An Anionic, Tetragonal Copper(II) Superoxide Complex

Abstract

Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH₃CN)]OTf, and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (H-atom abstracting) reagent in reactions with phenols.

An important first step in copper-promoted aerobic oxidations in biology¹ and catalysis² is the formation of a 1:1 Cu/O₂ adduct, in which the O₂ molecule is activated for subsequent reactions, either with substrate or to form different copper-oxygen species. In one approach aimed at understanding such adducts, synthetic 1:1 Cu/O₂ complexes have been targeted for detailed structural, spectroscopic, and reactivity studies.³ To date, three types have been identified: (a) end-on, triplet Cu(II)-superoxos supported by tetradentate tripodal⁴ or, in one case, tridentate⁵ N-donor ligands, (b) side-on, singlet Cu(II)-superoxos supported by facially coordinating tris(pyrazolyl)hydroborates,⁶ and (c) side-on, singlet Cu(III)-peroxos supported by strongly electron-donating, bidentate β-diketiminates or anilido-imines.^3,7 A key finding from reactivity studies of type (a) compounds is that they are electrophilic, with the ability to perform biologically relevant H-atom abstractions from phenols and weak C-H bonds; ^4b,5 investigations of the reactivity of type (b) compounds have not been reported, and type (c) compounds are relatively unreactive with external organic substrates. Herein we report that in seeking to expand the repertoire of available 1:1 Cu/O₂ structures for comparative evaluations, we have discovered an end-on Cu(II)-superoxide complex that displays unique characteristics, including a tetragonal geometry and non-electrophilic reactivity.

Inspired by a recent report,⁸ we prepared complex 1 (Scheme 1) by treating N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide⁹ with NaOMe followed by CuCl₂ in the presence of CH₃CN.¹⁰ In methanol or THF solution, 1 is green, whereas it is red-brown in the presence of CH₃CN or pyridine. Consistent with this solvatochromism (Figure S2), attempts to obtain crystals of 1 suitable for X-ray crystallographic analysis were complicated by apparent CH₃CN lability. The addition of 4-tBu-pyridine, however, yielded X-ray quality dark red crystals of 2 (Scheme 1). The complex is square planar with a geometry similar to other known 2,6-pyridinedicarboxamide Cu(II) complexes.¹¹

Scheme 1.

Compounds prepared in this work, and a representation of the X-ray crystal structure of 2 shown as 50% thermal ellipsoids (H atoms omitted for clarity). Selected bond distances (Å) and angles (deg): Cu-N1, 2.004(2); Cu-N2, 1.920(2); Cu-N3, 1.994(2); Cu-N4, 1.973(2); N1-Cu-N2, 80.56(10); N1-Cu-N3, 161.19(9); N1-Cu-N4, 99.57(9); N2-Cu-N3, 80.84(10); N2-Cu-N4, 173.28(10); N3-Cu-N4, 99.23(10).

Addition of 1 to a slurry of KO₂ and 18-crown-6 in DMF/THF (1:1 v/v) at −80 °C immediately yielded an azure solution comprising compound 3. This species is stable at −80 °C for hours, but decomposes upon warming above −60 °C as indicated by bleaching of its UV-vis spectral features. These features are compared to those of the starting material 1 in Figure 1. Particularly notable is an absorption at 627 nm (ε ~ 1700 M⁻¹ cm⁻¹) with sufficient intensity to suggest that it is a charge transfer transition. Laser excitation at 647 nm yields a resonance Raman (rR) spectrum with an O-isotope sensitive peak at 1104 cm⁻¹ (Δ¹⁸O = 60 cm⁻¹; Figure 2a). These data support formulation of 3 as a Cu(II)-superoxo complex with ν(O-O) = 1104 cm⁻¹, a value slightly lower than reported for other end-on Cu(II)-superoxos (1120 cm⁻¹).^4,5 Attempts at characterizing 3 by negative-ion electrospray ionization mass spectrometry have not been successful due to its thermal instability. Monitoring by EPR spectroscopy (X-band, parallel and perpendicular modes, 2 K) of a titration of 3 by KO₂/18-crown-6 showed only disappearance of the axial signal for 1 (Figure S1) and appearance of the g ~ 2 signal for superoxide; complex 3 is apparently EPR silent, which is consistent with a singlet or a triplet ground state.^{12 1}H NMR spectra of 1 and 3 in 1:1 d₈-THF/d₇-DMF at −80 °C only showed residual solvent peaks, with the spectrum of 3 also showing peaks in the diamagnetic region for 18-crown-6 overlapping that of CH₃CN. These data rule out an S = 0 ground state formulation and are consistent with both 1 and 3 being paramagnetic species, with the absence of paramagnetically shifted peaks for both suggesting very fast relaxation leading to extensive peak broadening.

