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. 2022 Sep 29;27(19):6421.
doi: 10.3390/molecules27196421.

From Gas Phase Observations to Solid State Reality: The Identification and Isolation of Trinuclear Salicylaldoximato Copper Complexes

Benjamin D Roach  1 , Ross S Forgan  1 , Eduardo Kamenetzky  2 , Simon Parsons  1 , Paul G Plieger  3 , Fraser J White  1 , Sidney Woodhouse  3 , Peter A Tasker  1
Affiliations

From Gas Phase Observations to Solid State Reality: The Identification and Isolation of Trinuclear Salicylaldoximato Copper Complexes

Benjamin D Roach et al. Molecules. .

Abstract

Conditions have been identified in which phenolic aldoximes and ketoximes of the types used in commercial solvent extraction processes can be doubly deprotonated and generate polynuclear Cu complexes with lower extractant:Cu molar ratios than those found in commercial operations. Electrospray mass spectrometry has provided an insight into the solution speciation in extraction experiments and has identified conditions to allow isolation and characterization of polynuclear Cu-complexes. Elevation of pH is effective in enhancing the formation of trinuclear complexes containing planar {Cu3-μ3-O}4+ or {Cu3-μ3-OH}5+ units. DFT calculations suggest that such trinuclear complexes are more stable than other polynuclear species. Solid structures of complexes formed by a salicylaldoxime with a piperidino substituent ortho to the phenolic OH group (L9H2) contain two trinuclear units in a supramolecular assembly, {[Cu3OH(L9H)3(ClO4)](ClO4)} 2, formed by H-bonding between the central {Cu3-μ3-OH}5+ units and oxygen atoms in the ligands of an adjacent complex. Whilst the lower ligand:Cu molar ratios provide more efficient Cu-loading in solvent extraction processes, the requirement to raise the pH of the aqueous phase to achieve this will make it impractical in most commercial operations because extraction will be accompanied by the precipitation (as oxyhydroxides) of Fe(III) which is present in significant quantities in feed solutions generated by acid leaching of most Cu ores.

Keywords: Cu extraction; Electrospray mass spectrometry; oxime proligands; solvent extraction; trinuclear complexes.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The pH-dependent extraction of Cu(II) by phenolic oximes.
Figure 2
Figure 2
The structure of the hexanuclear complex [{Cu3O}2H(L1H2)3]3− derived from L1H4 and a representation of the triangular cores [21].
Figure 3
Figure 3
The phenolic oxime extractants and related proligands studied in this work. a—Details of synthesis and characterization can be found in the Experimental Section.
Figure 3
Figure 3
The phenolic oxime extractants and related proligands studied in this work. a—Details of synthesis and characterization can be found in the Experimental Section.
Figure 4
Figure 4
The [Cu3Dy3(L3H)O]7+ component of the heterometallic cluster [Cu8Dy3(L3H)6(μ4-O)2Cl6(H2O)8]Cl8 formed by the 2-hydroxymethyl-substituted phenolic oxime L3H3. All three Dy atoms are displaced to the same side of the Cu3 triangle [23].
Figure 5
Figure 5
A representation of the Cu3O and Cu3OH cores present in complexes formed by 2-pyridinaldoxime, LAH [26,34], and some related α-imino-oximes (LBH–LDH) [35,36,37] which have only one ionizable H atom.
Figure 6
Figure 6
Suggested structures, together with their observed and calculated monoisotopic masses, for the dominant monoanionic species (a–d) in an acetonitrile solutions with different Cu(II):L4H2 molar ratios.
Figure 7
Figure 7
The high-resolution mass spectrum of a 50µM solution of L5H2 and copper acetate in acetonitrile. The observed and the calculated isotopic peak distribution and masses for [Cu3O(L5)3]− are inset.
Figure 8
Figure 8
A representation of the origins of the stabilization of the monoanions [Cu(L)(LH)]− by H-bonding in the lower part of the complexes (---) and of the destabilization by repulsion between electronegative atoms (double-headed red arrows in the upper part of the complexes).
Figure 9
Figure 9
The mass spectrum obtained from a MeCN solution of Cu(II) acetate (50 µM) and L6H2 and L8H2 (each 25 µM). Proposed structures are inset with observed and calculated monoisotopic masses.
Figure 10
Figure 10
UV/Vis spectra of 80µM solutions of L7H2 in MeCN containing varying concentrations of Cu(OAc)2.H2O (L7H2to Cu2+ molar ratio ranging from 1:3 to 8:1).
Figure 11
Figure 11
Low and high resolution (left and right) plots of the peak assigned to [Cu3O(L9H)3]2+ in a mass spectrum obtained from a 50 µM acetonitrile solution of [Cu3OH(L9H)3(ClO4)2]1.5H2O.
Figure 12
Figure 12
(a) The UV/Vis absorption spectra for varying concentrations of L9H2, [Cu(L9H)2] and {[Cu3OH(L9)3(ClO4)](ClO4)}2 and (b) their respective molar extinction coefficients, obtained by the absorbance vs. concentration plots form (a).
Figure 13
Figure 13
Components of the two crystalline forms (A,B) of the hexanuclear complexes {[Cu3OH(L9H)3(ClO4)](ClO4)}2.2H2O.2MeCN and {[Cu3OH(L9H)3(ClO4)](ClO4)}2.11H2O.MeCN. The trinuclear entities [Cu3OH(L9H)3]2+ and the hexanuclear units [Cu3OH(L9H)3(ClO4)]22+are shown on the left and right, respectively. H-atoms not involved in H-bonding have been removed for clarity.
Figure 14
Figure 14
The central Cu-chelating units of forms (A,B) of the hexanuclear complexes {[Cu3OH(L9H)3(ClO4)](ClO4)}2.1.5H2O and {[Cu3OH(L9H)3(ClO4)](ClO4)}2.11H2O.MeCN showing the secondary bonding which creates the dimeric structures. The shortest Cu…O contact distances and the O…O distances associated with hydrogen bonds formed by the µ3-OH groups listed are marked. The pendant piperidinium groups have been removed for clarity.
Figure 15
Figure 15
The energy-minimized structures for (a) [Cu3(L2)3μ3-OH]−, (b) [Cu3(L2)3μ3-OH], (c) [Cu3(L2)3μ3-O]2− and (d) [Cu3(L2)3μ3-O]−.
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