Pedro B. Groszewicz

Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides as Catalyst for the Chemoselective Hydrogenation of Substituted Aldehydes: a Remarkable Ligand Effect

Air-stable and homogeneous gold nanoparticles (AuNPs, 1a-5a) ligated by various secondary phosphine oxides (SPOs), [R(1)R(2)P(O)H] (R(1) = Naph, R(2) = (t)Bu, L1; R(1) = R(2) = Ph, L2; R(1) = Ph, R(2) = Naph, L3; R(1) = R(2) = Et, L4; R(1) = R(2) = Cy, L5; R(1) = R(2) = (t)Bu, L6), with different electronic and steric properties were synthesized via NaBH4 reduction of the corresponding Au(I)-SPO complex. These easily accessible ligands allow the formation of well dispersed and small nanoparticles (size 1.2-2.2 nm), which were characterized by the use of a wide variety of techniques, such as transmission electron microscopy, thermogravimetric analysis, UV-vis, energy-dispersive X-ray, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR), and cross polarization magic angle spinning (CP MAS) NMR spectroscopy. A pronounced ligand effect was found, and CP MAS NMR experiments enabled us to probe important differences in the polarity of the P-O bond of the SPOs coordinated to the nanoparticle surface depending on the type of substituents in the ligand. AuNPs containing aryl SPOs carry only SPO anions and are highly selective for aldehyde hydrogenation. AuNPs of similar size made with alkyl SPOs contain also SPOH, hydrogen bonded to SPO anions. As a consequence they contain less Au(I) and more Au(0), as is also evidenced by XPS. They are less selective and active in aldehyde hydrogenation and now show the typical activity of Au(0)NPs in nitro group hydrogenation.

Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQISwidth) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in 23Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.

Design of a Heterogeneous Catalyst Based on Cellulose Nanocrystals for Cyclopropanation: Synthesis and Solid-State NMR Characterization.

Heterogeneous dirhodium(II) catalysts based on environmentally benign and biocompatible cellulose nanocrystals (CNC-Rh2) as support material were obtained by ligand exchange between carboxyl groups on the CNC surface and Rh2(OOCCF3)4, as was confirmed by solid-state (19)F and (13)C NMR spectroscopy. On average, two CF3COO(-) groups are replaced during ligand exchange, which is consistent with quantitative analysis by a combination of (19)F NMR spectroscopy and thermogravimetry. CNC-Rh2 catalysts performed well in a model cyclopropanation reaction, in spite of the low dirhodium(II) content on the CNC surface (0.23 mmol g(-1)). The immobilization through covalent bonding combined with the separate locations of binding positions and active sites of CNC-Rh2 guarantees a high stability against leaching and allows the recovery and reuse of the catalyst during the cyclopropanation reaction.

Two linkers are better than one: enhancing CO2 capture and separation with porous covalent triazine-based frameworks from mixed nitrile linkers

Covalent triazine-based framework (CTF) materials were synthesized by combining two different nitrile building blocks: the tetranitrile tetrakis(4-cyanophenyl)ethylene (M) was reacted with either terephthalonitrile (M1), tetrafluoroterephthalonitrile (M2), 4,4′-biphenyldicarbonitrile (M3) or 1,3,5-benzenetricarbonitrile (M4) under ionothermal conditions (ZnCl2, 400 °C) to yield mixed-nitrile MM′-CTFs MM1 to MM4. Comparative 1H/13C and 19F/13C CP MAS analyses of MM2(300) (synthesized at 300 °C) suggest that the hydrogenated and fluorinated carbon atoms are in close vicinity (<5 A) to each other and support the formulation of the MM2(300) sample as a copolymeric CTF. Systematic N2, CO2 and CH4 gas sorption studies were performed up to 1 bar at 273 K and 293 K. The specific BET surface areas of MM1–MM4 were 1800, 1360, 1884 and 1407 m2 g−1, respectively. The CO2 uptake capacity of mixed-nitrile MM1, MM2 and MM4 was higher than the CO2 uptake of the respective individual single-nitrile M- or M′-CTF despite a higher surface area of the M-CTF PCTF-1 (2235 m2 g−1). The synergistic increase in the CO2 uptake of the mixed-nitrile MM′-CTFs is due to the higher CO2-accessible micropore volume Vmicro(CO2) and the higher micropore volume fraction V0.1/Vtot of the MM′-CTFs compared to the M- or M′-CTFs. The surface area of porous materials does not play the most important role in CO2 storage at low pressure but the CO2-accessible micropore volume is the more decisive factor. Further, MM2 shows the second highest (of known CTFs synthesized at 400 °C) CO2 uptake capacity of 4.70 mmol g−1 at 273 K and 1 bar because of its large micropore fraction (82%), which may be due to the release of fluorous decomposition products (‘defluorination carbonization’) during its synthesis. The CO2/N2 adsorption selectivities of mixed-nitrile MM1, MM2 and MM4 CTFs were also higher than those of the single-nitrile component M- or M′-CTFs.

