Synthesis, Solid-State Structure, and Bonding Analysis of a Homoleptic Beryllium Azide.

[Ph4 P]2 [Be(N3 )4 ] (1) and [PNP]2 [Be(N3 )4 ] (2; PNP=Ph3 PNPPh3 ) were synthesized by reacting Be(N3 )2 with [Ph4 P]N3 and [PNP]N3 . Compound 1 represents the first structurally characterized homoleptic beryllium azide. The electronic structure and bonding situation in the tetraazidoberyllate dianion [Be(N3 )4 ]2- were investigated by quantum-chemical calculations (NPA, ELF, LOL).

A Gallium-Substituted Distibene and an Antimony-Analogue Bicyclo[1.1.0]butane: Synthesis and Solid-State Structures.

RGa {R=HC[C(Me)N(2,6-iPr2C6H3)]2} reacts with Sb(NMe2)3 with insertion into the Sb-N bond and elimination of RGa(NMe2)2 (2), yielding the Ga-substituted distibene R(Me2N)GaSb=SbGa(NMe2 )R (1). Thermolysis of 1 proceeded with elimination of RGa and 2 and subsequent formation of the bicyclo[1.1.0]butane analogue [R(Me2N)Ga]2Sb4 (3).

Synthesis and characterization of heteroleptic 1-tris(pyrazolyl)borate beryllium complexes.

The synthesis and crystal structures of 1-tris(pyrazolyl)borate beryllium halides TpBeX (X = Cl 1, Br 2, I 3, F 4), the pyrazole adduct of TpBeF (5) and of 1-tris(pyrazolyl)borate beryllium hydride, deuteride and azide TpBeX (X = H 6, (2)H (D) 7, N3 8; Tp = 1-trispyrazolylborate) is described. In addition, (9)Be-NMR spectroscopy is introduced as suitable analytical tool for the in situ characterization of heteroleptic organoberyllium halides, pseudohalides and hydrides. Studies in different solvents and solvent mixtures allowed the formulation of a reference guide for the chemical shift of heteroleptic coordination complexes of beryllium only based on variation of the second substituent.

Temperature-dependent electron shuffle in molecular Group†13/15 intermetallic complexes.

Monovalent RAl (R=HC[C(Me)N(2,6-iPr2C6H3)]2) reacts with E2Et4 (E=Sb, Bi) with insertion into the weak E-E bond and subsequent formation of RAl(EEt2)2 (E=Sb 1; Bi 2). The analogous reactions of RGa with E2Et4 yield a temperature-dependent equilibrium between RGa(EEt2)2 (E=Sb 3; Bi 4) and the starting reagents. RIn does not interact with Sb2Et4 under various reaction conditions, but formation of RIn(BiEt2)2 (5) was observed in the reaction with Bi2Et4 at low temperature.

Synthesis and structure of base-stabilized germanium(II) diazide IPrGe(N3)2.

Coordination of a strong σ-base has been shown to be an effective method for the stabilization of low valent main group element complexes. This general method was now used for the synthesis of the divalent germanium diazide. IPrGe(N3)21 represents the first neutral homoleptic germanium diazide that could be structurally characterized.

An azobenzene unit embedded in a cyclopeptide as a type-specific and spatially directed switch.

By embedding an azobenzene unit into a chiral scaffold, switching of azobenzene from the trans-(P) isomer to the cis-(P) isomer and back was achieved (black arrows in picture). The embedding leads to a flipping process in which the phenyl rings can only move directly towards one another in the switching process.

First structural characterization of neutral, base-stabilized group 15-pentaazides: single crystal X-ray structures of dmap-As(N3)5 and dmap-Sb(N3)5.

Two neutral group 15-pentaazides dmap-As(N(3))(5) (1) and dmap-Sb(N(3))(5) (2) were synthesized and structurally characterized for the first time (dmap = 4-dimethylaminopyridine). Base-stabilization was confirmed to be very suitable for the kinetic stabilization of highly explosive covalent main group polyazides.

Syntheses and X-ray crystal structures of organoantimony diazides.

Antimony(III) complexes of the general type LSbF(2) (3: L(1) =[tBuC(NiPr)(2)]; 4: L(2) =[tBuC(NDipp)(2) , Dipp=2,6-iPr(2)C(6)H(3)) and LSb(N(3))(2) (6: L(1); 7: L(2)) were prepared in high yields and characterized by elemental analyses, NMR and IR spectroscopy and single-crystal X-ray diffraction. Moreover, the solid-state structures of [L(2)SbF(2)][L(2)Li] (5), [L(2) AlH(2)] (1), and [L'H][L'K(thf)(3)] (2; L'=HC(NDipp)(2)) are described, in which the Li (5) and K atoms (2) adopt rather unusual coordination modes.

