Equatorial Sites Research Articles

New Cobalt(II) and Copper(II) Thiocyanate Complexes of 2,5‐Bis(pyridin‐2‐yl)‐1,3,4‐oxadiazole Ligand: Synthesis, Crystal Structures, Spectral Properties and Hirshfeld Surface Analysis

AbstractThe synthesis and characterization of two new mononuclear coordination complexes based on 2,5‐bis(pyridin‐2‐yl)‐1,3,4‐oxadiazole (bpox) and thiocyanate ion as coligand, [M(bpox)2(SCN)2]nH2O (M=Co, n=1 for complex 1; M=Cu, n=0 for complex 2), are described. The obtained complexes crystallize in the monoclinic crystal system with space group C 2/c for complex 1 and P 21/n for complex 2. UV/Vis, FTIR and Raman data indicated the presence of bpox and thiocyanate groups, in isolated compounds. This finding has been confirmed by crystallographic studies, which shows that the two metals Co(II) and Cu(II), have an octahedral environment in complex 1 and complex 2, respectively, with both bpox ligands in equatorial sites and terminal SCN− in axial positions. The crystal structure cohesion of each compound is ensured by intramolecular metal‐nitrogen M–N (M= Co, Cu) interactions, by π–π stacking interactions and by different intermolecular‐hydrogen bonds, such as C–H⋯π, O–H⋯N, O–H⋯O, C–H⋯N, and/or C–H⋯S. The three‐dimensional Hirshfeld surface analysis and the two‐dimensional fingerprint plots reveal that the structure is dominated by C⋯H/H⋯C contacts.

Disentangling interglacial sea level and global dynamic topography: Analysis of Madagascar

Global inventories of stable sea-level markers for the peak of the last interglacial period, Marine Isotopic Stage (MIS) 5e, play a pivotal role in determining sea-level changes and in testing models of glacial isostatic adjustment. Here, we present surveying and radiometric dating results for emergent terraces from northern Madagascar, which is generally regarded as a stable equatorial site. Fossil coral specimens were dated using conventional and open-system corrected uranium series methods. Elevation of the upper (undated) terrace decreases from 33.8 m to 29.5 m over a distance of 35 km. An intermediate terrace has an average radiometric age of 130.7±13.2 ka (i.e. MIS 5e). Its elevation decreases from 9.3 m to 2.8 m over a distance of 80 km. The record of the lowest terrace is fragmentary and consists of beach rock containing rare corals with ages of 1.6–3.8 ka. The spatial gradient of the MIS 5e marker is inconsistent with glacio-isostatic adjustment calculations. Instead, we propose that variable elevations of this marker around Madagascar, and possibly throughout the Indian Ocean, at least partly reflect spatial patterns of dynamic topography generated by sub-plate mantle convection.

Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions.

As the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt=C5H2 tBu3‐1,2,4, 1‐Dy) exhibits magnetic hysteresis at 60 K, similar lanthanide (Ln) complexes have been targeted to provide insights into this remarkable property. We recently reported homologous [Ln(Cpttt)2][B(C6F5)4] (1‐Ln) for all the heavier Ln from Gd–Lu; herein, we extend this motif to the early Ln. We find, for the largest LnIII cations, that contact ion pairs [Ln(Cpttt)2{(C6F5‐κ1‐F)B(C6F5)3}] (1‐Ln; La–Nd) are isolated from reactions of parent [Ln(Cpttt)2(Cl)] (2‐Ln) with [H(SiEt3)2][B(C6F5)4], where the anion binds weakly to the equatorial sites of [Ln(Cpttt)2]+ through a single fluorine atom in the solid state. For smaller SmIII, [Sm(Cpttt)2][B(C6F5)4] (1‐Sm) is isolated, which like heavier 1‐Ln does not exhibit equatorial anion interactions, but the EuIII analogue 1‐Eu could not be synthesised due to the facile reduction of EuIII precursors to EuII products. Thus with the exception of Eu and radioactive Pm this work constitutes a structurally similar family of Ln metallocenium complexes, over 50 years after the [M(Cp)2]+ series was isolated for the 3d metals.

