Isolation and Reactivity of a Digerma Analogue of Vinyllithiums: a Lithium Digermenide

Organogermanium Analogues of Alkenes, Alkynes, 1,3-Dienes, Allenes, and Vinylidenes

In this review, the latest achievements in the field of multiply bonded organogermanium derivatives, mostly reported within the last two decades, are presented. The isolable Ge-containing analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes are discussed, and for each class of unsaturated organogermanium compounds, the most representative examples are given. The synthetic approaches toward homonuclear multiply bonded combinations solely consisting of germanium atoms, and their heteronuclear variants containing germanium and other group 14 elements, both acyclic and cyclic, are discussed. The peculiar structural features and nonclassical bonding nature of the abovementioned compounds are discussed based on their spectroscopic and structural characteristics, in particular their crystallographic parameters (double bond length, trans-bending at the doubly bonded centers, and twisting about the double bond). The prospects for the practical use of the title compounds in synthetic and catalytic fields are also briefly discussed.

A series of (E)-1,2-diaryldigermenes incorporating bulky Eind groups: structural characteristics and absorption properties

A series of (E)-1,2-diaryldigermenes, (Eind)ArGeGeAr(Eind) [Ar = phenyl (2), thiophen-2-yl (3), 9,9-dimethyl-2-fluorenyl (4) and 2,2'-bithiophen-5-yl (5)], supported by the fused-ring bulky 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups, have been obtained as yellow-orange to red crystalline solids by the reaction of 1,2-dibromodigermene, (Eind)BrGeGeBr(Eind) (1), with ArLi. In the crystals of 2-5, the digermene cores show a flexible nature adopting a trans-bent geometry with the trans-bent angles (θ) between the Ge-Ge vector and the C_Eind-Ge-C_Ar plane of 34.04(12)° (2), 38.3(3)° and 38.8(3)° (3), 33.69(12)° (4) and 39.30(13)° (5). In the UV-vis spectra, strong π-π* absorptions have been observed with an absorption maximum at 451 nm (ε = 1.3 × 10⁴) (2), 455 nm (ε = 9.7 × 10³) (3), 480 nm (ε = 1.3 × 10⁴) (4) and 497 nm (ε = 1.4 × 10⁴) (5), retaining the GeGe double bond in solution. The absorption data and DFT calculations provide evidence for the intrinsic π-conjugation between the GeGe chromophore and aromatic rings involving the narrowing of the HOMO-LUMO gaps (ΔE) with the extension of the carbon π-electron systems.

Metathesis Reactions of a NHC‐Stabilized Phosphaborene

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P‐protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N‐heterocyclic carbene (NHC)‐stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P‐terminal, as a room‐temperature‐stable crystalline solid accessible via facile NHC‐induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P‐centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X‐ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

Synthesis of Genuine Germenyl Lithiums and the First Persistent Germenyl Radicals

The first isolated genuine germenyl lithiums (R₃Si)(1‐Ad)C=Ge(SiMetBu₂)(Li⋅2 L) (R₃Si=tBu₂MeSi, L=THF (1  a), or L=12‐crown‐4 (1  b) and R₃Si=tBuMe₂Si, L=THF (2  a), or L=12‐crown‐4 (2  b)), were synthesized by reaction of the corresponding acyl germanes 3 and 4, respectively, with tBu₂MeSiLi in THF at 70 °C. The novel 1  a and 2  b were characterized by NMR and UV/Vis spectroscopy, and also by X‐ray crystallography (r(C=Ge)=1.865 Å for 1  a and 1.877 Å for 2  b). Nucleophilic addition reaction of 1  a with MeI and a C−H insertion reactions to the C=Ge bond of 1  a, 2  a and 2  b, are reported. Oxidation of 1  a and 2  b (toluene, 230 K) produces the first persistent germenyl radicals (R₃Si)(1‐Ad)C=Ge⋅‐(SiMetBu₂) (R₃Si=tBu₂MeSi (13  a), R₃Si=tBuMe₂Si (13  b)), which were characterized by EPR spectroscopy (t_1/2≈30 min at 230 K, g=2.029, a_av(⁷³Ge) is 55.0G for 13  a and 60.2G for 13  b). The experimental EPR parameters and DFT calculations indicate that 13  a and 13  b have a strongly bent structure at Ge (calc. ∡(C=Ge−Si)=136.7° (13  a), 135.9° (13  b)), and that the unpaired electron has a substantial s‐character.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Metathesis of Ge=Ge double bonds

The metathesis of carbon-carbon double bonds-the 'reshuffling' of their constituting carbene fragments-is a tremendously important preparative tool in industry and academia. Metathesis of heavier alkene homologues is restricted to occasional unproductive examples in phosphorus chemistry and cross-metathesis to mixed heavier alkynes. We now report the thermally induced, transition-metal-free metathesis of purpose-built unsymmetrically substituted digermenes. The A₂Ge=GeAB starting materials are thus converted to symmetrically substituted derivatives of the A₂Ge=GeA₂ and ABGe=GeAB types. The use of tethered auxiliary donors (dimethylaniline groups) in substituents B ensures intramolecular donor-acceptor stabilization of the transient germylene fragments, the intermediacy of which is proven by trapping experiments. Density functional theory calculations shed light on the thermodynamic driving force of the metathesis and validate the crucial role of the tethered donor. With an analogously equipped bridged tetragermadiene precursor (A₂Ge=GeB-X-BGe=GeA₂), heavier acyclic diene metathesis polymerization occurs, in analogy to the widespread acyclic diene metathesis (ADMET) polymerization in the carbon case, yielding a polydigermene.

A Mixed Heavier Si=Ge Analogue of a Vinyl Anion

The versatile reactivities of disilenides and digermenide, heavier analogues of vinyl anions, have significantly expanded the pool of silicon and germanium compounds with various unexpected structural motifs in the past two decades. We now report the synthesis and isolation of a cyclic heteronuclear vinyl anion analogue with a Si=Ge bond, potassium silagermenide as stable thf-solvate and 18-c-6 solvate by the KC8 reduction of germylene or digermene precursors. Its suitability as synthon for the synthesis of functional silagermenes is proven by the reactions with chlorosilane and chlorophospane to yield the corresponding silyl- and phosphanyl-silagermenes. X-ray crystallographic analysis, UV/Vis spectroscopy and DFT calculations revealed a significant degree of π-conjugation between N=C and Si=Ge double bonds in the title compound.

Persistent Digermenes with Acyl and α-Chlorosilyl Functionalities

We report the preparation of α‐chlorosilyl‐ and acyl‐substituted digermenes. Unlike the corresponding transient disilenes, these species with a Ge=Ge double bond show an unexpectedly low tendency for cyclization, but in turn are prone to thermal Ge=Ge bond cleavage. Triphenylsilyldigermene has been isolated as a crystalline model compound, and is the first fully characterized example of a neutral digermene with an A₂GeGeAB substitution pattern. Spectroscopic and computational evidence prove the constitution of 1‐adamantoyldigermene as a first persistent species with a heavy double bond conjugated with a carbonyl moiety.

An anionic heterosiliconoid with two germanium vertices

Reduction of dismutational 1,4-digermatetrasilabenzene isomer gives lithium digermatetrasila-benzpolarenide (R = Tip = 2,4,6-iPr₃C₆H₂). The positions of its Ge vertices yield mechanistic insights; its reaction with SiCl₄ proves its nucleophilicity.