Polymerization of 1-chloro-2-phenylacetylene by cationic monoanionic tridentate (: S, S)-bis(oxazolinylphenyl)amido-ligated palladium catalysts: Is it a coordination-insertion mechanism?

A series of air-stable monoanionic tridentate (S,S)-bis(oxazolinylphenyl)amido-ligated palladium (Pd) chloride complexes (R₂-(S,S)-BOPA)PdCl (1a-e) (1a: R = Ph; 1b: R = ⁱPr; 1c: R = Bn; 1d: R = Et; 1e: R = Me) have been synthesized and structurally characterized. The double decomposition reaction of these Pd chloride complexes 1a-e with one equivalent of silver trifluoromethanesulfonate (AgOTf) quantitatively affords the corresponding cationic monoanionic tridentate Pd species [(R₂-(S,S)-BOPA)Pd]⁺[OTf]^- (2a-e) as characterized by ¹H and ¹³C NMR spectroscopy. Such single-component or in situ generated cationic Pd species are active for the polymerization of once used 1-chloro-2-phenylacetylenes (CPAs) bearing nonpolar, halogen, and polar substituents (3a: Cl-CC-C₆H₄-p-Me; 3c: Cl-CC-C₆H₅; 3e: Cl-CC-C₆H₄-p-F; 3f: Cl-CC-C₆H₄-p-Cl; 3h: Cl-CC-C₆H₄-p-NO₂; 3j: Cl-CC-C₆H₄-p-COCH₃) even in air, affording partly soluble cis-selective poly(1-chloro-2-phenylacetylene)s (PCPAs) with high molecular weights and broad molecular weight distributions similar to those obtained by the known Pd catalysts. More significantly, the single-component cationic Pd species can also promote the polymerization of unused CPAs (3b: Cl-CC-C₆H₄-m-Me; 3d: Cl-CC-C₆H₄-p-OMe; 3g: Cl-CC-C₆H₄-p-COOEt; 3i: Cl-CC-(1-C₁₀H₇)) for the first time, providing novel cis-selective PCPAs which are unavailable from traditional catalysts. Similar to the known Pd catalysts, a possible coordination-insertion mechanism is proposed by means of ¹H NMR, IR, and ESI-MS spectroscopies, which provides a new insight into the initiation and termination polymerization process of CPAs catalyzed by these cationic monoanionic tridentate (S,S)-bis(oxazolinylphenyl)amido-ligated Pd catalysts.