Dinuclear Copper Complexes Based on Parallel β-Diiminato Binding Sites and their Reactions with O2: Evidence for a Cu-O-Cu Entity

Investigations concerning the system beta-diketiminato-Cu-I/O-2 have revealed, valuable insights that may be discussed in terms of the behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear Cu enzymes for the activation of O-2. With this background the ligand system [(Me2C6H3)Xanthdim](2-) containing two parallel beta-diiminato binding sites linked by a xanthene backbone with 2,3-dimethylphenyl residues at the diiminato units was investigated with respect to its copper coordination chemistry. The diimine [(Me2C6H3)Xanthdim]H-2 was treated with CuOtBu in the presence of acetonitrile, PPh3, and PMe3 to yield the corresponding complexes [(Me2C6H3)Xanthdim](Cu(L))(2) (L = CH3CN, 1, PPh3, 2, and PMe3, 3) that proved to be stable and were fully characterized. Single crystal X-ray diffraction analyses performed for the three complexes showed that considerable steric crowding within the binding pockets of 2 leads to a very long Cu-Cu distance while the structures of 1 and 3 are relaxed. Compounds 2 and 3 are relatively robust toward air, whereas 1 is very sensitive and quantitatively reacts with O-2 at room temperature (r.t.) within less than 2 min to give intractable compounds. At :low temperatures the formation of a green intermediate was observed that was identified as a Cu-II-O-Cu-II species spectroscopically and chemically. This finding is of relevance also in the context of the results obtained testing I as a catalyst for phenol oxidation using O-2: 1 efficiently catalyzes phenol coupling, while there was no evidence for any oxygenation reactions occurring.