Palladacycles incorporating a carboxylate-functionalized phosphine ligand: syntheses,characterization and their catalytic applications toward Suzuki couplings in water

A series of acetato-bridged C^X]-type (C = aryl carbanion, X = N, P) palladacycles (1–5) of the general formula Pd(μ-CH₃COO)(C^X)]₂ were synthesized as metal precursors via slightly modified procedures. However, in the case of complex 5 with Dpbp (Dpbp = 2′-(diphenylphosphino-κP)1,1′-biphenyl]-2-yl-κC) as the supporting C^P ligand, an unexpected dinuclear complex Pd(μ-CO₂)(Dpbp)]₂ (6) was obtained as a by-product and structurally determined by X-ray crystallography. The reactions of complexes 1–4 with 2-(diphenylphosphino)benzoic acid conveniently afforded four carboxylate-functionalized phosphine complexes Pd(C^N)(Dpb)] (Dbp = 2-(diphenylphosphino-κP)benzoato-κO, 7–10), two of which (9/10) are newly synthesized in the present work and have been fully characterized. A comparative catalytic study revealed that complex Pd(Ppy)(Dpb)] (7) (Ppy = 2-(2-pyridinyl-κN)phenyl-κC) is the best performer in Suzuki cross-couplings in H₂O. In addition, complex 7 exhibits much better catalytic activity compared to the non-functionalized phosphine equivalent Pd(OAc)(PPh₃)(Ppy)] (11), which clearly indicates the superiority of incorporating a carboxylate-functionalized phosphine ligand into the palladacycles. A preliminary mechanistic study uncovered a different precatalyst initiation pathway compared to other known analogues of catalyst precursors.