Periodic Reporting for period 4 - SILION (Design, Synthesis, Characterization and Catalytic Application of Silyliumylidene Ions)
Reporting period: 2020-06-01 to 2021-05-31
Silicon is the second most common element in the earth’s crust and so ubiquitous in everyday life that we tend to forget about its outstanding nature, reactivity and variety of applications. In general, silicon in its highest oxidation state (+4) forms stable bonds to a wide range of elements that gives access to a broad range of construction materials with great diversity in properties and uses, from refractory ceramics over glasses to highly flexible silicones. In its elemental form, besides the groundbreaking advancements in solar cells, silicon has allowed the development of a completely new industrial sector connected to micro processing and microelectronics. On the other hand, low-valent silicon compounds have received a great deal of research interest in recent years. As the field has developed, new classes of species previously thought to be unstable have been isolated and investigated in the condensed phase.
Silyliumylidene ions are silicon(II) cations with four valence electrons and two vacant orbitals, with two electrons localised as a lone pair on the silicon centre. As such, they can be considered as a combination of a silylene and a silylium ion, and their electrophilicity is even enhanced compared to that of a silylium ion.
Silyliumylidenes, understandably, are difficult to isolate as stable compounds. Development of new synthetic methodology and use of new ligand systems has been the driving force behind advancement in this still-young field. The aim of this proposal is the introduction of facile accessible silyliumylidene ions, which are combined the best properties of silylene and silylium ions, and development of its catalytic activity. The synthesis of silyliumylidene ions is straightforward and should allow the investigation of electronic and steric properties of the substituents and N-heterocyclic carbenes. It is anticipated that silyliumylidene ions can be used for a promising new building block for low-valent organosilicon compounds and high-performance catalyst.
Silyliumylidene ions are silicon(II) cations with four valence electrons and two vacant orbitals, with two electrons localised as a lone pair on the silicon centre. As such, they can be considered as a combination of a silylene and a silylium ion, and their electrophilicity is even enhanced compared to that of a silylium ion.
Silyliumylidenes, understandably, are difficult to isolate as stable compounds. Development of new synthetic methodology and use of new ligand systems has been the driving force behind advancement in this still-young field. The aim of this proposal is the introduction of facile accessible silyliumylidene ions, which are combined the best properties of silylene and silylium ions, and development of its catalytic activity. The synthesis of silyliumylidene ions is straightforward and should allow the investigation of electronic and steric properties of the substituents and N-heterocyclic carbenes. It is anticipated that silyliumylidene ions can be used for a promising new building block for low-valent organosilicon compounds and high-performance catalyst.
Recently we developed the straightforward synthetic route to silyliumylidene ions bearing bulky substituent from reaction of stable dichlorosilane with three equivalents of N-heterocyclic carbenes (NHCs) (Chem. Commun. 2014, 50, 12619-12622.; Bull. Chem. Soc. Jpn. 2017, 90, 255-277.). This combination of kinetic and thermodynamic stabilization effect and facile synthesis starting from silicon(IV) precursor led to the isolation of novel silyliumylidene ions in a single step. In fact, obtained silyliumylidne ions are stable enough to investigate their reactivity based on the nucleophilic silicon center as well as NHCs. Bulky silyl substituent has also been utilized in this system (Dalton Tran. 2019, 48, 10403-10406). It is of note that the use of NHCs for isolation of reactive low-valent main group compounds including the low-valent silicon compounds, we were able to publish the review article in Chem. Rev. 2018, 118, 9678-9842 which is highest impact factor journal in chemistry (IF = 53.613). The various substituents at central silicon atom has been investigated in order to understand the electronic and steric effect for corresponding silyliumylidene ions. Within this variation, the utilization of N-hetrocyclic imines (NHIs) as ligands for isolation of low-valent silicon species was conducted and these has been outlined and published in Chem. Soc. Rev. 2016, 45, 6327-6344 (IF = 40.182). As an initial investigation on reactivity of silyliumylidene ions, we found that it reacts with phenylacetylenem, affording the C-H bond activation product with concomitant elimination of imidazolium salts (Chem. Commun. 2014, 50, 12619-12622.). The reduction of carbon dioxide CO2 by silyliumylidene ions yielded the silaacylium ions, silicon analogue of acylium ion that is well known as an important intermediate species of Friedel-Crafts reaction (J. Am. Chem. Soc. 2015, 137, 5828-5836). In addition, we found that activation of elemental chalcogens such as sulfur, selenium, and tellurium gave corresponding heavier derivatives of silaacyclium ions, showing intriguing chalcogen transfer reaction as well as chalcogen exchange reaction (Dalton Tran. 2017, 46, 16014-16018). Furthermore, activation of S-H bond of hydrogen sulfide was also investigated and its reaction mechanism has been elucidated by using both experimental as well as computational methods (Inorganics, 2018, 6, 54-67). In addition, we could obtain the silicon analogue of aldehyde from the reaction of silyliumylidene ions with water in the presence of Lewis acid (Chem. Eur. J. 2019, 24, 1198-1202). Moreover, we could found that intriguing coordination behavior of silyliumylidene ions in various reactions towards transition metals (Chem. Commun. 2018, 54, 13658-13661: Chem, Eur, J. 2020, 26, 6271-6278) which would lead to the novel catalytic applications.
Since the seminal report on pentamethylcyclopentadienyl silyliumylidene ion by Jutzi, several silyliumylidene ions have been prepared and isolated. However, reactivity investigations remain limited to few specific reactions. It should be mentioned that our facile method to silyliumylidene ions enables to investigate reactivity study of silyliumylidene ions. In fact, we found several striking reactivity of silyliumylidene ions that involve the activation of carbon dioxide and E-H bond (E = O, S, C), could open the door of new field of low-valent silicon chemistry with silyliumylidene ions. In fact, we were able to show novel catalytic transformation using silyliumylidene ions. For examples, hydroboration of ketones, aldehydes, and carbon dioxdes in the presence of boranes were shown. Catalytic CO2 reduction by silyliumylidene ions with silanes have been realized. Moreover, catalytic N-methylation of amines and cyanosilylations of carbonyl compounds were successfully achieved.