Chemistry - A European Journal

The interfacial enzyme cutinase… shown at the air–water interface on the cover, was site-selectively modified with two different ECE-pincer–metal complexes. The resulting cutinase–pincer–metal hybrids crystallized under halide-rich conditions to give monomeric crystal structures, but also crystallized under halide-poor conditions to form a metal-induced dimer. See the Full Paper by R. J. M. Klein Gebbink, P. Gros, G. van Koten et al. on page 4270 ff., for details of the chemistry and the crystal structures. Photograph: View from the island of Saba (Netherlands Antilles) taken by Birgit Wieczorek. Design: Birgit Wieczorek and Cornelis A. Kruithof.

Free to decide: Various 4-(α-styryl) γ-lactams are synthesized in 61–88 % yield by a phosphine-free palladium-catalyzed carbopalladation/allylic alkylation domino sequence (see scheme). The cyclization is totally regio- and diastereoselective in favor of the 3,4-trans-disubstituted γ-lactam. The process is successfully applied to the synthesis of a new aza analogue of the naturally occurring lignan (+)-oxo-parabenzlactone.

Doing the hula twist? A photochemically stimulated inversion of an sp²-hybridized oxygen atom upon simultaneous rotation of two adjacent bonds may be possible in a pure singlet excited potential energy surface in uninhibited fluid media (see scheme).

A remarkable twosome: Four-coordinate square-planar complexes based on [Zn(salen)] derivatives are conveniently captured inside a crystal matrix using a supramolecular protecting strategy (see figure). The four-coordinate species is accompanied by a five-coordinate complex that binds an axial ligand able to hydrogen bond to an N-heterocycle positioned in the axial region of the coordinative unsaturated derivative. The size of the N-heterocycle is a decisive stabilization parameter.

Swap the coins! The Pt₂Au₂, Pt₂Au₂Cu₂, and Pt₂Au₂Ag₂ complexes of 3,5-dimethylpyrazolate exhibit yellow-green, orange, and sky-blue luminescence, respectively (see figure). The emission energies of Pt₂Au₂M₂ complexes can be controlled by the change of the third coinage metal ions M. The Pt₂Au₂M₂ complexes take the cis configuration with respect to the Au₂M₂ plane.

Atomic-level portrait: The mechanism of the reaction catalyzed by the puzzling enzyme farnesyltransferase is elucidated by using computational methods, allowing the obtainment of the first real detailed atomistic quantum-chemical transition-state structure (see figure) for the reaction catalyzed by this enzyme. The results obtained provide an atomic-level framework for the design of more potent and specific inhibitors for this important enzyme.

A precise jab to methane: The SO₂^.+ radical cation (see figure) effectively activates CH₄ at room temperature through a [H₃C^.⋅⋅⋅HOSO⁺] methyl intermediate isolated in the gas phase by mass spectrometry. Methanol and ionized methyl hydrogen sulfoxylate, CH₃OSOH^.+, are formed by selective, direct attack of the incipient methyl radical at the O atom of the intermediate. The reaction shows radical and charge effects in the activation of methane by metal-free radical cations.

Arrays of endless possibility: Without template or catalyst, high-purity ZnO nanoarrays were patterned though a rapid, tunable and repeatable method (see picture). Their potential contributions in both electrical and biomedical fields were also discussed and demonstrated.

Finding a clear route to new structures: The design of an adaptable time warping (ATW) methodology (see figure) for automatically, quickly, and reliably deciphering X-ray diffraction patterns is described.

Crystal-clear structures: The first crystal structures of organometallic pincer–cutinase hybrids (see figure) provide insight into the 3D structural arrangement of both the protein and the organometallic pincer moiety, and reveal different binding modes for different pincers.

Why bigger is better: A “steric wall” created by the N-(2,6-diisopropylphenyl) substituent on the bulky NHC ligand IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) guides the reactants to and from the Pd center through weak, fleeting (IPr)H–Pd interactions that help the oxidative addition intermediate escape “the anti-trap”. The alternative “side” approach leads to transmetalation (the rate-limiting step) for which a novel Pd–Zn interaction was identified.

A variety of differently substituted 6- and 7-arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o-quinonoid intermediates. A combined analysis of μs–ms (laser flash) and real-time kinetic data show that the o-quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating.

Mesocomplex versus helicate: A naphthalene-bridged bis(benzene-o-dithiol) ligand reacts with Ti⁴⁺ to give both dinuclear triple-stranded meso complexes and helicates, depending on the counterion employed during synthesis. DFT calculations performed with a simplified complex revealed that the interconversion of Λ to its Δ enantiomer proceeds via a C_3h-symmetric transition state (see figure).

Do the hydrogen shuffle! DFT calculations and experiments reveal low-barrier multistep pathways for hydrogen-atom transfer from organic radicals to [Co^II(por)]^. to form [Co(H)(por)], and for the radical pathway leading to net olefin insertion into the CoH bond of [Co(H)(por)] (see scheme). The results suggest that hydrogen-atom-transfer pathways could be important in numerous addition reactions of MH complexes with unsaturated substrates.

Sheet-like mimic enzyme: A novel nanostructure (see figure) of sheet-like FeS with peroxidase-like activity was synthesized as a mimic enzyme for the development of amperometric transducers and biocatalysts.

