Angewandte Chemie International Edition

Theoretical considerations and chemical reasoning lead us to claim that Ag−F bonding in Ag^III and Ag^II fluorides is substantially covalent, and in this way will resemble Cu−O bonding in Cu^II and Cu^III oxides. The prospect is that if appropriately modified, such high oxidation-state silver fluorides (for example, CsAgF₃ (shown here), made by Hoppe and co-workers) might be superconducting.

An extraordinary enzyme system for studying the fundamental chemistry of both dioxygen and C−H activation as well as for investigating the regulatory mechanism employed by a multicomponent oxygenase is the soluble methane monooxygenase system. The enzyme comprises three protein components (see picture). Activation of dioxygen occurs at a non-heme carboxylate-bridged diiron center in the α subunit of the hydroxylase protein MMOH, and results in spectroscopically characterized high-valent intermediates capable of oxidizing a wide variety of hydrocarbon substrates. MMOR provides the electrons for this reaction after oxidizing NADH to NAD⁺, while a third protein, MMOB, serves several regulatory functions.

The structure of the singlet state (¹A₁) of [Fe(CO)₄] in the gas phase (see picture) has been determined by a combination of laser photochemistry of [Fe(CO)₅] and electron diffraction imaging. The ground state of [Fe(CO)₄] is known to be a triplet species (³B₂), and this is the species detected in picosecond time-resolved IR experiments with [Fe(CO)₅] in solution. This is an appropriate moment to survey the state of knowledge on [Fe(CO)₄], beginning from the first low-temperature matrix experiments.

A single-electron transfer (SET) is the key step in new reactions using the hypervalent iodine compound 2-iodoxybenzoic acid (IBX) 1 as a reagent. Efficient oxidations of alcohols, oxidation of the benzylic position, or introduction of α,β-double bonds into carbonyl compounds are other new applications of this reagent.

Use of boron enolates in water! Boron enolates can be generated and used for aldol reactions in water by using a catalytic amount of Ph₂BOH (see scheme). This is the first example of a catalytic use of a boron source in stereoselective aldol reactions mediated by boron enolates. The mechanism of the reaction is discussed.

The through-space CH/π attraction between the η⁶-arene ligand on Ru and the carbonyl aryl substituent (see transition states in picture) plays a key role in the enantioselective transfer hydrogenation of aromatic carbonyl compounds with 2-propanol or formic acid, catalyzed by chiral η⁶-arene–Ru^II complexes.

Graft polymerization of photofunctional core–shell polymer microspheres of polystyrene-block-poly(4-vinylpyridine) in methyl methacrylate locks the microspheres in a body-centered cubic arrangement and yields a polymeric superstructure that exhibits a blue color tone (see picture). Similar optical properties are also observed in a diblock polymer with a lamellar morphology.

Olefinic analogues of effective peptide-based catalysts for the kinetic resolution of functionalized racemic secondary alcohols have been synthesized. The isosteric replacement of the peptide amide bond in 1 with an (E)-alkene to form 2 has enabled the evaluation of the kinetic role of particular amides within these catalysts.

G-Quartets can bind anions as well as cations: Solid-state and solution data indicate that self-assembled ion-pair receptors are formed from 16 guanosine monomers, 2 divalent cations, and 4 picrate anions. Hydrogen-bonding, ion–dipole, and base-stacking interactions combine to give a tubular complex with a cation-loaded interior. An array of hydrogen-bond donors on the receptor's surface then enables anion coordination (see schematic representation, shaded rectangles=G-quartets, shaded circles=cations).

Nanoporosity, good thermal stability, antiferromagnetic ordering, and hydrogenation with basic catalytic character are four important properties of the large-pore (24MR), zeolitic nickel(II) phosphate, VSB-5 (Ni₂₀[(OH)₁₂(H₂O)₆][(HPO₄)₈(PO₄)₄]⋅12 H₂O), which has been prepared under alkaline hydrothermal conditions. The structure of VSB-5 is depicted: NiO₆ octahedra: green; PO₄ tetrahedra: red.

