Hydroboration with pyridine borane at room temperature

Treatment of pyridine borane (Py.BH3) with iodine, bromine, or strong acids affords activated Py.BH2X complexes that are capable of hydroborating alkenes at room temperature. Evidence is presented for an unusual hydroboration mechanism involving leaving group displacement. In contrast to THF.BH3, hydroboration with Py.BH2I selectively affords the monoadducts. The crude hydroboration products are converted into synthetically useful potassium alkyltrifluoroborate salts upon treatment with methanolic KHF2.     

Conformational analysis of self-organized monolayers with scanning tunneling microscopy at near-atomic resolution

We describe the synthesis and a novel approach to the conformational analysis of 2,2'-bipyridines (bpy) bearing aromatic rich Frechet-type dendritic wedges of the first and second generation as substituents. The evaporation of solutions of these new ligands on graphite surfaces under ambient conditions results in the formation of self-organized monolayers. Scanning tunneling microscopy (STM) investigations of the monolayers under ambient conditions (air, 298 K) gave images at submolecular and near-atomic resolution. The analysis of the STM images includes the following processes: (i) identification and reproduction of potential homoconformational domains, (ii) exclusion of improper data using quality criteria for drift and feedback artifacts, (iii) compilation of running averages and checking for averaging artifacts, (iv) analysis of three-dimensional and contour plots, (v) calculation of the HOMO properties of the free molecules, and (vi) final conformational assignment based on all accessible information. Following this procedure, two different conformations could be assigned to domains observed in the monolayers of the first-generation (G1) and second-generation (G2) dendritic compounds. Homoconformational domains are observed side-by-side. The different conformations arise from syn or anti arrangements at the ether substituents. An additional conformational effect is found upon treating the G1 domains with HCl gas, when a partial rearrangement of the bpy from trans to cis occurs, concomitant with protonation.     

Photoelectron spectra of cyclopolysilanes

Summary He (I) PE spectra of cyclopentasilane and cyclohexasilane show excellent agreement with STO-3 G + ab initio calculations. The HOMO in both compounds is of (SiSi) 3 p character. First IP's appear at 9.4 and 9.6 eV respectively.

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Photoelektronenspektren von Cyclopolysilanen (Kurze Mitt.)

Zusammenfassung Die He (I) PE-Spektren der Cyclosilane Si₅H₁₀ und Si₆H₁₂ zeigen ausgezeichnete Übereinstimmung mit STO-3 G + ab initio Rechnungen. Beide Ringe besitzen HOMO's mit (SiSi) 3 p Charakter. Die ersten Ionisierungspotentiale liegen bei 9.4 bzw. 9.6 eV.

     

Crystal and molecular structure of the cycloaddition product between maleic anhydride,cyclopentadiene, and 1,2-bis(dibromomethyl)benzene as DMF solvate (1∶2)

Crystal structure determination reveals that the unknown cycloaddition product between maleic anhydride, cyclopentadiene, and 1,2-bis(dibromomethyl)benzene has a nona-cyclic centrosymmetric structure. The compound was studied as DMF solvate (12). There is no significant intermolecular interaction between the title molecule and the DMF. Packing is in layers for both molecular species. A possible mechanism of formation of the title compound is discussed.     

Conducting polymers containing in-chain metal centers: electropolymerization of oligothienyl-substituted {M(tpy)2} complexes and in situ conductivity studies, M = Os(II), Ru(II)

The electropolymerization of a series of Ru and Os bis-terpyridine complexes that form rodlike polymers with bithienyl, quaterthienyl, or hexathienyl bridges has been studied. Absorption spectroscopy, scanning electron microscopy, and cyclic voltammetry have been used to characterize the monomers and resulting polymer films. The absolute dc conductivity of the quaterthienyl-bridged {Ru(tpy)2} and {Os(tpy)2} polymers is unusually large and independent of the identity of the metal center at 1.6 x 10(-3) S cm(-1). The maximum conductivity occurs at the formal potential of each redox process, which typically is observed for systems where redox conduction is the dominant charge transport mechanism. Significantly, the dc conductivity of the metal-based redox couple observed in these polymers is 2 orders of magnitude higher thanthat of a comparable nonconjugated system.     

