Hydrogen-bonding and resonance stabilisation effects in cationic bis(iminium) phenoxide diacids.

The synthesis and characterisation of a bis(iminium)phenoxide diacid cation [4-tBu-C6H2-2,6-(HCN(H)Dipp)-1-O]+ ([H2tBu,DippL]+), is discussed. [H2tBu,DippL][BF4] (1) and [H2tBu,DippL][H2N{B(C6F5)3}2] (2) were synthesised in high yields via protonation of the bis(imino)phenol conjugate base with ethereal HBF4 or Bochmann's acid ([H(OEt2)2][H2N{B(C6F5)3}2]). Both species were fully characterised using NMR and IR spectroscopy as well as X-ray crystallography. The cationic fragment adopts an unusual tautomeric form in which both acidic protons are located on the nitrogen atoms: [HN〈O〉NH]+. This bis(iminium) phenoxide tautomer is stabilised by delocalisation of electron density from oxygen, into the extended π-system of the planar cation, and was found to be 22.6 and 263.1 kJ mol-1 lower in energy (ΔG) than the alternative [N〈OH〉NH]+ and [N〈OH2〉N]+ tautomers respectively. Topological analysis confirmed the presence of two electrostatic N+H⋯O- hydrogen bonds which contribute -111.2 kJ mol-1 towards the stabilisation of the diacid. The pKa values of the cations were estimated, from NMR experiments, to be 4.2 in THF (1) and 11.4 in acetonitrile (2).

Recent advances in direct air capture by adsorption.

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO2 capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO2 adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO2 concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO2 adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

Multimetallic Permethylpentalene Hydride Complexes.

The synthesis and characterization of group 4 permethylpentalene (Pn* = C8Me6) hydride complexes are explored; in all cases, multimetallic hydride clusters were obtained. Group 4 lithium metal hydride clusters were obtained when reacting the metal dihalides with hydride transfer reagents such as LiAlH4, and these species featured an unusual hexagonal bipyramidal structural motif. Only the zirconium analogue was found to undergo hydride exchange in the presence of deuterium. In contrast, a trimetallic titanium hydride cluster was isolated on reaction of the titanium dialkyl with hydrogen. This diamagnetic, mixed valence species was characterized in the solid state, as well as by solution electron paramagnetic resonance and nuclear magnetic resonance spectroscopy. The structure was further probed and corroborated by density functional theory calculations, which illustrated the formation of a metal-cluster bonding orbital responsible for the diamagnetism of the complex. These permethylpentalene hydride complexes have divergent structural motifs and reactivity in comparison with related classical cyclopentadienyl analogues.

Ring-Opening Polymerisation of Low-Strain Nickelocenophanes: Synthesis and Magnetic Properties of Polynickelocenes with Carbon and Silicon Main Chain Spacers.

Polymetallocenes based on ferrocene, and to a lesser extent cobaltocene, have been well-studied, whereas analogous systems based on nickelocene are virtually unexplored. It has been previously shown that poly(nickelocenylpropylene) [Ni(η5 -C5 H4 )2 (CH2 )3 ]n is formed as a mixture of cyclic (6x ) and linear (7) components by the reversible ring-opening polymerisation (ROP) of tricarba[3]nickelocenophane [Ni(η5 -C5 H4 )2 (CH2 )3 ] (5). Herein the generality of this approach to main-chain polynickelocenes is demonstrated and the ROP of tetracarba[4]nickelocenophane [Ni(η5 -C5 H4 )2 (CH2 )4 ] (8), and disila[2]nickelocenophane [Ni(η5 -C5 H4 )2 (SiMe2 )2 ] (12) is described, to yield predominantly insoluble homopolymers poly(nickelocenylbutylene) [Ni(η5 -C5 H4 )2 (CH2 )4 ]n (13) and poly(tetramethyldisilylnickelocene) [Ni(η5 -C5 H4 )2 (SiMe2 )2 ]n (14), respectively. The ROP of 8 and 12 was also found to be reversible at elevated temperature. To access soluble high molar mass materials, copolymerisations of 5, 8, and 12 were performed. Superconducting quantum interference device (SQUID) magnetometry measurements of 13 and 14 indicated that these homopolymers behave as simple paramagnets at temperatures greater than 50 K, with significant antiferromagnetic coupling that is notably larger in carbon-bridged 6x /7 and 13 compared to the disilyl-bridged 14. However, the behaviour of these polynickelocenes deviates substantially from the Curie-Weiss law at low temperatures due to considerable zero-field splitting.

