Daniel Stern

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.

A comparison of a microfocus X-ray source and a conventional sealed tube for crystal structure determination

Experiments are described in which a direct comparison was made between a conventional 2 kW water-cooled sealed-tube X-ray source and a 30 W air-cooled microfocus source with focusing multilayer optics, using the same goniometer, detector, radiation (Mo Kα), crystals and software. The beam characteristics of the two sources were analyzed and the quality of the resulting data sets compared. The Incoatec Microfocus Source (IµS) gave a narrow approximately Gaussian-shaped primary beam profile, whereas the Bruker AXS sealed-tube source, equipped with a graphite monochromator and a monocapillary collimator, had a broader beam with an approximate intensity plateau. Both sources were mounted on the same Bruker D8 goniometer with a SMART APEX II CCD detector and Bruker Kryoflex low-temperature device. Switching between sources simply required changing the software zero setting of the 2θ circle and could be performed in a few minutes, so it was possible to use the same crystal for both sources without changing its temperature or orientation. A representative cross section of compounds (organic, organometallic and salt) with and without heavy atoms was investigated. For each compound, two data sets, one from a small and one from a large crystal, were collected using each source. In another experiment, the data quality was compared for crystals of the same compound that had been chosen so that they had dimensions similar to the width of the beam. The data were processed and the structures refined using standard Bruker and SHELX software. The experiments show that the IµS gives superior data for small crystals whereas the diffracted intensities were comparable for the large crystals. Appropriate scaling is particularly important for the IµS data.

Template and pH‐Mediated Synthesis of Tetrahedral Indium Complexes [Cs⊂{In4(L)4}]+ and [In4(HNL)4]4+: Breaking the Symmetry of N‐Centered C3 (L)3− To Give Neutral [In4(L)4]

There are two classes of well known T-symmetric complexes, in which four octahedrally coordinated metal ions are located in the apices of a tetrahedron, and each of the six edges are bridged by linear C2-symmetric bis(bidentate) chelators (L) and (L) . In [Cs {FeFe3(L)6}] (1), [M {Fe4(L )6}] + (2 ; M = NH4 , K, Cs), and [R4N {M4(L )6}] 11 (3 ; M = Fe, Ga), a cation is endohedrally encapsulated in the center of the tetrahedron, whereas in the complexes [M4\{M4(L)6}] (4) (M = NH4, RNH3: empty, K, Cs: H2O as guest; M 3 = Mg, Co, Ni, Mn), four cations are exohedrally centered above the four tetrahedral triangular faces (Figure 1). However, there are far fewer examples known of Tsymmetric complexes, in which the octahedrally coordinated metal centers in the vertices of the tetrahedra are linked by C3-symmetric tris(bidentate) chelators (L ) or (L) , which occupy the faces of the tetrahedra. Examples thereof

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.

Reactivity studies of a Ge(I)-Ge(I) compound with and without cleavage of the Ge-Ge bond.

This Communication describes two strikingly different reactivities of a digermylene [{PhC(NtBu)(2)}(2)Ge(2)] (1) featuring a Ge(I)-Ge(I) single bond. In the reaction with azobenzene, 1 affords the oxidative addition product LGeN(Ph)N(Ph)GeL [2; L = PhC(NtBu)(2)], with simultaneous cleavage of the Ge-Ge bond, whereas treatment of 1 with Fe(2)(CO)(9) yields the Lewis acid-base adduct LGe[Fe(CO)(4)]Ge[(Fe(CO)(4)]L (3). Both compounds were characterized by single-crystal X-ray diffraction, NMR spectroscopy, electrospray ionization mass spectrometry, and elemental analysis.

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

Experimental charge density distribution of non-coordinating sp3 carbanions in [Mg{(pz*)3C}2]

In this communication we present the experimental charge density distribution in [Mg{(pz*)(3)C}(2)] (1), (pz* = 3,5-dimethylpyrazolyl), containing two non-coordinating sp(3) carbanionic lone-pairs.

Addition of Dimethylaminobismuth to Aldehydes, Ketones, Alkenes, and Alkynes

Bi-O chemistry: A direct regioselective route to bismuth bis(amino)naphthalene compounds, incorporating Bi-O and Bi-C bonds is described, in which an amide precursor is treated with aldehydes, ketones, alkenes, and alkynes, leading to insertion into the Bi-NMe(2) bond.

