High‐resolution NMR spectroscopic trends and assignment rules of metal‐free, metallated and substituted corroles

Abstract

Major advances over the last few years have facilitated the synthesis of a large variety of meso-only substituted corroles that display interesting catalytic, therapeutic and photophysical properties. This work is the first to study extensively the NMR spectral characteristics of both metallated and non-metallated triarylcorroles in various organic solvents and provide guidelines for easy and reliable assignments of 1D 1H spectra from trends of J coupling constants and chemical shifts. An excellent correlation is found between C=C bond lengths derived from 3J(H,H) values and experimental lengths determined by x-ray crystallography of the same molecules. The nuclear Overhauser effect provides a robust 1D 1H NMR tool for determining the selectivity of electrophilic substitutions. Variable-temperature NMR and isotopic labelling reveal a single preferred tautomerization state and unsymmetric ring orientations at -70 degrees C. The beta-pyrrole protons demonstrate long-range heteronuclear couplings with the coordination core (15N) and with the ortho-19F nuclei of the meso-carbon aryl rings. In sum, application of multinuclear magnetic resonance to corroles and their metal complexes, through the compilation of chemical shifts and J couplings and the recognition of trends therein, provides basic information essential to reliable spectral assignments. Additionally, the conclusions drawn about the structures of corroles and the electron densities at various positions of the corrole macrocycle resulting from the application of high-resolution NMR techniques are of importance to an in-depth understanding of the molecular interactions and processes of this relatively new and rapidly expanding class of compounds.

Insights into the Tautomerism in meso‐Substituted Corroles: A Variable‐Temperature 1H, 13C, 15N, and 19F NMR Spectroscopy Study

“…N correlation spectra of 1 and 1 a, respectively. The present assignments differ in some points from those reported by Gross et al [11] The chemical shifts of the pyrrole protons, the J-coupling constants 3 J CHCH between the CH pyrrole protons, and the 4 J CHNH coupling constants as well as 6 J FCH between the CH protons and the 19 F nuclei were evaluated in iterative fits to the CH spectra at low temperatures (see the Experimental Section).…”

“…This assignment stems from the fact that in a spectrum of 1 a at 201 K it shows J-couplings of 69.3 and 18.0 Hz to the 15 N A and 15 N B atoms (in Ref. [11], the corresponding values are incorrectly assigned to the J HNB and J HNC coupling constants). This observation is consistent (see Scheme 1) with the supposition that even at such a low temperature 1 occurs in the form of a rapid dynamic equilibrium of two tautomers, 1' with the proton located on atom N A , and 1'' with the proton on N B .…”

“…At this point it is worth mentioning that if the proton sponge units were formed by either rings B and C (as claimed earlier [11] ) or A and D, symmetrical NH and CH spectra at low temperature would result, what is not observed. Furthermore, coplanarity of the rings A and B, observed in the X-ray structure of 1 [14] and confirmed by our DFT calculations for isolated molecules of 1 ( Figure S1 in the Supporting Information) corroborate the present assignment.…”

“…An extensive NMR study on this process was recently published by Gross and co-workers. [11] The authors suggest that the NMR line-shape effects observed in corrole 1 (see Figure 1) are due to both the NH exchange and hindered rotation of the pentafluorophenyl substituents, but they do not formulate any definite conclusions in this regard. We therefore decided to reinvestigate these phenomena to find that hindered rotation of the phenyl substituents does not contribute to the line-shape effects.…”

Corrole NH Tautomers: Spectral Features and Individual Protonation

“…30 Therefore, the ratio of quickly reacting to slowly reacting NH tautomers, which undergo protonation, would increase dramatically with temperature since the former becomes a trap with respect to the slowly reacting one due to the much higher probability of protonation of the quickly reacting NH tautomer.…”