Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

ATI Stabilized Germylene Cation as a Cyanosilylation Catalyst for Aldehydes and Ketones

The aminotroponiminate (ATI) ligand stabilized germylene cation [(i-Bu)2ATIGe][B(C6F5)4] (2) is found to be an efficient low-valent main-group catalyst for the cyanosilylation of aldehydes and ketones (ATI=aminotroponiminate). It was synthesized by reacting [(i-Bu)2ATIGeCl] (1) with Na[B(C6F5)4]. The catalytic cyanosilylation of diverse aliphatic and aromatic carbonyl compounds (aldehydes and ketones) using 0.075-0.75 mol% of compound 2 was completed within 5-45 min. The catalytic efficiency seen with aliphatic aldehydes was around 15,800 h-1, making compound 2 a capable low-valent main-group catalyst for the aldehyde and ketone cyanosilylation reactions. Further, DFT calculations reveal a pronounced charge localization at the germanium atom of compound 2, leading to its superior catalytic performance.

Catalytic hydroboration of aldehydes and ketones with an electron-rich acyclic metallasilylene

The application of main group metal complexes in catalytic reactions is of increasing interest. Here we show that the electron-rich, acyclic metallasilylene L'(Cl)GaSiL C (L' = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3; L = PhC(NtBu)2) acts as a precatalyst in the hydroboration of aldehydes with HBPin. Mechanistic studies with iso-valeraldehyde show that silylene C first reacts with the aldehyde with [2 + 1] cycloaddition in an oxidative addition to the oxasilirane 1, followed by formation of the alkoxysilylene LSiOCH[Ga(Cl)L']CH2CHMe2 (2), whose formation formally results from a reductive elimination reaction at the Si center. Alkoxysilylene 2 represents the active hydroboration catalyst and shows the highest catalytic activity with n-hexanal (reaction time: 40 min, yield: >99%, TOF = 150 h-1) at room temperature with a catalytic load of only 1 mol%. Furthermore, the hydroboration reaction catalysed by alkoxysilylene 2 is a living reaction with good chemoselectivity. Quantum chemical calculations not only provide mechanistic insights into the formation of alkoxysilylene 2 but also show that two completely different hydroboration mechanisms are possible.

Activation of Ge-H and Sn-H Bonds with N-Heterocyclic Carbenes and a Cyclic (Alkyl)(amino)carbene

A study of the reactivity of several N-heterocyclic carbenes (NHCs) and the cyclic (alkyl)(amino)carbene 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene (cAAC^Me ) with the group 14 hydrides GeH₂ Mes₂ and SnH₂ Me₂ (Me=CH₃ , Mes=1,3,5-(CH₃ )₃ C₆ H₂ ) is presented. The reaction of GeH₂ Mes₂ with cAAC^Me led to the insertion of cAAC^Me into one Ge-H bond to give cAAC^Me H-GeHMes₂ (1). If 1,3,4,5-tetramethyl-imidazolin-2-ylidene (Me₂ Im^Me ) was used as the carbene, NHC-mediated dehydrogenative coupling occurred, which led to the NHC-stabilized germylene Me₂ Im^Me ⋅GeMes₂ (2). The reaction of SnH₂ Me₂ with cAAC^Me also afforded the insertion product cAAC^Me H-SnHMe₂ (3), and reaction of two equivalents Me₂ Im^Me with SnH₂ Me₂ gave the NHC-stabilized stannylene Me₂ Im^Me ⋅SnMe₂ (4). If the sterically more demanding NHCs Me₂ Im^Me , 1,3-di-isopropyl-4,5-dimethyl-imidazolin-2-ylidene (iPr₂ Im^Me ) and 1,3-bis-(2,6-di-isopropylphenyl)-imidazolin-2-ylidene (Dipp₂ Im) were employed, selective formation of cyclic oligomers (SnMe₂ )_n (5; n=5-8) in high yield was observed. These cyclic oligomers were also obtained from the controlled decomposition of cAAC^Me H-SnHMe₂ (3).

