Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity.

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C-C and one C-N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals.

Control of spin coupling through a redox-active bridge in a dinickel(II) porphyrin dimer: step-wise oxidations enable isolations of a chlorin-porphyrin heterodimer and a dication diradical with a singlet ground state.

A dinickel(II)porphyrin dimer has been used here in which the redox-active pyrrole-moiety, similar to the tryptophan residue in diheme enzymes such as MauG and bCcP, has been placed between two Ni(II)porphyrin centers connected a flexible, but unconjugated methylene bridge. This arrangement provides a large physical separation between the two metal centers and thus displays almost no communication between them through the bridge. Upon treatment with DDQ as an oxidant, the dinickel(II) porphyrin dimer slowly gets converted into an indolizinium-fused chlorin-porphyrin heterodimer.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions.

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers.

Intermacrocyclic Interaction Triggers Facile One-Pot Synthesis of a Chlorin-Porphyrin Heterodimer.

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.

Probing Bis-Fe MauG: Isolation of Highly Reactive Radical Intermediates.

MauG is a diheme enzyme that utilizes two covalently bound c-type heme centers. We report here step-wise oxidations of a synthetic analogue of MauG in which two heme centers are bridged covalently through a flexible linker containing a pyrrole moiety. One- and two-electron oxidations produce monocation radical and dication diradical intermediates, respectively, which, being highly reactive, undergo spontaneous intramolecular rearrangement involving the pyrrole bridge itself to form indolizinium-fused chlorin-porphyrin and spiro-porphyrinato heterodimers.

Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations.

The interaction between two porphyrin macrocycles, connected covalently through either a rigid ethylene or a flexible ethane bridge, in the metalloporphyrin dimers (M: 2H, Zn(2+)) have been investigated upon stepwise oxidations. Upon 1e-oxidation, two porphyrin macrocycles come closer and cofacial to each other while 2e-oxidation forces them to be separated as far as possible. This has resulted in the conversion of the cis isomer to trans for the ethylene bridged porphyrin dimer with the stabilization of an unusual "U" form, which has unique spectral and geometrical features.