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.

A unique type of Cu2/O2 adduct with orthogonal (close to 90°) Cu-O-O-Cu arrangement has been proposed for initial stages of O2 binding at biological type III dicopper sites, and targeted ligand design has now allowed us to emulate such an adduct in a pyrazolate-based µ-η1 :η1-peroxodicopper(II) complex (2) with Cu-O-O-Cu torsion φ of 87°, coined ⊥ P intermediate. Full characterization of 2, including X-ray diffraction (d O-O = 1.452 Å) and Raman spectroscopy (ν̃O-O = 807 cm-1), completes a series of closely related Cu2/O2 intermediates featuring µ-η1 :η1-peroxodicopper(II) cores with φ ranging from 55° (A, cis-peroxo C P; Brinkmeier A.et al., J. Am. Chem. Soc.2021, 143, 10361) via 87° (2, ⊥ P type) up to 104° (B, approaching trans-peroxo T P; Kindermann N.et al., Angew. Chem., Int. Ed.2015, 54, 1738). SQUID magnetometry revealed ferromagnetic interaction of the CuII ions and a triplet (S t = 1) ground state in 2. Frequency-domain THz-EPR has been employed to quantitatively investigate the spin systems of 2 and B. Magnetic transitions within the triplet ground states confirmed their substantial zero-field splittings (ZFS) suggested by magnetometry. Formally forbidden triplet-to-singlet transitions at 56 (2) and 157 cm-1 (B), which are in agreement with the exchange coupling strengths J iso inferred from SQUID data, are reported for the first time for coupled dicopper(II) complexes. Rigorous analysis by spin-Hamiltonian-based simulations attributed the corresponding nonzero transition probabilities and the ZFS to substantial antisymmetric (Dzyaloshinskii-Moriya) exchange d and provided robust values and orientations for the d , J , and g tensors. These interactions can be correlated with the Cu-O-O-Cu geometries, revealing a linear increase of J iso with the Cu-O-O-Cu torsion and a strong linear decrease with the Cu-O-O angle. Relevance of the ⊥ P intermediate for O2 activation at type III dicopper sites and a potential role of antisymmetric exchange in the concomitant intersystem crossing are proposed.

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

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 Cux(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 Cux/O2 chemistry.

Ambiphilicity of a mononuclear cobalt(III) superoxo complex.

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.