Synthesis, structure, and properties of palladium(II) complex of α-formyl pyrrolyl dipyrromethene

A simple α-formyl pyrrolyl dipyrromethene ligand was synthesized by deboronation of a BF₂ complex of α-formyl pyrrolyl dipyrrin under Lewis acid catalyzed conditions. The α-formyl pyrrolyl dipyrrin ligand was treated with PdCl₂ in CH₃CN/Et₃N at room temperature under inert atmosphere conditions for 3 h followed by recrystallization to afford a Pd(II) complex of α-formyl pyrrolyl dipyrrin in 61% yield. The X-ray structure revealed that Pd(II) was coordinated to three pyrrole Ns of the α-formyl pyrrolyl dipyrrin ligand and the fourth position was occupied by a water molecule with an overall distorted square planar geometry around the Pd(II) ion. The formyl group present in the appended pyrrole ring was not involved in bonding with the Pd(II) ion but helps in stabilizing the complex via hydrogen bonding interaction with the coordinated water molecule. The Pd(II) complex was further characterized and studied thoroughly by 1D and 2D NMR, absorption, cyclic voltammetry and DFT/TD-DFT techniques. The absorption spectrum of the Pd(II) complex showed one sharp intense band at 630 nm and less-intense bands at 585, 435 and 348 nm and the electrochemical studies indicated the electron-deficient nature of the Pd(II) complex. The DFT/TD-DFT studies were in agreement with the experimental observations. The Pd(II) complex was tested as a catalyst for the Suzuki-Miyaura coupling of aryl bromides with aryl boronic acid and the studies supported the catalyst's efficiency in the coupling reaction to form biaryl compounds.