Dioxygen Activation by Gold(I)-Distorted Porphyrin Dinuclear Complexes.

Interactions between gold-based materials and dioxygen (O2) have motivated researchers to understand reaction mechanisms for O2 activation by homo- and heterogeneous gold catalysts. In this work, gold(I) porphyrin dinuclear complexes were synthesized with a saddle-distorted porphyrin ligand. The gold(I) porphyrin complexes showed unprecedented O2 activation in the presence of protic solvents to form gold(III) tetradentate porphyrin complexes. Mechanistic insights into the O2 activation by the gold(I) center were elucidated by spectroscopic measurements and theoretical calculations, revealing that dissociation of halides on the gold(I) center by alcohol solvents and hydrogen bonding of an N-H proton in the distorted porphyrin with dioxygen played important roles in establishing the unique reactivities of gold(I) complexes.

Isomer-Selective Conversion of Au Clusters by Au(I) Thiolate Insertion.

Isomer-selective conversion is a challenging goal in the rational design of Au clusters. Herein, we demonstrate the isomer-selective conversion of Au18(ScC6)14 (ScC6 = cyclohexanethiolate) into Au24(SR)x(ScC6)20-x in high yields by reactions with gold(I) thiolate (AuSR) complexes. Electrospray ionization mass spectrometry indicated that even numbers of AuSR units are inserted into Au18(SR)x(ScC6)14-x to generate Au24(SR)x(ScC6)20-x through intermediates Au20(SR)x(ScC6)16-x or Au22(SR)x(ScC6)18-x. These results suggest that the number of constituent atoms in surface Au(I)SR oligomers only increases, while the number of electrons in an Au core is maintained. UV-vis analysis revealed the generation of one of two Au24(SR)x(ScC6)20-x isomers in the reactions of Au18(ScC6)14 with AuSR complexes, in contrast to the formation of both isomers by reactions with thiols. When the structures of Au18(SR)14 are compared with those of the Au24(SR)20 isomers, the partial structure in the Au cores is preserved in the isomer-selective conversion with AuSR complexes, regardless of the structures of the thiolate moiety.

Recent progress in the chemistry of heavy aromatics.

The aromaticity and synthetic application of "heavy benzenes", i.e., benzenes containing a heavier Group 14 element (Si, Ge, Sn, and Pb) in place of skeletal carbon, have been the targets of many theoretical and synthetic studies. Although the introduction of a sterically demanding substituent enabled us to synthesize and isolate heavy aromatic species as a stable compound by suppressing their high reactivity and tendency to polymerize, the existence of a protection group is an obstruction to the development of functional materials based on heavy aromatics. This review will delineate the most recent topics in the chemistry of heavy aromatics, i.e., the chemistry of "metallabenzenyl anions", which are the heavier Group 14 element analogs of phenyl anions stabilized by taking advantage of charge repulsion instead of steric protection.

Synthesis of Twisted [n]Cycloparaphenylene by Alkene Insertion.

Mono-alkene-inserted [n]cycloparaphenylenes 1 [(ene)-[n]CPP] with n=6, 8, and 10, mono-ortho-phenylene-inserted [6]CPP 2, and di-alkene-insertved [n]CPP 3 [(ene)2 -[n]CPP] with n=4, 6, and 8 were synthesized by fusing CPP precursors and alkene or ortho- phenylene groups through coupling reactions. Single-crystal X-ray diffraction analyses reveal that the strips formed by the π-surfaces of 1 and 2 exhibited a Möbius topology in the solid state. While the Möbius topology in the parent 1 and 2 in solution was lost due to the free rotation of the paraphenylene unit even at low temperatures, ene-[6]CPP 4 with eight 1-pyrrolyl groups preserved the Möbius topology even in solution. Despite a twist, 1 has in-plane conjugation and possesses a unique size dependence of the electronic properties: namely, the opposite size dependency of the HOMO-LUMO energy relative to conventional π-conjugated molecules.

Control over Ligand-Exchange Positions of Thiolate-Protected Gold Nanoclusters Using Steric Repulsion of Protecting Ligands.

