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 σ-donation of singlet carbenes. Cyclic aklyl(amino) carbene (cAAC) is regarded as a stronger σ-donor and a better π-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.
Diradical (cAAC&z.rad;)
2
SiCl
2
is isolated in two polymorphic forms. The crystals of one of the polymorphs are stable in open air for over a week.
Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and ...bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.
The present review highlights recent developments in the domain of cyclic (alkyl) (amino) carbenes with the emphasis on their applications in the field of catalysis, activation of small molecules, radical chemistry, organic light‐emitting diodes (OLEDs), medicinal chemistry, and single‐molecule magnets (SMMs).
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Two ways to relax: A defect‐dicubane Co2Dy2 single‐molecule magnet (SMM) displays slow relaxation of magnetization with a blocking temperature of 22 K (at 1500 Hz), the highest reported for a ...3d–4f‐based SMM. Analysis of the relaxation reveals two distinct blocking regimes, one of which is intraionic, localized on the DyIII ions, while the other is exchange‐based.
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A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air‐stable radical cation 1.+, and dication 12+ have been synthesized. The redox property of 1.+ was studied by cyclic ...voltammetry. EPR and theoretical calculations show that the unpaired electron in 1.+ is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis.
Carbons get excited! Commercially available CBr4 is utilized as a source of dicarbon for the preparation of the C4 cumulene that contains a cyclic alkyl(amino) carbene (cAAC). Lithium or potassium is used as a reducing agent. The corresponding air‐stable radical cation and dication of the C4 cumulene were also synthesized and their interconversion was studied by cyclic voltammetry.
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Abstract
Silver clusters are attractive candidates for their promising optical properties, and biomedical activities. Herein, we report on the first syntheses and isolation of three ...homo‐/mixed‐valence silver nanoclusters (NCs) with Ag
12
Cl
3
, Ag
29
, and Ag
8
cores ((cAAC)P)
6
Ag
12
Cl
3
(OTf)
3
(
1
), ((cAAC)P)
6
Ag
29
(
2
), and ((cAAC)P)
4
Ag
8
(
3
) having three‐/twofold symmetry, employing cyclic alkyl(amino) carbene (cAAC)‐supported phosphinidenide (cAACP
−
) as the π‐accepting stabilizing ligand. The average diameters of Ag NCs
1
, and
2
are approximately 1.6 to 2 nm. The redox non‐innocent mono‐atomic phosphorus anions (P
−
) anchored with cAAC ligands are generated in situ by the reaction of AgOTf with a boryl‐phosphaalkene (cAAC)P−B(N
i
Pr
2
)
2
through cleavage of the P−B bond with the help of a triflate anion (OTf
−
) as a weak nucleophile. Equivalent number of the (cAAC)P
−
anions generated in situ are oxidized to produce the corresponding bis‐phosphinidene (cAAC)
2
P
2
leading to the generation of Ag
0
ions in solution for the formation of the unprecedented mixed‐valence Ag NC
2
. Complex
3
is achieved by treating potassium phosphinidenide cAACPK with AgNTf
2
. The ligand field and the steric hindrance of the (cAAC)P units play crucial roles in stabilizing complexes
1
–
3
, further providing a three‐ (
1
,
2
)/two‐ (
3
) fold stand. The Ag
12
Cl
3
NC (
1
) with a tricationic core Ag
I
12
Cl
3
was found to be diamagnetic, and fluorescent, emitting green light at 563 nm when excited at 400 nm. In contrast, the neutral Ag
29
(
2
) and Ag
8
(
3
) clusters were found to be paramagnetic, and NMR silent showing characteristic EPR signals for Ag
0
at room temperature.
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6.
Conversion of a Singlet Silylene to a stable Biradical Mondal, Kartik Chandra; Roesky, Herbert W.; Schwarzer, Martin C. ...
Angewandte Chemie (International ed.),
February 4, 2013, Volume:
52, Issue:
6
Journal Article
Peer reviewed
Silicon becomes colored: Stable biradicals were prepared from an N‐heterocyclic carbene stabilized SiCl2 and a cyclic alkyl(amino)carbene, and characterized as two polymorphs. The deep‐blue crystals ...of one polymorph are stable upon exposure to air for about a week, while the solution in THF decomposes rapidly when exposed to air. In a side reaction, the different carbene species react with each other under CH activation and CC bond formation in the presence of the biradical.