Iminophosphonamido-Supported Plumbylenes and Plumbyliumylidenes: Synthesis and Properties

A series of low-valent lead(II) species, i.e., plumbylenes and plumbyliumylidenes supported by an N,N'-di-tert-butyliminophosphonamide ligand, have been synthesized. Chloroplumbylene [Ph₂P(N^tBu)₂PbCl] (1) was readily prepared by the reaction of the corresponding lithium iminophosphonamide and PbCl₂ in THF. The substitution reaction of 1 with KN(SiMe₃)₂ in THF afforded the corresponding aminoplumbylene [Ph₂P(N^tBu)₂PbN(SiMe₃)₂] (2) in the form of colorless crystals. The structures of these plumbylenes in the solid state and in solution were exhaustively characterized using multinuclear NMR spectroscopy and X-ray diffraction analysis. In the crystalline state, 1 adopts a nearly linear polymeric structure in the direction of the c axis via Pb-Cl bonds, with alternating four-membered PbN₂P rings. The chloride-abstraction reaction of 1 using Na[B(C₆F₅)₄] in fluorobenzene proceeded efficiently at ambient temperature to furnish plumbyliumylidene [Ph₂P(N^tBu)₂Pb:]⁺ (3⁺), which was isolated as the air-sensitive yellow borate salt 3[B(C₆F₅)₄]. Plumbyliumylidene 3[B(C₆F₅)₄] acts as a Lewis acid catalyst for the hydroboration of benzophenone and benzaldehyde at catalyst loadings of as low as 0.1 mol % under ambient conditions. Furthermore, the details of the reaction mechanism are discussed on the basis of the results of DFT calculations.

Hydroboration and reductive amination of ketones and aldehydes with HBpin by a bench stable Pd(II)-catalyst

A palladium(II) complex [(κ⁴-{1,2-C₆H₄(NCH-C₆H₄O)₂}Pd] (1) supported by a dianionic salen ligand [1,2-C₆H₄(NCH-C₆H₄O)₂]^2- (L) was synthesised and used as a molecular pre-catalyst in the hydroboration of aldehydes and ketones. The molecular structure of Pd(II) complex 1 was established by single-crystal X-ray diffraction analysis. Complex 1 was tested as a competent pre-catalyst in the hydroboration of aldehydes and ketones with pinacolborane (HBpin) to produce corresponding boronate esters in excellent yields at ambient temperature under solvent-free conditions. Further, the complex 1 proved to be a competent catalyst in the reductive amination of aldehydes with HBpin and primary amines under mild and solvent-free conditions to afford a high yield (up to 97%) of corresponding secondary amines. Both protocols provided high conversion, superior selectivity and broad substrate scope, from electron-withdrawing to electron-donating and heterocyclic substitutions. A computational study based on density functional theory (DFT) revealed a reaction mechanism for Pd-catalysed hydroboration of carbonyl species in the presence of HBpin. The protocols also uncovered the dual role of HBpin in achieving the hydroboration reaction.

Geminal C-Cl and Si-Cl bond activation of chloromethanes and chlorosilanes by gallanediyl LGa

The activation of relatively inert E-X σ-bonds by low-valent main group metal complexes is receiving increasing interest. We here confirm the promising potential of gallanediyl LGa (L = HC[C(Me)N(Dip)]₂, Dip = 2,6-i-Pr₂C₆H₃) to activate E-Cl (E = C, Si) σ-bonds of group 14 element compounds. Equimolar reactions of LGa with chloromethanes and chlorosilanes EH_xCl_4-x (E = C, x = 0-2; E = Si, x = 0, 1) occurred with E-Cl bond insertion and formation of gallylmethanes and -silanes L(Cl)GaEH_xCl_3-x (E = C, x = 2 (1), 1 (2), 0 (3); E = Si, x = 1 (4)). In contrast, consecutive insertion into a geminal E-Cl bond was observed with two equivalents of LGa, yielding digallyl complexes [L(Cl)Ga]₂EH_xCl_2-x (E = C, x = 2 (5); E = Si, x = 1 (6), 0 (7)). Compounds 1-7 were characterized by heteronuclear NMR (¹H, ¹³C, ²⁹Si (4, 6)), IR spectroscopy and elemental analysis, and their solid-state structures were determined by single-crystal X-ray diffraction (sc-XRD).

