Julian Henn

Lewis Base Stabilized Dichlorosilylene

The chemistry of silicon is mainly based on silicon(IV), whereas that of silicon(II) is still in its infancy. Silylene (R2Si:) is a molecule with a divalent neutral silicon atom having a lone pair of electrons. Silylenes are key intermediates in various photochemical, thermal, and metal reduction reactions of organosilicon compounds. Therefore, silylenes play a vital role in the field of silicon chemistry. Until 1994, silylenes were generally considered to be very reactive unstable species that decompose or polymerize readily at temperatures above 77 K. This situation changed when West et al. reported the first N-heterocyclic silylene that is stable at room temperature. Since then, a fair number of cyclic silylenes and a dialkyl silylene have been reported. Only one example of a monochlorosilylene that is stable at room temperature has been published, and it was characterized by X-ray diffraction; no dihalosilylenes that are stable at room temperature have been reported to date. Gaseous dichlorosilylene has been known for many years; at room temperature, it condenses to polymeric (SiCl2)n. Its properties were already studied by Schmeisser and Schenk in 1964. Some reactions of condensed SiCl2 with acetylene and with benzene were carried out by Timms in 1968, and resulted in brown polymeric products of unknown composition. A literature survey reveals that there have been few reports on the synthesis and properties of cyclic and linear polydichlorosilanes. A polymeric perchloropolysilane (SiCl2)n was reported by West et al. in 1998 and was studied by single crystal Xray diffraction. However, access to dichlorosilylenes that are stable at room temperature and with electronic structure and properties are quite similar or close to those of unstable examples, such as SiCl2, is still a challenge. Of course, efforts of making a stable version of a very reactive species may cause a change of its behavior. How an unstable species can be stabilized is a matter of keen interest for the scientific and industrial community. Addition reactions of HSiCl3 with different organic compounds in the presence of various tertiary amine bases were reviewed by Benkeser, and the existence of the trichlorosilyl anion as an intermediate was postulated. Moreover, Jung et al. reported the trapping of dichlorosilylene, which was generated by the reaction of HSiCl3 in the presence of phosphonium chloride in a stainless steel cylinder at 150 or 180 8C, by conjugated dienes and by an alkyne. In their proposed mechanism, species of dichlorosilylene or the trichlorosilyl anion as intermediates were discussed but not isolated. In 1995, Kuhn et al. prepared N-heterocyclic carbene (NHC) adducts of SiCl4. We have used N-heterocyclic carbenes as dehydrochlorinating agents for the preparation of germylene and metal hydroxides. Very recently, Robinson and co-workers reported a Lewis base stabilized low-valent silicon compound in which the silicon atom is present in the formal oxidation states + 1 or zero, but no dichlorosilylene was isolated. Almost all silylenes or low-valent silicon compounds reported to date were prepared by reductions of their parent compounds using strong reducing agents, such as potassium metal or KC8. Herein, we report the synthesis of the first basestabilized dichlorosilylene that is stable at room temperature, LSiCl2 (1; L 1 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). This compound is formed under mild reaction conditions by reductive elimination of HCl from trichlorosilane in the presence of the NHC (L; Scheme 1), and was

High yield access to silylene RSiCl (R = PhC(NtBu)2) and its reactivity toward alkyne: synthesis of stable disilacyclobutene.

Two new approaches for synthesizing RSiCl, (R = PhC(NtBu)(2)) are reported by the reaction of RSiHCl(2) with bis-trimethyl silyl lithium amide and N-heterocyclic carbene respectively. In the former method silylene is produced in 90% yield. The silylene was treated with biphenyl alkyne to afford the disilacyclobutene system. This is a rare example of two five-coordinate silicon centers arranged adjacent to each other in a four-membered ring. Furthermore, we fluorinated the four-membered ring by trimethyltin fluoride to obtain the fluoro substituted disilacyclobutene.

Foundations of residual-density analysis

New and concise descriptors of the residual density are presented, namely the gross residual electrons, the net residual electrons and the fractal dimension distribution. These descriptors indicate how much residual density is present and in what way it is distributed, i.e. the extent to which the distribution is featureless. The amount of residual density present accounts for noise in the experimental data as well as for modeling inadequacies. Therefore, the minimization of the gross residual electrons during refinement serves as a quality criterion. In the case where only Gaussian noise is present in the residual density, the fractal distribution is parabolic in shape. Deviations from this shape therefore serve as an indicator for systematic errors. The new measures have been applied to simulated and experimental data in order to study the effects of noise, model inadequacies and truncation in the experimental resolution. These measures, although designed and examined with particular regard to applications of space residual density, are very general and can in principle also be applied to space and momentum residual densities in a one-, two-, three- or higher-dimensional Euclidean space.

