Synthesis of bipyridylene-bridged bisporphyrin by nickel-mediated coupling reaction: ON-OFF control of cofacial porphyrin unit by reversible complexation

Novel bipyridylene-bridged bisporphyrin la, in which two porphyrin units were attached directly to symmetrical 4,4'-positions of the 2,2'-bipyridyl group, was synthesized by a nickel(0)-mediated homocoupling reaction of 5,10,15-tris(n-heptyl)-20-(2'-bromo-4'-pyridyl)porphyrinatozinc (3a) in 58% yield. Spatial geometries of two porphyrins in la were regulated by reversible complexation of the bipyridyl part with PdCl(2). Thus, the addition of 2.2 equiv of palladium chloride to la converted the freely rotating conformation to the cofacial bisporphyrin 2a. The subsequent addition of 4,4'-dimethyl-2,2'-bipyridine 9 regenerated the initial bisporphyrin 1a.