Preparation of two dimensional layered double hydroxide nanosheets and their applications.

Layered double hydroxides (LDHs) with their highly flexible and tunable chemical composition and physical properties have attracted tremendous attention in recent years. LDHs have found widespread application as catalysts, anion exchange materials, fire retardants, and nano-fillers in polymer nanocomposites. The ability to exfoliate LDHs into ultrathin nanosheets enables a range of new opportunities for multifunctional materials. In this review we summarize the current available LDH exfoliation methods. In particular, we highlight recent developments for the direct synthesis of single-layer LDH nanosheets, as well as the emerging applications of LDH nanosheets in catalyzing oxygen evolution reactions and preparing light emitting devices, supercapacitors, and flame retardant nanocomposites.

Fabrication of Lithium Silicates As Highly Efficient High-Temperature CO2 Sorbents from SBA-15 Precursor.

A series of lithium silicates with improved CO2 sorption capacity were successfully synthesized using SBA-15 as the silicon precursor. The influence of Li/Si ratio, calcination temperature, and calcination duration on the chemical composition and CO2 capture capacity of obtained lithium silicates was systematically investigated. The correlation between CO2 sorption performance and crystalline phase abundance was determined using X-ray diffraction and a normalized reference intensity ratio method. Under the optimized condition, Li-SBA15-4 prepared using Li/Si = 4 that contains mainly Li4SiO4 achieved an extremely high CO2 capture capacity of 36.3 wt % (corresponding to 99% of the theoretical value of 36.7 wt % for Li4SiO4), which is much higher than the Li4SiO4 synthesized from conventional SiO2 sources. It also showed very high cycling stability with only 1.0 wt % capacity loss after 15 cycles. Li-SBA15-10 (Li/Si = 10) that mainly contains Li8SiO6 displayed an extremely high CO2 uptake of 62.0 wt %, but its regeneration capacity was poor, with only 10.5 wt % of reversible CO2 capture capacity. The influence of CO2 concentration on the CO2 capture performance of Li-SBA15-4 and Li-SBA15-10 samples was also studied. With the decrease in CO2 concentration, relatively lower temperatures are needed for its maximum CO2 capture capacity. The CO2 sorption kinetics and mechanism for Li-SBA15-4 and Li-SBA15-10 samples were explored. Overall, we have shown that the lithium silicates synthesized from SBA-15 possessed much improved CO2 sorption performance than that attained from conventional SiO2.

Controlling the Surface Hydroxyl Concentration by Thermal Treatment of Layered Double Hydroxides.

Layered double hydroxides (LDHs) are important materials in the field of catalyst supports, and their surface hydroxyl functionality makes them interesting candidates for supporting well-defined single-site catalysts. Here, we report that the surface hydroxyl concentration can be controlled by thermal treatment of these materials under vacuum, leading to hydroxyl numbers (αOH) similar to those of dehydroxylated silica, alumina, and magnesium hydroxide. Thermal treatment of [Mg0.74Al0.26(OH)2](SO4)0.1(CO3)0.03·0.62(H2O)·0.04(acetone) prepared by the aqueous miscible organic solvent treatment method (Mg2.84Al-SO4-A AMO-LDH) is shown to yield a mixed metal oxide above 300 °C by a combination of thermogravimetric analysis, powder X-ray diffraction (PXRD), BET surface area analysis, and FTIR spectroscopy. PXRD shows the disappearance of the characteristic LDH 00l peaks at 300 °C indicative of decomposition to the layered structure, coupled with a large increase in the BET surface area (95 vs 158 m2 g-1 from treatment at 275 and 300 °C, respectively). Titration of the surface hydroxyls with Mg(CH2Ph)2(THF)2 indicates that the hydroxyl number is independent of surface area for a given treatment temperature. Treatment at 450 °C under vacuum produces a mixed metal oxide material with a surface hydroxyl concentration (αOH) of 2.14 OH nm-2 similar to the hydroxyl number (αOH) of 1.80 OH nm-2 for a sample of SiO2 dehydroxylated at 500 °C. These materials appear to be suitable candidates for use as single-site organometallic catalyst supports.

Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTi-LDH Precursors: An Active Water Oxidation Electrocatalyst.

Faceted NiO nanoparticles preferentially exposing high surface energy planes demand attention due to their excellent electrocatalytic properties. However, the activity of faceted NiO nanoparticles generally remains suboptimal due to their large lateral size and thickness, which severely limits the availability of coordinatively unsaturated active reactive edge and corner sites. Here, ultrafine NiO nanosheets with a platelet size of ∼4.0 nm and thickness (∼1.1 nm) stabilized by TiO2 were successfully prepared by calcination of a monolayer layered double hydroxide precursor. The ultrafine NiO nanosheets displayed outstanding performance in electrochemical water oxidation due to a high proportion of reactive NiO {110} facets, intrinsic Ni(3+) and Ti(3+) sites, and abundant interfaces, which act synergistically to promote H2O adsorption and facilitate charge-transfer.

Simultaneous Differential Scanning Calorimetry-Synchrotron X-ray Powder Diffraction: A Powerful Technique for Physical Form Characterization in Pharmaceutical Materials.

We report a powerful new technique: hyphenating synchrotron X-ray powder diffraction (XRD) with differential scanning calorimetry (DSC). This is achieved with a simple modification to a standard laboratory DSC instrument, in contrast to previous reports which have involved extensive and complex modifications to a DSC to mount it in the synchrotron beam. The high-energy X-rays of the synchrotron permit the recording of powder diffraction patterns in as little as 2 s, meaning that thermally induced phase changes can be accurately quantified and additional insight on the nature of phase transitions obtained. Such detailed knowledge cannot be gained from existing laboratory XRD instruments, since much longer collection times are required. We demonstrate the power of our approach with two model systems, glutaric acid and sulfathiazole, both of which show enantiotropic polymorphism. The phase transformations between the low and high temperature polymorphs are revealed to be direct solid-solid processes, and sequential refinement against the diffraction patterns obtained permits phase fractions at each temperature to be calculated and unit cell parameters to be accurately quantified as a function of temperature. The combination of XRD and DSC has further allowed us to identify mixtures of phases which appeared phase-pure by DSC.

Exchange of Coordinated Solvent During Crystallization of a Metal-Organic Framework Observed by In Situ High-Energy X-ray Diffraction.

Using time-resolved monochromatic high energy X-ray diffraction, we present an in situ study of the solvothermal crystallisation of a new MOF [Yb2(BDC)3(DMF)2]⋅H2O (BDC=benzene-1,4-dicarboxylate and DMF=N,N-dimethylformamide) under solvothermal conditions, from mixed water/DMF solvent. Analysis of high resolution powder patterns obtained reveals an evolution of lattice parameters and electron density during the crystallisation process and Rietveld analysis shows that this is due to a gradual topochemical replacement of coordinated solvent molecules. The water initially coordinated to Yb(3+) is replaced by DMF as the reaction progresses.

In†Situ Observation of Successive Crystallizations and Metastable Intermediates in the Formation of Metal-Organic Frameworks.

Understanding the driving forces controlling crystallization is essential for the efficient synthesis and design of new materials, particularly metal-organic frameworks (MOFs), where mild solvothermal synthesis often allows access to various phases from the same reagents. Using high-energy in situ synchrotron X-ray powder diffraction, we monitor the crystallization of lithium tartrate MOFs, observing the successive crystallization and dissolution of three competing phases in one reaction. By determining rate constants and activation energies, we fully quantify the reaction energy landscape, gaining important predictive power for the choice of reaction conditions. Different reaction rates are explained by the structural relationships between the products and the reactants; larger changes in conformation result in higher activation energies. The methods we demonstrate can easily be applied to other materials, opening the door to a greater understanding of crystallization in general.

Time-Resolved In†Situ X-ray Diffraction Reveals Metal-Dependent Metal-Organic Framework Formation.

