Limits of Activity: Weakly Coordinating Ligands in Arylphosphinesulfonato Palladium(II) Polymerization Catalysts

The coordination strength of various phosphine oxides
OPR3 toward the olefin polymerization catalyst (P∧O)­PdMe
(P∧O = κ2-P,O-Ar2PC6H4SO2O with Ar = 2-MeOC6H4) as
compared to that of dmso has been determined. Equilibrium constants KL for the reaction 1-dmso + L ⇆ 1-L + dmso range from 3.5 for electron-rich OPBu3 to 10–3 for electron-poor OP­(p-CF3C6H4)3. Complexes derived from more strongly
coordinating phosphine oxides, i.e. [(P∧O)­PdMe­(L)]
(1-L; L = OPBu3, OPOct3, OPPh3) have been isolated and fully characterized. Additionally, 1-OPBu3 and 1-OPPh3 were analyzed by X-ray diffraction analyses.
Complexes derived from weakly coordinating phosphine oxides have eluded
isolation due to loss of phosphine oxide and formation of barely soluble
multinuclear palladium complexes 1n by bridging coordination of the sulfonate group to various
Pd centers. Hence, the (P∧O)­PdMe fragment 1 exhibits an intrinsic limitation with respect to coordination
of weak donors. Species 1 generated in situ in the absence
of additional ligand (L) has been identified in homo- and copolymerization
experiments as well as NMR insertion studies as the most active possible
catalyst. Since 1 is generated from the easily available
precursor [{(1-Cl)-μ-Na}2)], these findings
give rapid access to highly active (P∧O)­PdMe catalysts.