Lewis-Base-Stabilized Dichlorosilylene: A Two-Electron σ-Donor Ligand

The first structurally described cobalt(I) Lewis-base-stabilized silylene complex [Co(CO)(3){SiCl(2)(IPr)}(2)](+)[CoCl(3)(THF)](-) [1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] was prepared by applying the two-electron sigma-donor ligand SiCl(2)(IPr) through coordination with Co(2)(CO)(8). The bonding situation between ligand SiCl(2)(IPr) and the cobalt(I) metal center in [Co(CO)(3){SiCl(2)(IPr)}(2)](+) of 1 was investigated by (1)H NMR and IR spectroscopy, single-crystal X-ray structural analysis, and density functional theoretical calculations.

Carbanion or Amide? First Charge Density Study of Parent 2-Picolyllithium†

The negative charge originating from deprotonation of the methyl group is distributed over the 2-picolyl ring. Bonding properties derived from the electron density distribution support the enamide character of picolyllithium (PicLi; the picture shows the deformation density of [2-PicLi x PicH](2)), but electrophilic attack occurs at the deprotonated C atom. This reactivity is rationalized by the electrostatic potential, which guides electrophiles towards the nucleophilic C atom.

Anharmonic Motion in Experimental Charge Density Investigations

In the charge density study of 9-diphenylthiophosphinoylanthracene the thermal motion of several atoms needed an anharmonic description via Gram-Charlier coefficients even for data collected at 15 K. As several data sets at different temperatures were measured, this anharmonic model could be proved to be superior to a disorder model. Refinements against theoretical data showed the resemblance of an anharmonic model and a disorder model with two positions very close to each other (~0.2 Å), whereas these two models could be clearly distinguished if the second position is 0.5 Å apart. The refined multipole parameters were distorted when the anharmonic motion was not properly refined. Therefore, this study reveals the importance of detecting and properly handling anharmonic motion. Unrefined anharmonic motion leads to typical shashlik-like residual density patterns. Therefore, careful analysis of the residual density and the derived probability density function after the refinement of the Gram-Charlier coefficients proved to be the most useful tools to indicate the presence of anharmonic motion.

Solvent-Separated and Contact Ion Pairs of Parent Lithium Trimethyl Zincate†

Bimetallic reagents, which are composed of one alkali metal ion, a second metal atom (Zn, Al, or Mg), and variable ligands have gained much attention over the last few years since they show a reactivity pattern that easily outperforms simple monometallic species. Especially zincate complexes, although known to chemists for more than 150 years, recently have been discovered to metalate substituted aromatic substrates at positions that can be reached neither by common organolithium nor by organomagnesium compounds alone. In their pioneering work, Mulvey and co-workers introduced the meta deprotonation into aromatic chemistry, complementing the established directed ortho metalation (DoM) by Snieckus and co-workers. They were able to metalate the previously inaccessible meta position of N,Ndimethylaniline employing the amido zincate [(tmeda)Na(m-tBu)(m-tmp)ZntBu)] and of toluene using [(tmeda)Na(m-Bu)(m-tmp)Mg(tmp)] (tmeda = tetramethylethylenediamine; tmp = tetramethylpiperidide). Another aspect is the smooth zincation of many substituted aromatics for which the traditionally used organolithium compounds fail because of their incompatibility with many functional groups. These advantages of ate complexes over their monometallic congeners have been accredited to synergistic effects. In their milestone paper on alkali-metal organics in 1917, Schlenk and Holtz elaborated that the reaction of diethylzinc and lithium or sodium yields only zinc and an alkali-metal ethylzinc compound; hence, transmetalation does not occur. To favor transmetalation, they employed diorganomercury compounds instead. Nowadays there are many examples of transmetalation reactions involving the R2Hg/R’Li system, [8] but the R2Zn/R’Li system normally yields lithium zincates of the general composition Li2ZnR2R’2 or LiZnR2R’. [9–11] An unexpected transmetalation reaction of an organolithium compound and dimethyl zinc to give a diorganozinc complex was reported as well. For a cooperative effect to facilitate alkali-metal-mediated zincation (AMMZ), however, close proximity of the two metal ions is necessary. The degree of aggregation of these novel synthetic reagents can be modulated by the donor solvents as well as the size of the substituents at the zinc atom. The whole ensemble of solvent, donor, and substituents determines whether a solvent-separated ion pair (SSIP) or a contact ion pair (CIP) is formed. Surprisingly, apart from powder diffraction data of parent [Li2ZnMe4], [10] no crystal structure of the basic methyl zincates is available in the current CCDC. Only zincates with bulky substituents (tBu, SiMe3, aryl) at the zinc atom are structurally characterized as SSIPs. 16] X-ray crystal structures of zincates as CIPs show four-membered Li-L-L-Zn (L = ligand) rings as the central structural motif, and recently the structure of [Zn{(m-Me)2Li(tmeda)}2] was determined by Hevia and co-workers. Obviously, the amide ligand tmp plays an important role in bridging the lithium and zinc centers. Like ring stacking and ladder formation in lithium amides, the amide ligand apparently promotes the proximity of the two metals and facilitates the bridging coordination of the alkyl group, although very recent examples were published with two bridging alkyl groups. Herein we present the results of low-temperature aggregation and deaggregation experiments with the donor bases N,N,N’,N’,N’’-pentamethyldiethylenetriamine (pmdeta) and diglyme. Under otherwise identical conditions, the first gives a CIP of the lithium cation and the ZnMe3 zincate anion, while the latter gives a SSIP with the lithium cation embedded in two diglyme donors (Scheme 1). Both samples were

