Palladium(ii) complexes of pyrazolated thio/selenoethers: syntheses, structures, single source precursors of Pd4Se and PdSe nano-particles and potential for catalyzing Suzuki–Miyaura coupling

Abstract

The reactions of 4-bromo-1-(2-chloroethyl)-1H-pyrazole prepared from 4-bromopyrazole with the in situ generated PhSNa, PhSeNa, Na₂S and Na₂Se have resulted in thio/selenoether ligands L1–L4 respectively. The complexes [PdL1/L2Cl₂](1–2) and [PdL3/L4Cl]BF₄ (3–4) of these ligands have been synthesized by reacting them with [PdCl₂(CH₃CN)₂] in CH₃CN at 70 °C. The L1–L4 and their complexes (1–4) have been characterized with spectroscopic techniques viz.¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR, IR and HR-MS. Single crystal structures of 1–4 determined by X-ray diffraction reveal nearly square planar geometry around Pd in each case. Thermally stable, moisture/air insensitive complexes 1–4 have been found to be efficient pre-catalysts for Suzuki–Miyaura coupling reactions (yield up to 96% in 2 h). Nano-particles (NPs) (size ∼3–19 nm) were formed in the beginning of these reactions. They are composed of Pd and S or Se with more % of Pd. These NPs also catalyze Suzuki coupling and appear to play an important role in catalysis. Single source one pot synthesis of Pd₄Se and PdSe NPs (size ranges ∼8–26 nm), capped with trioctylphosphine (TOP), has been developed by thermolysis of 2 and 4 at 200–250 °C in TOP. HR-TEM, SEM, SEM-EDX and powder XRD have been used to authenticate these nano-particles. The NPs of PdP₂ are formed when attempts were made to prepare nano-sized phases of palladium-sulfide by thermolysis of 1 and 3 in TOP.