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

Stable, readily accessible and in part even structurally established paramagnetic complex cations [Cu-2(mu-eta(4)-BL)L-2](+.), BL=bridging ligand, L=Ph2P(CH2)(n)PPh2 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 Delta g was found for the system with BL=azodi-tert-butyl formate and L=Ph2P(CH2)(6)PPh2, and the smallest (axial) Delta g splittings for 1,2,4,5-tetrazine-bridged species. There is a clear relation between Delta g as obtained from the high-field measurements and the Cu-63,Cu-65 hyperfine coupling as measured from solution spectra in the X band.