High-frequency (245 GHz) and X-band EPR study of stable dicopper radical complexes

Abstract

Stable, readily accessible and in part even structurally established paramagnetic complex cations [Cu₂(µ-η⁴-BL)L₂]⁺˙, BL = bridging ligand, L = Ph₂P(CH₂)_nPPh₂ or 2 PPh₃, were studied by high-frequency (245 GHz) and X-band EPR spectroscopy. The dicopper(I)/anion radical ligand formulation as opposed to a mixed-valent dimetal description is supported by the high-frequency EPR results which show g components between 2.0220 and 1.9968. The largest (rhombic) g anisotropy Δg was found for the system with BL = azodi-tert-butyl formate and L = Ph₂P(CH₂)₆PPh₂, and the smallest (axial) Δg splittings for 1,2,4,5-tetrazine-bridged species. There is a clear relation between Δg as obtained from the high-field measurements and the ^63,65Cu hyperfine coupling as measured from solution spectra in the X band.