D'Souza Francis, Zandler Melvin E., Tagliatesta Pietro, Ou Zhongping, Shao Jianguo, Van Caemelbecke Eric, Kadish Karl M.
Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, Dipartimento di Scienze e Tecnologie Chimiche, II Università degli Studi di Roma, 00133 Roma, Italy, and Department of Chemistry, University of Houston, Houston, Texas 77024-5641.
Inorg Chem. 1998 Sep 7;37(18):4567-4572. doi: 10.1021/ic980336y.
The electronic, spectral, and electrochemical characterization of (meso-tetraphenylporphyrinato)zinc(II) complexes bearing between 0 and 8 bromo substituents at the beta-pyrrole positions is reported. The investigated compounds are represented as (TPPBr(x)())Zn where TPPBr(x)() is the dianion of brominated 5,10,15,20-tetraphenylporphyrin and x varies between 0 and 8. Each porphyrin undergoes four well-defined one-electron transfer reactions to yield porphyrin pi-cation radicals and dications upon oxidation and porphyrin pi-anion radicals and dianions upon reduction. Half-wave potentials for the first reduction of (TPPBr(x)())Zn can be described by a single linear free energy relationship, and plots of E(1/2) versus the number of Br groups on the complex show a linear correlation with a positive slope of 63 mV per Br group. This is not the case for the other three electron transfer processes of the compounds where plots of E(1/2) versus the number of Br groups show distinctly different linear correlations for derivatives with 0-4 Br groups and those with 4-8 Br groups. The effect of increasing number of Br groups on the spectral and electrochemical properties of the neutral complexes was examined over the whole series of compounds, and these experimental results are compared to results of theoretical calculations by semiempirical molecular orbital AM1 methods using configurational interactions (CI) over the four Gouterman frontier pi-orbitals. The dihedral angle containing the four porphyrin macrocycle ring nitrogens is proposed as a measure of porphyrin ring nonplanarity, and this value increases with increasing number of Br substituents on (TPPBr(x)())Zn. Results of the AM1-CI = 4 calculations indicate that the spectrally determined HOMO-LUMO gap, i.e., the energy corresponding to the low-energy absorption band, varies in a nonlinear fashion with increasing number of Br substituents on the macrocycle and this is due to both the electronic effect of the substituents and the macrocycle nonplanarity. The HOMO-LUMO gaps theoretically calculated by AM1-CI = 4 methods thus parallel values which are experimentally obtained by electrochemistry or spectroscopy. The lack of well-defined linear free energy relationships for all processes except for the first reduction can be explained on the basis of electronic effects caused by the halogen substituents and nonplanar macrocyclic distortions induced by steric interactions among the peripheral substituents. In the case of porphyrin dication formation, the redox potentials are virtually independent of the bromo substituents.
报道了在β-吡咯位置带有0至8个溴取代基的（中-四苯基卟啉）锌（II）配合物的电子、光谱和电化学表征。所研究的化合物表示为（TPPBr(x)()）Zn，其中TPPBr(x)()是溴化5,10,15,20-四苯基卟啉的二价阴离子，x在0至8之间变化。每个卟啉经历四个明确的单电子转移反应，氧化时生成卟啉π-阳离子自由基和二价阳离子，还原时生成卟啉π-阴离子自由基和二价阴离子。（TPPBr(x)()）Zn首次还原的半波电位可用单一的线性自由能关系来描述，E(1/2)对配合物上Br基团数目的作图显示出线性相关性，每个Br基团的正斜率为63 mV。对于该化合物的其他三个电子转移过程则并非如此，其中E(1/2)对Br基团数目的作图显示，对于具有0 - 4个Br基团的衍生物和具有4 - 8个Br基团的衍生物，线性相关性明显不同。在整个系列化合物中研究了增加Br基团数目对中性配合物光谱和电化学性质的影响，并将这些实验结果与使用半经验分子轨道AM1方法并在四个Gouterman前沿π轨道上进行构型相互作用（CI）的理论计算结果进行了比较。提出包含四个卟啉大环环氮的二面角作为卟啉环非平面性的量度，并且该值随着（TPPBr(x)()）Zn上Br取代基数目的增加而增加。AM1 - CI = 4计算结果表明，光谱测定的HOMO - LUMO能隙，即对应于低能吸收带的能量，随着大环上Br取代基数目的增加以非线性方式变化，这是由于取代基的电子效应和大环非平面性共同作用的结果。因此，通过AM1 - CI = 4方法理论计算得到的HOMO - LUMO能隙与通过电化学或光谱学实验获得的值相似。除首次还原外，所有过程缺乏明确的线性自由能关系可基于卤素取代基引起的电子效应以及外围取代基之间的空间相互作用引起的非平面大环畸变来解释。在卟啉二价阳离子形成的情况下，氧化还原电位实际上与溴取代基无关。