Organic ligands on gold nanoclusters play important roles in regulating the structures of gold cores. However, the impact of the number and positions of the protecting ligands on gold-core structures remains unclear. We isolated thiolate-protected Au25 cluster anions, [Au25(SC2Ph)17(Por)1]- and [Au25(SC2Ph)16(Por)2]- (SC2Ph = 2-phenylethanethiolate), obtained by ligand exchange of [Au25(SC2Ph)18]- with one or two porphyrinthiolate (Por) ligands as mixtures of regioisomers. The ratio of two regioisomers in [Au25(SC2Ph)17(Por)1]- as measured by 1H NMR spectroscopy revealed that the selectivity could be controlled by the steric hindrance of the incoming thiols. Extended X-ray absorption fine structure studies of a series of porphyrin-coordinated gold nanoclusters clarified that the Au13 icosahedral core in the Au25 cluster was distorted through steric repulsion between porphyrin thiolates and phenylethanethiolates. This paper reveals interesting insights into the importance of the steric structures of protecting ligands for control over core structures in gold nanoclusters.

Characterization of resonance structures in aromatic rings of benzene and its heavier-element analogues.

We present the experimental visualization of the valence-electron-density distribution in benzene and its kinetically stabilized heavier-element analogues, i.e., 1,2-disilabenzene and 1,2-digermabenzene. The valence-electron-density-distribution (EDD) analysis on the 1,2-disila- and 1,2-digermabenzenes revealed that these contain incompletely delocalized π electrons on their cyclic conjugation systems, making them less aromatic compared to benzene. Based on the results of this EDD analysis in combination with anisotropy of the current-induced density (ACID) calculations, considerable contributions from the characteristic resonance structures of 1,2-disila- and 1,2-digermabenzenes with cleaved EE bonds can be expected.

A Novel Reactivity of Phosphanylalumane (>P-Al<): Reversible Addition of a Saturated Interelement Bond to Olefins.

The reversible addition of olefins to a phosphanylalumane, P-Al single-bond species, was investigated. The P-Al bond added to ethylene and relatively small terminal alkenes (propylene and hex-1-ene) at room temperature to give the corresponding alkene adducts. Heating the terminal alkene adducts released the corresponding alkenes and regenerated the P-Al bond, but no release of ethylene was observed even under vacuum conditions. The reactivity of ethylene adduct as a new saturated C2 vicinal P/Al-based FLP was also investigated. The ethylene adduct was found to undergo complexation with nitriles to give the corresponding nitrile adducts to the Al center, which retained the ethylene tether as in the case of the corresponding P/B-based FLP. However, the reactivity of ethylene toward CO2 and benzaldehyde differed from that of the P/B system giving the corresponding adducts.

Fused-Nonacyclic Multi-Resonance Delayed Fluorescence Emitter Based on Ladder-Thiaborin Exhibiting Narrowband Sky-Blue Emission with Accelerated Reverse Intersystem Crossing.

Developing organic luminophores with unique capability of strong narrowband emission is both crucial and challenging for the further advancement of organic light-emitting diodes (OLEDs). Herein, a nanographitic fused-nonacyclic π-system (BSBS-N1), which was strategically embedded with multiple boron, nitrogen, and sulfur atoms, was developed as a new multi-resonance thermally activated delayed fluorescence (MR-TADF) emitter. Narrowband sky-blue emission with a peak at 478 nm, full width at half maximum of 24 nm, and photoluminescence quantum yield of 89 % was obtained with BSBS-N1. Additionally, the spin-orbit coupling was enhanced by incorporating two sulfur atoms, thereby facilitating the spin-flipping process between the excited triplet and singlet states. OLEDs based on BSBS-N1 as a sky-blue MR-TADF emitter achieved a high maximum external electroluminescence quantum efficiency of 21.0 %, with improved efficiency roll-off.

Discovery of Self-Assembling Small Molecules as Vaccine Adjuvants.

