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

Abstract

Tris(pentafluorophenyl)corrole and its (15)N-enriched isotopomer were studied in [D8]toluene solution by 1D and 2D variable-temperature NMR techniques to establish the mechanisms of tautomerization of the NH protons inside the interior of the corrole macrocycle. Three such rate processes could be identified of which two modulate the spectral line shapes at temperatures above 205 K and the third is NMR-inaccessible as it is very fast. The latter involves the proton engaged in an unsymmetrical proton sponge unit formed by two pyrrole nitrogen atoms. Temperature and concentration dependences of the two remaining processes were determined. One of them is purely intramolecular and the other is intermolecular at low temperatures, with growing contribution of an intramolecular mechanism at elevated temperatures. The proposed microscopic mechanisms of all these processes are semi-quantitatively confirmed by quantum chemical calculations using density functional theory.

Excited State N−H Tautomer Selectivity in the Singlet Energy Transfer of a Zinc(II)‐Porphyrin–Truxene–Corrole Assembly

“…Measurements at 237 K show three distinct NH signals at circa 0.03, −5.34, and −6.35 ppm for all three of the investigated compounds. Measurements at 300 K show significant peak broadening, which indicates that two different species exist in a dynamic equilibrium . Peak assignments and VT NMR spectra can be found in Figure S10.…”

“…The absorption spectra (Figure ) of [COR] and [COR 2 ] both exhibit the presence of the expected Soret and Q‐bands derived from a classical free‐base corrole . The slight splitting of the Soret bands at 298 K (seen upon magnification) are associated with the presence of the T1 and T2 NH‐tautomers of the free‐base corrole and become much more apparent at 77 K. The 0–1 Q‐bands of the T1 and T2 NH‐tautomers are difficult to assign with precision owing to the strong overlap of the Q‐bands from the two tautomers, but the 0–0 absorption bands of the T1 and T2 tautomers can be easily distinguished.…”

“…Furthermore, the low energy gap between the ground states of the corrole tautomers coupled with the low barrier for tautomerization (ca. 2.5 to 3.5 kcal mol −1 ) allows us to assume a nearly 50:50 distribution of the corrole tautomers at the ground state. If we examine the 0–0 absorptions of the T1 and T2 tautomers, we can extract the energy difference between the S 1 states of T1 and T2 by taking into consideration the gap obtained by the van't Hoff plot.…”

“…Since then, a few studies on the nature of the corrole tautomers have been reported. These investigations include a multinuclear NMR analysis of the measurements of the rate of ground state tautomerization, and the examination of the individual basicity of the tautomers . In their ground state, the corrole tautomers have been demonstrated to co‐exist with nearly the same energy level.…”

The 1H, 13C, 15N, and 19F NMR chemical shifts assignments in 5,10,15‐tris (pentafluorophenyl)tetra–15N corrole at 191 K

“…However, the tautomerization processes affects the line shape of the signals in the spectra in the temperature range 325–190 K to the extent that individual assignments are impossible. With the aid of uniformly 15 N‐labeled corrole (Scheme ), it was therefore undertaken in this account to assign all nuclei in a molecule, i.e., 1 H, 13 C, 15 N, and 19 F, at 191 K. We recently revealed that these assignments made it possible to elucidate the tautomerization process . Although this process was recently studied in several laboratories , presented data challenge both spectral process assignments to physical processes in a molecule and the mechanism of tautomerization, which was never studied in detail.…”