Disorder and Halide Distributions in Cesium Lead Halide Nanocrystals as Seen by Colloidal 133Cs Nuclear Magnetic Resonance Spectroscopy.

Colloidal nuclear magnetic resonance (cNMR) spectroscopy on inorganic cesium lead halide nanocrystals (CsPbX3 NCs) is found to serve for noninvasive characterization and quantification of disorder within these structurally soft and labile particles. In particular, we show that 133Cs cNMR is highly responsive to size variations from 3 to 11 nm or to altering the capping ligands on the surfaces of CsPbX3 NCs. Distinct 133Cs signals are attributed to the surface and core NC regions. Increased heterogeneous broadening of 133Cs signals, observed for smaller NCs as well as for long-chain zwitterionic capping ligands (phosphocholines, phosphoethanol(propanol)amine, and sulfobetaines), can be attributed to more significant surface disorder and multifaceted surfaces (truncated cubes). On the contrary, capping with dimethyldidodecylammonium bromide (DDAB) successfully reduces signal broadening owing to better surface passivation and sharper (001)-bound cuboid shape. DFT calculations on various sizes of NCs corroborate the notion that the surface disorder propagates over several octahedral layers. 133Cs NMR is a sensitive probe for studying halide gradients in mixed Br/Cl NCs, indicating bromide-rich surfaces and chloride-rich cores. On the contrary, mixed Br/I NCs exhibit homogeneous halide distributions.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection.

Copper(II)-containing mordenite (CuMOR) is capable of activation of C-H bonds in C1 -C3 alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with CuI sites, formed upon reduction of CuII -oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.

Deciphering the Mechanism of Crystallization of UiO-66 Metal-Organic Framework.

Zirconium-containing metal-organic framework (MOF) with UiO-66 topology is an extremely versatile material, which finds applications beyond gas separation and catalysis. However, after more than 10 years after the first reports introducing this MOF, understanding of the molecular-level mechanism of its nucleation and growth is still lacking. By means of in situ time-resolved high-resolution mass spectrometry, Zr K-edge X-ray absorption spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray diffraction it is showed that the nucleation of UiO-66 occurs via a solution-mediated hydrolysis of zirconium chloroterephthalates, whose formation appears to be autocatalytic. Zirconium-oxo nodes form directly and rapidly during the synthesis, the formation of pre-formed clusters and stable non-stoichiometric intermediates are not observed. The nuclei of UiO-66 possess identical to the crystals local environment, however, they lack long-range order, which is gained during the crystallization. Crystal growth is the rate-determining step, while fast nucleation controls the formation of the small crystals of UiO-66 with a narrow size distribution of about 200 nanometers.

The coordination chemistry of 2,4,6-oxy functionalised 1,3,5-triphosphinines.

A number of stable group 6 metal complexes bearing 2,4,6-oxy functionalised 1,3,5-triphosphinines, phosphorus containing heterocyclic ligands with a central C3P3 core, were synthesised such that a complete series of [M{P3C3(OX)3}(CO)3] compounds is obtained [M = Cr(0), Mo(0), W(0); X = H, SitBuPh2, B(ipc)2]. In all complexes, the triphosphinine coordinates in a η6-binding mode via the delocalized 6π-system of the ring. The ligand properties can be tuned by changing the substituent on the oxygen centre. The π-electron accepting properties of the ligand increases in the following order: P3C3(OH)3 < P3C3(OSitBuPh2)3 < P3C3(OB(ipc)2)3. This trend is reflected in the structures determined by X-ray crystallography, and the ν(CO) stretching frequencies determined by IR spectroscopy. The collected data raise questions with respect to the frequently made assumption that phosphinines act as stronger π-acceptors with respect to arenes and thereby deplete electron density at the metal centres. With P3C3(OH)3 as an η6-coordinated ligand further molecules can be coordinated in the second coordination sphere via hydrogen bonds, which may be of interest for the construction of coordination polymers.

Unraveling the molecular mechanism of MIL-53(Al) crystallization.

The vast structural and chemical diversity of metal-organic frameworks (MOFs) provides the exciting possibility of material's design with tailored properties for gas separation, storage and catalysis. However, after more than twenty years after first reports introducing MOFs, the discovery and control of their synthesis remains extremely challenging due to the lack of understanding of mechanisms of their nucleation and growth. Progress in deciphering crystallization pathways depends on the possibility to follow conversion of initial reagents to products at the molecular level, which is a particular challenge under solvothermal conditions. The present work introduces a detailed molecular-level mechanism of the formation of MIL-53(Al), unraveled by combining in situ time-resolved high-resolution mass-spectrometry, magic angle spinning nuclear magnetic resonance spectroscopy and X-ray diffraction. In contrast to the general belief, the crystallization of MIL-53 occurs via a solid-solid transformation mechanism, associated with the spontaneous release of monomeric aluminum. The role of DMF hydrolysis products, formate and dimethylamine, is established. Our study emphasizes the complexity of MOF crystallization chemistry, which requires case-by-case investigation using a combination of advanced in situ methods for following the induction period, the nucleation and growth across the time domain.

