Phase transition of [2,2]-paracyclophane--an end to an apparently endless story.

In this contribution, the solid-state low-temperature phase structure of [2,2]-paracyclophane is unambiguously characterised by single-crystal X-ray analysis. Additionally, a heat capacity measurement was undertaken, which proves the existence of a λ-type phase transition at 45 K, a transition that is connected with the formation of a secondary Cp/T feature at 60 K. The low-temperature phase (<45 K) crystallises in the lower symmetry space group P4n2, whereas the high-temperature phase (>60 K) crystallises in space group P4(2)/mnm. This proves what has been postulated both by experimental and theoretical chemists but has repeatedly been dismissed as speculation many times.

Heteroaromaticity approached by charge density investigations and electronic structure calculations.

In this paper we present the results of a high-resolution single crystal X-ray diffraction experiment at 15 K on a benzothiazol-substituted phosphane and a subsequent charge density study based on multipole refinement and a topological analysis according to Bader's quantum theory of atoms in molecules. Although two valence shell charge concentrations (VSCCs) in the non-bonding region of each phosphorus and sulfur atom were found, the integration of both heteroatomic basins emphasizes charge depletion. Nevertheless they are attractive for C-H···P and C-H···S hydrogen bonding in the solid state. The nature of the P-C bonds and the question of aromaticity in the heterocycles were subject to our investigations. The ellipticities along the bonds were analysed to approach delocalization. The source function is employed to visualise atomic contributions to aromaticity. Theoretical calculations have been carried out to compute nuclear chemical shifts, induced ring currents and a variety of delocalization indices. All applied measures for delocalization point in the same direction: while heteroaromaticity is present in the benzothiazolyl substituents, the bridging P-C bonds are only involved marginally, almost preventing total conjugation of the phosphane. The charge density distributions around the phosphorus and the sulfur atoms have very similar features but turn out to be chemically very different from each other. Commonly used simplifying concepts have difficulties in providing a comprehensive view on the electronic situation in the molecule. Our results raise doubts on the validity of the common interpretation of VSCCs as one-to-one representations of Lewis lone pairs.

A unifying bonding concept for metal hydrosilane complexes.

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

Pentafluoropyridine as a fluorinating reagent for preparing a hydrocarbon soluble ß-diketiminatolead(II) monofluoride.

A well-designed method for the preparation of a ß-diketiminatolead(II) monofluoride has been developed using LPbNMe(2) (L = [CH{C(Me)(2,6-iPr(2)C(6)H(3)N)}(2)]) and pentafluoropyridine (C(5)F(5)N). The resulting LPbF was used for the synthesis of amidinatosilicon(II) monofluoride. Moreover the activation of a ketone was observed when the LPbF was treated with PhCOCF(3).

A stable silicon(II) monohydride.

A stable silicon(II) monohydride is accomplished through a covalent shared interaction of the silylene lone-pair and a sp(3)-hybridized boron atom of the Lewis acidic BH(3). Experimental charge density investigations reveal a central positively charged silicon atom bound to a negatively charged hydrogen atom. The positively charged H-Si-BH(3) moiety is coordinated by the lone-pairs of electrons of the benzamidinate ligand. This coordination is reinforced by a transannular Si1···C1 privileged exchange channel.

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.

Experimental charge density studies of disordered N-phenylpyrrole and N-(4-fluorophenyl)pyrrole.

The static electron densities of the title compounds were extracted from high-resolution X-ray diffraction data using the nucleus-centered finite multipole expansion technique. The interpretation of the data collected for the N-phenylpyrrole crystal revealed a static disorder that could be successfully resolved within the aspherical-atom formalism. The local and integrated topological properties of the density obtained via a constrained multipole refinement are in statistical agreement with those calculated at the B3LYP/cc-pVTZ level of theory for the isolated molecule and for those derived from the experimental density of the para-fluorinated derivative N-(4-fluorophenyl)pyrrole. The topological analysis of the densities indicates neither pyramidal character of the pyrrole N-atom nor a quinoidal structure of the phenyl rings in either molecule. The fluorine substitution appears to have only a minor effect on the density of the remaining constituents but it results in markedly different features of the electrostatic potential of the two compounds. The consistency of the multipole refinement is validated by residual density analysis.

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.

Catalytic Abilities of [(C6F5)2BR] (R=NC4H4 and NC4H8) deduced from experimental and theoretical charge-density investigations.

Marginal difference, huge impact: The topological analyses of the electron-density distributions obtained from experiment (see figure) and quantum-chemical calculations in the two title compounds can consistently explain marginal changes in bonding and rationalize different catalytic abilities. The electronic structures of the compounds bis(pentafluorophenyl)(N-pyrrolyl)borane (1) and bis(pentafluorophenyl)(N-pyrrolidinyl)borane (2) were investigated by low-temperature high-resolution X-ray diffraction experiments and subsequent multipole refinements. Additionally, DFT calculations were performed. The topological analyses of the electron-density distributions obtained from experiments and from quantum-chemical calculations are described and discussed. In this paper reasons for the different reactivities of the compounds are provided.

