283
Chemical Shifts and Coupling Constants in Pentafluorophenyl Derivatives. I. Correlations of Chemical Shifts, Coupling Constants, and r-Electronic Interactions' M. G. Hogben and W. A. G. Graham
Contribution from the Department of Chemistry, Uniuersity of Alberta, Edmonton, Alberta, Canada. Receiaed March 2, 1968 Abstract: The fluorine chemical shifts and coupling constants of 61 pentafluorophenyl derivatives with widely varied substituents are examined. It is found that two of the meta coupling constants form good straight line relationships with the chemical shift of the p-fluorine and it is argued that these relationships are mainly dependent upon the a-electron donor or acceptor character of the substituent. uclear magnetic resonance (nmr) has proved a useful method in furthering the understanding of electronic interaction between the benzene ring and its substituents. The chemical shifts of p - and m-carbon nuclei2+ and of other nuclei (protons,2 and borong) attached to the benzene ring at these positions have been related to 7r-electron charge densities. The increasing availability of pentafluorophenyl derivatives has led us to explore the possibility that nmr parameters of the pentafluorophenyl group might be correlated with the electronic interaction at the bond between substituent and ring. It was anticipated that coupling constants among the fluorine atoms, as well as their chemical shifts, would reflect changes in the ring-substituent interaction. This paper elaborates upon our previous communicationlo which outlined (a) an empirical coupling constant-chemical shift relation, (b) a proposal that this relation differentiated between 7r-donor and 7r-acceptor substituents, and (c) the application of this proposal to certain phosphine complexes. Paper I1 of this series examines the results of this work in terms of bonding within the series of organometallic compounds studied.
N
Anal. Calcd for C18H,oBC13F6P: C, 46.1; H, 2.2. Found: C, 46.8; H , 2.3. The crystalline complex was insufficiently soluble to produce an adequate spectrum, although very small peaks of the correct chemical shift were sufficient to distinguish the compound from the HCI adduct of the phosphine. The CsFoCNcomplexes with BF,, BCI3, and BBrs were not isolated or characterized due to dissociation of the adducts. For this reason, some caution is necessary in interpreting the spectra of the C6F5CNcomplexes. All other new compounds prepared were characterized either by elemental or by mass spectral analysis. The various phosphines and their complexes were prepared in this laboratory by reaction of pentafluorophenyllithium and the appropriate phosphorus chloride and subsequent reaction with the metal carbonyl.11 The various pentafluorophenyltin compounds were prepared in this laboratory by the reaction of pentafluorophenyllithium with the appropriate halotin compound. l 2 The pentafluorophenoxy and pentafluoroaniline derivatives were also prepared in this laboratory by the reaction of the appropriate organometallic halide with pentafluorophenol or its sodium salt, and with N-lithiopentafluoroaniline. 1 3 Nmr Measurements, The 19F spectra were obtained at either 56.4 Mcps or 94.1 Mcps using Varian HA spectrometers. Signals more than 2000 cps from the reference were observed by disabling the manual oscillator and substituting a Hewlett Packard 241 A oscillator. Hexafluorobenzene was used as internal reference and chemical shifts are presented relative to CFCI, by adding 163.0 ppm (see Discussion) to the chemical shift from C6F6. The temperature of measurement was ca. 30". The spectra were taken from 20 to 3 0 z (v/v or w/w) solutions in benzene except for some of the complexes, especially those of the metal carbonyls, which were sparExperimental Section ingly soluble and were measured as saturated solutions. Materials. All pentafluorophenyl reagents used in this work Nmr Analysis. The pentafluorophenyl system was treated as an were obtained from Imperial Smelting Corporation. The boron AA'KXX' system. The first-order spectrum of the p-fluorine is halides and CFCI3 were obtained from Matheson of Canada Ltd., analyzed initially yielding& and J34,andJ14as well if the substituent and other chemicals from the usual commercial sources. The atom has a magnetic moment (e.g., phosphorus). Thc numbering boron halide and hydrogen chloride complexes of C ~ F ~ P ( C B H & sequence is clockwise around the benzene ring with the substituent and C6FjCN were prepared