Structure and Framework Association of Lewis Acid Sites in MOR Zeolite.

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D 1H, 15N, and 27Al MAS NMR spectroscopy, carried out at different temperatures (from 298 down to 107 K), we were able to obtain the NMR spectroscopic signatures of LAS and Brønsted acid sites (BAS) in mordenite zeolite in the presence and the absence of adsorbed pyridine (Py). Combined with DFT modeling, this information enabled the structure of LAS to be revealed, namely (≡SiO)3Al sites interacting with pyridine, thus indicating that the corresponding base-free framework-associated sites are pseudo tricoordinated Al sites, namely tricoordinated Al sites interacting with an additional coordinated adjacent siloxane bridge. With this information in hand, we propose a molecular-level understanding on how the AlIV and AlVI framework and framework-associated sites evolve upon dehydration and exposure to Py into BAS and LAS, and their associated Py adducts. By measuring and analyzing the changes in quadrupolar coupling constants (CQ) that reflect electrical charge distribution around the nuclei, we further show that the lower CQ values observed at 298 K are due to residual dynamics that makes the electric field around aluminum nuclei more symmetric. Thus, NMR spectroscopic signatures of 27Al greatly vary with temperature; this information illustrates the importance of accounting for the temperature effect when confronting experimental and calculated CQ values of the corresponding aluminum sites in zeolites in order to obtain accurate structural assessment.

Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts.

The nitrogen oxides NO2 , NO, and N2 O are among the most potent air pollutants of the 21st century. A bimetallic RhI -PtII complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3-4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2 O) to water and N2 was recorded, making these RhI -PtII complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON=38) or nitrogen dioxide (NO2 , TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2 O, followed by the generation of N2 O and H2 O, which is then further converted to N2 and H2 O. The nitrogen-nitrogen bond-forming step takes place in the conversion from NO to N2 O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2 O2 2- ) complex, which was detected as an intermediate.

Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance.

Lead-halide perovskites increasingly mesmerize researchers because they exhibit a high degree of structural defects and dynamics yet nonetheless offer an outstanding (opto)electronic performance on par with the best examples of structurally stable and defect-free semiconductors. This highly unusual feature necessitates the adoption of an experimental and theoretical mindset and the reexamination of techniques that may be uniquely suited to understand these materials. Surprisingly, the suite of methods for the structural characterization of these materials does not commonly include nuclear magnetic resonance (NMR) spectroscopy. The present study showcases both the utility and versatility of halide NMR and NQR (nuclear quadrupole resonance) for probing the structure and structural dynamics of CsPbX3 (X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong quadrupole couplings, which originate from the interaction between the large quadrupole moments of, e.g., the 35Cl, 79Br, and 127I nuclei, and the local electric-field gradients, are highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. It is shown that space-averaged site-disorder is greatly enhanced in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals. The findings from NMR and NQR were corroborated by ab initio molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.

Lead-Halide Scalar Couplings in 207Pb NMR of APbX3 Perovskites (A = Cs, Methylammonium, Formamidinium; X = Cl, Br, I).

Understanding the structure and dynamics of newcomer optoelectronic materials - lead halide perovskites APbX3 [A = Cs, methylammonium (CH3NH3+, MA), formamidinium (CH(NH2)2+, FA); X = Cl, Br, I] - has been a major research thrust. In this work, new insights could be gained by using 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopy at variable temperatures between 100 and 300 K. The existence of scalar couplings 1JPb-Cl of ca. 400 Hz and 1JPb-Br of ca. 2.3 kHz could be confirmed for MAPbX3 and CsPbX3. Diverse and fast structure dynamics, including rotations of A-cations, harmonic and anharmonic vibrations of the lead-halide framework and ionic mobility, affect the resolution of the coupling pattern. 207Pb NMR can therefore be used to detect the structural disorder and phase transitions. Furthermore, by comparing bulk and nanocrystalline CsPbBr3 a greater structural disorder of the PbBr6-octahedra had been confirmed in a nanoscale counterpart, not readily captured by diffraction-based techniques.

Quantitative measures of myo-IP6 in soil using solution 31P NMR spectroscopy and spectral deconvolution fitting including a broad signal.

