Characterization and Structural Insights of the Reaction Products by Direct Leaching of the Noble Metals Au, Pd and Cu with N,N′-Dimethyl-piperazine-2,3-dithione/I2 Mixtures

Abstract

In the context of new efficient and safe leaching agents for noble metals, this paper describes the capability of the Me2pipdt/I2 mixture (where Me2pipdt = N,N′-dimethyl-piperazine-2,3-dithione) in organic solutions to quantitatively dissolve Au, Pd, and Cu metal powders in mild conditions (room temperature and pressure) and short times (within 1 h in the reported conditions). A focus on the structural insights of the obtained coordination compounds is shown, namely [AuI2(Me2pipdt)]I3 (1), [Pd(Me2pipdt)2]I2 (2a) and [Cu(Me2pipdt)2]I3 (3), where the metals are found, respectively, in 3+, 2+ and 1+ oxidation states, and of [Cu(Me2pipdt)2]BF4 (4) and [Cu(Me2dazdt)2]I3 (5) (Me2dazdt = N,N′-dimethyl-perhydrodizepine-2,3-dithione) compared with 3. Au(III) and Pd(II) (d8 configuration) form square–planar complexes, whereas Cu(I) (d10) forms tetrahedral complexes. Density functional theory calculations performed on the cationic species of 1–5 help to highlight the nature of the bonding in the different complexes. Finally, the valorization of the noble metals-rich leachates is assessed. Specifically, gold metal is quantitatively recovered from the solution besides the ligands, showing the potential of these systems to promote metal recycling processes.

Iodine‐Catalysed Dissolution of Elemental Gold in Ethanol

“…Elemental halogens (I 2 , Br 2 ) are also introduced to the concept as safer oxidants. Commonly, halogens are coupled in a charge-complex [34,[38][39][40][41][42][43][44][45][46] (Figure 1, c) with various S-donor ligands, in particular, dithiooxamides, [38][39][40][41][42] dithiocarbamates, [42][43][44] thiourea [44,45] and thioimidophosphinic acid [46] compounds. The latter then stabilize Au III and Au I centres in THF, [38][39][40][41] MeCN, [41,47,48] DCM, [45,48] chloroform, [39] DMSO, [49] acetone, [41][42][43] MEK, [41] Et 2 O, [44,46] ionic liquids, [50,51] deep eutectic solvent, [52] or even in water.…”

“…Commonly, halogens are coupled in a charge‐complex[ 34 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ] (Figure 1 , c) with various S ‐donor ligands, in particular, dithiooxamides,[ 38 , 39 , 40 , 41 , 42 ] dithiocarbamates,[ 42 , 43 , 44 ] thiourea[ 44 , 45 ] and thioimidophosphinic acid [46] compounds. The latter then stabilize Au III and Au I centres in THF,[ 38 , 39 , 40 , 41 ] MeCN,[ 41 , 47 , 48 ] DCM,[ 45 , 48 ] chloroform, [39] DMSO, [49] acetone,[ 41 , 42 , 43 ] MEK, [41] Et 2 O,[ 44 , 46 ] ionic liquids,[ 50 , 51 ] deep eutectic solvent, [52] or even in water. [34] Similar to aqueous iodine‐iodine leaching, over stoichiometric amounts of halogen are needed for quantitative dissolution of Au.…”

Quantitative, room‐temperature, solvent-free mechanochemical oxidation of elemental gold into organosoluble gold salts

“…The recovery of these soluble gold salts in a simple, rapid, and sustainable fashion for direct use is hindered by the low volatility of water, and the water-soluble nature of excess reagents and byproducts. There have been numerous recent attempts to address these problems, e.g., by selective precipitation, 9, 10 chelation of aurate species using macrocyclic hosts, [11][12][13] 3D-printed scavenger filters, 14 direct one-pot conversion of elemental gold to organo-soluble complexes. [15][16][17][18] We have previously shown that mechanochemistry, which encompasses the use of mechanical forces like grinding and shearing to initiate, sustain, and conduct chemical transformations, 19 is a simple, rapid, and environmentally-friendly method to convert noble metals such as Pd, Au, and Pt into water-soluble salts.…”

Iodine‐Catalysed Dissolution of Elemental Gold in Ethanol

“…Elemental halogens (I 2 , Br 2 ) are also introduced to the concept as safer oxidants. Commonly, halogens are coupled in a charge‐complex [34, 38–46] (Figure 1, c) with various S ‐donor ligands, in particular, dithiooxamides, [38–42] dithiocarbamates, [42–44] thiourea [44, 45] and thioimidophosphinic acid [46] compounds. The latter then stabilize Au III and Au I centres in THF, [38–41] MeCN, [41, 47, 48] DCM, [45, 48] chloroform, [39] DMSO, [49] acetone, [41–43] MEK, [41] Et 2 O, [44, 46] ionic liquids, [50, 51] deep eutectic solvent, [52] or even in water [34] .…”

“…Commonly, halogens are coupled in a charge-complex [34,[38][39][40][41][42][43][44][45][46] (Figure 1, c) with various S-donor ligands, in particular, dithiooxamides, [38][39][40][41][42] dithiocarbamates, [42][43][44] thiourea [44,45] and thioimidophosphinic acid [46] compounds. The latter then stabilize Au III and Au I centres in THF, [38][39][40][41] MeCN, [41,47,48] DCM, [45,48] chloroform, [39] DMSO, [49] acetone, [41][42][43] MEK, [41] Et 2 O, [44,46] ionic liquids, [50,51] deep eutectic solvent, [52] or even in water. [34] Similar to aqueous iodine-iodine leaching, over stoichiometric amounts of halogen are needed for quantitative dissolution of Au.…”