Dialumene-Mediated Production of Phosphines through P4 Reduction.

The activation and further functionalization of white phosphorus (P4) by main group complexes has become an increasingly studied topic in recent times. Herein, we report the controlled formation of phosphorus-rich alanes featuring butterfly-like geometries from the selective reaction of P4 with dialumenes, ([L(IiPr)Al]2) (1: L = Tripp; 2: L = tBu2MeSi; IiPr = {MeCN(iPr)}2C)). The two-electron-reduction product of P4 features a P42- structure and is shown to be able to act as a source of P3-. Treatments of different electrophiles (e.g., chlorotrimethylsilane (Me3SiCl), iodotrimethylsilane (Me3SiI), HCl, or acetyl chloride (CH3COCl)) with these phosphorus-rich alanes under mild conditions gave the corresponding phosphine (e.g., P(SiMe3)3, PH3, or P(COCH3)).

Tetryliumylidene ions in synthesis and catalysis.

Tetryliumylidene ions ([R-E:]+), recognised for their intriguing electronic properties, have attracted considerable interest. These positively charged species, with two vacant p-orbitals and a lone pair at the E(ii) centre (E = Si, Ge, Sn, Pb), can be viewed as the combination of tetrylenes (R2E:) and tetrylium ions ([R3E]+), which makes them potent Lewis ambiphiles. Such electronic features highlight the potential of tetryliumylidenes for single-site small molecule activation and transition metal-free catalysis. The effective utilisation of the electrophilicity and nucleophilicity of tetryliumylidenes is expected to stem from appropriate ligand choice. For most of the isolated tetryliumylidenes, electron donor- and/or kinetic stabilisation is necessary. This minireview highlights the developments in tetryliumylidene syntheses and the progress of research towards their reactivity and applications in catalytic reactions.

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.

Dearomatization of C6 Aromatic Hydrocarbons by Main Group Complexes.

The dearomatization reaction is a powerful method for transformation of simple aromatic compounds to unique chemical architectures rapidly in synthetic chemistry. Over the past decades, the chemistry in this field has evolved significantly and various important organic compounds such as crucial bioactive molecules have been synthesized through dearomatization. In general, photochemical conditions or assistance by transition metals are required for dearomatization of rigid arenes. Recently, main-group elements, especially naturally abundant elements in the Earth's crust, have attracted attention as they have low toxicity and are cost-effective compared to the late transition metals. In recent decades, a variety of low-valent main-group molecules, which enable the activation of stable aromatic compounds under mild conditions, have been developed. This minireview highlights the developments in the chemistry of dearomatization of C6 aromatic hydrocarbons by main-group compounds leading to the formation of seven-membered EC6 (E=main-group elements) ring or cycloaddition products.

An Aluminum Telluride with a Terminal Al=Te Bond and its Conversion to an Aluminum Tellurocarbonate by CO2 Reduction.

Facile access to dimeric heavier aluminum chalcogenides [(NHC)Al(Tipp)-µ-Ch]2 (NHC=IiPr (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene); Tipp=2,4,6-iPr3 C6 H2 ; Ch=Se, Te) by treatment of NHC-stabilized aluminum dihydrides with elemental Se and Te is reported. The higher affinity of IMe4 in comparison with IiPr toward the Al center in [(NHC)Al(Tipp)-µ-Ch]2 can be used for ligand exchange. Additionally, the presence of excess IMe4 allows for cleavage of the dimers to form a rare example of a neutral multiply bonded heavier aluminum chalcogenide in the form of a tetracoordinate aluminum complex, (IMe4 )2 (Tipp)Al=Te. This species reacts with three equivalents of CO2 across two Al-CNHC and the Al=Te bond affording a pentacoordinate aluminum complex containing a dianionic tellurocarbonate ligand [CO2 Te]2- , which is the first example of tellurium analogue of a carbonate [CO3 ]2- .

Isolation and Reactivity of Stannylenoids Stabilized by Amido/Imino Ligands.

The reaction of the lithium aryl(silyl)amide Dipp(i Pr3 Si)NLi (Dipp=2,6-i Pr2 C6 H3 ) with one equivalent of SnCl2 in THF gave a novel stannylenoid Dipp(i Pr3 Si)NSnCl⋅LiCl(THF)2 . Heating the solution of amidostannylenoid in toluene to 80 °C resulted in dimeric amido(chloro)stannylene [Dipp(i Pr3 Si)NSnCl]2 , which can be converted to bis(amido)stannylene Sn[N(Dipp)(i Pr3 Si)]2 and amido(imino)stannylene Sn[N(Dipp)(i Pr3 Si)][IPrN] (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-imino). Treatment of bis(imino)stannylenoid [IPrN]2 Sn(Cl)Li with N2 O resulted in the dimeric complex [IPrNSn(Cl)OLi]2 . All compounds were characterized by NMR, elementary analysis, and X-ray structural determination.

Isolation and Reactivity of Tetrylene-Tetrylone-Iron Complexes Supported by Bis(N-Heterocyclic Imine) Ligands.

