Forschung im HLRN-Verbund 2011

17Figure 1: (A) MFU-1 (the Co and N atoms that define the above mentioned Co-N-N-Co dihedral angle are shown asball and stick model), (B) η 2 -Superoxo and (C) vdW complex of MFU-1 with O 2 . (The adsorbed dioxygenis shown as ball and stick model)tion energy. Their computed adsorption enthalpiesat room temperature from PBE+D are -7.7 and -52.7 kJ/mol respectively, whereas the experimentalvalue is -11.1 kJ/mol. Although the η 2 -Superoxocomplex has the largest adsorption enthalpy, theexperimental observations below 120 ◦ C suggestthat only the van der Waals complex is observed,because no additional peaks due to the superoxocomplex around 1100 cm −1 appear in the IR spectrum.This implies that the formation of the η 2 -superoxo-Co(III) complex is prevented by an energybarrier.The reactions of the dioxygen with the cobaltatoms of the MFU-1 are complicated and createproducts of different spin multiplicity. The reactionscan be either exothermic or endothermic.However, this does not explain how fast theseCo-dioxygen complexes are formed or if they areblocked by a barrier. The height of this point withrespect to the initial state determines how fast theproduct is formed. In order to estimate the crossingpoint between the two surfaces, the method developedby Harvey and co-workers is employed.[4]The minimum energy crossing point is calculated44.4 kJ/mol above the reactant. The height of thebarrier at 44.4 kJ/mol provides a possible explanationwhy no superoxo species is formed below120 ◦ C, whereas at elevated temperatures O 2 reactsand starts decomposing MFU-1.More Information1. M. Tonigold, Y. Lu, B. Bredenkotter, B. Rieger,S. Bahnmuller, J. Hitzbleck, G. Langstein andD. Volkmer, Angew. Chem. Int. Ed. 2009, 48,7546-7550.2. M. Tonigold, Y. Lu, A. Mavrandonakis, A. Puls,R. Staudt, V. Hagen, J. Mollmer, J. Sauer and D.Volkmer, Chem. Eur. J. 2010, accepted.3. T. Kerber, M. Sierka and J. Sauer, J. Comput.Chem. 2008, 29, 2088-2097.4. J.N. Harvey, M. Aschi, H. Schwarz and W. Koch,Theor. Chem. Acc. 1998, 99, 95-99.FundingDFG Priority Programme 1362Chemie