Solid-State NMR Investigations of the Immobilization of a BF4- Salt of a Palladium(II) Complex on Silica

The structure of the silica supported palladium(II) complex [Pd(dppp)(S2C-NEt2)]BF4 (abbreviated as [Pd(dppp)(dtc)]BF4, where dppp is Ph2P(CH2)(3)PPh2) and interactions between the [Pd(dppp)(dtc)](+) cation, the BF4- anion, and the silica surface are studied using solid-state NMR spectroscopy. The unsupported, crystalline form of [Pd(dppp)(dtc)]BF4 is also investigated, both by X-ray diffraction and NMR. The structures of the cation and anion are found to be essentially the same in both unsupported and supported complex. The [Pd(dppp)(dtc)]BF4 loading has been determined by quantitative measurements of B-11, F-19, and P-31 intensities, whereas the arrangement of anions and cations on the surface of silica has been established by two-dimensional heteronuclear correlation experiments involving H-1, B-11, C-13, F-19, Si-29, and P-31 nuclei. At low coverages, the [Pd(dppp)(dtc)](+) cations are located near the BF4- anions, which in turn are immobilized directly on the surface near the Q(4) sites. At higher loadings, which in this study corresponded to 0.06-0.15 mmol/g, the complexes stack on top of each other, despite the fact that the directly adsorbed molecules take up less than 10% of the silica surface. The relevance of these findings to heterogeneous catalysis is discussed.