Is Iron the New Ruthenium?

Ruthenium complexes with polypyridine ligands are very popular choices for applications in photophysics and photochemistry, for example, in lighting, sensing, solar cells, and photoredox catalysis. There is a long-standing interest in replacing ruthenium with iron because ruthenium is rare and expensive, whereas iron is comparatively abundant and cheap. However, it is very difficult to obtain iron complexes with an electronic structure similar to that of ruthenium(II) polypyridines. The latter typically have a long-lived excited state with pronounced charge-transfer character between the ruthenium metal and ligands. These metal-to-ligand charge-transfer (MLCT) excited states can be luminescent, with typical lifetimes in the range of 100 to 1000 ns, and the electrochemical properties are drastically altered during this time. These properties make ruthenium(II) polypyridine complexes so well suited for the abovementioned applications. In iron(II) complexes, the MLCT states can be deactivated extremely rapidly (ca. 50 fs) by energetically lower lying metal-centered excited states. Luminescence is then no longer emitted, and the MLCT lifetimes become much too short for most applications. Recently, there has been substantial progress on extending the lifetimes of MLCT states in iron(II) complexes, and the first examples of luminescent iron complexes have been reported. Interestingly, these are iron(III) complexes with a completely different electronic structure than that of commonly targeted iron(II) compounds, and this could mark the beginning of a paradigm change in research into photoactive earth-abundant metal complexes. After outlining some of the fundamental challenges, key strategies used so far to enhance the photophysical and photochemical properties of iron complexes are discussed and recent conceptual breakthroughs are highlighted in this invited Concept article.     

Dispirocycles: Platforms for the Construction of High-Performance Host Materials for Phosphorescent Organic Light-Emitting Diodes

Spirocyclic compounds such as 9,9′-spirobifluorene (SBF) are becoming more and more attractive for use as host materials in organic optoelectronic devices. Herein, two dispirocycles, namely, dispiro[fluorene-9,9′-anthracene-10′,9′′-fluorene] and 10,10′′-diphenyl-10H,10′′H-dispiro[acridine-9,9′-anthracene-10′,9′′-acridine], were used for the construction of host materials 1 – 4 . The attached triphenylamino group determines the thermal, photophysical, electrochemical, and charge-transport properties, and therefore they have different electroluminescent performances. The device based on dispiro[fluorene-9,9′-anthracene-10′,9′′-fluorene] ( 2 ) and 10,10′′-diphenyl-10H,10′′H-dispiro[acridine-9,9′-anthracene-10′,9′′-acridine] ( 3 ) molecular platforms exhibited external quantum efficiencies of greater than 21 % with a very high power efficiency (≈100 lm W⁻¹). These results demonstrate the potential of extending the application of dispirocyclic molecular platforms with inherent rigidity for developing highly efficient host materials for organic light-emitting diodes.     

Electrochemical and Scalable Dehydrogenative C(sp3)−H Amination via Remote Hydrogen Atom Transfer in Batch and Continuous Flow

A hydrogen atom transfer-directed electrochemical intramolecular C−H amination has been developed in which the N-radical species are generated at the anode, and the base required for the reaction is generated at the cathode. A broad range of valuable pyrrolidines were prepared in good yields and with high chemoselectivity. The reaction was easily scaled up in both batch and continuous flow systems.     

Optoelectronic,Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max=427–464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL=525–578 nm) with quantum yields of up to 18 % and lifetimes of 1.1–1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1=0.62–1.02 V vs. Fc/Fc⁺) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.     

Chemical and Electrochemical Lithiation of van der Waals Tetrel-Arsenides

Lithiation of van der Waals tetrel-arsenides, GeAs and SiAs, has been investigated. Electrochemical lithiation demonstrated large initial capacities of over 950 mAh g⁻¹ accompanied by rapid fading over successive cycling in the voltage range 0.01–2 V. Limiting the voltage range to 0.5–2 V achieved more stable cycling, which was attributed to the intercalation process with lower capacities. Ex situ powder X-ray diffraction confirmed complete amorphization of the samples after lithiation, as well as recrystallization of the binary tetrel-arsenide phases after full delithiation in the voltage range 0.5–2 V. Solid-state synthetic methods produce layered phases, in which Si-As or Ge-As layers are separated by Li cations. The first layered compounds in the corresponding ternary systems were discovered, Li_0.9Ge_2.9As_3.1 and Li₃Si₇As₈, which crystallize in the Pbam (No. 55) and P2/m (No. 10) space groups, respectively. Semiconducting layered GeAs and SiAs accommodate the extra charge from Li cations through structural rearrangement in the Si-As or Ge-As layers and eventually by replacement of the tetrel dumbbells with sets of Li atoms. Ge and Si monoarsenides demonstrated high structural flexibility and a mild ability for reversible lithiation.     

