Synthesis of Triple-Stranded Complexes Using Bis(dipyrromethene) Ligands

The reaction of an alpha-free, beta,beta'-linked bis(dipyrromethene) ligand with Fe3+ or Co3+ led to noninterconvertible triple-stranded helicates and mesocates. In the present context, a stable alpha-free ligand 2 has been developed and complexation of ligands 1 and 2 with diamagnetic Co3+, Ga3+, and In3+ has been studied. The triple-stranded M(2)1(3) (M = Ga, In) and M(2)2(3) (M = Co, Ga, In) complexes were characterized using matrix-assisted laser desorption ionization time-of-flight spectrometry, H-1 NMR and UV-vis spectroscopy, and X-ray crystallography. Again, the H-1 NMR analysis showed that both the triple-stranded helicates and mesocates were generated in this metal-directed assembly. Consistent with our previous finding on coordinatively inert Co3+ complexes, variable-temperature NMR spectroscopy indicated that the triple-stranded helicate and mesocate of labile In3+ did not interconvert in solution, either. However, the diastereoselectivity of the M(2)2(3) complexes was found to improve with an increase in the reaction temperature. Taken together, this study complements the coordination chemistry of poly(dipyrromethene) ligands and provides further insight into the formation of helicates versus mesocates.