A study of solid [{Cu(MePY2)}(2)O-2](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 reso nance Raman and X-ray absorption spectroscopy, for which the former techniq ue probes the nature of the O-O bond and the latter defines the Cu-Cu inter action. In contrast to the crystal structure obtained for [{Cu(MePY2)}(2)O- 2](2+), which shows an "intermediate" Cu2O2 core (Cu-Cu = 3.3 Angstrom and O-O = 1.6 Angstrom), resonance Raman peaks characteristic of both a side-on peroxide-bridged dicopper(II) core and bis-mu-oxo dicopper(III) core are o bserved. The bis-mu-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 Angstrom (characterist ic of a side-on peroxide-bridged copper core) and 2.8 Angstrom (associated with a bis-mu-oxo dicopper core). Analysis of the edge region places an upp er limit on the amount of bis-mu-oxo isomer present in the solid at 40%. Th e factors governing the presence of bis-mu-oxo and/or side-on peroxide core s in solution for differing ligand systems are considered, and the contribu tion 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 th e presence of the bis-mu-oxo core, using frontier molecular orbital theory.