Homometallic Rare‐Earth Metal Phosphinidene Clusters: Synthesis and Reactivity

Two new trinuclear μ₃‐bridged rare‐earth metal phosphinidene complexes, [{L(Ln)(μ‐Me)}₃(μ₃‐Me)(μ₃‐PPh)] (L=[PhC(NC₆H₄iPr₂‐2,6)₂]^?, Ln=Y ( 2 a ), Lu ( 2 b )), were synthesized through methane elimination of the corresponding carbene precursors with phenylphosphine. Heating a toluene solution of 2 at 120 °C leads to an unprecedented ortho C? H bond activation of the PhP ligand to form the bridged phosphinidene/phenyl complexes. Reactions of 2 with ketones, thione, or isothiocyanate show clear phospha‐Wittig chemistry, giving the corresponding organic phosphinidenation products and oxide (sulfide) complexes. Reaction of 2 with CS₂ leads to the formation of novel trinuclear rare‐earth metal thione dianion clusters, for which a possible pathway was determined by DFT calculation.     

Steuerliganden für Katalysatoren

Ever since the isolation of the first free N‐heterocyclic carbene more than 20 years ago, this ligand class has become essential in modern chemistry. This development is decisively owed to the stability of the metal‐carbon bond, the smooth fine‐tuning of ligand properties, and the facile synthetic access to such complexes. Acting as steering ligands, they are integral parts of many organometallic compounds that currently push the frontiers of chemistry in many areas, for example the synthesis and application of immobilized, water soluble, or asymmetric systems that act as efficient catalysts in organic transformations. Additionally, metal NHC complexes find promising applications in fields beyond catalysis, such as medicine, optics, and material science.     

When VSEPR Fails: Experimental and Theoretical Investigations of the Behavior of Alkaline‐Earth‐Metal Acetylides

The synthesis, structural, and spectral characterization as well as a theoretical study of a family of alkaline‐earth‐metal acetylides provides insights into synthetic access and the structural and bonding characteristics of this group of highly reactive compounds. Based on our earlier communication that reported unusual geometry for a family of triphenylsilyl‐substituted alkaline‐earth‐metal acetylides, we herein present our studies on an expanded family of target derivatives, providing experimental and theoretical data to offer new insights into the intensively debated theme of structural chemistry in heavy alkaline‐earth‐metal chemistry.     

Recent Advances in the Synthesis of Heterocycles and Related Substances Based on α‐Imino Rhodium Carbene Complexes Derived from N‐Sulfonyl‐1,2,3‐triazoles

In recent years, α‐imino rhodium carbene complexes derived by ring‐opening of N‐sulfonyl‐1,2,3‐triazoles have attracted much attention from organic chemists. Many transformations of these species have been reported that involve, in most cases, nucleophilic attack at the carbene center of the α‐imino rhodium carbene, facilitating the synthesis of a wide range of novel and useful compounds, particularly heterocycles. This Minireview mainly focuses on advances in the transformation of N‐sulfonyl‐1,2,3‐triazoles during the past two years.     

Boron(II) Cations: Interplay between Lewis‐Pair‐Acceptor and Electron‐Donor Properties

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late‐transition‐metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π‐acceptor character of the neutral σ‐donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron‐donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.     

Bis(η5:η1‐pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives

Transition‐metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low‐valent bis(η⁵:η¹‐(di‐p‐tolyl)‐pentafulvene)niobium chloride. Owing to the π‐η⁵:σ‐η¹ coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η⁵:η¹‐pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η⁵:η¹ cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α‐hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N?H bond activation of primary amines to niobium imido complexes.     

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes,gem‐Difluoroalkenes,and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value‐added fluorinated organic compounds. Recent progress has been mainly focused on the transition‐metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B?H activation of N‐heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom‐economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem‐difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition‐metal free, and the regioselectivity obtained is complementary to the reported transition‐metal‐catalysis in many cases.     

Chiral N‐Heterocyclic Carbene Ligands Enable Asymmetric C−H Bond Functionalization

The asymmetric functionalization of C?H bond is a particularly valuable approach for the production of enantioenriched chiral organic compounds. Chiral N‐heterocyclic carbene (NHC) ligands have become ubiquitous in enantioselective transition‐metal catalysis. Conversely, the use of chiral NHC ligands in metal‐catalyzed asymmetric C?H bond functionalization is still at an early stage. This minireview highlights all the developments and the new advances in this rapidly evolving research area.     

