Transition-metal Complexes of Pyrrole Pigments. XI. Zinc(II) Chelates of Some Dipyrromethenes

Abstract

Abstract The zinc (II) chelates of five dipyrromethenes which have methyl substituents to a various extent, as well as that of 5,5′-diphenyldipyrromethene, have been prepared. The behaviors of methyl resonances upon metalcoordination were classified into three cases: (1) a down-field shift, but not greater than that of the corresponding ligand hydrochloride (3,3′-CH3); (2) nearly no change in chemical shift (4,4′-CH3); and (3) a profound up-field shift, the methyl resonance of the corresponding ligand hydrochloride showing a down-field shift on the contrary (5,5′-CH3). A marked change in chemical shift for the methine proton upon zinc(II)-coordination is related to conjugation in the chelate ring–the presence of aromaticity. As a whole, methyl resonances for the zinc chelates are consistent with the tetrahedral coordination geometry. Electronic absorption peaks for the zinc chelates, which were located in the lowest energy region with the highest intensity, were assigned to the intra-ligand π→π* transition and the metal→ligand charge-transfer as well. Vibrational absorption bands appearing in the 1600 cm−1 range and in the 400–350 cm−1 region have been assigned to the skeletal stretching mode of the dipyrromethene moiety and the metal-ligand stretching mode, respectively, for the zinc chelates.