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

Abstract

A unique type of Cu2/O2adduct with orthogonal (close to 90°) Cu-O-O-Cu arrangement has been proposed for initial stages of O2binding 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 (dO-O= 1.452 Å) and Raman spectroscopy (νO-O= 807 cm-1), completes a series of closely related Cu2/O2intermediates featuring μ-η1:η1-peroxodicopper(II) cores with φ ranging from 55° (A, cis-peroxo CP; Brinkmeier, A. et al., J. Am. Chem. Soc. 2021, 143, 10361) via 87° (2, ⊥P type) up to 104° (B, approaching trans-peroxo TP; Kindermann, N. et al., Angew. Chem., Int. Ed. 2015, 54, 1738). SQUID magnetometry revealed ferromagnetic interaction of the CuIIions and a triplet (St= 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 Jisoinferred 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 Jisowith the Cu-O-O-Cu torsion and a strong linear decrease with the Cu-O-O angle. Relevance of the ⊥P intermediate for O2activation at type III dicopper sites and a potential role of antisymmetric exchange in the concomitant intersystem crossing are proposed.