Synthesis of Organosilyl-Functionalized Cage-Type Germanoxanes Containing Fluoride Ions

Eight corners of a double-four ring cage-type germanoxane, containing a fluoride ion, were successfully silylated by the combination of chlorosilanes and silazanes. Three different silyl groups, trimethylsilyl, dimethylsilyl, and dimethylvinylsilyl, were attached on the corners of germanoxane cage. The solubility and reactivity of the cage modified with dimethylvinylsilyl groups were significantly increased, allowing for further reaction. Hydrosilylation reaction between dimethylvinylsilylated cage geramanoxanes and dimethylsilylated cage siloxanes afforded porous solids. Functionalization of the corners of germanoxanes with silyl groups should provide valuable building blocks in various functional materials.     

The Hydrogen Bond and Beyond: Perspectives for Rotational Investigations of Non-Covalent Interactions

In the last decade, experiment and theory have expanded our vision of non-covalent interactions (NCIs), shifting the focus from the conventional hydrogen bond to new bridging interactions involving a variety of weak donor/acceptor partners. Whereas most experimental data originate from condensed phases, the introduction of broadband (chirped-pulse) microwave fast-passage techniques has revolutionized the field of rotational spectroscopy, offering unexplored avenues for high-resolution studies in the gas phase. We present an outlook of hot topics for rotational investigations on isolated intermolecular clusters generated in supersonic jet expansions. Rotational spectra offer very detailed structural data, easily discriminating the isomeric or isotopic composition and effectively cancelling any solvent, crystal, or matrix bias. The direct comparison with quantum mechanical predictions provides insight into the origin of the inter- and intramolecular interactions with much greater precision than any other spectroscopic technique, simultaneously serving as test-bed for fine-tuning of theoretical methods. We present recent examples of rotational investigations around three topics: oligomer formation, chiral recognition, and identification of halogen, chalcogen, pnicogen, or tetrel bonds. The selected examples illustrate the benefits of rotational spectroscopy for the structural and energetic assessment of inter-/intramolecular interactions, which may help to move from fundamental research to applications in supramolecular chemistry and crystal engineering.     

Asymmetric Reactions involving Lewis Base Catalyst Tethered Dearomatized Intermediates

Numerous protocols have been developed for the functionalization of aromatic substances. Among them, the strategy by which aromatic substrates are activated in situ to generate dearomatized intermediates is highly efficient but challenging, especially in the field of asymmetric catalysis. In this Concept article, the application of some well-established chiral Lewis base catalysis, including primary/secondary amines and N-heterocyclic carbenes, that can covalently form catalyst-tethered dearomatized ortho/para-quinodimethane species with diverse heteroaryl and aryl carbonyl substrates is summarized in a number of asymmetric cycloaddition and addition reactions with diverse reagents generally having electrophilic properties. As a result, a variety of enantioenriched aromatic products with higher molecular complexity are constructed effectively through a rearomatization process.     

Beyond Biological Chelation: Coordination of f-Block Elements by Polyhydroxamate Ligands

The promise of polyhydroxamic acid ligands for the selective chelation of the f-block elements is becoming increasingly more apparent. The initial studies of polyhydroxamic acid siderophores showed the formation of highly stable complexes with Pu^IV, but a higher preference for Fe^III hindered effective applications. The development of synthetic routes toward highly pure and customizable ligands containing multiple hydroxamic acids allowed for the growth of new classes of compounds. Although the first round of these ligands focused on the incorporation of siderophore-like frameworks, the new synthetic strategies led to small molecules of various frameworks and even resins for applications in the field of f-block element separations and biological desorption. Unfortunately, a lack of consistent stability-constant data makes direct comparisons across this body of work difficult. More studies into the stability constants and separations of the f-block elements in a variety of pH ranges is necessary to truly realize the potential for polyhydroxamic acid ligands.