Tetryliumylidene ions in synthesis and catalysis

Abstract

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.

---

Sebastian Stigler

Sebastian Stigler received his MSc degree in chemistry in 2021 from the Technical University of Munich under the supervision of Prof. Shigeyoshi Inoue on the application of NHC-stabilised silyliumylidene ions in hydroboration and hydrosilylation catalysis. Since then he has continued his stay in the group of Prof. Shigeyoshi Inoue at the Technical University of Munich as a PhD student. His current research interests focus on the reactivity of silyliumylidene ions and their utilisation in catalytic reactions.

Shiori Fujimori

Shiori Fujimori obtained her MSc and PhD degrees in chemistry in 2019 at Kyoto University under the supervision of Prof. Norihiro Tokitoh on the synthesis of novel aromatic compounds containing heavier group 14 elements. In October 2019, she was awarded a Eurotech-Marie Curie Fellowship and joined the group of Prof. Inoue at the Technical University of Munich. She continued her stay at the TUM as an Alexander von Humboldt Fellow where she pursued her interest in low-valent silicon chemistry.

Arseni Kostenko

Arseni Kostenko obtained his bachelor's degree in Molecular Biochemistry at the Technion Israel Institute of Technology, subsequently pursuing a Master's degree and a PhD in the field of silicon chemistry under the supervision of Prof. Yitzhak Apeloig. In 2016, he received the Alexander von Humboldt fellowship and joined the group of Prof. Matthias Driess at the Technical University of Berlin. Since 2021, Dr Kostenko has been working in the group of Prof. Shigeyoshi Inoue at the Technical University of Munich, computationally studying the structure and reactivity of low-valent main group element-based compounds.

Shigeyoshi Inoue

Shigeyoshi Inoue obtained his PhD from the University of Tsukuba in 2008 supervised by Prof. Akira Sekiguchi. Following postdoctoral study at the TU Berlin with Prof. Matthias Driess, he established an independent research group within the framework of the Sofja Kovalevskaja program in 2010. Since 2015 he has been on the faculty at the Technical University of Munich (TUM). His research interests include the synthesis and reactivity of main-group compounds with unusual structures and unique electronic properties, with the goal of finding novel applications in synthesis, catalysis and materials science.

---

Introduction

Recent developments in the chemistry of low-valent/oxidation state main-group compounds have revealed their inherent electronic properties. These allow them to mimic the behaviour of transition-metal complexes, with regard to their ability to form coordination complexes, engage in multiple bonding, undergo redox reactions, and exhibit catalytic activity.^1–5 An important class of low-valent main group species capable of such behaviour are tetrylenes [R₂E:] (E = Si, Ge, Sn, Pb) – the heavier analogues of carbenes (R₂C:). Numerous representatives of these species are able to activate small molecules under mild conditions and can be used as ligands in catalytic organic reactions due to their ambiphilic nature.^6–12 The heavier congeners of carbonium ions, namely tetrylium cations [R₃E]⁺ (E = Si, Ge, Sn, Pb), have also garnered significant interest in recent decades attributed to their pronounced electrophilic nature, leading to various applications in catalysis and bond activation chemistry.^13–16 A related and less explored class of compounds are the tetryliumylidene ions (E = Si, Ge, Sn, Pb), which exhibit distinctive electronic properties, merging the strong electrophilicity observed in tetrylium cations with the Lewis ambiphilic character found in tetrylenes. In their idealised form, these mono-substituted cations have two vacant degenerate p orbitals and a lone pair at the E(II) centre (Fig. 1).¹⁷

Fig. 1 Frontier orbitals of free (left) and donor-stabilised tetryliumylidene ions (centre, right) (D = Lewis base).

The bonding situation in parent tetryliumylidenes can be examined from the point of view of the valent ns and np orbitals and their proclivity to hybridise. The inert s-pair effect, which reflects the situation where the diminishing hybridisation between ns and np orbitals makes only np orbitals available for bonding, has a great influence on the electronic structure of p-block element species. Consideration of the relative roles of s and p orbitals can be done using Weinhold's natural bond orbital (NBO) scheme,¹⁸ through which natural atomic orbital (NAO) hybridisations of natural localised molecular orbitals (NLMOs) can be obtained.¹⁹ It is known that tetrylenes exhibit higher tendency to the inert pair effect as we go down the period – the heavier elements tend to retain a low oxidation state, and forming [R₂E:] species becomes easier – thus, lead, for example, prefers Pb^IIR₂ species over Pb^IVR₄. Calculations (at the B3LYP^20–23/def2-TZVP²⁴ level of theory using Gaussian 16 (ref. 25) and NBO7 (ref. 26) software) show that the inert s-pair effect is significantly more pronounced in tetryliumylidenes in comparison with tetrylenes, even for lighter elements. In [H₂E:] series, there is a high p character of the lone pair in the parent carbene (0.68), and a significant drop in the p/s ratio in the silylene (0.30) (Fig. 2, top). The ratio gradually decreases when going down the period, and is 0.13 in [H₂Pb:]. In the case of [H–E:]⁺, even the methylidyne ion lone pair has a s/p ratio (0.13) similar to that of [H₂Pb:]. The decrease is less significant in [H–Si:]⁺, and only a slight change is observed when going down the period.

Fig. 2 Lone pair (top) and E–H bond (bottom) NAO/NLMO p/s hybridisation ratios in tetrylenes and tetryliumylidenes at the B3LYP/def2-TZVP level of theory.

An analogous situation is observed when comparing the E–H bonds in tetrylenes and tetryliumylidenes (Fig. 2, bottom). The C–H bond in [H₂C:] has a low p/s ratio of 3.67 which increases to 13.61 in plumbylene. In contrast, in the tetryliumylidene series, the methylidyne ion already has a high p/s ratio of 7.68, which is comparable with that of [H₂Ge:]. Overall, when going down the period the p composition of the hybrid orbital increases from 87.8% in [H–C:]⁺ to 95.4% in [H–Pb:]⁺. These results indicate that while hybridisation is increasingly more difficult in the case of tetrylenes when going down the periodic table, in tetryliumylidenes the effect is less dramatic, since sp hybridisation is difficult to begin with.

[R–E:]⁺ have the potential for versatile synthetic applicability due to their unique electronic structure with multiple reactive sites, originating from the two electrophilic vacant p-orbitals and one nucleophilic lone pair. Such arrangement should in principle allow the formation of up to three new bonds in a single reaction.²⁷ Thus, tetryliumylidene ions are expected to be highly reactive, and in general, electron donor stabilisation by inter- or intramolecular coordination of Lewis bases is required to isolate these species as stable compounds.²⁸

