Molecular and Immobilized Tripodal Phosphine Ligands and Their Trinuclear Palladium Complexes

The synthesis and characterization of the tripodal phosphines RSi(CH₂CH₂PPh₂)₃ (R = Me, OMe, OEt) (<b>1</b>-<b>3</b>) is described. The ¹H NMR spectra of all phosphines display virtual coupling patterns. The ligands form the corresponding trinuclear Pd complexes [RSi(CH₂CH₂PPh₂)₃]₂(PdCl₂)₃ (<b>4</b>-<b>6</b>) with three PdCl₂ moieties sandwiched between two tripodal ligands. The complexes <b>4</b>, <b>5</b>, and <b>7</b> (R = OH) have been analyzed by single crystal X-ray diffraction. The coordination at the Pd center is square planar with the phosphine groups occupying <i>trans</i> positions. The ³¹P{¹H} MAS NMR spectra of polycrystalline <b>1</b> are in accordance with the packing motif of the molecules in the unit cell. The tripodal ligand <b>3</b> has successfully been immobilized on silica as <b>3i</b>. It coordinates PdCl₂ on the surface, as demonstrated by ³¹P{¹H} MAS NMR. Hereby, the <i>cis</i> coordination is prevalent when <b>3i</b> has maximal surface coverage. At low surface coverage, one tripodal linker can accommodate <i>trans</i> coordination at the metal center. A surface-bound trinuclear Pd complex has been generated, as well as a heterobimetallic Pd/Cu complex. All surface species have been characterized by ³¹P{¹H} MAS NMR.