Germylene

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General structure of germylene Ger2.png
General structure of germylene

Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. [1] However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. [2] [3] Unprotected carbene analogs, including germylenes, has a dimerization nature. [4] Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. [5] The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991. [6]

Contents

Structures and bonding

Bonding situation for germylene is distinctively different from that for its light analog carbene. The carbon atom from carbene is sp2 hybridized. Although germylenes still have some sp2 hybridization character, the larger energy gap between s and p-orbitals for germanium permits the retainment of 4s24p2 electron configuration to some degree. The bond angle for H2Ge and Me2Ge was found to be: H-Ge-H 93° and C-Ge-C: 98°, which is smaller than 120°, the ideal bond angle for sp2 hybridized structure and thus proves the 4s24p2 valence electron configuration nature of germylene. [1] [7] [8] The lone pair of germylene tends to stay in the high-s-character orbital which is relatively inert, making germylene exclusively singlet. [3]

Dimerization of germylenes lead to the formation of germylene dimers (R2Ge=GeR2). [9] Digermylene dimers (as well as the higher-order digermynes) have a trans-bent structure quasi-tetrahedral at each germanium atom. Consequently the dimerization is believed to proceed through two donor-acceptor adducts instead of the triplet double-pairing found during carbene dimerization. [3]

Orbitals and electron configuration of triplet carbene and singlet germylene, double donor-acceptor interaction in a germylene dimer Stcg.png
Orbitals and electron configuration of triplet carbene and singlet germylene, double donor-acceptor interaction in a germylene dimer

Synthesis

Stabilization

Thermodynamic stabilization of germylene Thermo111.png
Thermodynamic stabilization of germylene

Dimerization of free germylenes does not have a noticeable energy barrier, which means that the dimerization reaction is almost spontaneous and diffusion limited, so the free germylene monomers without stabilization could dimerize or further polymerize once they form. [4] Free germylenes have to be stabilized kinetically or thermodynamically due to their high reactivity originating from the vacant p-orbital. Thermodynamical stabilization of this p-orbital is usually realized by coordination of a pentamethylcyclopentadiene (Cp*) ligand or of nitrogen (N), oxygen (O) or phosphorus (P) containing ligands, which are able to donate electrons and thus deactivate the vacant p-orbitals. [1] At the same time, stabilization can be accomplished by steric protection of bulky R groups like mesityl groups (Mes) to prevent nucleophiles from getting close to the germanium center. [10]

Synthesis of carbon substituted germylenes

Carbon substituents is different from other heteroatom N, O, P substituents which have lone pairs in that they provide less electronic perturbations. [11] As a result, a stronger steric and electronic stabilization is required to guarantee a monomeric product. Carbon substituted germylenes can be synthesized using various methods: (1) reduction of dibromogermanes with reducing agents like lithium naphthalene (LiNp) or potassium graphite (KC8), etc., (2) photolysis of strained cyclogermanes or Ge(IV) species, (3) substitution of a dihalo Ge(II) precursor species with nucleophiles like organometallic ligands (e.g. RLi/RMgBr). [1]

Synthetic methods for stable germylenes Sys111.png
Synthetic methods for stable germylenes

Synthesis of n-heterocyclic germylene and cyclic(alkyl)(amino)germylene

The introduction of heteroatom in the ligand backbone enhances the stability of reactive Ge(II) center by electron donation from N lone pair to vacant p-orbitals of germanium center. Typically, the strategy for synthesizing five-membered N-heterocyclic tetrylene involves the reaction between N-substituted 1,4-diaza-1,3-butadiene, the alkali metal based reducing agents and group 14 halides. [12] In the case of n-heterocyclic germylene (NHGe) synthesis, the method involves an initial reduction of N-substituted 1,4-diaza-1,3-butadiene by lithium. The following cyclization of the dianion with the corresponding Ge(II) halides gives the final product. [12]