Immune potentiators, termed adjuvants, trigger early innate immune responses to ensure the generation of robust and long-lasting adaptive immune responses of vaccines. Presented here is a study that takes advantage of a self-assembling small-molecule library for the development of a novel vaccine adjuvant. Cell-based screening of the library and subsequent structural optimization led to the discovery of a simple, chemically tractable deoxycholate derivative (molecule 6, also named cholicamide) whose well-defined nanoassembly potently elicits innate immune responses in macrophages and dendritic cells. Functional and mechanistic analyses indicate that the virus-like assembly enters the cells and stimulates the innate immune response through Toll-like receptor 7 (TLR7), an endosomal TLR that detects single-stranded viral RNA. As an influenza vaccine adjuvant in mice, molecule 6 was as potent as Alum, a clinically used adjuvant. The studies described here pave the way for a new approach to discovering and designing self-assembling small-molecule adjuvants against pathogens, including emerging viruses.

Synthesis and Properties of Thiophene-Fused Thiopyrylium Salts.

A new family of thiophene-fused thiopyrylium salts has been synthesized via Lewis-acid-induced Rieche formylation, followed by an intramolecular Friedel-Crafts cyclization of a series of diarylthioethers. Moreover, in the case of diarylthioethers that bear formyl groups, Lewis-acid-promoted intramolecular cyclizations afforded novel thiophene-fused bisthiopyrylium salts in good yield. The electronic structures of the new compounds were determined experimentally by NMR and UV-vis absorption spectroscopy and theoretically investigated by density functional theory calculations. The results of our examinations revealed effective conjugation of the π-electrons over the entire linearly fused heteroacene framework.

Reversible Isomerizations between 1,4-Digermabenzenes and 1,4-Digerma-Dewar-benzenes: Air-Stable Activators for Small Molecules.

The first examples of stable, crystalline, and air-sensitive 1,4-digermabenzenes were isolated. These species photochemically isomerize into the corresponding air-stable digerma-Dewar-benzenes. More importantly, alkyl-substituted Dewar-type-1,4-digermabenzenes can be considered as reversible "air-stable activators" for small molecules such as dihydrogen, carbon dioxide, and acetylene at room temperature. The regeneration of these activators can be accomplished via a thermal retro-isomerization that affords the corresponding 1,4-digermabenzenes.

A Mixed-Anion System Consisting of a Germyl Anion and Anions Delocalized on Conjugated Carbon Ring Skeletons.

A trianion consisting of one germyl anion and two anions delocalized on conjugated carbon ring skeletons was synthesized by trimerization of the germanium analogue of the anthryl anion, which exhibits high germylene character. In addition, the generation of the putative intermediate of this reaction, the radical anion of 9-germaanthracene, was confirmed by the formation of the corresponding dimeric dianion. The isolated trianion and dianion were characterized by X-ray crystallographic analysis together with NMR spectroscopy and theoretical calculations.

Near-Infrared Circularly Polarized Luminescence through Intramolecular Excimer Formation of Oligo(p-phenyleneethynylene)-Based Double Helicates.

Carbon-based double helicates consisting of two anthracene-containing oligo(p-phenyleneethynylene) units and two flexible chiral 1,1'-binaphthyl units or two rigid chiral 9,9'-spirobifluorene units were developed. The curved oligo(p-phenyleneethynylene) fragments in the double helicates were successfully constructed by tin-mediated reductive aromatization. Helical oligo(p-phenyleneethynylene) double strands fixed by two rigid spirobifluorene units showed little structural change under photoirradiation, thereby emitting circularly polarized luminescence (CPL) in the visible region with a high quantum yield (ΦPL =0.93). In contrast, flexible binaphthyl units induced dynamic structural change of the oligo(p-phenyleneethynylene) luminophores under photoirradiation, leading to strong CPL (|glum |=1.1×10-2 ) in the near-infrared (NIR) region. UV/Vis, circular dichroism (CD), CPL and NMR spectroscopic analyses of the binaphthyl-hinged double helicate suggested excimer formation between two π-conjugated strands in the excited state. Theoretical calculations highlight the importance of the tightly interlocked excimer structure of the carbon-based double helicate in controlling the angle between the electric and magnetic transition dipole moments for strong NIR CPL generation.

A Purified, Solvent-Intercalated Precursor Complex for Wide-Process-Window Fabrication of Efficient Perovskite Solar Cells and Modules.

A high-purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH3 NH3 PbI3 ⋅DMF, is introduced as an effective precursor material for fabricating high-quality solution-processed perovskite layers. Spin-coated films of the solvent-intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low-speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power-conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm2 , eight-cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).

