Chemistry - A European Journal

The cover picture shows a modeled structure (PM3(tm)-level) of a TADDOL derived monoligated bulky phosphorus amidite palladium–allyl complex. The allyl fragment, shown in green, has the syn,syn conformation and is clearly blocked by the aryl fragment of the S,S ligand upon counterclockwise rotation. Upon nucleophilic attack only clockwise rotation can occur, which explains the formation of the experimentally observed formation of the R product. A scheme of the reaction is shown in the background. For more information see the article by P. W. N. M. van Leeuwen et al. on p. 6232 ff.

Delving deeper into self-assembly: Multifunctionalization of synthetic polymer systems through self-assembly is an important milestone in supramolecular chemistry. It has the potential to rapidly optimize complex materials, and create new avenues to previously inaccessible ones. This concept requires multiple recognition units to perform the desired functions. In the schematic diagram shown here, two polymer chains are cross-linked with recognition unit A, while recognition unit B instills an additional functionality.

A new type of a photomediator containing a covalently linked substrate binding site is represented by a flavin–zinc(II)-cyclen complex. It efficiently oxidizes 4-methoxybenzyl alcohol to the corresponding aldehyde by photoinduced intramolecular electron transfer in water and in acetonitrile (see picture).

Versatile ligands: Bulky monodentate phosphoramidites are versatile ligands in allylic substitution reactions. An interesting memory effect was observed and a rationalisation for this observation will be presented. Furthermore, the use of these monodentate ligands results in high enantioselectivities up to 99 %. The figure shows the modelled structure of a syn,syn Pd–allyl complex (left); a schematic view along the Pd–allyl axis is presented on the right (the plane represents the aryl-fragment of the ligand blocking the allyl moiety upon rotation, empty circles represent minor steric interactions).

Sensitive sugars: Efficient, cooperative quenching of sugar-substituted poly(para-phenylene ethynylene) was achieved by using mercury trifluoroacetate or lead acetate. The schematic representation here shows the above-mentioned mercury quencher (grey sphere) with sugar substituents as crossed arrows. Stern–Volmer constants of up to 72×10³ were found for these systems.

True blue: Two geometrical isomers of the Os^II complex [Os(CO)₂(bptz)₂], where bptz denotes 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazolate, were synthesized in an effort to attain room-temperature blue phosphorescence. Comprehensive structural characterization, relaxation dynamics, and theoretical analyses (see picture) have been performed to gain an insight into the remarkably different photophysical properties of these two isomers. The results demonstrate the crucial role of metal–ligand bonding in radiationless deactivation.

Isomeric edge-bridging and face-capping alkenyl derivatives (see picture) are the products of the reactions of [Ru₃(μ-H)(μ₃-η²-R¹NNR²Me)(CO)₉] with alkynes. The regioselectivity of these reactions depends on the alkyne reagent used and the nature of the R¹ and R² groups of the bridging hydrazido ligand. The DFT-calculated thermodynamic stability of all the possible isomeric products of selected reactions has been used to rationalize the observed regioselectivity.

A MoP catalyst has been shown experimentally to be active in the hydrodenitrogenation (HDN) of o-propylaniline. The structure and the energetics of the o-propylaniline adsorption on the Mo-terminated MoP(001) surface have been investigated (see picture), and the reaction path for one of the branches of the HDN reaction network that leads to the formation of propylbenzene has been studied.

Depending on the reducing agent used, complexes with NH,NH- or NH,NOH-stabilized carbene ligands are obtained by reduction of the nitro group in complexes with the 2-nitrophenyl isocyanide ligand. The NH,NOH-stabilized carbene ligand undergoes stepwise alkylation, first at the oxygen atom and then at the nitrogen atom (see scheme).

Size matters! A variety of tetraazaporphyrins (TAPs), phthalocyanines (Pcs), naphthalocyanines (Ncs), and anthracocyanines (Acs; see picture) was synthesized and their electronic absorption, magnetic circular dichroism (MCD), IR, and voltammetric properties were interpreted with the help of quantum-mechanical calculations.

Insights into the mechanism and role played by complexes of the type [ReMeO₂(η²-alkyne)] in the aldehyde-olefination reaction (see scheme) were gained after several oxorhenium compounds were screened as to their activity. The formation of ylides seems to be of minor importance, and therefore these catalysts enable the synthesis of olefins from diazo compounds corresponding to unreactive ylides that would otherwise not be accessible by a normal Wittig reaction.

Highly reactive polyisobutenes with a high content of terminal olefinic groups can be synthesized by the use of hexakis(acetonitrile)manganese(II) initiators with bulky, noncoordinating counteranions in homogeneous solution. These easily accessible complexes initiate polymerization already at room temperature and form gel-free polyisobutenes with high viscosities. Furthermore, the initiators can be used for the copolymerization of isobutene with isoprene. In the presence of water the catalysts are deactivated by displacement of nitrile and coordination of aqua ligands, as in the complex shown in the picture.