Synthesis of NHGe NHGE.png
Synthesis of NHGe

The cyclic(alkyl)(amino)carbenes (CAACs) has already been known as both a better donor and better acceptor than n-heterocyclic carbenes (NHCs) due to its higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO). [13]

The synthetic strategy of CAAGe involves the synthesis of a α-β-unsaturated imine from a ketone and an amine via condensation followed by the treatment with GeCl2·dioxane. The resulting product is then reduced with KC8 to give CAAGe. [14] Analogous to CAAC, the electrophilicity of the germanium center can be obviously enhanced by the substitution of a π-donating and σ-withdrawing amino group along with σ-donating trimethylsilyl groups. [11]

Synthesis of CAAGe CaaGe.png
Synthesis of CAAGe

Synthesis of a unique homoconjugation stabilized germylene

In 2016, Muller et al reported the synthesis of a unique homoconjugation stabilized germylene in a relatively high yield by the reaction between hafnocene dichloride and dipotassium germacyclopentadienediide in THF at -80 °C. [15] [16] The product is stabilized by a remote interaction between a C=C double bond and vacant p-orbital of Ge center through homoconjugation. [17] This stabilization strategy results in a special structural which possesses unusual reactivity. [11]

Synthesis of a homoconjugation stabilized germylene ConjGe.png
Synthesis of a homoconjugation stabilized germylene

Synthesis of PGeP pincer compounds

The pincer based germylene is of great importance not only for their ability to stabilize transition metal species via chelation effects in homogeneous catalysis, but also for its serving as a good luminescence source. [18] [19] A PNHNHP ligand was used to synthesize the PGeP pincer stabilized germylene by treatment with two equivalents of potassium hexamethyldisilazide (KHMDS) and GeCl2·dioxane, which finally leads to the formation of the PGeP pincer compound. [20]

Synthesis of PGeP pincer compounds Pancerrr.png
Synthesis of PGeP pincer compounds

Reactivity

Oligomerization and polymerization

Dimerization of carbon substituted germylenes gives R2Ge=GeR2 dimers which could further polymerize to form polygermanes (R2Ge)n compounds. [5] The dimer could show a certain stability if prepared in an independent way. [21] Bulkier substituents are able to reduce the polymerization rate by steric effect. [22] More steric hindrance could even stop the polymerization or dimerization reactions and renders a germylene thermodynamically stable. [23] [24] [25]

Dimerization and polymerization of germylene Di&polyGe.png
Dimerization and polymerization of germylene

Insertion into σ bond

R2Ge insertion into C-C bonds has not been reported so far. [5] However, going down the group 14, C-E (E = Si, Ge, Sn, Pb) bonds become more accessible for R2Ge insertion. [5] The strained C-Ge bonds allow insertion of germylene to 7,7-dialkyl-7-germanorbornadienes in the melt, forming digermabicy-clooctadienes. [26]

Insertion into C-Ge bond GeC.png
Insertion into C-Ge bond

C-H bonds are generally unreactive toward germylene insertion. [27] However, strain release may help to overcome the activation energy barrier. [25]

Insertion into C-H bond C-H Ge.png
Insertion into C-H bond

Insertion to carbon-halide bonds is common for germylene. The mechanism for insertion of free Me2Ge into the C-Br bond of benzyl bromide was reported to be a two-step, radical abstraction-recombination process under thermal and photolytical conditions. [28] [29] An identical mechanism through a caged singlet radical pair was proposed for C-Cl bond insertion. [29] However, the interaction between halogen electrons and empty p-orbital of the germylene may result in the formation of a donor-acceptor complex before occurrence of any of the insertion mechanisms. [5]

Reaction mechanisms for insertion into C-Hal bonds CX Ge.png
Reaction mechanisms for insertion into C-Hal bonds

The insertion into the C-Hal bond in alkyne compounds go by a one-step mechanism under thermal or photolytical conditions. [30]

Reaction for insertion into C-Hal bonds in alkynes CHal Ge.png
Reaction for insertion into C-Hal bonds in alkynes

For C-O, the R2Ge insertion product could only remain stable at a very low temperature. [31]

Insertion into C-O bond CO Ge.png
Insertion into C-O bond

Addition to unsaturated systems

Addition reaction of Me2Ge to unsaturated systems is well studied. As mentioned above, dimerization and polymerization of Me2Ge does not have a noticeable activation energy barrier and is only controlled by diffusion. As a result, addition reactions should be rapid enough complete before getting polygermanes as products.

