Guidelines to design new spin crossover materials

This review focuses on new families of spin crossover (SCO) complexes based on polynitrile anions as new anionic ligands or on polyazamacrocycles as neutral macrocyclic ligands. We have shown that the structural and electronic characteristics (original coordination modes and high electronic delocalization) of the polynitrile anions can be tuned by slight chemical modifications such as substitution of functional groups or variation of the negative charge to design new discrete or polymeric SCO systems.In our ongoing work on the design of new molecular systems based on new ligands that can be fine-tuned via chemical modifications, another promising way which has been recently developed in our group concerns the use of new neutral polydentate ligands which are able to tune the ligand field energy around the metal centre. Here we report some recent original Fe(II) SCO complexes based on such polydentate ligands.     

Spin‐Übergänge in supramolekularen Strukturen. Für Speicherbausteine von morgen?

We reported about octahedrally coordinated Fe2+‐complexes, which are able to switch between two stable spin states (LS and HS) with different magnetic properties. This phenomenon is called spin crossover (SCO). The interaction between metal ion and ligand determines the actual spin state and whether an extern stimulus can trigger a spin crossover. Due to this fact it is possible for the chemist through the choice of the ligands to manipulate the character and the temperature region of the SCO. Some metal complexes assemble into highly ordered structures on graphite by molecular self assembly. The substitution of the metal complexes with alkyl chains and the interaction of these chains with the highly ordered graphite is crucial for a periodic arrangement of the complexes on the surface. For the future we are curious to see whether through the cooperative effort of coordination chemistry (SCO phenomenon) and surface science (self assembly of SCO complexes on a surface) the vision of a molecular memory will turn into a reality.     

Spin crossover active iron(II) complexes of selected pyrazole-pyridine/pyrazine ligands

This review begins with a brief introduction to pyrazole and to spin crossover. The focus then moves to a detailed consideration of the synthesis and magnetic properties of structurally characterized iron(II) spin crossover (SCO) active complexes of pyrazole- and pyrazolate-based ligands that also contain at least one pyridine or pyrazine unit within the ligand motif. The syntheses and crystallization methods reported in the original publications are emphasized in this review. The reason for this is that these factors often affect the exact nature of the final product, including the amount and nature of the crystallization solvent molecules present and/or what polymorph is obtained, and hence they can impact strongly on the SCO properties of the resulting materials, as can be seen in this review.     

A Joint Study of Experimental and Computational Chemistry to Probe the Effect of Coordination Modes on the Luminescence of Uranyl-Organic Coordination Polymers

Despite the potential application of uranyl-organic coordination polymers (UOCPs) in detecting metal cations and radiation rays, their luminescence properties have not been sufficiently studied at the molecular level. Herein, we synthesized a series of UOCPs ( UOCP1 – 9 ) with new construction based on bipyridinium salts and diverse auxiliary ligands. The physicochemical properties of the complexes with high purity were systematically characterized, especially the luminescence spectra. A deep theoretical investigation was conducted to illustrate the relation between uranyl coordination and the luminescence spectral property. The results show that the influence of ligand type on the uranyl luminescent spectral properties increases in the order of hydroxide>oxalate>aromatic carboxylate, which can be attributed to the magnitude of orbital interaction between uranyl and ligand. This work helps to profoundly understand uranyl optical properties and electronic structure at the molecular level, providing important hints for recognizing the nature of luminescent spectral features of uranyl-containing materials, as well as the fundamental chemistry of actinide elements.     

Ligand-driven light-induced spin transition in spin crossover compounds

Spin crossover (SCO) coordination compounds that show bistability between low spin and high spin states are promising light-controllable molecular switches. Selective wavelength irradiation of the coordination centre at low temperatures is known as a light-induced excited spin state trapping (LIESST effect) and it leads to the modulation of physical properties of SCO materials on the macroscopic as well as on the molecular level. Another way to trigger the spin state conversion by light is based on the isomerization of photoactive ligand moieties. The ligand field strength is changed due to light-induced photoisomerization and, therefore, corresponding cis–trans or ring-closing/ring-opening isomeric couples might exhibit different spin states at isothermal conditions. Such an approach is called as ligand driven light-induced spin change (LD LISC effect). From the application point of view, it presents a promising alternative to the LIESST effect because it can operate at room temperature. This article is focused on the most interesting iron and cobalt SCO compounds with photoisomerizable ligands and provides the overview of achieving results based on the LD LISC effect.     

A New Family of Anionic FeIII Spin Crossover Complexes Featuring a Weak‐Field N2O4 Coordination Octahedron

Unprecedented anionic Fe^III spin crossover (SCO) complexes involving a weak‐field O,N,O‐tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO?1 of a ligand might contribute to the enhancement of a ligand‐field splitting energy. The present anionic SCO complex also exhibited the light‐ induced excited‐spin‐state trapping effect.     

