From recovered metal waste to high-performance palladium catalysts

Abstract

The catalytic activity of a series of neutral and cationic, homo- and heteroleptic, mono- and bimetallic palladium(II) compounds based on dithiocarbamate and dithiooxamide S,S-donor ligands is described. High activity was observed in the regio- and chemo-selective C–H functionalization of benzo[h]quinoline to 10-alkoxybenzo[h]quinoline and 8-methylquinoline to 8-(methoxymethyl)quinoline in the presence of the oxidant PhI(OAc)2. The best performance was found for [Pd(Me2dazdt)2]I6 (Me2dazdt = N,N′-dimethyl-perhydrodiazepine-2,3-dithione), [PdI2(Me2dazdt)] and [Pd(Cy2DTO)2]I8 (Cy2DTO = N,N′-dicyclohexyl-dithiooxamide) which are all obtained directly as products of sustainable Pd-metal leaching processes used to recover palladium from scrap metal. These compounds provided almost quantitative yields under milder conditions (50 °C, 1–3 mol% Pd loading) and much shorter reaction times (1–3 h) than reported previously. These results illustrate how the complexes obtained from the selective and sustainable recovery of Pd from automotive heterogeneous Three Way Catalysts (TWC) can be employed directly in homogeneous catalysis, avoiding further metal recovery steps and valorising the metal complex itself in a ‘circular economy’ model

From Recovered Palladium to Molecular and Nanoscale Catalysts

“…of the complexes to be employed in catalysis, starting from both a commercial salt, cis- were prepared accordingly to Scheme 3, which shows the addition of K[L1] and K[L2] to cis-[PdCl2(PPh3)2] in the presence of NH4PF6. Complexes 3 and 4 were fully characterized by 1 H NMR, 13 C{ 1 H} and 31 P{ 1 H} NMR and IR spectroscopy as well as mass spectrometry and elemental analysis. The 1 H and 31 P{ 1 H} NMR spectra both showed an upfield shift in their resonances due to the electron back-donation from the palladium center to the dithiocarbamate ligand 26 and the σ-donation of the adjacent dithiocarbamate ligand.…”

“…In addition to the new resonance observed at 30.5 ppm for both 3 and 4 in the 31 P{ 1 H} NMR spectra, the presence of the hexafluorophosphate counteranion was evidenced by a characteristic septet. 13 C{ 1 H} NMR analysis revealed characteristic low-field singlets for the CS2 carbon nuclei at around 203 ppm. Further support for the formulation came from the solid state infrared spectra, which showed the presence of typical vibrational modes for phenyl rings attached to the phosphorous center (1000 and 1470 cm -1 ) and the ν(P-F) vibrational mode of the PF6counter-ion (830 cm -1 ).…”

“…10 These complexes were found to be as effective as the [Pd(OAc)2] typically employed in such reactions. 11,12 These investigations have been extended to catalysts supported by dithiocarbamate ligands, 13 which are simple and straightforward to prepare. [14][15][16] Moreover, in tune with the 'circular economy' concept, which aims to limit the use of critical raw materials, 17 we will show how it is possible to use a Pd recovery product, derived from a different kind of secondary resources (such as spent automotive three-way catalysts, WEEE etc.)…”

“…19 These leaching reactions are highly selective for palladium and are very promising for practical applications due to the low environmental impact of the reactants and working conditions, 20 in contrast to the dangerous and energy-intensive conventional pyro-and hydrometallurgical methods used in industry. [21][22][23] The palladium complexes obtained from these sustainable leaching reactions have recently been demonstrated to work very well in homogeneous Pd(II)-catalyzed C-H activation 13 and as suitable precursors of Pd(0) photocatalysts for H2 production. 24 These discoveries were the starting point for this contribution, which explores the potential of using the recovery products as precursors for a new magnetically-recoverable catalyst system.…”

“…Functionalization of benzo[h]quinoline in methanol with a Pd(II) catalyst and PhI(OAc)2 as the oxidant. 11 Dithiocarbamate-based Pd(II) catalysts were selected as promising for this purpose, due to their demonstrated catalytic activity in this reaction, 13 as well as the versatility offered by these ligands with regard to the incorporation of additional functionality (e.g., tethering groups) at the NR2 unit. 14…”