Figure 1.

(top) UV-vis spectra of compounds 1, 3, and 4 (−80 °C, DMF/THF). (middle) Resonance Raman difference spectrum (¹⁶O-¹⁸O) for compound 3 prepared from K¹⁶O₂ or K¹⁸O₂ (−196 °C, λ_ex = 647 nm). (bottom) Resonance Raman spectra for compound 4 prepared from 3(¹⁶O) (black line) or 3(¹⁸O) (blue line), overlaid with spectrum of [(tmpa)₂Cu₂(¹⁶O₂)](OTf)₂ (red dotted line) (−196 °C, λ_ex = 647 nm).

Figure 2.

Calculated structure of end-on triplet Cu(II)-superoxo complex 3. Carbon (grey), nitrogen (blue), oxygen (red), and copper (green) are shown. Hydrogens omitted for clarity.

In order to better characterize the electronic structure of 3, unrestricted DFT calculations were performed using the mPW functional and a broken-symmetry (BS) formalism for the singlet state.¹⁰ Various initial Cu(II)-superoxo complex geometries for either spin state all smoothly minimized to a structure showing end-on coordination of the superoxide to the copper center (Figure 2). The triplet state is computed to be lower in energy than the spin-purified singlet state by 6.2 kcal/mol. Previous analysis of various computational methods has suggested that the mPW functional may underestimate the energy of an end-on bound singlet state meaning that the triplet state may be even more strongly favored.¹³ The Cu-O distances were calculated to be 1.966 Å and 2.778 Å, with a Cu-O-O angle of 115.1°, and an O-O bond length of 1.292 Å, all consistent with superoxide character. The superoxide orients perpendicular to the plane of the N-donor ligand. A saddle point on the potential energy surface for superoxide rotation was identified with the O₂ moiety in the plane of the ligand. After inclusion of zero-point energy, the minimum and saddle point structure are essentially degenerate in energy, suggesting that rotation about the Cu–O bond is effectively barrierless. A singlet structure with symmetrical η² coordination of the superoxide was also located and identified as a transition-state structure corresponding to interconversion of one η¹ superoxide species with another; the barrier on the singlet surface is 19.8 kcal/mol. Computed ν(O-O) frequencies for the end-on minima are 1182 cm⁻¹ (Δ¹⁸O = 66 cm⁻¹) for the triplet state and 1160 cm⁻¹ (Δ¹⁸O = 64 cm⁻¹) for the singlet state (for the side-on singlet TS structure, ν(O-O) is 1042 cm⁻¹ (Δ¹⁸O = 58 cm⁻¹)). The computed frequencies are somewhat high compared to the rR values, likely because the calculations do not include a counterion or solvation, thereby reducing the tendency for negative charge to concentrate on the O₂ fragment and raising the stretching frequencies. Time dependent DFT calculations using the B98 functional¹⁰ identified a UV-vis transition at 625 nm. The orbitals involved correspond to the transition from the superoxide π^* orbital to an antibonding combination of the Cu dx²-y² and the four ligand sigma donors (Figure S8). The calculated data are collectively consistent with assigning 3 as an end-on, Cu(II)-superoxo species.

