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.
Although tetrameric Al(I) compounds have been known for a long time, the monomeric Al(I) compounds that are analogous to carbenes are very recent entrants in Al(I) chemistry. They possess novel ...structural features and exhibit distinct reactivity. This has resulted in the isolation and characterization of various unusual aluminum(III) compounds such as the aluminatetrazoles and aluminacyclopropenes. In comparison to the recent emergence of monomeric aluminum(I) compounds, stable silylenes and germylenes (carbene analogues of silicon and germanium) were recognized much earlier. This led to the evolution of the Si(II) and Ge(II) chemistry that at times surpasses the sophistication achieved in divalent carbon chemistry. Thus, while carbon lacks an example of a stable chlorocarbene (LCCl), for silicon there is one example of LSiCl, and for germanium there are a fair number of LGeCl compounds. While reactivity studies on LSiCl are anticipated, the utility of RGeCl as a synthon is well documented. Exotic compounds such as a germanethioacid chloride, a germanium(II) hydride, and a germanium(II) hydroxide are some of the examples that were derived from LGeCl. Recent results from our laboratory at Göttingen have helped in the development of these interesting areas of research, and the present account summarizes our contributions to the chemistry of Al(I), Si(II), and Ge(II).
Isolating stable compounds with low-valent main group elements have long been an attractive research topic, because several of these compounds can mimic transition metals in activating small ...molecules. In addition, compounds with heavier low-valent main group elements have fundamentally different electronic properties when compared with their lighter congeners. Among group 14 elements, the heavier analogues of carbenes (R2C:) such as silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:) are the most studied species with low-valent elements. The first stable carbene and silylene species were isolated as N-heterocycles. Among the dichlorides of group 14 elements, CCl2 and SiCl2 are highly reactive intermediates and play an important role in many chemical transformations. GeCl2 can be stabilized as a dioxane adduct, whereas SnCl2 and PbCl2 are available as stable compounds. In the Siemens process, which produces electronic grade silicon by thermal decomposition of HSiCl3 at 1150 °C, chemists proposed dichlorosilylene (SiCl2) as an intermediate, which further dissociates to Si and SiCl4. Similarly, base induced disproportionation of HSiCl3 or Si2Cl6 to SiCl2 is a known reaction. Trapping these products in situ with organic substrates suggested the mechanism for this reaction. In addition, West and co-workers reported a polymeric trans-chain like perchloropolysilane (SiCl2) n . However, the isolation of a stable free monomeric dichlorosilylene remained a challenge. The first successful attempt of taming SiCl2 was the isolation of monochlorosilylene PhC(NtBu)2SiCl supported by an amidinate ligand in 2006. In 2009, we succeeded in isolating N-heterocyclic carbene (NHC) stabilized dichlorosilylene (NHC)SiCl2 with a three coordinate silicon atom. (The NHC is 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes).) Notably, this method allows for the almost quantitative synthesis of (NHC)SiCl2 without using any hazardous reducing agents. Dehydrochlorination of HSiCl3 with NHC under mild reaction conditions produces (NHC)SiCl2. We can separate the insoluble side product (NHC)HCl readily and recycle it to form NHC. The high yield and facile access to dichlorosilylene allow us to explore its chemistry to a greater extent. In this Account, we describe the results using (NHC)SiCl2 primarily from our laboratory, including findings by other researchers. We emphasize the novel silicon compounds, which supposedly existed only as short-lived species. We also discuss silaoxirane, silaimine with tricoordinate silicon atom, silaisonitrile, and silaformyl chloride. In analogy with N-heterocyclic silylenes (NHSis), oxidative addition reactions of organic substrates with (NHC)SiCl2 produce Si(IV) compounds. The presence of the chloro-substituents both on (NHC)SiCl2 and its products allows metathesis reactions to produce novel silicon compounds with new functionality. These substituents also offer the possibility to synthesize interesting compounds with low-valent silicon by further reduction. Coordination of NHC to the silicon increases the acidity of the backbone protons on the imidazole ring, and therefore (NHC)SiCl2 can functionalize NHC at the C-4 or C-5 position.
Lewis Base Stabilized Dichlorosilylene Ghadwal, Rajendra S; Roesky, Herbert W; Merkel, Sebastian ...