Equatorial Active Site Compaction and Electrostatic Reorganization in Catechol-O-methyltransferase.

Catechol-O-methyltransferase (COMT) is a model S-adenosyl-l-methionine (SAM) dependent methyl transferase, which catalyzes the methylation of catecholamine neurotransmitters such as dopamine in the primary pathway of neurotransmitter deactivation in animals. Despite extensive study, there is no consensus view of the physical basis of catalysis in COMT. Further progress requires experimental data that directly probes active site geometry, protein dynamics and electrostatics, ideally in a range of positions along the reaction coordinate. Here we establish that sinefungin, a fungal-derived inhibitor of SAM-dependent enzymes that possess transition state-like charge on the transferring group, can be used as a transition state analog of COMT when combined with a catechol. X-ray crystal structures and NMR backbone assignments of the ternary complexes of the soluble form of human COMT containing dinitrocatechol, Mg2+ and SAM or sinefungin were determined. Comparison and further analysis with the aid of density functional theory calculations and molecular dynamics simulations provides evidence for active site “compaction”, which is driven by electrostatic stabilization between the transferring methyl group and “equatorial” active site residues that are orthogonal to the donor–acceptor (pseudo reaction) coordinate. We propose that upon catecholamine binding and subsequent proton transfer to Lys 144, the enzyme becomes geometrically preorganized, with little further movement along the donor–acceptor coordinate required for methyl transfer. Catalysis is then largely facilitated through stabilization of the developing charge on the transferring methyl group via “equatorial” H-bonding and electrostatic interactions orthogonal to the donor–acceptor coordinate.

A global analysis of coral bleaching over the past two decades

Thermal-stress events associated with climate change cause coral bleaching and mortality that threatens coral reefs globally. Yet coral bleaching patterns vary spatially and temporally. Here we synthesize field observations of coral bleaching at 3351 sites in 81 countries from 1998 to 2017 and use a suite of environmental covariates and temperature metrics to analyze bleaching patterns. Coral bleaching was most common in localities experiencing high intensity and high frequency thermal-stress anomalies. However, coral bleaching was significantly less common in localities with a high variance in sea-surface temperature (SST) anomalies. Geographically, the highest probability of coral bleaching occurred at tropical mid-latitude sites (15–20 degrees north and south of the Equator), despite similar thermal stress levels at equatorial sites. In the last decade, the onset of coral bleaching has occurred at significantly higher SSTs (∼0.5 °C) than in the previous decade, suggesting that thermally susceptible genotypes may have declined and/or adapted such that the remaining coral populations now have a higher thermal threshold for bleaching.

Synthesis and characterization of iridium hydride complexes with meso-Ph2PCH2P(Ph)CH2P(Ph)CH2PPh2 (meso-dpmppm) as an unsymmetric pincer ligand

