Redetermination of (E)-N,N′-bis­(4-bromo­phen­yl)formamidine

In comprison with the previous structural study [Anulewicz et al. (1991 ▶). Pol. J. Chem. 65, 465–471], for which only the coordinates of all non-H atoms and of some H atoms were reported, the current redetermination of the title compound, C13H10Br2N2, additionally reports anisotropic displacement parameters for all non-H atoms and the coordinates of all H atoms, accompanied by higher accuracy of the geometric parameters. Two independent half-mol­ecules are present in the asymmetric unit, which are completed by a twofold rotation axis as symmetry element. In the crystal, inter­molecular N—H⋯N hydrogen bonds link the mol­ecules into dimers. Linear chains parallel to [102] are formed by inter­molecular Br⋯Br inter­actions of 3.4328 (7) Å between two Br atoms of adjacent mol­ecules. The dihedral angles between the benzene rings are 50.05 (15) and 75.61 (11)° in the two independent molecules. Owing to the twofold symmetry of the mol­ecules, H atoms attached to the N atoms are only half-occupied, leading to them being disordered over two positions of equal occupancy

Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization

Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.

Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

The complex [Cp*RuCl(iPr2PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C6H4CF3) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru≡C≡CHR(iPr2PNHPy)]Cl (R = COOMe (2a- Cl), C6H4CF3 (2b-Cl)), whereas reaction of 1 with NaBPh4 in MeOH followed by addition of HC≡CR (R = COOMe, C6H4CF3) yields the metastable π-alkyne complexes [Cp*Ru(η2-HC≡CR)(iPr2PNHPy)][BPh4] (R = COOMe (3a-BPh4), C6H4CF3 (3b-BPh4)). The transformation of 3a-BPh4/3b-BPh4 into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HC≡CR (R = COOMe, C6H4CF3) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η2-HC≡CR)(iPr2PNHPy)]Cl (R = COOMe (3a-Cl), C6H4CF3 (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh4]− anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanolassisted hydrogen migrations. Calculations suggest participation of a hydrido−alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles

Unresolved issues and new challenges in teaching English to young learners:the case of South Korea

The introduction of languages, especially English, into the primary curriculum around the world has been one of the major language-in-education policy developments in recent years. In countries where English has been compulsory for a number of years, the question arises as to what extent the numerous and well-documented challenges faced by the initial implementation of early language learning policies have been overcome and whether new challenges have arisen as policies have become consolidated. This article therefore focuses on South Korea, where English has been compulsory in primary school since 1997. The issues raised by the introduction of English into the primary curriculum are reviewed and the current situation in South Korea is investigated. The results of a mixed methods study using survey data from 125 Korean primary school teachers, together with data from a small-scale case study of one teacher are presented. The study shows that, while some of the initial problems caused by the introduction of early language learning appear to have been addressed, other challenges persist. Moreover, the data reveal the emergence of a number of new challenges faced by primary school teachers of English as they seek to implement government policy

Counteranion-Dependent Reaction Pathways in the Protonation of Cationic Ruthenium−Vinylidene Complexes

The tetraphenylborate salts of the cationic vinylidene complexes [Cp*Ru=C=CHR(iPr2PNHPy)]+ (R = p-C6H4CF3 (1a-BPh4), Ph (1b-BPh4), p-C6H4CH3 (1c- BPh4), p-C6H4Br (1d-BPh4), tBu (1e-BPh4), H (1f-BPh4)) have been protonated using an excess of HBF4·OEt2 in CD2Cl2, furnishing the dicationic carbyne complexes [Cp*Ru≡CCH2R(iPr2PNHPy)]2+ (R = p-C6H4CF3 (2a), Ph (2b), p-C6H4CH3 (2c), p-C6H4Br (2d), tBu (2e), H (2f)), which were characterized in solution at low temperature by NMR spectroscopy. The corresponding reaction of the chloride salts 1a-Cl, 1b-Cl, 1c-Cl, and 1d-Cl followed a different pathway, instead affording the novel alkene complexes [Cp*RuCl(κ1(N),η2(C,C)-C5H4N-NHPiPr2CH=CHR)][BF4] (3a−d). In these species, the entering proton is located at the α- carbon atom of the former vinylidene ligand, which also forms a P−C bond with the phosphorus atom of the iPr2PNHPy ligand. To shed light on the reaction mechanism, DFT calculations have been performed by considering several protonation sites. The computational results suggest metal protonation followed by insertion. The coordination of chloride to ruthenium leads to alkenyl species which can undergo a P−C coupling to yield the corresponding alkene complexes. The noncoordinating nature of [BPh4]− does not allow the stabilization of the unsaturated species coming from the insertion step, thus preventing this alternative pathway

