Search for: All records

Evidence for the amphoteric behavior of an acridinium unit has been gathered through the investigation of two rhodium complexes in which the cationic moiety acts either as a Z- or L-type ligand, depending on the coordination environment of the metal 

The entrance channel of bimolecular reactions sometimes involves the formation of outer complexes as weakly bound, fleeting intermediates. Here, we characterize such an outer complex in a system that models the bimolecular, C-O bond–forming reaction of a phosphine oxide Lewis base with a carbenium Lewis acid. Crystallographic studies show that the C-O distance in the outer form exceeds that of the final or inner adduct by 1.1 angstroms. As the system samples the two forms of the complex, which correspond to minima on the corresponding potential energy surface, the C-O linkage switches from a secondary interaction in the outer complex to a dative bond in the inner complex. This phenomenon is harnessed as a functional feature to stabilize xanthylium-based photoredox catalysts. 

In the introduced range, the successful establishment of an invasive species may be influenced by positive plant-plant interactions. Pre-existing vegetation, particularly conspecific mature trees, can shape the availability and attributes of microsites, thus potentially creating favorable conditions for the establishment of conspecific seedlings through facilitation. Pines are widely introduced in the Southern Hemisphere primarily as forestry plantations; these pines can become invasive, causing detrimental effects on local ecosystems and economies. In the high-elevation grasslands of the Sierras de Córdoba, Central Argentina, pines have begun to invade the native grassland as a result of improper (or lack of) management of pine plantations. During early pine invasion in this semi-arid grassland, we aimed to quantify the influence of adult live pines and on congeneric pine seedling recruitment and survival. For this, 48,000 seeds of Pinus elliottii and P. taeda were sown in three consecutive field trials, under different tree status treatments: live pines, dead pines, and no pines (i.e., open grassland). Seed were sown with and without irrigation and seeded microsites were oriented to the north and south of the live and dead trees. We also considered the hillslope aspect where the sites were located. Our results show that pine seedling recruitment was 57 % higher under live pines compared to dead pines and no pines treatment, but only in the trials that were not irrigated. Microsites south of the live pine trees, more shaded from direct sunlight in the Southern hemisphere, presented 36 % more pine seed germination than those to the north. In terms of topography, hillslope aspects with lower solar incidence (wetter hillslopes) also showed higher pine seedling recruitment. Our results suggest that moisture availability is a dominant factor driving further invasion, and that adult pines may be facilitating the invasion process by creating moister microsites for germination and pine seedling establishment. Thus, the early removal of adult pines is important to consider in the management of pine invasions. They do not only serve as a source of propagules, but also positively affect the establishment of their congeneric seedlings. 

Context.The C-19 stellar stream is the most metal-poor stream known to date. While its wth and velocity dispersion indicate a dwarf galaxy origin, its metallicity spread and abundance patterns are more similar to those of globular clusters (GCs). If it is indeed of GC origin, its extremely low metallicity ([Fe/H]=−3.4, estimated from giant stars) implies that these stellar systems can form out of gas that is as extremely poor in metals as this. Previously, only giant stream stars were observed spectroscopically, although the majority of stream stars are unevolved stars. Aims.We pushed the spectroscopic observations to the subgiant branch stars (G≈ 20) in order to consolate the chemical and dynamical properties of C-19. Methods.We used the high-efficiency spectrograph X-shooter fed by the ESO 8.2m VLT telescope to observe 15 candate subgiant C-19 members. The spectra were used to measure radial velocities and to determine chemical abundances using the MyGIsFOS code. Results.We developed a likelihood model that takes metallicity and radial velocities into account. We conclude that 12 stars are likely members of C-19, while 3 stars (S05, S12, and S13) are likely contaminants. When these 3 stars are excluded, our model implies a mean metallicity 〈[Fe/H]〉 = −3.1 ± 0.1, the mean radial velocity is 〈v_r〉 = −192 ± 3km s⁻¹, and the velocity dispersion is σ_vr= 5.9_−5.9^+3.6km s⁻¹. This all agrees within errors with previous studies. The A(Mg) of a sample of 15 C-19 members, including 6 giant stars, shows a standard deviation of 0.44 dex, and the mean uncertainty on Mg is 0.25 dex. Conclusions.Our preferred interpretation of the current data is that C-19 is a disrupted GC. We cannot completely rule out the possibility that the GC could have belonged to a dwarf galaxy that contained more metal-rich stars, however. This scenario would explain the radial velocity members at higher metallicity, as well as the wth and velocity dispersion of the stream. In either case, a GC formed out of gas as poor in metals as these stars seems necessary to explain the existence of C-19. The possibility that no GC was associated with C-19 cannot be ruled out either. 

Aiming to study the interaction of gold with the highly Lewis acidic fluorenyl cation, we synthesised ( o -[Ph 2 P(C 6 H 4 )Flu)AuCl(tht)][BF 4 ] ([2][BF 4 ]) and ( o -Ph 2 P(C 6 H 4 )Flu)AuCl 2 (3) (Flu = 9-fluorenyl) and found that the latter could be converted into [( o -Ph 2 P(C 6 H 4 )Flu)AuCl] + ([4] + ) upon treatment with NaBArF 24 (BArF 24 = B(3,5-C 6 H 3 (CF 3 ) 2 ) 4 ). [4] + , which has been isolated as a chloride-bridged dimer, readily catalyses the cycloisomerisation of 2-allyl-2-(2-propynyl)malonate. Computational results show that [4] + possesses a strong Au → C + bond and readily activates enynes. 

We describe a novel gold chloride complex supported by an ambiphilic phosphine/xanthylium ligand in which the AuCl moiety interacts with the π + surface of the xanthylium unit as indicated by structural studies. Energy decomposition analyses carried out on a model system indicates the prevalence of non-covalent interactions in which the electrostatic and dispersion terms cumulatively dominate. The presence of these AuCl–π + interactions correlates with the high catalytic activity of this complex in the cyclisation of 2-(phenylethynyl)phenylboronic acid, N -propargyl- t -butylamide, and 2-allyl-2-(2-propynyl)malonate. Comparison with the significantly less active acridinium and the 9-oxa-10-boraanthracene analogues reinforces this conclusion. 

Here we present a new method to monitor fluoride transmembrane transport into liposomes using a europium( iii ) complex. We take advantage of the long emission lifetime of this probe to measure the transport activity of a fluorescent transporter. The high sensitivity, selectivity, and versatility of the assay allowed us to study different types of fluoride transporters and unravel their mechanisms of action. 

We report the synthesis and characterization of two phosphine gold carbinol species designed to support intramolecular Au⋯H–O hydrogen bonding. Increasing the metallobasicity of gold through chloride to phenyl ligand substitution produced an observable increase in this hydrogen bond's strength which was analyzed experimentally and computationally.