Reaction of [IrCl(CO)(PPh3)2] with meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso-dpmppm) afforded a mononuclear IrI complex, [Ir(meso-dpmppm−κ3)(CO)2]Cl (1), which showed excellent reactivity towards HX, H2, HCOOH, and R3SiH to yield a series of IrIII hydride complexes, [IrH(meso-dpmppm−κ3)(CO)2]X2 (X = Cl (2), PF6 (2*)), [Ir(H)2(meso-dpmppm−κ3)(CO)]Cl (4), and [IrH(SiR3)(meso-dpmppm−κ3)(CO)]Cl (R3 = Me2Ph (5a), Ph2H (5b)). The hydride IrIII complexes with isocyanides, [IrH(meso-dpmppm−κ3)(RNC)2](PF6)2 (R = Xyl (2,6-xylyl) (3a), Mes (2,4,6-mesityl) (3b), Cy (cyclohexyl) (3c), tBu (tert-butyl) (3d)), were also prepared by reacting [IrCl(cod)]2 with meso-dpmppm and RNC in the presence of NH4PF6. Complexes 2–5 were characterized by 1H and 31P NMR and ESI−MS spectroscopies and X-ray diffraction analyses (3a, 4, 5a,b) to have distorted octahedral IrIII structures supported by a meso-dpmppm in meridional mode as an unsymmetrical PPP−κ3 pincer ligand, coordinating with two outer and one inner phosphorus atoms to form fused six- and four-membered chelate rings and bearing an uncoordinate inner phosphine unit. The terminal hydride occupied the axial open site surrounded by the equatorially oriented phenyl groups of meso-dpmppm and is trans to the carbonyl (2, 4) and isocyanide ligands (3) nested in the closed site with respect to the {Ir(meso-dpmppm−κ3)} pincer plane. The remaining equatorial site is coordinated by CO (2), RNC (3), hydride (4), and silyl (5) ligand. These structural features demonstrated that oxidative additions of H+, H2, and R3SiH occurred at the axial open site of 1. The uncoordinate inner phosphine of 1 is readily reacted with [Cp*IrCl2]2 to give [Ir(μ−meso-dpmppm−κ3)(η5−Cp*IrCl2)(CO)2]Cl (1∙Cp*IrCl2, Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl), which further transformed by oxidative addition of H2 and HCl to [Ir(H)2(μ−meso-dpmppm−κ3)(η5−Cp*IrCl2)(CO)]Cl (4∙Cp*IrCl2) and [IrH(μ−meso-dpmppm−κ3) (η5−Cp*IrCl2)(CO)2]Cl2 (2∙Cp*IrCl2), respectively, and however addition of bulky Me2PhSiH resulted in a disproportionation mixture of [IrH(Me2PhSi)(μ−meso-dpmppm−κ3)(η5-Cp*IrCl2)(CO)]Cl (5a∙Cp*IrCl2) as well as 4∙Cp*IrCl2, indicating an allosteric influence by attaching Cp*IrCl2 unit on the uncoordinate phosphine of the {Ir(meso-dpmppm−κ3)} unsymmetric pincer unit in 1.

The reduction of cis-platinum(iv) complexes by ascorbate and in whole human blood models using 1H NMR and XANES spectroscopy.

The efficacy of platinum(iv) prodrugs depends on their relative resistance to reduction in the extra- and intra-cellular environments. In the study reported here we investigated the influence of the nature of the axial and equatorial ligands on the pathway of reduction of the platinum(iv) complexes by the endogenous reductant, ascorbate, and their relative resistance to reduction in human blood serum and in a whole human blood model. The pathway of reduction of platinum(iv) complexes in the presence of excess ascorbate was found to be dependent on the nature of their axial and equatorial ligands in that complexes with chloride in the equatorial sites lost either both axial ligands or combinations of axial and equatorial ligands while those with oxalate occupying the equatorial sites lost both axial ligands only. Using XANES spectroscopy, complexes with axial hydroxide ligands were found to be highly resistant to reduction in blood serum and were only slowly and incompletely reduced in whole blood. The dihydroxide complex with an oxalate ligand occupying the equatorial leaving group sites was more resistant to reduction, both in serum and in whole blood, than the complex with chloride ligands in these sites. cis, trans-[PtCl2(OAc)2(en)] and trans-[Pt(OAc)2(ox)(en)] were observed to be reduced rapidly and almost completely in whole blood but the latter was substantially resistant to reduction in human blood serum, and consequently demonstrates many of the features of an optimal platinum(iv) anticancer agent.

Spatial heterogeneity in the relationship between precipitation and carbon isotopic discrimination in C3 plants: Inferences from a global compilation