using the conventional high vacuum at 1 and the o-fluorines at 2 and 6. The value of JZ4thus obtained technique by distilling an excess of the gas directly into the nmr may then be used t o separate the two overlapping parts of the tube containing a degassed benzene solution of the pentafluorom-fluorine spectrum. The result is the upfield half of an AA'XX' phenyl compound and hexafluorobenzene. In the case of C6F5system which is analyzed after the manner of Grant, Hirst, and P(C6H5)z.BCI3, the nmr spectra were recorded immediately upon Gutowsky14 yielding Jz3, Jz5,&, and &. The low-field half, the warming since the complex crystallized within a few hours. The ortho spectrum, is then computed with J12equal to zero and thence crystalline complex was isolated and recrystallized from methylene by inspection JI2 can be determined. The couplings JIZ,J 1 3 , and chloride. J I ~ and any coupling to nuclei bonded to the substituent atom can safely be considered first-order interactions. Figures 1 and 2 exemplify spectra ofp- and m-fluorines. (1) Taken in part from the Ph.D. thesis of M. G. Hogben, University Figure 3 shows how the ortho spectrum is derived and also the of Alberta, Dec 1967. (2) H. Spiesecke and W. G. Schneider, J . Chem. Phys., 35, 731 (1961). steps involved in determining Jlz. Figure 3a shows the half-spec(3) P. C. Lauterbur, J . Am. Chem. Soc., 83, 1838 (1961). trum of the AA'XX' system as derived from the meta spectrum. (4) G. B. Savitsky, J . Phys. Chem., 67, 2723 (1963). Figure 3b shows the computed ortho spectrum, achieved by adding (5) G . E. Maciel and J. J. Natterstad, J. Chem. Phys., 42, 2427 (1965). the J N coupling. The observed ortho spectrum for C6FjP(C6H&
(6) R. W. Taft and J. W. Rakshys, Jr., J . Am. Chem. Soc., 87, 4387 (1965), and other references cited therein. (7) V. Boden, J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Mol. Phys., 8, 133 (1964). (8) I. J. Lawrenson, J . Chem. Soc., 117 (1965). (9) H . C. &leachell and D. W. Beistel, Inorg. Chem., 3, 1028 (1964). (lo) M. G. Hogben, R . S. Gay, and W. A. G. Graham, J. Am. Chem. Soc., 88, 3457 (1966).
(11) R . S.Gay and W. A. G. Graham, to be publislfed. (12) J. A . J. Thompson and W. A . G . Graham, Inorg. Chem., 6, 1875 (1967). (13) A. J. Oliver and W. A . G. Graham, to be published. (14) D. M. Grant, R. C. Hirst, and H. S.Gutowsky, J. Chem. Phys., 38, 470 (1963).
Hogben, Graham
1 I9F Chemical ShiJfs in Pentafluorophenyl Derivatives
284
5.9
IO,(
1.2
a',?'
3.1 I',Z K)',b' 111
12.8
3.1'
7',l?
1?',8'
Figure 1. The para and mera spectra of CeFaCOCI, showing the effect of double irradiation on the meta spectrum. Figure 3. (a) Half-spectrum of AA'XX'derived from meta spectrum. (b) Theoretical ortho spectrum, J12 = 0. (c) Observed orrho spectrum of CGFBP(CGH~)L. (d) Observed ortho spectrum of CGFSP(CGH~)~~'~O(CO)~. The correctness of the analysis was verified by comparison with the results given by the TWOSUM computer program.15 The program gives a listing of all transitions together with the energy levels associated with each transition. For a given set of chemical shifts and coupling constants, spectra were computed with different signs of J24. The resulting sets of spectra were identical in appearance although a given line was derived from diff'erent energy levels in the two cases, as shown in Table I. With reference to Figure 1 , irradiating CGF~COCI at a frequency corresponding to line 9 in the para spectrum produced three lines in place of the original singlets 1,2 and 1',2' in the meta spectrum. The computed spectrum showed that when J Z Iwas of the opposite sign to J Z 3the , two energy levels (7 and 17) involved in line 9 of the para spectrum are also found in the meta spectrum at lines 1,2 and 1',2', as designated by the symbol b in Table I. Had J24been of same sign as . J 2 3 , line 9 would have involved energy levels 1 and 4 and the lines 3,4 and 3 ' A ' would then have been affected (case a). This latter possibility was in fact found to be the case for C6FjNH2 (Figure 2) and it was concluded that JZ4was of' different sign in CsFjNHs than in CyFBCOC1 Lines 1,2 and 1',2' in the meta spectrum are each comprised of two transitions. One of these did not have an energy level derived from line 9 it1 the puru spectrum and remained unaffected. The affected transition caused two small peaks to appear on either side of the original.
Figure 2. The para and meta spectra of C6FsNH2, showing the effect of double irradiation on the mero spectrum.
