Axial coordination behaviors and photochemistry in complexis of early transition metals

研究了铬、锆、铌和六亚甲基卟啉的轴向配位行为。通过NMR、ESR和可见光谱法跟踪配体交换反应。轴向配体与酚类（邻苯二酚、对苯二酚、硫酚）交换的机理建立如下。1.质子从苯酚转移到配合物中，消除轴向配体，形成苯酚根阴离子和具有空配位的配合物阳离子的离子对。2.酚盐阴离子向络合物阳离子发生电子转移，形成一对苯氧基自由基和还原络合物。3.自由基对的取代产生苯氧络合物。通过电化学方法测定各阶段配合物的氧化还原电位，证实了这一机理。步骤2在酚类的情况下受到干扰，酚类的阴离子状态高度稳定，因此离子对，即氧化的不饱和络合物是保守的。如果苯酚或卟啉中存在较大的取代基，或者通过形成配位键而稳定化的程度较低，则步骤3会因还原的不饱和络合物和自由基的产生而受阻。铌和钼卟啉，从这个角度来看，光化学还原产生减少不饱和络合物。还原后的金属卟啉活化了分子氧，催化烯烃氧化反应。从还原的金属卟啉的电子转移，和重组，从而产生的自由基对是推测负责形成超氧化物复合物。以几种底物为例，阐明了超氧阴离子与底物的反应机理。

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.