Abstract Understanding the causes of large variations in carbon isotopic composition of C3 plants (δ13CC3) is a key challenge for paleoenvironmental reconstruction. Several investigations have attempted to study the global patterns in the δ13CC3 values and their correlation with the precipitation variability. Sparse representation of dataset from low-latitude and equatorial regions in the existing global compilation hinders to understand the precipitation-δ13CC3 gradient at different latitudinal-bands. In the present study, available global compilation has been extended with the newly measured dataset, and the new compilation (ca. 1.5 times larger than previous dataset) comprises results from 720 sites across the world. All the δ13CC3 values have been converted to carbon isotopic discrimination between air and leaf (Δair-leaf). Mann-Whitney analysis indicates distinct region-specific distribution of the Δair-leaf values. The average Δair-leaf value in mid-latitude region of the Northern Hemisphere is ca. 2‰ lower than equatorial sites. Multiple linear models have been used to investigate relative influence of climatic (precipitation and temperature) and geographical factors (altitude and latitude) on the Δair-leaf values. This analysis shows that precipitation controls the Δair-leaf values in every latitudinal-bin though the effect of precipitation is negligible beyond 1500 mm. Furthermore, the Δair-leaf-precipitation gradient has a large spatial variability and the most prominent changes in the Δair-leaf values have been observed at arid regions in mid-latitudes of the Southern Hemisphere (maximum 1.3‰ 100 mm−1). Region-specific distribution of δ13C values of plant and Δair-leaf-precipitation gradient prompt to recalculate the relative abundance of C3–C4 plants in the Siwalik sections of the Himalayan foreland basin. Our results show that earlier investigations in the Siwaliks for the Middle-Late Miocene have underestimated the abundance of C4 plants by ca. 20%. The recalculation shows that C4 plants existed in these regions earlier (ca. 17 Ma) than the previously proposed timings.

Three closely related 1-[(1,3-benzodioxol-5-yl)methyl]-4-(halobenzo-yl)piperazines: similar mol-ecular structures but different inter-molecular inter-actions.

In each of the compounds 1-[(1,3-benzodioxol-5-yl)methyl]-4-(3-fluoro-benzo-yl)piperazine, C19H19FN2O3 (I), 1-[(1,3-benzodioxol-5-yl)methyl]-4-(2,6-di-fluoro-benzo-yl)piperazine, C19H18F2N2O3 (II), and 1-[(1,3-benzodioxol-5-yl)methyl]-4-(2,4-di-chloro-benzo-yl)piperazine, C19H19Cl2N2O3 (III), the piperazine rings adopt a chair conformation with the (1,3-benzodioxol-5-yl)methyl substituent occupying an equatorial site: the five-membered rings are all slightly folded across the O⋯O line leading to envelope conformations. The dihedral angle between the planar amidic fragment and the haloaryl ring is 62.97 (5)° in (I) but 77.72 (12)° and 75.50 (5)° in (II) and (III), respectively. Despite their similarity in constitution and conformation, the supra-molecular inter-actions in (I)-(III) differ: in (I), a combination of C-H⋯O and C-H⋯π(arene) hydrogen bonds links the mol-ecules into a three-dimensional framework structure, but there are no hydrogen bonds of any sort in either (II) or (III), although the structure of (III) contains a short Cl⋯Cl contact between inversion-related pairs of mol-ecules.

The crystal structure of 1-(2-iodo-benzo-yl)-4-(pyrimidin-2-yl)piperazine: a three-dimensional hydrogen-bonded framework, augmented by π-π stacking inter-actions and I⋯N halogen bonds.

In 1-(2-iodo-benzo-yl)-4-(pyrimidin-2-yl)piperazine, C15H15IN4O, the central piperazine ring adopts an almost perfect chair conformation with the pyrimidine substituent in an equatorial site. The planar amide unit makes a dihedral angle of 80.44 (7)° with the phenyl ring. A combination of C-H⋯O and C-H⋯π(arene) hydrogen bonds links the mol-ecules into a complex three-dimensional network structure, augmented by a π-π stacking inter-action and an I⋯N halogen bond, all involving different pairs of inversion-related mol-ecules. Comparisons are made with the structures of a number of related compounds.