In the pioneering studies, Jutzi and co-workers reported tetryliumylidene ions [Cp*E:]⁺ (E = Ge, Sn, Pb; Cp* = η⁵-C₅Me₅) stabilised by the hyper-coordinating cyclopentadienyl group.^29,30 Following these discoveries, researchers have successfully isolated a range of tetryliumylidene ions.³¹ This was made possible by employing sterically demanding ligands for kinetic stabilisation, including bulky amide groups,³² β-diketiminato,³³ aminotroponiminate,³⁴ and cyclophane groups.³⁵ Alternatively, Lewis basic ligands, such as carbenes,^36,37 imines,³⁸ or weakly coordinating arenes,³⁹ have been utilised for the enhancement of the thermodynamic stability. Several decades after the discovery of Jutzi's heavier tetryliumylidene ions [Cp*E:]⁺ (E = Ge, Sn, Pb), in 2004, the same group succeeded in the synthesis of the first isolable silyliumylidene ion, the pentamethylcyclopentadienyl-silicon(II) cation [Cp*Si:][B(C₆F₅)₄].⁴⁰ Since these seminal discoveries, the reactivity of tetryliumylidene ions has been studied to some extent. It has been demonstrated that these species can be used as synthons for the isolation of novel low-valent compounds containing a heavier group 14 element. For example, a new class of fascinating main-group compounds, tetrylones, which are two-coordinate E⁰ species (L:→E←:L; E = Si,⁴¹ Ge⁴²), were synthesised from the reduction of the corresponding chloro-tetryliumylidenes. In addition, the ambiphilic character of tetryliumylidene ions, which mimics the frontier d-orbitals found in transition metals, enables them to activate small molecules (e.g., CO₂, N₂O, H₂S, H₂O, and heavier chalcogens).^43–47 Furthermore, they have been successfully applied in several catalytic transformations without transition metals.^48–51 Various sterically bulky electron donor ligands have been developed in the last few decades, allowing the preparation of donor-stabilised hydro-tetryliumylidenes [H–E:]⁺ with superior potency for widespread applications. For instance, hydrosilylation and hydroboration reactions were catalysed by [H–E:]⁺.^52–54 In this minireview, we have summarised the recent developments in the chemistry of tetryliumylidene ions, including their reactivity towards small molecules, as precursors to a variety of additional low-valent species and catalytic applications. While some tetryliumylidene ions were isolated and employed in catalysis by taking advantage of cooperative effects between transition metals and group 14 elements,^55–66 this review highlights only the main-group examples.

Silyliumylidenes

The first synthesis of a silyliumylidene ions in 2004 by Jutzi et al. was achieved by the reaction of decamethylsilicocene, (Cp*)₂Si, with the proton transfer reagent [Cp*H₂][B(C₆F₅)₄] to form [Cp*Si:][B(C₆F₅)₄] 1a – the first isolable silyliumyidene ion.⁴⁰ In a similar fashion, in 2018, the group of Filippou showed the successful synthesis of 1a by protonation of (Cp*)₂Si with one equivalent of [H(Et₂O)₂][B(C₆F₅)₄].⁵⁷ Another procedure to form this compound was reported more recently, in 2019, by Fritz-Langhals with the hydride abstraction of by a tritylium salt, resulting in tetramethylfulvene and 1a on a technical scale.⁴⁸ Jutzi et al. also reported the formation of penta-iso-propylcyclopentadienylsilicon(II) cation iPr₅C₅Si⁺1b by reacting the mixed silicocene (ⁱPr₅C₅)(Me₅C₅)Si with H(OEt₂)₂⁺Al(O^tBu^F)₄⁻ (Fig. 3).⁶⁷

Fig. 3 Reported silyliumylidene ions. (O^tBu^F = OC(CF)₃; Dipp = 2,6-ⁱPr₂C₆H₃; SIDipp = 1,3-bis(Dipp)imidazolidin-2-ylidene; BAr₄^F = B[C₆H₃-3,5-(CF₃)₂]₄; NHC = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene; E_Mind = 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl; Tbb = ((5-(tert-butyl)-1,3-phenylene)bis(methanetriyl))tetrakis(trimethylsilane); mTer = 2,6-Mes₂C₆H₃; Mes = 2,4,6-Me₃C₆H₂; Tipp = 2,4,6-ⁱPr₃C₆H₂; E_ind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl; DMAP = 4-(N,N-dimethylamino)pyridine).

Cyclopentadienyls and their derivatives are 2σ, 4π electron donor ligands and are routinely used for stabilisation of electron-deficient species. Their proclivity to afford hyper-coordinated centres allowed, in this case, for stabilisation of highly reactive silyliumylidene ions. Due to this, 1a and 1b cannot be considered true mono-substituted silyliumylidene ion complexes. It took almost two decades after the isolation of 1a, when Hinz reported the first achievements towards the mono-substituted silicon(II) cation [RSi:]⁺ (R = bulky carbazolyl substituent) 2 by halide abstraction from a base-free halosilylene with Ag[Al(O^tBu^F)₄]. Even though there are arene interactions between the Si atom and the carbazolyl scaffold, this silyliumylidene bears no other σ-donors except for the carbazolyl substituent.⁶⁸

The first two-coordinated silyliumylidene ion 3a was reported by Driess et al. in 2006. Here, the authors protonated the sterically demanding β-diketiminate ligand backbone of the corresponding zwitterionic N-heterocyclic silylene with Jutzi's oxonium acid [H(Et₂O)₂][B(C₆F₅)₄].⁶⁹ More recently, the group of Aldridge reported the silyliumylidene ion 3b stabilised by a β-diketiminate ligand featuring a backbone with NMe₂ groups.⁷⁰ In 2014, Filippou et al. reacted a chromium silylidyne complex salt, containing the first Cr–Si triple bond, with CO to achieve the four-legged piano–stool complex cation 4.⁴³ Very recently, in 2022, Kato et al. showed the formation of the Ni(0)-stabilised Si(II) species 5. The dative Ni → Si σ-interaction and π-donations from the amino- and Ni-moieties stabilise the highly electrophilic Si centre.⁷¹

Most of the known silyliumylidene ions are three-coordinated, stabilised kinetically, via sterically demanding ligands, and electronically, via donation to the silicon centre by Lewis bases. A majority of reported silyliumylidene ions use stabilisation by N-heterocyclic carbenes (NHCs). The first example of this class of compounds was reported by the group of Filippou in 2013.⁷² Here, the NHC stabilised silicon(II) diiodide was reacted with a sterically more demanding N-heterocyclic carbene to get to the silyliumylidene ion 6. At essentially the same time, Driess et al. isolated the chloro-silyliumylidene ion 7 stabilised by a cyclic bis-NHC.⁴¹ In 2014, Tokitoh et al. and our group independently used NHCs to synthesise aryl-substituted silyliumylidene ions. The group of Tokitoh treated dibromodisilenes with an excess of NHCs forming the bis-NHC adduct [ArSi(NHC)₂][Br] 8.⁷³ Meanwhile, our group utilised dichloroarylsilanes and three equivalents of NHCs in a facile one-pot reaction to obtain the corresponding silyliumylidene ions 9a,b.⁷⁴ In 2016, the group of So showed the formation of the silyliumylidene iodide 10, stabilised by both an NHC and a cyclic alkyl(amino) carbene (cAAC), by reacting the corresponding cAAC stabilised silicon(II) diiodide with NHC.⁷⁵ One year later, the same group isolated the first NHC-stabilised parent-silyliumylidene ion 11 by reacting an NHC–iodosilicon(I) dimer with four equivalents of NHC.⁷⁶ Matsuo et al. reported a similar approach to Tokitoh in 2018. Here, they used E_ind-substituted 1,2-dibromodisilene to form with four equivalents of NHCs the corresponding bis-NHC adduct of the formal aryl-silyliumylidene ion 12.⁷⁷ In 2019, utilising the already reported approach, our group could show the formation of the silyl-substituted NHC-stabilised silyliumylidene ions 9c,d.⁷⁸ Additionally, very recently we reported the synthesis of a neutral bidentate NHI ligand (NHI = N-heterocyclic imine) with a saturated imidazoline backbone and the formation of the corresponding chloro-silyliumylidene ion 13.^79,80

Apart from NHCs, the group of Driess utilised a bis(iminophosphorane) chelate ligand to form the chloro-silyliumylidene 14. The two N=PⁿBu₃ ylide moieties can act as very strong Brønsted and Lewis bases.⁸¹ In 2013, So et al. isolated the silyliumylidene ion 15 stabilised by an amidinate ligand and 4-dimethylaminopyridine (DMAP).⁸² More recently, Kato et al. reported in 2022 the use of different Lewis bases, such as SMe₂, PMe₃, and DMAP, and even observed silylene-stabilised silyliumylidene complexes 16.⁸³

Reactivity of silyliumylidenes and their use in catalytic reactions

Among tetryliumylidenes, silyliumylidene ions show the widest diversity in reactivity reported so far. The first stable silyliumylidene 1 reacts with the metalate Na[TpMeMo(CO)₂(PMe₃)] to afford the silylidyne complex [TpMe(CO)₂MoSi(η³-Cp*)] (TpMe = κ³-N,N′,N′′-hydridotris(3,5-dimethyl-1-pyrazolyl)borate) featuring a delocalised Mo–Si bond with a partial triple bond character.⁵⁷ Despite the high steric demand of the carbazolyl moiety, pseudo-mono-coordinated silyliumylidene ion 2 retains high reactivity and reacts with an amine to form three bonds at the silicon atom in one reaction 17 – justifying being called a “supersilylene” (Scheme 1).^27,68

Scheme 1 Reaction of 2 with two equivalents of amines to form 17 (R = bulky carbazolyl ligand).

