Geometric and Electronic Structure of [{Cu(MeAN)}2(μ-η2:η2(O22-))]2+ with an Unusually Long O–O Bond: O–O Bond Weakening vs Activation for Reductive Cleavage

Certain side-on peroxo-dicopper(II) species with particularly low νO-O (710-730 cm-1) have been found in equilibrium with their bis-μ-oxo-dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O-O cleavage. In a previous study (Liang, H.-C., et al. J. Am. Chem. Soc.2002, 124, 4170), we showed that oxygenation of the three-coordinate complex [CuI(MeAN)]+ (MeAN = N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{CuII(MeAN)}2(μ-η2:η2-O22-)]2+ peroxo species with low νO-O (721 cm-1), as characterized by UV-vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF6- salt, and an X-ray structure indicates the presence of an unusually long O-O bond (1.540(5) Å) consistent with the low νO-O. Extended X-ray absorption fine structure and rR spectroscopic and reactivity studies indicate the exclusive formation of [{CuII(MeAN)}2(μ-η2:η2-O22-)]2+ without any bis-μ-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo-dicopper(II) species with a significantly long and weak O-O bond. DFT calculations show that the weak O-O bond results from strong σ donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O-O bond does not reflect an increase in backbonding into the σ* orbital of the peroxide. Thus, although the O-O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-μ-oxo dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity. [ABSTRACT FROM AUTHOR]