Daniel O. Cicero

The corrole and ferrocene marriage: 5,10,15-triferrocenylcorrolato Cu.

Two synthetic routes have been defined for the preparation of a 5,10,15-triferrocenylcorrole Cu derivative. This complex has been characterized and the preliminary electrochemical investigation shows a strong interaction among the corrole and meso ferrocenyl substituents. The results obtained suggest that peculiar properties are gained by combining the eccentric characteristics of ferrocenyl substitution with the corrole macrocycle.

Phenyl derivative of iron 5,10,15-tritolylcorrole.

The phenyl–iron complex of 5,10,15-tritolylcorrole was prepared by reaction of the starting chloro–iron complex with phenylmagnesium bromide in dichloromethane. The organometallic complex was fully characterized by a combination of spectroscopic methods, X-ray crystallography, and density functional theory (DFT) calculations. All of these techniques support the description of the electronic structure of this phenyl–iron derivative as a low-spin iron(IV) coordinated to a closed-shell corrolate trianion and to a phenyl monoanion. Complete assignments of the 1H and 13C NMR spectra of the phenyl–iron derivative and the starting chloro–iron complex were performed on the basis of the NMR spectra of the regioselectively β-substituted bromo derivatives and the DFT calculations.

5,10,15-Triferrocenylcorrole Complexes

Complexes of 5,10,15-triferrocenylcorrole were synthesized from the crude free-base corrole product obtained by the reaction of ferrocenyl aldehyde and pyrrole. Direct formation of the complex in this manner leads to an increase of the reaction yield by protecting the corrole ring toward oxidative decomposition. The procedure was successful and gave the expected product in the case of the copper and triphenylphosphinecobalt complexes, but an unexpected result was obtained in the case of the nickel derivative, where metal insertion led to a ring opening of the macrocycle at the 5 position, giving as a final product a linear tetrapyrrole nickel complex bearing two ferrocenyl groups. The purified 5,10,15-triferrocenylcorrole complexes have been fully characterized by a combination of spectroscopic methods, electrochemistry, spectroelectrochemistry, and density functional theory calculations. Copper derivatives of 10-monoferrocenyl- and 5,15-diferrocenylcorrole were prepared to investigate how the number and position of the ferrocenyl groups influenced the spectroscopic and electrochemical properties of the resulting complexes. A complete assignment of resonances in the (1)H and (13)C NMR spectra was performed for the cobalt and nickel complexes, and detailed electrochemical characterization was carried out to provide additional insight into the degree of communication between the meso-ferrocenyl groups on the conjugated macrocycle and the central metal ion of the ferrocenylcorrole derivatives.

New example of hemiporphycene formation from the corrole ring expansion.

The reaction of 5,10,15-tris(4-tert-butylphenyl)corrole with 2,3-bis(bromomethyl)-5,6-dicyanopyrazine provides a new example of corrole ring expansion to a hemiporphycene derivative. The ring expansion is regioselective, with insertion of the pyrazine derivative at the 5-position of the corrole ring, affording the corresponding 5-hemiporphycene. Different macrocyclic products accompany formation of the 5-hemiporphycene, depending on the reaction experimental conditions. Br-substitued 5-hemiporphycenes and the 2-Br substituted corrole were obtained in 1,2,4-trichlorobenzene, while in refluxing toluene traces of an inner core substituted corrole were observed together with a significant amount of the unreacted corrole. These results provide an important indication of the reaction pathway. The coordination behavior of the 5-hemiporphycene, together with detailed electrochemical characterization of the free-base and some metal complexes, provides evidence for the reactivity of the peripheral pyrazino group.

Widening the scope of the corrole sulfonation

The sulfonation reaction has been particularly useful to prepare amphiphilic derivatives of 5,10,15-tris(pentafluorophenyl)corrole, but it has been limited to this corrole. We have studied the scope of this reaction, using 5,10,15-triphenylcorrole as the prototypical example of corrole structure. The reaction protocol has been modified to avoid the corrole aggregation, which limited its reactivity in the neat chlorosulfonic acid. Surprisingly, the reaction shows an unprecedented regioselectivity for corrole derivatives, affording 3 as the main product, but also corrole 5, which is the first example of monosubstituted corrole on pyrrole B. By modulating the corrole:chlorosulfonic acid reagent ratio, it has been possible to obtain the antipodal disubstituted product 6, demonstrating again a not usual regioselectivity for corrole substitution. These results allow the preparation of novel amphiphilic corrole derivatives, which open up their potential applications in different fields.

