Metalloporphyrin Mixed-Valence π-Cation Radicals:  Solid-State Structures

Abstract

The synthesis and structural characterization of three mixed-valence π-cation radical derivatives of (octaethylporphinato)metal(II) species, [M(OEP•/2)]2 +, (M = Ni, Cu, and (Zn,H2)), are reported. The species thus have the same formal ring oxidation level as the photooxidized special pair of photosynthetic reaction centers. The complexes display unique binuclear structures with cofacial porphyrin rings with mean plane separations of 3.20−3.32 Å, ring center to ring center distances of 3.50−3.55 Å, and lateral shifts between the two rings of 1.16−1.53 Å. These inter-ring geometries are distinctly different from either those of “classical” π-cation radical derivatives or those of dimers formed by neutral species. The different inter-ring geometries suggests that there are inherent inter-ring geometries that depend principally on the oxidation level of the porphyrin rings.

Structural deformations and bond length alternation in porphyrin π-cation radicals

“…The high degree of ring-ring overlap in the p-cation radical derivatives, which is much stronger than that seen in the solid-state structures of neutral species [5], suggested that the degree of ring overlap might be related to the oxidation level of the porphyrin ring. This idea was further supported by the degree of overlap found for the structures of three mixed-valence species [22,23]. In these species, which are called mixedvalence species because the one electron that is removed in the ring oxidation appears to be delocalized over two porphyrin rings.…”

“…The value of the lateral shift in these three mixed-valence radical derivatives are intermediate to the tightest values seen in the neutral species and the completely overlapped cores of the fMOEP Á g 2 2 dimers. Scheidt et al [23] thus suggested that the ring-ring p-p interactions are sensitive to the oxidation level of the porphyrin ring. Subsequently, it was shown that one form of fNiOEP Á g 2 2 [24] and fMgOEP Á g 2 2 [20] did not conform to this pattern; each of these derivatives has lateral shifts similar to those displayed by tightly interacting neutral species.…”

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…Further, I would take notice of two bodies of work that have not been discussed in this paper, work on the other oxynitrogen-ligated porphyrins (the ligands NO 2 − and NO 3 − ) [112][113][114][115][116][117][118][119][120][121][122][123][124] and the work on the characterization of π-cation radical complexes [125][126][127][128][129][130][131][132][133][134][135][136][137][138][139]. Diagram Illustrating the effect of axial ligand orientation on the axial bond distances in iron (III) species.…”

Conformational and structural studies of meso monosubstituted metalloporphyrins—Edge-on molecular interactions of porphyrins in crystals

“…In comparison the two available structures for metal complexes of mono-tertbutylporphyrin exhibit more typical -stacked aggregates. 21 In the nickel(II) derivative Ni(II)2c, with one crystallographically independent molecule in the asymmetric unit, four 'dimers' fill the unit cell with the meso substituents offset by 90º against each other.…”