Characterization of the side-on coordinated bissuperoxo complexes of aluminum FAl(O2)2, ClAl(O2)2, and BrAl(O2)2 with triplet electronic ground states:: A combined matrix IR and quantum chemical study

Matrix isolation has been used to study the photolytically induced reaction of AlX (X = F, Cl, or Br) with O-2. The peroxo and bisperoxo compounds XAlO2 and XAl(O-2)(2) are found to be the products of these reactions. While the peroxo species XAlO2 were already addressed in a separate work, we concentrate herein on the bissuperoxo complexes XAl(O-2)(2), which are to our knowledge the first examples of such complexes with Al centers. Our IR spectroscopic results taking in the effect of isotopic substitution (O-16/O-18) allied with quantum chemical calculations show that the O-2 Moieties in these complexes are side-on coordinated, leading to an overall C-2v symmetry of the complexes and a spin multiplicity of 3. The O-O distance of about 1.366 Angstrom argues for the presence of superoxide units. The force constants are, however, somewhat smaller than expected for a superoxide anion and indicate that the bonding in the complexes cannot be described simply on the basis of an ionic model. Interestingly a photoinduced intramolecular isotopic scrambling process is observed for the compounds resulting in partial conversion of the XAl(O-16(2))(O-18)(2) isotopomer into XAl((OO)-O-16-O-18)((OO)-O-16-O-18). The properties of the complexes will be compared to those of complexes to transition metal centers.