A. Claudia Stückl

Easy Access to Silicon(0) and Silicon(II) Compounds

Two different synthetic methodologies of silicon dihalide bridged biradicals of the general formula (L(n)•)2SiX2 (n = 1, 2) have been developed. First, the metathesis reaction between NHC:SiX2 and L(n): (L(n): = cyclic akyl(amino) carbene in a 1:3 molar ratio leads to the products 2 (n = 1, X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C-C bond formation. The formation of the coupled NHCs (L(m) = cyclic alkyl(amino) carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction between NHC:SiBr2 and L(n): when compared with that of NHC:SiCl2. Second, the reaction of L(1):SiCl4 (8) (L(1): =:C(CH2)(CMe2)2N-2,6-iPr2C6H3) with a non-nucleophilic base LiN(iPr)2 in a 1:1 molar ratio shows an unprecedented methodology for the synthesis of the biradical (L(1)•)2SiCl2 (2). The blue blocks of silicon dichloride bridged biradicals (2, 4) are stable for more than six months under an inert atmosphere and in air for one week. Compounds 2 and 4 melt in the temperature range of 185 to 195 °C. The dibromide (6, 7) analogue is more prone to decomposition in the solution but comparatively more stable in the solid state than in the solution. Decomposition of the products has been observed in the UV-vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated (L(n):)2Si(0) (n = 1 (9), 2 (10)) under KC8 reduction. Compounds 2 and 4 were also reduced to dehalogenated products 9 and 10, respectively when treated with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements show that 10 can irreversibly undergo both one electron oxidation and reduction.

Formation of Trichlorosilyl-Substituted Carbon-Centered Stable Radicals through the Use of π-Accepting Carbenes†

Carbon or silicon radicals with silyl substituents are important intermediates in organometallic and organic chemistry. Already in 1970, Bassindale et al. had reported the persistent tris(trimethylsilyl)methyl radical, (Me3Si)3CC, which has a lifetime of several days at 298 K. The groups of Ingold, Apeloig, Bravo-Zhivotovskii, Lee, Sekiguchi, and others have shown that the lifetime of the radicals largely depends on the steric bulk of the substituents. In 2002, Sekiguchi et al. reported the first stable silicon-centered radical without any p conjugation. Several reports describe the preparation of this class of radicals. The most successful ones proceed through the photolytic or thermal cleavage of a Si Si bond, and R3Si SiHCl2 can be reacted with bulky reagents, such as (tBu)2MeSiLi, according to the Apeloig– Sekiguchi method. Several radical species of main-group elements, such as PNC, P2C , phosphinyl radical cations, HBC, and ketenes with biradical character, were also stabilized by cyclic alkyl(amino) carbenes (cAACs). We have observed that the chemical response of cAACs toward silylenes is very different from that of N-heterocyclic carbenes (NHCs). Recently, we have demonstrated that NHC!SiCl2 1 reacts with Me2-cAAC in a redox reaction to form the biradical (Me2-cAACC)2SiCl2 2 in a two-electron redox step. [8g] Consequently, we were curious to investigate the one-electron redox process. Therefore, we reacted cAAC!SiCl4 3 with KC8 in an equimolar ratio in n-hexane to yield stable radicals 4, of the general formula (cAACC)–SiCl3. The highly reactive trichloromethane radical CCl3C [9a] and its congener SiCl3C [9b] are frequently generated by flash photolysis. These species are active radical intermediates in many photochemical transformations. Recently, the chemical reactivity of the TEMPO radical towards SiCl4 [9c] and metal ions was explored, and the catalytic and fashionable magnetic properties of these adducts were studied. Although handling and controlling the chemical behavior of radicals can be very difficult, the chemistry of radicals has always been captivating. To the best of our knowledge, stable radicals with the SiCl3 group next to the radical center have not been reported so far. The carbene carbon atom of an NHC is bound to two both swithdrawing and p-donating nitrogen atoms. In a cAAC, one nitrogen atom of the NHC is replaced by a s-donating quaternary carbon atom. Theoretical calculations showed that the HOMO–LUMO energy gap is smaller in cAACs. Thus, cAACs are both more nucleophilic and more electrophilic than NHCs. Recently, P NMR analysis of a number of carbene–phenylphosphinidene adducts revealed that cAACs are better p acceptors than NHCs. These inherent differences may play a pivotal role in the replacement of an NHC by a cAAC and their behavior in a reaction. The NHC!SiCl4 adduct was reduced to NHC!SiCl2, (NHC! SiCl)2, and NHC!Si=Si !NHC through the use of KC8. In detail, an equimolar mixture of cAAC!SiCl4 3 (1 mmol) and KC8 (1 mmol) in n-hexane (85 mL) was initially reacted at 78 8C. The resulting suspension was slowly warmed to room temperature to obtain a clear colorless solution and an unreacted deposit of insoluble KC8. Upon stirring for 24 h, the color of the solution changed to a clear light yellow with the black deposit of graphite; after filtration, the solution was concentrated to a volume of 2–3 mL to obtain fluorescent yellow plates/needles of the (cAACC)-SiCl3 radical 4. Herein, we report the synthesis, structural correlation, DFT calculations, and EPR studies of the two carbon-centered radicals 4a and 4b (Scheme 1). The syntheses of 3a and 3b are described in the Supporting Information. The melting points of compounds 3a, 3b, 4a, and 4b range from 105 8C to 118 8C. The color of compounds 4a and 4b is a fluorescent yellow, whereas 3a and 3b are colorless, as expected. Compounds 4a and 4b are soluble in toluene, benzene, and THF, whereas 3a is only soluble in THF, and 3b is only sparingly soluble in this solvent. Compounds 3a, 3b, 4a, and 4b are stable in inert [*] Dr. K. C. Mondal, Prof. Dr. H. W. Roesky, Dr. A. C. St ckl Institut f r Anorganische Chemie, Universit t Gcttingen Tammannstrasse 4, 37077 Gcttingen (Germany) E-mail: hroesky@gwdg.de