Angewandte Chemie (International ed.),
July 20, 2009, Letnik:
48, Številka:
31
Journal Article
Recenzirano
Stable? You can bottle it! The base‐stabilized dichlorosilylene L1SiCl2 (see picture; L1=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) is stable at room temperature. L1SiCl2 can undergo a ...reaction with diphenylacetylene to form a trisilacyclopentene derivative. These compounds have been characterized by X‐ray crystallography and computational studies.
Quantum chemical calculations using density functional theory have been carried out for the cyclic (alkyl)(amino)carbene (cAAC) complexes of the group 11 atoms TM(cAAC)2 (TM = Cu, Ag, Au) and their ...cations TM(cAAC)2+. The nature of the metal–ligand bonding was investigated with the charge and energy decomposition analysis EDA-NOCV. The calculations show that the TM–C bonds in the charged adducts TM(cAAC)2+ are significantly longer than in the neutral complexes TM(cAAC)2, but the cations have much higher bond dissociation energies than the neutral molecules. The intrinsic interaction energies ΔE int in TM(cAAC)2+ take place between TM+ in the 1S electronic ground state and (cAAC)2. In contrast, the metal–ligand interactions in TM(cAAC)2 involve the TM atoms in the excited 1P state yielding strong TM p(π) → (cAAC)2 π backdonation, which is absent in the cations. The calculations suggest that the cAAC ligands in TM(cAAC)2 are stronger π acceptors than σ donors. The trends of the intrinsic interaction energies and the bond dissociation energies of the metal–ligand bonds in TM(cAAC)2 and TM(cAAC)2+ give the order Au > Cu > Ag. Calculations at the nonrelativistic level give weaker TM–C bonds, particularly for the gold complexes. The trend for the bond strength in the neutral and charged adducts without relativistic effects becomes Cu > Ag > Au. The EDA-NOCV calculations suggest that the weaker bonds at the nonrelativistic level are mainly due to stronger Pauli repulsion and weaker orbital interactions. The NBO picture of the C–TM–C bonding situation does not correctly represent the nature of the metal–ligand interactions in TM(cAAC)2.
The effective catalytic activity of organoaluminum compounds for the monohydroboration of carbodiimides has been demonstrated. Two aluminum complexes, 2 and 3, were synthesized and characterized. The ...efficient catalytic performances of four aluminum hydride complexes L1AlH2 (L1=HC(CMeNAr)2, Ar=2,6‐Et2C6H3; 1), L2AlH2(NMe3) (L2=o‐C6H4F(CH=N‐Ar), Ar=2,6‐Et2C6H3; 2), L3AlH (L3=2,6‐bis(1‐methylethyl)‐N‐(2‐pyridinylmethylene)phenylamine; 3), and L4AlH(NMe3) (L4=o‐C6H4(N‐Dipp)(CH=N‐Dipp), Dipp=2,6‐iPr2C6H3; 4), and an aluminum alkyl complex L1AlMe2 (5) were used for the monohydroboration of carbodiimides investigated under solvent‐free and mild conditions. Compounds 1–3 and 5 can produce monohydroborated N‐borylformamidine, whereas 4 can afford the C‐borylformamidine product. A suggested mechanism of this reaction was explored, and the aluminum formamidinate compound 6 was characterized by single‐crystal X‐ray, also a stoichiometric reaction was investigated.
Monohydroboration of carbodiimides: The catalytic activity of five aluminum compounds for the monohydroboration of carbodiimides were investigated. The suggested mechanism was supported by aluminum formamidinate compound formation and the stoichiometric reaction.
The reaction of (LSi:)2 (1; L = PhC(NtBu)2) with 2 equiv of Me3SiC2C2SiMe3 resulted in the formation of (Me3SiC2)2(Me3Si)2C4Si2(L)2 (2). 2 exhibited a one-electron transfer when treated with 1 equiv ...of Ph3C+B(C6F5)4− to yield (Me3SiC2)2(Me3Si)2C4Si2(L)2·+B(C6F5)4− (3) and Ph3CCPh3, respectively. When compound 2 was treated with 2 equiv of AgOSO2CF3 a transfer of two electrons occurred to produce (Me3SiC2)2(Me3Si)2C4Si2(L)22+·2OSO2CF3− (4) and elemental silver. The 1,4-disilabenzene 2 is disclosed of an open-shell singlet diradical character, and 3 and 4 are, respectively, the elusive stable radical cation and dication species of the 1,4-disilabenzene (2). Furthermore, 2 reacted with group 16 elements of O, S, and Se by oxidative addition to form (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-O2) (5) and (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-E) (E = S (6) and Se (7)), respectively.
