Suzuki Coupling Reactions Catalyzed with Palladacycles and Palladium(II) Complexes of 2‐Thiophenemethylamine‐Based Schiff Bases: Examples of Divergent Pathways for the Same Ligand

Abstract

AbstractActivation of Suzuki coupling with a complex of palladium varies with the mode of coordination of its ligand. The moisture‐/air‐insensitive palladacycles and palladium complexes designed using the same ligand (Schiff base, coordinating as an N,C– and N,O– ligand, respectively) have been found to follow different pathways. Palladacycles are more efficient, as their 0.001 mol‐% loading gives good conversion (yield > 90 %) in several cases. Higher loading than this is required for Pd complexes to obtain a similar yield. Activation with palladacycles involves the role of nanosized Pd‐containing species generated in situ during catalysis, in which, as with PdII complexes, no such particle is formed and Pd0 probably remains protected by the sulfur of the thienyl group. The Schiff bases used here were designed by the reaction of 2‐thiophenemethylamine with 2‐hydroxybenzophenone/2‐hydroxy‐4‐methoxybenzophenone/2‐hydroxyacetophenone (L1/L2/L3). Upon treatment with [PdCl2(CH3CN)2] and [Na2PdCl4] they gave palladacycles [PdL1/L2(CH3CN)Cl] (1/3) and palladium(II) complexes [Pd(L1/L2/L3)2] (2/4/5), respectively. Compounds L1–L3 and their complexes 1–5 were authenticated with 1H and 13C{1H} NMR spectroscopy, and HRMS. Single crystal structures of 1, 2, 4, and 5 reveal nearly square‐planar geometry around Pd in each case. The PPh3/Hg poisoning and two‐phase tests indicate that the catalysis is homogeneous for both the palladacycle and palladium complexes, probably through leaching of Pd0 from NPs in the case of the former.