Self-assembled tetrameric phosphine-bis(arenesulfonate) (OPO) palladium catalysts (Figure 1, A) based on a Li4S4O12 cage were discovered to produce high molecular weight linear polyethylene (PE) and incorporate higher levels of vinyl fluoride compared to the non-self-assembled mononuclear analogues (Figure 1, B). Cooperative effects involving the two proximal Pd centers and steric blockage of an axial site at each Pd center by the cage are proposed to explain the high MW PE production. However, the Li4S4O12 cages undergo partial dissociation to monomeric species (Pd1) under polymerization conditions, which complicates mechanistic studies and applications in polymer synthesis. It is interesting to investigate other Pd4 catalysts based on different cage structures and explore their polymerization behaviors. Two types of Pd4 compounds based on metal phosphonate cages have been targeted for study in this work. Phosphine-phosphonate-sulfonate (OPPSO) palladium catalysts (Figure 1, C) that are based on zinc phosphonate cage structures are designed to be structural analogues of the OPO Pd4 catalyst. The sulfonate group will have one oxygen coordinate to Pd and another one cap the Zn corner. Borophosphonate cages that contain pendant phosphine-arensulfonate ligands covalently attached to the B corner of the cage are designed to form Pd4 catalysts in which the Pd centers are well separated (Figure 1, D). The synthesis, structure, and reactivity of the cage compounds will be discussed.

Synthesis of metal phosphonate cage compounds for tetranuclear Pd polymerization catalysts

251st National Meeting (2016)

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