and trimetallic cores as nodes

To investigate the relationship between network connectivity and metal nuclearity, we designed and synthesized a series of three-dimensional (3D) entangled coordination frameworks based on different metal cores, namely [Zn(2)(bdc)(2)(L)(2)]2H(2)O (1), [Zn(bdc)(L)(0.5)] (2), [Zn(oba)(L)(0.5)] (3) and [Cd(3)(bdc)(3)(L)(2)(H(2)O)(2)] (4) by self-assembly of d(10) metal salts with the flexible long-chain ligand 1,4-bis(1,2,4-triazol-1-yl)butane (L), and with the rigid and nonrigid aromatic dicarboxylate ligands 1,4-benzenedicarboxylate (bdc) and 4,4'-oxybis(benzoate) (oba). Compound 1 exhibits a threefold interpenetrated diamondoid array typically based on a tetrahedral second building unit (SBU) at a single Zn center. Compound 2 adopts a threefold interpenetrated alpha-polonium-type network that is built from bimetallic cores as six-connected vertices. The structure of 3 also consists of dinuclear units; it comprises a novel (3,4)-connected threefold interpenetrated net with complex (4610)(46(2)10(3)) topology when single zinc centers act as four-connected nodes (or the alpha-Po topology if dinuclear units are considered as six-connected nodes). Compound 4, derived from a crosslinked fivefold interpenetrated diamond-like substructure, is an unusual example of a self-penetrating coordination framework displaying an unprecedented eight-connected 4(20)6(8) topology with trinuclear cadmium clusters as eight-connected nodes which, to our knowledge, not only defines a new topology for eight-connected coordination networks, but also represents the highest connected topology presently known for self-penetrating systems. Detailed structural comparison of these complexes indicates that the increase in metal nuclearity induces the progressive increase in the connectivities of the ultimate nets: that is, the metal nuclearity plays a significant role in tuning the connectivity of a specific network. The thermal and luminescent properties of these compounds are discussed.     

Dithiolene complexes containing N coordinating groups and corresponding tetrathiafulvalene donors

The chemistry of transition metal dithiolene complexes containing N coordinating groups and the corresponding TTF donors, is reviewed starting from the ligand synthesis to the coordination structures where these dithiolene complexes are used as bridging units. The dithiolene ligands containing N coordinating atoms present two coordination poles which can selectively bind different metals and act as bridging units in a variety of coordination architectures. The transition metal dithiolene complexes based on these N containing ligands and the corresponding TTF donors can be themselves regarded as ligands. These can be used to coordinate other metals, potentially leading to a diversity of hetero metallic coordination architectures. With the use of appropriate auxiliary ligands they can lead to discrete metal complexes. In addition they can lead to more extended polymeric structures of different dimensionality such as 1D chains, 2D layers or even 3D polymers can also be obtained.     

Synthesis and Characterization of Bis(pyridylimino)isoindolide Alkali Metal Complexes in Three Redox States

Non-innocent ligands (NILs) like bis(pyridylimino)isoindolide (BPI) play crucial roles in coordination chemistry, biosciences, catalysis and material sciences. Investigating the isolated redox states of NILs is inevitable for understanding their redox-activity and fine-tuning the properties of corresponding metal complexes. The limited number of fundamental studies on the coordination behavior and redox chemistry of reduced BPI species is suggested to hamper further applications of the title compounds. This work describes for the first time the isolation of alkali metal complexes of BPI and Me₂BPI in three different oxidation states and their characterization by means of NMR or EPR spectroscopy, DFT calculations, and SC-XRD studies. The latter revealed the connection between bond orders in the ligand scaffold and its oxidation state. The paramagnetic compound Me₂BPI-K₂ was isolated as a coordination copolymer with 18-crown-6, which enabled the characterization of the dianionic BPI radical. Furthermore, the so-far unknown trianionic state of BPI was reported by the isolation of BPI-K₃. This divulges an unprecedented bis(amidinato)isoindolide coordination mode.     

