Synthesis, characterization and solvent/structural effects on spectral and redox properties of cobalt triphenylcorroles in nonaqueous media

Abstract

Corroles and metallocorroles have attracted a great deal of interest in recent years [1][2][3][4][5][6][7][8], in part because of improved synthetic methods which make them more readily available than in the past [1,2,6,7] and in part because these compounds have potential applications as catalysts for a variety of reactions [4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22]. One of the most frequently studied groups of metallocorroles are the cobalt derivatives which have been characterized as to their spectral and electrochemical properties under many different solution conditions [1,2,6,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40].Part of our own research effort has been directed towards the synthesis and electrochemical characterization of four and five coordinate cobalt corroles with different macrocyclic substituents [14-18, 21-23, 25-27, 41]. This ABSTRACT: Five cobalt(III) triphenylcorroles with different electron-withdrawing or electrondonating substituents and an axially bound triphenylphosphine ligand were synthesized and characterized by spectroscopic and electrochemical techniques. The investigated compounds are represented as (4-XPh) 3 CorCo(PPh 3 ), where Ph 3 Cor is the trianion of a triphenylcorrole and X is a OMe, Me, H, F or Cl substituent on the meso-phenyl rings. Each corrole was examined by UV-vis, 1 H NMR and IR spectroscopy, mass spectrometry, electrochemistry and thin-layer spectroelectrochemistry. Redox potentials and spectra of each oxidized and reduced species were examined in dichloromethane and N,N ′-dimethylformamide containing 0.1 M tetra-n-butylammonium perchlorate. Each Co(III) corrole undergoes up to five one-electron transfer reactions, some of which are reversible and others which are not. The Co III /Co II process is irreversible in both solvents due to the loss of the triphenylphosphine axial ligand following electron transfer. The Co II /Co I process is reversible in DMF but irreversible in CH 2 Cl 2 due to a homogenous chemical reaction between the electrogenerated Co(I) corrole and the chlorinated solvent. The potential for the first oxidation of the investigated corroles varies little with change of solvent, consistent with the lack of solvent binding to the neutral and singly oxidized forms of (4-XPh) 3 CorCo(PPh 3 ). However, a single DMF molecule strongly binds to the doubly oxidized corrole in DMF or DMF/CH 2 Cl 2 mixtures. This results in an easier oxidation and a negative shift of ~200 mV in E 1/2 upon going from CH 2 Cl 2 to DMF as solvent. The effect of substitutents and solvent on redox potentials is discussed and an overall electroreduction/electrooxidation mechanism is proposed.

Carbazole–corrole and carbazole–prophyrin dyads: synthesis, fluorescence and electrochemical studies

“…Copper containing dyad 5 displayed three irreversible oxidations ranging from 0.16 to 1.62 V and three irreversible reductions between À0.74 to À1.7 V. On the other hand, copper corrole (Cu-TPC) 20 had shown only one oxidation and one reduction wave (Table 5). Cobalt corrole was reported to have three oxidations processes between 0.51 to 1.56 V and two reductions around À0.75 and À1.61 V. 43 The cobalt containing dyad 6 exhibited two oxidations processes, one quasi-reversible at 0.79 V and one reversible at 1.23 V. Dyad 7 having free base porphyrin unit, undergoes two irreversible oxidations at 1.24 and 1.69 V along with three reductions processes in between À0.99 to À1.58 V. While H 2 TPP displayed two oxidations peaks at 1.03 and 1.30 V in addition to the two reductions processes at À1.23 and À1.55 V. 44 Dyad 8 with zinc metal exhibited two reversible oxidations at 0.97 and 1.25 V along with one reversible reduction at À1.15 V and one quasi-reversible one at À1.56 V. Dyad 9 with copper metal displayed two oxidations, one quasi-reversible and one reversible at 1.21 V and 1.36 V respectively. Also dyad 9 showed one reversible reduction at À1.13 V and one quasi-reversible reduction at À1.61 V. The oxidation and reduction potential values of dyads 4-9 are very different from their corresponding monomers, suggesting that the coupling with the carbazole moiety at the meso position affected the electronic properties of the corrole/porphyrin units in all the compounds.…”

Old Dog, New Tricks: Innocent, Five-coordinate Cyanocobalt Corroles

“…Numerous four-and five-coordinate metallocorroles with copper, cobalt, and iron have been shown to possess an oxidized corrole ligand and a reduced metal ion, [1][2][3]26 and these complexes were assigned as having a noninnocent macrocyclic ligand in its cation-radical (Cor complexes with noninnocent macrocycles have also been shown to undergo multiple reductions and oxidations in nonaqueous media, and a conversion between the different oxidation states of both the central metal ion and the corrole macrocyclic ligand is easily achieved through either electrochemical or synthetic methods, 37,42,56 consequently leading to a shift from a noninnocent to an innocent metallocorrole system. Both direct and indirect spectroscopic criteria, such as X-ray absorption spectroscopy (XAS), electron paramagnetic resonance spectroscopy, and UV−visible spectroscopy, laid out by Ganguly and Ghosh 57 have been used to classify ligand noninnocence.…”

Electronic Structure of Cobalt–Corrole–Pyridine Complexes: Noninnocent Five-Coordinate Co(II) Corrole–Radical States

“…Cobalt–corrole–bispyridine complexes, Co­[Cor]­(py) 2 , are currently of great interest as efficient catalysts of both proton reduction and water oxidation under ambient conditions. − An essential aspect of the catalytic mechanisms is the lability of the axial pyridine ligands which allows the generation of five- and four-coordinate Co corrole intermediates that can engage in further reactivity. − The coordinatively unsaturated character of these intermediates also facilitates their attachment to carbon nanotubes and other nanomaterials, affording nanoconjugates with potentially improved catalytic properties relative to the original molecular catalysts. Somewhat surprisingly, the five-coordinate Co–corrole–pyridine intermediates, Co­[Cor]­(py), remain poorly characterized; indeed, except for Co–corrole–triphenylphosphine complexes, , which are stable and readily amenable to structural characterization, five-coordinate Co corroles in general remain relatively little explored. In this study, we have investigated the nature of the Co center in Co­[Cor]­(py) intermediates, in particular, whether it is low-spin Co­(III), intermediate-spin Co­(III), or for that matter even Co­(II), the last in conjunction with an oxidized corrole •2– ligand.…”

“…6−8 The coordinatively unsaturated character of these intermediates also facilitates their attachment to carbon nanotubes 9 and other nanomaterials, affording nanoconjugates with potentially improved catalytic properties relative to the original molecular catalysts. Somewhat surprisingly, the fivecoordinate Co−corrole−pyridine intermediates, Co[Cor](py), remain poorly characterized; indeed, except for Co−corrole− triphenylphosphine complexes, 10,11 which are stable and readily amenable to structural characterization, five-coordinate Co corroles in general remain relatively little explored. In this study, we have investigated the nature of the Co center in Co[Cor](py) intermediates, in particular, whether it is low-spin Co(III), intermediate-spin Co(III), or for that matter even Co(II), the last in conjunction with an oxidized corrole •2− ligand.…”