| dc.description.abstracteng | The µ-η²:η²-peroxodicopper(II) core found in the oxy forms of the active sites of type III dicopper proteins have been a key target for bioinorganic model studies. Here, it is shown that simple bis(oxazoline)s (BOXs), which are classified among the so-called “privileged ligands” and have found widespread use in catalysis, provide a suitable scaffold for supporting such biomimetic copper/dioxygen chemistry. Ligands with different backbone substituents have been used. Their BOX-copper(I) complexes bind dioxygen to yield biomimetic μ-η²:η²-peroxodicopper(II) species. O2 can be reversibly released upon an increase in temperature. Their formation kinetics have been studied under cryo-stopped-flow conditions, evidencing a slow and unusually strong entropy-driven O2 activation mechanism. The copper-oxygen complexes have been isolated as surprisingly stable solids and investigated in depth by a variety of methods, both in solution and in the solid state. A slight butterfly-shape of the Cu2O2 core has been derived from EXAFS data and DFT calculations.
Furthermore, employing a deprotonated ligand, a bis(µ-oxo)dicopper(III) complex could be yielded instead of the peroxo complex. An equilibrium with the corresponding peroxodicopper(II) species as the minor component was indicated. Interestingly, the bis(µ-oxo) complex could alternatively be yielded by ligand deprotonation of the corresponding (protonated) peroxo compound in a stepwise manner. Cleavage of the peroxo O-O bond is therefore controlled by the mere protonation status of the supporting BOX ligand. Considering the vast number of known BOX derivatives, a rich and versatile Cu/O2 chemistry based on this platform is anticipated.
Besides this main part, redox non-innocence in BOX ligands was observed for the first time. This is induced by the coordination of redoxactive copper(II) chloride. The observed reactions are highly selective and lead to aerobic oxygenation in the case of a sterically more demanding ligand and to dimerisation by C-C coupling in the sterically unhindered ligands. Finally, the bulkiest ligand forms a stable copper(II) complex. Transient intermediates with distinct organic radical/copper(I) character could be postulated based on DFT computation and EPR spin-trap experiments. Furthermore, the anti-oxidant DPPH free radical assay successfully validated the found trends.
Finally, the tautomerism of BOX ligands could be structurally and spectroscopically evidenced. In solution, one of the BOX ligands is present as a 1:1 mixture of the diimine and iminoenamine tautomers. The thermodynamics and kinetics for the tautomeric equilibrium were investigated, showing that the process of isomer interconversion is slow and proceeds via an intramolecular reaction pathway with a particularly high reaction barrier. | de |