Formation and electronic properties of ring-oxidized and ring-reduced radical species of the phthalocyanines and porphyrins

Abstract

<jats:p> The 18-π electron ring in the phthalocyanines and porphyrins largely controls their chemical and spectroscopic properties. It is the aim of this microsymposium to explore the formation and reactivities and the spectroscopic and theoretical properties of the ring-oxidized and ring-reduced phthalocyanines and porphyrins, essentially the π cation and π anion radical species. Redox chemistry of the ring, through ring oxidation and reduction to the radical species, offers powerful, new synthetic capabilities in which photochemical as well as electrochemical techniques may be used to effect highly localized reactions. The chemistry of these radical species can be moderated by control of the redox states of the central metal, the addition of electron-withdrawing or electron-donating peripheral substituents, and by the extent of intermolecular association applying both solvent polarity and peripheral substitution. </jats:p>

Application of Lever’s EL Parameter Scale toward Fe(II)/Fe(III) versus Pc(2-)/Pc(1-) Oxidation Process Crossover Point in Axially Coordinated Iron(II) Phthalocyanine Complexes

“…In both cases, the formation of new bands at 690–695 nm and 515 nm was observed. The relative intensities of the transitions at 690–695 nm and 515 nm are again rather suggestive of an electrogenerated phthalocyanine cation-radical species in solution. ,− In agreement with this, no shift in the first oxidation potential of PcFe II Py 2 was observed in the Py:DMF/0.1 M TBAP system that has the DMF component varied between 0 and 75%, which indicates the relative stability of the Fe-N­(Py) bonds under these solution conditions (Figure S21, Supporting Information). However, when a freshly prepared solid PcFe II Py 2 was dissolved in a DMF/0.3 M TBAP or was in situ -generated using a DCM/Py mixture (consisting of only 2 mol equiv of pyridine)/0.1 M TBAP, the spectral changes associated with electrooxidation were accompanied by the appearance of 850, 691, 565, and 527 nm bands (Figure d) for the latter system ( i.e.…”

“…Indeed, oxidation of these iron­(II) phthalocyanines under an applied oxidizing potential or addition of a chemical oxidant results in the disappearance of the initial Q-band and the appearance of transitions at ∼700, ∼525, and ∼420 nm with the band at ∼525 nm being the most intense in the visible region of the spectra (Figure and Figure S16, Supporting Information). The final UV–vis spectral envelopes of these red-colored species are independent of the electrolyte (TBAP and TFAB were tested for the PcFe II (P­(OBu) 3 ) 2 complex) and clearly indicate the formation of well-known phthalocyanine cation-radical species. ,− The EPR spectra of these oxidized compounds have a very characteristic isotropic signal with the g -value close to that of the free electron, and thus, the oxidation of PcFe II L 2 (L = t -BuNC, FcNC, and P­(OBu) 3 ; Σ E L (L ax ) = 0.72–0.84) is assigned to occur at the macrocycle, resulting in the formation of [Pc­(1-)­Fe II L 2 ] + • species. In each case, the electrooxidation is fully reversible in both DCM/0.3 M TBAP and DCM/0.15 M TFAB systems (Figure S17, Supporting Information).…”

Ligand-Centered Triplet Diradical Supported by a Binuclear Palladium(II) Dipyrrindione

“…Coordination compounds of redox-active ligands, in which oxidizing and/or reducing equivalents can be stored on the ligand π system, have generated broad interest owing to their potential applications in areas including homogeneous catalysis, molecular magnetism, photovoltaic devices, and quantum information processing (e.g., spin qubits) . Macrocyclic oligopyrroles, such as porphyrins, corroles, and several expanded porphyrinoids, are well-studied examples of redox-active ligands, and their ability to host unpaired spins has been investigated extensively. Besides macrocyclic structures, early examples of linear oligopyrrolic radicals in metal complexes were found in the study of bilindiones (Chart a), , but more recent reports highlighted the formation of stable ligand-based radicals in metal complexes of tetradentate bis­(phenolate)-dipyrrins (Chart b), , bis­(2-aminophenyl)-dipyrrins, and diimino-dipyrrins, as well as tridentate tripyrrindione (Chart c) − and dihydrazonopyrrole scaffolds.…”

Tripyrrindione as a Redox‐Active Ligand: Palladium(II) Coordination in Three Redox States

“…Thedesign and study of such ligands is key to the engineering of new catalytic systems featuring additional pathways for redox reactivity. [1][2][3][4] Within biological catalysts,t he storage and release of redox equivalents on tetrapyrrolic ligands is critical in numerous heme enzymes including catalases and cytochrome P450 monooxygenases.S ynthetic tetrapyrroles,s uch as porphyrins, [5] corroles, [6][7][8] and bilins, [9,10] as well as expanded porphyrin macrocycles [11][12][13][14][15] and tetradentate bis(phenolate)-dipyrrins, [16] display rich ligand-based redox chemistry.Incontrast, the stabilization of unpaired electrons is not well documented for smaller dipyrrolic and tripyrrolic fragments.H erein, we describe the one-electron redox chemistry of astable tripyrrindione framework in palladium-(II) complexes.…”