Synthesis of new diaryl-substituted triple-decker and tetraaryl-substituted double-decker lanthanum(III) porphyrins and their porphyrin ring rotational speed as compared with that of double-decker cerium(IV) porphyrins

Tetraaryl-substituted cerium(IV) double-decker porphyrin (2D), tetraaryl-substituted lanthanum(III) double-decker porphyrin (3D), and diaryl-substituted lanthanum(III) triple-decker porphyrins (4T•Me and 4T•MeO) were newly synthesized and their porphyrin ring rotation rates were systematically estimated by means of a VT NMR spectroscopic method. In 2D the coalescence temperature (T _c) for the porphyrin ring rotation was higher than 110 °C, whereas in 3D it appeared at ca. 0 °C. In diaryl-substituted cerium(IV) double-decker porphyrin (5D) it appeared at 13 °C whereas in 4T•Me and 4T•MeO they were lower than -80 °C. These results consistently support the view that the porphyrin ring rotation rates in lanthanum(III)-based porphyrins are much faster than those in cerium(IV)-based porphyrins. The difference is reasonably explained by the difference in the ion size between these two metal ions. Since these sandwich-type porphyrins can act as novel scaffolds for designing positive allosteric recognition systems, La(III) complexes which feature the faster porphyrin ring rotation should be useful to develop more efficient positive allosteric systems.