A Highly Emissive Water-Soluble Phosphorus Corrole

The synthesis, spectroscopic, and optical properties of the water-soluble phosphorus complex of a 2-sulfonato-10-(4-sulfonatophenyl)-5,15-dimesitylcorrole have been investigated. The compound was prepared by adopting a novel strategy for the corrole sulfonation, leading to the regioselective isomer in an almost quantitative yield. The phosphorus coordination has a key role in determining the corrole substitution pattern, limiting the formation of poly-substituted species, which affected the reaction of the corrole free base. The resulting complex shows excellent optical properties in terms of emission quantum yield, also in polar protic solvents, including water. 31 P NMR spectroscopy in CD3 OD indicates that the P sulfonate complex has been isolated in a hexacoordinated geometry with two different ligands (L1=-OH, L2=-OCH3 ), and it is prone to axial ligand exchange with methanol, with no evidence of intermediate pentacoordinated species. The morphological characterization of thin layers of the P corrole deposited onto an Au(111) surface showed that the addition of an intermediate layer of reduced graphene oxide allows for a better control of corrole aggregation, inducing also transformation of the Au(111) reconstructed surface.

The scope of the β-halogenation of triarylcorroles

Functionalization of corrole at its peripheral positions is an intriguing field of research, since the unusual reactivity of this macrocycle usually makes it difficult to predict the reaction products. We have investigated the introduction of halogen atoms at the corrole β-positions by using haloacids as reagents. Different behavior, in terms of number and position of the units introduced, was observed: chlorination yielded mono and disubstituted corrole, whereas bromination only afforded mono-substitution, even if on different positions. Iodination did not occur on the corrole free base, while the protection of the inner core by chelation with silver ion gave better results and a symmetric diiodinated corrole was isolated. 2D NMR analysis and X-ray crystallography provided useful information about the site of these corrole functionalizations.

Synthesis and functionalization of β-alkyl-meso-triarylcorroles

After the definition of efficient synthetic routes for the preparation of triarylcorroles, the functionalization of these macrocycles is becoming a necessary and challenging field of research. One important synthetic step is the introduction of substituents able to influence the electronic distribution in the macrocyclic ring. A valuable target would be a corrole macrocycle with some β-pyrrole positions occupied by methyl groups, while exploiting other positions to introduce electron-withdrawing substituents. To explore the scope of this approach, we investigated the bromination and the nitration of the corrole ring and the desired products have been obtained in moderate to good yield. The successful preparation of selectively halogenated corroles is particularly interesting since they are suitable substrates for the preparation of more complex partially alkylated structures using modern cross coupling methodologies.

Iron, iron everywhere: synthesis and characterization of iron 5,10,15-triferrocenylcorrole complexes

The chloroiron complex of 5,10,15-triferrocenylcorrole was synthesized by direct metalation of the crude free-base corrole product obtained by the reaction of ferrocenyl aldehyde with pyrrole. This procedure eliminates the need to isolate the unstable free-base corrole and gives a reasonable yield for the desired product, as earlier reported for structurally related Cu and Co derivatives. The isolated chloroiron corrole was then subjected to axial ligand exchange, leading to the generation of a diamagnetic iron nitrosyl complex and a paramagnetic σ-phenyl-bonded corrole derivative. The three related corrole complexes were then characterized using 1H NMR, electrochemical and spectroelectrochemical techniques.

NMR spectroscopy of the phenyl derivative of germanium(IV) 5,10,15-tritolylcorrole

A thoroughly structural characterization of (TTC)GePh (TTC = 5,10,15-tritolylcorrole; Ph = phenyl) in solution has been carried out through a combination of 2D NMR (1H-1H COSY, 1H-1H ROESY, 1H-13C HSQC and 1H-13C HMBC) experiments and density functional theory (DFT) calculations of the molecular and electronic structure and the shielding constants. The 1H and 13C chemical shifts computed at DFT-S12g and DFT-SAOP levels of theory nicely reproduce the experimental values, the agreement between theory and experiment being especially good for the DFT-S12g results. The calculations prove to be able to capture the fine details of the NMR spectra and to resolve some assignment ambiguities related to the inherent conformational flexibility of the macrocycle. The calculations also provide an explanation of the observed chemical shift trends in terms of diamagnetic and paramagnetic components of the shielding tensor.