Immobilization of Carbonylnickel Complexes: A Solid-State NMR Study

Silica is commonly modified with bifunctional phosphines like PPh2CH2CH2Si(OEt)3 prior to immobilization of catalysts. Here, besides PPh2(CH2)3Si(OEt)3, ligands that are more stable toward oxidation, namely PPh2(C6H4)Si(OEt)3, PPh2(C6H4)SiMe2(OEt), and PPh2(CH2)4OH, are applied. The diand tricarbonylnickel complexes of these ligands are synthesized, characterized by 61Ni, 31P, 13C, and 1H NMR and IR spectroscopy, and immobilized on silica. Alternatively, the corresponding phosphine-modified silicas are treated with Ni(CO)4. The obtained materials are investigated by 31P and 13C solid-state NMR spectroscopy. Diand tricarbonylnickel species can be distinguished by their different chemical shift anisotropies and also by their IR data. While the first route allows immobilization of both diand tricarbonylnickel complexes, reaction of surface-bound phosphines with Ni(CO)4 gives the tricarbonyl species exclusively. Among the different ligands, PPh2(CH2)4OH has optimal qualities: It gives high surface coverages without oxidation or cross-linking, and therefore narrow NMR lines, and it allows 13C CP/MAS NMR spectroscopy.