Synthesis, structure and spectral and electrochemical properties of 3-pyrrolyl BODIPY-metal dipyrrin complexes

The stable dipyrrin substituted 3-pyrrolyl BODIPY (alpha-dipyrrin 3-pyrrolyl BODIPY) was synthesized by oxidation of dipyrromethane substituted 3-pyrrolyl BODIPY with 2,3-dichloro-5,6-dicyano-benzoquinone in CH2Cl2 at room temperature. The alpha-dipyrrin 3-pyrrolyl BODIPY is characterized by using HR-MS, 1D, 2D NMR and absorption spectroscopic techniques. The absorption spectrum of alpha-dipyrrin 3-pyrrolyl BODIPY showed a characteristic absorption band at 630 nm and a charge transfer band at 717 nm due to intramolecular charge transfer from the dipyrrin unit to the 3-pyrrolyl BODIPY unit. The 3-pyrrolyl BODIPY-metal dipyrrin complexes (Pd(II), Re(I) and Ru(II)) were prepared by treating alpha-dipyrrin 3-pyrrolyl BODIPY with appropriate metal salts in toluene-triethylamine at 100 degrees C and purified by silica gel column chromatography. The crystal structure obtained for the 3-pyrrolyl BODIPY-Pd(II) dipyrrin complex showed that the 3-pyrrolyl BODIPY and metal dipyrrin moieties are aligned to each other with an angle of 41.9 degrees. The absorption studies showed a strong band at similar to 620 nm corresponding to 3-pyrrolyl BODIPY moiety and a weak band at similar to 530 nm corresponding to metal dipyrrin unit with complete disappearance of the charge transfer band at 717 nm. The complexes are electron deficient and exhibited only reversible/quasi-reversible reductions in cyclic voltammetry.