CPMAS NMR Research Articles

New insight into the preferred valency of interlayer anions in hydrotalcite-like compounds: The effect of Mg/Al ratio

Abstract Magnesium-aluminum hydrotalcite-like compounds with molar ratios of Mg/Al ranging from 1.93 to 6.63 were synthesized and characterized with elemental analysis, XRD, SEM, TG/MS, XPS, 27Al solid state MAS NMR, 13C solid state CP MAS NMR, and vibrational spectroscopy (FTIR and Raman). The results of physico-chemical studies show that the materials are free of impurities and consistently point to the steady evolution of the interlayer anion composition upon increase of Mg/Al ratio. In particular, it is observed that the growing content of magnesium within the hydrotalcite layer is accompanied by the gradual substitution of dinegative interlayer carbonate anions with mononegative bicarbonates and/or nitrates. The observation contradicts the generally accepted view that the hydrotalcite-like structures have greater affinities for multivalent anions compared with monovalent ones. It is argued that in materials with low degree of Al for Mg substitution, hence larger distance between the charge generating sites, compensation by monovalent anions enhances Coulombic interactions within the hydrotalcite-like structure.

Physicochemical and catalytic properties of acylase I from aspergillus melleus immobilized on amino- and carbonyl-grafted stöber silica.

Acylase I from Aspergillus melleus was immobilized on supports consisting of unmodified and modified silica. Modification was performed using 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde (GA). The effectiveness of immobilization was investigated using the standard Bradford method in addition to a number of physicochemical techniques, including spectroscopic methods (FTIR, 29 Si and 13 C CP MAS NMR), porous structure and elemental analysis, and zeta potential measurement. A determination of catalytic activity was made based on the deacetylation reaction of N-acetyl-l-methionine. Furthermore, the effect of pH and temperature on the catalytic activity of the free and immobilized enzyme, as well as the reusability of the silica-bound aminoacylase, were determined. The immobilized systems demonstrated a high degree of catalytic activity. The best catalytic parameters were those of aminoacylase immobilized on silica modified with APTES (apparent activity 3937 U/g, relative activity 61.6%). © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 34:767-777, 2018.

Ionic dynamics of the cation in organic-inorganic hybrid compound (CH3NH3)2MCl4 (M = Cu and Zn) by 1H MAS NMR, 13C CP MAS NMR, and 14N NMR.

The ionic dynamics of (CH3NH3)2MCl4 (M = Cu, Zn) by 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR), 13C cross-polarization (CP) MAS NMR, and 14N NMR are investigated as a function of temperature with a focus on the role of the CH3NH3+ cation. The molecular motions in (CH3NH3)2MCl4 are also discussed based on the 1H spin–lattice relaxation time in the rotating coordinate frame T1ρ. From the 1H T1ρ results, the activation energies for the tumbling motion of 1H for CH3 and NH3 were similar, and the uniaxial rotations occurred within a large temperature range. The molecular motions for 13C and 14N of the main chain in the CH3NH3+ cation were rigid, whereas those for 1H of the side chain in the CH3NH3+ cation were very free at high temperatures. T1ρ provides insight into the changes in the cation reorientation rates induced by heating at high temperatures.

Molybdenum(0) tricarbonyl and tetracarbonyl complexes with a cationic pyrazolylpyridine ligand: synthesis, crystal structures and catalytic performance in olefin epoxidation.

The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)3(ptapzpy)Br] (1) and cis-[Mo(CO)4(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)6 (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)4(pip)2] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125–150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo3O9([ptapzpy]Br)2]·nH2O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and 13C{1H} CP MAS NMR spectroscopy. Compounds 1–3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H2O2 or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L)x[Mo8O26], were formed from 1–3 with TBHP and from 1 with H2O2, while soluble oxoperoxo species were formed from 3 with H2O2. These findings helped to explain the different catalytic performances obtained.

Effect of SiO2 amount on heterogeneous base catalysis of SiO2@Mg-Al layered double hydroxide.