Mechanochemical syntheses, crystal structures, and photo-luminescent properties of a new hydrazone and its nickel and cadmium complexes

A new hydrazone, ethyl Z-2-(pyridine-2yl-methylene)hydrazinecarboxylate (C9H11N3O2; Hpyec) and its metal complexes bis(ethyl Z-2-(pyridine-2yl-methylene)hydrazinecarboxylate)nickel(II) monohydrate, [Ni(pyec)2]·H2O (1) and bis(acetato)(ethyl Z-2-(pyridine-2yl-methylene)hydrazinecarboxylate)cadmium(II), [Cd(Hpyec)(CH3COO)2] (2), have been prepared by mechanochemical syntheses and characterized by elemental analysis, UV–vis, IR and NMR spectroscopies, TG-DTA, and solid state emission techniques. The X-ray single crystal structures of both complexes were determined. In 1, the Ni2+ ion is coordinated by two N,N,O-tridentate anionic ligands to generate a distorted cis-NiO2N4 octahedron. In 2, the cadmium ion is seven-coordinate by one neutral N,N,O-tridentate ligand and two chelating acetate ions to generate a very-distorted CdN2O5 pentagonal bipyramid with the Hpyec atoms occupying both axial and one equatorial site. Crystal data: 1, C18H22N6NiO5, Mr = 461.12, monoclinic, P21/c, a = 10.8759(3) Å, b = 11.7055(4) Å, c = 16.8424(5) Å, β = 108.583(1)°, V = 2032.38(11) Å3, Z = 4, R(F) = 0.023, wR(F2) = 0.065; 2, C13H17CdN3O6, Mr = 423.69, monoclinic, Cc, a = 13.0215(10) Å, b = 15.8104(11) Å, c = 7.99(6) Å, β = 96.692(2)°, V = 1621.7(2) Å3, Z = 4, R(F) = 0.017, wR(F2) = 0.038.

Exploring the DNA binding, oxidative cleavage, and cytotoxic properties of new ternary copper(II) compounds containing 4-aminoantipyrine and N,N-heterocyclic co-ligands

Herein we report on the synthesis and characterization of new water-soluble complexes with the basic formula [Cu(ON)(NN)(ClO4)2], where ON = 4-aminoantipyrine and NN = 1,10-phenanthroline (1) or 2,2ʹ-bipyridine (2). Both complexes have distorted tetragonal geometry around each copper centre, which is coordinated to both bidentate ligands in equatorial sites with two perchlorate ions weakly bonded in the axial positions. The compounds bind to DNA and induce oxidative DNA damage mediated by reactive oxygen species (ROS). Both complexes inhibit the growth of K562 cells in a concentration-dependent manner with IC50 values lower than the corresponding free ligands. Significantly, the most cytotoxic agent (complex 1) presented high in vitro nucleolytic activity by generating single- and double-strand breaks, besides of inhibiting topoisomerase I enzymatic activity at low micromolar concentration.

Crystal structures of [Cu2(2,2′-bipyridine-N,N′)2(H2O)2(μ2-OH)2](barbiturate)2·2H2O and [Cu(2,2′-bipyridine-N,N′)(H2O)(barbiturate-O)Cl]·2H2O

Two mixed-ligand Cu(II) complexes [Cu2(Bipy)2(H2O)2(OH)2](Hba)2·2H2O (1) and [Cu(Bipy)(H2O)(Hba)Cl]·2H2O (2) (Bipy = 2,2′-bipyridine and Hba− = barbiturate anion) were synthesized and structurally characterized by single crystal X-ray diffraction. In 1, structural data revealed distorted square-pyramidal geometry for each of two crystallographic independent copper(II) atom with the basal plane formed by two nitrogen atoms of the 2,2′-bipyridine ligand and two oxygen atoms of bridging hydroxo groups. The apical positions are filled by the oxygen atoms from a water molecule. In 2, the Cu atom is also in distorted square pyramidal geometry, where the equatorial sites are occupied by two N atoms of one Bipy ligand, one O atom of the barbiturate ion and one chloride ion and the apical position by one aqua ligand. Numerous intermolecular hydrogen bonds OH⋯O, NH⋯O, CH⋯O in the structures 1–2 and in addition OH⋯Cl, CH⋯Cl in the structure 2 form the 3D networks. Topological analysis showed that these H-bonded networks are new. An additional factor that plays a role in the crystal packing of the discussed complexes is the π-π stacking interaction between the aromatic rings of Hba− and Bipy forming infinite chains –Hba–Bipy–Bipy–Hba–. The spectroscopic and thermal properties of compounds were analyzed.

