The synâanti conformational switching has been demonstrated in the ethane-bridged dicobalt(II)bisporphyrin which is present in the syn-form only. The addition of either perylene or axial ligands to CoII(bisporphyrin) completely transforms the syn form into the anti because of strong ÏâÏ interaction and axial coordination, respectively. The complex undergoes four 1e-oxidations in CH2Cl2 which are indicative of strong through space interactions between the two cofacial Co-porphyrins at 295 K. The first oxidation is a metal centered one and occurs at a potential much lower than that of the monomeric analog. However, the second oxidation, which is again metal centered, was at a significantly higher potential. The large difference between the first two oxidations, as observed here, is due to much stronger inter-porphyrin interactions. The step-wise oxidations have been performed both chemically and electro-chemically while the progress of the reactions was monitored by UV-visible and 1H NMR spectroscopy. After 1e-oxidation, a very broad 1H NMR signal results with increased difference between two meso resonances, which indicates that the two macrocycles are in the syn-form with lesser interplanar separation as also observed by DFT. However, 2e-oxidation results in the stabilization of the anti form. The addition of axial ligands to CoII(bisporphyrin) also completely transforms the syn form into the anti form. While additions of THF and I2/Iâ both result in the formation of five-coordinate complexes, Co(II) is oxidized to Co(III) in the case of the latter. However, additions of 1-methylimidazole, pyridine and pyrazine as axial ligands result in the formation of a six-coordinate complex in which Co(II) is spontaneously oxidized to Co(III) in air.
乙烷桥接的二
钴(II)双
卟啉仅以同位形式存在,这种同反构象转换在二
钴(II)双
卟啉中得到了证实。在 CoII(双
卟啉)中加入过烯
配体或轴向
配体后,由于ÏâÏ相互作用和轴向配位作用分别很强,因此能将同位形式完全转变为反位形式。该复合物在
CH2Cl2 中发生了四次 1e- 氧化反应,这表明在 295 K 的温度下,两个共面
卟啉之间存在强烈的空间相互作用。然而,第二次氧化也是以
金属为中心的氧化,其电位明显更高。这里观察到的前两次氧化之间的巨大差异,是由于
卟啉之间的相互作用要强得多。分步氧化是通过
化学和电
化学方法进行的,同时通过紫外可见光谱和 1H NMR 光谱监测反应的进展。1e 氧化后,会产生非常宽的 1H NMR 信号,两个中位共振之间的差异增大,这表明两个大环处于合成形式,平面间的分离较小,DFT 也观察到了这一点。然而,2e-氧化会导致反式的稳定。向 CoII(双
卟啉)中添加轴向
配体也会使同向形式完全转变为反向形式。加入
四氢呋喃和 I2/Iâ 都会形成五配位络合物,但加入 I2/Iâ 时,Co(II)会被氧化成 Co(III)。然而,加入 1-甲基
咪唑、
吡啶和
吡嗪作为轴
配体会形成六配位络合物,其中的 Co(II) 会在空气中自发氧化成 Co(III)。