Is it True That the Normal Valence‐Length Correlation Is Irrelevant for Metal–Metal Bonds?

The most intriguing feature of metal–metal bonds in inorganic compounds is an apparent lack of correlation between the bond order and the bond length. In this study, we combine a variety of literature data obtained by quantum chemistry and our results based on the empirical bond valence model (BVM), to confirm for the first time the existence of a normal exponential correlation between the effective bond order (EBO) and the length of the metal–metal bonds. The difference between the EBO and the formal bond order is attributed to steric conflict between the (TM)_n cluster (TM=transition metal) and its environment. This conflict, affected mainly by structural type, should cause high lattice strains, but electron redistribution around TM atoms, evident from the BVM calculations, results in a full or partial strain relaxation.     

The Nitrogenase FeMo‐Cofactor Precursor Formed by NifB Protein: A Diamagnetic Cluster Containing Eight Iron Atoms

The biological activation of N₂ occurs at the FeMo‐cofactor, a 7Fe–9S–Mo–C–homocitrate cluster. FeMo‐cofactor formation involves assembly of a Fe_6–8–S_X–C core precursor, NifB‐co, which occurs on the NifB protein. Characterization of NifB‐co in NifB is complicated by the dynamic nature of the assembly process and the presence of a permanent [4Fe–4S] cluster associated with the radical SAM chemistry for generating the central carbide. We have used the physiological carrier protein, NifX, which has been proposed to bind NifB‐co and deliver it to the NifEN protein, upon which FeMo‐cofactor assembly is ultimately completed. Preparation of NifX in a fully NifB‐co‐loaded form provided an opportunity for Mössbauer analysis of NifB‐co. The results indicate that NifB‐co is a diamagnetic (S=0) 8‐Fe cluster, containing two spectroscopically distinct Fe sites that appear in a 3:1 ratio. DFT analysis of the ⁵⁷Fe electric hyperfine interactions deduced from the Mössbauer analysis suggests that NifB‐co is either a 4Fe²⁺–4Fe³⁺ or 6Fe²⁺–2Fe³⁺ cluster having valence‐delocalized states.     

Oligosilane‐ and Oligosiloxane‐bridged Phosphines – Synthesis,Structures, and Metalation

The range of molecular silicon phosphorus compounds has been extended by some new species containing oligosilane ((R₂Si)_n; n ≥ 2) or oligosiloxane ((R₂SiO)_mSiR₂; m ≥ 1) fragments bound to phosphorus atoms. Primary and secondary compounds of these types allow for the synthesis of metal derivatives. Such metalated species usually form oligomers and exhibit a versatile structural chemistry with cyclic, polycyclic, and cage‐like patterns. The main results obtained in the field of oligosilane‐ and oligosiloxane‐bridged phosphines will be presented below and the structures of the metal derivatives will be discussed. Moreover, the synthesis of an inorganic ligand on the basis of siloxane‐bridged phosphines will be presented. This compound opens up a new chapter in host‐guest chemistry.     

Synthesis of an All‐Ferric Cuboidal Iron–Sulfur Cluster [FeIII4S4(SAr)4]

An unprecedented, super oxidized all‐ferric iron–sulfur cubanoid cluster with all terminal thiolates, Fe₄S₄(STbt)₄ ( 3 ) [Tbt=2,4,6‐tris{bis(trimethylsilyl)methyl}phenyl], has been isolated from the reaction of the bis‐thiolate complex Fe(STbt)₂ ( 2 ) with elemental sulfur. This cluster 3 has been characterized by X‐ray crystallography, zero‐field ⁵⁷Fe Mössbauer spectroscopy, cyclic voltammetry, and other relevant physico‐chemical methods. Based on all the data, the electronic ground state of the cluster has been assigned to be S_tot=0.     

Control of the Transition between Ni‐C and Ni‐SIa States by the Redox State of the Proximal FeS Cluster in the Catalytic Cycle of [NiFe] Hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H₂. The electrons produced by the H₂ cleavage pass through three Fe–S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni‐SI_a, Ni‐C, and Ni‐R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni‐C and Ni‐SI_a states remain unrevealed. In this study, the FT‐IR spectra under light irradiation at 138–198 K show that the Ni‐L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni‐C and Ni‐SI_a states. The transition of the Ni‐C state to the Ni‐SI_a state occurred when the proximal [Fe₄S₄]_p^2+/+ cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe₄S₄]_p^2+/+ cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.     

