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.

Keywords: DFT calculations; X-ray diffraction; copper; dithiones; gold; iodine; palladium; precious metals; recycling.

Scheme 1

Acyclic and cyclic hexa- and hepta-atomic dithiooxamides.

Scheme 2

Reversible redox processes of α-dithiones and typical bond distances on the C₂S₂ moiety [26].

Scheme 3

Reaction scheme for gold, palladium and copper powder dissolution by Me₂pipdt/I₂ mixtures. Reaction conditions: THF or CH₃CN as solvent; room temperature; stirring. Complete leaching time: 1 h; Stoichiometry of product crystallization: (1) n = 1, m = 2.5; (2) n = 2, m = 3; (3) n = 2, m = 1.5.

Figure 1

(a) Molecular structure of complex [Au(Me₂pipdt)I₂]I₃ (1) with thermal ellipsoids plotted at the 50% probability level. (b) Portion of the crystal packing, showing intermolecular interactions as dashed bonds. Selected interaction distances (Å) and angles are indicated.

Figure 2

(a) Molecular structure of complex [Pd(Me₂pipdt)₂](I)₂ (2a) with thermal ellipsoids plotted at the 50% probability level. (b) Portion of the crystal packing, showing intermolecular interactions as dashed bonds.

Figure 3

(a) Molecular structure of complex [Cu(Me₂pipdt)₂]I₃ (3) with thermal ellipsoids plotted at the 50% probability level. (b,c) Portions of the crystal packing, showing intermolecular interactions as dashed bonds.

Figure 4

(a) Molecular structure of complex [Cu(Me₂pipdt)₂]BF₄ (4) with thermal ellipsoids plotted at the 50% probability level. One molecular entity of the two comprising the asymmetric unit is reported for clarity. (b) Portion of the crystal packing, showing intermolecular interactions as dashed bonds.

Figure 5

(a) Molecular structure of complex [Cu(Me₂dazdt)₂]I₃ (5) with thermal ellipsoids plotted at the 50% probability level. (b) Portion of the crystal packing. Intermolecular interactions are depicted as dashed bonds. Halogen bond interaction is reported in Å.

Figure 6

Depiction of the molecular planes used to calculate the dihedral angle between the S,S-chelate ligands (a) and between the thioamido groups (b). See also Tables S2 and S3.

Figure 7

DFT calculated molecular orbitals and orbital diagram for cationic complexes 1′, 2′ and 3′. Starting from the upper part, the figure depicts all the MO ranging from the LUMO+2 to the HOMO-2; for each complex, HOMO and LUMO are highlighted in pale green and yellow colors, respectively (isovalue = 0.04).