The Myth of d8 Copper(III)

Abstract

Seventeen Cu complexes with formal oxidation states ranging from Cu^I to Cu^III are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally Cu^III species is extremely covalent, resulting in Cu K-edge and L_2,3-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L_2,3-edge data reveals that all formally Cu^III species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs). DFT calculations provide further validation of the orbital composition analysis, and excellent agreement is found between the calculated and experimental results. The finding that Cu has limited capacity to be oxidized necessitates localization of electron hole character on the supporting ligands; consequently, the physical d⁸ description for these formally Cu^III species is inaccurate. This study provides an alternative explanation for the competence of formally Cu^III species in transformations that are traditionally described as metal-centered, 2-electron Cu^I/Cu^III redox processes.

Figure 1.

Ligand field molecular orbital (MO) bonding regimes, ranging from a classical Werner-type field in which the frontier unoccupied Ψ* MO is characterized by predominantly metal-centered 3d-character (left), a covalent regime in which the M−L bond receives equal contribution from the metal 3d orbitals and ligand orbitals (middle), and an inverted ligand field in which Ψ* has predominantly ligand orbital character (right).

Figure 2.

Cu complexes investigated in this study. Formal oxidation states are shown below each complex.

Figure 3.

(a) L_2,3-edge mainline (black) of 4 and RIXS energy-transfer slices corresponding to incident excitation at 8989.6 eV (green), 8985.6 eV (blue), and 8982.0 eV (red). (b) Corresponding experimental HERFD Cu K-edge XAS spectra (solid) and TDDFT-calculated (B3LYP, CP(PPP) on Cu, ZORA-def2-TZVP(-f) on all other atoms) Cu K-edge XAS (dashed) highlighting acceptor MOs accessed at 8982.0 eV (red), 8985.6 eV (blue), and 8989.6 eV (green). 1s2p RIXS energy-transfer data in (a) are reproduced from ref .

Figure 4.

Correlation between summed L_2,3-edge area of 1−3 and Cu 3d contribution to acceptor molecular orbitals. Error in the slope is estimated at ±0.14, and the error in the intercept is estimated at ±0.6.

Figure 5.

Correlation between experimental and calculated Cu d-character in L_2,3-edge acceptor MOs for formally Cu^II (red), Cu^III (blue), and Cu^I (green) species. Error in the slope is estimated at ±0.04, and error in the intercept is estimated at ±1.55.

Figure 6.

LUMOs (4−10, 12, and 14) and SOMOs (17) of formally Cu^III species. Experimental (red) and calculated (black) % Cu 3d contributions are given, showing that ligand field inversion is operative in all cases. Orbitals shown are the QROs plotted at an isolevel of 0.03 au.

Figure 7.

Frontier MO diagrams (calculated using B3LYP with CP(PPP) on Cu and ZORA-def2-TZVP(-f) on all other atoms) corresponding to 11 (left) and 12 (right). Cu-localized orbitals are shown in red, and the ligand-localized MOs are shown in black; both are plotted at an isolevel of 0.03 au.

Figure 8.

Sampling of additional previously reported, stable Cu^III complexes spanning a range of geometries and ligand compositions.

Figure 9.

DFT calculated frontier LUMO orbitals along with corresponding Löwdin orbital composition for reported Cu^III compounds shown in Figure 8. Unrestricted Kohn−Sham α MOs plotted at an isolevel of 0.03 au.

Figure 10.

DFT-calculated frontier molecular orbitals of (a) 17 and (b) [(L^TEEDCu)₂(O₂)]²⁺ highlighting large N 2p and O 2p orbital contribution to the SOMOs, respectively. QRO orbitals are plotted at an isolevel of 0.03 au.

Figure 11.

DFT-calculated frontier molecular orbitals of (a) (^ArL)Fe(NAd) and (b) (^ArL)Fe(N(C₆H₄ ^tBu)) highlighting large N 2p orbital contribution in the β SOMO, respectively. Unrestricted corresponding orbitals (UCOs) are plotted at an isolevel of 0.03 au.

Figure 12.

Charge on bound C’s in [Cu(benzyl)(CF₃)₃]¹⁻ and [Cu(CF₃)₄]¹⁻. Cu 3d counts obtained Löwdin population analyses following hybrid DFT single point using the B3LYP functional, CP(PPP) basis on Cu, and ZORA-def2-TZVP(-f) on all other atoms. C-atom charges (indicated by arrows) were obtained from IBBA analysis of the aforementioned single-point calculations.