Not only fullerenes but also inorganic cage molecules can be highly spherical aromatic. The molecular allotropes 1 (E=N, P, As, Sb, Bi) of the Group V elements and a variety of Zintl anions are even double spherical aromatic and obey the 2(N+1)² rule, since both their σ and π shells are completely filled. This is reflected by a pronounced diatropic character.

In a radiationless process, electronic excitation energy can be transported in the photonic antennae presented herein from the borders to the center of cylindrical zeolite L crystals (ca. 2 μm). These antennae are formed by supramolecular organization (see schematic representation) of a cationic and a neutral dye in the parallel channels of the crystal. The rectangles symbolize adsorption sites, for which the red ones are filled with red-emitting dyes and blue ones with blue-emitting dyes.

Innocent behavior of a typical non-innocent ligand has been observed for the first structurally characterized discrete metal complex with tetranucleating TCNQ. The coordination of four [Re(CO)₃(bpy)]⁺ units (see picture: C: black, N: green, O: blue, Re: red) facilitates the reduction of the already excellent π-acceptor molecule TCNQ by a further 0.74 V! TCNQ=7,7,8,8-tetracyano-p-quinodimethane, bpy=2,2′-bipyridine.

Pseudoprolines (ΨPro) have been developed as tools for inducing bioactive conformations that allow for optimal spatial complementation in protein–protein interactions. This dual function of ΨPro was explored for tuning proline-rich peptides as potent ligands for SH3 domains (see picture).

Squaring the cycle: Ligand L¹ has been developed for the fabrication of chiral molecular squares (see scheme), which can also be considered as circular helicates. By using a chirally modified ligand, one of the two forms of the complex can be generated selectively. bpy=2,2′-bipyridin-6-yl.

Stable product quality without the interfering influences of other isoenzymes and hydrolases is possible through the application of recombinant pig liver esterase (rPLE), for which functional expression has now been achieved for the first time. In the hydrolysis of 1-phenyl-2-butyl acetate rPLE leads to substantially higher enantioselectivity than commercial PLE preparations (see scheme).

Mild solid/gas or solid/liquid reactions can be used to prepare crystalline organosilicates, a class of silica-based hybrid materials, from the corresponding solid chlorosilanes (see picture). Hydrolysis and polycondensation in the solid state lead to the formation of the highly anisotropic organosilicates.

The right ligand enables the efficient carbonylation of unactivated chloroarenes. A general synthesis of benzoic acid derivatives is possible with the palladium–ferrocenylphosphane catalysts (see scheme).

The short thermolysis of the butterfly molecule [{Cp′′′(OC)₂Fe}₂P₄] affords, besides [Cp′′′FeP₅] and [{Cp′′′Fe}₂P₄], the diphosphadiferratetrahedrane [{Cp′′′Fe}₂(μ-CO)(μ-η²:η²-P₂)] with a Fe−Fe double bond in 18 % yield. The P₂ building block was split photochemically into two μ-P ligands with elimination of CO. In the resulting dinuclear complex 1, the ³¹P NMR signal is shifted to extremely low field (δ=1406.9), and the rhombic Fe₂P₂ four-membered ring is almost orthogonal to the eclipsed Cp′′′ five-membered rings (see picture); Cp′′′=tBu₃C₅H₂.

1,3,5-tripyrrolidinobenzene is the first example of an aromatic amine that forms σ dimers during oxidation/reduction cycles [Eq. (1); X⁻=ClO₄⁻], and thus turns out to be an efficient molecular switch. Surprisingly, such σ dimers are relatively stable intermediates during the formation of conjugated oligomers and polymers.

Metal–metal bonds are not essential for multiply bridging hydride ligands. A tetranuclear μ₄-hydride complex [(μ₄-H)Rh₄(PNNP)₂(CO)₄]⁺ (PNNP=3,5-bis(diphenylphosphanylmethyl)pyrazolate), in which the quadruply bridging hydrido ligand holds the four isolated metal centers together to form a tetrahedral metal array, has been prepared and characterized by spectroscopic methods and single-crystal X-ray diffraction (see picture).

