Synthesis, Electrochemistry, and Reversible Interconversion among Perhalogenated Hydroxyphenyl Ni(II) Porphyrins, Porphodimethenes, and Porpho-5,15-bis-paraquinone Methide

摘要

Two octahalogenated nickel(II) hydroxyphenylporphyrins were synthesized and characterized as to their electrochemical and spectroscopic properties as well as their reactivity in neutral, acidic, and basic nonaqueous media. The newly synthesized complexes are represented as NiPorCl(8) and NiPorBr(8), where Por is the dianion of meso-tetrakis(3,5-ditert-butyl-4-hydroxyphenyl)porphyrin. The UV-vis spectra of NiPorCl(8) and NiPorBr(8), vary with the solvent and degree of axial coordination but are almost identical to each other in a given solvent. These spectra are also substantially different from that of the unhalogenated NiPor parent porphyrin (which resembles nickel tetraphenylporphyrin, NiTPP), and they also differ from the spectra of beta-octahalogenated NiTPPCl8 and NiTPPBr8 under the same solution conditions. The NiPorX(8) spectra are stable with time and interpreted in terms of 4- or 6-coordinate derivatives in 13 different nonaqueous solvents. This is not the case, however, in DMF or DMSO, where a transient six-coordinate complex is initially formed upon dissolving the NiPorCl(8), followed by the formation of an air-oxidized porphodimethene-like product called porpho-5,15-bis-paraquinone methide, with the time of this chemical transformation depending upon the concentration of the porphyrin in solution. The initial species formed from NiPorCl(8) and NiPorBr(8), after the first one-electron addition in CH2C12 is stable for short times at -60 degrees C, but this is not the case at room temperature, where a rapid homogeneous chemical reaction occurs. Four additional redox reactions are also observed in CH2C12, and the UV-visible spectra of several in-situ-generated electroreduction products are compared with that of chemically synthesized porphodimethenes formed in neutral, acidic, and basic solutions of CH2C12 containing acid in the form of TFA or base in the form of TBA(+)X, where X = OAc-, CN-, and OH-. Finally, a reversible electrochemically driven conversion between the Ni(II) hydroxyphenylporphyrin and a reduced porphodimethene or oxidized porphyrin-like product, porpho-5,15-bis-paraquinone methide, is described.