Metal vs Ligand Oxidation: Coexistence of Both Metal-Centered and Ligand-Centered Oxidized Species.

A series of two-electron-oxidized cobalt porphyrin dimers have been synthesized upon controlled oxidations using halogens. Rather unexpectedly, X-ray structures of two of these complexes contain two structurally different low-spin molecules in the same asymmetric unit of their unit cells: one is the metal-centered oxidized diamagnetic entity of the type CoIII(por), while the other one is the ligand-centered oxidized paramagnetic entity of the type CoII(por•+). Spectroscopic, magnetic, and DFT investigations confirmed the coexistence of the two very different electronic structures both in the solid and solution phases and also revealed a ferromagnetic spin coupling between Co(II) and porphyrin π-cation radicals and a weak antiferromagnetic coupling between the π-cation radicals of two macrocycles via the bridge in the paramagnetic complex.

Highly Oxidized Cobalt Porphyrin Dimer: Control of Spin Coupling via a Bridge.

A cobalt porphyrin dimer is constructed in which two Co(II)porphyrins are connected covalently through a redox-active diethylpyrrole moiety via a flexible but "nonconjugated" methylene bridge. Upon oxidation with even a mild oxidant such as iodine, each cobalt(II) center and porphyrin ring undergo 1e- oxidation, leading to the formation of a 4e--oxidized cobalt(III)porphyrin dication diradical complex. Other oxidants such as Cl2 and Br2 also produce similar results. To stabilize such highly oxidized dication diradicals, the "nonconjugated" methylene spacer undergoes a facile and spontaneous oxidation to form a methine group with a drastic structural change, thereby making the bridge fully π-conjugated and enabling through-bond communication. This results in a strong spin coupling between two π-cation radicals which stabilizes the singlet state. The experimental observations are also strongly supported by extensive density functional theory calculations. The present study highlights the crucial role played by the nature of the bridge in the long-range electronic communication.

Stepwise Oxidations in a Cofacial Copper(II) Porphyrin Dimer: Through-Space Spin-Coupling and Interplay between Metal and Radical Spins.

cis and trans-copper(II) porphyrin dimers have been synthesized, in which two CuII porphyrin macrocycles are bridged through a rigid ethene linker for possible through-space and through-bond spin-couplings between the paramagnetic CuII centers. It has been found that the two macrocycles come closer after 1 e- oxidation, however, they move far apart upon further 1 e- oxidation leading to transformation of the cis to the trans isomer. Detailed investigations are performed here on the interactions between the two porphyrin macrocycles, between two unpaired spins of closely spaced CuII centers, and also between the unpaired spins of metal and porphyrin π-cation radicals. Spectroscopic investigations along with the X-ray structure of the 2 e- -oxidized complex displayed strong electronic communications through the bridge between two porphyrin π-cation radicals. The counterion I9 - is stabilized in an unusual trigonal-pyramidal structure in the 2 e- -oxidized complex in which the central iodide ion is bound with four iodine (I2 ) molecules. Variable-temperature magnetic study revealed strong antiferromagnetic coupling between the two porphyrin π-cation radical spins (Jr-r ) in the 2 e- -oxidized complex. DFT calculations suggest stabilization of the triplet state, which is also in good agreement with the experiment. Ab initio molecular dynamics allowed the variation of the structural details to be followed upon stepwise oxidations and also the final isomerization process including its associated energy barrier.

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. 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.

Diheme cytochromes: Effect of mixed-axial ligation on the electronic structure and electrochemical properties with cobalt porphyrin dimer.

A series of six-coordinate diCo(III) porphyrin dimers, as synthetic analogues of diheme cytochromes, have been reported here having bis(imidazole), bis(pyridine) and mixed thiophenolate-pyridine/imidazole axial ligands. In the X-ray structures of bis(imidazole) and bis(pyridine) complexes, the axial ligands are in perpendicular orientation while they are parallelly oriented in their monomeric analog. The porphyrin rings are also highly ruffle-distorted in dimer but planar in monomer which reflect the effect of intramolecular interaction between two Co(porphyrin) units in dimers. In the X-ray structure of diCo(III) thiophenolate-pyridine mixed-ligated complex, the axial Co-S and Co-N(py) distances are 2.256(1) and 2.063(2) Å, respectively. The Co-N(py) distance of 2.063(2) Å is much longer than the distances of 1.961(3) and 1.972(3) Å observed in bis(pyridine) complex and the Co-S distance is larger than Co-N(py) in the mixed ligated complex which results in a displacement of Co by 0.15 Štowards the pyridine ligand from the mean porphyrin plane. Indeed, this is the first X-ray structure of a metalloporphyrin with mixed thiophenolate-pyridine axial ligands. The effect of mixed-axial ligation is demonstrated by a blue-shift of the Soret band in the UV-visible spectroscopy and also a positive shift of the Co(III)/Co(II) redox couple as compared to their bis(pyridine) analogue. The redox potentials are shifted to a large negative value just upon replacing the metal from iron to cobalt. The present investigation emphasizes the role of axial ligation, metal ions, and also the effect of heme-heme interaction in controlling the spectral and electrochemical properties.