Nature and strength of C-H···O interactions involving formyl hydrogen atoms: computational and experimental studies of small aldehydes.

Structural and electronic properties of C-H···O contacts in compounds containing a formyl group are investigated from the perspective of both hydrogen bonding and dipole-dipole interactions, in a systematic and graded approach. The effects of α-substitution and self-association on the nature of the formyl H-atom are studied with the NBO and AIM methodologies. The relative dipole-dipole contributions in formyl C-H···O interactions are obtained for aldehyde dimers. The stabilities and energies of aldehyde clusters (dimer through octamer) have been examined computationally. Such studies have an implication in crystallization mechanisms. Experimental X-ray crystal structures of formaldehyde, acrolein and N-methylformamide have been determined in order to ascertain the role of C-H···O interactions in the crystal packing of formyl compounds.

Weak C-H···O hydrogen bonds in anisaldehyde, salicylaldehyde and cinnamaldehyde.

In situ cryocrystallization has been employed to grow single crystals of 4-methoxybenzaldehyde (anisaldehyde), C(8)H(8)O(2), 2-hydroxybenzaldehyde (salicylaldehyde), C(7)H(6)O(2), and (2E)-3-phenylprop-2-enal (cinnamaldehyde), C(9)H(8)O, all of which are liquids at room temperature. Several weak C-H···O interactions of the types C(aryl)-H···O, C(formyl)-H···O and Csp(3)-H···O are present in these related crystal structures.

Intramolecular electronic interactions between nonconjugated arene and quinone chromophores.

The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result from broad absorption bands in the visible range (1, lambda(max) = 537 nm and 2, lambda(max) = 423 and lambda(shoulder) =515 nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-substituted duroquinone 31, even though the clips 1 and 2 only contain insulated pi systems as chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3, the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones 4, 5, and 6, the so-called "half-clips". The "half-clips" 6 and 5 show a similar color change and the same trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already rules out that the color of these systems is a result of "through-space" pi-pi interactions between the aromatic sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra of the clip quinones 2, 3, and 1 and the "half-clip" quinones 4, 5, and 6 with an increasing number of rings in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration interaction between a n --> pi* excitation of the quinoid component and a pi --> pi* excitation with intramolecular charge transfer (CT) character. The initial pi orbitals involved here and in higher lying transitions mainly stem from through-space interactions between pi orbitals of the aromatic sidewalls and pi orbitals of the quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1-3, the "half-clips" 4-6, and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well with the experimental spectra.

Co-crystals with acetylene: small is not simple!

Acetylene is an amazingly versatile component for the formation of co-crystals. It requires careful handling and special techniques for crystallisation, but the efforts seem to be rewarding when attaining co-crystals with small molecules as partners. Many basic questions such as the dominance of specific heterogeneous intermolecular interactions, their driving force for the formation of multicomponent crystals instead of neat ones are expected to be easily analysed. The underlying packing patterns and resulting stoichiometries based on the known supramolecular synthons seem to be straightforward for such small molecules and crystal engineering, considered as the prototype of supramolecular synthesis, should be a simple task. Nineteen co-crystals with acetylene are presented in this paper, some of which have been previously reported individually. An attempt has been made to find features shared by the groups of co-crystals, including those that could not be co-crystallised. But in spite of clear ideas and experiences from previous experiments, surprisingly almost none of systems reached our expectations. Our intuitive approach was not fulfilled, which demonstrates that multicomponent crystals even of small molecules will remain a great challenge for theoretical methods and the crystal structures shown herein represent good candidates for future testing. On the other hand, we wish to encourage other groups to present their views on the crystal structures with an unbiased approach that may offer a better explanation than we are able to outline in this article.

Synthesis and theoretical characterization of an acetylene-ammonia cocrystal.

One of the smallest molecular cocrystals consisting of acetylene and ammonia is grown in situ on the X-ray diffractometer. The resulting 1:1 molecular cocrystal is composed of antiparallel layers of zigzag chains. Subunits of the crystal structure are already stable as small molecular aggregates as shown by ab initio calculations. These aggregates may be considered as infant stages of cocrystal formation.

[123]Tetramantane: parent of a new family of sigma-helicenes.