Consequences of the ‘Pedersen papers’ on crown type chemistry at Würzburg and Bonn Universities: From heteroaromatic crowns and podands to large molecular and crystalline cavities including multisite receptors,cascade molecules,chromoionophores, siderophores,surfactant-type,and extreme ligands

We thank the editors of this issue for the opportunity to present the historic development of crown chemistry at the Universities of Wurzburg and Bonn in memory of C. J. Pedersen, the originator of the crown ethers. His legacy of science has tremendously stimulated research at both universities.This paper is dedicated to the memory of the late Dr C. J. Pedersen.     

Solid-state binding of dimethyl sulphoxide involving carboxylic host molecules. X-ray crystal structures of four inclusion species

The crystal structures of four dimethyl sulphoxide (DMSO) inclusion compounds with different carboxylic acid hosts,1–4, have been studied by single crystal X-ray analysis. Crystals of thetrans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid inclusion compound (1a), [1 · DMSO (1: 1)] show monoclinic (P2₁/n) symmetry with the unit cell dimensionsa = 11.522(4),b = 18.658(2),c = 8.709(1) Å and = 98.92(2)°. The clathrate of the 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (2a), [2 · DMSO (1: 2)] is triclinic (P) with the cell dimensionsa = 15.043(7),b =9.657(4),c = 8.118(7) Å, = 101.81(5), = 96.05(4) and = 100.04(4)°. Triclinic (P) symmetry is shown also by the inclusion compound of 9,10-dihydro-9,10-ethanoanthracene-11-monocarboxylic acid (3a) [3 · DMSO (1:1)] with the cell dimensionsa=6.3132(1),b=7.9846(2),c=17.5314(4) Å, = 96.46(2), = 87.08(2) and = 106.02(2)°. The 9,9-bianthryl-2-monocarboxylic acid clathrate (4a) [4 · DMSO (1:1)] is monoclinic (P2₁/n) and the cell dimensions area = 19.625(18),b = 8.817(1),c = 14.076(8) Å and = 97.92(6)°. In all these structures, the hosts show the same basic recognition pattern for the DMSO guest, involving a strong O-H ... O bond from the COON to the S=O group, and a possible C-H ... O type interaction between the carbonyl O atom of the host and a CH₃ group of the guest. The crystals consist of discrete host-guest aggregates which are mainly held together by weak intermolecular interactions of the Van der Waals' type. The stoichiometries of the aggregates are, however, different.     

On the Correlation between Ionization Potentials and Excitation Energies,part III: Pyrazine

A recently suggested scheme for relating ionisation potentials (IP) to excitation energies (E) has been applied to pyrazine and 2,6-dimethylpyrazine. The results indicate that in these systems the energy gap (ΔE) between the two lowest lying ¹n,π*-states is significantly smaller than that (ΔIP) between the corresponding ²n-states of the parent radical cations. The values estimated for ΔE on the basis of ΔIP, measurable by photoelectron spectroscopy, disagree with those calculated theoretically but seem to be supported by the experimental evidence available. Since ΔE = ΔIP would normally be expected, the relationship ΔE < ΔIP actually present in these systems is discussed and related to the different shape of the n-MO's involved in the ionization and excitation processes.     

Quadricyclanes. Part II: Electronic structure and chemical reactivity

The electronic structure of quadricyclane and 3-methylidenequadricyclane obtained by photoelectron spectroscopy, is used as a basis for the discussion of cycloadditions to these systems. The electronic structure of 3-heteroquadricyclanes, arrived at by theoretical calculations, agrees well with that expected from the above measured systems. A surprising outcome is that the orbital most responsible for the observed 2,4-cycloadditions to these heterosystems in not the HOMO but the third highest orbital which lies well below the former. This strongly suggests that these 2,4-cycloadditions proceed not in a concerted fashion but presumably involve as rate-determining step the formation of a resonance-stabilized zwitterionic intermediate. The nature of this intermeiate is discussed and the feasability of its formation investigated on the basis of thermochemical considerations.