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

[Image: see text] A unique type of Cu(2)/O(2) adduct with orthogonal (close to 90°) Cu–O–O–Cu arrangement has been proposed for initial stages of O(2) 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-η(1):η(1)-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 (ν̃(O–O) = 807 cm(–1)), completes a series of closely related Cu(2)/O(2) intermediates featuring μ-η(1):η(1)-peroxodicopper(II) cores with φ ranging from 55° (A, cis-peroxo (C)P; A. Brinkmeieret al., J. Am. Chem. Soc.2021, 143, 1036134191490) via 87° (2, (⊥)P type) up to 104° (B, approaching trans-peroxo (T)P; N. Kindermannet al., Angew. Chem., Int. Ed.2015, 54, 1738). SQUID magnetometry revealed ferromagnetic interaction of the Cu(II) 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 O(2) activation at type III dicopper sites and a potential role of antisymmetric exchange in the concomitant intersystem crossing are proposed.