Cocatalyst effects in Hf-catalysed olefin polymerization: taking well-defined Al-alkyl borate salts into account.

Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of CS-symmetric (Hf-CS-Met) and C2-symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu2(PhNMe2)Al]2(µ-H)}+[B(C6F5)4]- (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity (e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al.

A Hydrocarbon Soluble, Molecular and "Complete" Al-Cocatalyst for High Temperature Olefin Polymerization.

The dinuclear aluminum salt {[iBu2(DMA)Al]2(µ-H)}+[B(C6F5)4]- (AlHAl; DMA = N,N-dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant (s-AlHAl) bearing p-hexadecyl-N,N-dimethylaniline (DMAC16) units, which enhances solubility in aliphatic hydrocarbons. The novel s-AlHAl was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process.

Efficient Hydrogenation of N-Heterocycles Catalyzed by NNP-Manganese(I) Pincer Complexes at Ambient Temperature.

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions.

Halide perovskites as disposable epitaxial templates for the phase-selective synthesis of lead sulfochloride nanocrystals.

Colloidal chemistry grants access to a wealth of materials through simple and mild reactions. However, even few elements can combine in a variety of stoichiometries and structures, potentially resulting in impurities or even wrong products. Similar issues have been long addressed in organic chemistry by using reaction-directing groups, that are added to a substrate to promote a specific product and are later removed. Inspired by such approach, we demonstrate the use of CsPbCl3 perovskite nanocrystals to drive the phase-selective synthesis of two yet unexplored lead sulfochlorides: Pb3S2Cl2 and Pb4S3Cl2. When homogeneously nucleated in solution, lead sulfochlorides form Pb3S2Cl2 nanocrystals. Conversely, the presence of CsPbCl3 triggers the formation of Pb4S3Cl2/CsPbCl3 epitaxial heterostructures. The phase selectivity is guaranteed by the continuity of the cationic subnetwork across the interface, a condition not met in a hypothetical Pb3S2Cl2/CsPbCl3 heterostructure. The perovskite domain is then etched, delivering phase-pure Pb4S3Cl2 nanocrystals that could not be synthesized directly.

Classical Force-Field Parameters for CsPbBr3 Perovskite Nanocrystals.

Understanding the chemico-physical properties of colloidal semiconductor nanocrystals (NCs) requires exploration of the dynamic processes occurring at the NC surfaces, in particular at the ligand-NC interface. Classical molecular dynamics (MD) simulations under realistic conditions are a powerful tool to acquire this knowledge because they have good accuracy and are computationally cheap, provided that a set of force-field (FF) parameters is available. In this work, we employed a stochastic algorithm, the adaptive rate Monte Carlo method, to optimize FF parameters of cesium lead halide perovskite (CsPbBr3) NCs passivated with typical organic molecules used in the synthesis of these materials: oleates, phosphonates, sulfonates, and primary and quaternary ammonium ligands. The optimized FF parameters have been obtained against MD reference trajectories computed at the density functional theory level on small NC model systems. We validated our parameters through a comparison of a wide range of nonfitted properties to experimentally available values. With the exception of the NC-phosphonate case, the transferability of the FF model has been successfully tested on realistically sized systems (>5 nm) comprising thousands of passivating organic ligands and solvent molecules, just as those used in experiments.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

The Reactivity of CsPbBr3 Nanocrystals toward Acid/Base Ligands.

The interaction of lead bromide perovskite nanocrystals with charged ligands, such as salts, zwitterions, or acid-base pairs, has been extensively documented over the past few years. On the other hand, little is known about the reactivity of perovskite nanocrystals toward neutral ligands. To fill this gap, in this work we study the interaction of CsPbBr3 nanocrystals passivated with didodecyldimethylammonium bromide (DDABr) toward a series of exogenous acid/base ligands using a combined computational and experimental approach. Our analysis indicates that DDABr-capped nanocrystals are inert toward most ligands, except for carboxylic, phosphonic, and sulfonic acids. In agreement with the calculations, our experimental results indicate that the higher the acidity of the ligands employed in the treatment, the more etching is observed. In detail, dodecylbenzenesulfonic acid (pKa = -1.8) is found to etch the nanocrystals, causing their complete degradation. On the other hand, oleic and oleylphosphonic acids (pKa 9.9 and 2, respectively) interact with surface-bound DDA molecules, causing their displacement as DDABr in various amounts, which can be as high as 40% (achieved with oleylphosphonic acid). Despite the stripping of DDA ligands, the optical properties of the nanocrystals, as well as structure and morphology, remain substantially unaffected, empirically demonstrating the defect tolerance characterizing such materials. Our study provides not only a clear overview on the interaction between perovskite nanocrystals and neutral ligands but also presents an effective ligand stripping strategy.

How Divalent Cations Interact with the Internal Channel Site of Guanine Quadruplexes.

