Silicon based radicals, radical ions, diradicals and diradicaloidsChem. Soc. Rev.


Kartik Chandra Mondal, Sudipta Roy, Herbert W. Roesky
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This journal is©The Royal Society of Chemistry 2015 Chem. Soc. Rev.

Cite this:DOI: 10.1039/c5cs00739a

Silicon based radicals, radical ions, diradicals and diradicaloids

Kartik Chandra Mondal, Sudipta Roy and Herbert W. Roesky*

Radicals are an important class of species which act as intermediates in numerous chemical and biological processes. Most of the radicals have short lifetimes. However, radicals with longer lifetimes can be isolated and stored in a pure form. They are called stable radicals. Over the last five decades, the syntheses of several stable radicals have been reported. Recently, highly unstable radicals have been successfully stabilized via strong s-donation of singlet carbenes. Cyclic aklyl(amino) carbene (cAAC) is regarded as a stronger s-donor and a better p-acceptor when compared with that of an N-heterocyclic carbene (NHC). In this article we review preferentially the results of our group to generate stable radical centers on the carbene carbon atoms by employing the so far hidden and unique ability of the cAACs. We focus on the development of new synthetic routes to stable and isolable radicals containing silicon atoms. All the compounds have been well characterized by single crystal X-ray analysis; the mono-radicals have been distinguished by EPR spectroscpy and the ground state of the diradicals has been studied by magnetic susceptibility measurements and theoretical calculations. Many of these compounds are studied by cyclic voltammetry and are often converted to their corresponding radical cations or radical anions via electron abstraction or addition processes. Some of them are stable, having long lifetimes and hence are isolated and characterized thoroughly. Not much information has been obtained on the short lived persistent radical species.

Herein, we discuss some of the examples of such a type of species and focus on what kind of chemical reactions are initiated by these short-lived radical species in solution. We also briefly mention the syntheses and charaterization of the so far reported stable silicon centered radicals.


Radicals are captivating chemical species which have attracted the interest of experimental as well as theoretical chemists and

Institut fu¨r Anorganische Chemie, Georg-August-Universita¨t, 37077 Go¨ttingen,

Germany. E-mail:; Fax: +49 551 39 33373;

Tel: +49 551 39 33001

Kartik Chandra Mondal

Kartik Chandra Mondal received his PhD in 2011 from Karlsruhe

Institute of Technology (KIT) under the supervision of Professor Annie

K. Powell. He worked on mixed 3d–4f ion based single molecule magnets. After a short stay as a postdoctoral researcher in the same group he moved to University of Go¨ttingen in October, 2011.

Since then he has been working with Professor Herbert W. Roesky as a postdoctoral fellow. His research interest mainly focuses on the stabilization of low valent low coordinate group 14 elements and transition metal complexes. Dr Mondal has coauthored more than 45 peer-reviewed publications in leading scientific journals.

Sudipta Roy

Sudipta Roy received her PhD in 2012 from the University of

Regensburg under the supervision of Professor Oliver Reiser. Afterwards she worked as a postdoctoral fellow at the Institute of Organic and Biomolecular

Chemistry, University of Go¨ttingen on transition metal catalyzed C–H bond activation. Since 2014 she has been working as a postdoctoral fellow with Professor Herbert W.

Roesky. Her research interests include the synthesis of cyclic alkyl(amino) carbene stabilized highly sensitive Si–P compounds, and transition metal complexes with their application in catalysis.

Received 27th September 2015

DOI: 10.1039/c5cs00739a

Chem Soc Rev


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Chem. Soc. Rev. This journal is©The Royal Society of Chemistry 2015 physicists for over a century.1 The first organic radical (trityl radical; Ph3C) was synthesized in 1897 by the Russian scientist

Moses Gomberg and was reported in 1900.1a Trityl radical is the first organic radical which was produced within a glass container in a laboratory. It was accidentally obtained in an attempt to synthesize sterically crowded Ph3C–CPh3 by reduction of Ph3C–Cl with Zn or Ag metal. A trityl radical (Ph3C) mostly exists in the dimeric form and a small percentage of the monomeric radical form is in equilibrium with its dimer.

Seven decades later the molecular structure of Ph3C analogues were studied and confirmed by X-ray single crystal diffraction.

The chemists then tried to understand and unlock the hidden mystery behind the unusual stability of such open shell species.

Finally, it has been concluded that substitution with six chlorine atoms (at the 2,6-position of each phenyl ring) is enough tomake this class of radicals to be stable in air. Radicals are mostly stabilized by the steric effect preventing them from taking part in further chemical processes such as dimerization and proton abstraction. A large number of chemical2,3 and biological4 procedures are functioning via radical pathways.2–4 Radicals play crucial roles in our daily biological systems to laboratory chemical processes. A large number of radical species have been even found in the interstellar atmosphere such as methylidyne (CH), ethynyl radical (C2H), methylene diradical (CH2), amino radical (NH2), formyl radical (HCO), hydroxyl radical (OH), and cyanogen radical (CN). They could not combine with each other due to the very low density of the interstellar medium.

In general most of the radicals are short-lived species.1h,5

The lifetimes of some radicals are extremely short and thus they cannot be even characterized by electron paramagnetic resonance (EPR) measurements. Radicals having relatively longer lifetimes can be studied by EPR spectroscopy at low temperatures. The instability of the unstable radicals arises mainly from their high reactivity due to thermodynamic and kinetic reasons. Highly reactive radicals are prone to undergo dimerization, polymerization, bond activation, fragmentation or atom abstraction from solvents, etc. The radicals are mainly divided into two categories, viz. persistent radicals5 and stable radicals. The former have sufficiently long lifetimes and can be characterized in solution by EPR. The persistent radicals cannot be isolated in a pure form, while the stable radicals are isolable and storable even at room temperature (rt).1h,5 Stable radicals can be structurally characterized by X-ray single crystal diffraction.6 They are utilized in many areas of chemistry for different means. They can catalyse polymerization reactions and hence are of huge interest in polymer chemistry.7 They have been combined with metal salts for oxidation of alcohols to the corresponding aldehydes.8 Stable radicals have often been used for trapping unstable radical intermediates in organic and inorganic chemical transformations.9 When radicals are combined with anisotropic metal ions at the molecular level, the composite can act as single molecule magnets (SMMs)10 or single chain magnets (SCMs),11 which are promising candidates for the construction of molecule based electronics.12 Moreover, radicals have the ability to act as antioxidants.13