Stannylene cyanide and its use as a cyanosilylation catalyst

The usefulness of stannylene cyanide (ATISnCN (5); ATI = aminotroponiminate) as a catalyst for the cyanosilylation of aldehydes is demonstrated.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Main Group Catalysis: Cationic Si(II) and Ge(II) Compounds as Catalysts in Organosilicon Chemistry

Cyclopentadienyl (Cp)-coordinated cationic Si(II) (1) and Ge(II) compounds (2) are a new class of catalysts for various transformations in organosilicon chemistry. This review demonstrates that these compounds effectively catalyze technically important reactions, such as the hydrosilylation of carbon-carbon multiple bonds and various types of siloxane-coupling reactions, e.g., the Piers-Rubinsztajn reaction and the oxidative siloxane coupling reaction. Whereas the cationic Si(II) compounds are sensitive to air and moisture, the corresponding cationic Ge(II) compounds are bench stable, thus offering further advantages. The new catalysts contribute to the growing need for the substitution of transition metals and heavier main group metals by their lighter congeners, especially in industrially relevant organosilicon chemistry.

Hydrostannylation of carbon dioxide by a hydridostannylene molybdenum complex

Reaction of the aryltin(II) hydrides {Ar^iPr₄Sn(μ-H)}₂ or {Ar^iPr₆Sn(μ-H)}₂ (Ar^iPr₄ = -C₆H₃-2,6-(C₆H₃-2,6-ⁱPr₂)₂, Ar^iPr₆ = -C₆H₃-2,6-(C₆H₂-2,4,6-ⁱPr₃)₂) with two equivalents of the molybdenum carbonyl [Mo(CO)₅(THF)] afforded the divalent tin hydride transition metal complexes, Mo(CO)₅{Sn(Ar^iPr₆)H}, (1), or Mo(CO)₅{Sn(Ar^iPr₄)(THF)H} (2), respectively. Complex 1 effects the facile hydrostannylation of carbon dioxide, to yield Mo(CO)₅{Sn(Ar^iPr₆)(κ²-O,O'-O₂CH)}, (3), which features a bidentate formate ligand coordinating the tin atom. Reaction of 3 with the pinacolborane, HBpin (pin = pinacolato) in benzene regenerated 1 in quantitative yield. All complexes were characterized by X-ray crystallography, as well as UV-visible, IR, and multinuclear NMR spectroscopies. The isolation of 1 and 2 is consistent with the existence of monomeric forms of {Ar^iPr₄Sn(μ-H)}₂ and {Ar^iPr₆Sn(μ-H)}₂ in solution. Regeneration of 1 from 3via reaction with pinacolborane as the hydrogen source shows the catalytic potential of 1 in the hydrogenation of CO₂.

Synthesis of Cationic Silaamidinate Germylenes and Stannylenes and the Catalytic Application for Hydroboration of Pyridines

The N-heterocyclic germylenes and stannylenes LSi(NAr)₂EX (L = PhC(NtBu)₂, Ar = 2,6-iPr₂C₆H₃; E = Ge, Sn; X = Cl, CF₃SO₃, BPh₄) supported by the bulky silaamidinate ligand [LSi(NAr)₂]^- have been synthesized and fully characterized. The germylene triflate LSi(NAr)₂GeOTf (3b) and dimeric borate [LSi(NAr)₂Ge]₂ClBPh₄ (3a) enabled highly regio- and chemoselective catalytic hydroboration of pyridines and may represent the most active catalytic system for the transformation. DFT calculations disclosed that the cationic germylene [LSi(NAr)₂Ge]⁺ with a low-lying LUMO energy initiated the catalytic process. In contrast, the analogous amidinate germylene triflates are almost inactive, indicating the silaamidinate ligand is essential for the stabilization of cationic species.

