Cyclophane‐Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18‐Porphyrinyl Dicyanomethyl Diradicals

2,18‐Bis(dicyanomethyl)‐substituted Ni II porphyrin 8 and Zn II porphyrin 11 were prepared and subjected to oxidation with PbO 2 in CH 2 Cl 2 at 298 K to give cyclophane‐type chlorin dimers ( 9 ) 2 and ( 12 ) 2 as a consequence of double recombination of biradicals 9 and 12 , respectively. Dimer ( 9 ) 2 takes a syn ‐conformation of two distorted Ni II chlorins but ( 12 ) 2 takes an anti ‐conformation of relatively planar Zn II chlorins. At 298 K, dimer ( 9 ) 2 is stable and its 1 H NMR spectrum is sharp but becomes broad at high temperature, while the 1 H NMR spectrum of ( 12 ) 2 is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of ( 12 ) 2 is much less than that of ( 9 ) 2 . The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer ( 16 ) 2 to give homo dimers ( 9 ) 2 and ( 15 ) 2 .