HCl, the secret ingredient! Reaction of alkenyl β-keto esters, alkenyl α-aryl ketones, and alkyl alkenyl ketones with a catalytic amount of [PdCl₂(CH₃CN)₂] in the presence of Me₃SiCl or HCl leads to intramolecular hydroalkylation to form 2-substituted cyclohexanones in good yield with high regioselectivity (see scheme).

More from the world of palladium catalysis. Reaction of 4-pentenyl-β-diketones and substituted 4-pentenyl-β-keto esters with a catalytic amount of [PdCl₂(CH₃CN)₂] (5 mol %) and a stoichiometric amount of CuCl₂ (2.5 equiv) led to intramolecular oxidative alkylation to form the corresponding cyclohexenones in good yield (see scheme).

A stem peptide bearing two masked aldehydes such as 2 has been used as an effective synthon for building the branched oxime-linked conjugate 1 from MUC1 tandem repeat sequences and a universal T-helper epitope. This original tri-branched construct induces antibodies which recognize a recombinant MUC1 protein.

Reducing dendrimers! The electrochemical behavior, and chemical and photosensitized reduction of two dendrimers containing 9 and 21 viologen units, respectively, in their branches have been investigated. The obtained results indicate that only part of the viologen units present in the dendrimers can be reduced. For the larger dendrimer, the number of reducible viologens (out of the 21 present) is 14 in electrochemical experiments, 9 on chemical reduction, and 13 by photoinduced electron transfer (see diagram). The reduced viologen units undergo partial dimerization.

Controlled drug release: Microcapsules of vitamin K₃, biotin, or insulin were prepared by forming multilayered polymeric shells of varying thickness around crystalline cores. Exposure of coated VK₃ crystals to ethanol, or of enveloped insulin crystals to 0.01 n HCl, resulted in hollow, polymeric shells (see figure). The release rates of vitamin K₃ and insulin decreased as the number of polyelectrolyte layers increased. The release time varied by a factor of more than ten, depending on the number of layers added.

Shuttling and switching: Three neutral [2]rotaxanes have been synthesized in a thermodynamically-controlled manner (slippage) from their respective dumbbell precursors, themselves constructed in a modular fashion. The kinetics and thermodynamics associated with the shuttling processes in two degenerate rotaxanes have been probed by dynamic ¹H NMR spectroscopy. The electrochemically controlled switching of the neutral bistable rotaxane was demonstrated by cyclic voltammetry. Chemical switching (illustrated here), which could be controlled by Li⁺ ions, was monitored by both UV/Vis and ¹H NMR spectroscopy.

Supramoleculer special-pair/electron-acceptor systems (see figure) were synthesized to elucidate the role of the special-pair arrangement in the electron-transfer process in photosynthesis.

CH/π interaction is more favored in a hydrophobic cavity than π–π interaction and this preference causes an intramolecular rearrangement without bond rupture in the coordination sphere of a ruthenium(II)–pyridylamine complex (see scheme).

CO₂⁺ions undergo isotope exchange with CO, through a C₂O₃⁺ ion of OCOCO connectivity. The reaction efficiency is critically affected by the internal energy deposited into the initially formed C₂O₃⁺ adduct, whose decomposition is favoured at higher internal energies with respect to the O exchange occurring through a double-minimum potential-energy surface. Under highly energetic conditions, an indirect ulterior route (dashed lines in figure) to dissociation of carbon dioxide into CO and oxygen atoms is provided by the reaction of excited CO₂⁺ ions with CO₂.

The magnetism of the bulk ferromagnetpara-(methylthio)phenyl nitronyl nitroxide crystal (YUJNEW) has been theoretically reinvestigated by using our recently proposed bottom-up approach. We first define the crystal magnetic topology (see figure) from the computed J_AB radical⋅⋅⋅radical magnetic interactions. Then the macroscopic magnetic properties are computed numerically. They fully reproduce the experimental data when low-temperature crystal structures are used.

The next generation: An efficient, convergent route to the main chain type of organometallic dendrimers, in which platinum moieties are linked by 1,3,5-triethynylbenzene, has been developed. The platinum–acetylide dendrimers were prepared up to the third generation (shown here; PT=[Pt(PEt₃)₂]) by reacting dendrons with a triplatinum core and a tetraplatinum core.

The intimate electron transfer pathways involved in electrochemical reduction of the coordinated diazene unit in [Cp₂Mo₂(μ-SMe)₃(μ-η¹:η¹-HNNPh)]⁺ (see figure) are revealed by density functional calculations. The metal centres play an active part in the reduction process, acting as a source of additional reducing equivalents for the cleavage of the NN bond.

Criteria for a better predictability of the catalytic asymmetric hydroboration performance after small variations on the reactants and catalysts are described. The relation between the enantiocontrol and the role of the hydroborating reagent, the metal and the electronic nature of the catalytic system (see scheme) has been rationalised from structural evidences and the relative stability of the most abundant isomers.

New improved MP2: Reactions and activation energies for pericyclic reactions (Diels–Alder, [3,3] shifts, and electrocyclizations, see figure) calculated with the new SCS-MP2 method are as good as those at G3.