Alkoxide Clusters of Molybdenum and Tungsten as Templates for Organometallic Chemistry: Comparison with Carbonyl Clusters of the Later Transition Elements

Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant d_π orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled d_π orbitals. Both ligands readily form bridges that span M? M bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)₃], [CpW(CO)₂], [Co(CO)₃], and CH are related by the isolobal analogy. Thus the compounds [(RO)₃W ? W(OR)₃], [Cp(CO)₂W?W(CO)₂Cp], hypothetical [(CO)₃Co?Co(CO)₃], and HC?CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W₃(μ₃-CR)(OR′)₉] versus [Co₃(μ₃-CR)(CO)₉] and [W₄(C)(NMe)(OiPr)₁₂] versus [Fe₄(C)(CO)₁₃]—but differ with respect to M? M bonding. The carbonyl clusters use e_g-type orbitals for M? M bonding whereas the alkoxide clusters employ t_2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M? M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M₂(OR)₆] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry.     

Interpenetrated Cage Structures

This Review covers design strategies, synthetic challenges, host–guest chemistry, and functional properties of interlocked supramolecular cages. Some dynamic covalent organic structures are discussed, as are selected examples of interpenetration in metal–organic frameworks, but the main focus is on discrete coordination architectures, that is, metal‐mediated dimers. Factors leading to interpenetration, such as geometry, flexibility and chemical makeup of the ligands, coordination environment, solvent effects, and selection of suitable counter anions and guest molecules, are discussed. In particular, banana‐shaped bis‐pyridyl ligands together with square‐planar metal cations have proven to be suitable building blocks for the construction of interpenetrated double‐cages obeying the formula [M₄L₈]. The peculiar topology of these double‐cages results in a linear arrangement of three mechanically coupled pockets. This allows for the implementation of interesting guest encapsulation effects such as allosteric binding and template‐controlled selectivity. In stimuli‐responsive systems, anionic triggers can toggle the binding of neutral guests or even induce complete structural conversions. The increasing structural and functional complexity in this class of self‐assembled hosts promises the construction of intelligent receptors, novel catalytic systems, and functional materials.     

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”.     

New Developments in Nitrogen Fixation

The production of ammonia from atmospheric dinitrogen at room temperature and ambient pressure in analogy to nature is a long-term goal for coordination chemists. Novel reactions of N₂-containing transition metal complexes with H₂, the first side-on N₂-bridged structure of an actinide complex, and an interesting variation of synthetic N₂ fixation are the key points addressed in this contribution. The results are related to the known chemistry of N₂ complexes, and their significance is discussed with respect to enzymatic N₂ fixation.     

Metallacarboranes of the C2B4-cage systems

A comprehensive review of the chemistry of polyhedral cluster complexes in whichs,p,d, andf-block metals are incorporated into C₂B₄-carborane cages to form the respective metallacarboranes is presented. The main focus of the review is directed toward systematizing the chemistry of metallacarboranes in order to promote their use as possible electronic, ceramic, and/or catalytic materials in addition to providing information on which we can test and expand our knowledge about the fundamental interactions that are at work in determining the structures and properties of these cluster complexes.     

Luminescent Metal–Organic Frameworks (MOFs) as a Chemopalette: Tuning the Thermochromic Behavior of Dual‐Emissive Phosphorescence by Adjusting the Supramolecular Microenvironments

Two classical copper(I)‐cluster‐based luminophores, namely, Cu₄I₄ and [Cu₃Pz₃]₂ (Pz=pyrazolate), are immobilized in a supramolecular system through the formation of metal–organic framework (MOF) materials. This series of luminescent MOF materials, namely, [Cu₄I₄(NH₃)Cu₃( L1 )₃]_n, [Cu₄I₄(NH₂CH₃)Cu₃( L1 )₃]_n, and [Cu₄I₄Cu₃( L2 )₃]_n ( L1 =3‐(4‐pyridyl)‐5‐(p‐tolyl)pyrazolate; L2 =3‐(4‐pyridyl)‐5‐(2,4‐dimethylphenyl)pyrazolate), exhibit diverse thermochromism attributed to the relative functioning efficacy of the two coordination luminophores. Such an intriguing chemopalette effect is regulated by the different supramolecular microenvironments between the two‐dimensional layers of these MOFs, and in particular, by the fine‐tuned Cu–Cu distances in the excimeric [Cu₃Pz₃]₂ luminophore. The structure–property elucidation of the thermochromic behavior allows one to understand these optical materials with unusual dual‐emissive properties.     

