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 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 ; 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 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.