Hexakis-adducts of [60]fullerene as molecular scaffolds of polynuclear spin-crossover molecules

A family of hexakis-substituted [60]fullerene adducts endowed with the well-known tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand for spin-crossover (SCO) systems has been designed and synthesized. It has been experimentally and theoretically demonstrated that these molecular scaffolds are able to form polynuclear SCO complexes in solution. UV-vis and fluorescence spectroscopy studies have allowed monitoring of the formation of up to six Fe(ii)–bpp SCO complexes. In addition, DFT calculations have been performed to model the different complexation environments and simulate their electronic properties. The complexes retain SCO properties in the solid state exhibiting both thermal- and photoinduced spin transitions, as confirmed by temperature-dependent magnetic susceptibility and Raman spectroscopy measurements. The synthesis of these complexes demonstrates that [60]fullerene hexakis-adducts are excellent and versatile platforms to develop polynuclear SCO systems in which a fullerene core is surrounded by a SCO molecular shell.

Polynuclear spin-crossover molecules showing both thermal and photoinduced spin transitions have been prepared using a [60]fullerene hexakis-adduct endowed with Fe(ii) complexes of tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand.     

2-[(2,4-二氯苯胺基)羰基]苯甲酸与过渡金属配合物的研究

本文报道了2-[2,4-二氯苯胺基)羰基]苯甲酸与5种过渡金属配合物的制备和表征,证实配合物是通过羧酸根上羟基氧原子和酰按羰基上氧原子配位成键,除Cu(Ⅱ)配合物分子为平面正方形结构外,其余均为八面体结构,只是扭曲程度不同。     

1,10-Phenanthroline: A versatile building block for the construction of ligands for various purposes

This review will cover the developments in the chemistry of phenanthroline-based ligands in the last 10–15 years. 1,10-Phenanthroline (phen) is a classic ligand in coordination chemistry, which couples versatility in metal ion binding with peculiar properties of its complexes. For instance, metal complexes with phenanthroline can be featured by an intense luminescence or can interact with DNA in an intercalative fashion inducing, in some cases, DNA cleavage. For this reason a number of phenanthroline-containing ligands has been recently synthesized by inserting phenanthroline within open-chain or macrocyclic backbone, in order to develop new molecular chemosensors for metal cations and anions, ionophores as well as new intercalating agents for polynucleotides. Furthermore, phenanthroline is rigid and its insertion within cyclic or acyclic structures can impart to the resulting ligand a high degree of pre-organization, affording selective complexing agents. This review will discuss on the coordination, luminescence and intercalating and/or DNA cleaving properties as well as on analytical applications of metal complexes with phenanthroline-based ligands. Particular attention will be devoted to macrocyclic receptors or open-chain ligands that, beside the phenanthroline nitrogen atoms, contain other donor atoms able to interact with the metal cations or anions.     

Review: Recent advances of one-dimensional coordination polymers as catalysts

This review aims to provide reports of one dimensional (1-D) coordination polymers that have been used as catalysts in various organic reactions in the last decade, covering the literature from 2007 and onwards. The CPs have been mainly categorized into homometallic and heterometallic compounds; additional parameters such as the metal and ligand selection for the CP are discussed to provide a more detailed look into each system.     

Main-chain organometallic polymers: synthetic strategies, applications, and perspectives

Main-chain organometallic polymers utilize transition metal-organic ligand complexes as primary components of their backbones. These hybrid materials effectively integrate the physical and electronic properties of organic polymers with the physical, electronic, optical, and catalytic properties of organometallic complexes. Combined with the rich and continuously growing array of ligands for transition metals, these materials have outstanding potential for use in a broad range of applications. This tutorial review discusses the major classes of main-chain organometallic polymers, including coordination polymers, poly(metal acetylide)s, and poly(metallocene)s. Emphasis is placed on their synthesis, characterization, physical properties, and applications, as well as ongoing challenges and limitations. These discussions are supplemented with highlights from the recent literature. The review concludes with perspectives on the current status of the field, as well as opportunities that lie just beyond its frontier.     

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.     

Spin-crossover in cobalt(II) compounds containing terpyridine and its derivatives

In this review article we discuss the unique and novel magnetic properties for the cobalt(II) compounds with a variety of terpy derivatives including substituents at the 4-position. These are also compared with the unsubstituted terpy cobalt(II) complex. Since the first SCO cobalt(II) complex with terpy ligand was reported, this system has been widely studied. SCO cobalt(II) complexes possessing terpy or OH-terpy ligand reveal incomplete or gradual SCO behavior. The pyterpy-appended cobalt(II) complex shows SCO depending on the guest molecules involved. Cobalt(II) complexes with long-alkylated terpy ligands, [Co(Cn-terpy)₂](BF₄)₂ (n = 16, 14 and 12) have been synthesized and some were characterized by single crystal X-ray analysis. Furthermore, variable-temperature magnetic susceptibility indicated that the non-solvated compounds [Co(Cn-terpy)₂](BF₄)₂ (n = 16, 14 and 12) exhibit “reverse spin transition” phenomenon with wide thermal hysteresis around room temperature. In addition, the solvated compound [Co(C12-terpy)₂](BF₄)₂·EtOH·0.5H₂O shows “re-entrant SCO” behavior. The long alkyl chains in SCO cobalt(II) complexes can lead to novel physical properties resulting from a synergetic effect between SCO and response of the flexibility toward external stimuli.     

