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

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