Results and Discussion A summary of data on 61 C6FB-X compounds from thisI6 arid other ~ o r k 7 ~ ~ ~is~ 7presented -** in Table 11.
is shown in Figure 3c. The coupling of the o-fluorine to phosphorus (A2= 39 cps) causes the spectrum to show two overlapping multiplets, each the same as Figure 3b. When the phosphine is complexed t o a metal carbonyl in the compound CaF5P(C&)2Mo(CO)G, Jlz decreases to less than 2 cps. Comparing Figure 3d with Figure 3b, we find that only the two innermost lines in Figure 3d are derived from single transitions. Since these give no indication of splitting, JI2is taken to be less than this peak width (2 cps).
(15) A FORTRAN IV program by G . R. Demare, J. S. Martin, and F. W. Birss, University of Alberta! based on NMRIT and NMREN by R . C. Ferguson and D. W. Marquardt, J . Chem. Phys., 41,2087 (1964). (16) To avoid excessive length in Table 11, only single values, averaged where necessary, are given. For values determined in this work, but shown as part of' an average in Table 11, see ref 1. (17) P. Bladon, D. W. A. Sharp, and J. M. Winfield, Spectrochim. Acta, 20, 1033 (1964). (18) M. G. Barlow, M. Green, R. N. Haszeldine, and H. G. Nigson, J . Chem. SOC.,B, 1025 (1966).
Joiirnai of the American Chemical Society / 91:2
Janiiary 15, 1969
285 Table I. Energy Levels Computed for Transitions in the p and m-Fluorine Spectra of Pentafluorobenzoyl Chloride5
I 1 0
-Lineblevels for each transition-para -Energy JN opposite sign to J z ~ spectrum J24same sign as Jza 1 2 3 4 5
HQ
6
ffo
Y
29-32 24-30, 25-31 16-26 19-27, 18-28 11-20, 13-22, 10-21 12-23 6-14; 5-15
5,9 10,6 1,2 5',9' 3,4 10',6' 7,11 1',2' 12,8 3',4' 7',11'
(b) -
(a) (b) -
(a)
P
I
P
0 L 0
+DCN
I I
$
P
WCOCl m C F 3
16-26 24-30, 25-31 29-32 6-14, 5-15 10-21, 12-23, 11-20 13-22 19-27, 18-28
-
9-21, 23-30 22-31, 8-20 17-27, 27-32 3-10, 12-24 4-14, 14-26 13-25, 2-1 1 9-22, 20-30 19--29, 7-19 21-31, 8-23 6-16, 1-6 3-13, 11-24
OM
P
@NH
O M
I
120
I
130
I
IA0
+
150
I
160
D O H
I
,
170
180
(ppm)
Figure 4. The chemical shifts of the o-, p ,and m-fluorines for the common organic substituents.
= Symbols a_ and refer to energy levels common to both para and mera spectra, depending on the sign of Jzd (see text). *The same numbering of lines is also used in Figures 1 and 2. Primed numbers refer to lines in the upfield half of the meta spectrum which result from coupling to the p-fluorine.
A comparison of values from different sources is complicated by variations due to conditions of measurement. Reported values of Jz3 and J34vary in some instances over a l-cps range (the experimental error is usually given as 0.2 cps); the range for other smaller coupling constants is proportionately less. Chemical shifts are more sensitive to conditions, and values differing by as much as 2 ppm have been reported by various workers. One factor involved in the variation of reported chemical shifts is the choice of reference. Although 19Fchemical shifts have commonly been measured by using trichlorofluoroniethane as reference and solvent, hexafluorobenzene has been found in this work to be a preferred reference and benzene a more suitable solA recent quantitative approach to solvent effects on fluorine chemical shiftPb supports this and shows that if CFCL is used as a universal reference, measured chemical shifts may differ by 2-3 ppm depending upon choice of solvent. However, if CFC1, is used for aliphatic compounds and C6F6 for aromatic (19) P. L. Timms, D. D. Stump, R . A . Kent, and J. L. Margrave, J . Am. Chem. Soc., 88, 940 (1966). (20) J. Cooke, M. Green, and F. G. A . Stone, J . Chem. SOC.,A , 170 (1968). (21) F. J. Hopton, A . J. Rest, D. T. Rosevear, and F.G. A. Stone, ibid., 1326 (1966). (22) R. B. Johannesen, F. E. Brinkman, and T. D. Colle, J . P h j s . Chenz., 72, 660 (1968). (23) (a) Our preference for CGFB results from (a) the proximity of its chemical shift to those of pentafluorophenyl compounds generally; (b) the high concentration of fluorme nuclei it provides; and (c) its single sharp resonance. Trichlorofluoromethane, in view of its low fluorine content, must be used in such high concentration that it becomes, for practical purposes, the solvent; moreover, it gives rise to a rather broad signal with definite shoulders. (b) J . W. Emsley and L.Phillips, Mol. Phys., 11, 437 (1966).