Structure/reactivity studies on an alpha-lithiated benzylsilane: chemical interpretation of experimental charge density.

Modern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2 x quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R,S)-2 x quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr > or = 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2 x quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the alpha-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomeric ratios.

Chemical interpretation of molecular electron density distributions.

In this study, the two small molecules HS(CH)(CH(2)), 1, and F(CH)(4)F, 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)(4)F 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.

Dilithiation of Bis(benzene)molybdenum and subsequent isolation of a molybdenum-containing paracyclophane.

The homoleptic sandwich complex bis(benzene)molybdenum, [Mo(eta6-C6H6)2], was successfully dilithiated by employing an excess of BuLi in the presence of N,N,N',N'-tetramethylethylenediamine (up to 6 equiv each) at slightly elevated temperatures furnishing the highly reactive, ring metalated species [Mo(eta6-C6H5Li)2].tmeda in high yields. Alternatively, this compound was synthesized upon prolonged sonication with 5 equiv of tBuLi/tmeda without heating. An X-ray crystal structure determination revealed a symmetrical, dimeric composition in the solid state, i.e., a formula of [Mo(eta6-C6H5Li)2]2.(thf)6, where the six-membered rings are connected by two pairs of bridging lithium atoms. The synthesis of an elusive ansa-bridged complex failed in the case of a [1]bora and a [1]sila bridge due to the thermal lability of the resulting compounds. Instead, reverse addition of the dilithio precursor to an excess of the appropriate element dihalide facilitated the isolation of several unstrained, 1,1'-disubstituted derivatives, namely, [Mo{eta6-C6H5(BN(SiMe3)2X)}2] (X = Cl, Br) and [Mo{eta6-C6H5(SiiPr2Cl)}2], respectively. However, the incorporation of a less congesting [2]sila bridge was accomplished. In addition to the formation of [Mo{(eta6-C6H5)2Si2Me4}], a molybdenum-containing paracylophane complex was isolated and characterized by means of crystal structure analysis. The ancillary formation of 1 equiv of bis(benzene)molybdenum strongly suggests that this species is generated by deprotonation of the ansa-bridged complex by the dilithiated precursor and subsequent reaction with a second equivalent of the disilane.

[2]Borametallocenophanes of Zr and Hf: synthesis, structure, and polymerization activity.

The first [2]borametallocenophanes of Zr and Hf are reported, which were obtained from 1,2-bis(dimethylamino)-1,2-dicyclopentadienyldiborane(4). The constitution of these complexes in solution and, for the Hf complex, in the crystalline state is discussed. The title complexes were studied as Ziegler-Natta-type catalyst precursors for the polymerization of ethylene. Both complexes produced PE of high molecular mass up to 1.6 x 106 g/mol, and very interestingly, the Hf complex proved to be as active as its Zr analogue.

Synthesis and ligand properties of thianthrenophane.

Thianthrenophane 1 has a cavity which offers enough room to potentially enable endohedral coordination to small ions or molecules. For the complexation of silver(I) perchlorate the complex stability constants of 1 logK1=5.45 +/- 0.13 and of thianthrene logK2=9.16 +/- 0.10 were determined by UV/Vis titration. Single competition transport experiments with ten metal salts demonstrate a very high selectivity of 1 as a carrier for silver(I) and a distinctly higher transport rate compared to carriers such as thianthrene and 1,4,8,11-tetrathiacyclotetradecane (14-ane-S4). Although the X-ray crystal structure analysis of the polymeric [Ag(1)]ClO4.(dioxane)7 complex shows an exohedral coordination to silver(I) we suggest that the formation of an endohedral [Ag(1)]+ complex is the explanation for the unusual carrier selectivity of silver(I) by 1 in bulk liquid membrane.

Metal coordination to the formal P=N bond of an iminophosphorane and charge-density evidence against hypervalent phosphorus(V).

The iminophosphorane Ph(2)P(CH(2)Py)(NSiMe(3)) (1) was treated with deprotonating alkali metal reagents to give [(Et(2)O)Li[Ph(2)P(CHPy)(NSiMe(3))]] (2), [[Ph(2)P(CH(2)Py)(NSiMe(3))]Li[Ph(2)P(CHPy)(NSiMe(3))]] (3) and [[Ph(2)P(CH(2)Py)(NSiMe(3))]Na[Ph(2)P(CHPy)(NSiMe(3))]] (4). We report their coordination behaviour in solid-state structures and NMR spectroscopic features in solution. Furthermore, we furnish experimental evidence against hypervalency of the phosphorus atom in iminophosphoranes from experimental charge-density studies and subsequent topological analysis. The topological properties, correlated to the results from NMR spectroscopic investigations, illustrate that the formal P=N double bond is better written as a polar P(+)--N(-) single bond. Additionally, the effects of metal coordination on the bonding parameters of the iminophosphorane and the related anion are discussed.

How delocalized is N,N,N',N'-tetraphenylphenylenediamine radical cation? An experimental and theoretical study on the electronic and molecular structure.

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus-Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of 1(+) on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.