Synthesis of an adj-Dicarbaporphyrin and the Formation of an Unprecedented Tripalladium Sandwich Complex

An adj-dicarbaporphyrin was prepared by carrying out a base-catalyzed MacDonald reaction between bis­(3-indenyl)­methane and a dipyrrylmethane dialdehyde. The porphyrinoid system exhibited highly diatropic characteristics, and the proton NMR spectrum gave resonances at −5.74 and −6.24 ppm for the internal NH and CH protons, respectively. The UV–vis spectrum was also porphyrin-like, giving a Soret band at 455 nm and a series of Q bands at longer wavelengths. Addition of trifluoroacetic acid gave a C-protonated monocation, and at higher acid concentrations a dicationic species was observed. Addition of 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU) afforded a monodeprotonated porphyrinoid anion. All of these species retained highly diatropic characteristics. Density functional theory calculations showed that a nonplanar tautomer with four internal hydrogens was favored, in agreement with the spectroscopic data. Nucleus-independent chemical shift calculations also confirmed the aromatic characteristics of the free-base, cationic, and anionic structures. The dicarbaporphyrin reacted with palladium­(II) acetate in refluxing acetonitrile to give an unusual tripalladium sandwich complex consisting of two dianionic palladium­(II) dicarbaporphyrin units surrounding a palladium­(IV) cation with unique η5 interactions involving meso-carbon atoms.