Carbocation Mechanism Revelation of Molecular Iodine-Mediated Dehydrogenative Aromatization of Substituted Cyclic Ketones to Phenols.

Dehydrogenative aromatization (DA) of cyclic ketones is central to the development of functionalized aromatic precursors and hydrogen transfer-related technologies. Traditional DA strategies require precious metals with oxidants and are typically performed at high temperatures (100-150 °C) to overcome the high energy barrier of aliphatic C-H bond activation. Recently, a mild alternative approach based on I2 has been proposed to realize DA on substituted unsaturated cyclic ketones under ambient conditions. However, depending on the solvent, the product selectivity may vary between phenol ether and phenol, and the reaction mechanisms remain unclear. Herein, based on time-resolved proton nuclear magnetic resonance, DFT calculation, and mass spectrometric analyses, we established a unified mechanism to account for the product distribution. Through substrate scope and desorption electrospray ionization-mass spectrometry, we discovered the formation of a carbocation, which has been overlooked in previous studies. An expanded substrate scope study coupled with spectroscopic observation provided strong evidence to elucidate the formation mechanism and the location of the carbocation. With a renewed understanding of the mechanism, we achieved a phenolic product yield of 17-96% while controlling the selectivity. Moreover, some reactants could undergo DA in H2O, achieving 95-96% yield at below water-boiling temperature.

Toward Property-Based Regulation.

An expanding web of adverse impacts on people and the environment has been steadily linked to anthropogenic chemicals and their proliferation. Central to this web are the regulatory structures intended to protect human and environmental health through the control of new molecules. Through chronically insufficient and inefficient action, the current chemical-by-chemical regulatory approach, which considers regulation at the level of chemical identity, has enabled many adverse impacts to develop and persist. Recognizing the link between fundamental physicochemical properties and hazards, we describe a new paradigm─property-based regulation. By regulating physicochemical properties, we show how governments can delineate and enforce safe chemical spaces, increasing the scalability of chemical assessments, reducing the time and resources to regulate a substance, and providing transparency for chemical designers. We highlight sparse existing property-based approaches and demonstrate their applicability using bioaccumulation as an example. Finally, we present a path to implementation in the United States, prescribing roles and steps for government, nongovernmental organizations, and industry to accelerate this transition, to the benefit of all.

Selective adsorption of arsenic over phosphate by transition metal cross-linked chitosan.

The ability of transition metal chitosan complexes (TMCs) of varying valence and charge to selectively adsorb As(III) and As(V) over their strongest adsorptive competitor, phosphate is examined. Fe(III)-chitosan, Al(III)-chitosan, Ni(II)-chitosan, Cu(II)-chitosan, and Zn(II)-chitosan are synthesized, characterized via Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) and X-ray Diffractometry (XRD), and their selective sorption capabilities towards As(III) and As(V) in the presence of phosphate are evaluated. It was found that the stability of the metal-chitosan complexes varied, with Al(III)- and Zn(II)-chitosan forming very unstable complexes resulting in precipitation of gibbsite, and Wulfingite and Zincite, respectively. Cu(II)-, Ni(II)-, and Fe(III)-chitosan formed a mixture of monodentate and bidentate complexes. The TMCs which formed the bidentate complex (Cu(II)-, Ni(II)-, and Fe(III)-) showed greater adsorption capability for As(V) in the presence of phosphate. Using the binary separation factor ∝t/c, it can be shown that only Fe(III)-chitosan is selective for As(V) and As(III) over phosphate. Density Functional Theory (DFT) modeling and extended X-ray adsorption fine structure (EXAFS) determined that Fe(III)-chitosan and Ni(II)-chitosan adsorbed As(V) and As(III) via inner-sphere complexation, while Cu(II)-chitosan formed mainly outer-sphere complexes with As(V) and As(III). These differences in complexation likely result in the observed differences in selective adsorption capability towards As(V) and As(III) over phosphate. It is hypothesized that the greater affinity of Fe(III)- and Ni(II)-chitosan towards As(V) and As(III) compared to Cu(II)-chitosan is due to their forming less-stable, more reactive chitosan complexes as predicted by the Irving Williams Series.

Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption.

Development of novel adsorbents often neglects the competitive adsorption between co-occurring oxo-anions, overestimating realistic pollutant removal potentials, and overlooking the need to improve selectivity of materials. This critical review focuses on adsorptive competition between commonly co-occurring oxo-anions in water and mechanistic approaches for the design and development of selective adsorbents. Six "target" oxo-anion pollutants (arsenate, arsenite, selenate, selenite, chromate, and perchlorate) were selected for study. Five "competing" co-occurring oxo-anions (phosphate, sulfate, bicarbonate, silicate, and nitrate) were selected due to their potential to compete with target oxo-anions for sorption sites resulting in decreased removal of the target oxo-anions. First, a comprehensive review of competition between target and competitor oxo-anions to sorb on commonly used, nonselective, metal (hydr)oxide materials is presented, and the strength of competition between each target and competitive oxo-anion pair is classified. This is followed by a critical discussion of the different equations and models used to quantify selectivity. Next, four mechanisms that have been successfully utilized in the development of selective adsorbents are reviewed: variation in surface complexation, Lewis acid/base hardness, steric hindrance, and electrostatic interactions. For each mechanism, the oxo-anions, both target and competitors, are ranked in terms of adsorptive attraction and technologies that exploit this mechanism are reviewed. Third, given the significant effort to evaluate these systems empirically, the potential to use computational quantum techniques, such as density functional theory (DFT), for modeling and prediction is explored. Finally, areas within the field of selective adsorption requiring further research are detailed with guidance on priorities for screening and defining selective adsorbents.

Insight into the Interactions of Amyloid ß-Sheets with Graphene Flakes: Scrutinizing the Role of Aromatic Residues in Amyloids that Interact with Graphene.

The interaction of amyloid ß-sheet segments with graphene-flake models is investigated by using DFT calculations. The structure of ß-sheets of selected amyloid segments is based on crystal structures obtained from the Protein Data Bank. Our study, based on DFT calculations for model systems, indicates that the interaction in amyloid-graphene aggregates can be stronger than the interaction in the respective amyloid-amyloid aggregates. The results also indicate an important specific role of aromatic sidechains in amyloid-graphene interactions. This work confirms recent experimental evidence that graphene and its modifications inhibit the aggregation of ß-amyloid peptides.

Methane Activations by Titanium Neopentylidene Complexes: Electronic Resilience and Steric Control.

The titanium neopentylidene complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2-) is capable of activating both sp2 and sp3 C-H bonds under mild conditions. In addition to methane C-H activation, competition between the initial hydrogen abstraction reaction to form the methane activation product and the tautomerization reaction of this product to form a terminal methylidene was also explored. Several modifications of the PNP and CHtBu ligands were explored to determine the effect of these changes on C-H bond activation. In general, on the one hand, the modifications involving electronic effects have small and inconsistent influence on the stability of the intermediates and products and on the reaction barriers. On the other hand, the use of bulky groups in the ligands favors the methane activation process. By replacing the iPr groups in the PNP ligand with tBu groups, both methane activation and tautomerization reactions become more energetically favorable than in the unmodified complex. On the one hand, the largest acceleration of the methane C-H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine. On the other hand, replacing the nitrogen in the PNP ligand by phosphorus results in lower barriers for the tautomerization reaction and the stabilization of the product of the tautomerization although it remains slightly less stable than product of methane C-H activation. While several ligand modifications related to the electronic effects were examined, it is interesting that most of them did not make a significant change on the barriers for either reaction, indicating a significant resilience of this titanium complex, which could be used to enhance the practical aspects of the complex without a significant loss of its activity.

Unexpected Importance of Aromatic-Aliphatic and Aliphatic Side Chain-Backbone Interactions in the Stability of Amyloids.

