Conformational dynamics of bis(BF₂)‐2,2′‐bidipyrrins revealed by through‐space ¹³C¹⁹F and ¹⁹F¹⁹F couplings

Abstract

The conformation of [bis-(N,N'-difluoroboryl)]-3,3'-diethyl-4,4',8,8',9,9',10,10'-octamethyl-2,2'-bidipyrrin (1) in solution was studied by analyzing the 13C-19F and 19F-19F through-space spin-spin couplings. The 1H and 13C NMR spectra were assigned on the basis of nuclear Overhauser effect spectroscopy (NOESY), heteronuclear single-quantum correlation (HSQC), and heteronuclear multiple-bond correlation (HMBC) experiments. The 19F spectrum of 1 was compared with that of 2-ethyl-1,3,5,6,7-pentamethyl-4,4-difluoro-4-bor-3a,4a-diaza-s-indacen (2). The 19F-19F through-space spin--spin coupling in 1 was thus assigned and the coupling constant was obtained by simulating the coupling patterns. The obtained conformation of 1 was compared with those of the known complexes [bis-(N,N'-difluoroboryl)]-3,3',8,8',9,9'-hexaethyl-4,4',10,10'-tetramethyl-6,6'-(4-methylphenyl)-2,2'-bidipyrrin (3)and [bis-(N,N'-difluoroboryl)]-9,9'-diethyl-4,4',8,8',10,10'-hexamethyl-3,3'-bis(methoxycarbonylethyl)-2,2'-bidipyrrin (4). The conformational dynamics of 1, 3, and 4 was surveyed by observing the temperature dependence of the through-space coupling constants between 253 and 333 K. The 13C-19F and 19F-19F through-space spin-spin couplings thus confirm similar conformations of different BisBODIPYs in solution in contrast to earlier findings in the solid state.

Sulfur‐Bridged BODIPY DYEmers

“…In the 19 F NMR spectrum of 5, two broad and unspecific signals at d % À139 and À148 ppm appear at ambient temperature. [11,34] In contrast, the disulfide 6 and the sulfide 10 give a single quartet in the 19 F NMR spectrum (d % À142 and À147 ppm, respectively) under the same conditions, which is typical for BODIPYs with equal positions of the fluorine atoms, like for example for 8 or 11.…”

Synthesis, Molecular Structure and Properties of a Ferrocene‐Based Difluoropyrrolo‐Oxaborole Derivative

“…From inspection of FBF we might expect, because of restricted rotation, that the two fluorine nuclei for each ketopyrrole are inequivalent, resulting in F–F coupling. Typical J values are around 100 Hz for vicinal fluorines and 20 Hz for intramolecular through‐space interactions 10. The actual spectrum consists of two signals at δ = –149.9 (broad) and –158.0 ppm (doublet, J = 26 Hz).…”

Application of fluorine NMR for structure identification of steroids

“…Throughspace couplings are especially interesting as they give information about close proximity of nuclei and they have been used to evaluate internuclear distances. [16,55,56] These studies, however, have been performed on closely related compounds and care has to be taken to relate long-range couplings to distances as, for example, the correlation does not hold for molecules with spatially crowded fluorines. [57] The name 'through-space' also has to be considered descriptive as the coupling is caused by overlapping of lone-pair orbitals of fluorine substituents in proximity.…”

“…[59 -62] These long-range couplings, like long-range J FF , are considered through-space couplings as they depend on a favorable spatial relationship, although it has to be remembered, as mentioned previously, that the coupling is a result of orbital overlap. [16,63] In steroids, long-range proton-proton couplings can be detected from 17α to 18-methyl through a W-coupling. [25] It is interesting to note that compounds that are fluorinated at 17α do not display coupling from 17α to 18-methyl, but fluorination at 17β does present coupling from 17β to 18-methyl, indicating that indeed spatial proximity is important in long-range fluorine-proton couplings.…”

“…[57] The name 'through-space' also has to be considered descriptive as the coupling is caused by overlapping of lone-pair orbitals of fluorine substituents in proximity. [16] Although the different fluorine couplings, J FF , J CF and J HF , give information about the structure of a compound, they are not sufficient to determine the dihedral angles and distances. The analysis of the coupling pattern in terms of bond angles and thus stereochemistry is in general agreement with the preliminary assignment of Fα and Fβ.…”