Startling facts are often revealed by the quantitative investigation of a reaction thought to be understood. The photoionization of dimethylaminobenzonitrile, which was previously regarded as biphotonic, was found to occur by the successive absorption of three photons, and to produce two electrons per starting molecule [Eq. (1)]. The explanation is an intervening rapid chemical reaction: deprotonation followed by intramolecular charge transfer.

The suicidal complex ⁴2, which inactivates cytochrome P450 during olefin epoxidation, was shown by density functional calculations to be formed from the same high-spin intermediate (⁴1-III) that leads to stereochemical scrambling.

Chameleon states: the ruthenium and iron metalloporphyrin analogues of compound I of cytochrome P450 (1; L = thiolate) possess low-lying states that change their electronic structure with solvent polarization. The ground state of the ruthenium complex is a low-spin electrophilic state, whereas the ground state of the iron complex is triradicaloid.

Eight- and six-ring channels built from NbO₆ octahedra and SiO₄ or GeO₄ tetrahedra, respectively, are contained in the structures of the first organically templated niobium silicate and germanate open-framework structures. The organic template (piperazine) occupies a well-defined position at the intersection of the channels (see picture).

Resistance to leaching and re-usability are characteristic of the novel heterogeneous Lewis acid catalyst that was prepared by anchoring tin(IV) chloride on silica grafted with tetraalkylammonium or pyridinium chloride groups. The catalyst displays high activity and selectivity in the synthesis of 3-methyl-3-buten-1-ol by the Prins condensation of isobutene with formaldehyde [Eq. (1)].

Rhodium supported on titania–silica is modeled by 1, which was obtained from [{Cp*TiMe(μ-O₂SiPh₂)}₂] (2) and [{Rh(OH)(cod)}₂]. Complex 2 and its triply bridged derivative [Cp*Ti(μ-O₂SiPh₂)₃TiCp*] (3) can be envisaged as molecular models of titania–silica systems. Compounds 1–3 could potentially provide insights into the nature of the catalytically active sites in these systems; cod=1,5-cycloctadiene, Cp*=η⁵-C₅Me₅.

A variety of terminal alkenes are produced in excellent yields by the rhodium(I)-catalyzed methylenation of aldehydes using TMSCHN₂ and PPh₃ [Eq. (1)]. These mild reaction conditions allowed the conversion of enolizable substrates and the chemoselective methylenation of aldehydes over ketones. TMS=trimethylsilyl.

The titanium-stabilized lithium carbanion 1 represents a new class of organodimetallic compounds that are easily accessible by a dilithiation–transmetalation sequence [Eq. (1)]. The X-ray structure of 1 revealed an aggregate consisting of two titanated lithium phosphonate units and two molecules of LiCl as the aggregation nucleus. Density functional calculations on a model system showed excellent agreement with the solid-state structure and suggest a configurational stability comparable to those of lithium phosphonates.

The spontaneous formation of the contested ferryl ion is evident in ab initio molecular dynamics calculations on the Fe^II/H₂O₂ system in aqueous solution (Fenton reagents). Not only is the ferryl ion preferred over the hydroxyl radical as the active oxidative species, but the solvent water molecules play a crucial role in the mechanism. The picture shows the unit cell containing the iron complex surrounded by solvent water molecules 1.8 ps after the start of the simulation, when the ferryl ion is being formed (blue: iron, red: oxygen, white: hydrogen).

At least three phthalocyanine units can be incorporated into phosphonium salts to give compounds such as 1, following unprecedented exchange reactions between phthalocyanine moieties bound to a palladium center and phenyl groups bound to the coordinated phosphane. Preliminary second-order nonlinear optics studies of the phthalocyanine–phosphonium salts have been carried out.

A new way of forming P−C bonds: Catalytic amounts of F⁻ ions facilitate the reaction of P^III−F compounds with CF₃SiMe₃ to give P^III−CF₃ compounds (see scheme). These compounds can also be obtained in a one-pot procedure from P^III−OAr precursors (also shown).

