Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations

The interaction between two porphyrin macrocycles, connected covalently through either a rigid ethylene or a flexible ethane bridge, in the metalloporphyrin dimers (M: 2H, Zn2+) have been investigated upon stepwise oxidations. Upon 1e-oxidation, two porphyrin macrocycles come closer and cofacial to each other while 2e-oxidation forces them to be separated as far as possible. This has resulted in the conversion of the cis isomer to trans for the ethylene bridged porphyrin dimer with the stabilization of an unusual “U” form, which has unique spectral and geometrical features. Detailed ultraviolet–visible–near-infrared (UV-vis-NIR), infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopic investigations, along with X-ray structure determination of the 2e-oxidized complexes, have demonstrated strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. Such extensive π-conjugation also results in strong antiferromagnetic coupling between the radical spins of both of the macrocycles, which generates a diamagnetic compound. The experimental observations are also strongly supported by density functional theory (DFT) calculations.本研究针对金属卟啉二聚体（M：2H、Zn²⁺）中，通过刚性乙烯桥或柔性乙烷桥共价连接的两个卟啉大环（porphyrin macrocycles）之间的相互作用，开展了分步氧化过程的系统探究。 当发生单电子氧化时，两个卟啉大环会彼此靠近并呈共面构型；而双电子氧化则会促使二者尽可能远离。这一现象使得乙烯桥连卟啉二聚体的顺式异构体转化为反式异构体，同时稳定得到一种罕见的"U"型结构，该结构具备独特的光谱与几何特性。 通过紫外-可见-近红外（UV-vis-NIR）、红外（IR）、电子顺磁共振（EPR）、核磁共振（NMR）等光谱学表征，结合双电子氧化配合物的X射线晶体结构解析，研究证实：两个卟啉π阳离子自由基之间可通过桥连乙烯基团实现强烈的电子相互作用。这种高度延展的π共轭体系还会促使两个大环的自由基自旋之间产生强反铁磁耦合，进而生成抗磁性化合物。 本研究的实验观测结果同时得到了密度泛函理论（density functional theory, DFT）计算的有力支撑。