Bis-Palladium Complex of α-Benzimidazole 9-Pyrrolyl Dipyrromethene: Synthesis, Structure, and Spectral and Catalytic Properties.

A new ligand is designed and synthesized in two steps starting from α-formyl 3-pyrrolyl BODIPY. In the first step, the α-formyl 3-pyrrolyl BODIPY was condensed with 1,2-diaminobenzene in toluene at reflux and afforded α-benzimidazole 3-pyrrolyl BODIPY in 16% yield. In the second step, α-benzimidazole 3-pyrrolyl BODIPY was decomplexed upon being treated with Lewis acid AlCl3 and afforded the desired ligand α-benzimidazole 9-pyrrolyl dipyrromethene. However, the ligand was not very stable and reacted further with PdCl2 in CH3CN for 1 h at reflux followed by recrystallization and afforded a novel bis-palladium complex of α-benzimidazole 9-pyrrolyl dipyrromethene in 36% yield. The bis-palladium complex was characterized and studied by high-resolution mass spectrometry, one- and two-dimensional nuclear magnetic resonance, X-ray crystallography, absorption, and density functional theory/time-dependent DFT (DFT/TD-DFT) studies. The X-ray structure revealed that two ligands and two Pd(II) ions were involved in forming a unique complex in which each Pd(II) ion was coordinated to three pyrrole N atoms of the first ligand and the benzimidazole N atom of the second ligand in a distorted square planar geometry. The absorption spectrum of the bis-palladium complex shows ill-defined, broad, and less intense bands in the region of 345-425 nm along with split bands in the higher-wavelength region of 600-630 nm. The bis-palladium complex was nonfluorescent, and the results of DFT/TD-DFT studies were in agreement with the experimental observations. The preliminary studies indicated that the bis-palladium complex can act as an efficient catalyst for coupling different aryl bromides with phenylboronic acid.