The effects of SiO2 amount on the base catalysis of highly active finely crystallized Mg–Al type layered double hydroxides prepared by the co-precipitation method with coexistence of SiO2 spheres, denoted as SiO2@LDHs, were investigated. With the Si/(Mg + Al) atomic ratios of 0–0.50, the highest activity for the Knoevenagel condensation was observed in the case of Si/(Mg + Al) = 0.17, as the reaction rate of 171.1 mmol g(cat)−1 h−1. The base activity increased concomitantly with decreasing LDH crystallite size up to Si/(Mg + Al) atomic ratio of 0.17. However, above the Si/(Mg + Al) atomic ratio of 0.17, the reaction rate and TOFbase were decreased although the total base amount was increased. Results of TEM-EDS and 29Si CP-MAS NMR suggest that the co-existing SiO2 causes advantages for dispersion and reduction of the LDH crystallite to improve the base catalysis of SiO2@Mg–Al LDH, whereas the excess SiO2 species unfortunately poisons the highly active sites on the finely crystallized LDH crystals above a Si/(Mg + Al) atomic ratio of 0.17. According to these results, we inferred that the amount of spherical SiO2 seeds in the co-precipitation method is an important factor to increase the base catalysis of SiO2@LDHs; i.e. the control of Si/(Mg + Al) atomic ratio is necessary to avoid the poisoning of highly active base sites on the LDH crystal.

Aerobic reactions of antitumor active dirhodium(II) tetraacetate Rh2(CH3COO)4 with glutathione.

The aerobic reaction between glutathione (H3A) and dirhodium(II) tetraacetate, Rh2(AcO)4 (AcO-=CH3COO-), in aqueous solution (pH 7.4) breaks up the direct RhII-RhII bond and its carboxylate framework, as evidenced by UV-Vis spectroscopy. After purifying the reaction product using size exclusion chromatography, electrospray ionization mass spectrometry (ESI-MS) of the solution showed binuclear [Formula: see text] and [Formula: see text] ions. Evaporation yielded a solid compound, [Formula: see text], for which Rh K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed~2 Rh-O (2.08±0.02 Å) and~4 Rh-S (2.33±0.02 Å) bond distances around each RhIII center, and the RhIII··RhIII distance 3.11±0.02 Å, close to that in dirhodium(III) complexes with three bridging thiolates connecting [Formula: see text] units. The 13C CPMAS NMR spectrum of the RhIII-glutathione complex showed a change ∆δ C>6ppm in the chemical shift of the COO- signal, indicating some carboxylate coordination to the Rh(III) ions. This study shows that under aerobic conditions glutathione enables oxidation of Rh2(AcO)4 and thus reduces its antitumor efficiency. The reaction of Rh2(AcO)4 with glutathione was investigated by ESI-MS, UV-Vis, 13C NMR and X-ray absorption spectroscopy, revealing that glutathione breaks down the carboxylate framework enabling oxidization of the [Formula: see text] core to Rh(III) dimeric units, bridged by three thiolates.

Microstructural analyses of loaded and/or unloaded semisynthetic porous material for understanding of superadsorption and optimization by response surface methodology

Physicochemically modified goat collagen fiber (PCMGCF), a porous semisynthetic adsorbent of excellent reusability/recyclability, adsorption capacities (ACs) and waste remediation efficiency, was synthesized from scraps collagen to report individual and synergistic superadsorption of chemically distinct hazardous textile/leather anionic dyes, like mono-azo (Acid Red 131/AR131), bis-azo (Acid Blue 113/AB113) and metal-complex-azo (Acid Brown 369/AB369) and cationic triphenylmethane dyes, like Methyl Violet (MV) and Malachite Green (MG). A new strategy was introduced for comprehensive understanding of the comparative physicochemical changes in microstructures, adsorption mechanism and dye allocations within macro-, meso- and micro-pores of PCMGCF, through extensive analyses of 13C CP MAS NMR, O1s-/N1s-/C1s-/S2p3/2-/Cr2p3/2-XPS, FTIR, TGA, DTG, DSC, XRD, SEM and EDX analyses and by measuring the isotherm, kinetics, diffusion and thermodynamics parameters of adsorption. Appearance of several physicochemical effects in simultaneous adsorption, like metachromasia, H-/J-aggregate, dye–dye complex, cation-anion-PCMGCF adduct, hydrophobic interaction of alkylamino groups, H-bonding and orientation effects of dyes, like side on, tilted and peripheral, were determined through UV–Vis analyses. Individual and interactive effects of input variables on performance potential were measured through response surface methodology (RSM). Diffusion mechanism of thermodynamically favourable chemisorption process was studied by intraparticle diffusion model of Weber-Morris. However, ACs of AR131, AB113 and AB369 were found to be 32.02, 31.52 and 27.70mgg−1, respectively, at optimum conditions (i.e. 200mgL−1, 299K, pHi=4 and 0.25g of PCMGCF).