The supramolecular organization of [Cu₄L]⁵⁺moieties, assembled by the action of the polynucleating ligand H₃L on CuX₂ salts (see scheme), is dramatically affected by small changes in the ligand:metal:base ratio or by the nature of X⁻. Thus, methoxide-bridged [Cu₈L₂] dimers are formed in methanol in the presence of excess base. These moieties organize as cluster polymers (X⁻ = NO₃⁻, Cl⁻, Br⁻) or as discrete cages (X⁻ = ClO₄⁻). At lower ratios, OMe⁻ bridging leads to one-dimensional [Cu₄]_∞ coordination chains. All structural types show strong antiferromagnetic coupling within the tetrametallic building blocks.

Not only regular alkanes, but also alkylbenzenes can be nitrated under mild conditions using vanadium-substituted Keggin-type phosphomolybdates such as [H₄PVMo₁₁O₄₀], [H₅PV₂Mo₁₀O₄₀], and [H₆PV₃Mo₉O₄₀] as efficient catalyst precursors. The kinetic and mechanistic studies show a catalytic reaction mechanism involving a radical-chain path (see scheme).

An electroactive receptor consisting of a crown-tetrathiafulvalene (TTF) unit has been covalently linked to electropolymerizable pyrrole or 3,4-ethylenedioxythiopene monomers. These systems have been electropolymerized and the resulting modified electrodes have been electrochemically characterized. The electrodes constitute the first example of conducting polymer-based modified electrodes incorporating a TTF redox probe, able to interact with a guest ion, such as Ba²⁺ (see scheme).

Structure and bonding: As more complexes of the type M(CC)_nM (M=transition metal; see example here) with long sp-carbon chains become available, many unusual properties are emerging. This paper details 1) a comprehensive model for structure and bonding in the title compounds, 2) rationales for various “chain-length effects”, 3) models for principal electronic transitions, and 4) energy profiles for endgroup rotation and chain bending.

Layering up! Thin films consisting of a fulleropyrrolidine derivative and a water-soluble porphyrin have been prepared by the Langmuir–Shäfer method. Evidence of the effective interactions between the two moieties at the air–water interface was obtained from Langmuir curves, Brewster angle microscopy, and UV/Vis reflection spectroscopy. Photoaction spectra recorded from films deposited by only one horizontal lifting onto indium–tin–oxide (ITO) electrodes (see scheme) have shown that the photocurrent increases notably with increasing transfer surface pressure.

Chiral molecular glass: Synthesis of highly-functionalized thiophene-based[7]helicenes is based upon kinetic resolution of the intermediate diketone, followed by the annelation step forming the center benzene ring (intramolecular McMurry reaction). One such enantiopure [7]helicene, which adopts a tetrahedral-like shape in the racemic crystal (illustrated here), would not crystallize but formed a rare chiral molecular glass.

Iterative Redlich–Kister analyses of mixture vapor pressure (MVP) data yield significantly less error with the experimental data if the temperature and pure vapor pressures are also parameters. These results suggest a vast amount of literature MVP data should be reanalyzed. Results on the H₂O₂–H₂O system demonstrate this approach; solution complexities not evident in previous works (upper three curves, left figure) are revealed (lower two curves, left figure).

No matter what the surroundings, be it solution, a self-assembled monolayer on gold, or a solid-state polymer matrix, the same molecular electromechanical switching mechanism is obeyed for bistable rotaxanes (see illustration) and bistable catenanes in appropriate settings. However, the kinetics and thermodynamic parameters change quite dramatically from one environment to another.

Selenenyl fluorides ArSeF, generated by the reaction of Ar₂Se₂ or ArSeSiMe₃ with XeF₂, are detected for the first time by ⁷⁷Se and ¹⁹F NMR measurements (see picture). Other types of ArSeF are unstable and disproportionate to ArSeF₃ and Ar₂Se₂ (Ar = Ph, o-PrC₆H₄, o-MeOCH₂C₆H₄, o-(S)-Et(MeO)CHC₆H₄, 2,6-(MeOCH₂)₂C₆H₃.

The accepted mathematical conditions for a degree of chirality of a geometrical object such as a tetrahedron lead to an infinite variety of such indices. Moreover, to every chiral geometric tetrahedron (see picture), no matter how close to an achiral or degenerate limit, there is some legitimate functional form for degree of chirality that makes this the “most chiral” tetrahedron.

A series of new hydrophobic ionic liquids, consisting of [R_FBF₃]⁻ (R_F=CF₃, C₂F₅, nC₃F₇, nC₄F₉) anion with 1-alkyl(alkyl ether)-3-methylimidazolium ([C_mmim]⁺ or [C_mO_nmim]⁺ (where C_m is 1-alkyl, C_m=nC_mH_2m+1, m=1–4 and 6; C_mO_n is 1-alkyl ether, C₂O₁=CH₃OCH₂, C₃O₁=CH₃OCH₂CH₂, and C₅O₂=CH₃(OCH₂CH₂)₂) cation; see structure), were synthesized and characterized. The key features for these new ionic liquids are their low melting points (−42 to 35 °C) or extremely low glass transition (between −87 and −117 °C) without melting, and considerably low viscosities (26–77 cP at 25 °C).