Versatile scorpionates and new developments in the denticity changes of NNCp hybrid scorpionate/cyclopentadienyl ligands in Sc and Y compounds: from kappa1-Neta5-Cp to kappa2-NNeta5-Cp

Reaction of hybrid scorpionate/cyclopentadienyl ligands in the form of the lithium derivatives [Li(bpzcp)(THF)] [bpzcp=2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethylcyclopentadienyl], [Li(bpztcp)(THF)] [bpztcp=2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-butylethylcyclopentadienyl], and the in situ-generated "Li(bpzpcp)" [bpzpcp=2,2-bis(3,5-dimethylpyrazol-1-yl)-1-phenylethylcyclopentadienyl] with MCl3(THF)3 afforded the group 3 halide compounds [MCl2(bpzcp)(THF)] (M=Sc, 1; Y, 2), [MCl2(bpztcp)(THF)] (M=Sc, 3; Y, 4), and [MCl2(bpzpcp)(THF)] (M=Sc, 5; Y, 6). The H2O adduct of 4, [YCl2(bpztcp)(H2O)] (7), was formed when a solution of 4 was allowed to stand at room temperature in the presence of moisture. Complexes 1-7 adopt a pseudo-octahedral structure with heteroscorpionate ligands kappa2-NNeta5-Cp coordinated to the metal through the cyclopentadienyl group and two imino nitrogens of pyrazole rings. The alkyl heteroscorpionate scandium and yttrium complexes recently reported by our group, [M(CH2SiMe3)2(bpzcp)], react with 2,6-dimethylphenol and 3,5-dimethylphenol to give the bis(aryloxide) derivatives [M(OAr)2(bpzcp)] (M=Sc, OAr=2,6-dimethylphenoxide, 8; M=Y, OAr=2,6-dimethylphenoxide, 9; M=Y, OAr=3,5-dimethylphenoxide, 10). Complex 9 underwent an interesting hydrolysis process to give the tetranuclear complex [{Y(bpzcp)}(micro-OH)2(micro3-OH){Y(OAr)2}]2 (11). Variable-temperature 1H NMR experiments on 9 and 10 revealed a rapid fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers in which the scorpionate ligand can be coordinated in a kappa1-Neta5-Cp form. The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structures of 2, 7, and 11 were also established. Complexes 1 and 2 are active olefin polymerization catalysts after activation with methylaluminoxane. These compounds gave atactic polystyrenes with narrow molecular weight distribution (Mn/Mw 1.26-1.91) and with low molecular weights.     

Continuum percolation of long lifespan clusters in a simple fluid

We present results on the percolation loci for chemical clusters and physical clusters of long lifespan. Chemical clusters are defined as sets of particles connected through particle-particle bonds that last for a given time tau. Physical clusters are sets of particles that remain close together at every instant for a given period of time tau. By using molecular dynamics simulations of a Lennard-Jones system we obtain the percolation loci at different values of tau as the lines in the temperature-density plane at which the system presents a spanning cluster in 50% of the configurations. We find that the percolation loci for chemical clusters shifts rapidly toward high densities as tau is increased. For moderate values of tau this line converges to the low-density branch of the liquid-solid coexistence curve. This implies that no stable chemical clusters can be found in the fluid phase. In contrast, the percolation loci for physical clusters tend to a limiting line, as tau tends to infinity, which is far from the liquid-solid transition line.     

NHC-Adducts of Cyclopentadienyl-Substituted Alanes

The mono- and bis-iodo-substituted NHC-stabilized alanes (NHC) ⋅ AlH₂I and (NHC) ⋅ AlHI₂ offer a convenient entry for further substitution reactions at aluminum. Reactions of (NHC) ⋅ AlH₂I 1 – 4 with one equivalent of NaCp afforded the adducts (NHC) ⋅ AlH₂Cp 9 – 12 (NHC=Me₂Im^Me ( 9 ), iPr₂Im^Me ( 10 ), iPr₂Im ( 11 ), Dipp₂Im ( 12 )). Alane adducts with two Cp substituents (NHC) ⋅ AlHCp₂ 13 – 16 (NHC=Me₂Im^Me ( 13 ), iPr₂Im^Me ( 14 ), iPr₂Im ( 15 ), Dipp₂Im ( 16 )) were prepared by the analogous reaction of (NHC) ⋅ AlHI₂ 5 – 8 using two equivalents of NaCp. The unusual dimeric adducts ((NHC) ⋅ AlH₂Cp ⋅ CpMgI)₂ 17 – 19 (NHC=Me₂Im^Me ( 17 ), iPr₂Im^Me ( 18 ), iPr₂Im ( 19 )) were obtained from the reaction of 1 – 3 with MgCp₂.     

