Structurally Characterized μ-1,2-Peroxo/Superoxo Dicopper(II) Pair

Abstract

Superoxo complexes of copper are primary adducts in several O2-activating Cu-containing metalloenzymes as well as in other Cu-mediated oxidation and oxygenation reactions. Because of their intrinsically high reactivity, however, isolation of Cu x (O2 •–) species is challenging. Recent work (J. Am. Chem. Soc. 2017, 139, 9831; 2019, 141, 12682) established fundamental thermochemical data for the H atom abstraction reactivity of dicopper­(II) superoxo complexes, but structural characterization of these important intermediates was so far lacking. Here we report the first crystallographic structure determination of a superoxo dicopper­(II) species (3) together with the structure of its 1e– reduced peroxo congener (2; a rare cis-μ-1,2-peroxo dicopper­(II) complex). Interconversion of 2 and 3 occurs at low potential (−0.58 V vs Fc/Fc+) and is reversible both chemically and electrochemically. Comparison of metric parameters (d(O–O) = 1.441(2) Å for 2 vs 1.329(7) Å for 3) and of spectroscopic signatures (ν̃(16O–16O) = 793 cm–1 for 2 vs 1073 cm–1 for 3) reflects that the redox process occurs at the bridging O2-derived unit. The CuII–O2 •––CuII complex has an S = 1/2 spin ground state according to magnetic and EPR data, in agreement with density functional theory calculations. Computations further show that the potential associated with changes of the Cu–O–O–Cu dihedral angle is shallow for both 2 and 3. These findings provide a structural basis for the low reorganization energy of the kinetically facile 1e– interconversion of μ-1,2-superoxo/peroxo dicopper­(II) couples, and they open the door for comprehensive studies of these key intermediates in Cu x /O2 chemistry.

Antisymmetric Spin Exchange in a μ-1,2-Peroxodicopper(II) Complex with an Orthogonal Cu–O–O–Cu Arrangement and S = 1 Spin Ground State Characterized by THz-EPR

“…Another possible explanation for 2 is the formation of isomers in solution due to the decreased symmetry of the nonsymmetric ligand scaffold [L 1,2 ] − combined with the shallow potential energy surface associated with changes of the Cu–O–O–Cu torsion. 11 A further isotope-sensitive band for solid 2 at 515 cm –1 (Δ( 16 O 2 – 18 O 2 ) = 23 cm –1 ) is assigned to the Cu–O stretch, and again this is very similar to the value observed for B (512 cm –1 , Δ( 16 O 2 – 18 O 2 ) = 22 cm –1 ). Overall, the electronic absorption and Raman spectra indicate close similarity of the peroxodicopper(II) cores of 2 and B .…”

“…The Cu···Cu distance is 4.081 Å and 4.060 Å for the two crystallographically independent molecules, which is in between the values previously reported for dicopper(I) complexes [L 1,1 Cu 2 ]BPh 4 (4.153 Å) and [L 2,2 Cu 2 ]BPh 4 (3.968 Å), demonstrating the tunability of the metal···metal separation by targeted ligand design. 11 , 16 Diamagnetic complex 1 was also characterized by NMR spectroscopy ( SI, Section 3.1 ).…”

“…These geometries appear to be imposed by the specific binding pockets as they are essentially the same as in the respective symmetric congeners A (τ 5 = 0.60) and B (τ 5 = 0.20) with two short or long side arms, respectively. 11 , 16 …”

“… Series of peroxodicopper(II) complexes A , 11 2 , and B ( 16 ) with Cu–O–O angles θ (left) and their core structures. The Cu–O–O–Cu torsion angle φ is visualized by looking along the O1–O2 bond of the core structures determined by X-ray diffraction (right).…”

Modulation of a μ-1,2-Peroxo Dicopper(II) Intermediate by Strong Interaction with Alkali Metal Ions

“…Deep purple peroxo complex 1 forms upon adding dry O 2 to a colorless solution of the dicopper(I) complex [LCu I 2 ](BPh 4 ) in MeCN at −40 °C, as reported recently. 23 1 has a characteristic UV−vis absorption spectrum showing maxima at 527 nm (ε = 5000 M −1 cm −1 ) and 648 nm (ε = 3900 M −1 cm −1 ) and a shoulder at 456 nm (ε = 2600 M −1 cm −1 ). The spectrum looks similar to those of related T P systems, whose electronic structure has been thoroughly investigated.…”