Solvent-tailored Pd3P0.95 nano catalyst for amide–nitrile inter-conversion, the hydration of nitriles and transfer hydrogenation of the CO bond

Abstract

For the first time, a one pot thermolysis of [Pd(PPh₃)₂Cl₂] prepared by reacting Ph₃P with PdCl₂ in a 2 : 1 molar ratio in MeOH at 280 °C in a trioctylphosphine (TOP) and oleylamine(OA)-octadecane(ODE) mixture (1 : 1) was used to prepare quantum dots (QDs; size ∼2–3 nm) and nanoparticles (NPs; size ∼13–14 nm), respectively, of composition Pd₃P₀.₉₅. TEM, SEM-EDX, powder-XRD and XPS (for QDs only) were used to authenticate the two nanophases. ³¹P{¹H}NMR experiments performed to monitor the progress of thermolysis reactions revealed that the phosphorus present in the Pd₃P₀.₉₅ QDs had come from TOP, whereas in Pd₃P₀.₉₅ NPs, its source is triphenylphosphine. The nature of the solvent did not affect the chemical composition of the nano-phase but controlled its size. Probably, solvent dependent, unique, single source precursors (SSPs) of palladium were generated in situ, and controlled the size. The catalytic activity of both Pd₃P₀.₉₅ QDs and NPs was explored. The QDs were found to be efficient as a catalyst for the amide–nitrile interconversion at room temperature (yield up to 92% in 4 h), hydration of nitriles and transfer hydrogenation (TH) of carbonyl compounds with yields up to 96% in 3–4 h. The yields and reaction rates of amide–nitrile inter-conversion and TH when catalyzed by Pd₃P_0.95 QDs were found to be higher compared to the ones observed with the Pd/C catalyst. The binding energy of Pd(3d) in the X-ray photoelectron spectrum (XPS) of Pd₃P_0.95 indicated an electron transfer from the metal to phosphorus, resulting in electron deficient palladium, which facilitates the coordination of a substrate to Pd and drives the reaction. The reusability of Pd₃P₀.₉₅ QDs for the interconversion was found to be up to 4-times, while for the transfer hydrogenation of carbonyl compounds it was up to 6-times, but with a diminished yield. Pd₃P₀.₉₅ NPs were found to be less active (yield up to 36% in optimized reaction conditions) in comparison to Pd₃P₀.₉₅ QDs. The mercury poisoning test suggested that the catalysis predominantly proceeded heterogeneously on the surface of the QDs. The PXRD and XPS results did not suggest a significant variation in the phase of QDs after the third catalytic cycle. The bleeding of Pd during catalysis (determined by flame AAS) and the agglomeration of QDs as supported by the SEM-EDX and TEM results are probably responsible for the reduction in the catalytic activity of QDs after reusing three times.