The past two decades have brought remarkable advances in organosilicon chemistry with the isolation of stable silylenes, persila-allene, and disilynes. The extension of this list gives an impression ...that it will continue to flourish. The judicous employment of sterically appropriate ligands has enabled the synthesis and isolation of compounds with low-valent silicon. Recently, for example, interconnected bis-silylenes were isolated where the two Si atoms are connected by a σ-bond and each Si atom is possessing a lone pair of electrons. The formal oxidation state of each Si atom in the interconnected bis-silylene is +1, so bis-silylenes can be considered as the valence isomers of disilynes. In this Account, we describe the synthesis of interconnected bis-silylenes and assess their potential as a new building block in organosilicon chemistry. In 2009, we reported the isolation of a bis-silylene ((PhC(NtBu)2)2Si2) stabilized by a sterically bulky benz-amidinato ligand with tBu substituents on the nitrogen atoms. Prior to our work, Robinson and co-workers described the synthesis of a N-heterocyclic carbene stabilized bis-silylene. In following years, just two more interconnected bis-silylenes have been reported. Density functional theory calculations to establish the geometric and electronic structures of the reported bis-silylenes have shown that the Wiberg bond index (WBI) for all the reported bis-silylenes is ∼1. The synthesis of stable (PhC(NtBu)2)2Si2 prompted explorations of its reactivity. An important facet of silylene chemistry involves oxidative addition at the Si(II) center with unsaturated substrates, a reaction also available for bis-silylenes. Due to the three reaction sites (two lone pairs of electrons and a labile Si(I)–Si(I) single bond) in the interconnected bis-silylenes, we expect novel product formation. A labile Si–Si bond facilitates the reactions of (PhC(NtBu)2)2Si2 with diphenyl alkyne or adamantyl phosphaalkyne which afforded 1,4- disilabenzene and 1,3-disilacarbaphosphide (CSi2P) derivatives, respectively. The former is a noteworthy addition to the silicon analogues of benzene, and the latter serves as a heavy cyclobutadiene. With white phosphorus, a cyclic Si2P2 derivative, an analogue of cyclobutadiene was obtained. The most predominant structural feature of these heavy cyclobutadienes is the presence of two-coordinate P atoms.
A successful selective reduction of X2B-Tip (Tip = 1,3,5- i Pr3-C6H2, X = I, Br) with KC8 and Mg metal, respectively, in the presence of a hybrid ligand (C6H4(PPh2)LSi) leads to a stable low-valent ...five-membered ring as a boryl radical C6H4(PPh2)LSiBTipBr (1) and neutral borylene C6H4(PPh2)LSiBTip (2). Compound 2 reacts with 1,4-cyclohexadiene, resulting in hydrogen abstraction to afford the radical C6H4(PPh2)LSiB(H)Tip (3). Quantum chemical studies reveal that compound 1 is a B-centered radical, and compound 2 is a phosphane and silylene stabilized neutral borylene in a trigonal planar environment, whereas compound 3 is an amidinate-centered radical. Although compounds 1 and 2 are stabilized by hyperconjugation and π-conjugation, they display high H-abstraction energy and basicity, respectively.
Over the past few decades, β-diketiminate ligands have been widely used in coordination chemistry and are capable of stabilizing various metal complexes in multiple oxidation states. Recently, the ...chemistry of aluminum and gallium in their +1 oxidation state has rapidly emerged. NacNacM(
i
) (M = Al, Ga; NacNac = β-diketiminate ligand) shows a two coordinate metal center comparable with singlet carbene-like species. The metal center also possesses a formally vacant p-orbital. In this article we present an overview of the last 10 years for aluminum(
i
) and gallium(
i
) stabilized by β-diketiminate ligands that have been widely explored in bond breaking and forming species.
Herein we present an overview of the last 10 years for aluminum(
i
) and gallium(
i
) stabilized by β-diketiminate ligands that undergo a series of oxidative addition reactions with molecules containing single and multiple bonds.
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