β-Diketiminate stabilised silyliumylidene 3b can react with NH₂^tBu via formal N–H oxidative addition to form a Si(H)(NH^tBu) moiety.⁷⁰ When two-coordinated silyliumylidene ion 4 is exposed to an N₂O atmosphere, rapid formation of metallosilanone occurs.⁴³5 shows multiple examples of activation of small molecules, such as MeOTf 18, DMAP 19, H₂20, diphenylacetylene 21, and 2,3-dimethyl-1,3-butadiene 22 (Scheme 2).⁷¹

Scheme 2 Multiple examples for small molecule activations of 5 (R₂P = P(N^tBu)₂SiMe₂, NHC = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene).

Silyliumylidenes could be used as precursors for the formation of additional exotic low-valent species. Thus, dehalogenation of chloro-silyliumylidene 7 yielded the cyclic bis-NHC stabilised silylone 23 (Scheme 3),⁴¹ and mixed NHC–cAAC stabilised silyliumylidene 10 could be reduced with KC₈ to form the bent silaallene 33 (Scheme 4).⁷⁵

Scheme 3 Reductive dechlorination of 7.

Scheme 4 Reductive dehalogenation of 10.

Aryl- and silyl-substituted silyliumylidene ions 9 showed a plethora of reactivities with small molecules, such as the C–H insertion reaction with three equivalents of phenylacetylene to form 1-alkenyl-1,1-dialkynylsilane 24.⁷⁴9 can activate CO₂ to form the silaacylium ion 25,⁴⁴ as well as the NHC-stabilised heavier silaacylium ions 26 with sulphur, selenium, and tellurium via chalcogen-atom transfer.⁴⁵ Moreover, 9 can activate the S–H bond of hydrogen sulfide to form the thiosilaaldehyde 27,⁴⁶ react with GaCl₃ and water to afford the silaaldehyde 28,⁴⁷ and convert to the Si(IV) complex 29 by reaction with triflic acid (Scheme 5).⁸⁴ In some of these reactions, one NHC acts as a H–Cl scavenger, which is a substantial thermodynamic driving force for the reaction progress. Furthermore, compound 9 derivatives are capable of forming transition metal complexes. They can either react with coinage metals, such as copper, silver, and gold, to form the metal chloride adducts 30,⁸⁵ or transition metal carbonyls (M = Cr, Wo, W, Fe) to form the M(CO)_n (n = 4 or 5) silyliumylidene adducts 31.⁸⁶ Additionally, 9 can undergo a facile insertion into M–Cl bonds (M = Ru, Rh), forming the chlorosilylene transition-metal complexes 32 (Scheme 5).⁸⁷

Scheme 5 Reported reactivity of 9 (X = Cl, OTf; p-cym = 1-Me-4-ⁱPr-benzene).

When reacted with elemental sulphur, the bis(iminophosphorane) coordinated chloro-silyliumylidene 14 is oxidised to the chlorosilathionium salt 33 (Scheme 6).⁸¹ This reactivity is similar to the reactivity of 9 with chalcogens.

Scheme 6 Oxidation of 14 with elemental sulphur.

Bis-NHI stabilised silyliumylidene 13 shows the formation of the heavier silaacylium ions (E = S, Se, Te) 34 in reaction with chalcogens, coinage metal complexes 35 with CuCl, AgCl, and (Me₂S)AuCl, and the heterobimetallic gold–iron complex 36 in reaction with a gold–chloride complex in the presence of tetracarbonylferrate (Scheme 7).⁷⁹

Scheme 7 Reported reactivity of 13.

Amidinate-substituted silyliumylidene 15 can undergo oxidative addition at the silicon centre with the amidinate silicon(I) dimer to form the disilylenylsilylium triflate 37, react with nucleophilic reagents, such as K[HB(ⁱBu)₃], to afford the corresponding silylsilylene 38, and form the silanethionium triflate 39 with elemental sulphur (Scheme 8).⁸²

Scheme 8 Reported reactivity of 15.

Lewis base stabilised silyliumylidene 16 is able to undergo the [1+2]-cycloaddition with ethylene at the silicon centre to form the silacyclopropyl cation 40, [4+1]-cycloaddition with 2,3-dimethyl-1,3-butadiene (41), [2+1]-cycloaddition with diphenylacetylene (42), oxidation of the Si(II) centre with CO₂ to generate a cationic silanone 43, and oxidative addition of triethylsilane, diphenylphosphine, or pinacolborane to give the corresponding silylium ion 44 (Scheme 9).^83,88

Scheme 9 Reported reactivity of 16 (R¹ = Ph, SiMe₃, Mes; H–E = HSiEt₃, HPPh₂, HBpin; R₂P = P(N^tBu)₂SiMe₂).

While there are a multitude of examples of silyliumylidene ion reactivities, to date there are only several examples of their use as catalysts. The first isolable silyliumylidene 1 proved to be an efficient non-metallic catalyst for the hydrosilylation of unsaturated carbon–carbon bonds at low catalyst loadings of <0.01 mol% and the Piers–Rubinsztajn reaction.⁴⁸ In the proposed mechanism for the hydrosilylation, the silyliumylidene first activates the silicon hydrogen bond to form the hydrogen-bridged complex 45. The substrate can insert into the silicon–hydrogen bond of the silane to form 46. Subsequent elimination of the hydrosilylated product regenerates the catalyst (Scheme 10).

Scheme 10 Proposed mechanism of the hydrosilylation of carbon–carbon multiple bonds (R₃ = Me₂OTMS, Me(OTMS)₂, Et₃, Me₂Ph, Me₂Cl, MeCl₂; R′ = nC₄H₉, SiMe₃, Ph, cyclo-hexene, norbornene).

The NHC-stabilised parent-silyliumylidene ion [NHC₂SiH]⁺11 shows unprecedented abilities in the catalytic hydroboration of CO_2, aldehydes, ketones, and pyridines with a catalyst loading of 10 mol%, as well as isocyanates with a catalyst loading of 1 mol% (Scheme 11).^52,54

Scheme 11 Hydroboration of aldehydes, ketones, CO₂, pyridines, and isocyanates with pinacolborane.

Furthermore, 11 catalyses the chemoselective N-formylation of amines using CO₂ and phenylsilane with a catalyst loading of 5 mol%. The reaction mechanism was studied using density-functional theory (DFT) calculations. The silyliumylidene sequentially activates first CO₂, then simultaneously phenylsilane and amines via transition state 47. Following a dihydrogen elimination mechanism, formamides, siloxanes, dihydrogen gas, and the regenerated catalyst are formed (Scheme 12).⁵³

Scheme 12 N-Formylation of amines using CO₂ and phenylsilane (HNR₂ = primary and secondary amines).

Germyliumylidenes

The richest chemistry among tetryliumylidenes, in terms of the number of different reported examples, is that of germyliumylidenes. Over two decades before the silicon analogue, the cyclopentadienyl moiety was utilised by Jutzi's group to achieve the synthesis of the first isolable germyliumylidene ion 48 by reaction of decamethylgermanocene (Cp*)₂Ge with HBF₄.²⁹ Following this outstanding work, numerous germyliumylidenes have been reported. There are several synthetic strategies for the isolation of Ge(II)-derived monocations, most of them utilising neutral ancillary ligands (Fig. 4).