The germanium iron carbonyl complex 3 was prepared by the reaction of dimeric chloro(imino)germylene [IPrNGeCl]2 (IPrN=bis(2,6-diisopropylphenyl)imidazolin-2-iminato) with one equivalent of Collman's reagent (Na2 Fe(CO)4 ) at room temperature. Similarly, the reaction of chloro(imino)stannylene [IPrNSnCl]2 with Na2 Fe(CO)4 (1 equiv) resulted in the Fe(CO)4 -bridged bis(stannylene) complex 4. We observed reversible formation of bis(tetrylene) and tetrylene-tetrylone character in complexes 3 vs. 5 and 4 vs. 6, which was supported by DFT calculations. Moreover, the Li/Sn/Fe trimetallic complex 12 has been isolated from the reaction of [IPrNSnCl]2 with cyclopentadienyl iron dicarbonyl anion. The computational analysis further rationalizes the reduction pathway from these chlorotetrylenes to the corresponding complexes.

Carbon Monoxide in Main-Group Chemistry.

The usage of carbon monoxide (CO) as a C1 feedstock for carbonylation has been an important subject of numerous studies for over a century. The chemistry in this field has evolved significantly, and several processes (e.g., Fischer-Tropsch, Monsanto, and Cativa process) have even been industrialized to serve humankind in our daily lives. CO is also a crucial ligand (carbonyl) in organometallic chemistry, and transition-metal carbonyl complexes have been widely used as homogeneous catalysts in various chemical transformations. Historically, transition-metal carbonyls have been considered to be dominant for these purposes. In recent decades, main-group elements, especially naturally abundant elements in the Earth's crust such as silicon and aluminum, have gained much attention, as they are eco-friendly and have low toxicity compared to the late transition metals. Recent developments in main-group chemistry have revealed reactivity which can mimic that of transition-metal complexes toward small molecules such as H2, alkenes, and alkynes, along with carbon monoxide. This Perspective highlights CO activation by main-group compounds which leads to the formation of carbonyl complexes or CO insertion into the main-group element center as well as the reductive homologation of CO.

Small Molecule Activation by Two-Coordinate Acyclic Silylenes.

In recent decades, the chemistry of stable silylenes (R2Si:) has evolved significantly. The first major development in this chemistry was the isolation of a silicocene which is stabilized by the Cp* (Cp* = η5-C5Me5) ligand in 1986 and subsequently the isolation of a first N-heterocyclic silylene (NHSi:) in 1994. Since the groundbreaking discoveries, a large number of isolable cyclic silylenes and higher coordinated silylenes, i.e. Si(II) compounds with coordination number greater than two, have been prepared and the properties investigated. However, the first isolable two-coordinate acyclic silylene was finally reported in 2012. The achievements in the synthesis of acyclic silylenes have allowed for the utilization of silylenes in small molecule activation including inert H2 activation, a process previously exclusive to transition metals. This minireview highlights the developments in silylene chemistry, specifically two-coordinate acyclic silylenes, including experimental and computational studies which investigate the extremely high reactivity of the acyclic silylenes.

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.

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.

Generation of stannabenzenes and their monomer-dimer equilibration.

Stannabenzene has not been isolated so far because of its high reactivity. In order to synthesize and isolate a stable stannabenzene, we have introduced two steric protection groups, the Tbt (2,4,6-tris[bis(trimethylsilyl)methyl]phenyl) or the Bbt (2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl) group on the tin atom and the t-butyl group on the 2-position, to the stannabenzene skeleton. Such a combination of steric protection groups was found to suppress the dimerization of stannabenzene to some extent, giving an equilibrated mixture of the corresponding stannabenzene and its dimer.

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.

Ru-Complexes of an anionic germabenzenyl ligand.

Ruthenium complexes having an anionic germabenzenyl ligand, [Cp*Ru{η1,η3-GeC5(t-Bu)H4}]2 (1) and [Cp*Ru{η1-GeC5(t-Bu)H4}{[η1,η5-GeC5(t-Bu)H4]RuCp*}2] (2), have been synthesized by the reaction of germabenzenylpotassium, KGeC5(t-Bu)H4, with [Cp*RuCl]4. Complexes 1 and 2 exhibit both σ- and π-type coordination and can be regarded as the first examples of a transition-metal substituted benzenoid of heavier Group 14 elements. The isolated complexes 1 and 2 have been characterized using NMR and UV-vis spectroscopy, X-ray crystallographic analysis, and theoretical calculations.

Germabenzenylpotassium: A Germanium Analogue of a Phenyl Anion.

Reduction of the stable germabenzene, 1-Tbt-2-tert-butyl-germabenzene (Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), with KC8 resulted in the formation of 2-tert-butylgermabenzenylpotassium, that is, the germanium analogue of phenylpotassium, under concomitant elimination of the aryl group from the Ge atom. Under an inert atmosphere, this germabenzenylpotassium could be isolated in the form of stable yellow crystals. In the crystalline state, as well as in solution, the germabenzenyl moiety adopts a monomeric form, even though the X-ray diffraction analysis suggests the presence of highly reactive Ge=C double bonds. The spectroscopic and X-ray crystallographic analyses, in combination with theoretical calculations indicate an ambident character for this germabenzenyl anion, with contributions from aromatic and germylene resonance structures.