To further corroborate the formulation of 3, we sought to trap it with a Cu(I) complex to generate a (peroxo)dicopper species, a transformation known to occur in reactions of Cu(I) complexes with O₂.¹⁴ Treatment of a solution of 3 with [(tmpa)Cu(CH₃CN)]OTf¹⁵ at −80 °C yielded a purple solution with intense UV-vis absorptions at 624 nm (ε ~ 8300 M⁻¹ cm⁻¹) and a shoulder around 550 nm (4, Figure 1). The rR spectrum of this solution using 647 nm excitation showed an isotope sensitive peak at 832 cm⁻¹ (Δ¹⁸O = 44 cm⁻¹; Figure 1b) that we assign as the ν(O-O) for a (peroxo)dicopper complex. Taken together, the available evidence supports formulation of 4 as a (1,2-peroxo)dicopper species (Scheme 1). The similarities of the data to those reported for trans-1,2-peroxo complexes^4d,16 suggest a similar geometry in 4, although the intensity pattern for the UV-vis absorptions is reversed from the typical pattern (higher energy peroxo π_σ* → Cu(II) more intense than lower energy peroxo π_v* → Cu(II) transition), perhaps due to distortions of the CuOOCu atoms from planarity. To confirm that the UV-vis and rR features attributed to 4 are not due to [(tmpa)₂Cu₂(O₂)]²⁺, which might be envisioned to form from reaction of [(tmpa)Cu(CH₃CN)]OTf with adventitious O₂ or KO₂, we collected UV-vis and rR data for [(tmpa)₂Cu₂(O₂)](OTf)₂ generated by purposeful addition of O₂ to [(tmpa)Cu(CH₃CN)]OTf in 1:1 DMF/THF at −80 °C. The pattern of UV-vis absorptions (Figure S2) and the ν(O-O) value (822 cm⁻¹, Figure 2b) are similar to literature data¹⁶ but are different from the data obtained for 4.

Preliminary examination of the reactivity of 3 with phenols and acids revealed behavior distinct from that reported for other Cu(II)-superoxo complexes. For example, no reaction was observed upon addition of excess 4-t-butyl-, 2,4-di-t-butyl- or 2,4,6-tri-t-butyl-phenols to solutions of 3 at −80 °C, whereas other Cu(II)-superoxo complexes generate phenoxyl radicals or derived coupled products via H-atom abstraction from these substrates.^4b,5 Even upon warming of the solution, no hydroxylated, coupled or other phenolic products could be identified from the reaction mixtures (GC/MS). Addition of PPh₃ to a solution of 3 at −80 °C similarly yielded no change of the UV-vis spectral features and upon warming to room temperature and acidic workup, GC/MS analysis showed recovery of 78% of the PPh₃ and no OPPh₃. On the other hand, addition of HOAc or 4-nitrophenol to 3 resulted in immediate quenching of the features due to 3 and, for the latter case, the growth of an intense absorption at 407 nm attibutable to the 4-nitrophenoxide anion (likely complexed to Cu(II)). This was confirmed by observation of the same UV-vis spectrum upon addition of sodium 4-nitrophenoxide to a solution of 1 (Figure S3). Upon acidic workup of the product solution from the reaction of p-nitrophenol with 3, only 4-nitrophenol was identified in the reaction mixture (80% yield by GC/MS) suggesting that the Cu(II)-superoxo species was being protonated and not undergoing any radical processes. We surmise that the unique reactivity exhibited by 3 is due to the fact that it is anionic and thus more prone to act as a base or nucleophile, whereas other end-on Cu(II)-superoxo complexes are supported by neutral N-donor ligands and are therefore cationic and electrophilic.

In summary, a new type of tetragonal, anionic, end-on Cu(II)-superoxo species has been characterized by spectroscopy and theory. The reactivity of this species is significantly different from that of previously reported Cu(II)-superoxo complexes. Taken together, these results provide new insights into the chemistry of copper-oxygen species proposed to be key intermediates in oxidation catalysis.