Inositol phosphates, particularly myo-inositol hexakisphosphate (myo-IP6), are an important pool of soil organic phosphorus (P) in terrestrial ecosystems. To measure concentrations of myo-IP6 in alkaline soil extracts, solution 31P nuclear magnetic resonance (NMR) spectroscopy is commonly used. However, overlap of the NMR peaks of myo-IP6 with several other peaks in the phosphomonoester region requires spectral deconvolution fitting (SDF) to partition the signals and quantify myo-IP6. At present, two main SDF approaches are in use; the first fits a Lorentzian/Gaussian lineshape to the myo-IP6 peaks directly to the baseline without an underlying broad signal, and the second fits a Lorentzian/Gaussian lineshape to the myo-IP6 peaks simultaneously with an underlying broad peak. The aim of this study was to compare the recovery of added myo-IP6 to soil extracts using both SDF procedures for six soil samples of diverse origin and differing concentrations of organic P (112 to 1505 mg P per kgsoil). The average recovery of total added myo-IP6 was 95% (SD 5) and 122% (SD 32) using SDF with and without an underlying broad signal, respectively. The recovery of individual peaks of myo-IP6 differed, most notably, the C5 phosphate peak of myo-IP6 was overestimated by up to 213% when a broad peak was not included in SDF. Based on the SDF procedure that includes a broad peak, concentrations of myo-IP6 ranged from 0.6 to 90.4 mg P per kgsoil, which comprised 1-23% of total phosphomonoesters. Our results demonstrate that the SDF procedure with an underlying broad signal is essential for the accurate quantification of myo-IP6 in soil extracts.

Pathways of Methane Transformation over Copper-Exchanged Mordenite as Revealed by In†Situ NMR and IR Spectroscopy.

The reaction of methane with copper-exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic CuII species. Several types of surface-bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono- and bidentate formates, CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper-exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.

Low-valent homobimetallic Rh complexes: influence of ligands on the structure and the intramolecular reactivity of Rh-H intermediates.

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(i) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two C[triple bond, length as m-dash]C bonds of the trop ligand. This reaction is chemoselective and converts C[triple bond, length as m-dash]C bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H2 activation by interfacial sites of heterogeneous Rh/C catalysts.

Substituent-controlled, mild oxidative fluorination of iodoarenes: synthesis and structural study of aryl I(iii)- and I(v)-fluorides.

We report a mild approach to the synthesis of difluoro(aryl)-λ3-iodanes (aryl-IF2 compounds) and tetrafluoro(aryl)-λ5-iodanes (aryl-IF4 compounds) using trichloroisocyanuric acid (TCICA) and potassium fluoride (KF). Under these reaction conditions, selective access to either the I(iii)- or I(v)-derivatives is predictable based solely on the substitution pattern of the iodoarene starting material. Moreover, the discovery of this TCICA/KF approach prompted detailed dynamic NMR, kinetic, computational, and crystallographic studies on the relationship between the IF2 group and the ortho-substituents on carefully designed probe molecules. It was during these experiments that the role of the ortho-substituent in inhibiting further oxidative fluorination of I(iii)-compounds to I(v)-compounds during the reaction with TCICA and KF was revealed. Additionally, a notable exception to this empirical trend is discussed herein.

Determining the predominant tautomeric structure of iodine-based group-transfer reagents by 17O NMR spectroscopy.

Cyclic benziodoxole systems have become a premier scaffold for the design of electrophilic transfer reagents. A particularly intriguing aspect is the fundamental II-IIII tautomerism about the hypervalent bond, which has led in certain cases to a surprising re-evaluation of the classic hypervalent structure. Thus, through a combination of 17O NMR spectroscopy at natural abundance with DFT calculations, we establish a convenient method to provide solution-phase structural insights for this class of ubiquitous reagents. In particular, we confirm that Shen's revised, electrophilic SCF3-transfer reagent also adopts an "acyclic" thioperoxide tautomeric form in solution. After calibration, the approach described herein likely provides a more general and direct method to distinguish between cyclic and acyclic structural features based on a single experimental 17O NMR spectrum and a computationally-derived isotropic shift value. Furthermore, we apply this structural elucidation technique to predict the constitution of an electrophilic iodine-based cyano-transfer reagent as an NC-I-O motif and study the acid-mediated activation of Togni's trifluoromethylation reagent.

Elucidating the Distribution and Speciation of Boron and Cesium in BCsX Zeolite Catalysts for Styrene Production.

