Abstract:Presented herein is a study of the acid‐induced demetalation of two sterically hindered copper corroles, Cu β‐octabromo‐meso‐triphenylcorrole (Cu[Br8TPC]) and β‐octakis(trifluoromethyl)‐meso‐tris(p‐methoxyphenyl)corrole (Cu[(CF3)8TpOMePC]). Unlike reductive demetalation, which affords the free‐base β‐octabromocorrole, demetalation of Cu[Br8TPC] under non‐ reductive conditions (CHCl3/H2SO4) resulted in moderate yields of free‐base 5‐ and 10‐hydroxy isocorroles. The isomeric free bases could be complexed to CoII… Show more
“…Although the Co(II) center lies within the plane of the N4 core of the isocorrole, the coordination environment is not strictly square planar, as the bond lengths between Co(II) and the two pyrrole nitrogen atoms bridged by the sp 3 -hybridized meso-carbon (N1 and N4) are 1.902 Å and 1.906 Å, which are longer than the distances between Co(II) and the nitrogens (N2 and N3) of the pyrroles that are directly fused (1.879 Å and 1.873 Å). We note that a similar deviation from rigorous square planar coordination has also been observed for highly brominated isocorrole complexes of Co(II) 67 and Pd(II). 71 The two ancillary pentafluorophenyl groups of Co those previously described for other metalloisocorrole complexes of copper, nickel, and palladium.…”
Section: Resultssupporting
confidence: 78%
“…[18][19][20]24,48 Recent work has been aimed at development of nonaromatic tetrapyrrole scaffolds (e.g. phlorins, [49][50][51][52][53][54][55][56] biladienes, [57][58][59][60] and isocorroles [61][62][63][64][65][66][67] ) that include an sp 3 -hybridized meso-carbon and which support unique photophysical and multielectron redox properties. These nontraditional tetrapyrroles provide alternate platforms to consider for ORR and other small molecule activation processes and may offer distinct reactivity profiles and catalytic activities.…”
The selective 4e–/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. How- ever, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e –/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based upon a non-aromatic isocorrole scaffold that is competent for 4e–/4H+ ORR. This Co(II) 10,10-dimethyl isocorrole (Co[10- DMIC]) is obtained in a just four simple steps and excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co(II) porphyrin ([Co(TPFPP)]) and corrole ([Co(TPFPC)(PPh3)]) homologs demonstrated that the light harvesting and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ~85% selective for the 4e–/4H+ reduction of O2 to H2O over the more kinetically facile 2e–/2H+ process that delivers H2O2. By contrast, the same ORR evaluations conducted for the Co(II) porphyrin and corrole homologs demonstrated that these traditional aromatic systems catalyze the 2e–/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups, ditopic molecular clefts, or which impose cofacially ori- ented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and which shows a privileged performance for the 4e–/4H+ peractivation of O2 to water as compared to other simple Co(II) tetrapyrroles.
“…Although the Co(II) center lies within the plane of the N4 core of the isocorrole, the coordination environment is not strictly square planar, as the bond lengths between Co(II) and the two pyrrole nitrogen atoms bridged by the sp 3 -hybridized meso-carbon (N1 and N4) are 1.902 Å and 1.906 Å, which are longer than the distances between Co(II) and the nitrogens (N2 and N3) of the pyrroles that are directly fused (1.879 Å and 1.873 Å). We note that a similar deviation from rigorous square planar coordination has also been observed for highly brominated isocorrole complexes of Co(II) 67 and Pd(II). 71 The two ancillary pentafluorophenyl groups of Co those previously described for other metalloisocorrole complexes of copper, nickel, and palladium.…”
Section: Resultssupporting
confidence: 78%
“…[18][19][20]24,48 Recent work has been aimed at development of nonaromatic tetrapyrrole scaffolds (e.g. phlorins, [49][50][51][52][53][54][55][56] biladienes, [57][58][59][60] and isocorroles [61][62][63][64][65][66][67] ) that include an sp 3 -hybridized meso-carbon and which support unique photophysical and multielectron redox properties. These nontraditional tetrapyrroles provide alternate platforms to consider for ORR and other small molecule activation processes and may offer distinct reactivity profiles and catalytic activities.…”
The selective 4e–/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. How- ever, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e –/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based upon a non-aromatic isocorrole scaffold that is competent for 4e–/4H+ ORR. This Co(II) 10,10-dimethyl isocorrole (Co[10- DMIC]) is obtained in a just four simple steps and excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co(II) porphyrin ([Co(TPFPP)]) and corrole ([Co(TPFPC)(PPh3)]) homologs demonstrated that the light harvesting and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ~85% selective for the 4e–/4H+ reduction of O2 to H2O over the more kinetically facile 2e–/2H+ process that delivers H2O2. By contrast, the same ORR evaluations conducted for the Co(II) porphyrin and corrole homologs demonstrated that these traditional aromatic systems catalyze the 2e–/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups, ditopic molecular clefts, or which impose cofacially ori- ented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and which shows a privileged performance for the 4e–/4H+ peractivation of O2 to water as compared to other simple Co(II) tetrapyrroles.
“…Furthermore, multiple transitions with a similar intensity then cluster in the typical Soret region (~3.0 eV), whose cumulative effect is a deceptively porphyrin-like overall spectrum. Finally, since the LUMO has large amplitudes at the meso positions and the majority of the low-energy transitions have significant LUMO character, it stands to reason that the UV-vis-NIR spectra should exhibit a strong dependence on meso substituents, as is indeed observed 1 – 5 . Figure 7 OLYP/STO-TZP π-type MOs of Ni[IsoPh 2 MeCor], which involve homoconjugative interactions at the C10 meso position, along with their orbital energies (eV).…”
Section: Resultsmentioning
confidence: 87%
“…Isocorroles are fascinating macrocyclic ligands with a sterically constrained N 4 cavity characteristic of corroles and with the 2– charge of porphyrins (Fig. 1 ) 1 – 5 . With significant absorption in 700–1000 nm range, they are of considerable interest as near-IR dyes 6 .…”
Density functional theory calculations of magnetically induced current densities have revealed high diatropic ring currents in unsubstituted isocorrole consistent with homoaromatic character. An examination of the Kohn-Sham molecular orbitals showed clear evidence of homoconjugative interactions in four occupied π-type molecular orbitals as well as in the LUMO. Remarkably, substituents at the saturated meso position were found to exert a dramatic influence on the overall current density pattern. Thus, whereas bis(trimethylsilyl)-substitution strongly enhanced the peripheral diatropic current (consistent with enhanced homoaromaticity), difluoro-substitution engendered a strong, net paratropic current (consistent with antihomoaromaticity). In this respect, isocorroles stand in sharp contrast to benzenoid aromatics, for which substituents typically exert a small influence on the current density distribution.
“…Nevertheless, a string of papers over the last decade have highlighted their potential importance. 4,5,6,7 They are relatively stable and practical to work with. They are also increasingly accessible, albeit in rather variable yields, via simple derivatization of corroles.…”
Free-base meso-triarylcorroles and pyrrole undergo oxidative coupling in dichloromethane in the presence of DDQ affording 35–60% yields of 5/10-pyrrole-appended isocorroles in a matter of seconds.
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