Vacatadithiasapphyrins: Synthesis, Structure, and Spectral and Redox Properties

Four examples of novel aromatic [22]vacatadithiasapphyrins were synthesized by refluxing appropriate [22]telluradithiasapphyrins in 1,2-dichlorobenzene in the presence of excess HCl followed by simple column chromatographic purification. The [22]vacatadithiasapphyrins can exist in three conformers "in", "out", and "zigzag" w.r.t the butadiene moiety, and under our experimental conditions, the "out" conformer was the major compound. The X-ray structure obtained for one of the "out" conformers of vacatadithiasapphyrins revealed that the macrocycle was planar similar to its parent telluradithiasapphyrin and showed effective π-delocalization over the entire macrocyclic core. NMR studies supported the formation of the "out" conformer and suggested that the vacatadithiasapphyrins were less aromatic than the parent telluradithiasapphyrins. Density functional theory, time-dependent DFT, and nuclear independent chemical shift studies indicated that the protonated form of vacatadithiasapphyrin was more aromatic than the parent protonated telluradithiasapphyrin. The absorption spectra of vacatadithiasapphyrins showed a typical strong Soret band at ∼488 nm and four relatively broad Q-bands in the region of 590-860 nm, and electrochemical studies suggest that vacatadithiasapphyrins were easier to oxidize and easier to reduce compared to the parent telluradithiasapphyrins.