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Silver clusters are attractive candidates for their promising optical properties, and biomedical activities. Herein, we report on the first syntheses and isolation of three homo‐/mixed‐valence silver ...nanoclusters (NCs) with Ag12Cl3, Ag29, and Ag8 cores ((cAAC)P)6Ag12Cl3(OTf)3 (1), ((cAAC)P)6Ag29 (2), and ((cAAC)P)4Ag8 (3) having three‐/twofold symmetry, employing cyclic alkyl(amino) carbene (cAAC)‐supported phosphinidenide (cAACP−) as the π‐accepting stabilizing ligand. The average diameters of Ag NCs 1, and 2 are approximately 1.6 to 2 nm. The redox non‐innocent mono‐atomic phosphorus anions (P−) anchored with cAAC ligands are generated in situ by the reaction of AgOTf with a boryl‐phosphaalkene (cAAC)P−B(NiPr2)2 through cleavage of the P−B bond with the help of a triflate anion (OTf−) as a weak nucleophile. Equivalent number of the (cAAC)P− anions generated in situ are oxidized to produce the corresponding bis‐phosphinidene (cAAC)2P2 leading to the generation of Ag0 ions in solution for the formation of the unprecedented mixed‐valence Ag NC 2. Complex 3 is achieved by treating potassium phosphinidenide cAACPK with AgNTf2. The ligand field and the steric hindrance of the (cAAC)P units play crucial roles in stabilizing complexes 1–3, further providing a three‐ (1, 2)/two‐ (3) fold stand. The Ag12Cl3 NC (1) with a tricationic core AgI12Cl3 was found to be diamagnetic, and fluorescent, emitting green light at 563 nm when excited at 400 nm. In contrast, the neutral Ag29 (2) and Ag8 (3) clusters were found to be paramagnetic, and NMR silent showing characteristic EPR signals for Ag0 at room temperature.
Silver stableware: We have demonstrated the first syntheses and solid‐state isolation of three unprecedented silver clusters ((cAAC)P)6Ag12Cl3(OTf)3 (1), ((cAAC)P)6Ag29 (2), and ((cAAC)P)4Ag8 (3) by employing cyclic alkyl(amino) carbene (cAAC)‐supported phosphinidenide (cAACP−) generated in situ as the stabilizing ligand from the corresponding phosphaalkene and potassium phosphinidenide, respectively.
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8.
Experimental Charge Density Study of a Silylone Niepötter, Benedikt; Herbst-Irmer, Regine; Kratzert, Daniel ...
Angewandte Chemie (International ed.),
March 3, 2014, Volume:
53, Issue:
10
Journal Article
Peer reviewed
An experimental and theoretical charge density study confirms the interpretation of (cAAC)2Si as a silylone to be valid. Two separated VSCCs present in the non‐bonding region of the central silicon ...are indicative for two lone pairs. In the experiment, both the two crystallographically independent SiC bond lengths and ellipticities vary notably. It is only the cyclohexyl derivative that shows significant differences in these values, both in the silylones and the germylones. Only by calculating increasing spheres of surrounding point charges we were able to recover the changes in the properties of the charge density distribution caused by weak intermolecular interactions. The nitrogen–carbene‐carbon bond seems to have a significant double‐bond character, indicating a singlet state for the carbene carbon, which is needed for donor acceptor bonding. Thus the sum of bond angles at the nitrogen atoms seems to be a reasonable estimate for singlet versus triplet state of cAACs.
Two non‐bonding VSCCs found in a silylone: An experimental and theoretical charge‐density study confirms the interpretation of (cAAC)2Si as a silylone to be valid. The two separated VSCCs present in the non‐bonding region of the central silicon are indicative for two lone pairs. The nitrogen–carbene‐carbon bond seems to have a significant double bond character, indicating a singlet state for the carbene carbon atom.
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Spin frustration is a unique phenomenon in magnetic complexes, where if a spin fails to lower the total energy of the system, by orienting itself in relative to its adjacent spins, it ...is termed as frustrated. In this work, we discuss about the magnetic properties, and the possibility of spin frustration in our isolated NiII2MnIII(hep)4(PhCO2)(N3)2 (Hhep = 2-(2-hydroxyethyl) pyridine) having an isosceles NiII2MnIII triangle. The triangular MnIIINiII2 core was held together by two μ3-alkoxide bridges. The magnetic properties of this complex were studied in detail. Variable temperature magnetic susceptibility data of the complex in the range of 1.8 to 300 K, indicates the presence of stronger intramolecular antiferromagnetic coupling between two NiII centres than between NiII and MnIII ions. Experimentally, the ground state of the complex was found to be a mixture of E(2,0) and E(1,1) states, suggesting our complex to be close to a spin frustrated system which was supported by ab initio calculations.
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Reduction of the neutral carbene tetrachlorosilane adduct (cAAC)SiCl4 (cAAC=cyclic alkyl(amino) carbene :C(CMe2)2(CH2)N(2,6‐iPr2C6H3) with potassium graphite produces stable (cAAC)3Si3, a ...carbene‐stabilized triatomic silicon(0) molecule. The Si−Si bond lengths in (cAAC)3Si3 are 2.399(8), 2.369(8) and 2.398(8) Å, which are in the range of Si−Si single bonds. Each trigonal pyramidal silicon atom of the triangular molecule (cAAC)3Si3 possesses a lone pair of electrons. Its bonding, stability, and electron density distributions were studied by quantum chemical calculations.
Three‐sided: Reaction of potassium graphite with a cyclic alkyl(amino) carbene tetrachlorosilane adduct results in the formation of a stable and isolable carbene‐stabilized triatomic silicon(0) cluster (see figure). The triangular compound was characterized by single crystal X‐ray structural crystallography, and its bonding and electron density distributions were studied by quantum chemical calculations.
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