An improved understanding of the nature and distribution of boron and cesium species in BCsX zeolites is a prerequisite to guide future developments in the environmentally attractive, yet challenging, production of styrene through the side-chain alkylation of toluene with methanol. Herein, standard characterization and catalytic tests are complemented by integrated visualization through time-of-flight secondary-ion mass spectrometry and energy-dispersive X-ray spectroscopy and detailed assessment by 133 Cs and 11 B NMR spectroscopy, to correlate the properties and performance during successive ion-exchange and impregnation steps in the preparation of both powders and millimeter-sized granules. The results highlight a significant impact of catalyst scaleup on the effective introduction of boron species, which originates chemical heterogeneity that is linked to selectivity losses. They also illustrate the complexity of elucidating the role of this promotor, which interacts with cesium cations and exhibits different coordination states and chemical environments, depending on the pretreatment.

CO2 -to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface.

Methanol synthesis by CO2 hydrogenation is a key process in a methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. A mechanistic study of the hydrogenation of CO2 on Cu/ZrO2 is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO2 hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.

Selective Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate.

A catalytic process is demonstrated for the selective conversion of methane into carbon monoxide via oxychlorination chemistry. The process involves addition of HCl to a CH4 -O2 feed to facilitate C-H bond activation under mild conditions, leading to the formation of chloromethanes, CH3 Cl and CH2 Cl2 . The latter are oxidized in situ over the same catalyst, yielding CO and recycling HCl. A material exhibiting chlorine evolution by HCl oxidation, high activity to oxidize chloromethanes into CO, and no ability to oxidize CO, is therefore essential to accomplish this target. Following these design criteria, vanadyl pyrophosphate (VPO) was identified as an outstanding catalyst, exhibiting a CO yield up to approximately 35 % at 96 % selectivity and stable behavior. These findings constitute a basis for the development of a process enabling the on-site valorization of stranded natural-gas reserves using CO as a highly versatile platform molecule.

Probing the molecular character of periodic mesoporous organosilicates via photoluminescence of Lewis acid-base adducts.

Photoluminescence decay was used as a structure-sensitive method to compare the distribution of emitting sites in periodic mesoporous organosilicates (PMOs) to their respective molecular analogs. The observed close similarity of PL decays confirms the molecular nature of PMOs and high homogeneity of emitting sites.

Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts.

Re2O7 supported on γ-alumina is an alkene metathesis catalyst active at room temperature, compatible with functional groups, but the exact structures of the active sites are unknown. Using CH3ReO3/Al2O3 as a model for Re2O7/Al2O3, we show through a combination of reactivity studies, in situ solid-state NMR, and an extensive series of DFT calculations, that µ-methylene structures (Al-CH2-ReO3-Al) containing a Re═O bound to a tricoordinated Al (AlIII) and CH2 bound to a four-coordinated Al (AlIVb) are the precursors of the most active sites for olefin metathesis. The resting state of CH3ReO3/Al2O3 is a distribution of µ-methylene species formed by the activation of the C-H bond of CH3ReO3 on different surface Al-O sites. In situ reaction with ethylene results in the formation of Re metallacycle intermediates, which were studied in detail through a combination of solid-state NMR experiments, using labeled ethylene, and DFT calculations. In particular, we were able to distinguish between metallacycles in TBP (trigonal-bipyramidal) and SP (square-pyramidal) geometry, the latter being inactive and detrimental to catalytic activity. The SP sites are more likely to be formed on other Al sites (AlIVa/AlIVa). Experimentally, the activity of CH3ReO3/Al2O3 depends on the activation temperature of alumina; catalysts activated at or above 500 °C contain more active sites than those activated at 300 °C. We show that the dependence of catalytic activity on the Al2O3 activation temperature is related to the quantity of available AlIII-defect sites and adsorbed H2O.

The reactivity of acyl chlorides towards sodium phosphaethynolate, Na(OCP): a mechanistic case study.

The reaction of Na(OCP) with mesitoyl chloride delivers an ester functionalized 1,2,4-oxadiphosphole in a clean and P-atom economic way. The reaction mechanism has been elucidated by means of detailed NMR-spectroscopic, kinetic and computational studies. The initially formed acyl phosphaketene undergoes a pseudo-coarctate cyclization with an (OCP)- anion under the loss of carbon monoxide to yield a five-membered ring anion. Subsequently, the nucleophilic attack of the formed heterocyclic anion on a second acyl chloride molecule results in the 1,2,4-oxadiphosphole. The transient acyl phosphaketene is conserved during the reaction in the form of four-membered ring adducts, which act as a reservoir. Consequently, the phosphaethynolate anion has three different functions in these reactions: it acts as a nucleophile, as an en-component in [2 + 2] cycloadditions and as a formal P- transfer reagent.