Local structure of the B-site in BNT-xBT investigated by 47,49Ti NMR:Effect of barium content

Towards a deeper understanding of the local structure of the B-site in (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT) with 0 ≤ x ≤ 15, solid-state nuclear magnetic resonance (NMR) spectra of the titanium isotopes 47Ti and 49Ti were investigated. The 47,49Ti NMR spectra of BNT-xBT indicate a disordered local structure for the B-site of these perovskites. The line-shape of the titanium NMR spectra of BNT-xBT samples is found to be independent on the barium content. This fact implies that the local structure of the B-site remains invariant with respect to the structural changes that result from the chemical modification with barium. The analysis of 47,49Ti NMR spectra supports the hypothesis that the main structural changes across the morphotropic phase boundary in these solid-solutions are constrained to the A-site and are related to the tilting of rigid oxygen octahedra.

Thermoreversible Self-Assembly of Perfluorinated Core-Coronas Cellulose-Nanoparticles in Dry State

Self-assembly of nanoparticles (NPs) forming unique structures has been investigated extensively over the past few years. However, many self-assembled structures by NPs are irreversible, because they are generally constructed using their suspensions. It is still challenging for NPs to reversibly self-assemble in dry state, let alone of polymeric NPs with general sizes of hundreds of nm. Herein, this study reports a new reversible self-assembly phenomenon of NPs in dry state, forming thermoreversible strip-like supermolecular structures. These novel NPs of around 150 nm are perfluorinated surface-undecenoated cellulose nanoparticles (FSU-CNPs) with a core-coronas structure. The thermoreversible self-assembled structure is formed after drying in the air at the interface between FSU-CNP films and Teflon substrates. Remarkably, the formation and dissociation of this assembled structure are accompanied by a reversible conversion of the surface hydrophobicity, film transparency, and anisotropic properties. These findings show novel feasibility of reversible self-assembly of NPs in dry state, and thereby expand our knowledge of self-assembly phenomenon.

Topochemical Fluorination of La2NiO4+d: Unprecedented Ordering of Oxide and Fluoride Ions in La2NiO3F2

The Ruddlesden-Popper (K2NiF4) type phase La2NiO3F2 was prepared via a polymer-based fluorination of La2NiO4+ d. The compound was found to crystallize in the orthorhombic space group Cccm ( a = 12.8350(4) Å, b = 5.7935(2) Å, c = 5.4864(2) Å). This structural distortion results from an ordered half occupation of the interstitial anion layers and has not been observed previously for K2NiF4-type oxyfluoride compounds. From a combination of neutron and X-ray powder diffraction and 19F magic-angle spinning NMR spectroscopy, it was found that the fluoride ions are only located on the apical anion sites, whereas the oxide ions are located on the interstitial sites. This ordering results in a weakening of the magnetic Ni-F-F-Ni superexchange interactions between the perovskite layers and a reduction of the antiferromagnetic ordering temperature to 49 K. Below 30 K, a small ferromagnetic component was found, which may be the result of a magnetic canting within the antiferromagnetic arrangement and will be the subject of a future low-temperature neutron diffraction study. Additionally, density functional theory-based calculations were performed to further investigate different anion ordering scenarios.

Novel Dirhodium Coordination Polymers: The Impact of Side Chains on Cyclopropanation

Seven novel dirhodium coordination polymers (Rh2–Ln) (n = 1–7) are prepared by employing bitopic ligands to connect dirhodium nodes. The formation of the framework is confirmed by attenuated total reflectance Fourier transform infrared (ATR-FTIR) and 1H → 13C cross polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR) spectroscopy. Defect sites resulting from incomplete ligand substitution are revealed by 19F MAS NMR. The random stacking behavior of the frameworks in the obtained solid is analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Rh2/O interaction in neighboring layers is investigated by diffuse reflectance ultra-violet visible light (DR-UV-vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). This interaction is relevant to understand the catalytic behavior of various Rh2–Ln catalysts in the cyclopropanation of styrene with ethyl diazoacetate (EDA). In this context, the structure-reactivity relationship is discussed by taking into consideration both interlayer Rh2/O interactions and steric effects of side chains.

Compensating the asymmetric probe response in broad MAS NMR spectra of quadrupolar nuclei

The spinning sidebands envelope of satellite transitions often display an asymmetric shape, which is caused by an asymmetric response of the NMR probe circuit. In this work, we revisit the basic concepts of the RLC circuit at the heart of every NMR probe and present two approaches capable of minimizing this artifact. While the first one consists of deliberately mistuning the probe, the second one relies on measuring the probes response function and deconvoluting its contribution from the spectra. Both approaches are validated with 23Na NMR spectra of a lead-free relaxor ferroelectric (BNT-1BT). This material is particularly suitable as an example of the applicability of both strategies for samples with a disordered local structure.