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。     

Coordination complexes incorporating pyrophosphate: Structural overview and exploration of their diverse magnetic,catalytic and biological properties

Current attention continues to revolve around the chemistry and biochemistry associated with polyphosphate anions because of their importance in biology. A pivotal intermediate within this family is the pyrophosphate tetraanion, P₂O₇^4?. Considering its biological relevance and the multidentate nature that makes it an ideal ligand in the field of the coordination chemistry, there is a growing interest in the use of this anion in building new class of molecules/compounds for different purposes. While the total number of characterized structures still remains modest, several new pyrophosphate-containing coordination complexes have been reported in the last decade, as well as different solid-state structures. This review focuses on the structural, magnetic, and biological properties of coordination complexes incorporating the pyrophosphate ligand reported to date.     

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。     

Formation and stability of phytate complexes in solution

1,2,3,4,5,6 hexakis (di-hydrogen phosphate) myo-inositol, best known as phytic acid, is a very important molecule from a biological, environmental and technological point of view. For a thorough understanding of phytate properties and the mechanisms involving this ligand, a careful study of its acid–base behavior and of the formation and stability of its complexes in solution is necessary. Unfortunately, regarding the thermodynamic data on phytate complexes in solution, some are lacking, while some others exhibit large discrepancies between different authors. This motivated a detailed evaluation of the literature on this topic, aimed at identifying the most accurate data on phytate coordination chemistry in solution. This review presents the results of this, reporting and analyzing the most significant thermodynamic parameters published for both phytate protonation and complex formation with several metal and organometal cations, as well as polyammonium ligands.     

Effect of substitution and the counterion on the structural and spectroscopic properties of CuII complexes of methylated pyrazoles

The coordination chemistry of pyrazole and three of its methyl derivatives with the chloride and nitrate salts of copper(II) under strictly controlled reaction conditions is systematically explored to gain a better understanding of the effect of counterion coordination strength and ligand identity on the structure and electronic absorption spectra of their resulting complexes. Despite the initial 2 : 1 ligand to metal ratio in water, copper(II) nitrate forms exclusively 4 : 1 ligand to metal complexes while copper(II) chloride forms a 4 : 1 ligand to metal complex only with pyrazole, with the methyl derivatives forming 2 : 1 ligand to metal complexes, as determined by single-crystal X-ray diffraction (XRD). This is attributed to a combination of ligand sterics and stronger coordination of chloride relative to nitrate. Electronic absorption spectroscopy in both water and methanol reveals a surprisingly strong effect of the pyrazole methyl position on the Cu^II d–d transition, with 4-methylpyrazole producing a higher energy d–d transition relative to the other ligands studied. In addition, the number of methyl groups plays a determining role in the energy of the pz π→Cu^II d_xy LMCT band, lowering the transition energy as more methyl groups are added.     

Synthesis,pyrolytic and bioassay studies of complexes Ni(II), Cu(II) and Zn(II) bromides with a chelating ligand of N,N donor sites

Coordination complexes of modified hydrazine are prepared with Ni(II), Cu(II), and Zn(II) metal ions. The ligand is synthesized by removing the methoxy moiety of methyl anthranilate with nitrogen of hydrazine hydrate, creating new coordination site. The coordination complexes are synthesized by reacting the ABH ligand with dehydrated M(II) [Cu²⁺, Zn²⁺ and Ni²⁺] bromide in an inert environment. The structures of the coordination complexes are elucidated basing on the physical measurements including elemental analysis, NMR, IR, UV–Vis spectra, magnetic and conductance measurements. These results reflect the M(ABH)Br₂ composition of the corresponding complexes. Thermal studies show the Irving William trend for the stability of complexes. Antibacterial activities and antifungal studies are also carried out in order to investigate the biological activity upon complexation.     

Synthesis and structures of cadmium(II) complexes of a series of multinucleating N/S donor ligands

The coordination chemistry of a series of bis-bidentate ligands with cadmium(II) ions has been investigated. The ligands, containing two N,S-donor chelating (pyrazolyl/thioether) fragments, have afforded complexes of a variety of structural types (dinuclear M₂L₂ ‘mesocate’ complexes, a one-dimensional chain coordination polymer and a simple mononuclear complex) according to whether the bis-bidentate ligands act as bridges spanning two metal ions, or a tetradentate chelate to a single metal ion. The p-phenylene and m-biphenyl spaced ligands L¹ and L³ form dinuclear M₂L₂ complexes where the ligands are arranged in a ‘side-by-side’ fashion. In contrast the m-phenylene spaced ligand L² forms a one-dimensional coordination polymer where the ligands adopt a highly folded conformation. The 1,8-naphthalene spaced ligand L⁴ adopts a tetradendate chelating mode and affords a simple mononuclear complex.