Self-assembled phosphine-bis(arenesulfonate) (OPO) tetranuclear palladium catalysts based on Li4S4O12 cage were discovered to produce high molecular weight linear polyethylene (PE), however the dissociation of Li4S4O12 cage into the monomeric Pd1 species under polymerization condition causes a broad molecular weight distribution. Pd4 catalysts based on borophosphonate cage structures are designed to overcome the dissociation problem due to the stronger Lewis acid-base interactions. Synthesis of simple borophosphonate cages was studies first. Aryl borophosphonate cage compounds [ArPO3BAr’]n with n = 4 or 6 were synthesized by condensation reactions of ArP(O)(OH)2 and Ar’B(OH)2. (3,5-tBu2-Ph)P(O)(OH)2 (1) reacts with arylboronic acids that contain electron-withdrawing substituents to form borophosphonate tetramers [Ar1PO3BAr2]4 (Ar1 = 3,5-tBu2-Ph; Ar2 = o-Br-Ph, o-CF3-Ph, p-CF3-Ph, p-CHO-Ph; 3a-d) and hexamers [Ar1PO3BAr2]6 (Ar2 = p-CF3-Ph, p-CHO-Ph; 4c-d) in 80 - 93 % NMR yield. For Ar2 = p-CF3-Ph and p-CHO-Ph, since both reactants are less steric demanding, both products were observed, with the tetramer being favored under dilute reaction conditions and the hexamer favored under concentrated reaction conditions. Interconversion between tetramer (3c or 3d) and hexamer (4c or 4d) was observed at room temperature and above in toluene. The phosphine phosphonic acid (2-PPh2-Ph)P(O)(OH)2 (6) reacts with arylboronic acids that contain electron-withdrawing substituents to form tetramers [Ar1PO3BAr2]4 (Ar1 = 2-PPh2-Ph; Ar2 = p-CF3-Ph, p-CHO-Ph; 7c-d) in 70 - 75 % NMR yield. The reactions of 1 or 6 with (p-tolyl)B(OH)2 (2f), and the reaction of 6 with (o-CF3-Ph)B(OH)2 (2b), yield only trace amounts of borophosphonate cage compounds, and instead afford the corresponding [ArBO]3 boroxines and condensation products with unknown structures. Recent studies on borophosphonate cage functionalization and other related cage structures will also be discussed.

Tetranuclear Pd catalysts based on metal phosphonate cages for olefin polymerization

250th National Meeting (2015)

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