Novel functional groups of low-valent silicon
Final Report Abstract
The concept of donor-acceptor stabilization using N-heterocyclic carbenes (NHCs) has enabled the isolation of a considerable number of low-valent main-group element complexes in the last decade. Utilizing a chelating bis-NHC ligand, we could realize the bis-NHC supported monoatomic zero-valent silicon complex (silylone) in 2013. The overarching interest we pursued with this project is achieving unprecedented low-valent silicon species by extending our previous investigation on the first silylone, (bis-NHC)Si. Featuring four valence electrons as two lone pairs at the central silicon(0) atom, (bis-NHC)Si can serve as a useful building block for other low-valent silicon compounds and the same is true for the germylone (bis-NHC)Ge. For instance, we generated the Lewis adducts (bis- NHC)Si→GaCl3 and (bis-NHC)Si→(ZnCl2)2 as unusual silicon(0) complexes. The latter (bis- NHC)Si→GaCl3 has further been used to synthesize the isolable :Si=Se complex (bis- NHC)Si(=Se)GaCl3 with divalent silicon, representing molecular heavy congeners of CO, and the silicon ditelluride Te=Si=Te complex (bis-NHC)Si(=Te)2 as the first isolable complex as a molecular silicon analogue of CO2. Moreover, (bis-NHC)Si could even be employed to activate CO2, affording the molecular silicon dicarbonate (bis- NHC)Si(CO3)2 via the formation of SiO and SiO2 complexes as elusive intermediates. To extend the silylone chemistry based on the bis-NHC ligand, we successfully introduced the xanthene-based bis-NHSi (N- heterocyclic silylene) with stronger Si(II) donors compared to the NHC-C(II) centers for synthesizing the new isolable silicon(0) complex (bis-NHSi)Si. Strikingly, the bis-silylene supported silylone can not only react with N2O to yield novel silicon oxides but is also able to activate two molecules of NH3, affording the respective 1,3- diaminotrisilane, representing the first case of N−H bond cleavage of ammonia by a silylone. Remarkably, heterolytic cleavage of H2 and ethylene addition are also accessible through cooperative bond activation of the electron-rich (bis-NHSi)Si with BPh3. For the first time, a silylone could be applied as a Lewis base in Frustrated Lewis Pair (FLP) chemistry. For comparison, we also studied the related low-valent chemistry of germanium as the heavier homologue of silicon. While the PCO chemistry could not be accomplished with lowvalent silicon, due to the oxygen affinity of silicon which prevents the elimination of CO from the PCO moiety, we were able to isolate a series of Ge=X- (X= P, As) containing novel compounds, starting from NaPCO and NaAsCO. With the germylone (bis-NHC)Ge in hand, we could obtain stable complexes featuring Ge=E (E = S, Se, Te) moieties in parallel to the lowvalent silicon analogues. Utilizing the chelating bis-NHSi ligand, the zero-valent germanium species (bis-NHSi)Ge (germylone) has also been realized. Our achievements in the chemistry of silylones and germylones demonstrate that the characteristics of monatomic zero-valent silicon and its analogues can provide novel reaction patterns to gain access to unprecedented low-valent silicon and germanium species and even extend the series of functional groups of these group 14 elements. With this, we envision that more interesting zero-valent complexes of silicon and its heavier homologues with unprecedented reactivity will follow in the near future.
Publications
- From a Phosphaketenyl-Functionalized Germylene to 1,3-Digerma-2,4-diphosphacyclobutadiene. Angew. Chem. Int. Ed. 2016, 55, 4781– 4785
S. Yao, Y. Xiong, T. Szilvási, H. Grützmacher, M. Driess
(See online at https://doi.org/10.1002/anie.201600701) - Heavier congeners of CO and CO 2 as ligands: from zero-valent germanium (‘germylone’) to isolable monomeric GeX and GeX 2 complexes (X = S, Se, Te). Chem. Sci. 2016, 7, 5462–5469
Y. Xiong, S. Yao, M. Karni, A. Kostenko, A. Burchert, Y. Apeloig, M. Driess.
(See online at https://doi.org/10.1039/c6sc01839d) - Facile Access to NaOC≡As and Its Use as an Arsenic Source To Form Germylidenylarsinidene Complexes. Angew. Chem. Int. Ed. 2017, 56, 7465–7469
S. Yao, Y. Grossheim, A. Kostenko, E. Ballestero-Martínez, S. Schutte, M. Bispinghoff, H. Grützmacher, M. Driess
(See online at https://doi.org/10.1002/anie.201703731) - Unexpected Photodegradation of a Phosphaketenyl-Substituted Germyliumylidene Borate Complex. Angew. Chem. Int. Ed. 2017, 56, 4333–4336
Y. Xiong, S. Yao, T. Szilvási, E. Ballestero-Martínez, H. Grützmacher, M. Driess
(See online at https://doi.org/10.1002/anie.201701337) - An isolable β-diketiminate chlorosilylene. Dalt. Trans. 2018, 2152–2155
Y. Xiong, S. Yao, A. Kostenko, M. Driess
(See online at https://doi.org/10.1039/c8dt00121a)