Applications of Pillar[n]arene‐Based Supramolecular Assemblies

Macrocycles are an important player in supramolecular chemistry. In 2008, a new class of macrocycles, “pillar[n]arenes”, were first discovered. Research efforts in the area of pillar[n]arenes have elucidated key properties, such as their shape, reaction mechanism, host–guest properties, and their versatile functionality, which has contributed to the development of pillar[n]arene chemistry and their applications to various fields. This Minireview describes how pillar[n]arene‐based supramolecular assemblies can be applied to supramolecular gel formation, reactions, light‐harvesting systems, drug‐delivery systems, biochemical applications, separation and storage materials, and surface chemistry.     

Synthesis of a new iron–sulfur cluster compound and its photocatalytic H2 evolution activity through visible light irradiation

A new iron–sulfur cluster compound, namely [(μ‐BNT)Fe₂(CO)₆] ( A ; BNT = (R)‐1,1′‐binaphthalene‐2,2′‐dithiol), was synthesized by self‐assembly of BNT with [Fe₃(CO)₁₂] and characterized using ¹H NMR, ¹³C NMR, infrared spectra and elemental analysis. The H₂ evolution activity of A was evaluated in a constructed homogeneous photocatalytic system by combining A as catalyst, xanthene dyes as photosensitizer and triethylamine as sacrificial reagent, to give efficient H₂ generation under visible‐light irradiation (λ > 420 nm). The maximum H₂ evolution of 404 turnovers (versus catalyst) was recorded under optimal conditions in CH₃CN–H₂O (1:1, v/v) after 4 h irradiation. The mechanism of H₂ evolution is briefly discussed using fluorescence spectra and electrochemical analysis. Copyright © 2016 John Wiley & Sons, Ltd.     

An Anti CuO2‐type Metal Hydride Square Net Structure in Ln2M2As2Hx (Ln=La or Sm,M=Ti,V, Cr,or Mn)

Using a high pressure technique and the strong donating nature of H^?, a new series of tetragonal La₂Fe₂Se₂O₃‐type layered mixed‐anion arsenides, Ln₂M₂As₂H_x, was synthesized (Ln=La or Sm, M=Ti, V, Cr, or Mn; x≈3). In these compounds, an unusual M₂H square net, which has anti CuO₂ square net structures accompanying two As^3? ions, is sandwiched by (LaH)₂ fluorite layers. Notably, strong metal–metal bonding with a distance of 2.80 Å was confirmed in La₂Ti₂As₂H_2.3, which has metallic properties. In fact, these compounds are situated near the boundary between salt‐like ionic hydrides and transition‐metal hydrides with metallic characters.     

Sequential Chemistry Toward Core–Shell Structured Metal Sulfides as Stable and Highly Efficient Visible‐Light Photocatalysts

A universal sequential synthesis strategy in aqueous solution is presented for highly uniform core–shell structured photocatalysts, which consist of a metal sulfide light absorber core and a metal sulfide co‐catalyst shell. We show that the sequential chemistry can drive the formation of unique core–shell structures controlled by the constant of solubility product of metal sulfides. A variety of metal sulfide core–shell structures have been demonstrated, including CdS@CoS_x, CdS@MnS_x, CdS@NiS_x, CdS@ZnS_x, CuS@CdS, and more complexed CdS@ZnS_x@CoS_x. The obtained strawberry‐like CdS@CoS_x core–shell structures exhibit a high photocatalytic H₂ production activity of 3.92 mmol h^?1 and an impressive apparent quantum efficiency of 67.3 % at 420 nm, which is much better than that of pure CdS nanoballs (0.28 mmol h^?1), CdS/CoS_x composites (0.57 mmol h^?1), and 5 %wt Pt‐loaded CdS photocatalysts (1.84 mmol h^?1).     

