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.

Carbene stabilized interconnected bis-germylene and its silicon analogue with small methyl substituents.

The cyclic alkyl(amino) carbene (cAAC) [:C{N-C6H3(2,6-IPr2)}(CMe2)2CH2] stabilized MeGeGeMe has been isolated in the molecular form with composition (cAAC)MeGe-GeMe(cAAC) (1) at room temperature. Compound 1 was synthesized from the reduction of MeGeCl3 using three equivalents of KC8 in the presence of one equivalent of cAAC. The corresponding silicon compound (cAAC)MeSi-SiMe(cAAC) (2) was also prepared. Compounds 1 and 2 are the first examples of REER compounds (E = Ge, Si) carrying the smallest organic group. Furthermore the structures of compounds 1 and 2 have been investigated by using theoretical methods. The theoretical analysis of the structure of 1 is in agreement with the formation as an unprecedented carbene stabilized bis-germylene whereas compound 2 can be equally described as carbene stabilized bis-silylene with coordinate bonds as with classical double bonds of a 2,3-disila-1,3-butadiene. The compounds were also characterized by X-ray crystallography.

Synthesis, characterization and oxygen atom transfer reactivity of a pair of Mo(iv)O- and Mo(vi)O2-enedithiolate complexes - a look at both ends of the catalytic transformation.

Two new molybdenum complexes (Bu4N)2[MoIVO(ntdt)2] (1) and (Ph4P)2[MoVIO2(ntdt)2] (2) (ntdt = 2-naphthyl-1,4-dithiolate) were synthesized using asymmetric dithiolene precursors and were characterized as structural models for the active site of arsenite oxidase, a molybdopterin bearing enzyme. The ligand was obtained readily by a two-step synthesis starting from 2-bromo-2'-acetonapthone. Complexes 1 and 2 were obtained by reaction of the resulting 4-naphthyl-1,3-dithiol-2-one with metal precursors trans-[MoO2(CN)4]4- and cis-[MoO2(NCS)4]2- respectively. Notably and to the best of our knowledge, this work constitutes the first utilization of the latter in dithiolene chemistry. 1 and 2 were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and in case of 1 by single-crystal X-ray diffraction. The molecular structure of compound 1 exhibits the less common cis isomeric form (i.e. the naphthyl groups of the 2-naphthyl-1,4-dithiolate ligands are located on the same side of the MoS4 square base). Structural, spectroscopic and electrochemical data are discussed in context. The catalytic oxo-transfer properties of 1 and 2 were investigated by oxo-transfer reactions from DMSO to PPh3 with varied catalyst : PPh3 ratios. Interestingly, the oxygen atom transfer reaction from DMSO to PPh3 starting from compound 2 was found to be more efficient under the given conditions than when the reduced catalyst 1 was employed as initial species. The two catalytic systems are discussed and compared in terms of their reactivity.

NMR analysis of an Fe(i)-carbene complex with strong magnetic anisotropy.

A tricoordinated FeI complex with two cyclic-alkyl(amino) carbene (cAAC) and one chlorido ligand, (cAAC)2FeCl (1), is studied by means of 1H NMR spectroscopy and DFT calculations. Due to the cAAC ligands, which can take significant amounts of spin density from the metal center, and due to the magnetic anisotropy of the FeI ion (P. P. Samuel et al., J. Am. Chem. Soc., 2014, 136, 11964-11971), compound 1 is a rare example of a paramagnetic d-block compound which is expected to have significant contributions from both contact and pseudocontact terms to the hyperfine NMR shift. Compound 1 is fluxional, which makes the analysis of its 1H NMR spectrum more difficult but allows a preliminary assignment from EXSY spectra. Then, a software-aided approach enabled a satisfactory signal assignment of all protons which are distanced from the FeI center and carbene cyclic core, and thereby the extraction of the axial and rhombic components of the magnetic susceptibility anisotropy tensor (Δχ). Components of Δχ enable the calculation of zero-field spitting D and E parameters from solution NMR measurements of 1, and these parameters are compared to previously reported experimental and theoretical values.

An unprecedented 1,4-diphospha-2,3-disila butadiene (-P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P-) derivative and a 1,3-diphospha-2-silaallyl anion, each stabilized by the amidinate ligand.

The first acyclic 4π-electron -P[double bond, length as m-dash]Si-Si[double bond, length as m-dash]P- motif with two four coordinate silicon substituents supported by the amidinate ligand and two coordinate phosphorus has been synthesized from the reaction of heteroleptic chlorosilylene LSiCl (1), TripPCl2 (Trip = 2,4,6-iPr3C6H2) and KC8 in a 1 : 1 : 3 ratio. The same reaction in a 1 : 2 : 6 ratio in the presence of one equivalent of 18-crown-6 ether affords the 1,3-diphospha-2-silaallyl anion.

Insertion of Cyclic Alkyl(amino) Carbene into the Si-H Bonds of Hydrochlorosilanes.

