Lohmiller, T.; Spyra, C.J.; Dechert, S.; Demeshko, S.; Bill, E.; Schnegg, A.; Meyer, F: 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. Journal of the American Chemical Society 2 (2022), p. 1134–1143
10.1021/jacsau.2c00139
Open Access Version (externer Anbieter)
Abstract:
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 (dO–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 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 CuII ions 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 Jiso 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 Jiso 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.