Equatorial fluorine

In spite of bearing nonequivalent fluorine substituents, both PF5 and all compounds of the type R-PF4 exhibit only a single signal in their fluorine NMR spectra. The observed magnetic equivalence of the fluorines in such compounds is believed to derive from a rapid intramolecular, pseudorotational exchange process that is too rapid, even at -80 °C, to allow distinction of the axial and equatorial fluorine atoms (Scheme 7.7). 

In order to correctly predict which ligands occupy which sites in such compounds, one must recognize that, as a general rule, fluorines will always prefer the axial site in a trigonal bipyramidal system, perhaps because of fluorine s small size, but probably also because of its preference to bind to orbitals with as little s-character as possible. The orbitals used by P to make its axial bonds have less s-character than those used to make its equatorial bonds. This is reflected by the larger F—P coupling constants to the equatorial fluorine substituents. 

One must assume that the two examples below (ones with more electronegative R groups) that only exhibit one signal in their fluorine NMR spectra must be undergoing rapid intramolecular exchange of their axial and equatorial fluorines (Scheme 7.8). 

There are some unique structural aspects of some of the sulfur fluorides that will need to be discussed in order to understand the 19F NMR spectra. The geometry of tetracoordinate group VI compounds is predicted on the basis of Gillespie s electron-pair repulsion theory to be trigonal bipyramid, with an electron pair occupying one of the equatorial sites.2 Thus, the SF3 substituent as well as the molecule SF4 have structures as depicted in Scheme 7.12, with nonequivalent (axial and equatorial) fluorines, and thus their 19F NMR spectra consist of two 19F signals, with the fluorines being coupled if the system is scrupulously dry. 

Sulfur tetrafluoride appears as two broad singlets at room temperature, as one broad singlet at 85 °C, and (when dry) as two sharp triplets at -30 °C. SF6, with its symmetrical octahedral geometry, appears as a sharp singlet at all temperatures. The activation energy for pseudorotation of SF4, which interconverts its axial and equatorial fluorines, is 12kcalmor1.3... 

Scheme 7.14 provides fluorine NMR data for some organic SF3 compounds, each of which exhibits peaks of vastly different chemical shift for their axial and their equatorial fluorines. The peaks in the fluorine NMR spectrum of CH3SF3 are given in Fig. 7.1 as an example of this type of compound. 

The equatorial fluorines of these nitrogen-bound SF3 groups are greatly deshielded as compared to those in their carbon-bound counterparts in Scheme 7.13. 

Whether the SF5 group is aliphatic-, vinylic-, or aromatic bound does not seem to have much influence upon the fluorine chemical shifts observed. However, an SF5 group that is proximate to a functional group can have its chemical shifts, particularly those of the equatorial fluorines, influenced somewhat by that functional group. 

On the other hand, P-halogen substituents appear not to affect the axial fluorine much, but deshield the four equatorial fluorines (Scheme 7.25). 

The effect of such a carbonyl function is to shield the axial fluorine, but it has little effect on the equatorial fluorines. 

There is a remarkable difference in the ab4 systems of SF5-acetylenes as compared to the respective SF5-alkenes or SF5-alkanes. Most noticeably, the order of appearance of the ab4 signals switches for SF5-acety-lenes, with the four fluorine signals due to the equatorial fluorines appearing downfield of the one fluorine signal due to the axial fluorine (Scheme 7.31). Relative to SF5-ethane, the SF5-acetylene equatorial fluorines have shifted 20ppm downfield, whereas its axial fluorine is shifted 10ppm upfield compared to those of SF5-ethane. 

Figure 7.7 Contour maps of L for CIF3 (a) in the plane of the molecule and (b) in the symmetry plane through the equatorial fluorine atom.

Nuclear magnetic resonance spectra were obtained for S02C1F solutions at -80 °C. b The symbols, FA and FB, denote axial and equatorial fluorine atoms, respectively. c The anion parameters apply to all carbocation salts and to the Br(OTeF5)2+ salt of Sb(OTeF5)6" also see ref 73. d Predicted from pairwise additivity parameters as described in the Chemical Shifts and Coupling Constant Trends section.f See ref 84 and 85.e The l23Te satellites were not observed. 

In both cases it is not possible to distinguish between axial and equatorial fluorine atoms this is only possible in those cases in which the phosphorus has heavier substituent groups<1968,38 39 and42) as for ... 

A number of interacting factors precludes any generalisations to be made but it should be noted that the clear distinction of the proton couplings to axial and equatorial fluorine atoms in pentavalent compounds is again obtained. The same order is found as for 2/(H-P-F) namely 13/(H-F ax) 3/(H-Feq). ... 

However, equatorial and axial fluorines rapidly interchange. The two out-of-plane equatorial fluorines bend outward at the same time as the top ar/a/fluorine bends downward and the bottom axial fluorine bends upward. 

Finally, a square-based pyramid structure is reached (the transition state for pseudorotation) in which the two ax/a/ fluorines and two of the equatoriar fluorines are equivalent. Motion continues until the two equatorial fluorines occupy axial positions and the two axial fluorines occupy equatorial positions (the remaining equatorial fluorine does not move). 

There are two nonequivalent pairs of S-F bonds in the sulfur tetrafluoride molecule, the shorter equatorial S-F bonds (1.545 A) and the longer axial S-F bonds (1.646 A). The bond angles arc equally 101 33 and 186" 56 for equatorial and axial bonds, respectively.5 The shorter and the longer S-F bond energies are, respectively, —20.31 and — 13.74 eV. The electric charge distribution in the sulfur tetrafluoride molecule is as follows + 1.70 on the sulfur atom, — 0.35 on the equatorial fluorine atoms and —0.50 on the axial fluorine atoms.5... 

Perhaps more subtle is a molecule such as PF3 (Fig. 6.le), which has symmetry. The three equatorial fiuonne atoms can be interchanged by reflection or by rotation about the C3 axis. Similarly, the two axial atoms can be reflected or rotated into each other. However, no operation allows interchange of an axial and an equatorial fluorine atom. Thus we have two sets of symmetry (and chemically) equivalent fluorine atoms. As a consequence, we would not expect P—F,v bond lengths to be the same as P—Teq bond lengths (and they are not), nor would we expect the flve fiuonne... 

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