Isolation of Neutral Mononuclear Copper Complexes Stabilized by Two Cyclic (Alkyl)(amino)carbenes

Two (cAAC)2Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et2-cAAC)2Cu(+)I(-) with metallic sodium in THF, and by a one-pot synthesis using Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d(10) electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with (63,65)Cu isotopes, (14)N, and (1)H nuclei.

Metal telluride clusters composed of niobocene carbonyl, telluride, and cobalt carbonyl units: syntheses, structures, and reactivity

Abstract: The reaction of [Cp#2NbTe2H] (1#; Cp# = Cp* (C5Me5) or Cp(x) (C5Me4Et)) with two equivalents of [Co2(CO)8] gives a series of cobalt carbonyl telluride clusters that contain different types of niobocene carbonyl fragments. At 0 degrees C, [Cp#2NbTe2CO3(CO)7] (2#) and [Co4Te2(CO)10] (3) are formed which disappear at higher temperatures: in boiling toluene a mixture of [cat2][Co9Te6(CO)8] (5#) (cat= [Cp#2Nb(CO)2]+) and [cat2][Co11Te7(CO)10] (6#) is formed along with [cat][Co(CO)4] (4#). Complexes 6# transform into [cat][Co11Te7(CO)10] (7#) upon interaction with HPF6 or wet SiO2. The molecular structures of 2(Cp(x)), 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) have been determined by X-ray crystallography. The structure of the neutral 2(Cp(x)) consists of a [Co3(CO)6Te2] bipyramid which is connected to a [(C5Me4Et)2Nb(CO)] fragment through a mu4-Te bridge. The ionic structures of 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) each contain one (4, 7) or two (5, 6) [Cp#2Nb(CO)2]+ cations. Apart from 4, the anionic counterparts each contain an interstitial Co atom and are hexacapped cubic cluster anions [Co9Te6(CO)8]2- (5) or heptacapped pentagonal prismatic cluster anions [Co11Te7(CO)10]n- (n=2: [6]2- , n=1: [7]-), respectively. Electrochemical studies established a reversible electron transfer between the anionic clusters [Co11,Te7(CO)10]- and [Co11Te7(CO)10]2in 6# and 7# and provided evidence for the existence of species containing [Co11Te7(CO),0] and [Co11Te7(CO)0]3-. The electronic structures of the new clusters and their relative stabilities are examined by means of DFT calculations.