Carbenes are known for their ability to abstract HCl from hydrochlorosilanes to form carbene hydrochloride adducts. In contrast, the Si-H bond insertion products RSiCl2(cAACH) (2, 4, 6, and 8) have been formed in the reaction of RSiHCl2 [R = Ar(SiMe3)N (1), Cp* (3), PhC(NtBu)2 (5), Cl (7); Ar = 2,6-iPr2C6H3] with a cyclic alkyl(amino) carbene (cAAC:) irrespective of the steric demand of the R group. The new products have been characterized by various analytical tools including X-ray crystallography, electron ionization mass spectrometry, and NMR spectroscopy. Theoretical investigations have also been performed to understand why cAAC prefers insertion into the Si-H bond rather than the dehydrohalogenation pathway.

Multiple Cycloaddition Reactions of Ketones with a ß-Diketiminate Al Compound.

A ß-diketiminate Al compound (1) with an exocyclic double bond reacts with two equivalents each of benzophenone and 2-benzoylpyridine in a [4+2] cycloaddition to generate bicyclic and tricyclic compounds 2 and 3, respectively. Compound 2 consists of six- and eight-membered aluminium rings, whereas 3 has two five- and one eight-membered ring. Compounds 2 and 3 were characterized by a number of analytical tools including single-crystal X-ray diffraction. The quantum mechanical calculations suggest that the dissociation of the solvent molecule from 1 would lead to an active species 1A having two 1,4-dipolar 4π electron moieties, in which the electrophilic site is the Al atom and the nucleophilic positions are polarized exocyclic and endocyclic C=C π bonds. The detailed mechanistic study shows that the dipolarophiles, benzophenone, and 2-benzoylpyridine undergo double cycloaddition with two 1,4-dipolar 4π electron moieties of 1A. Herein, the addition of one molecule of the dipolarophile promotes the addition of the second one.

A stable dimer of SiS2 arranged between two carbene molecules.

The Me-cAAC:-stabilized dimer of silicon disulfide (SiS2 ) has been isolated in the molecular form as (Me-cAAC:)2 Si2 S4 (2) at room temperature [Me-cAAC:=cyclic alkyl(amino) carbene]. Compound 2 has been synthesized from the reaction of (Me-cAAC:)2 Si2 with elemental sulfur in a 1:4 molar ratio under oxidative addition. This is the smallest molecular unit of silicon disulfide characterized by X-ray crystallography, electron ionization mass spectrometry, and NMR spectroscopy. Structures with three sulfur atoms arranged around a silicon atom are known; however, 2 is the first structurally characterized silicon-sulfur compound containing one terminal and two bridging sulfur atoms at each silicon atom. Compound 2 shows no decomposition after storing for three months in an inert atmosphere at ambient temperature. The bonding of 2 has been further studied by theoretical calculations.

Cr(i)Cl as well as Cr+ are stabilised between two cyclic alkyl amino carbenes.

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)2CrCl2 (1) with equivalent amount of KC8 results in the formation of (cAAC)2CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C6H3(CF3)2)4] resulting in [(cAAC)2Cr]+[B(C6H3(CF3)2)4]-, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.

Aspherical-atom modeling of coordination compounds by single-crystal X-ray diffraction allows the correct metal atom to be identified.

Single-crystal X-ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid-state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical-atom least-squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld-atom refinement (Acta Crystallogr. Sect. A- 2008, 64, 383-393; IUCrJ. 2014, 1,61-79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B- 2013, 69, 91-104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear-coordinate 3d metal complexes, for which the wrong element is found if standard independent-atom model scattering factors are relied upon, are studied, and it is shown that only aspherical-atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed.

Electronic structure and slow magnetic relaxation of low-coordinate cyclic alkyl(amino) carbene stabilized iron(I) complexes.

Cyclic alkyl(amino) carbene stabilized two- and three-coordinate Fe(I) complexes, (cAAC)2FeCl (2) and [(cAAC)2Fe][B(C6F5)4] (3), respectively, were prepared and thoroughly studied by a bouquet of analytical techniques as well as theoretical calculations. Magnetic susceptibility and Mössbauer spectroscopy reveal the +1 oxidation state and S = 3/2 spin ground state of iron in both compounds. 2 and 3 show slow magnetic relaxation typical for single molecule magnets under an applied direct current magnetic field. The high-frequency EPR measurements confirm the S = 3/2 ground state with a large, positive zero-field splitting (∼20.4 cm(-1)) and reveal easy plane anisotropy for compound 2. CASSCF/CASPT2/RASSI-SO ab initio calculations using the MOLCAS program package support the experimental results.

One-electron-mediated rearrangements of 2,3-disiladicarbene.

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

C4 cumulene and the corresponding air-stable radical cation and dication.

A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(·+) , and dication 1(2+) 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.

