The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon–climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

/pdf/acceleration-of-global-warming-due-to-carbon-cycle-feedbacks-3788793i7v.pdf

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

Water gelation by small organic molecules.

Dynamic combinatorial chemistry.

Supramolecular chemistry has developed over the last forty years as chemistry beyond the molecule. Starting with the investigation of the basis of molecular recognition, it has explored the implementation of molecular information in the programming of chemical systems towards self-organisation processes, that may occur either on the basis of design or with selection of their components. Supramolecular entities are by nature constitutionally dynamic by virtue of the lability of non-covalent interactions. Importing such features into molecular chemistry, through the introduction of reversible bonds into molecules, leads to the emergence of a constitutional dynamic chemistry, covering both the molecular and supramolecular levels. It considers chemical objects and systems capable of responding to external solicitations by modification of their constitution through component exchange or reorganisation. It thus opens the way towards an adaptive and evolutive chemistry, a further step towards the chemistry of complex matter.

From supramolecular chemistry towards constitutional dynamic chemistry and adaptive chemistry

Since the first reports in the late 1970s on transition metal complexes contain- ing pincer-type ligands—named after the particular coordination mode of these ligands—these systems have at- tracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typical- ly, such a ligand comprises an anionic aryl ring which is ortho,ortho-disubsti- tuted with heteroatom substituents, for example, CH2NR2 ,C H 2PR2 or CH2SR, which generally coordinate to the met- al center, and therefore support the MC s bond. This commonly results in a terdentate and meridional coordina- tion mode consisting of two metalla- cycles which share the MC bond. Detailed studies of the formation and the properties of a large variety of pincers containing platinum group metal complexes have provided direct access to both a fundamental under- standing of a variety of reactions in organometallic chemistry and to a range of new applications of these complexes. The discovery of alkane dehydrogenation catalysts, the mecha- nistic elucidation of fundamental transformations (for example, CC bond activation), the construction of the first metallodendrimers for sustain- able homogeneous catalysis, and the engineering of crystalline switches for materials processing represent only a few of the many highlights which have emanated from these numerous inves- tigations. This review discusses the synthetic methodologies that are cur- rently available for the preparation of platinum group metal complexes con- taining pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineer- ing of sensors, switches, and catalysts.

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts

Several geminal bis-urea compounds were synthesised by means of an acid-catalysed condensation of various benzaldehydes with different monoalkylureas. Many of these compounds form thermoreversible gels with a number of organic solvents at very low concentrations (<3 mM) and which are stable to temperatures higher than 100 °C. Electron microscopy revealed a three-dimensional (3D) network of intertwined fibres, which are several tens of micrometers long and have a width ranging from approximately 30 to 300 nm. The possible aggregate forms and aggregate symmetries were evaluated by means of molecular mechanics calculations. 1H NMR, 2D NMR, 13C NMR and 13CCP/MAS NMR techniques were used to obtain information about the aggregation and possible aggregate symmetry of geminal bis-ureas in solution, in the gel state, and in the solid state.

Geminal Bis‐ureas as Gelators for Organic Solvents: Gelation Properties and Structural Studies in Solution and in the Gel State

Rosettes that are held together by hydrogen bonds (see sketch on the right) were synthesized from metallodendrimers constructed by coordination chemistry. Two orthogonal, noncovalent interactions (metal-ligand and hydrogen bonding) were employed to build these nanosized dendrimers (M 7-28 kDa).

/pdf/noncovalent-synthesis-of-nanostructures-combining-4e9ztkrxyk.pdf

Noncovalent Synthesis of Nanostructures: Combining Coordination Chemistry and Hydrogen Bonding

Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3-bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen-bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen-bonded assembly, but this effect very much depends on the exact positions of the groups.

/pdf/noncovalent-assembly-of-functional-groups-on-calix-4-arene-2gba2ar2e8.pdf

Noncovalent assembly of functional groups on calix[4]arene molecular boxes

The results of a systematic study of the structural isomerism in more than 30 noncovalent hydrogen-bonded assemblies are described. These dynamic assemblies, composed of three calix[4]arene dimelamines and six barbiturates/cyanurates, can be present in three isomeric forms with either D3, C3h, or Cs symmetry. The isomeric distribution can be readily determined via a combination of 1H NMR and 13C NMR spectroscopy. In one case it is shown that the covalent capture of the dynamic assemblies via a ring-closing metathesis (RCM) reaction provides a novel analytical tool to distinguish between the D3 and C3h isomeric forms of the assembly. For the D3 isomer the RCM results in the formation of a cyclic trimer, comprising three dimelamines, whereas for the C3h isomer a cyclic monomer is formed. Molecular dynamics simulations in chloroform are qualitatively in agreement with the experimental data and reveal that the isomeric distribution is determined by a combination of steric, electronic, and solvation effects. A wide range of isomeric distributions covering all extremes has been found for the studied assemblies. Those with 5,5-disubstituted barbituric acid derivatives exclusively form the D3 isomer, because steric hindrance between the barbiturate substituents prevents formation of the C3h and Cs isomers. In contrast, assemblies with isocyanuric acid derivatives exhibit increased stability of the C3h and Cs isomers upon increasing the size of the isocyanurate substituent. The outcome of the assembly process can be controlled to a large extent via chiral substituents in the calix[4]arene dimelamines, due to the preferred orientation of the chiral centers.

Ron Hulst

Papers

Geminal Bis‐ureas as Gelators for Organic Solvents: Gelation Properties and Structural Studies in Solution and in the Gel State

Noncovalent Synthesis of Nanostructures: Combining Coordination Chemistry and Hydrogen Bonding

Noncovalent assembly of functional groups on calix[4]arene molecular boxes

Control of structural isomerism in noncovalent hydrogen-bonded assemblies using peripheral chiral information

Libraries of non-covalent hydrogen-bonded assemblies; combinatorial synthesis of supramolecular systems