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While analogues of allenes with heavier main-group elements (group 14-16) have been extensively studied, 2-heteraallenes are rare chemical species whose properties are mostly unknown. It is also notable that the synthesis and isolation of allene-type molecules are not widespread, despite the extensive study of two-coordinated low-valent chemical species.

Tricyclic 1,4-dihydro-1,4-phosphasilines 3a,b were synthesized from Si(NR 2 ) 2 -bridged imidazole-2-thione compounds 2a,b. Based on calculated FMOs of 3b, forecasting a possible P-selective P-N bond cleavage reduction, a redox cycle could be established using solutions of P-centred anionic derivative K[4b]. The cycle started with the oxidation of the latter to give the P-P coupled product 5b which could be chemically reduced by KC 8 to yield K[4b], again. All new products have been unambiguously confirmed in solution and solid state.

Persistent radicals have attracted increasing attention owing to their fascinating properties, including multi-step redox and long-wavelength absorption characteristics. In this study, boron complexes with π-extended nitroxide ligands were synthesised via Buchwald-Hartwig amination reactions of the corresponding boron-nitroxide complex and diarylamines. These nitroxide complexes exhibited two-step and reversible one-electron oxidation processes, suggesting stable redox triads involving anionic aminoxide, neutral nitroxide-radical, and cationic oxoammonium ligands. Cationic boron complexes of the nitroxide radical ligands were obtained via chemical oxidation of the aminoxide complexes and structurally characterised. The cationic nitroxide-radical complexes exhibited strong absorptions at approximately 1100 nm, demonstrating their potential as near-infrared (NIR)-II-active functional dyes.

Several examples of stable low-coordinated species of heavier group 14 elements (Si, Ge, Sb, Pb) such as divalent species and multiple-bond compounds have been reported. With the goal in mind to create unprecedented low-coordinated species of heavier group 14 elements that exhibit considerably increased redox stability, ferrocenyl (Fc)-substituted low-coordinated species of heavier group 14 elements were designed. In this short account article, recent progress on the synthesis of Fc-based low-coordinated species of heavier group 14 elements is summarized.

1,2-Insertion reactions of alkynes into the Ge-C bonds in dibromodigermenes afford stable crystalline bromovinylgermylenes. In contrast to previously reported Lewis-base-supported vinylgermylenes, the bromovinylgermylene obtained from reaction of the bromogermylene with 3-hexyne via such an 1,2-insertion is a donor-free monomer. A feasible reaction mechanism, proposed on the basis of the observed experimental results in combination with theoretical calculations, suggests that the [1+2]-cycloadduct and the insertion product are the kinetic and thermodynamic product, respectively.

A stable 1,4-disilabenzene was generated from the reaction of a stable disilyne with 3-hexyne. Its photochemical isomerization afforded the corresponding disilabenzvalene, which was structurally characterized by a single-crystal X-ray diffraction analysis.

A stable 3,5-diphenyl-1,2-disilabenzene was selectively synthesized by the reaction between the isolable disilyne TbbSi[triple bond, length as m-dash]SiTbb (Tbb = 2,6-[CH(SiMe3)2]2-4-t-Bu-phenyl) with phenylacetylenes. Its molecular structure and physical properties were examined and compared to those of the 1,2-disilabenzene that was obtained from the reaction between TbbSi[triple bond, length as m-dash]SiTbb and acetylene. Moreover, a plausible formation mechanism for this reaction is discussed.

1,2-Dihalodigermenes, (E)-(Eind)XGe[double bond, length as m-dash]GeX(Eind) (X = Br and Cl), bearing the fused-ring bulky Eind group (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) have been isolated as orange-yellow crystals by the ligand redistribution reaction between the diarylgermylene, (Eind) 2 Ge:, and GeX 2 ·dioxane in toluene via the cleavage and recombination of the Ge-C and Ge-X bonds. The dihalodigermenes have a Ge[double bond, length as m-dash]Ge double bond character in the crystalline state, but dissociate into halogermylenes, (Eind)XGe:, in solution. The addition of excess LiBr to the THF solution of the bromogermylene, (Eind)BrGe:, led to the formation of an equilibrium mixture containing the lithium bromogermylenoid, [Li + ][(Eind)GeBr 2 ] - . The bromogermylenoid can be isolated as a cryptand-separated potassium ion pair, [K + (crypt-222)][(Eind)GeBr 2 ] - , which has been structurally characterized.

In this work, we investigated the effects of counter anions, P-substituents, and solvents on the optical and photophysical properties of 2-phenylbenzo[b]phospholium salts in solution. A series of 2-phenylbenzo[b]phospholium salts was prepared by P-alkylation or P-phenylation of 1,2-diphenylbenzo[b]phosphole followed by anion exchange reactions. X-ray crystallographic analyses of six benzo[b]phospholium salts showed that each phosphorus center has an onium nature with an essentially tetrahedral geometry. 1 H NMR and steady-state UV-vis absorption and fluorescence spectroscopic measurements of these phospholium salts revealed the pivotal role of counter-anion solvation. The observed results are discussed on the basis of the association-dissociation equilibrium between a contact ion pair (CIP) and a solvent-separated ion pair (SSIP) in solution. The hexafluorophosphates exist as SSIPs and emit intense fluorescence, irrespective of the P-substituents and solvents. In contrast, the iodides are present as SSIPs in methanol but exist as equilibrium mixtures of the two emitting species, SSIP and CIP, in dichloromethane. As a consequence, fluorescence intensities of the iodides varied significantly depending on the solvents, P-substituents, and solution concentrations. These findings were studied in more detail using time-resolved fluorescence spectroscopy and fluorescence titration measurements. The light-emitting properties of the 2-phenylbenzo[b]phospholium halides in the CIPs rely on heavy atom effects derived from the counter halide anions on the S 1 state of the adjacent cationic benzo[b]phosphole π-systems. The present study suggests that 2-arylbenzo[b]phospholium salts would be promising scaffolds for developing new phosphole-based ionic fluorophores that are capable of responding to external stimuli such as anionic species and solvents.

The reactivity of a diaryl-substituted disilyne, Ar-Si[triple bond, length as m-dash]Si-Ar, with alkynes was examined. Reaction of the disilyne with acetylene yielded a 1,2-disilabenzene as the sole product.

The first stable neutral stannaaromatic compound, 2-stannanaphthalene , was synthesized by taking advantage of an extremely bulky and efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of were discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy, cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene , which has a framework similar to that of , was synthesized for comparison, and systematic elucidation was made for the properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).

There has been much interest in the synthesis and properties of doubly bonded systems between heavier Group 15 elements, i. e. heavier analogues of azo-compounds (dipnictenes), from the viewpoints of fundamental and material chemistry. Although such double-bond compounds between heavier main group elements are known to be highly reactive, too much so to be isolated as stable compounds, a number of reports on the synthesis of kinetically stabilized diphosphenes (RP[double bond, length as m-dash]PR), diarsenes (RAs[double bond, length as m-dash]AsR), and phosphaarsenes (RP[double bond, length as m-dash]AsR) bearing bulky substituent have been published since 1980. We have also succeeded in the synthesis of the first stable distibene (RSb[double bond, length as m-dash]SbR) and dibismuthene (RBi[double bond, length as m-dash]BiR) by taking advantage of efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt), and revealed their structures and properties systematically. Thus, the doubly bonded compounds between heavier Group 15 elements are no longer imaginary species but are those with real existence which are stable, even in the case of the heaviest non-radioactive element bismuth, when they are appropriately protected by bulky substituents. This Perspective describes our research on the chemistry of kinetically stabilized double-bond compounds between heavier Group 15 elements.