Intermacrocyclic Interaction Triggers Facile One‐Pot Synthesis of a Chlorin–Porphyrin Heterodimer

Abstract

This work reports a highly facile one-pot synthesis of a new series of fully π-conjugated unsymmetric chlorin-porphyrin heterodimers with quantitative yields by utilizing intermacrocyclic interactions. One-electron oxidations of dicopper(II) and dipalladium(II) porphyrin dimers using mild oxidants such as iodine at room temperature resulted in the formation of a strongly interacting cofacial mixed-valent π-cation radical dimers. The radical, being highly reactive, drives spontaneous and rapid transformation involving a new N=C bond formation, 1,2-ethyl migration, and the generation of a new indolizinium ring that bridges between the two macrocycles. X-ray structural characterization of the heterodimers reveals that the two macrocycles are nearly coplanar and thereby extends the π-conjugation from one end to the other. DFT calculations that reproduce the experimental results are also reported.

Unusual Stabilization of Dication Diradical Intermediate of Dizinc(II) Porphyrin Dimer

“…All these spectral changes are due to the formation of a fully π‐conjugated dication diradical complex 3· Zn · I, which was isolated as a solid in excellent yield and structurally characterized. Compared to 3· Zn · I, the fused chlorin‐porphyrin heterodimers 2· Cu · I 3 and 2· Pd · I 3 are characterized[10a] by the presence of two low energy bands near 832 and 802 nm, respectively (Figure ). It is quite interesting to note here that complex 3· Zn · I exhibits two intense bands (at 415 and 525nm) along with a broad low‐energy band (at 840 nm), which are completely different from the bands observed for the heterodimers 2· Cu · I 3 and 2· Pd · I 3 .…”

“…In sharp contrast, the radical signal of 1· Pd · I 3 sharply reduces its intensity with time due to the concomitant formation of the heterodimer 2· Pd · I 3 . [10a] Both the complexes 1· Zn · I and 2· Zn · I show an isotropic EPR signal at g = 2.0031 and g = 2.0035 respectively, indicating the generation of free radical in the complex due to formation of porphyrin π‐cation radicals (Figure S5, Supporting Information).…”

“…Metalloporphyrin π‐cation radicals have been involved as key intermediates in a large variety of enzymatic catalysis and photosynthesis . To unveil the key roles and fundamental molecular features that are important for relevant biochemical processes, π‐cation radicals of synthetic metalloporphyrin have been investigated continuously . We have been actively working on the structure‐function correlation of various diheme enzymes such as MauG[4a], [4b] and cytochrome c peroxidases (bCcP)[4c], [4d] using covalently linked porphyrin dimers .…”

“…To unveil the key roles and fundamental molecular features that are important for relevant biochemical processes, π‐cation radicals of synthetic metalloporphyrin have been investigated continuously . We have been actively working on the structure‐function correlation of various diheme enzymes such as MauG[4a], [4b] and cytochrome c peroxidases (bCcP)[4c], [4d] using covalently linked porphyrin dimers . Besides many other interesting features of these covalently linked porphyrin dimers, their ability of forming fully π‐conjugated networks upon oxidation and the stability of the unusual electronic structure and coordination arrangements of the oxidized metal complexes are worth mentioning here ,…”

“…Quite recently, we have reported[10a] that one‐electron oxidations of diethylpyrrole‐bridged dicopper(II) and dipalladium(II) porphyrin dimers using mild oxidants such as iodine at room temperature resulted in the formation of strongly interacting cofacial mixed‐valent π‐cation radical dimers. The radical species, being highly reactive, undergo a facile and rapid transformation into a fully π‐conjugated unsymmetrical chlorin‐porphyrin heterodimer involving a new N=C bond formation, 1,2‐ethyl migration and the generation of a new indolizinium ring that bridges between the two macrocycles.…”