Structural and Variable-Temperature NMR Studies of 9-Diisopropylphosphanylanthracenes and 9,10-Bis(diisopropylphosphanyl)anthracenes and Their Oxidation Products

The diisopropylphosphanyl-substituted anthracenes i-Pr2P(C14H9) (1a), i-Pr2P(C14H8)Br (2a), and (i-Pr2P)2(C14H8) (3a) and some of their oxidation products were prepared from 9-bromoanthracene and 9,10-dibromoanthracene, respectively. Low-temperature (1)H NMR spectra of the 9-monophosphanyl-substituted anthracenes 1a and 2a are in accordance with a staggered conformer, while above room temperature dynamic processes occur. The low-temperature NMR spectrum of the 9,10-diphosphanylanthracene 3a indicates the presence of two different rotational isomers. The rotational barrier for 1a was determined from variable-temperature (1)H NMR spectra to be 56 kJ mol(-1) (DeltaG(298K)). The crystal structure determinations show the solid-state conformers to be consistent with the prevailing conformer at low temperature.

Consecutive donor-base exchange in anthracenyllithium compounds.

Organolithium compounds are of outstanding importance as starting materials for numerous products in synthetic chemistry and also in industrial processes. Their remarkably wide range of applications spans from deprotonation of weakly acidic compounds to transfer reactions of organic groups or even anionic polymerization reactions. Since Schlenk and Holtz reported the first syntheses of organolithium compounds, and the first crystal structure of a compound from this class (tetrameric ethyllithium) was successfully determined by Dietrich in 1963, a large number of alkyl and aryl lithium reagents has been structurally characterized. In hydrocarbon solvents, organolithium reagents form oligomers and their size significantly influences the reactivity. For the two compounds most commonly used in synthesis, nBuLi and tBuLi, the degree of oligomerization in solution was identified early on by cryoscopical and spectroscopical measurements. It was found that nBuLi forms a hexamer whereas tBuLi forms a tetramer in non-donating solvents. By addition of ethers, such as diethyl ether or THF, and especially by addition of tertiary amine donor bases, such as TMEDA or PMDETA, these oligomers can be disaggregated to smaller fragments, resulting in an enhanced reactivity. This is illustrated by the benzylic deprotonation of toluene employing nBuLi, which is only feasible upon the addition of TMEDA. In practice, mixtures of different donor bases are frequently used; however, the exact percentage of the single donor bases to give the most suitable complex is usually determined empirically, and the exact constitution of the reactive complex is unknown. It is most commonly presumed that the entire coordination sphere of the lithium atom is occupied by the strongest donor molecules available. Structural evidence for complexes in which organolithium compounds are coordinated by two different donor bases simultaneously are rare and their reproducibility is difficult. Using sterically demanding organolithium compounds, we embarked on monitoring the selective and consecutive donor base exchange. Herein, the complexes [R(C14H8)Li·{Et2O}n· {THF}m]2 were isolated and structurally characterized, and the lengths of the Li C bonds of 1a (n = 1, m = 0, R = Br), 2b (n = 1, m = 1/2, R = Me), 3 a (n = 1, m = 1, R = Br), 4c (n = 1/2, m = 3/2, R = Cl), and 5a (n = 0, m = 2, R = Br) are monitored, because they provide information on the polarity of the bonds and thus shed some light on the expected reactivities of the formed complexes. The degree of oligomerization, which mainly depends on the donor-base capacity, is additionally influenced by the steric demand of the alkyl or aryl carbanion, and is decreased with increasing size; for example, nBuLi is a hexamer whereas tBuLi forms a tetramer. By coordination of multidentate donor bases, the size of these aggregates can further be decreased, even to monomers: for example, [PhLi·Et2O]4 > [PhLi·TMEDA]2 > [PhLi·PMDETA]. Upon combination of weak monodentate donor bases such as diethyl ether with organolithium compounds of substantial steric bulk, small, mostly dimeric aggregates are formed. In these dimers, the lithium atoms are sterically shielded in a way that the coordination sphere can not be filled entirely to the preferred coordination number of four. Systems such as these are particularly suitable for monitoring the process of donor base uptake and exchange. In the following investigation we selected anthracene, which can formally be described as a diortho-substituted benzene. It is suitable as a proof-of-concept to facilitate the comparison to diortho-substituted phenyllithium derivatives omnipresent in the literature. The brominated anthracene derivatives can easily be obtained by the reaction of the monosubstituted anthracenes with elemental bromine in solution. Reacting nBuLi with bromoanthracenes gives the lithiated species in almost quantative yields (Scheme 1).