Site Selectivity of Hydride in Early-Transition-Metal Ruddlesden-Popper Oxyhydrides.

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH2 reduction from Ruddlesden-Popper Sr n+1Ti nO3 n+1 phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered Sr2TiO3.91(2)D0.14(1) material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H- has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system Sr2TiO3.875H0.125 is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered Sr3Ti2O6.20H0.12 material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the Sr3Ti2O6.875H0.125 model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V3+ electronic polaron is suggested to play a role in stabilizing H- on the equatorial site in Sr2VO4- xH x for x = 0.125.

Improvements of the MODIS Gross Primary Productivity model based on a comprehensive uncertainty assessment over the Brazilian Amazonia

Tropical forests and savannas are responsible for the largest proportion of global Gross Primary Productivity (GPP), a major component of the global carbon cycle. However, there are still deficiencies in the spatial and temporal information of tropical photosynthesis and its relations with environmental controls. The MOD17 product, based on the Light Use Efficiency (LUE) concept, has been updated to provide GPP estimates around the globe. In this research, the MOD17 GPP collections 5.0, 5.5 and 6.0 and their sources of uncertainties were assessed by using measurements of meteorology and eddy covariance GPP from eight flux towers in Brazilian tropical ecosystems, from 2000 to 2006. Results showed that the MOD17 collections tend to overestimate GPP at low productivity sites (bias between 111% and 584%) and underestimate it at high productivity sites (bias between −2% and −18%). Overall, the MOD17 product was not able to capture the GPP seasonality, especially in the equatorial sites. Recalculations of MOD17 GPP using site-specific meteorological data, corrected land use/land cover (LULC) classification, and tower-based LUE parameter showed improvements for some sites. However, the improvements were not sufficient to estimate the GPP seasonality in the equatorial forest sites. The use of a new soil moisture constraint on the LUE, based on the Evaporative Fraction, just showed improvements in water-limited sites. Modifications in the algorithm to account for separate LUE for cloudy and clear sky days presented noticeably improved GPP estimates in the tropical ecosystems investigated, both in magnitude and in seasonality. The results suggest that the high cloudiness makes the diffuse radiation an important factor to be considered in the LUE control, especially over dense forests. Thus, the MOD17 GPP algorithm needs more updates to accurately estimate productivity in tropical ecosystems.

Solvent and intermolecular nitrogen coordination dictated formation of self-assembled organostannane-macrocycles based on monomers and coordination polymers with unsymmetrical 5-[(E)-2-(4-pyridyl)-1-diazenyl]-2-hydroxybenzoate ligand: Structural topologies and dimensionality

