Mono- and binuclear palladacycles via regioselective C–H bond activation: syntheses, mechanistic insights and catalytic activity in direct arylation of azoles

Abstract

Regioselective C–H bond palladation of the hybrid pincer-type ligands, 3-R₂PO-C₆H₄-1-CH₂NⁱPr₂ [^R₂POCN^iPr₂–H; R = Ph (1a), R = Et₂N (1b)] has been described to accomplish mono- and binuclear palladacycles. The reactions of the ligands ^R₂POCN^iPr₂–H (1a, 1b) with Pd(COD)Cl₂ in the presence of K₃PO₄ afforded the mononuclear pincer complexes {κ^P,κ^C,κ^N-2-(R₂PO)-C₆H₃-6-(CH₂NⁱPr₂)}PdCl (^R₂POCN^iPr₂)PdCl [R = Ph (2a), R = Et₂N (2b)], whereas similar reactions in the presence of Et₃N produced the chloro-bridged binuclear palladacycles [{κ^P,κ^C-2-(R₂PO)-C₆H₃-4-(CH₂NⁱPr₂)}(μ-Cl)Pd]₂ {R = Ph (4a), R = Et₂N (4b)} via the regioselective ligands C(2)–H and C(4)–H bond activation, respectively. Similarly, the reaction of a previously reported ligand ^iPr₂POCN^iPr₂–H (1c) with Pd(COD)Cl₂ in the presence of Et₃N affords a chloro-bridged binuclear complex [{κ^P,κ^C-2-(ⁱPr₂PO)-C₆H₃-4-(CH₂NⁱPr₂)}(μ-Cl)Pd]₂ (4c) via the regioselective C(4)–H bond palladation. On the contrary, using Pd(OAc)₂ instead of Pd(COD)Cl₂ in the palladation reaction of 1a, in the presence or absence of base, leads to the exclusive formation of the acetate-bridged binuclear palladacycle [{κ^P,κ^C-2-(Ph₂PO)-C₆H₃-4-(CH₂NⁱPr₂)}(μ-OAc)Pd]₂ (5a). Mechanistic detail for this regioselective C–H bond activation to achieve mono- and binuclear palladacycles has been demonstrated, which highlights that the coordinating ability of the external base, sterics between the coordinated base and N-arm of the POCN ligand as well as the electrophilicity of the palladium center are crucial to the regioselective palladation. The palladium complex 4c was shown to be the active catalyst for the direct arylation of azoles with aryl iodides under mild reaction conditions.