Molecular Rare‐Earth‐Metal Hydrides in Non‐Cyclopentadienyl Environments

Molecular hydrides of the rare‐earth metals play an important role as homogeneous catalysts and as counterparts of solid‐state interstitial hydrides. Structurally well‐characterized non‐metallocene‐type hydride complexes allow the study of elementary reactions that occur at rare‐earth‐metal centers and of catalytic reactions involving bonds between rare‐earth metals and hydrides. In addition to neutral hydrides, cationic derivatives have now become available.     

β-Tosylamino and/3-Hydroxyl Substituted α-Diazocarbonyl Compounds： X-ray Structure and 1,2-Migrations under Photochemical Condition

As a continuation of the investigation on 1,2-migration of metal carbene and free carbene, two X-ray structures of β-substituted α-diazocarbonyl compounds were obtained. The relationship between the structure and 1,2-migratory aptitude was discussed and an exploratory photochemical study of the diazo compounds presented.     

Asymmetric Catalysis Mediated by the Ligand Sphere of Octahedral Chiral‐at‐Metal Complexes

Due to the relationship between structure and function in chemistry, access to novel chemical structures ultimately drives the discovery of novel chemical function. In this light, the formidable utility of the octahedral geometry of six‐coordinate metal complexes is founded in its stereochemical complexity combined with the ability to access chemical space that might be unavailable for purely organic compounds. In this Minireview we wish to draw attention to inert octahedral chiral‐at‐metal complexes as an emerging class of metal‐templated asymmetric “organocatalysts” which exploit the globular, rigid nature and stereochemical options of octahedral compounds and promise to provide new opportunities in the field of catalysis.     

Dimeric Rare‐Earth BINOLate Complexes: Activation of 1,4‐Benzoquinone through Lewis Acid Promoted Potential Shifts

Reaction of p‐benzoquinone (BQ) with a series of rare‐earth metal/alkali metal/1,1′‐BINOLate (REMB) complexes (RE: La, Ce, Pr, Nd; M: Li) results in the largest recorded shift in reduction potential observed for BQ upon complexation. In the case of cerium, the formation of a 2:1 Ce/BQ complex shifts the two‐electron reduction of BQ by greater than or equal to 1.6 V to a more favorable potential. Reactivity investigations were extended to other RE^III (RE=La, Pr, Nd) complexes where the resulting highly electron‐deficient quinone ligands afforded isolation of the first lanthanide quinhydrone‐type charge‐transfer complexes. The large reduction‐potential shift associated with the formation of 2:1 Ce/BQ complexes illustrate the potential of Ce complexes to function both as a Lewis acid and an electron source in redox chemistry and organic‐substrate activation.     

f‐Element–Metal Bonding and the Use of the Bond Polarity To Build Molecular Intermetalloids

Metal–metal bonding in heterobimetallic complexes is of fundamental interest due to its implications to both bonding theory and new reactivities. In this Concept, structurally authenticated molecular compounds with direct bonds between rare‐earth metals or actinoids and transition or main group metals are summarized. Special attention is given to the use of bond polarity as a tool for designing molecular intermetalloids incorporating rare‐earth atoms and transition metals.     

Recent Advances in Rare Earth‐Doped Hydrides

The optical properties of rare earth ions in different inorganic host materials, for instance oxides, silicates, borates, or nitrides, have been used in applications for many years, from color TV to fluorescent tubes, lasers, or pc‐LEDs. However, rare earth metal ion‐doped hydrides have not really been considered as host lattices and up to now only been studied in a relatively small number of investigations. Yet, for certain metal hydrides these studies, e.g., allowed the determination of the crystal field strength and nephelauxetic effect of the hydride anion using the Eu²⁺ 5d excited state. In air‐sensitive hydrides, the use may be restricted to fundamental studies and local probes. But recently more and more air‐stable mixed anionic hydrides have been discovered, which may serve as hosts. This short review summarizes the synthesis and characterization of rare earth metal ion‐doped hydrides reported so far.     