A series of six new organotin(IV) compounds, namely, [nPr3Sn(HL)(MeOH)] 1, [nBu3Sn(HL)]n2, [Bz3Sn(HL)]n3, [Ph3Sn(HL)]n⋅nC6H64, [Ph3Sn(HL)(EtOH)] 5 and [nBu2Sn(L)]n6 were synthesized by reacting appropriate tri- and di-organotin(IV) precursors with the flexible pro-ligand 5-[(E)-2-(4-pyridyl)-1-diazenyl]-2-hydroxybenzoic acid. Their solid-state structures were deduced from single crystal X-ray diffraction data and, in the case of compound 6, also by solid-state 13C and 119Sn NMR spectroscopy. Compounds 1 and 5 are mononuclear as a consequence of methanol/ethanol coordination preventing the propagation of the polymers. The monodentate oxygen atom from the carboxylate group of HL− and the tin-coordinated oxygen atom of alcohol are situated at the axial positions, while Sn-R substituents are at the equatorial sites, defining the distorted trigonal-bipyramidal coordination polyhedron. Both 1 and 5 form one-dimensional (1D) chains created by virtue of O−Hsolv⋯Npy hydrogen bonding interactions, and antiparallel running chains in 1 furnish 34-membered dinuclear macrocyclic rings occupied by SnPr3 groups. Compounds 2–4 feature a one-dimensional coordination polymer facilitated by intermolecular N→Sn bond formation with the pyridyl substituent of LH−. The axially located OOCO and Npy donor atoms and equatorial Sn-R groups give rise to a trigonal-bipyramidal geometry. While similar coordination geometries are observed for 2–4, the tri-n-butyl derivative 2 generates 60-membered macrocyclic rings where four n-butyl groups from two neighboring molecules occupy the cavities. The triphenyltin analogue 4 forms two-dimensional layers with voids filled by benzene solvate molecules, and the sterically demanding tribenzyltin groups in compound 3 result in only relatively weak C−H⋯π, C−H⋯N and van der Waals contacts. On the other hand, compound 6 is a two-dimensional (2D) coordination polymer displaying a distorted pentagonal-bipyramidal geometry around the tin atom where four oxygen atoms of two ligand molecules and the nitrogen atom of a ligand constitute the equatorial plane, while the carbon atoms of the n-butyl groups complete the axial positions. The molecules of 6 self assembled to an interesting dimeric tecton based on a four-membered Sn2O2 ring, generating large 48-membered hexanuclear macrocycles, where the cavities are filled by four di-n-butyl groups from two neighboring molecules. The solution behaviors of compounds 1–5 were judged from the results of the 119Sn NMR spectroscopic characterization.

Long‐term limnological changes in the Ecuadorian páramo: Comparing the ecological responses to climate warming of shallow waterbodies versus deep lakes

Abstract Páramos are high‐altitude ecosystems of grasslands and shrubs that sustain high levels of biodiversity and contain numerous lakes, ponds and wetlands that are a crucial source of water for millions. In the Andes, limnological data are rare from páramos and particularly so from shallow waterbodies that are prominent features of the landscape. Here, we analyse fossil diatom assemblages using dated sediment cores from three shallow lakes in the páramo of southern Ecuador and document their response to recent climate changes. The two shallowest sites were <0.5 m deep and contained nearly identical diatom assemblages, dominated by Achnanthidium minutissimum and other benthic taxa. The deepest study site, at 4 m depth, differed notably from its shallower counterparts in that the dominant taxa were tychoplanktonic Aulacoseira species. All three study sites showed only minor assemblage shifts over the past ˜200 years. This contrasts sharply with paleolimnological data from nearby deep lakes (Zmax > 17 m) that recorded abrupt changes in diatom phytoplankton coincident with the onset of higher temperatures, reduced wind speeds and the onset of thermal stratification in recent decades. In temperate and high‐latitude regions, an overriding factor influencing freshwater ecosystems is the duration and extent of ice cover, which itself is closely linked to climate variables. In contrast, there is no winter ice cover and the growing season is continuous year round in our equatorial Andean sites. Instead, rising temperatures are affecting deep lakes primarily by altering the physical structure of the water column resulting in greater periods of thermal stratification, which in turn drives changes in biota and other lake processes. However, this mechanism of change only affects deep lakes because shallow waterbodies are easily mixed by wind. Our data demonstrate the differential response of large, deep lakes in the páramo compared to the relative complacency of changes in shallow waterbodies, which is in marked contrast to similar sites affected by seasonal ice cover.

Effect of Disturbance Electric Fields on Equatorial Electrojet Over Indian Longitudes

