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

Abstract

2,18‐Bis(dicyanomethyl)‐substituted NiII porphyrin 8 and ZnII porphyrin 11 were prepared and subjected to oxidation with PbO2 in CH2Cl2 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 NiII chlorins but (12)2 takes an anti‐conformation of relatively planar ZnII chlorins. At 298 K, dimer (9)2 is stable and its 1H NMR spectrum is sharp but becomes broad at high temperature, while the 1H 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.

Direct observation of reversible bond homolysis by 2D EXSY NMR

“…7,9,10 Conventionally (Fig. 1a), reversible bond homolysis has been studied most frequently using a temperature-dependent reversible change of UV-Vis, [11][12][13][14][15][16][17][18][19][20][21][22] IR, 15,23,24 EPR, 17,18,[20][21][22][25][26][27][28][29] or NMR [16][17][18][21][22][23][24][30][31][32][33][34][35][36][37] spectra. In a few cases, reversible bond homolysis has been observed in the solid state by powder X-ray diffractometry.…”