Metalation Control of Open‐Shell Character in meso‐meso Linked Porphyrin meso‐Oxy Radical Dimers

Abstract

Control of open-shell character of meso-meso linked porphyrin meso-oxy radical dimers has been demonstrated by core metalation. Namely, Ni -porphyrin dimer 6Ni exhibits a clear H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature-dependent H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis-imidazole-coordinated Zn -porphyrin dimer 6ZnIm exhibits a perpendicular structure consisting of two planar Zn -porphyrins and has been characterized as a distinct open-shell diradical on the basis of its non-observable H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm is an extremely stable molecule.

Stable meso–meso‐Linked 2NH‐Corrole Radical Dimers as a Key Intermediate to Corrole Tape

“…The structure of 3 has been confirmed by X‐ray diffraction analysis (Figure a). Curiously, the dihedral angle between the two corrole units is 49.7°, being considerably smaller than that of the precursor 1 (87.5° as shown in Figure b) . The CNC bond angles of a pyrrole subunit are useful to assign the pyrrolic segment as an imine‐type or an amine‐type.…”

“…The theoretical spin‐exchange integral ( J S‐T =−2.02 kcal mol −1 ) is roughly in accordance with the experimental value (−2.19 kcal mol −1 ) estimated by the Bleaney–Bowers equation, although the J S‐T values are highly dependent on the dihedral angle between the two corrole planes . Therefore, the observed stability of 3 may be ascribed largely due to the intramolecular antiferromagnetic interaction . It was thought that 3 is an intermediate to triply linked 2NH ‐corrole dimer 2 .…”

Stable meso–meso‐Linked 2NH‐Corrole Radical Dimers as a Key Intermediate to Corrole Tape

“…Curiously, the dihedral angle between the two corrole units is 49.78, being considerably smaller than that of the precursor 1 (87.58 as shown in Figure 1 b). [6,8] The CNC bond angles of a pyrrole subunit are useful to assign the pyrrolic segment as an imine-type or an amine-type. In 3, the angles of CN 1 C and CN 3 C (106-1088) are distinctly smaller than those of CN 2 C and CN 4 C (110-1128), while only one pyrrole subunit shows an imine-type feature (]CN 1 C = 108.18) in 1.…”

“…[13] Therefore, the observed stability of 3 may be ascribed largely due to the intramolecular antiferromagnetic interaction. [8,14,15] It was thought that 3 is an intermediate to triply linked 2NH-corrole dimer 2. Indeed, oxidation of 3 with 2 equiv of DDQ in refluxing CHCl 3 for 30 minutes gave 2 in 83 % yield.…”

Phenylene‐bridged Porphyrinmeso‐Oxy Radical Dimers

“…[6] By taking advantage of the radical-stabilizinga bilities and variousf unctionalization methods, porphyrin-based oligo-radicals are easily accessible. [7] For example, meso-meso directly linked dimers 3M (M = H 2 ,Z n, Ni)s howedc entral-metal dependent diradical characters. [7a] Ni II complex 3Ni exists as aq uasiplanar closed-shell quinonoid, whileZ n II complex 3Zn-im possesses an open-shell singlet ground state with ap erpendicular geometry.I nterestingly,f ree-base 3H 2 was in an equilibrium between open-shell and closed-shells pecies.…”