WITHDRAWN: An example of S,S′-bidentate coordination mode of the dithiophosphate group in the heteroleptic tetraphenylantimony(V) complex, [Sb(C6H5)4{S2P(O-cyclo-C6H11)2}]: Multinuclear (13C, 31P) CP MAS NMR, single-crystal X-ray diffraction studies and thermal behaviour

Crystalline O,O′-di-cyclo-hexyl dithiophosphate (Dtph) tetraphenylantimony(V) complex, [Sb(C6H5)4{S2P(O-cyclo-C6H11)2}] (1) was prepared and studied by means of heteronuclear (31C, 31P) CP MAS NMR ...

Charcoal in Organic Horizon and Surface Mineral Soil in a Boreal Forest Fire Chronosequence of Western Quebec: Stocks, Depth Distribution, Chemical Properties and a Synthesis of Related Studies

Wildfires are a major driver of carbon stocks and ecosystem development in Canadian boreal forests, but there is little information on amounts and properties of the charcoal produced. Using data and samples available from a previous study, we determined amounts, depth distribution and chemical properties of visually-determined charcoal (> 2 mm) in a boreal chronosequence in the Abitibi region of Quebec, Canada. Sites ranged from 24 to 2355 years since fire (ysf) and originated from low- and high-severity soil burns (> 5 cm or < 5 cm organic horizon unburned, respectively). Two or three pits were sampled at 1-cm depth intervals from 20 jack pine (Pinus banksiana) sites (one low severity and 19 high severity) and 31 black spruce (Picea mariana) sites (12 low severity and 19 high severity). Site-level charcoal stocks ranged from 50 to 5527 kg ha-1 with high within-site variability and lower stocks for the oldest sites. Depth distributions typically peaked around the organic-mineral interface, but some low-severity sites also had charcoal layers within the organic horizon. Means from 30 samples were 569 mg g-1 total C, 4.1 mg g-1 total N and 140 C/N (molar), with total C and C/N showing a trend of decline with time since fire, and total N showing an increase. Solid-state 13C CPMAS NMR spectra of nine samples showed high variability among the younger samples, but a trend to higher aromaticity for the older ones. A literature survey focusing on boreal forests similarly showed highly variable stocks and chemical properties of charcoal in organic horizon and upper mineral soil, with reduction of variance and lower stocks after several hundred years. This initial variation was also consistent with reports of highly variable temperatures and duration of charring in wildfires. Adding reports available for char production, and considering that most studies of char stocks and production are limited to the organic horizon (forest floor), suggests that initial production of charred material from boreal wildfires might be around 5-10 tonnes ha-1.

Racemic crystals of trolox derivatives compared to their chiral counterparts: Structural studies using solid-state NMR, DFT calculations and X-ray diffraction

Trolox C (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) is a water-soluble vitamin E analogue that is available in enantiomeric forms R or S. Enantiomerically pure Trolox 1, its derivatives 2, 3 (R and S enantiomers) and racemic forms 1–3 were studied using solid-state 13C cross-polarisation (CP) magic angle spinning (MAS) NMR (13C CPMAS NMR). Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations of the shielding constants supported the assignment of 13C resonances in the solid-state NMR spectra. For the 13C CPMAS NMR spectra of 1, resonances of pure enantiomers were significantly broader than those of the racemic R/S form. In order to explain these effects, five of the available crystal structures were analysed (1R/S, 3R/S, 2S and the newly measured 2R/S and 3S). Cyclic dimers with one R and one S enantiomer linked by two OHOC2b hydrogen bonds were formed in 1R/S. Similar hydrogen-bonded dimers were present in 3S but not in 3R/S, in which interactions are water-mediated. A comparison of X-ray diffraction, CPMAS NMR data and the DFT GIPAW calculations of racemic forms and pure enantiomers was conducted for the first time. Our results, particularly the solid-state NMR data, were discussed in relation to Wallach's rule, that the racemic crystal appears as more ordered than its chiral counterpart.

