Benedikt Niepötter

A Stable Singlet Biradicaloid Siladicarbene: (L:)2Si

Silicon, the congener of carbon, frequently shows different chemistry than that of its sister element. It has been realized that silicon prefers a positive charge if bonded to a more electronegative atom. Compounds with silicon in lower oxidation states are very important, since they can activate small organic molecules which cannot be activated by transition metals. In 2008 Robinson et al. reported on the adduct of two N-heterocyclic carbene (NHC) molecules with disilicon A (Scheme 1). This unusual compound was prepared

Ruthenium(II)‐Catalyzed CH Activation/Alkyne Annulation by Weak Coordination with O2 as the Sole Oxidant

Aerobic oxidative CH functionalizations of weakly coordinating benzoic acids have been accomplished with versatile ruthenium(II) biscarboxylates under ambient oxygen or air. Mechanistic studies identified the key factors controlling the elementary step of the oxidation of the ruthenium(0) complex.

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.

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.

Synthesis, Characterization, and Theoretical Investigation of Two‐Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π‐Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π-accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C-Pd-C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange-colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)2 Pd/Pt] complexes can be quasi-reversibly oxidized to their corresponding [(cAAC)2 Pd/Pt](+) cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.

Empirical correction for resolution‐ and temperature‐dependent errors caused by factors such as thermal diffuse scattering

Charge density investigations of the compounds [Mg{(pz*)3C}2] (1) (pz* = 3,5-dimethylpyrazolyl) and (Ph2P=S)C14H9 (2) show two disturbing features in each case. First, the models derived from high-resolution data sets at different temperatures differ significantly. Additionally, residual density appears close to or even at the atomic positions, especially for data sets measured at 100 K. This indicates significant errors that could be caused by thermal diffuse scattering (TDS). A reduction of the integration box size leads to a substantial improvement in the model quality and removes the differences in the models. At the same time it indicates TDS to be the reason for these errors. However, this method is very time consuming and an alternative is needed. In endeavouring to improve the method, it was noticed that the refinement of resolution-dependent scale factors can be employed as a validation tool to detect such errors. In a nested interval approach, a correction factor {α = a[sin(θ)/λ]2 + b[sin(θ)/λ]3} is determined that minimizes these errors and improves the model quality. This model is now based on refinement of only a single scale factor.

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.

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.

Validation of experimental charge-density refinement strategies: when do we overfit?

A cross-validation method is supplied to judge between various strategies in multipole refinement procedures. It easily detects when the refinement of additional parameters leads to an improvement in the model or when it simply overfits the given data.

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.