Fig. 4 Reported germyliumylidene ions.

For example, N-heterocyclic carbenes (NHCs) have been used extensively to stabilise germyliumylidenes. Driess et al. reported the formation of bis-NHC stabilised chloro-germyliumylidene 49 by reacting GeCl₂·dioxane with the corresponding ligand.⁴² They also isolated parent-germyliumylidene hydride 50 stabilised by a bis(NHC)borate ligand.³⁷ By the addition of two equivalents of NHC to an aryl chloro-germylene, our group could isolate the corresponding three-coordinate germanium cation 51.⁸⁹

Utilising an imino-N-heterocyclic carbene ligand, Kinjo et al. obtained chloro- and methyl-substituted germyliumylidene 52a,b, respectively.^90,91 More recently, a diimino-carbene was utilised by Nikonov et al. to stabilise the germyliumylidene 53.⁹² The groups of Glorius and Hahn reported the first intramolecularly NHC-stabilised germyliumylidene 54.⁹³ Utilisation of a bis(N-heterocyclic silylenyl)pyridine pincer ligand led to the formation of chloro-germyliumylidene 55 reported by Driess et al.⁹⁴

In 2016, it was demonstrated that a single NHC with adequate steric bulk together with a sterically more demanding substituent is also sufficient for germyliumylidene stabilisation. Thus, Rit, Aldridge, et al. isolated the two-coordinate germyliumylidene 56a bearing a CH(SiMe₃)₂ substituent, stabilised by a Dipp-substituted NHC (Dipp = 2,6-diisopropylphenyl).⁹⁵ More recently, the same group reported NHC-stabilised germyliumylidenes 56b–d with smaller NHCs, but larger substituents at the Ge centre.⁹⁶ Rivard et al. utilised the steric protection provided by the extremely bulky trityl (CPh₃)–NHC to prepare the chloro-germyliumylidene 57.⁹⁷ Using a single Lewis base, Krossing, Jones, and co-workers reported the bulky amido germanium(II) monocation 58 stabilised by either weak intramolecular arene interactions or DMAP.³²

NHIs can also be used for germyliumylidene stabilisation. By utilising Dipp–NHIs, we could isolate the four-membered amino(imino)germyliumylidene 59 and the germylene-germyliumylidene 60 borate salts, respectively.^98–100 Computational analysis indicated the latter to have a considerable bis(germyliumylidene) character. In 2020, we used bidentate bis(N-heterocyclic imine) ligands to stabilise the three-coordinate chloro-germyliumylidenes 61.^80,101

A similar approach to [R–Ge:]⁺ stabilisation is the use of imino ligands. Recently, Majumdar et al. reported bis(chlorogermyliumylidene) stabilised within a flexible tetra-dentate 2,7-bis(2-pyridyl)-3,6-diazaocta-2,6-diene ligand 62 (ref. 102) and within bifunctional PNNP ligand frameworks.¹⁰³ In 2018, Kinjo et al. reported the bis(imidazolyl) supported chloro-germyliumylidene 63 in which the Ge₂N₂C₂ six-membered ring possesses two Ge–Cl units.¹⁰⁴ In 2012, the groups of Roesky and Stalke used the substituted Schiff base 2,6-diacetylpyridinebis(2,6-diisopropylanil) as Lewis base, which mediated the autoionisation of GeCl₂ to the germyliumylidene 64.³⁸ The group of Driess isolated the bis(iminophosphorane) chelate stabilised chloro-germyliumylidene 65 similar to the structurally equivalent silyliumylidene 3.¹⁰⁵ In 2013, Jambor et al. showed the synthesis of the chloro-germyliumylidene 66 by treating a neutral 2-[C(CH₃)=N(C₆H₃-2,6-ⁱPr₂)]-6-(CH₃O)C₆H₃N ligand with GeCl₂, which spontaneously dissociates.¹⁰⁶ Majumdar and co-workers reported acyclic flexible diiminodi(furan) and diiminodi(thiophene) ligands in which the two imino nitrogen coordinating sites can stabilise the chloro-germyliumylidene ion 67.¹⁰⁷

Another type of germyliumylidene stabilisation is the use of monoanionic ligands that form cyclic compounds, such as the aminotroponiminate derivatives 68a,b utilised by Dias et al. in 1997,³⁴ and, more recently, by the group of Nagendran in 2019.⁴⁹ Power et al. used a Dipp-substituted β-diketiminate ligand to form the two-coordinated germyliumylidene 69a.³³ In 2020, Aldridge et al. utilised a β-diketiminate ligand bearing two NMe₂ groups to form the germyliumylidene 69b.⁷⁰ The structurally similar Ge(II) cation 70 was isolated by Müller and co-workers by protonating the corresponding germylene.¹⁰⁸ Similar to silyliumylidene 2, stabilised by arene coordination, Hinz showed the formation of the carbazolyl-substituted pseudo-one-coordinate germyliumylidene 71 by halide abstraction.¹⁰⁹

Schmidbaur reported a related approach, for which he reacted cyclophane with one equivalent of GeCl₂ to form the [2.2.2]-paracyclophane chloro-germyliumylidene complex 72.³⁵

Two additional examples of methods to stabilise germyliumylidenes were provided by the groups of Alcarazo and Driess.^110,111 Alcarazo et al. isolated the two-coordinated chloro-germyliumylidene 73 stabilised by σ- and π-donation from a monodentate carbodiphosphorane;¹¹⁰ while Hadlington, Driess et al. reported the bisphosphinidine stabilised chloro-germyliumylidene 74.¹¹¹

Reactivity of germyliumylidenes and their use in catalytic reactions

Several examples of the reactivity of germyliumylidene ions have been reported, while only a few catalytic applications are known. Similarly to the silicon analogue, the chloro-germyliumylidene 49 can be dehalogenated (by sodium naphthalenide) to form the cyclic bis(NHC) Ge(0) complex 75 (Scheme 13).⁴² The reduction of 52a with KC₈ leads to the formation of the corresponding germylone 76 (Scheme 13), similar to germyliumylidene 64.^90,92 Germyliumylidene 51 reacts with N₂O to form the germa-acylium ion 77 (Scheme 14).¹¹²

Scheme 13 Dechlorination of 49 and 52a to form the corresponding germylone.

Scheme 14 Formation of germa-acylium ion 77.

By reacting it with K₂[Fe(CO)₄], germyliumylidene 55 forms the germylone–iron carbonyl complex 78 stabilised by a bis(NHSi)pyridine pincer ligand (Scheme 15).⁹⁴

Scheme 15 Reaction of 55 with K₂[Fe(CO)₄].

56a can undergo oxidative addition of dichloromethane (79), [2+1] cycloaddition with phenylacetylene (80), and formation of an imido complex (81) by reacting with Me₃SiN₃ (Scheme 16).⁹⁵

Scheme 16 Reported reactivity of 56a.

56b can insert into the Si–H bond of phenylsilane.⁹⁶ Germylene-germyliumylidene 60 reacts with Me₃SiOTf to form a bis(triflate).¹⁰⁰ Bis(NHI)-stabilised chloro-germyliumylidene 61 can react with NaBH₄ to the hydridogermyliumylidene–BH₃ adduct 82 and with LiAlH₄ to the corresponding aluminium dihydride 83 (Scheme 17).¹⁰¹

Scheme 17 Reactions of 61 with NaBH₄ and LiAlH₄.

