Recently a Cu(III)2 bis-μ-oxo dimer ligated by peralkylated amines has been found to interconvert
with the side-on peroxo-bridged, μ-η2:η2 isomer. The Cu(III)2(μ-O)2 dimer exhibits two intense charge transfer
(CT) features in the near-UV region of the electronic absorption spectrum. Laser excitation into the lower-energy CT absorption band at 25 000 cm-1 results in intense resonance enhancement of the Raman peaks at
609 and 118 cm-1 which profile this band and give overtone and combination progressions. The combined
application of a normal coordinate analysis of the Raman features and a time-dependent Heller theory analysis
of the electronic absorption spectrum and resonance Raman profiles provide the excited-state geometry. As
this transition corresponds to an oxo-to-Cu(III) CT, this excited state is formally an oxyl−Cu(II) species.
Density functional calculations correlated to these data (including the excited-state geometry and the relative
CT intensities) allow for an unambiguous assignment of the observed charge-transfer transitions. This assignment
shows that one of the CT features involves the same orbital origin as a corresponding transition in the side-on
peroxo dimer, while the new, low-energy band (∼25 000 cm-1) only observed for the bis-μ-oxo species
corresponds to an oxo σu* to Cu(III) d
xy
CT transition which is present when the O−O bond is cleaved. This
study provides electronic structural insight into the relationship between the bis-μ-oxo and side-on peroxo-bridged Cu species and their relative reactivities.
Copper(I)-dioxygen interactions are of great interest due to their role in biological O2-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-mu-eta2:eta2 (side-on) peroxo versus Cu(III)-bis-mu-oxo species formation. Drastic differences in O2-reactivity of Cu(I) complexes which differ only by a single -CH3 versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C6F5)4 (1) (MeAN = N,N,N',N',N'-pentamethyl-dipropylenetriamine) reacts with O2 at -80 degrees C to form almost exclusively the side-on peroxo complex [{CuII(MeAN)}2(O2)]2+ (3) in CH2Cl2, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C6F5)4 (2) (AN = 3, 3'-iminobis(N,N-dimethyl-propylamine) can support either Cu2O2 structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH2Cl2 solvent, where 1 reacts with O2 giving 3, while 2 forms exclusively the bis-mu-oxo species [{CuIII(AN)}2(O)2]2+ (4Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-mu-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O2 versus 2/O2 (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-mu-oxo versus side-on peroxo formation continue to be of interest.
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