TY - JOUR
T1 - Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies
AU - Delley, Murielle F.
AU - Lapadula, Giuseppe
AU - Núñez-Zarur, Francisco
AU - Comas-Vives, Aleix
AU - Kalendra, Vidmantas
AU - Jeschke, Gunnar
AU - Baabe, Dirk
AU - Walter, Marc D.
AU - Rossini, Aaron J.
AU - Lesage, Anne
AU - Emsley, Lyndon
AU - Maury, Olivier
AU - Copéret, Christophe
PY - 2017/7/5
Y1 - 2017/7/5
N2 - © 2017 American Chemical Society. 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(SiMe3)2)3vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2or HOSi(OtBu)3for 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-enhanced89Y 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.
AB - © 2017 American Chemical Society. 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(SiMe3)2)3vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2or HOSi(OtBu)3for 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-enhanced89Y 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.
U2 - 10.1021/jacs.7b02179
DO - 10.1021/jacs.7b02179
M3 - Article
SN - 0002-7863
VL - 139
SP - 8855
EP - 8867
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 26
ER -