By using organosilicon reductants bis(chlorogermyliumylidene) 62 can undergo reductive cyclisation to form the 2,3-di(pyridin-2-yl)-substituted piperazine 84 with high diastereoselectivity (Scheme 18).¹⁰²

Scheme 18 Reductive cyclisation of 62.

65 can undergo oxidation with elemental sulphur to form a chloro-germathionium salt.¹⁰⁵

In contrast to its Si analogue, germyliumylidene 69b does not undergo oxidative addition but reacts with NH₂^tBu to form the simple adduct.⁷⁰ Two-coordinated chloro-germyliumylidene 73 can form a DMAP adduct and reacts with elemental sulphur to form a dimeric S=Ge double-bond species.¹¹⁰

In terms of catalysis, germyliumylidenes have only rarely been applied in catalytic reactions. As already mentioned, 51 can form the germa-acylium ion 77, which is used as a precatalyst for the germanium-catalysed N-functionalisation of amines with CO₂.¹¹²51 can catalyse the reduction of CO₂ with amines and phenylsilane, the cyanosilylation of carbonyls with TMSCN, and the hydroboration of aldehydes with pinacolborane (HBpin) (Scheme 19).⁵⁰

Scheme 19 Multiple reactions catalysed by germyliumylidene 51.

The proposed mechanism of the hydroboration catalysed by 51 shows the initial substitution of IMe₄ (IMe₄ = 1,3,4,5-tetramethyl-1H-imidazole-3-ium-2-ide) by an aldehyde to form 86via transition state 85. The subsequent B–H bond activation in HBpin is mediated by the previously released IMe₄. The formed adduct transfers the hydride to the carbonyl carbon in 86, while the boron coordinates to the oxygen centre to form intermediate 88. Subsequently, the IMe₄ re-associates with the Ge centre to regenerate the catalyst and obtain the hydroboration product (Scheme 20).

Scheme 20 Proposed mechanism for the hydroboration of aldehydes catalysed by 51.

Germyliumylidene 68b can act as a precatalyst in the catalytic hydroboration of aldehydes and ketones with pinacolborane.⁴⁹ In the proposed mechanism, 68b reacts with HBpin to form the hydridogermylene 89 as the active catalyst. Subsequent reaction with the substrate via a four-membered heterocyclic transition state leads to formation of the germylene alkoxide intermediate 90. Through σ-bond metathesis with HBpin the catalyst is regenerated and the hydroboration product is formed (Scheme 21).

Scheme 21 Proposed mechanism of the hydroboration of carbonyls catalysed by 68b (L = aminotroponiminate ligand).

Stannyliumylidenes

The chemistry of stannyliumylidenes [R–Sn:]⁺ was commenced with the first isolation of [Cp*Sn:]⁺[BF₄]⁻91, reported by Jutzi and co-workers in 1980.²⁹91, like the other first examples of tetryliumylidenes was stabilised by the hyper-coordinating pentamethylcyclopentadienyl. Since then, several stannyliumylidenes have been isolated by sterically demanding ligands (kinetic stabilisation) and/or electronically stabilising ligands based on heteroatom substituents. Halogen-substituted stannyliumylidenes [X–Sn:]⁺ (X = halogen) are crucial intermediates since they have been employed as synthons for the isolation of novel low-valent tin compounds such as stannyliumylidene derivatives [R′–Sn:]⁺ and Sn(0) through substitution reactions and halide abstraction, respectively.

Unlike germyliumylidene, where several examples of stabilisation using carbenes were reported, in the case of tin, there is only one reported mixed NHC–cAAC-substituted species 92.¹¹³

Our group could isolate the N-heterocyclic imine (NHI) stabilised stannyliumylidenes 93,¹¹⁴94,¹⁰¹ and 95.¹¹⁵ Similar to germanium, neutral chelating ligands with nitrogen atoms have attracted attention for stannyliumylidene stabilisation due to the tunability of the steric and/or electronic nature by changing the substituents on the nitrogen atoms. Thus, iminopyridine 96,¹⁰⁶ diiminopyridine (DIMPY) 97,³⁸ and bis(α-iminopyridine) 98 (ref. 116) have been utilised.

Coordination by bisphosphinidene 99,¹¹¹ mixed phosphine-imino 100¹¹⁷ and ferrocene-bridged N-heterocyclic carbene-phosphinidene (NHCP) 101 (ref. 118) ligands have been reported.

Stannyliumylidene ions stabilised by monoanionic ligands such as aminotroponiminate 102,¹¹⁹ β-diketiminate 103a,b,^70,120 extremely hindered amide 104,³² bis(oxazoline) 105,¹²¹ organo-phophane oxide 106,¹²² and bulky carbazolyl 107 (ref. 109) were isolated. Schmidbaur et al. utilised a [2.2.2]–paracyclophane ligand to stabilise stannyliumylidene 108 (Fig. 5).³⁵

Fig. 5 Reported stannyliumylidenes.

Reactivity of stannyliumylidenes

The reactivity of stannyliumylidenes is scarcely studied, and only a few examples of their clearly identified transformations have been reported so far. The β-diketiminate stabilised stannyliumylidene 103b forms, like its germanium congener, with NH₂^tBu the simple adduct.⁷⁰ The organo-phosphane oxide substituted stannyliumylidene 106 reacts with elemental sulphur to form a heavier acylium ion intermediate, which upon head-to-tail dimerisation yields the corresponding dication 109 containing a four-membered Sn–S–Sn–S ring (Scheme 22).¹²² This type of reactivity would be expected from heavy ketone derivatives. The NHI-substituted stannyliumylidene has been demonstrated to undergo transmetalation reactions. Thus, in the reaction of 94 with NaBH₄ in a mixture of THF and 1,2-difluorobenzene (DFB), selective transformation to form the bisNHI-stabilised dihydroboronium 110 was observed. The precipitation of elemental tin accompanied this process. Analogously, treatment of 94 with LiAlH₄ resulted in the formation of the aluminium dihydride 111. In a similar fashion, transmetalation of 94 with an equivalent of GeCl₂·(dioxane) in acetonitrile at room temperature furnished the germyliumylidene cation 61 (Scheme 23).¹⁰¹

Scheme 22 Reaction of stannyliumylidene 106 with S₈ (L = DMAP).

Scheme 23 Transmetalation reactions of stannyliumylidene 94 (DFB = 1,2-difluorobenzene).

Like the NHI-substituted counterpart, NHCP-stabilised stannyliumylidene 101 could also be transmetalated. 101-I reacts with CuCl to provide the transmetalation product 112 (Scheme 24).¹¹⁸ In addition, chloro-stannyliumylidene 101-Cl was utilised to transfer the Sn(II) synthon from the bisphosphinidene to a bisimine. Treatment of the bisNHCP-supported 101-Cl with bisNHI in THF-d₈ gave a bisNHI-stabilised 94 and free bisNHCP (Scheme 25).¹¹⁸

Scheme 24 Transmetalation reaction of stannyliumylidene 101-I.

Scheme 25 Sn(II) transfer reaction between bisNHCP and bisNHI.

To date, to the best of our knowledge, except for the dinuclear tin complex 95 that was found to catalyze the hydroboration of aldehydes and ketones, no other examples of catalytic transformations that utilize stannyliumylidenes have been reported.¹¹⁵

Plumbyliumylidenes and catalytic reactions

Compared to lighter congeners, the chemistry of the heaviest plumbyliumylidenes [R–Pb:]⁺ remains underdeveloped. This can be attributed to the bad reputation of lead due to its toxicity. Like the lighter congeners, the first example of isolable plumbyliumylidene emerged from Jutzi's group. Accordingly, in 1989, Jutzi and Nöth reported 113, a plumbyliumylidene stabilised by a pentamethylcyclopentadienyl ligand (Fig. 6).³⁰ Since then, several examples of isolable plumbyliumylidenes stabilised by bulky and strongly donating substituents have been reported. Power and co-workers succeeded in isolating the plumbyliumylidene 114 with a terphenyl group in 2004.¹²³ Subsequent to this, Fulton and co-workers reported the plumbyliumylidenes 115 that are stabilised by a β-diketiminate ligand.¹²⁰ In 2006, Godwin et al. obtained the three-coordinated N₂S(alkylthiolate) stabilised plumbyliumylidene ion 116.¹²⁴ In addition, the coordination of platinum complex Pt(PCy₃)₂ (Cy = cyclohexyl) can stabilise the plumbyliumylidene 117.⁶⁶ The carbazole-based bulky substituent that has been utilised in Si, Ge, and Sn analogues was also used to isolate the corresponding plumbyliumylidene 118.¹⁰⁹

Fig. 6 Reported plumbyliumylidene ions (Cy = cyclohexyl; Y = =B(C₆F₅)₄ or Al(OC₄F₉)₄).