Extended Open-Chain Polyenides as Versatile Delocalized Anion Ligands for Metal Chain Clusters

Although small cyclic- and open-chain unsaturated hydrocarbon anions such as cyclopentadienide and open-chain pentadienide are used as the strongly electron-donating auxiliary ligands for metal complexes, more extended π-conjugated unsaturated hydrocarbon anions have rarely been used in coordination chemistry, despite their potential ability to serve as the multiply bridging π-ligands for metal clusters. This work reports isolation of metal chain clusters bearing the multi-dentate, open-chain extended unsaturated hydrocarbon anion ligands. The extended open-chain π-conjugated polyenyl ligands could effectively stabilize oxidized palladium chains, including an unprecedented [Pd₄]⁴⁺ chain.     

Metal Complexes of Indigo and of Some Related Ligands

While the first coordination compounds of indigo were reported over 100 years ago, a systematic study of the coordination chemistry of this important dye and also its structural “relatives” remained silent for over half a century. Only in the last two decades of the 20th century research on this topic started again keeps on growing ever since. It could be established, mainly by X-ray crystallography, but also by many other modern spectroscopic techniques, that deprotonated indigo, and its deprotonated oxidized and reduced forms act as mono- and bidentate chelate ligands in metal complexes     

Metal ion complexes of thioformin: A density functional study

Some metal ion complexes of thioformin have been investigated by the density functional approach in order to understand its coordination chemistry. Due to the presence of the soft sulfur atom in addition to the oxide ion, thiohydroxamates exhibit a different coordination chemistry compared to their hydroxamate counterparts. It is found that the oxide ion forms bonds with metal ions that are predominantly ionic in nature while the sulfur atom is involved in weak covalent bond formation. Most of the complexes are found to be tetrahedrally distorted square planar structures, showing distinct cis and trans isomers. The exception is Zn(II), which forms tetrahedral complexes. The stabilities of the complexes have been linked with their antibiotic activity.     

配位化学中新型金属配合物的电子结构与成键特点

研究配合物的几何电子构型、阐明配位键的本质是配位化学中重要的理论组成部分。本文在回顾配位化学基本的成键理论基础上,介绍几例近年来具有教科书级别的国内高水平原创工作,重点阐述具有独特芳香性、低氧化态、高配位数以及锕系金属的新型金属配合物的电子结构和成键特点,对丰富和拓展配位化学的基本理论具有重要意义。     

稀土-氨基多羧酸配合物的配位结构及变化规律I氨基三乙酸(nta)、乙二胺四乙酸(edta)和反式-1,2-环己二胺四乙酸(cydta)系列

本文综述了稀土金属离子与一系列氨基多羧酸配体(如nta(=氨基三乙酸),edta(=乙二胺四乙酸),cydta(=反式-1,2-环己二胺四乙酸),dtpa(=二乙三胺五乙酸)和ttha(=三乙四胺六乙酸)等)形成配合物的分子结构和晶体结构,发现了这些配合物的配位规律和结构变化,说明了稀土金属离子象其它过渡金属离子一样,与氨基多羧酸配体形成配合物的配位数和配位结构取决于稀土金属离子的离子半径,电子结构和配体形状以及配电离子等。     

Expanding the utility of proteins as platforms for coordination chemistry

Whether for constructing advanced materials and complex biological devices or for building sophisticated coordination complexes with diverse metal-based functions, proteins are nature's favorite building blocks. Yet, our ability to control the assembly of proteins or to use them as ligand platforms for inorganic chemistry has been somewhat limited. In this review, we highlight our work from the past four years, which has aimed to exploit the utility of a protein scaffold in both regards. First, by considering proteins as “simple” ligand platforms and controlling the metal coordination chemistry on their surfaces, we show how their self-assembly can be readily dictated by metal binding. Second, we show how metal-mediated protein self-assembly leads to novel metal centers buried within protein interfaces. While on one hand our studies have pointed out the challenges of using proteins as ligands, they have also revealed how the extensive, chemically-rich protein surfaces can be exploited to form a network of covalent and non-covalent interactions around interfacial metal centers, providing a powerful handle to control their coordination chemistry.