Palladium(ii)-(E,N,E) pincer ligand (E = S/Se/Te) complex catalyzed Suzuki coupling reactions in water via in situ generated palladium quantum dots

Abstract

The (E,N,E) pincer ligands (ArECH₂CH₂)₂NH (L1/L2: Ar = Ph, E = S/Se; L3: Ar = CH₃O-p-C₆H₄, E = Te) synthesized by reaction of PhS⁻/PhSe⁻/CH₃O-p-C₆H₄Te⁻ with bis(2-chloroethyl)amine react with Na₂PdCl₄ in aqueous ethanol, resulting in nearly square planar diamagnetic complexes [Pd(L)Cl]Cl (1–3), where L = L1–L3. All the ligands (L1–L3) and their complexes (1–3) have been characterised with ¹H, ¹³C{¹H}, ⁷⁷Se{¹H} and ¹²⁵Te{¹H} NMR spectra and high resolution mass spectrometry. The single crystal structures (determined with X-ray diffraction) of 2 and 3 have been solved (Pd–Se: 2.4104(5)/2.4222(6) Å; Pd–Te: 2.560(2)/2.588(2) Å). The conversions for Suzuki–Miyaura coupling (SMC) of various aryl bromides with phenylboronic and 4-formyl/acetyl phenylboronic acid in water using 2–3 mol% of each of the complexes 1–3 have been found good. Complexes 1 and 2 show better catalytic activity than 3, as higher yields were observed with them in a relatively short time. The coupling reactions appear to be catalyzed with Pd(0) nanoparticles (NPs) generated in situ in the course of reaction. The NPs have been isolated and HRTEM studies on them have revealed their size as ∼1–3 nm. The SEM-EDX indicates their protection with organochalcogen fragments. Addition of TBAB was essential in some cases to get good yield of cross coupled product. The isolated NPs show catalytic activity for SMC independently. The yields of cross coupled product were excellent when NPs were reused. The two phase test suggests a relatively low contribution of homogeneous Pd species in catalysis.