Versatility in metal substitution is one of the key aspects of metal-organic framework (MOF) chemistry, allowing properties to be tuned in a rational way. As a result, it important to understand why MOF syntheses involving different metals arrive at or fail to produce the same topological outcome. Frequently, conditions are tuned by trial-and-error to make MOFs with different metal species. We ask: is it possible to adjust synthetic conditions in a systematic way in order to design routes to desired phases? We have used in situ X-ray powder diffraction to study the solvothermal formation of isostructural M2 (bdc)2 dabco (M=Zn, Co, Ni) pillared-paddlewheel MOFs in real time. The metal ion strongly influences both kinetics and intermediates observed, leading in some cases to multiphase reaction profiles of unprecedented complexity. The standard models used for MOF crystallization break down in these cases; we show that a simple kinetic model describes the data and provides important chemical insights on phase selection.

Synthesis, Structure, and Bonding for Bis(permethylpentalene)diiron.

The synthesis of the first homoleptic double metallocene complex of iron, Fe2Pn*2 (Pn* = permethylpentalene, C8Me6) is described. The structural and electronic properties of Fe2Pn*2 have been characterized by NMR and EPR spectroscopy, single crystal X-ray diffraction, magnetic measurements, cyclic voltammetry, and DFT calculations. Fe2Pn*2 adopts a Ci symmetry in the solid state with a Fe-Fe distance of 2.3175(9) Å, slightly lower than the sum of radii in metallic iron. Magnetic measurements in solution, and of the solid phase between 60 and 300 K, indicate that Fe2Pn*2 is a triplet (S = 1) paramagnet, with effective magnetic moments (µeff) of 3.4 and 3.48 µB, respectively. DFT calculations indicate the origin of this high magnetic moment is likely to be unquenched orbital angular momentum contributions from two SOMOs which have metal d character. Cyclic voltammetry studies demonstrate that Fe2Pn*2 can access four charge states (-1, 0, +1, +2).

Ring-opening polymerization of a strained [3]nickelocenophane: a route to polynickelocenes, a class of S = 1 metallopolymers.

We report the synthesis, reactivity studies, and ring-opening polymerization of a tricarba[3]nickelocenophane. The resulting green polynickelocene (5) possesses a -(CH2)3- spacer between the nickelocene units and is shown to be of high molecular weight. SQUID magnetometry measurements indicate that 5 is a macromolecular material with an S = 1 repeat unit.

Ammonia-rich high-temperature superconducting intercalates of iron selenide revealed through time-resolved in situ X-ray and neutron diffraction.

The development of a technique for following in situ the reactions of solids with alkali metal/ammonia solutions, using time-resolved X-ray diffraction methods, reveals high-temperature superconducting ammonia-rich intercalates of iron selenide which reversibly absorb and desorb ammonia around ambient temperatures.

Boosting the effectiveness of UV filters and sunscreen formulations using photostable, non-toxic inorganic platelets.

We have studied the size-dependent optical scattering of aqueous suspensions containing Mg2Al-LDH platelets, which exhibit high total- and side-scatterings. By incorporating 3 wt% Mg2Al-LDH platelets (280 nm) in a commercial sunscreen formulation, we achieved a twofold Sun Protection Factor boost, providing a promising, high-efficient and non-toxic strategy to enhance sunscreen effectiveness.

Mixing and matching N,N- and N,O-chelates in anionic Mg(I) compounds: synthesis and reactivity with RNCNR and CO.

Reduction of [Mg(NON)]2 ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) affords Mg(I) species containing NON- and NNO-ligands ([NNO]2- = [N(Dipp)SiMe2N(Dipp)SiMe2O]2-). The products of reactions with iPrNCNiPr and CO are consistent with the presence of reducing Mg(I) centres. Extraction with THF affords [K(THF)2]2[(NNO)Mg-Mg(NNO)] with a structurally characterised Mg-Mg bond that was examined using density functional theory.

Europium insertion into MgAl hydrotalcite-like compound to promote the photocatalytic oxidation of nitrogen oxides.