There is no reaction between simple alkenes and free germylenes. [5] However, styrenes and α-substituted styrenes are able to react with Me2Ge. The resulting product is a 1:1 mixture of the syn and anti-isomers of 3,4-diphenyl-3,4-R-1,1-dimethyl-1-germacyclopentane. [32]

Addition to styrenes AddStyrene.png
Addition to styrenes

A variety of 1,2-substituted-vinylgermyl compounds can be synthesized in both high yield and high regioselectivity by addition of germylene to alkynes. [33]

Addition to alkynes AlkYne Ge.png
Addition to alkynes

1,4-Cycloaddition of conjugated (hetero-)dienes by free germylenes gives the corresponding 5-membered ring. [34] [35] [36]

Addition to conjugated (hetero-)dienes AddDiene.png
Addition to conjugated (hetero-)dienes

Germylenes reacts only with one of the strained double bonds in cumulated systems like allenes (C=C=C). [5] Germylenes prefer to react with more electron-deficient allenes. [5]

Addition to allenes =c= Ge.png
Addition to allenes

Complexation by donors

During complexation with donors, the germylenes stay in the singlet ground state, where the lone pair is placed in the high-s-character orbital, while the heteroatom-containing donors like R2O, ROH, R2S, R3P, R3N and RCl interact with the vacant p-orbital at germanium center, which could stabilize the singlet germylene and prevent further polymerization. Most of the complexes are stable in room temperature. [37] The absorption bands of adducts commonly exhibits shorter wavelengths in comparison to those of the free germylenes due to substituent-influenced n-p transitions at the Ge center. [31]

Germylene catalyzed reaction

Germylenes could also act as catalysts as transition metals do. [11] Oxidative addition and reductive elimination, along with the related Mn+/M(n+2)+ redox couples are of great significance to the transition metal catalysis. [38] Due to the electronic structure and chemical properties of germylenes, including the empty p-orbital, germylenes are able to activated small molecules and give the corresponding Ge(IV) complexes, which raised researchers' interests in germylenes' acting as spectator ligands in certain catalytic cycles. [39] However, subsequent regeneration of Ge(II) compound through reductive elimination is not thermodynamically favored for germylenes.[11] The key of germylene catalysis chemistry is to maintain a balance between oxidative addition and reductive elimination. [40] One example of germylene catalyzed reaction is hydroboration of CO2, where a preliminary hydrogermylation of CO2 step is followed by the formation of methanol derivatives with 3 equivalent of catecholborane to regenerate the germylene compound. [41]

Hydroboration of CO2 using germylene catalyst and its catalytic cycle Cat Ge CO2.png
Hydroboration of CO2 using germylene catalyst and its catalytic cycle

See also

Related Research Articles

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions.

<span class="mw-page-title-main">Organoruthenium chemistry</span>

Organoruthenium chemistry is the chemistry of organometallic compounds containing a carbon to ruthenium chemical bond. Several organoruthenium catalysts are of commercial interest and organoruthenium compounds have been considered for cancer therapy. The chemistry has some stoichiometric similarities with organoiron chemistry, as iron is directly above ruthenium in group 8 of the periodic table. The most important reagents for the introduction of ruthenium are ruthenium(III) chloride and triruthenium dodecacarbonyl.

