Determining the surface structure of silicated alumina catalysts via isotopic enrichment and dynamic nuclear polarization surface-enhanced NMR spectroscopy

We would like to thank SASOL and EPSRC (EP/L505079/1) for studentship funding for AGMR. SEA would also like to thank the Royal Society and Wolfson Foundation for a merit award. PBW would like to thank the Royal Society for the award of an Industry Fellowship. The University of Nottingham DNP MAS NMR Facility used in this research was funded by EPSRC and the University of Nottingham, and assistance from the Facility Manager (Subhradip Paul, University of Nottingham) is also acknowledged. This work was also supported by ERC Advanced Grant No. 320860. The research data (and/or materials) supporting this publication can be accessed at DOI: http://dx.doi.org/10.17630/00533fb3-e938-498d-bfe4-f07d82c309d6.Isotopic enrichment of 29Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt%) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense 1H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si-O-Si linkages at the surface (identified as Q4(3Al)-Q4(3Al)), and confirm that the anchoring of the surface overlayer with the alumina occurs through AlIV and AlV species only. This suggests the presence of Q3/Q4 Si at the surface affects the neighboring Al species, modifying the surface structure and making it less likely AlVI environments are in close spatial proximity. In contrast, Q1/Q2 species, bonded to the surface by fewer covalent bonds, have less of an effect on the surface and more AlVI species are consequently found nearby. The combination of isotropic enrichment and DNP provides a definitive and fully quantitative description of the Si-modified alumina surface, and we demonstrate that almost one-third of the silicon at the surface is connected to another Si species, even at the low level of coverage used, lowering the propensity for the formation of Brønsted acid sites. This suggests that a variation in the synthetic procedure might be required to obtain a more even coverage for optimum performance. The work here will allow for more rigorous future investigations of structure-function relationships in these complex materials.PostprintPeer reviewe

Self-complementary nickel halides enable multifaceted comparisons of intermolecular halogen bonds : fluoride ligands vs other halides

The syntheses of three series of complexes designed with self-complementary motifs for formation of halogen bonds between an iodotetrafluorophenyl ligand and a halide ligand at square-planar nickel are reported, allowing structural comparisons of halogen bonding between all four halides C6F4IX–Ni (X = F, Cl, Br, I). In the series, trans-[NiX(2,3,5,6-C6F4I)(PEt3)2] 1pX and trans-[NiX(2,3,4,5-C6F4I)(PEt3)2] (X = F, Cl, Br, I) 1oX, the iodine substituent on the benzene ring was positioned para and ortho to the metal, respectively. The phosphine substituents were varied in the series, trans-[NiX(2,3,5,6-C6F4I)(PEt2Ph)2] (X = F, I) 2pX. Crystal structures were obtained for the complete series 1pX, and for 1oF, 1oCl, 1oI and 2pI. All these complexes exhibited halogen bonds in the solid state, of which 1pF exhibited unique characteristics with a linear chain, the shortest halogen bond d(C6F4IF–Ni) = 2.655(5) Å and the greatest reduction in halogen bond distance (IF) compared to the sum of the Bondi van der Waals radii, 23%. The remaining complexes form zig-zag chains of halogen bonds with distances also reduced with respect to the sum of the van der Waals radii. The magnitude of the reductions follow the pattern F > Cl ~ Br > I, 1pX > 1oX, consistent with the halogen bond strength following the same order. The variation in the IX–Ni angles is consistent with the anisotropic charge distribution of the halide ligand. The temperature dependence of the X-ray structure of 1pF revealed a reduction in halogen bond distance of 0.055(7) Å on cooling from 240 to 111 K. Comparison of three polymorphs of 1oI shows that the halogen bond geometry may be altered significantly by the crystalline environment. The effect of the halogen bond on the 19F NMR chemical shift in the solid state is demonstrated by comparison of the magic-angle spinning NMR spectra of 1pF and 1oF with that of a complex incapable of halogen bond formation, trans-[NiF(C6F5)(PEt3)2] 3F. Halogen bonding causes deshielding of iso in the component of the tensor perpendicular to the nickel coordination plane. The results demonstrate the potential of fluoride ligands for formation of halogen bonds in supramolecular structures

Crystal structure of tetrabutylammonium bromide–1,2-diiodo-3,4,5,6-tetrafluorobenzene–dichloromethane (2/2/1)

The crystallization of a 1:1 molar solution of 1,2-diiodo-3,4,5,6-tetrafluorobenzene (o-DITFB) and tetrabutylammonium bromide (n-Bu4NBr) from dichloromethane yielded pure white crystals of a halogen-bonded compound, C16H36N+·Br−·C6F4I2·0.5CH2Cl2 or [(n-Bu4NBr)(o-DITFB)]·0.5CH2Cl2. The compound may be described as a quaternary system and may be classified as a salt–cocrystal solvate. The asymmetric unit contains one molecule of solvent, two o-DITFB molecules, two cations (n-Bu4N+) and two crystallographically distinct bromide ions [θI...Br-...I = 144.18 (1) and 135.35 (1)°]. The bromide ion is a bidentate halogen-bond acceptor which interacts with two covalently bonded iodines (i.e. halogen-bond donors), resulting in a one-dimensional polymeric zigzag chain network approximately along the a axis. The observed short contacts and angles are characteristic of the non-covalent interaction [dC—I...Br = 3.1593 (4)–3.2590 (5) Å; θC—I...Br = 174.89 (7) and 178.16 (7)°]. It is noted that iodine acts as both a halogen-bond donor and a weak CH hydrogen-bond acceptor, while the bromide ions act as acceptors for weak CH hydrogen bonds and halogen bonds

Core-Shell Structure of Organic Crystalline Nanoparticles Determined by Relayed Dynamic Nuclear Polarization NMR

The structure of crystalline nanoparticles (CNPs) is determined using dynamic nuclear polarization (DNP) enhanced NMR spectroscopy experiments. The CNPs are composed of a crystalline core containing an active pharmaceutical ingredient (compound P), coated with a layer of PEG (DSPE-PEG 5000) located at the crystal surface, in a D2O suspension. Relayed DNP experiments are performed to study H-1-H-1 spin diffusion and to determine the size of the crystalline core as well as the thickness of the PEG overlayer. This is achieved through selective doping to create a heterogeneous system in which the D2O contains glycerol and organic radicals, which act as polarization sources, and the CNPs are exempt of radical molecules. We observe features that are characteristic of a core-shell system: high and constant DNP enhancement for components located in the surrounding radical solution, short build-up times for the PEG layer, and longer build-up times and time dependent enhancements for compound P. By comparing numerical simulations and experimental data, we propose a structural model for the CNPs with a core-shell organization and a high affinity between the radical and the PEG molecules

Intracluster ligand rearrangement: an NMR-based thermodynamic study

Ligand and metal exchange reactions are powerful methods to tailor the properties of atomically precise metal nanoclusters. Hence, a deep understanding of the mechanisms behind the dynamics that rule the ligand monolayer is crucial for its specific functionalization. Combining variable-temperature NMR experiments and dynamic-NMR simulations, we extract the thermodynamic activation parameters of a new exchange reaction: the intracluster ligand rearrangement between the two symmetry-unique positions in [Ag25(DMBT)18]− and [Ag24Au(DMBT)18]− clusters. We report for the first time that this peculiar intracluster modification does not seem to proceed via metal–sulphur bond breaking and follows a first-order rate law, being therefore a process independent from the well-described collisional ligand exchange.</p