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

Novel bipyridylene-bridged bisporphyrin la, in which two porphyrin units we re attached directly to symmetrical 4,4'-positions of the 2,2'-bipyridyl gr oup, 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% yiel d. Spatial geometries of two porphyrins in la were regulated by reversible complexation of the bipyridyl part with PdCl2. Thus, the addition of 2.2 eq uiv 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.