Fensterbierscheiben in the Pena National Palace collection – chemical and iconographic relations

The stained‐glass collection from the Pena National Palace (Sintra, Portugal) includes around 130 ‘rural panels’, also known as Fensterbierscheiben, that were produced between the 16th and 19th centuries. The aim of this investigation is to characterise the glass composition of this collection of Fensterbierscheiben and relate it with the iconographic research made on these panels, in order to establish possible provenance of production. This is the first study on Fensterbierscheiben, where the chemical information of the glass is considered and related with historical information. The micro‐energy dispersive X‐ray fluorescence allowed performing non‐invasive analysis, mostly performed in situ. Micro‐particle‐induced X‐ray emission analysis was performed on the cross section of a small group of fragmented panels for obtaining quantitative chemical composition of the glass. Through the analysis of the colourless glass, and the comparison of micro‐energy dispersive X‐ray fluorescence and micro‐particle‐induced X‐ray emission data, it was concluded that the majority of the panes have a high lime low alkali glass composition. Furthermore, the Fensterbierscheiben panes form a cohesive group in terms of composition, suggesting that they were all manufactured with raw materials from the same region. This study also allowed one to observe the chronological evolution in terms of treatments applied to the used raw materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Assortment optimization under a multinomial logit model with position bias and social influence

Motivated by applications in retail, online advertising, and cultural markets, this paper studies the problem of finding an optimal assortment and positioning of products subject to a capacity constraint in a setting where consumers preferences can be modeled as a discrete choice under a multinomial logit model that captures the intrinsic product appeal, position biases, and social influence. For the static problem, we prove that the optimal assortment and positioning can be found in polynomial time. This is despite the fact that adding a product to the assortment may increase the probability of selecting the no-choice option, a phenomenon not observed in almost all models studied in the literature. We then consider the dynamics of such a market, where consumers are influenced by the aggregate past purchases. In this dynamic setting, we provide a small example to show that the natural and often used policy known as popularity ranking, that ranks products in decreasing order of the number of purchases, can reduce the expected profit as times goes by. We then prove that a greedy policy that applies the static optimal assortment and positioning at each period, always benefits from the popularity signal and outperforms any policy where consumers cannot observe the number of past purchases (in expectation).     

Relative Gorenstein Dimensions

In the last years (Gorenstein) homological dimensions relative to a semidualizing module C have been subject of several works as interesting extensions of (Gorenstein) homological dimensions. In this paper, we extend to the noncommutative case the concepts of G _C -projective module and dimension, weakening the condition of C being semidualizing as well. We prove that indeed they share the principal properties of the classical ones and relate this new dimension with the classical Gorenstein projective dimension of a module. The dual concepts of G _C -injective modules and dimension are also treated. Finally, we show some interesting interactions between the class of G _C -projective modules and the Bass class associated to C on one side, and the class of G\({_{C^{\vee}}}\) -injective modules (C ^∨ = Hom _R (C, E) where E is an injective cogenerator in R-Mod) and the Auslander class associated to C in the other.     

Analytical method for biomonitoring of endocrine-disrupting compounds (bisphenol A,parabens, perfluoroalkyl compounds and a brominated flame retardant) in human hair by liquid chromatography-tandem mass spectrometry

In this paper, a method for the determination of four groups of endocrine-disrupting compounds in human hair is proposed. Target compounds were a plastic monomer (bisphenol A), three parabens commonly used as preservatives (methylparaben, ethylparaben and propylparaben), six perfluoroalkyl compounds commonly used as water, oil and dirt repellents (perfluorooctane sulfonic acid and five perfluoroalkyl carboxylic acids, with alkyl chains from four to eight carbon atoms) and a brominated flame retardant (hexabromocyclododecane). All of them are of especial concern to human health because they are utilized in many everyday products. The method is based on hair incubation with methanol/acetic acid solution (85:15, v/v), extraction with acetone for 15 min in an ultrasonic bath and analysis by liquid chromatography-electrospray-tandem mass spectrometry in negative ionization mode. Limits of quantification in hair samples ranged from 0.6 ng g⁻¹ to 6.1 ng g⁻¹, except for hexabromocyclododecane (36 ng g⁻¹). Recoveries were higher than 69%. Intra-day and inter-day precision, expressed as relative standard deviation, were lower than 15% and 10%, respectively. The applicability of the method was proven by analyzing the target compounds in hair samples from six volunteers. High frequencies of detection and concentrations were obtained for bisphenol A (83% of samples; concentrations up to 158 ng g⁻¹) and parabens (100% of samples; concentrations up to 624 ng g⁻¹). Lower concentrations were detected for the perfluoroalkyl compounds (up to 13 ng g⁻¹). Hexabromocyclododecane was not detected.     

Melanopsin and the Non‐visual Photochemistry in the Inner Retina of Vertebrates

Melanopsin (Opn4), a member of the G‐protein‐coupled receptor family, is a vitamin A‐based opsin in the vertebrate retina that has been shown to be involved in the synchronization of circadian rhythms, pupillary light reflexes, melatonin suppression and other light‐regulated tasks. In nonmammalian vertebrates there are two Opn4 genes, Opn4m and Opn4x, the mammalian and Xenopus orthologs respectively. Opn4x is only expressed in nonmammalian vertebrates including reptiles, fish and birds, while Opn4m is found in a subset of retinal ganglion cells (RGCs), the intrinsically photosensitive (ip) RGCs of the inner retina of both mammals and nonmammalian vertebrates. All opsins described utilize retinaldehyde as chromophore, photoisomerized from 11‐cis‐ to all‐trans‐retinal upon light exposure. Visual retinal photoreceptor cones and rods, responsible for day and night vision respectively, recycle retinoids through a process called the visual cycle that involves the retinal pigment epithelium or glial Müller cells. Although Opn4 has been characterized as a bistable photopigment, little is known about the mechanism/s involved in its chromophore regeneration. In this review, we will attempt to shed light on the visual cycle taking place in the inner retina and discuss the state of the art in the nonvisual photochemistry of vertebrates.