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

<span class="mw-page-title-main">Boranylium ions</span>

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands:

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

<span class="mw-page-title-main">Stannylene</span> Class of organotin(II) compounds

Stannylenes (R2Sn:) are a class of organotin(II) compounds that are analogues of carbene. Unlike carbene, which usually has a triplet ground state, stannylenes have a singlet ground state since valence orbitals of tin (Sn) have less tendency to form hybrid orbitals and thus the electrons in 5s orbital are still paired up. Free stannylenes are stabilized by steric protection. Adducts with Lewis bases are also known.

<span class="mw-page-title-main">Diphosphagermylene</span> Class of compounds

Diphosphagermylenes are a class of compounds containing a divalent germanium atom bound to two phosphorus atoms. While these compounds resemble diamidocarbenes, such as N-heterocyclic carbenes (NHC), diphosphagermylenes display bonding characteristics distinct from those of diamidocarbenes. In contrast to NHC compounds, in which there is effective N-C p(π)-p(π) overlap between the lone pairs of planar nitrogens and an empty p-orbital of a carbene, systems containing P-Ge p(π)-p(π) overlap are rare. Until 2014, the geometry of phosphorus atoms in all previously reported diphosphatetrylenes are pyramidal, with minimal P-Ge p(π)-p(π) interaction. It has been suggested that the lack of p(π)-p(π) in Ge-P bonds is due to the high energetic barrier associated with achieving a planar configuration at phosphorus, which would allow for efficient p(π)-p(π) overlap between the phosphorus lone pair and the empty P orbital of Ge. The resulting lack of π stabilization contributes to the difficulty associated with isolating diphosphagermylene and the Ge-P double bonds. However, utilization of sterically encumbering phosphorus centers has allowed for the isolation of diphosphagermylenes with a planar phosphorus center with a significant P-Ge p(π)-p(π) interaction.

<span class="mw-page-title-main">Phosphenium</span> Divalent cations of phosphorus

Phosphenium ions, not to be confused with phosphonium or phosphirenium, are divalent cations of phosphorus of the form [PR2]+. Phosphenium ions have long been proposed as reaction intermediates.

<i>N</i>-heterocyclic silylene Chemical compound

An N-Heterocyclic silylene (NHSi) is an uncharged heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.

<span class="mw-page-title-main">Plumbylene</span> Divalent organolead(II) analogues of carbenes

Plumbylenes (or plumbylidenes) are divalent organolead(II) analogues of carbenes, with the general chemical formula, R2Pb, where R denotes a substituent. Plumbylenes possess 6 electrons in their valence shell, and are considered open shell species.

<span class="mw-page-title-main">Germanium(II) dicationic complexes</span>

Ge(II) dicationic complexes refer to coordination compounds of germanium with a +2 formal oxidation state, and a +2 charge on the overall complex. In some of these coordination complexes, the coordination is strongly ionic, localizing a +2 charge on Ge, while in others the bonding is more covalent, delocalizing the cationic charge away from Ge. Examples of dicationic Ge(II) complexes are much rarer than monocationic Ge(II) complexes, often requiring the use of bulky ligands to shield the germanium center. Dicationic complexes of Ge(II) have been isolated with bulky isocyanide and carbene ligands. Much more weakly coordinated Germanium (II) dications have been isolated as complexes with polyether ligands, such as crown ethers and [2.2.2]cryptand. Crown ethers and cryptands are typically known for their ability to bind metal cations, however these ligands have also been employed in stabilizing low-valent cations of heavier p-block elements. A Ge2+ ion's valence shell consists of a filled valence s orbital but empty valence p orbitals, giving rise to atypical bonding in these complexes. Germanium is a metalloid of the carbon group, typically forming compounds with mainly covalent bonding, contrasting with the dative bonding observed in these coordination complexes.

A Fischer carbene is a type of transition metal carbene complex, which is an organometallic compound containing a divalent organic ligand. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is electrophilic.