The front cover artwork is provided by the TheoCheM group of the Vrije Universiteit Amsterdam. The image shows that guanine quadruplexes have a stronger binding affinity for divalent cations than monovalent cations. Read the full text of the Article at 10.1002/cphc.202100529.

Hafnium vs. Zirconium, the Perpetual Battle for Supremacy in Catalytic Olefin Polymerization: A Simple Matter of Electrophilicity?

The performance of C2-symmetric ansa-hafnocene catalysts for isotactic polypropylene typically deteriorates at increasing temperature much faster than that of their zirconium analogues. Herein, we analyze in detail a set of five Hf/Zr metallocene pairs-including some of the latest generation catalysts-at medium- to high-polymerization temperature. Quantitative structure-activity relationship (QSAR) models for stereoselectivity, the ratio allyl/vinyl chain ends, and 2,1/3,1 misinsertions in the polymer indicate a strong dependence of polymerization performance on electrophilicity of the catalyst, which is a function of the ligand framework and the metal center. Based on this insight, the stronger performance decline of hafnocenes is ascribed to electrophilicity-dependent stabilization effects.

How Divalent Cations Interact with the Internal Channel Site of Guanine Quadruplexes.

The formation of guanine quadruplexes (GQ) in DNA is crucial in telomere homeostasis and regulation of gene expression. Pollution metals can interfere with these DNA superstructures upon coordination. In this work, we study the affinity of the internal GQ channel site towards alkaline earth metal (Mg2+ , Ca2+ , Sr2+ , and Ba2+ ), and (post-)transition metal (Zn2+ , Cd2+ , Hg2+ , and Pb2+ ) cations using density functional theory computations. We find that divalent cations generally bind to the GQ cavity with a higher affinity than conventional monovalent cations (e. g. K+ ). Importantly, we establish the nature of the cation-GQ interaction and highlight the relationship between ionic and nuclear charge, and the electrostatic and covalent interactions. The covalent interaction strength plays an important role in the cation affinity and can be traced back to the relative stabilization of cations' unoccupied atomic orbitals. Overall, our findings contribute to a deeper understanding of how pollution metals could induce genomic instability.

Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation.

The degradation pathways of highly active [Cp*Ir(κ2 -N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO3 ), 2 undergoes partial protonation of the amide moiety, inducing rapid κ2 -N,N to κ2 -N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ2 -N,N key intermediate of FA dehydrogenation (INH ), bearing a N-protonated pica, can easily transform into the κ2 -N,O analogue (INH2 ; ΔG≠ ≈11 kcal mol-1 , ΔG ≈-5 kcal mol-1 ). Intramolecular hydrogen liberation from INH2 is predicted to be rather prohibitive (ΔG≠ ≈26 kcal mol-1 , ΔG≈23 kcal mol-1 ), indicating that FA dehydrogenation should involve mostly κ2 -N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c. DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.

Iridium-Doped Nanosized Zn-Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts.

Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO4, turnover number up to ∼10,000). Nanomaterials 1-3 display the highest ever reported turnover frequency values (up to 402 min-1) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 µm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications.

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A-C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation.

Three M-doped LDHs (M = noble metal active site, LDH = layered double hydroxides; Ir-1, Ir-ZnAl; Ru, Ru-ZnAl; Ir-2, Ir-MgAl), containing small amounts of M (ca. 2 mol% and even <1 mol% for Ru and Ir, respectively), were prepared by following simple and established synthetic procedures. Their characterization indicates that M atoms are effectively incorporated into the brucite-like layers of LDH, without phase segregation. The resulting materials catalyse electrochemical water oxidation (WO), when immobilized in carbon paste electrodes, with performances that exceed those of the benchmark system IrO2, as probed by linear sweep voltammetry (LSV). Some of these catalysts undergo continuous activation upon chronoamperometric and chronopotentiometric treatments over several hours. The crystalline structure of all of them is preserved during electrocatalytic experiments, and no significant leaching of noble metal in solution is detected. The results herein reported highlight the remarkable potential of these doped M-LDHs and confirm that dispersing Ir and Ru centers in layered and cheap inorganic materials results in easily accessible metal centers, providing highly active catalysts, while minimizing the utilization of noble metals.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands.

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)2(µ-κ3-O,N,O-H4-EDTMP)] (Ir-H4-EDTMP, H4-EDTMP4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)2(µ-κ3-O,N,O-EDTA)] (Ir-EDTA, EDTA4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO2 nanocrystals obtaining Ir-H4-EDTMP@TiO2 and Ir-EDTA@TiO2 hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO4, providing almost quantitative yields, and TON values only limited by the amount of NaIO4 used. As for the molecular catalysts, Ir-H4-EDTMP (TOF up to 184 min-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H4-EDTMP@TiO2 (up to 33 min-1) and Ir-EDTA@TiO2 (up to 41 min-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

Aggregation induced emission of a new naphthyridine-ethynyl-gold(i) complex as a potential tool for sensing guanosine nucleotides in aqueous media.