Germyliumylidene: A Versatile Low Valent Group 14 Catalyst

Bis‐NHC stabilized germyliumylidenes [RGe(NHC)₂]⁺ are typically Lewis basic (LB) in nature, owing to their lone pair and coordination of two NHCs to the vacant p‐orbitals of the germanium center. However, they can also show Lewis acidity (LA) via Ge−C^NHC σ* orbital. Utilizing this unique electronic feature, we report the first example of bis‐NHC‐stabilized germyliumylidene [^MesTerGe(NHC)₂]Cl (1), (^MesTer=2,6‐(2,4,6‐Me₃C₆H₂)₂C₆H₃; NHC= IMe₄=1,3,4,5‐tetramethylimidazol‐2‐ylidene) catalyzed reduction of CO₂ with amines and arylsilane, which proceeds via its Lewis basic nature. In contrast, the Lewis acid nature of 1 is utilized in the catalyzed hydroboration and cyanosilylation of carbonyls, thus highlighting the versatile ambiphilic nature of bis‐NHC stabilized germyliumylidenes.

A Stable Homoleptic Divinyl Tetrelene Series

The synthesis of the new bulky vinyllithium reagent (Me IPr=CH)Li, (Me IPr=[(MeCNDipp)2 C]; Dipp=2,6-iPr2 C6 H3 ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [Me IPr=CH]- . Furthermore, a high-yielding route to the degradation-resistant SiII precursor Me IPr⋅SiBr2 is presented. The efficacy of (Me IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series (Me IPrCH)2 E: (E=Si to Pb). (Me IPrCH)2 Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the SiII center noted. Cyclic voltammetry shows this electron-rich silylene to be a potent reducing agent, rivalling the reducing power of the 19-electron complex cobaltocene (Cp2 Co).

The key role of R-NHC coupling (R = C, H, heteroatom) and M-NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal-ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.

Cooperative N-H bond activation by amido-Ge(ii) cations

N-heterocyclic carbene (NHC) and tertiary phosphine-stabilized germylium-ylidene cations, [R(L)Ge:]+, featuring tethered amido substituents at R have been synthesized via halide abstraction. Characterization in the solid state by X-ray crystallography shows these systems to be monomeric, featuring a two-coordinate C,N- or P,N-ligated germanium atom. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of N-H bonds in 1,2-fashion: the products resulting from reactions with carbazole feature a tethered secondary amine donor bound to a three-coordinate carbazolyl-GeII centre. In each case, addition of the components of the N-H bond occurs to the same face of the germanium amide function, consistent with a coordination/proton migration mechanism. Such as sequence is compatible with the idea that substrate coordination via the pπ orbital at germanium reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group.

Organogermanium(II) Hydrides as a Source of Highly Soluble LiH

The reactions of monomeric C,N-chelated organogermanium(II) hydride L(H)Ge⋅BH₃ with organolithium salts RLi yielded lithium hydrogermanatoborates (Li(THF)₂ {BH₃ [L(H)GeR]})₂ . Compound (Li(THF)₂ {BH₃ [L(H)GePh]})₂ was used as a source of LiH for the reduction of organic C=O or C=N bonds in nonpolar solvents accompanied by the elimination of a neutral complex L(Ph)Ge⋅BH₃ . The interaction of (Li(THF)₂ {BH₃ [L(H)GePh]})₂ with the polar C=O bond was further investigated by computational studies revealing a plausible geometry of a pre-reactive intermediate. The experimental and theoretical studies suggest that, although the Li atom of (Li(THF)₂ {BH₃ [L(H)GePh]})₂ coordinates the C=O bond, the GeH fragment is the active species in the reduction reaction. Finally, benzaldehyde was reduced by a mixture of L(H)Ge⋅BH₃ with PhLi in nonpolar solvents.