Coinage metal N-heterocyclic carbene (NHC) complexes refer to transition metal complexes incorporating at least one coinage metal center (M = Cu, Ag, Au) ligated by at least one NHC-type persistent carbene. A variety of such complexes have been synthesized through deprotonation of the appropriate imidazolium precursor and metalation by the appropriate metal source, producing MI, MII, or MIII NHC complexes. While the general form can be represented as (R2N)2C:–M (R = various alkyl or aryl groups), the exact nature of the bond between NHC and M has been investigated extensively through computational modeling and experimental probes. These results indicate that the M-NHC bond consists mostly of electrostatic attractive interactions, with some covalent bond character arising from NHC to M σ donation and minor M to NHC π back-donation. Coinage metal NHC complexes show effective activity as catalysts for various organic transformations functionalizing C-H and C-C bonds, and as antimicrobial and anticancer agents in medicinal chemistry.

<i>N</i>-Heterocyclic carbene boryl anion Isoelectronic structure

An N-heterocyclic carbene boryl anion is an isoelectronic structure of an N-heterocyclic carbene (NHC), where the carbene carbon is replaced with a boron atom that has a -1 charge. NHC boryl anions have a planar geometry, and the boron atom is considered to be sp2-hybridized. They serve as extremely strong bases, as they are very nucleophilic. They also have a very strong trans influence, due to the σ-donation coming from the boron atom. NHC boryl anions have stronger electron-releasing character when compared to normal NHCs. These characteristics make NHC boryl anions key ligands in many applications, such as polycyclic aromatic hydrocarbons, and more commonly low oxidation state main group element bonding.

<span class="mw-page-title-main">Bismuthinidene</span> Class of organobismuth compounds

Bismuthinidenes are a class of organobismuth compounds, analogous to carbenes. These compounds have the general form R-Bi, with two lone pairs of electrons on the central bismuth(I) atom. Due to the unusually low valency and oxidation state of +1, most bismuthinidenes are reactive and unstable, though in recent decades, both transition metals and polydentate chelating Lewis base ligands have been employed to stabilize the low-valent bismuth(I) center through steric protection and π donation either in solution or in crystal structures. Lewis base-stabilized bismuthinidenes adopt a singlet ground state with an inert lone pair of electrons in the 6s orbital. A second lone pair in a 6p orbital and a single empty 6p orbital make Lewis base-stabilized bismuthinidenes ambiphilic.

A molecular electron-reservoir complex is one of a class of redox-active systems which can store and transfer electrons stoichiometrically or catalytically without decomposition. The concept of electron-reservoir complexes was introduced by the work of French chemist, Didier Astruc. From Astruc's discoveries, a whole family of thermally stable, neutral, 19-electron iron(I) organometallic complexes were isolated and characterized, and found to have applications in redox catalysis and electrocatalysis. The following page is a reflection of the prototypal electron-reservoir complexes discovered by Didier Astruc.

Gallylenes are a class of gallium species which are electronically neutral and in the +1-oxidation state. This broad definition may include many gallium species, such as oligomeric gallium compounds in which the gallium atoms are coordinated to each other, but these classes of compounds are often referred to as gallanes. In recent literature, the term gallylene has mostly been reserved for low valent gallium species which may have a lone pair, analogous to NHC's or terminal borylenes. They are compounds of academic interest because of their distinctive electronic properties which have been achieved for higher main group elements such as borylenes and carbenes.