Recently in 2022, Nakata and co-workers prepared the plumbyliumylidene 119 supported by an N,N′-di-tert-butyl-iminophosphonamide ligand. 119 is one of the two examples of a cationic low-coordination lead compound capable of acting as a catalyst.⁵¹ In the proposed mechanism, which was also supported by quantum chemical calculation, the Lewis acidic 119 captures a carbonyl compound (benzophenone or benzaldehyde) to furnish the plumbyliumylidene–benzophenone complex Int-121-[B(C₆F₅)₄]. This reactive intermediate can further react with an HBpin (HBpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane) to form a four-membered intermediate Int-123. Subsequent reductive elimination of a boronate ester 124 regenerates the plumbyliumylidene 119. The catalytic cycle proceeds at room temperature with a low for a main-group complex catalyst loading of 0.1 mol% (Scheme 26).

Scheme 26 Plumbyliumylidene-catalysed hydroboration of benzaldehyde (R = H) and benzophenone (R = Ph).

At around the same time, the group of Kato obtained the phosphine or amine stabilised plumbyliumylidene 120, which readily reacts with phenylacetylene to the corresponding cationic vinylplumbylene 125via alkyne insertion into the Pb–L bond (Scheme 27).¹²⁵

Scheme 27 Reaction of 120 with phenylacetylene (R₂P = =P(N^tBu)₂SiMe₂, L = PMe₃, PPh₃, HNⁱPr₂, HNEt₂).

In the case of amine stabilisation, plumbyliumylidene 120 can catalyse the hydroamination of phenylacetylene to give the corresponding enamine 126 in a regioselective manner (Scheme 28). The yield improves drastically, when using HNEt₂ instead of HNⁱPr₂. The faster attack of amine on the activated acetylene bound to the cationic plumbylene prevents side reactions, which can deactivate the catalyst. Further reaction of 126 with Pb-activated phenylacetylene can lead to the formation of the corresponding diene 127. The use of an excess of amine can improve the selectivity of the reaction to enamine 126.¹²⁵

Scheme 28 Catalytic hydroamination of phenylacetylene (R′ = ⁱPr, Et).

Conclusions

Pioneering work by Jutzi and co-workers set the stage for the isolation and stabilisation of the unique E(II) cations (E = Si, Ge, Sn, Pb), which merge the electrophilicity of tetrylium cations with the ambiphilic character of tetrylenes.^29,30 The subsequent decades witnessed the isolation of various stable tetryliumylidene ions with different ligand stabilisation strategies. The following studies demonstrated promising reactivity of tetryliumylidene ions in various synthetic transformations. These developments expanded our understanding of tetryliumylidene capabilities and paved the way for their application in small molecule activation and catalysis. Despite these achievements, the chemistry of tetryliumylidenes remains largely underexplored. We hope that this review will highlight the topic and stimulate further research aimed at developing more tetryliumylidene-based systems capable of facilitating unique chemical transformations and fostering novel catalytic applications.

Author contributions

All authors discussed the concepts and contributed to the final manuscript. S. S. and S. F. performed literature reviews and wrote the original draft. A. K. carried out the quantum chemical calculations. A. K. and S. I. conceived the topic, strategised on content and information organisation, and edited the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This project has received funding from the Alexander von Humboldt Foundation for a Research Fellowship (S.F.) and the European Research Council (ALLOWE 101001591).