Chemical interpretation of molecular electron density distributions

In this study, the two small molecules HS(CH)(CH2), 1, and F(CH)4F, 2, are presented, which yield different chemical interpretations when one and the same density is interpreted either by means of Natural Bond Orbital and subsequent Natural Resonance Theory application or by the Quantum Theory of Atoms In Molecules. The first exhibits a SC bond in the orbital based approach, whereas the density based Quantum Theory of Atoms In Molecules detects no corresponding bond. In F(CH)4F a F···F bond is detected in the density based approach, whereas in the orbital based approach no corresponding bond is found. Geometrical reasons for the presence of unexpected and the absence of expected bond critical points are discussed.

Electron Densities of Three B12 Vitamins

The electron densities of the three natural B(12)-vitamins, two of them being essential cofactors for animal life, were determined in a procedure combining high-order X-ray data collection at low to very low temperatures with high-level density functional calculations. In a series of extensive experimental attempts, a high-order data set of adenosylcobalamin (AdoCbl) could be collected to a resolution of sin theta/lambda = 1.00 A(-1) at 25 K. This modification contains only minor disorder at the solvent bulk. For methylcobalamin (MeCbl), only a severely disordered modification was found (sin theta/lambda = 1.00 A(-1), 100 K, measured with synchrotron radiation). The already published data set of cyanocobalamin (CNCbl) (sin theta/lambda = 1.25 A(-1), 100 K) was reintegrated to guarantee similar treatment of the three compounds and cut to sin theta/lambda = 1.11 A(-1) to obtain a higher degree of completeness and redundancy. On the basis of these accurate experimental geometries of AdoCbl, MeCbl, and CNCbl, state-of-the-art density functional calculations, single-point calculations, and geometry optimizations were performed on model compounds at the BP86/TZVP level of theory to evaluate the electronic differences of the three compounds. AdoCbl and MeCbl are known to undergo different reaction paths in the body. Thus, the focus was directed toward the characterization of the dative Co-C(ax) and Co-N(ax) bonds, which were quantifed by topological parameters, including energy densities; the source function including local source; and the electron localizability indicator (ELI-D), respectively. The source function reveals the existence of delocalized interactions between the corrin macrocycle and the axial ligands. The ELI-D indicates unsaturated Co-C(ax) bonding basins for the two biochemically active cofactors, but not for CNCbl, where a population of 2.2e is found. This may be related to significant pi-backbonding, which is supported by the delocalization index, delta, of 0.15 between the Co atom and the N atom of the cyano ligand. Considering all results, the inherent electronic differences between AdoCbl and MeCbl are found to be small thus, supporting earlier findings that the interaction with the protein site mainly controls the type of Co-C(ax) bond cleavage.

The P(bth)2 anion as a Janus head staple between lithium and manganese (bth = benzothiazol-2-yl, C7H4NS)

Deprotonation of HP(bth)2 (1) affords the lithium phosphanide [(Et2O)2Li(bth)2P], (2) with both nitrogen atoms coordinated to the lithium atom while in the heterobimetallic complex [Li(bth)2P{Mn(CO)2Cp}2](n) (3) additionally the phosphorus atom micro-bridges two Mn(CO)2Cp residues.