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

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 NiPorCl8and NiPorBr8, where Por is the dianion of meso-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin. The UV−vis spectra of NiPorCl8andNiPorBr8vary 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 β-octahalogenated NiTPPCl8andNiTPPBr8under the same solution conditions. The NiPorX8spectra are stable with time and interpreted in terms of 4- or 6-coordinate derivatives in13 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 NiPorCl8, followed by the formation of an air-oxidized porphodimethene-like product calledporpho-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 NiPorCl8and NiPorBr8after the first one-electron addition in CH2Cl2isstable for short times at−60°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 CH2Cl2, 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 CH2Cl2containing 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.