Easy synthesis of efficient, non-toxic photocatalysts is a target to expand their potential applications. In this research, the role of Eu3+ doping in the non-toxic, affordable, and easily prepared MgAl hydrotalcite-like compounds (HTlcs) was explored in order to prepare visible light semiconductors. Eu doped MgAl-HTlcs (MA-xEu) samples were prepared using a simple coprecipitation method (water, room temperature and atmospheric pressure) and europium was successfully incorporated into MgAl HTlc frameworks at various concentrations, with x (Eu3+/M3+ percentage) ranging from 2 to 15. Due to the higher ionic radius and lower polarizability of Eu3+ cation, its presence in the metal hydroxide layer induces slight structural distortions, which eventually affect the growth of the particles. The specific surface area also increases with the Eu content. Moreover, the presence of Eu3+ 4f energy levels in the electronic structure enables the absorption of visible light in the doped MA-xEu samples and contributes to efficient electron-hole separation. The microstructural and electronic changes induced by the insertion of Eu enable the preparation of visible light MgAl-based HTlcs photocatalysts for air purification purposes. Specifically, the optimal HTlc photocatalyst showed improved NOx removal efficiency, ∼ 51% (UV-Vis) and 39% (visible light irradiation, 420 nm), with excellent selectivity (> 96 %), stability (> 7 h), and enhanced release of •O2- radicals. Such results demonstrate a simple way to design photocatalytic HTlcs suitable for air purification technologies.

Trends in Structure and Ethylene Polymerization Reactivity of Transition-Metal Permethylindenyl-phenoxy (PHENI*) Complexes.

A family of ansa-permethylindenyl-phenoxy (PHENI*) transition-metal chloride complexes has been synthesized and characterized (1-7; {(η5-C9Me6)Me(R″)Si(2-R-4-R'-C6H2O)}MCl2; R,R' = Me, tBu, Cumyl (CMe2Ph); R″ = Me, nPr, Ph; M = Ti, Zr, Hf). The ancillary chloride ligands could readily be exchanged with halides, alkyls, alkoxides, aryloxides, or amides to form PHENI* complexes [L]TiX2 (8-17; X = Br, I, Me, CH2SiMe3, CH2Ph, NMe2, OEt, ODipp). The solid-state crystal structures of these PHENI* complexes indicate that one of two conformations may be preferred, parametrized by a characteristic torsion angle (TA'), in which the η5 system is either disposed away from the metal center or toward it. Compared to indenyl PHENICS complexes, the permethylindenyl (I*) ligand appears to favor a conformation in which the metal center is more accessible. When heterogenized on solid polymethylaluminoxane (sMAO), titanium PHENI* complexes exhibit exceptional catalytic activity toward the polymerization of ethylene. Substantially greater activities are reported than for comparable PHENICS catalysts, along with the formation of ultrahigh-molecular-weight polyethylenes (UHMWPE). Catalyst-cocatalyst ion pairing effects are observed in cationization experiments and found to be significant in homogeneous catalytic regimes; these effects are also related to the influence of the ancillary ligand leaving groups in slurry-phase polymerizations. Catalytic efficiency and polyethylene molecular weight are found to increase with pressure, and PHENI* catalysts can be categorized as being among the most active for the controlled synthesis of UHMWPE.

Interplay between Defects and Short-Range Disorder Manipulating the Oxygen Evolution Reaction on a Layered Double Hydroxide Electrocatalyst.

Improving the efficiency of the oxygen evolution reaction (OER) is crucial for advancing sustainable and environmentally friendly hydrogen energy. Layered double hydroxides (LDHs) have emerged as promising electrocatalysts for the OER. However, a thorough understanding of the impact of structural disorder and defects on the catalytic activity of LDHs remains limited. In this work, a series of NiAl-LDH models are systematically constructed, and their OER performance is rigorously screened through theoretical density functional theory. The acquired results unequivocally reveal that the energy increase induced by structural disorder is effectively counteracted at the defect surface, indicating the coexistence of defects and disorder. Notably, it is ascertained that the simultaneous presence of defects and disorder synergistically augments the catalytic activity of LDHs in the context of the OER. These theoretical findings offer valuable insights into the design of highly efficient OER catalysts while also shedding light on the efficacy of LDH electrocatalysts.