Fire effects on litter chemistry and early development of Eucalyptus globulus

Fires affect what happens to litter in ecosystems. Biological and chemical effects of burnt litter on plants are not as of yet fully understood. We aimed to assess the effects of heat-treated leaf litter on germination and seedling root growth of Eucalyptus globulus. Litter from E. globulus, Acacia dealbata, Pinus pinaster, and Quercus suber was collected in Portugal, on Humic Cambisol, and heated between 25 °C and 600 °C. Those materials were then characterized by: 13C CPMAS NMR spectroscopy, proximate lignin and cellulose, and elemental analyses. Afterwards, they were used as substrate in bioassays with Eucalyptus seeds. Heating changed litter composition (P < 0.05), consistently across species: alkyl C, O-alkyl C, and methoxyl + N-alkyl C decreased more than 50%; and aromatic C increased more than 5-fold. Unheated and lightly heated litters inhibited germination and growth, with maximum inhibition by Quercus and Eucalyptus litters, down to 17% of the control (P < 0.05). Severely charred materials had neutral or stimulatory effects, up to 191% of the control (P < 0.05). These responses were associated with concentrations of dominant C types in litters (P < 0.05). Litter charring, as it occurs during wildfires, is potentially critical for the success of E. globulus’s regeneration from seeds.

Differences of C sequestration in functional groups of soil humic acid under long term application of manure and chemical fertilizers in North China

The differences of molecular characteristic of humic acid (HA) under long-term application of chemical fertilizer and manure for 34 years were determined by 13C cross polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy in order to investigate the carbon (C) sequestration in soils. The results showed that the relative content of alkyl C increased in manure treatments, compared to control and chemical fertilizer treatments. There were positive correlations between alkyl C and hydrogen to carbon (H/C) ratio (R2=0.84, P<0.01), alkyl C and amounts of HA (R2=0.57, P<0.01) as well as alkyl C and E4/E6 ratio (R2=0.55, P<0.01). Conversely, negative correlations were found between aromatic C and alkyl C (R2=0.76, P=0.01), H/C ratio and aromaticity (R2=0.37, P=0.04), aromaticity and amounts of HA (R2=0.71, P<0.01) as well as aromaticity and E4/E6 ratio (R2=0.65, P<0.01). Compared to control treatment, the amorphous C/crystalline C ratio increased for 0.02–0.14 and 0.30–0.78 in chemical fertilizer treatments and manure treatments, separately. Moreover, the short-chain (CH2)n/Long-chain (CH2)n ratio increased for 0.03–0.37 and 0.03–0.18 in chemical fertilizer treatments and manure treatments, separately.These findings suggest that application of manure favours alkyl C sequestration in HA and tends to be more closely linked to amorphous C than crystalline C, while incorporates less short-chain (CH2)n, compared to chemical fertilizer treatments. Moreover, the E4/E6 ratio of HA increased with more alkyl C.

Absorption of Cu(II) in layered diaminoalkyl- and monoaminoalkyl-polysilsesquioxane