Catalytic Transformations of Alkynes via Ruthenium Vinylidene and Allenylidene Intermediates

NOTICE: This is the peer reviewed version of the following book chapter: Varela J. A., González-Rodríguez C., Saá C. (2014). Catalytic Transformations of Alkynes via Ruthenium Vinylidene and Allenylidene Intermediates. In: Dixneuf P., Bruneau C. (eds) Ruthenium in Catalysis. Topics in Organometallic Chemistry, vol 48, pp. 237-287. Springer, Cham. [doi: 10.1007/3418_2014_81]. This article may be used for non-commercial purposes in accordance with Springer Verlag Terms and Conditions for self-archiving.Vinylidenes are high-energy tautomers of terminal alkynes and they can be stabilized by coordination with transition metals. The resulting metal-vinylidene species have interesting chemical properties that make their reactivity different to that of the free and metal π-coordinated alkynes: the carbon α to the metal is electrophilic whereas the β carbon is nucleophilic. Ruthenium is one of the most commonly used transition metals to stabilize vinylidenes and the resulting species can undergo a range of useful transformations. The most remarkable transformations are the regioselective anti-Markovnikov addition of different nucleophiles to catalytic ruthenium vinylidenes and the participation of the π system of catalytic ruthenium vinylidenes in pericyclic reactions. Ruthenium vinylidenes have also been employed as precatalysts in ring closing metathesis (RCM) or ring opening metathesis polymerization (ROMP). Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metalallenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Cα and Cγ, while Cβ is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the π system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy

Natural hazards in Australia: heatwaves

As part of a special issue on natural hazards, this paper reviews the current state of scientific knowledge of Australian heatwaves. Over recent years, progress has been made in understanding both the causes of and changes to heatwaves. Relationships between atmospheric heatwaves and large-scale and synoptic variability have been identified, with increasing trends in heatwave intensity, frequency and duration projected to continue throughout the 21st century. However, more research is required to further our understanding of the dynamical interactions of atmospheric heatwaves, particularly with the land surface. Research into marine heatwaves is still in its infancy, with little known about driving mechanisms, and observed and future changes. In order to address these knowledge gaps, recommendations include: focusing on a comprehensive assessment of atmospheric heatwave dynamics; understanding links with droughts; working towards a unified measurement framework; and investigating observed and future trends in marine heatwaves. Such work requires comprehensive and long-term collaboration activities. However, benefits will extend to the international community, thus addressing global grand challenges surrounding these extreme events

Mechanics and thermodynamics of a new minimal model of the atmosphere

The understanding of the fundamental properties of the climate system has long benefitted from the use of simple numerical models able to parsimoniously represent the essential ingredients of its processes. Here, we introduce a new model for the atmosphere that is constructed by supplementing the now-classic Lorenz ’96 one-dimensional lattice model with temperature-like variables. The model features an energy cycle that allows for energy to be converted between the kinetic form and the potential form and for introducing a notion of efficiency. The model’s evolution is controlled by two contributions—a quasi-symplectic and a gradient one, which resemble (yet not conforming to) a metriplectic structure. After investigating the linear stability of the symmetric fixed point, we perform a systematic parametric investigation that allows us to define regions in the parameters space where at steady-state stationary, quasi-periodic, and chaotic motions are realised, and study how the terms responsible for defining the energy budget of the system depend on the external forcing injecting energy in the kinetic and in the potential energy reservoirs. Finally, we find preliminary evidence that the model features extensive chaos. We also introduce a more complex version of the model that is able to accommodate for multiscale dynamics and that features an energy cycle that more closely mimics the one of the Earth’s atmosphere

Intensification of Northern Hemisphere subtropical highs in a warming climate

Semi-permanent high-pressure systems over the subtropical oceans, known as subtropical highs, influence atmospheric circulation, as well as global climate. For instance, subtropical highs largely determine the location of the world’s subtropical deserts, the zones of Mediterranean climate and the tracks of tropical cyclones. The intensity of two such high-pressure systems, present over the Northern Hemisphere oceans during the summer, has changed in recent years. However, whether such changes are related to climate warming remains unclear. Here, we use climate model simulations from the Intergovernmental Panel on Climate Change Fourth Assessment Report, reanalysis data from the 40-year European Centre for Medium-Range Weather Forecasts, and an idealized general circulation model, to assess future changes in the intensity of summertime subtropical highs over the Northern Hemisphere oceans. The simulations suggest that these summertime highs will intensify in the twenty-first century as a result of an increase in atmospheric greenhouse-gas concentrations. We further show that the intensification of subtropical highs is predominantly caused by an increase in thermal contrast between the land and ocean. We suggest that summertime near-surface subtropical highs could play an increasingly important role in regional climate and hydrological extremes in the future