Grafting of Group‐10 Organometallic Complexes on Silicas: Differences and Similarities, Surprises and Rationale

Abstract

Surface organometallic chemistry (SOMC) represents a unique synthetic platform for the preparation of model heterogeneous catalysts resembling those broadly applied in industry. SOMC techniques usually rely on the grafting of tailored molecular precursors onto the surface OH groups of oxide supports. The development of such precursors and the understanding of their reactivity with the supports are therefore crucial for the development of well-defined surface species. While a large number of organometallic precursors of early transition metals are known, only few examples of group-10 metal complexes are reported, in spite of the great interest for heterogeneous catalysts based on the Pt-group elements. Herein, we report the reactivity of a family of group-10 (Ni, Pd and Pt) alkyl complexes, towards partially dehydroxylated SiO 2 yielding well-defined supported species. We studied the effect of the metal, ligand, and support on the grafting mechanism of such precursors through a combined experimental and computational approach. Ultimately, we showed that at least two grafting pathways are possible for these compounds, namely the protonolysis of the M-alkyl bond by surface OH groups and the opening of strained siloxane bridges: the proportion of the two depending on the nature of the metal and its ancillary ligand.

Group 10 Metal Allyl Amidinates: A Family of Readily Accessible and Stable Molecular Precursors to Generate Supported Nanoparticles

“…1600 cm -1 ) and N-H stretching bands 3400-3200 cm -1 ) emerged, indicating that grafting occurs via a similar pathway as discussed for SiO2-700. This is further supported by 13 C-MAS SSNMR spectroscopy of 1-Ni/Al2O3-600 (Figure S14).…”

“…To further corroborate the structure of the surface site, we turned to solid-state NMR spectroscopy. 13 C-MAS SSNMR spectroscopy of 1-Ni/SiO2-700 (Figure 3c.) confirmed that both the h 3 -allyl, and the amidine ligand are present in the grafted materials.…”

“…confirmed that both the h 3 -allyl, and the amidine ligand are present in the grafted materials. In the 13 C MAS SSNMR spectrum of 1-Ni/SiO2-700, the signal around 158 ppm corresponds to the quaternary carbon of the amidine, the signal around 105 ppm is assigned to the central carbon of the allyl ligand. The terminal carbon atoms of the allyl and the central carbon of the iso-propyl group are overlapped (ca.…”

“…involvement of siloxane bridges rather than OH groups). 13 Overall, these precursors are either too inert or too reactive and unstable. The prominent examples are allyl derivatives; while convenient for early transition metals to generate well-defined surface species, the corresponding derivatives of group 10, the bis-h 3 -allyl-M(II) (M=Ni, Pd, Pt) compounds, are thermally unstable.…”

Group 10 Metal Allyl Amidinates: A Family of Readily Accessible and Stable Molecular Precursors to Generate Supported Nanoparticles

“…13 C-MAS SSNMR spectroscopy of 1-Ni/SiO 2−700 (Figure 3c) confirmed that both the η 3 -allyl and the amidine ligand are present in the grafted materials. In the 13 C-MAS SSNMR spectrum of 1-Ni/ SiO 2−700 , the signal around 158 ppm corresponds to the quaternary carbon of the amidine, and the signal around 105 ppm is assigned to the central carbon of the allyl ligand. The terminal carbon atoms of the allyl and the central carbon of the iso-propyl group are overlapped (ca.…”