Stabilization of Copper(III) Ions with Deprotonated Hydroxyiminoamide Ligands: Syntheses, Structures, and Electronic Properties of Copper(II) and Copper(III) Complexes

Abstract

Copper(II) and copper(III) complexes were prepared with two novel ligands, N,N′‐bis(2‐(1‐hydroxyimino‐2‐methyl‐1‐phenyl)propyl)dimethylmalondiamide (H4mal55) and N,N′ bis(‐(1‐hydroxyiminoethyl)phenyl)dimethylmalondiamide (H4mal66), both of which contain two amide and two oxime functionalities as potential ligand donor groups. The two copper(II) complexes (NEt4)[Cu(Hmal55)] (1) and (NEt4)[Cu(Hmal66)] (2) can be reversibly oxidized in acetonitrile at a redox potential of –0.120 and –0.075 V vs. the Fc/Fc+ redox couple, respectively. While the quantitative electrolysis of 1 results in the preparation of the oxidized complex [Cu(Hmal55)] (3), which is sufficiently stable to be isolated, isolation of the oxidation product of 2 was not attempted because of its long‐term instability. The properties of the complexes were investigated by means of various spectroscopic methods (UV‐vis, ESR, NMR, and IR spectroscopy) and by X‐ray structure analysis. The structure determinations and the spectroscopic investigations of the complexes reveal a square‐planar CuN4coordination environment for each complex in the solid state and in acetonitrile solution. In both the oxidized and reduced oxidation states of the complexes, the coordinated ligands remain triply deprotonated with a hydrogen atom bridging both oxime oxygen atoms. The ligands can therefore be regarded as pseudo‐macrocyclic. The characterization of the oxidation products clearly identifies the electron‐transfer reaction as being metal‐centered. For the first time, the structure of a copper(III) complex with a ligand containing oximes as donor groups was determined. The redox potentials of the copper complexes are compared to related CuIII/CuII redox couples.

Macrocyclization of the ATCUN Motif Controls Metal Binding and Catalysis

“…This agrees with previous work showing that side chain alterations to nonhistidine residues can similarly affect the redox properties of linear ATCUN peptides. , Cu- 1 has a reduction potential of 739 mV, essentially identical to the Cu- 6 complex. Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides and other macrocycles . Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Figure ).…”

“…Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides 23 and other macrocycles. 43 Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Fig. 4).…”

Macrocyclization of the ATCUN Motif Controls Metal Binding and Catalysis

“…This agrees with previous work showing that side chain alterations to nonhistidine residues can similarly affect the redox properties of linear ATCUN peptides. , Cu- 1 has a reduction potential of 739 mV, essentially identical to the Cu- 6 complex. Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides and other macrocycles . Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Figure ).…”

“…Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides 23 and other macrocycles. 43 Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Fig. 4).…”

Mechanistic Studies of Ullmann‐Type C–N Coupling Reactions: Carbonate‐Ligated Copper(III) Intermediates

“…If L 2 is added to a solution of 8 , the expected pentacoordinated Cu III complex 9 is formed. Experimental and theoretical spectra validate the formation of square‐planar 8 (electronic absorptions at λ =686, 788, 854, and 978 nm) and square‐pyramidal 9 (electronic absorptions at λ =612, 844, and 1048 nm; Figure S12 in the Supporting Information). These observations indicate a high reactivity and thus the effective participation of the relatively electron‐deficient 1 b in OA.…”

Defining the Electronic and Geometric Structure of One-Electron Oxidized Copper−Bis-phenoxide Complexes

“…Comparison of the metal−ligand bond lengths for Cu Sal and [Cu Sal ] + shows that the coordination sphere is contracted in the oxidized complex (Table ); on average the Cu−O and Cu−N bond lengths decrease by 0.04 and 0.03 Å respectively upon oxidation. This result is in contrast to the majority of reported metal−phenoxyl-radical complexes, in which the metal−phenoxyl bond lengthens, consistent with a reduction of the electron-donating ability of a phenoxyl ligand as compared to a phenolate. , Oxidation of the metal center to Cu(III), resulting in a decreased ionic radius for Cu and a gain in ligand-field stabilization energy, could explain the observed coordination sphere contraction for [Cu Sal ] + . − , The more rigorously square-planar structure for [Cu Sal ] + , as compared to Cu Sal , also supports this conclusion.…”