We present a new type of sigma-helical structure based on a diamondoid (nanodiamond) framework, C(2)-symmetric [123]tetramantane, whose (+) and (-) isomers could be enantioseparated by HPLC techniques. Bromination of the enantiopure hydrocarbon led to the isolation of (+)-7-bromo-[123]tetramantane, which could be crystallized and subjected to X-ray structure analysis. Using the anomalous dispersion, we have identified this compound as the P isomer for the hydrocarbon moiety. Experimental and computed optical rotatory dispersion (ORD) and vibrational circular dichroism (VCD) spectra independently and in agreement with the X-ray structure analysis gave M-(-) as the configuration of the second eluted parent hydrocarbon isomer.

1,3-Difluoro-benzene.

The weak electrostatic and dispersive forces between C(δ+)-F(δ-) and H(δ+)-C(δ-) are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of other dominant inter-actions, e.g. C-H⋯π. The title compound, C(6)H(4)F(2), Z' = 2, forms one-dimensional tapes along two homodromic C-H⋯F hydrogen bonds. The one-dimensional tapes are connected into corrugated two-dimensional sheets by further bi- or trifrucated C-H⋯F hydrogen bonds. Packing in the third dimension is controlled by C-H⋯π inter-actions.

1,2,3-Trifluoro-benzene.

In the title compound, C(6)H(3)F(3), weak electrostatic and dispersive forces between C(δ+)-F(δ-) and H(δ+)-C(δ-) groups are at the borderline of the hydrogen-bond phenomenon and are poorly directional and further deformed in the presence of π-π stacking inter-actions. The mol-ecule lies on a twofold rotation axis. In the crystal structure, one-dimensional tapes are formed via two anti-dromic C-H⋯F hydrogen bonds. These tapes are, in turn, connected into corrugated two-dimensional sheets by bifurcated C-H⋯F hydrogen bonds. Packing in the third dimension is furnished by π-π stacking inter-actions with a centroid-centroid distance of 3.6362 (14) Å.

Molecular clips with extended aromatic sidewalls as receptors for electron-acceptor molecules. Synthesis and NMR, photophysical, and electrochemical properties.

We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalls and (ii) a dimethylene-connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime, depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds (2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises, (ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible disassembling of the complex can be obtained by electrochemical stimulation.

(3S,4S,5R)-4-Hydr-oxy-3-methyl-5-[(2S,3R)-3-methyl-pent-4-en-2-yl]-4,5-dihydro-furan-2(3H)-one.

The relative configuration of the title compound, C(11)H(18)O(3), was corroborated by single-crystal X-ray diffraction analysis. In the crystal, mol-ecules are linked via a O-H⋯O hydrogen bond and a chain of mol-ecules is formed along [010].

Solvent-induced ion separation of a beryllium scorpionate complex.

Spontaneous ion separation of the scorpionate beryllium complex, TpBeI 1 (Tp = 1-trispyrazolylborate), occurs upon treatment with THF, yielding [TpBe(thf)]I 2, which was characterized by heteronuclear NMR spectroscopy (1H, 9Be, 13C) and structurally characterized by single crystal X-ray diffraction. 2 represents a rare example of a structurally characterized monocationic beryllium complex, and to the best of our knowledge, the synthesis of 2 by a solvent-induced ion separation has previously only been observed in the reactions of beryllium dihalides with strong Lewis bases.

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations.

A new pyrene-based fluorescent probe for the determination of critical micelle concentrations (CMC) is described. The title compound 1 is obtained in five steps, starting from pyrene. Fluorescence spectroscopic properties of 1 are studied in homogeneous organic solvents and aqueous micellar solutions. In a wide range of organic solvents, probe 1 exhibits a characteristic monomer emission of the pyrene fluorophore, with three distinct peak maxima at 382, 404, and 425 nm. The spectra change dramatically in aqueous solution, where no monomer emission of the pyrene fluorophore is detected. Instead, only strong excimer fluorescence with a broad, red-shifted emission band at lambda(max) = 465 nm is observed. In micellar aqueous solution, a superposition of the monomer and excimer emission is found. The appearance of the monomer emission in micellar solution can be explained on the basis of solubilization of 1 by the surfactant micelles. The ratio of the monomer to excimer fluorescence intensities of 1 is highly sensitive to changes in surfactant concentration. This renders 1 a versatile and sensitive probe molecule for studying the micellization of ionic and nonionic surfactants. For a representative selection of common surfactants, the critical micelle concentrations in aqueous solution are determined, showing excellent agreement with established literature data.