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

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)3 vs siloxides (M(OSi(OtBu)3)3·L with L = (THF)2 or HOSi(OtBu)3 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 oli...