AbstractPrompt penetration (PP) and ionospheric disturbance dynamo (Ddyn) are two expressions of the disturbed electric field developed during a geomagnetic storm in the Earth's ionosphere. Earlier investigations treated these two processes independently over longitudinal separation beyond 20°. In the present study, we investigate the conjunction of these phenomena on equatorial electrojet at 20° longitude separations using daytime magnetic field data from three equatorial sites viz., Minicoy, Vencode, and Campbell Bay and three low‐latitude sites viz., Alibag, Hyderabad, and Nabagram, located within 20° longitude over Indian region, during three intense storms (Dst <− 150 nT). We propose a statistical method to identify PP signatures from large data set. In addition, we distinguish the signatures of PP and Ddyn during early recovery phase of intense storms and compute pure Ddyn signature in the late recovery phase. It is seen that the effect of PP is similar at the three equatorial sites for each storm. We find that signatures of Ddyn are amplified at Minicoy and Vencode compared to Campbell Bay, and their magnitude decreases toward low‐latitude stations, also reflected in the current vector patterns. Our investigations of intense geomagnetic storms show that PP effect dominates the main phase, followed by a combined effect of PP and Ddyn in the early recovery phase. Eventually the Ddyn signature dominates in the late recovery phase with decreasing amplitudes of Ddyn with increasing time. Such investigations can provide new information about variations in ionospheric parameters.

Synthesis, molecular structure, vibrational and theoretical studies of a new non-centrosymmetric organic sulphate with promising NLO properties

The new non-centrosymmetric inorganic-organic hybrid material tetrakis(2,6-dimethylpiperazine-1,4-diium) tetrakis(sulfate) dihydrate (DMPS) has been prepared and crystallized in the non-centrosymmetric space group Cc of the monoclinic system with the crystallographic parameters: a = 25.044 (4) Å, b = 12.1046 (18) Å, c = 14.075 (2) Å, β = 107.289 (5) °, V = 4074.0 (10) Å3 and Z = 4. The tetrahedron (S(1)O42−) and (S(4)O42−) anions are connected to water molecules forming as infinite chains type C44(12) along the c-axis in y = ¼ and ¾. The 2,6-dimethylpiperazine-1,4-diium cations are correlated to the inorganic part by means of NH⋯O and CH⋯O hydrogen bonds. The diprotonated piperazine ring endorses a chair conformation, with the methyl groups placing in an equatorial site. Intermolecular contacts in the crystal structure were computed by Hirshfeld surface examination. Infrared spectrum proves the presence of the functional groups in the synthesised salt and different coordination modes are observed in the sulfate groups. Optical propriety affirms a noteworthy band gap energy signifying steadiness of the title compound. Furthermore, the polarizability and first order hyperpolarizability of the title molecule were calculated and interpreted. Subsequently, theoretical calculations indicate that first order hyperpolarizability is 13 times greater than urea. Moreover, the thermodynamic properties (heat capacity, entropy, and enthalpy) of the title compound at different temperatures were calculated in gas phase. Additionally, two neutral, one anionic and the hydrated species experimentally observed by X-ray diffraction were identified in the vibrational infrared spectrum of DMPS combining the experimental spectrum with the corresponding predicted by using DFT calculations and the scaled quantum mechanical force field (SQMFF) procedure.

Directed synthesis and magnetic properties of a hexanuclear ferric cluster

The hexanuclear iron(III) complex, [Fe6(µ3-3-bpp)4(µ3-O)2(µ2-OMe)3.67(µ2-OH)0.33Cl2]·0.33MeOH·H2O (1) was synthesized by a redox reaction between FeCl2, AgNO2, and 3-H2bpp in methanol (3-H2bpp = 2,6-bis(3-pyrazolyl)pyridine). The crystal structure of the complex is composed of two trinuclear subunits related by an inversion operation. This symmetry results in an overall octahedral arrangement of Fe(III) sites within the cluster. The four equatorial Fe sites are linked together by µ2-OMe− and µ2-OH− bridges, while each of the four 3-bpp2− bridges spans two axial and one equatorial Fe sites. The two Cl− ligands cap the axial Fe sites. The difference in the coordination environment for the equatorial and axial Fe sites is validated by the observation of two quadrupole doublets in the Mössbauer spectrum of 1, which also confirms that all metal sites correspond to the high-spin Fe(III) ions. Temperature-dependent magnetic susceptibility data reveal strong antiferromagnetic exchange coupling between the Fe(III) centers, which is also justified by quantum-chemical calculations at the density-functional level of theory.