The role of aromatic and nonaromatic amino acids in amyloid formation has been elucidated by calculating interaction energies between ß-sheets in amyloid model systems using density functional theory (B3LYP-D3/6-31G*). The model systems were based on experimental crystal structures of two types of amyloids: (1) with aromatic amino acids, and (2) without aromatic amino acids. Data show that these two types of amyloids have similar interaction energies, supporting experimental findings that aromatic amino acids are not essential for amyloid formation. However, different factors contribute to the stability of these two types of amyloids. In the former, the presence of aromatic amino acids significantly contributes to the strength of interactions between side chains; interactions between aromatic and aliphatic side chains are the strongest, followed by aromatic-aromatic interactions, while aliphatic-aliphatic interactions are the weakest. In the latter, that is, the amyloids without aromatic residues, stability is provided by interactions of aliphatic side chains with the backbone and, in some cases, by hydrogen bonds.

The stacking interactions of bipyridine complexes: the influence of the metal ion type on the strength of interactions.

The strength of the stacking interactions in the bipy complexes of nickel, palladium, and platinum, [M(CN)2 bipy]2 (M = Ni, Pd, Pt), was calculated using the ωB97xD/def2-TZVP method. The results show that for all considered geometries, interactions are the strongest for platinum, and weakest for nickel complexes, as a result of higher dispersion contributions of platinum over the palladium and nickel complexes. It was also shown that strength of interactions considerably rises with an increase of the stacking overlap area. As a consequence of the favorable electrostatic term, the strength of interactions also rises when metal atom and cyano ligands are involved in the overlap with bipy ligand. The strongest interaction was calculated in the platinum complex, for the geometry that has overlap of metal and cyano ligands with bipy ligand with an energy of -39.80 kcal mol(-1). The energies for similar geometries of palladium and nickel complexes are -34.60 and -32.45 kcal mol(-1). These energies, remarkably, exceed the strength of the stacking interactions between organic aromatic molecules. These results can be of importance in all systems with stacking interactions, from materials to biomolecules.

Experimental and theoretical investigations of the self-association of oxaliplatin.

Self-aggregation in water of anti-cancer agents such as oxaliplatin (1) or its palladium-containing parent (2) is suspected to be the main reason for the exceptional resistance of concentrated infusions of these complexes to hydrolysis; this hypothesis, i.e. the self-association of metal chelates, was investigated in a systematic manner by experimental and theoretical means. (1)H diffusion-ordered NMR spectroscopy (DOSY NMR) and UV-visible absorption titration were inconclusive as to the formation of a dimer of 1 in water or DMSO. Further isothermal titration calorimetry (ITC) methods allowed the accurate determination of the enthalpy of formation of only the homodimer [2]2 and putative heterodimer [1·2] together with an estimation of the formation constants, which indicate that dimer formation is not a spontaneous process in solution, whereas electrospray ESI mass spectroscopy tends to suggest the contrary in the gas phase. A dispersion-corrected DFT method, i.e. DFT-D (BLYP-D3), was used to model the aggregation in solution (COSMO) and to investigate the assisting role of London force in the cohesion of bimolecular aggregates. The concordance of experimental and theoretical thermodynamic parameters was judged reasonably even though the treatment of solvation by conventional continuum models does not account for specific interactions of the solute with molecules of solvent; nonetheless these results outline the importance of dispersion, a.k.a. London force. The role of the latter was further stressed by computing the affinities of 1 and 2 for the lipophilic cavity of cucurbit[7]uril in modeled water (COSMO-RS), which were preliminarily determined experimentally by ITC methods using pure water as solvent. From our investigations carried out in pure water the connection between the notorious chemical stability of "concentrated" infusions of 1 in aqueous media and the formation of oligomers remains unsettled.

Geometries of stacking interactions between phenanthroline ligands in crystal structures of square-planar metal complexes.

Stacking interactions of phenanthroline square-planar complexes in crystal structures were studied by analyzing data from the Cambridge Structural Database. In most of the crystal structures, two phenanthroline complexes were oriented "head to tail." Phenanthroline complexes show a wide range of overlap geometries in stacking interactions, while short metal-metal distances were not observed. Stacking chains with alternating overlaps were the predominant type of packing in the crystal structures.