Current advances in the chemistry of silicon: not exactly a carbon copy

Many contributions of chemistry to modern technology, and much of our understanding of chemical principles, have resulted from studies of the chemistry of carbon. Certainly organic compounds and carbon-based materials continue to provide important advances to the economy, the quality of life, and human health. Looking beyond carbon; however, silicon has always intrigued chemists as an element with exciting potential, especially given its position just below carbon in the periodic table and the fact that it is the second most abundant element in the earth's crust. Indeed, more than a century after Kipping's pioneering work on organosilanes, and 60 years after the discovery of the transistor, silicon plays a vital role in everyday life. Silicones, or polysiloxanes, are inorganic polymers that form the basis of a multi-million dollar business and provide a variety of very useful consumer products. They are valuable (for example) as insulators, sealants, release (non-stick) coatings, and as materials in medical applications. In addition, semiconductor silicon is ubiquitous in essentially all electronic devices in common, everyday use.

Contemporary silicon chemistry, which is the topic of this themed issue in Dalton Transactions, has developed way beyond the synthesis and conversion of Me₂SiCl₂ (the main starting material for silicones) and HSiCl₃ (an important intermediate for the synthesis of high purity silicon, used in the manufacture of silicon wafers for the semiconductor industry). The 25 articles compiled in this issue are a representative selection of current research directions in silicon chemistry. It contains articles that mostly deal with basic research, as well as those describing studies on applications in organic synthesis and catalysis. Other reports deal with specialized, new applications of silicon chemistry.

Jörg Wagler's contribution (DOI: 10.1039/c7dt00300e) is concerned with crystal structures of some common chlorosilanes such as Me₂SiCl₂. Since these are liquid at ambient temperature, little has been known about the structural chemistry of such compounds. Although the process of siloxane formation by the hydrolysis of chlorosilanes is of utmost industrial importance, methods to prepare silicon–oxygen bonds under different conditions are less well known and understood. Chunming Cui's paper (DOI: 10.1039/c6dt04882j) on related syntheses of siloxanes by the NHC-catalyzed hydrolytic oxidation of various hydrosilanes is dedicated to this topic.

An interesting anionic, thermally labile triazenide silicon compound with an SiNNNSi backbone is described by Hans-Wolfram Lerner and co-workers (DOI: 10.1039/c7dt00173h). Also along the lines of new molecular silicon chemistry, Carsten von Hänisch (DOI: 10.1039/c6dt04018g) reports on hybrid disila-crown ethers that complex alkaline earth metals, and Akihiro Tsurusaki and Soichiro Kyushin (DOI: 10.1039/c7dt00395a) describe silyl-substituted borate anions. Silicon atoms are larger than carbon atoms and as a consequence it is more feasible to access stable compounds with more than four substituents. Such “hypercoordinate” silicon compounds are therefore heavily investigated and the contributions by Reinhold Tacke (DOI: 10.1039/c6dt03264h) and Viatcheslav Jouikov (DOI: 10.1039/c7dt00447h) are dedicated to this topic.

Another interesting molecular property that extends from concepts developed for carbon concerns unsaturation and aromaticity. Thus, there has been considerable interest in the possible participation of heavier elements in compounds that exhibit these properties. Along these lines, Larissa Leites (DOI: 10.1039/c7dt00356k) and coworkers describe spectroscopic investigations on aromaticity in a series of benzannulated N-heterocyclic ylene compounds of C, Si, Ge, Sn and Pb. A related study by David Scheschkewitz and Gregor Jung (DOI: 10.1039/c7dt00397h) describes conjugation in oligomers containing one or two disilene (Si=Si) units.