Macrocyclization of the ATCUN Motif Controls Metal Binding and Catalysis

“…This agrees with previous work showing that side chain alterations to nonhistidine residues can similarly affect the redox properties of linear ATCUN peptides. , Cu- 1 has a reduction potential of 739 mV, essentially identical to the Cu- 6 complex. Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides and other macrocycles . Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Figure ).…”

“…Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides 23 and other macrocycles. 43 Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Fig. 4).…”

Mechanistic Studies of Ullmann‐Type C–N Coupling Reactions: Carbonate‐Ligated Copper(III) Intermediates

“…If L 2 is added to a solution of 8 , the expected pentacoordinated Cu III complex 9 is formed. Experimental and theoretical spectra validate the formation of square‐planar 8 (electronic absorptions at λ =686, 788, 854, and 978 nm) and square‐pyramidal 9 (electronic absorptions at λ =612, 844, and 1048 nm; Figure S12 in the Supporting Information). These observations indicate a high reactivity and thus the effective participation of the relatively electron‐deficient 1 b in OA.…”

Defining the Electronic and Geometric Structure of One-Electron Oxidized Copper−Bis-phenoxide Complexes

“…Comparison of the metal−ligand bond lengths for Cu Sal and [Cu Sal ] + shows that the coordination sphere is contracted in the oxidized complex (Table ); on average the Cu−O and Cu−N bond lengths decrease by 0.04 and 0.03 Å respectively upon oxidation. This result is in contrast to the majority of reported metal−phenoxyl-radical complexes, in which the metal−phenoxyl bond lengthens, consistent with a reduction of the electron-donating ability of a phenoxyl ligand as compared to a phenolate. , Oxidation of the metal center to Cu(III), resulting in a decreased ionic radius for Cu and a gain in ligand-field stabilization energy, could explain the observed coordination sphere contraction for [Cu Sal ] + . − , The more rigorously square-planar structure for [Cu Sal ] + , as compared to Cu Sal , also supports this conclusion.…”

Macrocyclization of the ATCUN Motif Controls Metal Binding and Catalysis

“…This agrees with previous work showing that side chain alterations to nonhistidine residues can similarly affect the redox properties of linear ATCUN peptides. , Cu- 1 has a reduction potential of 739 mV, essentially identical to the Cu- 6 complex. Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides and other macrocycles . Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Figure ).…”

“…Thus, for cyclic peptide 1 , macrocyclization is compatible with Cu(III) formation under oxidizing conditions, as has been observed for other cyclic peptides 23 and other macrocycles. 43 Also, the reduction current for the Cu- 1 complex is lower than that of Cu- 4 or Cu- 6 (Fig. 4).…”

Mechanistic Studies of Ullmann‐Type C–N Coupling Reactions: Carbonate‐Ligated Copper(III) Intermediates

“…If L 2 is added to a solution of 8 , the expected pentacoordinated Cu III complex 9 is formed. Experimental and theoretical spectra validate the formation of square‐planar 8 (electronic absorptions at λ =686, 788, 854, and 978 nm) and square‐pyramidal 9 (electronic absorptions at λ =612, 844, and 1048 nm; Figure S12 in the Supporting Information). These observations indicate a high reactivity and thus the effective participation of the relatively electron‐deficient 1 b in OA.…”

Defining the Electronic and Geometric Structure of One-Electron Oxidized Copper−Bis-phenoxide Complexes

“…Comparison of the metal−ligand bond lengths for Cu Sal and [Cu Sal ] + shows that the coordination sphere is contracted in the oxidized complex (Table ); on average the Cu−O and Cu−N bond lengths decrease by 0.04 and 0.03 Å respectively upon oxidation. This result is in contrast to the majority of reported metal−phenoxyl-radical complexes, in which the metal−phenoxyl bond lengthens, consistent with a reduction of the electron-donating ability of a phenoxyl ligand as compared to a phenolate. , Oxidation of the metal center to Cu(III), resulting in a decreased ionic radius for Cu and a gain in ligand-field stabilization energy, could explain the observed coordination sphere contraction for [Cu Sal ] + . − , The more rigorously square-planar structure for [Cu Sal ] + , as compared to Cu Sal , also supports this conclusion.…”