Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies

Abstract

Grafting molecular precursors on partially dehydroxylated silica followed by a thermal treatment yields silica-supported M(III) sites for a broad range of metals. They display unique properties such as high activity in olefin polymerization and alkane dehydrogenation (M = Cr) or efficient luminescence properties (M = Yb and Eu) essential for bioimaging. Here, we interrogate the local structure of the M(III) surface sites obtained from two molecular precursors, amides M(N(SiMe)) vs siloxides (M(OSi(OBu))·L with L = (THF) or HOSi(OBu) for M = Cr, Yb, Eu, and Y, by a combination of advanced spectroscopic techniques (EPR, IR, XAS, UV-vis, NMR, luminescence spectroscopies). For paramagnetic Cr(III), EPR (HYSCORE) spectroscopy shows hyperfine coupling to nitrogen only when the amide precursor is used, consistent with the presence of nitrogen neighbors. This changes their specific reactivity compared to Cr(III) sites in oxygen environments obtained from siloxide precursors: no coordination of CO and oligomer formation during the polymerization of ethylene due to the presence of a N-donor ligand. The presence of the N-ligand also affects the photophysical properties of Yb and Eu by decreasing their lifetime, probably due to nonradiative deactivation of excited states by N-H bonds. Both types of precursors lead to a distribution of surface sites according to reactivity for Cr, luminescence spectroscopy for Yb and Eu, and dynamic nuclear polarization surface-enhanced Y NMR spectroscopy (DNP SENS). In particular, DNP SENS provides molecular-level information about the structure of surface sites by evidencing the presence of tri-, tetra-, and pentacoordinated Y-surface sites. This study provides unprecedented evidence and tools to assess the local structure of metal surface sites in relation to their chemical and physical properties.

Recent developments in MAS DNP-NMR of materials

“…DNP has been applied to characterize the surface of nanoparticles used for catalysis [124,184,192,238,268], biomaterials [106,107], cements [269], polymer fillers [85,210] and optoelectronics devices [158,159]. The investigated nanoparticles included functionalized silica [193,227,238,270], alumina [80,124,184,196,225,231,[271][272][273], silica alumina [192,233,234,274], ceria [195,275], sulfated zirconia [276], calcium silicate hydrates [269], phosphates [106,107,210], partially oxidized Sn nanoparticles [151] and crystalline semiconductors, such as CdSe, CdS or InP, in the form of nanoparticles (also called quantum dots) [158,159]. DNP has also been applied to investigate calcium silicate hydrates [269] and crystalline semiconductors, such as CdSe and CdS, in the form of nanosheets (also called nanoplatelets) (see Figure 24) [159].…”

Silica‐Supported MnII Sites as Efficient Catalysts for Carbonyl Hydroboration, Hydrosilylation, and Transesterification

“…Elemental analysis reveals a loading of 1.8 % (0.33 mmol Mn per g). IR Spectroscopy shows the presence of remaining Me 3 SiO group (Figure S10), as previously observed when trimethylsilyl amide derivatives are used as precursor . Although EPR measurements of 2 show a fairly complex pattern (Figures S10 and S11), with an unusual g factor of 2.80 for 2 , that is likely due to spin‐orbit coupling and symmetry of molecule, a broad line at g =2.09 is observed for 2@SiO 2 .…”

“…IR Spectroscopy shows the presence of remaining Me 3 SiOg roup ( Figure S10), as previously observed when trimethylsilyl amide derivatives are used as precursor. [23,24] Although EPR measurements of 2 show af airly complex pattern (Figures S10 and S11), with an unusual g factor of 2.80 for 2,t hat is likely due to spin-orbit coupling ands ymmetry of molecule, [25] ab road line at g = 2.09 is observed for 2@SiO 2 .T he EPR signal of the manganese site free of organic ligand 2@SiO (FigureS12) is similar to that of 1@SiO with as inglei sotropic resonance andi dentical g = 2.01 (Figure S13), indicating as imilar environment after thermalt reatment and suggesting that monomeric Mn II sites could be present in both materials. The XANES spectra of 2, 2@SiO 2 ,a nd 2@SiO 2-400 ,s hare almost identicale dge energies and pre-edge features indicating that similar oxidation states and geometries are retained throughout (Figure 1b).…”

Model systems in heterogeneous catalysis: towards the design and understanding of structure and electronic properties

“…33 It has recently been suggested that butene formation results from ethylene oligomerization. 49,50 To shed more light on the structure of our model system, we carried out XPS measurements. The O 1s signal in the pristine lms on Ru(0001) contains a main signal at 531.6 eV, assigned to oxygen atoms in the various Si-O-Si bonds in the lm, and a shoulder at 529.4 eV, assigned to chemisorbed O atoms on Ru(0001) ( Fig.…”