A Study of Solid [{Cu(MePY2)}₂O₂]²⁺ Using Resonance Raman and X-ray Absorption Spectroscopies:  An Intermediate Cu₂O₂ Core Structure or a Solid Solution?

Solid [{Cu(MePY2)} 2 O 2 ] 2+ is spectroscopically characterized using resonance Raman and X-ray absorption spectroscopy, for which the former technique probes the nature of the O−O bond and the latter defines the Cu−Cu interaction. In contrast to the crystal structure obtained for [{Cu(MePY2)} 2 O 2 ] 2+, which shows an “intermediate” Cu 2 O 2 core (Cu−Cu = 3.4 Å and O−O = 1.6 Å), resonance Raman peaks characteristic of both a side-on peroxide-bridged dicopper(II) core and bis-μ-oxo dicopper(III) core are observed. The bis-μ-oxo isomer is estimated to be present at approximately 5−20%. A good fit is obtained for EXAFS data for solid [{Cu(MePY2)} 2 O 2 ] 2+ using an 80:20 ratio of Cu−Cu separations of 3.6 Å (characteristic of a side-on peroxide-bridged copper core) and 2.8 Å (associated with a bis-μ-oxo dicopper core). Analysis of the edge region places an upper limit on the amount of bis-μ-oxo isomer present in the solid at 40%. The factors governing the presence of bis-μ-oxo and/or side-on peroxide cores in solution for differing ligand systems are considered, and the contribution of the bite angle of the equatorial nitrogen atom donors is explored. The reactivity of [{Cu(MePY2)} 2 O 2 ] 2+ in solution is correlated with the presence of the bis-μ-oxo core, using frontier molecular orbital theory.