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  21. ↑ Marchand, Annette; Gerval, Pierre; Duboudin, Françoise; Gaufryau, M.-H.; Joanny, Marguerite; Mazerolles, Pierre (1984). "Mise en evidence de la formation de R2MMR2 a partir de disila-1,2 et digerma-1,2 cyclohexenes-4". Journal of Organometallic Chemistry. 267 (1): 93–106. doi:10.1016/0022-328x(84)80162-x. ISSN   0022-328X.
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  26. ↑ Bleckmann, Paul; Minkwitz, Rolf; Neumann, Wilhelm P.; Schriewer, Michael; Thibud, Manfred; Watta, Bärbel (1984). "Dimethyl germylene insertion into a strained C-Ge bond and matrix isolation of tetramethyl digermene Me2Ge=GeMe2". Tetrahedron Letters. 25 (23): 2467–2470. doi:10.1016/s0040-4039(01)81206-5. ISSN   0040-4039.
  27. ↑ Nefedov, O. M.; Skell, P. S. (1981). "THE GENERATION AND REACTIONS OF DIMETHYLSILYLENE AND DIMETHYLGERMYLENE IN VAPOR-PHASE". Doklady Akademii Nauk SSSR. 259 (2): 377–379.
  28. ↑ Koecher, Juergen; Lehnig, Manfred (1984). "Insertion reactions of dimethylgermylene, Me2Ge, and their mechanisms as studied by CIDNP". Organometallics. 3 (6): 937–939. doi:10.1021/om00084a022. ISSN   0276-7333.
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  30. ↑ Billeb, G.; Brauer, H.; Maslov, S.; Neumann, W.P. (1989). "Chemie der schweren Carben-Analogen R2M, M = Si, Ge, Sn". Journal of Organometallic Chemistry (in German). 373 (1): 11–19. doi:10.1016/0022-328X(89)85019-3.
  31. 1 2 Ando, Wataru.; Itoh, Hiroyuki.; Tsumuraya, Takeshi. (1989). "Electronic absorption spectra of diorganogermylenes in matrixes: formation of diorganogermylene complexes with heteroatom-containing substrates". Organometallics. 8 (12): 2759–2766. doi:10.1021/om00114a004. ISSN   0276-7333.
  32. ↑ Koecher, Jurgen; Neumann, Wilhelm P. (1985). "Germacyclopentanes via cycloadditions of free dimethylgermylene to styrenes". Organometallics. 4 (2): 400–402. doi:10.1021/om00121a038. ISSN   0276-7333.
  33. ↑ Billeb, Gilbert; Brauer, Hartmut; Neumann, Wilhelm P. (1990). "Regiospecific Synthesis of Vinylgermyl Compounds from Alkynes or Allenes and Free Singlet Dimethylgermylene". Synlett. 1990 (3): 113–114. doi:10.1055/s-1990-21006. ISSN   0936-5214. S2CID   98481510.
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  35. ↑ Michels, Erhard; Neumann, Wilhelm P. (1986). "Additions of free dimethylgermylene to vinyl ketones and α-diketones". Tetrahedron Letters. 27 (22): 2455–2458. doi:10.1016/s0040-4039(00)84554-2. ISSN   0040-4039.
  36. ↑ Bootz, Konrad; Neumann, Wilhelm P. (1989). "The first synthesis of aza-germacyclopentenes: Cycloadditions of 1-aza- and 1,4-diazabutadienes with free singlet dimethyl germylene". Tetrahedron Letters. 30 (48): 6669–6672. doi:10.1016/s0040-4039(00)70646-0. ISSN   0040-4039.
  37. ↑ Nefedov, O. M.; Kolesnikov, S. P.; Rogozhin, I. S. (1980). "Molecular complexes of germylene with n-donor ligands". Proceedings of the Academy of Sciences of the USSR Series Chemical: 170–173.
  38. ↑ Dunn, Nicole L.; Ha, Minji; Radosevich, Alexander T. (2012). "Main Group Redox Catalysis: Reversible P III /P V Redox Cycling at a Phosphorus Platform". Journal of the American Chemical Society. 134 (28): 11330–11333. doi:10.1021/ja302963p. ISSN   0002-7863. PMID   22746974.
  39. ↑ Herrmann, W. A. (2002). "N-heterocyclic carbene: a new concept in organometallic catalysis". Angew Chem Int Ed. 41 (8): 1290–1309. doi:10.1002/1521-3773(20020415)41:8<1290::AID-ANIE1290>3.0.CO;2-Y. PMID   19750753.
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