A new organometallic alkynyl-gold(i) complex capable of exhibiting aggregation induced emission was designed and synthesized. The linear complex structure possesses a central Au(i) atom, bearing two axial ligands: (1) 1,3,5-triaza-7-phosphaadamantane and (2) 2-acetamido-7-ethynyl-1,8-naphthyridine. While the former accounts for its partial solubility in an aqueous environment, the latter acts as a receptor unit for binding guanosine nucleotides and derivatives via multiple hydrogen bonding interactions. At high concentrations, aggregation of the complex was observed by the formation of new absorption (λmax∼ 400 nm) and emission bands (550-700 nm). Formation of aggregates of ca. 60 nm diameter was confirmed by Small Angle X-ray Scattering (SAXS). Disruption of the aggregates in the presence of guanosine derivatives resulted in a ratiometric signal with apparent association constants in the order of 105 M-1 and high sensitivity (around 63% signal change) which are, to the best of our knowledge, in line with the highest values recorded for nucleotide sensors based on hydrogen bonding and capable of working in water. Computational studies indicate the presence of additional hydrogen bonding interactions that account for the strong binding of the Au(i) complex to phosphorylated guanosine nucleotides.

Extraction of Reliable Molecular Information from Diffusion NMR Spectroscopy: Hydrodynamic Volume or Molecular Mass?

Measuring accurate translational self-diffusion coefficients (Dt ) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (VH ) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the Dt values of three isosteric Cp2 MCl2 complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that VH is the critical molecular property affecting Dt . This central concept served to clarify the assumptions behind the derivation of Dt =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (Dt ) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules.

Secondary Electrostatic Interaction Model Revised: Prediction Comes Mainly from Measuring Charge Accumulation in Hydrogen-Bonded Monomers.

The secondary electrostatic interaction (SEI) model is often used to predict and explain relative hydrogen bond strengths of self-assembled systems. The SEI model oversimplifies the hydrogen-bonding mechanisms by viewing them as interacting point charges, but nevertheless experimental binding strengths are often in line with the model's predictions. To understand how this rudimentary model can be predictive, we computationally studied two tautomeric quadruple hydrogen-bonded systems, DDAA-AADD and DADA-ADAD. Our results reveal that when the proton donors D (which are electron-donating) and the proton acceptors A (which are electron-withdrawing) are grouped together as in DDAA, there is a larger accumulation of charge around the frontier atoms than when the proton donor and acceptor groups are alternating as in DADA. This accumulation of charge makes the proton donors more positive and the proton acceptors more negative, which enhances both the electrostatic and covalent interactions in the DDAA dimer. The SEI model is thus predictive because it provides a measure for the charge accumulation in hydrogen-bonded monomers. Our findings can be understood from simple physical organic chemistry principles and provide supramolecular chemists with meaningful understanding for tuning hydrogen bond strengths and thus for controlling the properties of self-assembled systems.

RNA versus DNA G-Quadruplex: The Origin of Increased Stability.

DNA quadruplexes have been the subject of investigation because of their biological relevance and because of their potential application in supramolecular chemistry. Similarly, RNA quadruplexes are now gaining increasing attention. Although DNA and RNA quadruplexes are structurally very similar, the latter show higher stability. In this study we report dispersion-corrected density functional theory (DFT-D) quantum chemical calculations that were undertaken to understand the difference in stabilities of RNA and DNA quadruplexes. The smallest meaningful model of a stack of quartets, interacting with alkali metal cations, was simulated in an aqueous environment. The energy decomposition analysis allows for in-depth examination of the interaction energies, emphasising the role of noncovalent interactions and better electrostatics in determining RNA-GQs higher stabilities, particularly pinpointing the role of the extra 2'-OH groups. Furthermore, our computations present new insights on why the cation is required for self-assembly: unexpectedly the cation is not necessary to relieve the repulsion between the oxygen atoms in the central cavity, but it is needed to overcome the entropic penalty.

Oxidation of organic diselenides and ditellurides by H2O2 for bioinspired catalyst design.

The reactivity of diselenides and ditellurides of general formula (RX)2 (X = Se, Te; R = H, CH3, Ph) toward hydrogen peroxide was studied through a computational approach based on accurate Density Functional Theory (DFT) calculations. The aliphatic and aromatic dichalcogenides have been chosen in light of their activity in glutathione peroxidase (GPx)-like catalytic cycles and their promising features as efficient antioxidant compounds. The reaction products, the energetics and the mechanistic details of these oxidations are discussed. Analogous disulfides are included in our analysis for completeness. We find that the barrier for oxidation of dichalcogenides decreases from disulfides to diselenides to ditellurides. On the other hand, variation of the substituents at the chalcogen nucleus has relatively little effect on the reactivity.