Lamellar polysilsesquioxane containing two kinds of aminoalkyl groups were synthesized using alkanoic acids. Our intension with this synthesis was to increase the exposure to the absorbate of the absorption sites in the polymer solid without sacrificing its high density of sites. That is, this method is designed to weaken the interactions around the amine sites that inhibit diffusion of the aqueous cation pollutants. The mixing ratio, x: y, of 3-aminopropyltrimethoxysilane (AP silane) and N-(aminoethyl)-3-aminopropyltrimethoxysilane (AeAP silane) in the liquid precursor agrees well with that of the AP and AeAP groups in the solid, [(C13H27COOH)x(C12H23CH(COOH)CH2COOH)y(NH2C3H6)x(NH2C2H4NHC3H6)ySi10O15]n or [(MAS)x(C12SA)y(AP)x(AeAP)ySi10O1.5]n (x + y = 10). A good lamellar structure was obtained with x: y ratio higher than 2: 8. FT-IR spectroscopy reveals that increasing the x: y ratio enhances the bands due to hydrogen-bonded –COOH and dissociated –COO-, suggesting an increase in water molecule insertion around the carboxyl and amine groups with x: y. In 13C CP-MAS NMR, unresolved peaks due to carboxylate carbons disappear at x: y = 4: 6 and 2: 8, suggesting that carboxylate is isolated at these mixing ratios probably due to hydration. In addition, continuous growth of the peak due to ionized carboxylate is observed. The amount of absorbed aqueous Cu(II) is particularly large when x: y = 4: 6, which is revealed by analysis with the Langmuir equation. In contrast, the ESR spectrum of Cu(II) is independent on the x: y ratio, suggesting the accessibility of the absorption sites is simply improved when x: y = 4: 6. It is likely that this mixing ratio provides a particularly effective structure for exposing the absorption sites of aqueous Cu(II).

Synthesis of Metforminium Succinate by Melting. Crystal Structure, Thermal, Spectroscopic and Dissolution Properties

The reaction by melt mixing at 220 °C of the antihyperglycemic drug metformin hydrochloride 1 with dehydrated sodium succinate yields efficiently the organic salt [MET]2[SUC] 2 (H-MET+= metforminium and SUC2- = succinate). Solid state CPMAS NMR 13C spectroscopy experiments, powder X-ray diffraction and FT-IR results support the formation of the pharmaceutical salt 2 in good yields. Besides, the charged-assisted hydrogen bonding interactions of type N-H…-O(carboxylate) were thoroughly analyzed by single crystal X-Ray diffraction techniques. Thus, the pharmaceutical salt 2 possesses considerable thermal differences when compared to the pure starting reagents. In addition, intrinsic dissolution rate experiments in buffered aqueous solutions at pH= 6.8 showed a sustained-release behavior of the drug in 2 with a constant value of Kint = 0.885 mg/min * cm2.

Solid-state studies and antioxidant properties of the γ-cyclodextrin·fisetin inclusion compound.

Fisetin is a natural antioxidant with a wide range of nutraceutical properties, including antidiabetic, neuroprotecting, and suppression or prevention of tumors. The present work describes the preparation of a water-soluble, solid inclusion compound of fisetin with gamma-cyclodextrin (γ-CD), a cyclic oligosaccharide approved for human consumption. A detailed physicochemical analysis of the product is carried out using elemental analysis, powder X-ray diffraction (PXRD), Raman, infrared and 13C{1H} CP-MAS NMR spectroscopies, and thermal analysis (TGA) to verify fisetin inclusion and to present a hypothetical structural arrangement for the host–guest units. The antioxidant activity of the γ-CD·fisetin inclusion compound is evaluated by the DPPH assay.

Batch and Continuous-Flow Huisgen 1,3-Dipolar Cycloadditions with an Amphiphilic Resin-Supported Triazine-Based Polyethyleneamine Dendrimer Copper Catalyst

A polystyrene–poly(ethylene glycol) (PS–PEG) resin-supported triazine-based polyethyleneamine dendrimer copper catalyst (PS–PEG-TD2–CuSO4) was prepared and characterized by means of CP-MAS NMR, UV–vis/NIR, FTIR, SEM-EDX, XPS, and ICP-AES analyses. PS–PEG-TD2–CuSO4 was highly active in the Huisgen 1,3-dipolar cycloaddition of various organic azides with alkynes in water as well as the three-component reaction of alkynes, alkyl bromides, and sodium azide under batch conditions to give the corresponding triazoles in excellent yields with high recyclability of the catalyst. TEM analysis suggested that the copper nanoparticles generated in situ through reduction of PS–PEG-TD2–CuSO4 with sodium ascorbate serve as the active catalytic species. The application of PS–PEG-TD2–CuSO4 catalyst in a continuous-flow Huisgen reaction for the synthesis of 1,2,3-triazoles was also examined. The cycloaddition of organic azides with alkynes was completed within 22 s in the continuous-flow system containing PS–PEG-TD2–CuSO4 t...

