Immobilization of Carbonylnickel Complexes:  A Solid-State NMR Study

Silica is commonly modified with bifunctional phosphines like PPh 2 CH 2 CH 2 Si(OEt) 3 prior to immobilization of catalysts. Here, besides PPh 2 (CH 2 ) 3 Si(OEt) 3, ligands that are more stable toward oxidation, namely PPh 2 (C 6 H 4 )Si(OEt) 3, PPh 2 (C 6 H 4 )SiMe 2 (OEt), and PPh 2 (CH 2 ) 4 OH, are applied. The di- and tricarbonylnickel complexes of these ligands are synthesized, characterized by 61 Ni, 31 P, 13 C, and 1 H 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 31 P and 13 C solid-state NMR spectroscopy. Di- and 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 di- and tricarbonylnickel complexes, reaction of surface-bound phosphines with Ni(CO) 4 gives the tricarbonyl species exclusively. Among the different ligands, PPh 2 (CH 2 ) 4 OH has optimal qualities: It gives high surface coverages without oxidation or cross-linking, and therefore narrow NMR lines, and it allows 13 C CP/MAS NMR spectroscopy.