Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.
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. The lone pair of germylene tends to stay in the high-s-character orbital which is relatively inert, making germylene exclusively singlet. Dimerization of germylenes lead to the formation of germylene dimers (R2Ge=GeR2). 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.
Synthesis
Stabilization 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. 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. 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.
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. 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).
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. 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.
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). 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. 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.
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. The product is stabilized by a remote interaction between a C=C double bond and vacant p-orbital of Ge center through homoconjugation. This stabilization strategy results in a special structural which possesses unusual reactivity.
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. 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.
Reactivity
Oligomerization and polymerization Dimerization of carbon substituted germylenes gives R2Ge=GeR2 dimers which could further polymerize to form polygermanes (R2Ge)n compounds. The dimer could show a certain stability if prepared in an independent way. Bulkier substituents are able to reduce the polymerization rate by steric effect. More steric hindrance could even stop the polymerization or dimerization reactions and renders a germylene thermodynamically stable.
Insertion into σ bond R2Ge insertion into C-C bonds has not been reported so far. However, going down the group 14, C-E (E = Si, Ge, Sn, Pb) bonds become more accessible for R2Ge insertion. The strained C-Ge bonds allow insertion of germylene to 7,7-dialkyl-7-germanorbornadienes in the melt, forming digermabicy-clooctadienes.
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