Axial coordination behaviors and photochemistry in complexis of early transition metals

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Axial coordination behaviors were studied for chromium, zirconium, niobium, and molybdenumorphyrins. The ligand exchange reactions were followed by means of NMR, ESR, and visible spectroscopy. Mechanisms for axial ligand exchange with phenols (catechols, hydroquinones, thiophenols) were established as follows. 1. Proton transfer from phenol to the complex to eliminate an axial ligand and to form and ion pair of phenolate anion and a complex cation with vacant coordination site. 2. Electron transfer from phenolate anion to the complex cation to afford a radical pair of phenoxyl radical and reduced comples. 3. Recombination of the radical pair resulting phenoxo complex. This mechanism was confirmed by electrochemical determination of redox potentials of complexes in each stage. The step 2 is disturbed in the case of phenols, anion state of which is highly stabilized, so that the ion pair i.e. oxidized unsaturated complex is conserved. If sterically large substituent groups exist in the phenol or porphyrin, or if stabilization by forming coordination bond is small, the step 3 is hindered arising of reduced nusaturated complex and free radical, It is most essential to control the change in the unsaturated complex as constructing a catalytic system with metal complex. Niobium and molybdenum porphyrins were, in this viewpoint, photochemically reduced resulting reduced unsaturated complex. The reduced metalloporphyrin activated the molecular oxygen and catalyzed oxygenation of olefins. Electron transfer from the reduced metalloporphyrin, and recombination of thus generated radical pair is presumable responsible for formation of superoxide complex. Reaction mechanism of the superoxide and substrate was elucidated by using several substrates.