Functionalized Tri- and Tetraphosphine Ligands as a General Approach for Controlled Implantation of Phosphorus Donors with a High Local Density in Immobilized Molecular Catalysts

Supported phosphine ligands are auxiliaries of topical academic and industrial interest in catalysis promoted by transition metals. However, both controlled implantation and controlled conformation of ligands should be achieved to produce immobilized catalysts that are able to structurally ``copy'' efficient homogeneous systems. We provide herein a general synthetic strategy for assembling a new class of branched tetra- and triphosphine ligands with a unique controlled rigid conformation, and thus providing a high local density of phosphorus atoms for extended coordination to the metal center. We prepared new functionalized cyclopentadienyl (Cp) salts to design polyphosphines that were ``ready for immobilization''. These protected Cp fragments might also be of synthetic utility for generating other supported metallocenes. Tetra- and triphosphines were then formed and diversely functionalized for immobilization on virtually any kind of support. As evidenced by P-31 NMR spectroscopy and the strong ``through-space'' spin-spin (TS)J(P,P) coupling observed between P atoms, these modified polyphosphines induce, when immobilized on a support, a high local concentration of phosphorus donors that are spatially very close to each other (3-5 angstrom). These functionalized ligands have been incorporated into polystyrene to give either soluble or insoluble polymers and on inorganic supports such as silica. We report the catalytic performance at 0.05-0.5 mol% low palladium loading of the resulting immobilized polyphosphine ligands. This study provides proof-of-concept of supported catalytic materials built from modular polyphosphines with a novel approach to structurally controlled polydentate heterogeneous catalysts.