Comprehensive characteristic and potential application of POSS-coated MgO-SiO2 binary oxide system

This study concerns the possibility of functionalization of a binary oxide system with an organic agent in order to change its physicochemical properties and surface activity. The binary oxide material – MgO-SiO2–was obtained via a sol-gel route using organic precursors of magnesium oxide and silica, ammonia as catalyst and methanol as a solvent. The powder material prepared according to the proposed method of synthesis was functionalized with a commercial POSS compound – AminoethylaminopropylIsobutyl POSS®. The efficiency of immobilization of the modifying agent onto MgO-SiO2 material was evaluated by the FTIR, 29Si and 13C CP MAS NMR methods. Moreover, detailed physicochemical and structural characteristics of the resulting materials, including morphology, parameters of the porous structure, hydrophobic/hydrophilic nature and thermal stability, were determined. As a result of this functionalization, a novel group of oxide systems with defined physicochemical properties dedicated to specific applications was created. The functionalized form of the synthesized MgO-SiO2 material was successfully applied as a bond/stabilizer in polyol mixtures and as a flame retardant for polyurethane foams.

Novel thio-kaolinite nanohybrid materials and their application as heavy metal adsorbents in wastewater

A new clay nanohybrid adsorbent material was obtained by grafting the compound 3,6-dithia-1,8-octanediol (HO(CH2)2S(CH2)2S(CH2)2OH) onto the internal aluminol surfaces of both the urea- and dimethylsulfoxide (DMSO) precursors of two kaolinite clay minerals: the Source clay KGa-1b and a Portuguese clay sample collected from the Clays of Taveiro formation. All samples were characterised by X-ray diffraction and solid-state 13C cross-polarisation nuclear magnetic resonance (CP-MAS NMR) spectroscopy. Initial materials and final products were also characterised by thermal gravimetric analysis (TGA) and Fourier-Transform infrared (FTIR) spectroscopy. The results describe nanohybrid materials that are resistant to hydrolysis and exhibit different grafting fashions.The potential application of these materials as a heavy metal adsorbents was tested in ZnCl2 solutions as well as in zinc-spiked raw wastewater. The first assessment of the zinc adsorption capacity of the Source clay KGa-1b was made. Zinc concentrations were analysed concomitant with other metal species by inductively coupled plasma mass spectrometry (ICP-MS), demonstrating variable adsorption capacities of the different clay samples, and a consistently greater zinc removal in the wastewater system compared to simple zinc solutions. Measured concentrations of concomitantly analysed metals reveal a potentially worrying release of certain heavy metal species from some of the clay samples as a result of zinc adsorption.

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site

The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/SiO2) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C2H4 reduction of silica-supported CrVI ions to CrII ions in the precatalyst, exposure to ethylene results in the formation of organoCrIII sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol–gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined CrII end point. Ethylene causes rapid reoxidation of these CrII sites to CrIII, even at low temperatures. Solid-state 13C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO)2CrIII–CH═CH2 although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these ...

Diradicaloid or Zwitterionic Character: The Non-Tetrahedral Unsaturated Compound [Si4 {N(SiMe3 )Dipp}4 ] with a Butterfly-type Si4 Substructure.

The reduction of the tribromoamidosilane {N(SiMe3 )Dipp}SiBr3 (Dipp=2,6-iPr2 C6 H3 ) with potassium graphite or magnesium resulted in the formation of [Si4 {N(SiMe3 )Dipp}4 ] (1), a bicyclo[1.1.0]tetrasilatetraamide. The Si4 motif in 1 does not adopt a tetrahedral substructure and exhibits two three-coordinate and two four-coordinate silicon atoms. The electronic situation on the three-coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as 29 Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si4 S3 {N(SiMe3 )Dipp}4 ] (2).