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

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Abstract
Matrix isolation has been used to study the photolytically induced reaction of AlX (X = F, Cl, or Br) with O2. The peroxo and bisperoxo compounds XAlO2 and XAl(O2)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(O2)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 (16O/18O) allied with quantum chemical calculations show that the O2 moieties in these complexes are side-on coordinated, leading to an overall C2v symmetry of the complexes and a spin multiplicity of 3. The O−O distance of about 1.366 Å 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(16O2)(18O)2 isotopomer into XAl(16O18O)(16O18O). The properties of the complexes will be compared to those of complexes to transition metal centers.