Hypercorroles Formed via the Tail that Wagged the Dog: Charge Transfer Interactions from Innocent Corroles to Meso-Nitrophenyl Substituents

Abstract

A series of cobalt nitrophenylcorroles were spectrally characterized in CH 2 Cl 2 , and under certain solution conditions, several compounds were shown to exhibit hypercorrole spectra resulting from charge transfer interactions from the corrole π-system to the redox-active meso-NO 2 Ph substituents. The resulting spectral pattern has not previously been reported for metallocorroles and in the case of the cobalt derivatives was shown to depend upon the number and position of the meso-nitrophenyl groups on the macrocycle, the position of the NO 2 substituent on the meso-phenyl ring(s) (para or meta), and the electronic structure of the corrole, which can exist in its innocent or noninnocent form depending in large part upon the type and number of axial ligands. Cobalt corroles bearing p-nitrophenyl groups at the 5,15-or 5,10,15positions of the macrocycle exhibited the most marked hypercorrole spectra under solution conditions where the complex was innocent (i.e., Cor 3− Co III ), and a systematic analysis of the spectral data suggests the root of this perturbation to be a corrole-to-aryl interaction (i.e., ligand-to-ligand charge transfer or LLCT). The largest interaction between the π-system and the NO 2 Ph substituents was seen upon coordination of anionic cyanide (CN − ) axial ligands to the Co(III) center of the bis-(CN − )-5,15-dinitrophenyl derivative, resulting in a cobalt hypercorrole spectrum where the broad Q-band was red-shifted even further into the NIR region and located at 795 nm in CH 2 Cl 2 and 827 nm in pyridine. Cyclic voltammetry of the bis-CN − adducts showed that the first electrons are added to the LUMOs of the p-NO 2 Ph substituents rather than the corrole, while the same orbitals for the mono-CN − adducts are nearly degenerate. This redox behavior contrasts with what is seen for the noninnocent nitrophenyl corroles having "normal" unperturbed UV−vis spectra where the first reduction involves the π-system of the macrocycle, followed by reduction of the p-NO 2 Ph groups at more negative potentials.

Altering the Site of Electron Abstraction in Cobalt Corroles via meso-Trifluoromethyl Substituents

“…Our own laboratories have focused on elaborating an electrochemical litmus test for evaluating the innocence or noninnocence of transition-metal corroles ,,,,,− and found that the presence of a facile reduction at potentials more positive than −0.20 V vs SCE (saturated calomel electrode) accompanied by a relatively small HOMO-LUMO gap (<1.0 V) could be used as an electrochemical diagnostic criterion to establish ligand noninnocence in these systems. Modulation of the orbital interactions, and thus the noninnocence of the corrole system, has been clearly demonstrated by changing the number and/or type of axial ligands bound to the central metal ion (mainly in the case of Mn, − Fe, ,− and Co − ,− derivatives), while the introduction of specific peripheral substituents has been suggested, in only a few cases, to facilitate conversion between the noninnocent [Cor •2– M II ] and innocent [Cor 3– M III ] configuration of the molecules.…”

Synthesis, Spectral, Redox, and Sensing Studies of β-Dicyanovinyl-Appended Corroles and Their Metal Complexes

“…Silicon corroles have been utilized as excellent fluorescent F – sensors. , Lu and co-workers designed and synthesized a corrole probe exhibiting excellent sensitivity and selectivity for sulfide ions via chromogenic and fluorogenic signaling in living cells . Mono- and bis-cyano axially ligated cobalt triarylcorroles were synthesized and investigated spectroscopically and electrochemically in various solvents by Kadish and co-workers, − and from this work, Nardis and co-workers developed an optical sensor based on (PPh 3 )­CoTTC/TDACl, which detects cyanide ions up to 0.053 mg/L …”

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

“…The reason is that, although all derivatives are present in the solid state (X-ray determination) as 6-coordinate complexes, they easily lose either one or even both axial ligands in dilute solutions. 21 The electron-richness of corroles is a major factor in that regard, coming into effect by 3 orders of magnitude differences in the bis(pyridine)cobalt(III) complex dissociation constants. 21,22 A plausible explanation is that the original pyridine ligands in the least Lewis acidic complex 3b are fully dissociated (presumably replaced by the coordinating acetonitrile solvent) even prior to reduction.…”

“…21 The electron-richness of corroles is a major factor in that regard, coming into effect by 3 orders of magnitude differences in the bis(pyridine)cobalt(III) complex dissociation constants. 21,22 A plausible explanation is that the original pyridine ligands in the least Lewis acidic complex 3b are fully dissociated (presumably replaced by the coordinating acetonitrile solvent) even prior to reduction. Supporting evidence for this conclusion is based on two other observations: (a) the reoxidation process (i.e., the reverse scan) of [3b] − is maximal at −0.5 V, identical with the one assigned for PPh 3 -free [3t] − ; (b) while the second reduction of 2b occurs at a much less negative potential than that of 3b, consistent with the difference between them regarding the electron-withdrawing effect of CF 3 versus H, the opposite is true for their first reduction process.…”

“…This Co II /Co III transformation frequently proceeds by the well-known electrochemical "box mechanism" that includes ligand association/disassociation. 20,21 The 5-coordinate lowspin d 6 cobalt(III) complexes bind their PPh 3 ligand tightly, but the affinity is strongly reduced upon reduction to much more labile d 7 cobalt(II). A reversible Co II /Co III redox process is obtained only for 2t, thus indicating that this complex is reoxidized without losing its axial ligand.…”