Ge=B π-Bonding: Synthesis and Reversible [2+2] Cycloaddition of Germaborenes

Phosphine-stabilized germaborenes featuring an unprecedented Ge=B double bond with short B⋅⋅⋅Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene-phosphine Lewis pair (1). After oxidative addition of boron trihalides BX3 (X=Cl, Br), the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in yields of 78 % (4) and 57 % (5). These halide-substituted germaborenes were characterized by single-crystal structure analysis, and the electronic structures were studied by quantum-chemical calculations. According to an NBO NRT analysis, the dominating Lewis structure contains a Ge=B double bond. The germaborenes undergo a reversible, photochemically initiated [2+2] cycloaddition with the phenyl moiety of a terphenyl substituent at room temperature, forming a complex heterocyclic structure with GeIV in a strongly distorted coordination environment.

Activation of Protic, Hydridic and Apolar E-H Bonds by a Boryl-Substituted GeII Cation

The synthesis of a boryl-substituted germanium(II) cation, [Ge{B(NDippCH)₂ }(IPrMe)]⁺ , (Dipp=2,6-diisopropylphenyl) featuring a supporting N-heterocyclic carbene (NHC) donor, has been explored through chloride abstraction from the corresponding (boryl)(NHC)GeCl precursor. Crystallographic studies in the solid state and UV/Vis spectra in fluorobenzene solution show that this species dimerizes under such conditions to give [(IPrMe){(HCNDipp)₂ B}Ge=Ge{B(NDippCH)₂ }(IPrMe)]²⁺ (IPrMe = 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene), which can be viewed as an imidazolium-functionalized digermene. The dimer is cleaved in the presence of donor solvents such as [D₈ ]thf or [D₅ ]pyridine, to give monomeric adducts of the type [Ge{B(NDippCH)₂ }(IPrMe)(L)]⁺ . In the case of the thf adduct, the additional donor is shown to be sufficiently labile that it can act as a convenient in situ source of the monomeric complex [Ge{B(NDippCH)₂ }(IPrMe)]⁺ for oxidative bond-activation chemistry. Thus, [Ge{B(NDippCH)₂ }(IPrMe)(thf)]⁺ reacts with silanes and dihydrogen, leading to the formation of Ge^IV products, whereas the cleavage of the N-H bond in ammonia ultimately yields products containing C-H and B-N bonds. The facile reactivity observed in E-H bond activation is in line with the very small calculated HOMO-LUMO gap (132 kJ mol^-1 ).

A vinyl silylsilylene and its activation of strong homo- and heteroatomic bonds

A facile route to an two-coordinate acyclic silylene that can activate strong homo- and heteroatomic bonds is reported.

The facile synthesis of a rare two-coordinate acyclic silylene (R₂Si:) that is stabilized using a bulky vinylic N-heterocyclic olefin ligand and the strongly σ-donating hypersilyl group [Si(SiMe₃)₃]^– is reported. This vinyl-substituted silylene exhibits an excellent combination of prolonged thermal stability along with high reactivity towards small molecules. Despite being stable for months in solution, the reactivity of this new silylene is manifest in its ambient temperature activation of strong B–H, Si–Cl, C–O, P–P and C–H bonds.

Ge(ii) cation catalyzed hydroboration of aldehydes and ketones

The catalytic utility of a germylene cation 4 is reported. In the presence of compound 4, a variety of aldehydes and ketones can be hydroborylated using HBpin.

Recent advances in the catalytic hydroboration of carbonyl compounds

The latest development in the catalytic hydroboration of CO groups is summarized in this review. Access to borate ester intermediates provides a pathway to convert them into the corresponding valuable functionalized alcohols.