Notes and references

1. P. P. Power, Nature, 2010, 463, 171–177.
2. P. P. Power, Acc. Chem. Res., 2011, 44, 627–637.
3. H. Braunschweig, R. D. Dewhurst, F. Hupp, M. Nutz, K. Radacki, C. W. Tate, A. Vargas and Q. Ye, Nature, 2015, 522, 327–330.
4. M. M. Hansmann and G. Bertrand, J. Am. Chem. Soc., 2016, 138, 15885–15888.
5. C. Weetman, in Encyclopedia of Inorganic and Bioinorganic Chemistry, ed. R. A. Scott, John Wiley & Sons, Hoboken, New Jersey, 2021, vol. 2021, pp. 1–27.
6. S. Yadav, S. Saha and S. S. Sen, ChemCatChem, 2016, 8, 486–501.
7. B. Blom and M. Driess, in Functional Molecular Silicon Compounds II: Low Oxidation States, ed. D. Scheschkewitz, Springer International Publishing, Cham, 2014, pp. 85–123,  DOI:10.1007/430_2013_95.
8. S. Raoufmoghaddam, Y.-P. Zhou, Y. Wang and M. Driess, J. Organomet. Chem., 2017, 829, 2–10.
9. S. K. Mandal and H. W. Roesky, Acc. Chem. Res., 2012, 45, 298–307.
10. M. Soleilhavoup and G. Bertrand, Acc. Chem. Res., 2015, 48, 256–266.
11. J. Zheng, Z. H. Li and H. Wang, Chem. Sci., 2018, 9, 1433–1438.
12. M.-A. Légaré, G. Bélanger-Chabot, R. D. Dewhurst, E. Welz, I. Krummenacher, B. Engels and H. Braunschweig, Science, 2018, 359, 896–900.
13. H. F. T. Klare, L. Albers, L. Süsse, S. Keess, T. Müller and M. Oestreich, Chem. Rev., 2021, 121, 5889–5985.
14. T. Müller, in Organogermanium Compounds: Theory, Experiment, and Applications, ed. V. Y. Lee, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2023, pp. 299–338,  DOI:10.1002/9781119613466.ch6.
15. J. B. Lambert, in Tin Chemistry: Fundamentals, Frontiers, and Applications, ed. A. G. Davies, M. Gielen, K. H. Pannell and E. R. T. Tiekink, John Wiley & Sons, Chichester, 2008, pp. 152–159.
16. M. Janssen, S. Mebs and J. Beckmann, Chem. Commun., 2023, 59, 7267–7270.
17. V. S. V. S. N. Swamy, S. Pal, S. Khan and S. S. Sen, Dalton Trans., 2015, 44, 12903–12923.
18. A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899–926.
19. M. Kaupp, in The Chemical Bond, ed. G. Frenking and S. Shaik, 2014, pp. 1–24,  DOI:10.1002/9783527664658.ch1.
20. A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
21. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789.
22. S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200–1211.
23. P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Chem. Phys., 1994, 98, 11623–11627.
24. F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–3305.
25. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox, Gaussian 16 Rev. C.01, Wallingford, CT, 2016.
26. E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M. Morales, P. Karafiloglou, C. R. Landis, and F. Weinhold, NBO 7.0, Madison, 2018.
27. P. P. Gaspar, X. Liu, D. Ivanova, D. Read, J. S. Prell, and M. L. Gross, in Modern Aspects of Main Group Chemistry, American Chemical Society, 2005, ch. 4, vol. 917, pp. 52–65.
28. G. Bertrand, Science, 2004, 305, 783–785.
29. P. Jutzi, F. Kohl, P. Hofmann, C. Krüger and Y.-H. Tsay, Chem. Ber., 1980, 113, 757–769.
30. P. Jutzi, R. Dickbreder and H. Nöth, Chem. Ber., 1989, 122, 865–870.
31. R. Jambor and M. Novák, Eur. J. Inorg. Chem., 2023, 26, e202300505.
32. J. Li, C. Schenk, F. Winter, H. Scherer, N. Trapp, A. Higelin, S. Keller, R. Pöttgen, I. Krossing and C. Jones, Angew. Chem., Int. Ed., 2012, 51, 9557–9561.
33. M. Stender, A. D. Phillips and P. P. Power, Inorg. Chem., 2001, 40, 5314–5315.
34. H. V. R. Dias and Z. Wang, J. Am. Chem. Soc., 1997, 119, 4650–4655.
35. T. Probst, O. Steigelmann, J. Riede and H. Schmidbaur, Angew. Chem., Int. Ed., 1990, 29, 1397–1398.
36. P. A. Rupar, V. N. Staroverov, P. J. Ragogna and K. M. Baines, J. Am. Chem. Soc., 2007, 129, 15138–15139.
37. Y. Xiong, T. Szilvási, S. Yao, G. Tan and M. Driess, J. Am. Chem. Soc., 2014, 136, 11300–11303.
38. A. P. Singh, H. W. Roesky, E. Carl, D. Stalke, J.-P. Demers and A. Lange, J. Am. Chem. Soc., 2012, 134, 4998–5003.
39. A. Schäfer, F. Winter, W. Saak, D. Haase, R. Pöttgen and T. Müller, Chem.–Eur. J., 2011, 17, 10979–10984.
40. P. Jutzi, A. Mix, B. Rummel, W. W. Schoeller, B. Neumann and H.-G. Stammler, Science, 2004, 305, 849–851.
41. Y. Xiong, S. Yao, S. Inoue, J. D. Epping and M. Driess, Angew. Chem., Int. Ed., 2013, 52, 7147–7150.
42. Y. Xiong, S. Yao, G. Tan, S. Inoue and M. Driess, J. Am. Chem. Soc., 2013, 135, 5004–5007.
43. A. C. Filippou, B. Baars, O. Chernov, Y. N. Lebedev and G. Schnakenburg, Angew. Chem., Int. Ed., 2014, 53, 565–570.
44. S. U. Ahmad, T. Szilvási, E. Irran and S. Inoue, J. Am. Chem. Soc., 2015, 137, 5828–5836.
45. D. Sarkar, D. Wendel, S. U. Ahmad, T. Szilvási, A. Pöthig and S. Inoue, Dalton Trans., 2017, 46, 16014–16018.
46. A. Porzelt, J. I. Schweizer, R. Baierl, P. J. Altmann, M. C. Holthausen and S. Inoue, Inorganics, 2018, 6, 54.
47. D. Sarkar, V. Nesterov, T. Szilvási, P. J. Altmann and S. Inoue, Chem.–Eur. J., 2019, 25, 1198–1202.
48. E. Fritz-Langhals, Org. Process Res. Dev., 2019, 23, 2369–2377.
49. S. Sinhababu, D. Singh, M. K. Sharma, R. K. Siwatch, P. Mahawar and S. Nagendran, Dalton Trans., 2019, 48, 4094–4100.
50. D. Sarkar, S. Dutta, C. Weetman, E. Schubert, D. Koley and S. Inoue, Chem.–Eur. J., 2021, 27, 13072–13078.
51. K. Nakaya, S. Takahashi, A. Ishii and N. Nakata, Inorg. Chem., 2022, 61, 15510–15519.
52. B.-X. Leong, J. Lee, Y. Li, M.-C. Yang, C.-K. Siu, M.-D. Su and C.-W. So, J. Am. Chem. Soc., 2019, 141, 17629–17636.
53. B.-X. Leong, Y.-C. Teo, C. Condamines, M.-C. Yang, M.-D. Su and C.-W. So, ACS Catal., 2020, 10, 14824–14833.
54. Y.-C. Teo, D. Loh, B.-X. Leong, Z.-F. Zhang, M.-D. Su and C.-W. So, Inorg. Chem., 2023, 62, 16867–16873.
55. N. C. Breit, T. Szilvási, T. Suzuki, D. Gallego and S. Inoue, J. Am. Chem. Soc., 2013, 135, 17958–17968.
56. H.-X. Yeong, Y. Li and C.-W. So, Organometallics, 2014, 33, 3646–3648.
57. P. Ghana, M. I. Arz, G. Schnakenburg, M. Straßmann and A. C. Filippou, Organometallics, 2018, 37, 772–780.
58. P. Frisch and S. Inoue, Dalton Trans., 2020, 49, 6176–6182.
59. C. Seow, M. L. B. Ismail, H.-W. Xi, Y. Li, K. H. Lim and C.-W. So, Organometallics, 2018, 37, 1368–1372.
60. R. Jambor, B. Kašná, S. G. Koller, C. Strohmann, M. Schürmann and K. Jurkschat, Eur. J. Inorg. Chem., 2010, 2010, 902–908.
61. R. Dostálová, L. Dostál, A. Růžička and R. Jambor, Organometallics, 2011, 30, 2405–2410.
62. J. Martincová, L. Dostál, S. Herres-Pawlis, A. Růžička and R. Jambor, Chem.–Eur. J., 2011, 17, 7423–7427.
63. P. M. Keil and T. J. Hadlington, Chem. Commun., 2022, 58, 3011–3014.