A Stable Neutral Radical in the Coordination Sphere of Aluminum

The neutral radical (Me2 -cAAC)2 AlCl2 (2) is stabilized by cyclic (alkyl)(amino)carbenes (cAACs). Complex 2 was synthesized by reduction of the Me2 -cAAC:→AlCl3 (1) adduct with KC8 in the presence of another equivalent of Me2 -cAAC. The crystal structure of 2 shows that the Al-C bond lengths of the two carbenes bound to the Al center are considerably different, which is likely the result of intermolecular interactions. Quantum-chemical calculations from the gas phase give an equilibrium structure with identical Al-C bond lengths. Compound 2 exhibits monoradical character, which was confirmed by EPR measurements. A bonding analysis indicates that the unpaired electron resides mainly at the carbene carbon atoms. Compound 2 is an example for an unusual neutral Al radical.

Stable Radicals from Commonly Used Precursors Trichlorosilane and Diphenylchlorophosphine

Intermediate species dichlorosilylene was generated in situ from trichlorosilane and inserted into the P-Cl bond of diphenylchlorophosphine (Ph2P-Cl) to obtain Ph2P-SiCl3 (1). Monodechlorination of 1 by cyclic alkyl(amino) carbenes (cAACs)/KC8 in THF at low temperature led to the formation of stable radicals Ph2P-Si(cAAC·)Cl2 (2a,b). Compounds 2a,b were characterized by X-ray single crystal diffraction, mass spectrometry and studied by cyclic voltammetry and theoretical calculations. Radical properties of 2 are confirmed by EPR measurements that suggest the radical electron in 2 couples with (14)N (I = 1), (35/37)Cl (I = 3/2), and (31)P (I = 1/2) nuclei leading to multiple hyperfine lines. Hyperfine coupling parameters computed from DFT calculations are in good agreement with those of experimental values. Electronic distributions obtained from the theoretical calculations suggest that the radical electron mostly resides on the carbene C of 2.

Arene‐Ligand‐Free Ruthenium(II/III) Manifold for meta‐C−H Alkylation: Remote Purine Diversification

meta-Selective C-H alkylations of bioactive purine derivatives were accomplished by versatile ruthenium catalysis. Thus, the arene-ligand-free complex [Ru(OAc)2 (PPh3 )2 ] enabled remote C-H functionalizations with ample scope and excellent levels of chemo- and positional selectivities. Detailed experimental and computational mechanistic studies provided strong support for a facile C-H activation within a ruthenium(II/III) manifold.

A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical

Silicon difluoride (SiF2 ) is highly unstable at room temperature and condenses at this temperature rapidly to a polymeric material of unknown structure. Therefore, the isolation of a stable monomeric silicon difluoride species is a challenging task. The cyclic alkyl(amino) carbene (cAAC) coordinated silicon difluoride was isolated as (cAAC)2 SiF2 (2), synthesized from the reduction of cAAC-SiF4 (1) by using two equivalents of KC8 in the presence of one equivalent of cAAC. In the solid state, compound 2 is stable at room temperature for a long time under inert conditions. The reduction of compound 1 in the presence of one equivalent KC8 resulted in the first stable silicon trifluoride monoradical (cAAC)SiF3 (3).

Organosilicon Radicals with Si–H and Si–Me Bonds from Commodity Precursors

The cyclic alkyl(amino) carbene (cAAC) stabilized biradicals of composition (cAAC)2SiH2 (1), (cAAC)SiMe2-SiMe2(cAAC) (2), and (cAAC)SiMeCl-SiMeCl(cAAC) (3) have been isolated as molecular species. All the compounds are stable at room temperature for more than 6 months under inert conditions in the solid state. All radical species were fully characterized by single-crystal X-ray structure analysis and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1-3 have been investigated by theoretical methods. Compound 1 contains the SiH2 moiety and this is the first instance, where we have isolated 1 without an acceptor molecule.