Bulky Amido Ligands in Rare Earth Chemistry — Syntheses,Structures, and Catalysis

The amido metal chemistry of the rare earth elements is a rapid developing area in coordination chemistry. Especially bulky mono and bidentate amido and amidinates have been introduced as ligands in rare earth chemistry. Due to these sterically demanding ligands, the coordination numbers of the rare earth elements are significantly reduced. This article focuses on two of these bulky ligand systems: bis(trimethylsilyl)amide and aminotroponiminates. The homoleptic bis(trimethylsilyl)amides of rare earth elements, [Ln{N(SiMe₃)₂}₃], are well established compounds in synthetic chemistry. Therefore, this article reviews recent progress in the catalytic application of these compounds. In the second part of this research report, it is shown that N, N′‐disubstituted aminotroponiminates and mono bridged bisaminotroponiminates can be used as cyclopentadienyl alternatives. Achiral and chiral aminotroponiminates have been used. The structural properties, reactivities as well as the catalytic and synthetic applications of the aminotroponiminates complexes will be outlined in this article.     

Electro‐kinetic Separation of Rare Earth Elements Using a Redox‐Active Ligand

Purification of rare earth elements is challenging due to their chemical similarities. All of the deployed separation methods rely on thermodynamic properties, such as distribution equilibria in solvent extraction. Rare‐earth‐metal separations based on kinetic differences have not been examined. Herein, we demonstrate a new approach for rare‐earth‐element separations by exploiting differences in the oxidation rates within a series of rare earth compounds containing the redox‐active ligand [{2‐(t BuN(O))C₆H₄CH₂}₃N]³⁻. Using this method, a single‐step separation factor up to 261 was obtained for the separation of a 50:50 yttrium–lutetium mixture.     

Coumaraz‐2‐on‐4‐ylidene: Ambiphilic N‐Heterocyclic Carbenes with a Tunable Electronic Structure

Herein, a coumaraz‐2‐on‐4‐ylidene ( 1 ) as a new example of an ambiphilic N‐heterocyclic carbene, having electronic properties that can be fine‐tuned, is reported. The N‐carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the ⁷⁷Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine‐tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main‐group and transition‐metal chemistry.     

Ultra‐Tuning of the Rare‐Earth fcu‐MOF Aperture Size for Selective Molecular Exclusion of Branched Paraffins

Using isoreticular chemistry allows the design and construction of a new rare‐earth metal (RE) fcu ‐MOF with a suitable aperture size for practical steric adsorptive separations. The judicious choice of a relatively short organic building block, namely fumarate, to bridge the 12‐connected RE hexanuclear clusters has afforded the contraction of the well‐defined RE‐ fcu ‐MOF triangular window aperture, the sole access to the two interconnected octahedral and tetrahedral cages. The newly constructed RE (Y³⁺ and Tb³⁺) fcu ‐MOF analogues display unprecedented total exclusion of branched paraffins from normal paraffins. The resultant window aperture size of about 4.7 Å, regarded as a sorbate‐size cut‐off, enabled a complete sieving of branched paraffins from normal paraffins. The results are supported by collective single gas and mixed gas/vapor adsorption and calorimetric studies.     

Tris(pentafluorophenyl)borane‐Catalyzed Acceptorless Dehydrogenation of N‐Heterocycles

Catalytic acceptorless dehydrogenation is an environmentally benign way to desaturate organic compounds. This process is traditionally accomplished with transition‐metal‐based catalysts. Herein, a borane‐catalyzed, metal‐free acceptorless dehydrogenation of saturated N‐heterocycles is disclosed. Tris(pentafluorophenyl)borane was identified as a versatile catalyst, which afforded several synthetically important N‐heteroarenes in up to quantitative yield. Specifically, the present metal‐free catalytic system exhibited a uniquely high tolerance toward sulfur functionalities, and demonstrated superior reactivity in the synthesis of benzothiazoles compared to conventional metal‐catalyzed systems. This protocol can thus be regarded as the first example of metal‐free acceptorless dehydrogenation in synthetic organic chemistry.     

Total Synthesis of the Post‐translationally Modified Polyazole Peptide Antibiotic Goadsporin

The structurally unique polyazole antibiotic goadsporin contains six heteroaromatic oxazole and thiazole rings integrated into a linear array of amino acids that also contains two dehydroalanine residues. An efficient total synthesis of goadsporin is reported in which the key steps are the use of rhodium(II)‐catalyzed reactions of diazocarbonyl compounds to generate the four oxazole rings, which demonstrates the power of rhodium carbene chemistry in organic chemical synthesis.