All light and no heat? Structure–property relationships for photochromic and thermochromic N-salicylideneanils have been revised (see picture). One derivative has a packed crystal structure, is photochromic and exhibits an infinitely slow thermal back relaxation of technological interest. Complexation of the anils to transition metals has also been studied.

Mix and calcinate: The tin–tungsten mixed oxide (Sn–W oxide) prepared by calcination of the Sn–W hydroxide at 800 °C acts as an effective and reusable solid catalyst for CC bond-forming reactions, such as the cyclization of citronellal, the Diels–Alder reaction, and the cyanosilylation of carbonyl compounds with trimethylsilyl cyanide (see scheme). The observed catalysis was truly heterogeneous, and the recovered catalyst could be reused without loss of its high catalytic performance.

Zwitterionic “sandwiches”: Combined preparative, spectroscopic and theoretical studies on the syntheses, structures and electronic properties of zwitterionic iron(II) and cobalt(II) sandwich complexes [M(^MeTpmd)₂] containing κ³N-coordinated tris(pyrazolyl)methanide ligands (^MeTpmd; see picture) are presented.

Functional spheres: Monodisperse gold-doped titania spheres with tunable sizes under high concentration of titanium precursor have been synthesized by introducing trace amounts of chloroauric acid into the reaction system. Surface photovoltage, surface photocurrent, and transient photovoltage measurements (see figure) of annealed samples reveal that gold nanodots can act as both electron acceptors and donors under the illumination of different wavelengths of light.

Having the edge on aromatic–aromatic interactions: Model systems containing aromatic, fluoroaromatic, and heteroaromatic residues arranged in an edge-to-face disposition have been synthesized (see figure). Investigation of their structures and stereodynamic behavior shows a favorable interaction between an “edge” ring featuring a pyridine nitrogen lone pair and a perfluorinated “face” ring.

Bound to stay heteroleptic: Grafting of homoleptic benzyl and amide calcium reagents onto silica affords well-defined supported heteroleptic species, catalytically active in hydrosilylation, hydroamination, and styrene polymerization. The fact that attempts to generate a molecular model were thwarted by the Schlenk equilibrium affording the homoleptic compound (see scheme) emphasizes the importance of immobilization as an approach for the syntheses of heteroleptic calcium complexes.

Selecting the ring: tert-Butyldimethylsilyltriflate (TBSOTf)/NEt₃ treatment of alkynyl esters tethered to bicycloalkanones leads to the formation of tricyclic allenoates with total diasteroselectivity at the ring junction. An intramolecular alkynylogous Mukaiyama aldol reaction promoted by a TBSOTf/NEt₃ dual activation is involved. This novel methodology was illustrated by a formal total synthesis of (±)-hamigeran B.

Deceptively simple: Joint results obtained by EPR and IR spectroscopies and inhibited autoxidation studies allow an explanation, on quantitative grounds, of the factors determining the H-atom donating abilities of 2-alkoxyphenols (see graphic; blue: 2-methoxyphenol, red: cyclic phenol).

From urea to nucleobases: Freeze–thaw cycles in urea (1) solutions under methane/nitrogen atmospheres lead, with application of an energy source, to the synthesis of pyrimidines (mainly cytosine (2) and uracil (3)), triazines (such as cyanuric acid (4)), and adenine. This synthesis appears to be dependent on the atmosphere and the freezing conditions. At room temperature, hydantoin (5) is obtained. However, a freezing urea/water system subjected to an energy source under an inert atmosphere generates s-triazines.

Don't cry! The attachment of ferrocene moieties on the surface of carbon nano-onions influences the electrochemical properties of these moieties and the photophysical properties of the carbon nano-onions (see figure). Quantum chemical calculations confirm that the spectral properties of carbon nano-onions depend on their size and the degree of functionalisation.

Pepped up polymers: The synthesis and properties of novel chiral cyclic peptides designed to complex with suitable polymers through hydrogen bonding are described. A substituted cyclic peptide self-assembles into supramolecular nanotubes and develops noncovalent interactions with poly(vinyl alcohol) (PVA) by means of its carboxyl side chains.

Pd^IIate complex: A novel alkylated pincer thioimido–Pd complex generated from a catalyst precursor and basic organometallic reagents (RM) was observed by in situ IR, ¹H NMR, and ¹³C NMR spectroscopies for the first time and proved to be the active catalyst in stoichiometric and catalytic reactions of aryl iodides with RM (see scheme). The catalyst, as an electron-rich Pd^II species, promoted the Negishi coupling of aryl iodides and alkylzinc reagents with high efficiency, even at low temperatures (0 or −20 °C).

Under pressure: The changes occurring in [Co₂(CO)₆(XPh₃)₂] species at high pressure and low temperature have been investigated by experimental X-ray diffraction and ab initio periodic calculations. The transformation of the carbonyl conformation (from staggered to eclipsed; see figure) and the compression/expansion of the CoCo bond have been carefully analysed.

Super powers! Uranium(VI) ions can be extracted from aqueous solutions into supercritical CO₂ (sc-CO₂) via an ionic liquid phase that contained tri-n-butyl phosphate (TBP) as a complexing agent (see figure). This process has potential applications in the nuclear industry for removing actinides from nuclear waste.

At the double: DFT studies on the biosynthesis of prostacyclin (PGI₂, see scheme) from prostaglandin H₂ (PGH₂) show two reaction mechanisms through two different oxidation states, an Fe^IV–porphyrin intermediate and an Fe^III–porphyrin π-cation radical, followed by a proton-coupled electron-transfer process.