2,5-Digermaselenophenes: Germanium Analogues of Selenophenes.

A stable crystalline 2,5-digermaselenophene was synthesized. In contrast to hitherto reported selenophenes, this digermaselenophene exhibits a trans-pyramidalized structure, which is due to its electronic properties. The practical utility of this 2,5-digermaselenophene is reflected in its ability to activate dihydrogen and acetylene at room temperature in the absence of a transition-metal complex, and this behavior can be rationalized on the basis of its physicochemical properties, which are characterized by considerable electron-donating and -accepting abilities.

Stannabenzenylpotassium: The First Isolable Tin-Containing Benzene Derivative.

In contrast to the well-investigated sila- and germa-aromatic compounds, very few aromatic compounds containing tin have been reported. Specifically, no stable example of monomeric stannabenzene, that is, the tin analogue of benzene, the simplest aromatic compound, has been isolated until now. In this work, a tin analogue of the phenyl anion, the stannabenzenyl anion, was successfully isolated by utilizing the same strategy as employed for the germanium system. Stannabenzenylpotassium was characterized by X-ray crystallography, NMR (1 H, 13 C, and 119 Sn) and UV/Vis spectroscopy, together with theoretical calculations.

A Redox-Active Bis(ferrocenyl)germylene and Its Reactivity.

Bis(ferrocenyl)germylene Fc*2 Ge: [2; Fc*=2,5-bis(3,5-di-tert-butylphenyl)-1-ferrocenyl] was isolated in the form of red crystals from the reaction of the sterically demanding ferrocenyl lithium dimer (Fc*Li)2 and GeI2 . Bis(ferrocenyl)germylene 2 exhibits extraordinary thermal stability in hydrocarbon solution and the solid state, as well as stable redox behavior. Moreover, it undergoes a ligand-redistribution reaction with GeCl2 ⋅(dioxane) to afford the corresponding chlorogermylene, which was isolated upon coordination with PBu3 .

Neutral, Cationic and Hydride-substituted Siloxygermylenes.

The introduction of the labile trimethylsiloxy group to GeII centers in the presence of an N-heterocyclic carbene donor is reported. The new complex IPr⋅GeCl(OSiMe3 ) (IPr=[(HCNDipp)2 C:]; Dipp=2,6-iPr2 C6 H3 ) was readily converted into the structurally unique GeII siloxy(hydrido)germylene IPr⋅GeH(OSiMe3 )⋅BH3 by treatment with lithium borohydride. Additionally, the reactive siloxygermylene cation [IPr⋅Ge(OSiMe3 )]+ was synthesized and clean oxidative addition of CH2 Cl2 was demonstrated. The two-coordinate [IPr⋅Ge(OSiMe3 )]+ cation also promoted the catalytic hydroborylation of sterically hindered ketones under mild conditions, with enhanced reactivity stemming from an open coordination site at Ge.

Syntheses and Structures of d10 Coinage Metal Complexes of Electron-Accepting Phosphine Ligands Featuring a 3,3,4,4,5,5-Hexafluorocyclopentene Framework.

Cu(I), Ag(I), and Au(I) complexes of monophosphine or bisphosphine ligands based on the 3,3,4,4,5,5-hexafluorocyclopentene skeleton were synthesized and structurally characterized by X-ray crystallographic analysis. The electron-withdrawing nature of these polyfluorinated phosphines was experimentally revealed via UV/vis absorption studies and crystal structure analysis. Successful catalytic application of the Au(I) complex for alkyne hydration reactions was investigated.

Synthesis and Structural Properties of Axially Chiral Binaphthothiophene Dicarboxylic Acid.

Axially chiral binaphthothiophene dicarboxylic acid was prepared as a novel functionalized chiral dicarboxylic acid. The crystal structures of both the racemic form and its salt with chiral diamine revealed the intramolecular S···O interactions (chalcogen bonds) between the sulfur in the naphthothiophene rings and the oxygen of the carboxy groups. The negative-positive and the positive-negative Cotton effects from longer to shorter wavelengths were observed for (R)- and (S)-enantiomers, respectively, in the circular dichroism (CD) spectra.