Georgii I. Nikonov

Reaction of cyclic germylene NacNac′Ge (3, where NacNac′ = [ArNC(Me)CHC(═CH2)NAr]2– and Ar = 2,6-Pri2C6H3) with KOBut affords new germyl anion [NacNac′Ge(OBut)]−K+, which can be further converted into hydridogermylide [NacNac′Ge(H)]−K+ (8). Anionic compound 8 catalyzes hydrosilylation of ketones and aldehydes and hydroboration of N-heterocycles. Mechanistic studies show that these reactions proceed by a non-hydride mechanism in which the germylide serves as an activator for the reducing reagent, silane or borane.

Reduction of cationic stannylene [(dim(NHC))SnCl]⁺ (dim(NHC) = diimino N-heterocyclic carbene) by potassium graphite affords the Sn(0) compound (stannylone) (dim(NHC))Sn (14). SC-XRD study of 14 revealed a shortened C–Sn bond, indicating a multiple bond character. Stannylone 14 undergoes a [2 + 2] cycloaddition with isocyanates RNCO (R = 4-tolyl or 2,6-xylyl) to furnish novel alkyl carboxamido stannylenes 21 (R = 4-tolyl) and 22 (R = 2,6-xylyl), featuring a C–C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 22 with 4-tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C–C bond formation in the coordination sphere of the tin atom.

Attempts to generate NacNacAl‐based (NacNac=[ArNC(Me)CHC(Me)NAr]⁻; Ar=2,6‐iPr₂C₆H₃) alkylidenes via reactions with trimethyl(methylidene), triphenyl(methylidene) and triethyl(ethylidene) phosphiranes led to new and unexpected reactivity patterns. Reaction with Me₃P=CH₂ resulted in the intermolecular deprotonation of the NacNac −CCH₃ backbone. Reaction with Ph₃P=CH₂*LiI afforded an equimolar mixture of NacNacAlH₂ and the ortho‐metalated adduct, NacNac'Al(η(C,C)²−CH₂=PPh₂(o‐C₆H₄)) (NacNac’=[ArNC(Me)CHC(=CH₂)NAr]²⁻). Finally, the use of Et₃P=CHMe*LiBr resulted in the fragmentation of the NacNac moiety.

[4 + 1]- and [4 + 2]-cycloaddition reactions of a thiazole-2-thione-based 1,4-diphosphinine (I) is reported, focussing on reversible reactions. Their thermal [4 + 2]-cycloreversion is studied from an experimental as well as theoretical point of view.

The germylone dimNHCGe (5, dimNHC=diimino N‐heterocyclic carbene) undergoes a [2+2] cycloaddition with isocyanates RNCO (R=4‐tolyl or 3,5‐xylyl) to furnish novel alkyl carboxamido germylenes 7 (R=4‐tolyl) and 8 (R=3,5‐xylyl), featuring a C−C bond between the former carbene carbon and the isocyanate moiety. Heating a mixture of 8 with 4‐tolyl isocyanate to 100 °C results in isocyanate metathesis, demonstrating reversible C−C bond formation on the reduced germanium compound. DFT calculations suggest that this process occurs via the reductive dissociation of isocyanate from 8 that regenerates the parent Ge(0) compound 5.

Reaction of NacNacGa with phenylisocyante generates a transient species amenable to unusual 1,3-C–H bond addition of unactivated sp³ C–H and sp² C–H bonds of substrates featuring a hard donor atom. This reaction proceeds for pyridine oxide, dimethylsulfoxide, and dimethylacetamide, but not for pyridine, cyclohexanone, and ethyl acetate. C–H activation was also not observed for reactions with triethylphosphine oxide but, interestingly, in the presence of this compound isocyanate undergoes self-coupling on Ga(I) with a regioselectivity that is different when carried out in the absence of Et₃PO.

Diimino−carbene‐supported germylone dimNHCGe does not react with BPh₃ and does not activate dihydrogen in the FLP mode in the combination with this borane. However, it reacts with B(C₆F₅)₃ to give the zwitterionic borate dimNHCGe−(C₆F₄)BF(C₆F₅)₂. This compound can be converted into the hydroborate dimNHCGe−(C₆F₄)BH(C₆F₅)₂ (8) and further into [dimNHCGe−(C₆F₄)B(C₆F₅)₂]⁺ (4). Compound 4 is a Ge/B analogue of Stephan's FLP parent P/B compound (C₆H₂Me₃)₂P−C₆F₄−B(C₆F₅)₂ but unlike the latter cannot split dihydrogen. Moreover, attempts to prepare a Ge/B analogue of the zwitterion (C₆H₂Me₃)₂HP−C₆F₄−BH(C₆F₅)₂ by protonation of borate 8 resulted in immediate elimination of H_2.

The germylone dimNHCGe (dimNHC=diimino N‐heterocyclic carbene) reacts with azides N₃R (R=SiMe₃ or p‐tolyl) to furnish the first examples of germanium π‐complexes, i. e. guanidine‐ligated compounds (dimNHI−SiMe₃)Ge (NHI=N‐heterocyclic imine, R=SiMe₃) and (dimNHI−Tol)Ge (R=p‐tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI−SiMe₃)Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me₃SiNGe]₄ tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C−C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI−SiMe₃)Ge with tetrachloro‐o‐benzoquinone results in the net transfer of a germanium atom and formation of the free diimino‐guanidine ligand. This ligand also forms when (dimNHI−SiMe₃)Ge is treated with azide N₃(p−Tol), with the germanium product being [(p−Tol)NGe]_n

Oxidation of a base‐stabilized phosphinidene (κ²‐NNP)P (12, NNP=phosphinoamidinate) with N₂O afforded a labile phosphinidene oxide (κ²‐NNP)P=O (16) which was characterized by NMR spectroscopy. Further oxidation of 16 by N₂O or reaction of 12 with two equivalents of pyridine oxide afforded the isolable dioxide (κ²‐NNP)PO₂ which was characterized by NMR and SC XRD. Trapping of 16 with tolyl isocyanate resulted in P=O/N=C metathesis, eventually affording a urea‐ligated phosphine (κ¹‐NNP)P(NTol)₂C=O (17) The mechanism of this reaction was elucidated by DFT calculations. Reactions of phosphinidene 12 with azides generated transient imines (NNP)P=NR, which in the case of R=Tol underwent cycloaddition with tolyl Isocyanate to afford the urea product 17, and in the case of R=SiMe₃ reacts with N₃SiMe₃ via the addition of N−Si across the P=N bond affording, after the extrusion of dinitrogen, a P,N‐heterocyclic compound. Both products of the reactions with azides have been fully characterized, both in solution and the solid‐state. Finally, reaction of phosphinidene 12 with one equivalent of sulfur resulted in the isolation of the base‐stabilized phosphinidene sulfide (κ²‐NNP)P=S that has also been fully characterized.