Synthesis, Structure, and Optical and Redox Properties of Chlorophyll Derivatives Directly Coordinating Ruthenium Bisbipyridine at the Peripheral β-Diketonate Moiety

Abstract

The diketonate group of the peripheral position in chlorophyll derivatives 1 and 2 coordinated ruthenium bisbipyridine to give direct linkages 3-5 of the chlorin ring with the Ru(II) complex. Zinc metalation of the central position in the chlorin ring of free base 3 afforded the Ru-Zn binuclear complex 3-Zn. Because the diketonate group at the C3 position of chlorophyll derivatives coordinated to bulky Ru(bpy)2(2+), the plane of the diketonate group was twisted from the chlorin π ring in synthetic 3-5 and 3-Zn to lead to a partial deconjugation and a slight blue shift of the longest wavelength electronic absorption band in dichloromethane. A broad metal-to-ligand charge-transfer absorption band derived from the Ru complex was observed around 500 nm, in addition to visible absorption bands from the chlorophyll moiety. Chlorophyll derivatives 3-5 and 3-Zn directly coordinating the ruthenium complex were less fluorescent in dichloromethane than chlorophyll-diketonate ligands 1, 2, and 1-Zn due to the heavy atom effect of the ruthenium in a molecule. The coordination to the ruthenium complex moiety at the peripheral position shifted the electrochemical reduction of the chlorin part in acetonitrile to a negative potential, and the coordination to zinc at the central position decreased the redox potentials. Chemical modification of the bipyridine and diketonate ligands of the ruthenium complexes greatly affected the redox potentials of Ru(II)/(III) and/or Ru(II)/(I) but minimally the redox properties of the chlorin moiety. Substitution with electron-donating groups shifted the former to a negative potential but only barely shifted the latter. The zinc metalation caused no apparent shifts for the redox potentials of the Ru center.