Lanthanide octaethylprophyrins: preparation, association, and interaction with axial ligands

The synthesis of virtually all the lanthanide octaethylporphyrin complexes have been achieved by heating appropriate anhydrous lanthanide halide and octaethylporphyrin in imidazole melt at 210 degrees C for two hours. The lighter lanthanide porphyrin complexes are very susceptible to hydrolysis, the middle lanthanide porphyrin complexes are moderately stable, and the heavier lanthanide porphyrin complexes are relatively more stable to hydrolysis. Two out of four lanthanide porphyrin complexes studied in detail, namely ytterbium and lutetium octaethylporphyrins, aggregate in benzene and the Soret bands in their absorption spectra are about 6 nm shifted to higher energies upon a hundred-fold increase in their concentrations. The aggregations of these lanthanide porphyrin complexes in non-coordinating solvents have been further verified by 1H NMR spectral studies. This spectral behavior can be interpreted qualitatively in terms of the model of the molecular exciton interactions with stacking of at least two prophyrins. A dimeric structure of these lanthanide porphyrin complexes has been proposed on the basis of geometrical considerations. On the contrary, the europium and gadolinium octaethylporphyrins associate very weakly in benzene in the concentration range studied. All four lanthanide porphyrin complexes interact with pyridine and piperidine, and the Soret bands in their absorption spectra are about 8 nm shifted to low energies as compared with their values in pure benzene.