Alkaline‐Earth Metal Cations as Structure Building Blocks for Molecular Cages with Entrapment and Controlled Release of Quintuple Ionic Aggregates

Currently, main‐group metal cations are totally neglected as the structure‐building blocks for the self‐assembly of supramolecular coordination metallocages due to the lack of directional bonding. However, here we show that a common Arrhenius acid–base neutralization allows the alkaline‐earth metal cations to act as charged binders, easily connecting two or more highly directional anionic transition‐metal‐based metalloligands to coordination polymers. With a metal salt such as K⁺PF₆^? added during the neutralization, the main‐group metal‐connected skeleton can be templated by the largest yet reported ionic‐aggregate anion, K₂(PF₆)₃^?, formed from KPF₆ in solution, into molecular metallocages, encapsulating the ion. Crystal‐structure details, DFT‐calculation results, and controlled‐release behavior support the presence of K₂(PF₆)₃^? as a guest in the cage. Upon removal of PF₆^? ions, the cage stays intact. Other ions like BF₄^? can be put back in.     

Encapsulation of a CrIII Single‐Ion Magnet within an FeII Spin‐Crossover Supramolecular Host

Single functional molecules are regarded as future components of nanoscale spintronic devices. Supramolecular coordination chemistry provides unlimited resources to implement multiple functions to individual molecules. A novel coordination [Fe₂] helicate exhibiting spin‐crossover is demonstrated to be ideally suited to encapsulate a [Cr(ox)₃]^3? complex anion (ox=oxalate), unveiling for the first‐time single ion slow relaxation of the magnetization for this metal. A possibility of tuning the dynamics of this relaxation as well as the performance of the Cr^III center as qubit arises from the observation that metastable high spin Fe^II centers from the host can be generated by irradiation with green light at low temperature.     

Higher Nuclearity Rhenium Halide Clusters That Contain Re–Re Multiple Bonds

The discovery that the rhenium chloride cluster Re₃Cl₉ and its chloro-anion [Re₃Cl₁₂]^3- contain a triangular cluster of Re atoms and Re–Re bond orders of two was first reported in 1963. This led very quickly to the report by F. A. Cotton of the first species with a metal–metal quadruple bond, namely, the [Re₂Cl₈]^2- anion. While the field of dinuclear multiple bond chemistry has undergone explosive growth in the last 40 years, that involving the chemistry of higher nuclearity clusters with metal–metal multiple bonds is much more limited. One of the relatively few exceptions is encountered with the aforementioned rhenium halides that contain the triangular [Re₃]⁹⁺ core. In the present review, the synthesis, structures and reactivity of these halide clusters and those of higher nuclearity that contain Re–Re multiple bonds are surveyed. This article provides a comprehensive review of the literature covering the 40 year period 1963–2003.     

Controlling the Reactivity of the SeSe Bond by the Supramolecular Chemistry of Cucurbituril

We describe a new strategy to control the reactivity of Se?Se bond by using supramolecular chemistry of cucurbituril. We have demonstrated that selenocystamine (SeCy) and cucurbit[6]uril (CB[6]) can form a stable supramolecular complex (K_a=5.5×10⁶ M ^?1). Before complexation, the free Se?Se bond in SeCy is rather sensitive to redox stimuli and gets disrupted quickly with addition of reductant or oxidant. However, after binding with CB[6], the Se?Se bond becomes quite inert and hardly reacts with reductant or oxidant. One advantage of this supramolecular protection is that it can be applied in a wide pH range from weakly acidic to basic. Additionally, the supramolecular complex formed by SeCy and CB[6] can be reversibly dissociated simply with addition of Ba²⁺.     

Red‐Light‐Induced Decomposition of an Organic Peroxy Radical: A New Source of the HO2 Radical

The gas‐phase decomposition of the α‐hydroxy methylperoxy radical has been theoretically examined, and the results provide insight into a new source of the hydroperoxy radical (HO₂) in the troposphere. Bimolecular peroxy decomposition is promoted by the red‐light or near‐IR radiation excitation. The calculations suggest for the first time, an important chemical role for the H₂O?HO₂ radical complex that exist in significant abundance in the troposphere. In particular, the reaction of organic peroxy radicals with the HO₂ radical and the H₂O?HO₂ radical complex represent an autocatalytic source of atmospheric HO₂. This reaction is a new example of red‐light‐initiated atmospheric chemistry that may help in understanding the discrepancy between the observed and measured levels of the HO_x at sunrise.