Transition metal–silicon chemistry has played an important role in the development of efficient synthetic routes to silicon compounds and materials, and several articles in this issue describe new contributions to this field. Along these lines, Peter Roesky's (DOI: 10.1039/c7dt00483d) article focuses on the chemistry of amidinate-stabilized silylene fragments, including coordination to transition metal centers. Silylenes as metal ligands are also studied by Hisako Hashimoto and Hiromi Tobita (DOI: 10.1039/c7dt00505a), who present research on hydrogen-bridged bis(silylene) complexes of ruthenium and iron. Takeaki Iwamoto (DOI: 10.1039/c7dt01113j) contributes a paper showing the use of a well-designed silylene ligand for platinum complexes that catalyze hydrosilylation reactions with alkenes. Transition metal chemistry with silicon ligands is further highlighted by Sylviane Sabo-Etienne and Virginia Montiel-Palma (DOI: 10.1039/c7dt00727b), who investigated new tris(silyl)phosphine ligands and their complexation to rhodium. Transition metal–siloxide chemistry is another recently emerging area of interest. Thomas Rauchfuss and co-workers (DOI: 10.1039/c6dt04205h) describe several methods for the preparation of methoxysiloxide metal complexes.

Silicon plays a fairly important role in organic chemistry but arguably its main use is in silyl protection groups. Alternative organic silicon chemistry includes Keith Woerpel's (DOI: 10.1039/c6dt04612f) work on silylene transfer to organic substrates to achieve stereoselective silacyclopropanation, and Richard Laine's (DOI: 10.1039/c7dt00373k) examples of exploiting silsesquioxanes as nanoreactors, where attached phenyl groups are first functionalized and then released as phenols.

The idea that silicon chemistry should resemble that of carbon makes polysilanes, with a backbone of Si–Si bonds, inorganic analogues of alkanes. A particularly fascinating property of polysilanes is that of σ-bond electron delocalization. The fact that conjugation and conductivity in a polysilane chain are dependent on the chain's spatial orientation has made these compounds interesting for the development of molecular electronic devices. Articles by Rebekka Klausen and Arthur Bragg (DOI: 10.1039/c7dt00384f) and by Lisa Rosenberg (DOI: 10.1039/c7dt00546f) deal with physical investigations and chemical modifications of polysilanes, respectively.

There is a vast materials science associated with silicon, and important advances are still being made in this direction. Contributions by Nicola Hüsing (DOI: 10.1039/c7dt00558j) and Michael Elsässer (DOI: 10.1039/c7dt00571g) deal with silicon-containing aerogels and porous materials. Justin Hargreaves and Andrew Hector (DOI: 10.1039/c7dt00285h) prepared a composite material of vanadium nitride and silicon imidonitride [Si(NHMe)₄] which catalyzes the CO_x-free production of hydrogen from methane. A useful application of nanoparticles is described by Bhanu Chauhan (DOI: 10.1039/c7dt00544j) who found that polysiloxane-stabilized Pt nanoparticles are recyclable catalysts for the selective synthesis of vinylsilanes via the hydrosilylation of alkynes.

Two articles are concerned with medial applications of silicon chemistry. The one by Joseph Lichtenhan (DOI: 10.1039/c7dt00487g) deals with in vitro studies of the interaction of silsesquioxane (POSS) trisilanols with red blood cells and explores the use of this substance class for treatment of traumatic hemorrhage and wound care. The other article by Javier Sánchez-Nieves and Javier de la Mata (DOI: 10.1039/c6dt03791g) describes the use of gold nanoparticles stabilized by cationic carbosilane dendrons as gene carriers.

As editors of this issue it was a pleasure for us take the opportunity to highlight a few aspects of the vigorously growing directions in current silicon chemistry. We are very pleased to be associated with this project, which highlights the breadth and impact of silicon chemistry for the readers of Dalton Transactions.

We wish to extend sincere thanks to the editorial staff of Dalton Transactions. In particular, Alice Jensen, Mina Roussenova, Helen Lunn and Michelle Canning have all worked very hard in dealing with authors and referees to make this issue a success.

TDT would like to thank the National Science Foundation of the US for support of his research in silicon chemistry. CM would like to thank the Austrian Science Fund FWF.

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