64. P. M. Keil, A. Soyemi, K. Weisser, T. Szilvási, C. Limberg and T. J. Hadlington, Angew. Chem., Int. Ed., 2023, 62, e202218141.
65. A. Schulz, T. L. Kalkuhl, P. M. Keil and T. J. Hadlington, Angew. Chem., Int. Ed., 2023, 62, e202305996.
66. H. Braunschweig, M. A. Celik, R. D. Dewhurst, M. Heid, F. Hupp and S. S. Sen, Chem. Sci., 2015, 6, 425–435.
67. P. Jutzi, A. Mix, B. Neumann, B. Rummel and H.-G. Stammler, Chem. Commun., 2006, 3519–3521.
68. A. Hinz, Angew. Chem., Int. Ed., 2020, 59, 19065–19069.
69. M. Driess, S. Yao, M. Brym and C. van Wüllen, Angew. Chem., Int. Ed., 2006, 45, 6730–6733.
70. D. C. H. Do, A. V. Protchenko, M. Á. Fuentes, J. Hicks, P. Vasko and S. Aldridge, Chem. Commun., 2020, 56, 4684–4687.
71. S. Takahashi, M. Frutos, A. Baceiredo, D. Madec, N. Saffon-Merceron, V. Branchadell and T. Kato, Angew. Chem., Int. Ed., 2022, 61, e202208202.
72. A. C. Filippou, Y. N. Lebedev, O. Chernov, M. Straßmann and G. Schnakenburg, Angew. Chem., Int. Ed., 2013, 52, 6974–6978.
73. T. Agou, N. Hayakawa, T. Sasamori, T. Matsuo, D. Hashizume and N. Tokitoh, Chem.–Eur. J., 2014, 20, 9246–9249.
74. S. U. Ahmad, T. Szilvási and S. Inoue, Chem. Commun., 2014, 50, 12619–12622.
75. Y. Li, Y.-C. Chan, Y. Li, I. Purushothaman, S. De, P. Parameswaran and C.-W. So, Inorg. Chem., 2016, 55, 9091–9098.
76. Y. Li, Y.-C. Chan, B.-X. Leong, Y. Li, E. Richards, I. Purushothaman, S. De, P. Parameswaran and C.-W. So, Angew. Chem., Int. Ed., 2017, 56, 7573–7578.
77. N. Hayakawa, K. Sadamori, S. Mizutani, T. Agou, T. Sugahara, T. Sasamori, N. Tokitoh, D. Hashizume and T. Matsuo, Inorganics, 2018, 6, 30.
78. P. Frisch and S. Inoue, Dalton Trans., 2019, 48, 10403–10406.
79. F. Hanusch, D. Munz, J. Sutter, K. Meyer and S. Inoue, Angew. Chem., Int. Ed., 2021, 60, 23274–23280.
80. S. V. Hirmer, F. S. Tschernuth, F. Hanusch, R. Baierl, M. Muhr and S. Inoue, Mendeleev Commun., 2022, 32, 16–18.
81. Y. Xiong, S. Yao, S. Inoue, E. Irran and M. Driess, Angew. Chem., Int. Ed., 2012, 51, 10074–10077.
82. H.-X. Yeong, H.-W. Xi, Y. Li, K. H. Lim and C.-W. So, Chem.–Eur. J., 2013, 19, 11786–11790.
83. R. Nougué, S. Takahashi, A. Baceiredo, N. Saffon-Merceron, V. Branchadell and T. Kato, Angew. Chem., Int. Ed., 2023, 62, e202215394.
84. S. Stigler, M. Park, A. Porzelt, A. Kostenko, D. Henschel and S. Inoue, Organometallics, 2022, 41, 2088–2094.
85. P. Frisch and S. Inoue, Chem. Commun., 2018, 54, 13658–13661.
86. P. Frisch, T. Szilvási, A. Porzelt and S. Inoue, Inorg. Chem., 2019, 58, 14931–14937.
87. P. Frisch, T. Szilvási and S. Inoue, Chem.–Eur. J., 2020, 26, 6271–6278.
88. R. Nougué, S. Takahashi, A. Dajnak, E. Maerten, A. Baceiredo, N. Saffon-Merceron, V. Branchadell and T. Kato, Chem.–Eur. J., 2022, 28, e202202037.
89. D. Sarkar, C. Weetman, S. Dutta, E. Schubert, C. Jandl, D. Koley and S. Inoue, J. Am. Chem. Soc., 2020, 142, 15403–15411.
90. B. Su, R. Ganguly, Y. Li and R. Kinjo, Angew. Chem., Int. Ed., 2014, 53, 13106–13109.
91. B. Su, R. Ganguly, Y. Li and R. Kinjo, Chem. Commun., 2016, 52, 613–616.
92. M. T. Nguyen, D. Gusev, A. Dmitrienko, B. M. Gabidullin, D. Spasyuk, M. Pilkington and G. I. Nikonov, J. Am. Chem. Soc., 2020, 142, 5852–5861.
93. D. Paul, F. Heins, S. Krupski, A. Hepp, C. G. Daniliuc, K. Klahr, J. Neugebauer, F. Glorius and F. E. Hahn, Organometallics, 2017, 36, 1001–1008.
94. Y.-P. Zhou, M. Karni, S. Yao, Y. Apeloig and M. Driess, Angew. Chem., Int. Ed., 2016, 55, 15096–15099.
95. A. Rit, R. Tirfoin and S. Aldridge, Angew. Chem., Int. Ed., 2016, 55, 378–382.
96. R. J. Mangan, A. R. Davies, J. Hicks, C. P. Sindlinger, A. L. Thompson and S. Aldridge, Polyhedron, 2021, 196, 115006.
97. M. M. D. Roy, P. A. Lummis, M. J. Ferguson, R. McDonald and E. Rivard, Chem.–Eur. J., 2017, 23, 11249–11252.
98. T. Ochiai, D. Franz, X.-N. Wu and S. Inoue, Dalton Trans., 2015, 44, 10952–10956.
99. T. Ochiai and S. Inoue, Phosphorus, Sulfur Silicon Relat. Elem., 2016, 191, 624–627.
100. T. Ochiai, T. Szilvási, D. Franz, E. Irran and S. Inoue, Angew. Chem., Int. Ed., 2016, 55, 11619–11624.
101. F. S. Tschernuth, F. Hanusch, T. Szilvási and S. Inoue, Organometallics, 2020, 39, 4265–4272.
102. M. Majumdar, R. K. Raut, P. Sahoo and V. Kumar, Chem. Commun., 2018, 54, 10839–10842.
103. P. Sahoo, R. K. Raut, D. Maurya, V. Kumar, P. Rani, R. G. Gonnade and M. Majumdar, Dalton Trans., 2019, 48, 7344–7351.
104. Y. Su, Y. Li, R. Ganguly and R. Kinjo, Eur. J. Inorg. Chem., 2018, 2228–2231.
105. Y. Xiong, S. Yao, S. Inoue, A. Berkefeld and M. Driess, Chem. Commun., 2012, 48, 12198–12200.
106. M. Bouška, L. Dostál, A. Růžička and R. Jambor, Organometallics, 2013, 32, 1995–1999.
107. D. Maurya, J. Karmakar, P. Sahoo, R. K. Raut and M. Majumdar, Inorg. Chim. Acta, 2020, 503, 119380.
108. A. Schäfer, W. Saak, D. Haase and T. Müller, Chem.–Eur. J., 2009, 15, 3945–3950.
109. A. Hinz, Chem.–Eur. J., 2019, 25, 3267–3271.
110. S. Khan, G. Gopakumar, W. Thiel and M. Alcarazo, Angew. Chem., Int. Ed., 2013, 52, 5644–5647.
111. T. J. Hadlington, A. Kostenko and M. Driess, Chem.–Eur. J., 2021, 27, 2476–2482.
112. D. Sarkar, C. Weetman, S. Dutta, E. Schubert, C. Jandl, D. Koley and S. Inoue, J. Am. Chem. Soc., 2020, 142, 15403–15411.
113. R. S. P. Turbervill and J. M. Goicoechea, Aust. J. Chem., 2013, 66, 1131–1137.
114. T. Ochiai, D. Franz, E. Irran and S. Inoue, Chem.–Eur. J., 2015, 21, 6704–6707.
115. X.-X. Zhao, J. A. Kelly, A. Kostenko, S. Fujimori and S. Inoue, Z. Anorg. Allg. Chem., 2022, 648, e202200220.
116. R. K. Raut and M. Majumdar, J. Organomet. Chem., 2019, 887, 18–23.
117. I. Objartel, H. Ott and D. Stalke, Z. Anorg. Allg. Chem., 2008, 634, 2373–2379.
118. R. Baierl, A. Kostenko, F. Hanusch and S. Inoue, Dalton Trans., 2021, 50, 14842–14848.
119. H. V. R. Dias and W. Jin, J. Am. Chem. Soc., 1996, 118, 9123–9126.
120. M. J. Taylor, A. J. Saunders, M. P. Coles and J. R. Fulton, Organometallics, 2011, 30, 1334–1339.
121. H. Arii, M. Matsuo, F. Nakadate, K. Mochida and T. Kawashima, Dalton Trans., 2012, 41, 11195–11200.
122. M. Wagner, T. Zöller, W. Hiller, M. H. Prosenc and K. Jurkschat, Chem.–Eur. J., 2013, 19, 9463–9467.
123. S. Hino, M. Brynda, A. D. Phillips and P. P. Power, Angew. Chem., Int. Ed., 2004, 43, 2655–2658.
124. R. J. Andersen, R. C. diTargiani, R. D. Hancock, C. L. Stern, D. P. Goldberg and H. A. Godwin, Inorg. Chem., 2006, 45, 6574–6576.
125. A. Chandran, J. M. Léon Baeza, V. Timofeeva, R. Nougué, S. Takahashi, R. Ohno, A. Baceiredo, R. S. Rojas Guerrero, M. Syroeshkin, T. Matsuo, N. Saffon-Merceron and T. Kato, Inorg. Chem., 2022, 61, 16156–16162.

---