Synthesis and characterization of a triphenyl-substituted radical and an unprecedented formation of a carbene-functionalized quinodimethane.

The trichlorosilylcarbene monoradical (Cy-cAAC ·)SiCl3 (1) was directly converted to (Cy-cAAC ·)SiPh3 (2) by substitution of the three chlorine atoms with phenyl groups without affecting the radical center adjacent to the silicon atom. In addition to the structure determination, compound 2 was studied by EPR spectroscopy and DFT calculations. The three hyperfine lines in the EPR spectrum of 2 are due to the coupling with (14)N nucleus. Functionalized 1,4-quinodimethane Me2-cAAC=C6H4=CPh2 (7) was isolated, whereas carbon analogue of radical 2 was targeted. Cyclic voltammogram of 7 indicated that a stable radical-anion 7 ·-, as well as a radical-cation 7 ·+, can be prepared. Theoretical calculations showed that one-electron ionization energy and electron affinity of 7 are 5.1 and 0.7 eV mol(-1), respectively.

Experimental charge density study of a silylone.

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 Si-C 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.

Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes.

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:Co(II)(µ-Cl)Cl)2 (1) with KC8. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2(•-) and 2(•+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me2-cAAC:)2Co2](•+)OTf(-) (2(•+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(•+) reveals the coupling of the electron spin with 2 equiv (59)Co isotopes, leading to a (Co(0.5))2 state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2(•+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(•+)OTf(-) possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2(•+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(•+) is caused by intermolecular interactions.

Synthetic access to a hydrocarbon-soluble trifluorinated Ge(II) compound and its Sn(II) congener.

Trifluorinated germanium anions attracted attention of theoretical chemists already in the late 1990s to predict their physical and chemical properties. However these species were not synthesized in the laboratory, although substantial evidence for their existence was obtained from the mass spectrometry of GeF4. The present study shows that controlled fluorination of LMNMe2 (L = PhC(N(t)Bu)2, M = Ge, Sn) using HF·pyridine in toluene leads to the formation of [LH2](+)[MF3](-) under elimination of HNMe2. The products contain the trifluorinated Ge(II) and Sn(II) anionic species which are stabilized by interionic H···F bonds. The new compounds were characterized by single crystal X-ray structural analysis, NMR spectroscopy, and elemental analysis.

Synthesis of metallasiloxanes of group 13-15 and their application in catalysis.

Herein we report on the synthesis, characterization and catalytic application of metallasiloxanes of group 13-15. Reactions of R(Me)Si(OH)2 (R = N(SiMe3)-2,6-iPr2C6H3) (A) with Bi(NEt2)3, Sb(NEt2)3, Ge[N(SiMe3)2]2 and AlMe3 afforded [R(Me)SiO2BiNEt2]2 (1), [R(Me)SiO2SbOSi(OH)(Me)R]2 (2), [R(Me)SiO2]3(GeH)2 (3), and [R(Me)SiO2AlMe(THF)]2 (4), respectively. Reactions of RSi(OH)3 (B) with Bi(NEt2)3 and AlMe3 produced complexes (RSiO3Bi)4 (5) and (RSiO3)2[AlMe(THF)]3 (6). Compounds 1-6 have been characterized by IR and NMR spectroscopy, single crystal X-ray structure and elemental analysis. Each of the compounds 1, 2 and 4 features an eight-membered ring of composition Si2O4Bi2, Si2O4Sb2 and Si2O4Al2, while 3 and 6 exhibit a bicyclic structure with the respective skeletons of Si3O6Ge2 and Si2O6Al3. Compound 5 has a cubic core of Si4O12Bi4. Compounds 1-6 exhibit very good catalytic activity in the addition reaction of trimethylsilyl cyanide (TMSCN) with benzaldehyde. Compound 5 was found to be the best catalyst and its activity was probed in the reactions of TMSCN with a number of aldehydes and ketones.

Lewis-base stabilized diiodine adducts with N-heterocyclic chalcogenamides.

Oxidation reactions of stable chalcogenamides with iodine are intriguing due to their broad application in various organic syntheses. In the present study we report on the utilization of N-heterocyclic carbene and cyclic-alkyl-amino carbenes L(1-3): (L(1): = :C[N(2,6-iPr2-C6H3)CH]2, L(2): = :C(CH2)(CMe2)(C6H10)N-2,6-iPr2-C6H3, L(3): = :C(CH2)(CMe2)2N-2,6-iPr2-C6H3) for the syntheses of chalcogenamides L(1-3)=E (E = S, Se, Te) 1-4 and zwitterionic adducts L(1-3)=E-I-I 5-8. The synthesis of compounds 1-4 involved the addition reaction of ligand L(1-3): and elemental chalcogen. Treatment of 1-4 with iodine resulted in the formation of adducts 5-8. Compounds 5-8 are well characterized with various spectroscopic methods and single-crystal X-ray structural analysis.