5-Azido sialyl donors with O-acetyl protecting groups are useful α-selective glycosylation reagents, especially for primary hydroxyl groups as acceptors. This is shown with a variety of reactions using 1 as a sialyl donor. It was also possible to synthesize NeuAcα(2→9)NeuAc as a thioglycoside donor for use in subsequent glycosylations.

Clinically relevant relaxivity enhancement of a magnetic resonance imaging (MRI) contrast agent has been achieved by using prodrug Gd³⁺ complexes (see picture, DTPA=diethylenetriaminepentaaceto). Enzymatic cleavage of lysine residues from the prodrug exposes a group that has a high affinity to human serum albumin and promotes enhanced relaxivity, thus enabling the detection of targets at submicromolar concentrations.

According to a chiral memory protocol, a chiral glycolic acid equivalent, the butane-2,3-diacetal-desymmetrized glycolate 2, is obtained from chiral 3-halopropane-1,2-diols 1. Compound 2 is a new and effective building block for the synthesis of mono- and dialkylated α-hydroxy acids 3, which constitute a biologically and pharmacologically important structural motif.

DNA hybridization enables the three-dimensional assembly of Au nanoparticles and streptavidin (see picture). The high-density DNA-modified Au nanoparticles were stable to nonspecific binding of streptavidin. Structural and melting investigations on the assemblies showed their formation was reversible.

The precatalyst [Cp*RuCl(cod)] promotes the head-to-head cyclodimerization of propargylic alcohols and the formation of novel alkylidene cyclobutenes (see scheme; Cp*=C₅Me₅, cod=1,5-cyclooctadiene) by addition of carboxylic acid to the Ru(η⁴-cyclobutadiene) intermediate and dehydration.

Two new C−C bonds as well as a remote stereocenter are formed in the title reaction. With remarkable efficiency, this new reaction provides, through remote 1,5-asymmetric induction, anti-1,5 diols that are useful synthons for polyol synthesis (see scheme; Hacac=acetylacetone).

The presence of fluorine atoms in the ligand transforms titanium–salicylidene complex (see picture) into an effective catalyst for the living polymerization of ethylene. The catalyst displays very high activities (maximum turnover frequency 24 300 min⁻¹ atm⁻¹) and creates high molecular weight polyethylene (M_n>400 000) with extremely narrow polydispersities (M_w/M_n<1.20) at very high polymerization temperatures (25–50°C).

A retro-ene reaction that generates the parent aldehyde and a sigmatropic rearrangement are involved in the In(OTf)₃-catalyzed conversion of homoallylic alcohols 1 into the thermodynamically favored regioisomers 2. This method can be used for the stereocontrolled synthesis of linear homoallylic 22α-sterols from their readily accessible branched 22β isomers.

From two to five: A mononuclear Ru complex of salicylaldehyde thiosemicarbazone (H₃saltsc, 1), in which only two of the coordination sites of 1 participate in bonding ([Ru(bpy)₂(H₂saltsc)]ClO₄), could be used to synthesize the cyclic complex cation [{Ru(bpy)₂(saltsc)}₄Ni₄]⁴⁺ (bpy=2,2′-bipyridine). Its formation demonstrates that the Ru complex can act as an N,N,O donor ligand for the construction of polynuclear complexes.

Cores for thought! A new [Mn₄O₄]⁶⁺ “cubane” core complex (L₆Mn₄O₄) with six facially bridging phosphinate chelate ligands (L⁻=(MePh)₂PO₂⁻) was synthesized. Photo-excitation releases molecular O₂ by intramolecular coupling of two core oxygen atoms and selective rearrangement to a [Mn₄O₂]⁶⁺ “butterfly” core ([L₅Mn₄O₂]⁺; see scheme). Thus the Mn₄O₄ cubane core exhibits unique reactivity in O₂ evolution which may account for its presence in the photosynthetic enzyme.