compounds, variations are reduced to 0.5 ppm or lower. We have found these uncertainties to be of the same magnitude as the differences encountered in Table 11. The chemical shift of hexafluorobenzene relative to trichlorofluoromethane has been reported at values ranging from 162.237 to 163.9 ppm.21 In our work, concentrations in the range 20-30% wjw or v/v (approximately 10 mol %) were used, this being effectively the lowest concentration at which both chemical shifts and coupling constants could be conveniently measured. We have used the value 163.0 for conversion to the CFCl, scale. Under similar conditions, other workers* have found 162.9 ppni. The results of an investigation of concentration effects on chemical shift are summarized in Table 111, in which differences between measurements at 10 mol and values obtained by extrapolation to infinite dilution are compared for nine compounds. The differences are small, and even for those substituents such as O H and NH, where association might be expected, are less than the standard deviations of the correlations presented below. The coupling constant, Jz4,for these nine compounds changed by less than 0.1 cps in the same range of concentrations. To correlate data from the various sources in Table 11, we have adjusted the reported chemical shifts in two cases. Boden, et al., used C6F6as reference,7 but expressed their results relative to CFCl, by adding 162.23 ppm. To bring these into line with our scale (C6F6= 163.0), 0.77 ppm was added to each shift. Barlow, et al.,lS measured chemical shifts in 50 % solutions using internal CFC1, or external CF3COzH as reference. Comparing the four compounds of their work that were also analyzed in this work, it was found that the chemical shifts averaged 1.4 ppm higher. This discrepancy, no doubt due to concentration effects, was accounted for by subtracting 1.4 ppm from all chemical shifts from that source. Although it is realized that solvent effects change the chemical shifts of the o-,
Hogben, Graham
/ I9F Chemical Shifts in Pentajuorophenyl Dericatives
286 Table II.
l9F
Chemical Shifts and Coupling Constants of Pentafluorophenyl Compounds
Compd
-Chemical shiftsn40 @P @m 128.7 128.1 131.5 125.6 120.4 132.6 132.2 125.3 131.3 130.2 130.8 132.4 138.7 128.8 128.1 132.0 140.2 128.8 127.6 127.4 124.4 124.7 124.3 124.1 124.3 135.4 130.6 129.2 139.2 127.7 128.6 120.5 134.2 130.9 119.4 132.0 119.4 117.5 117.9 117.2 138.8 127.0 128.7 119.5 123.0 138.7 122.6 133.2 141.0 137.5 143.9 132.2 156.2 158.7 157.0 164.4 160.4 162.6 158.9 163.3 161.3
140.2 136.0 141.5 140.4 143.8 142.4 144.3 143.4 141.6 145.4 146.1 146.0 147.4 147.3 148.6 143.2 147.8 147.5 149.6 148.1 149.2 149.2 149.5 149.5 150.0 146.9 147.7 150.5 151.6 150.6 150.5 150.4 152.1 153.5 149.6 154.2 150.7 151.6 151.2 152.6 155.7 152.5 153.7 153.0 153.5 154.3 154.2 155.2 156.5 159.1 159.1 159.3 166.5 167.6 169.2 171.2 172.8 173.8 174.1 174.3 176.1
157.6 156.4 158.2 156.6 158.7 158.5 159.0 159.8 158.3 159.8 160.0 159.4 162.1 160.1 160.1 158.9 160.3 160.3 161.1 159.1 159.3 161.1 159.4 159.5 159.5 159.2 159.8 160.8 162.1 161.0 160.4 160.9 161.3 162.9 159.3 163.0 159.7 160.4 159.9 160.1 162.7 159.8 161.6 160.1 161.4 162.6 160.7 161.0 161.7 162.3 164.4 164.3 165.0 165.1 166.3 165.8 168.8 165.9 165.7 165.8 168.2
Coupling constantsb
7
i J 3 5
7
JLz
+J2a
i J 3 4
iJZ3
8.6 8.5 7.9 7.5 7.3 7.2 7.2 6.6 6.3 6.2 6.0 6.0 5.9 5.9 5.8 5.8 5.8 5.6 5.4 5.2 5.2 5.1 5.0 5.0 4.8 4.6 4.5 4.2 4.0 4.0 3.8 3.4 3.3 3.3 3.0 2.9 2.8 2.7 2.6 2.6 2.6 2.4 2.3 2.0 1.9 1.3 1.1 1.1 0.7 0 0 0 -4.3 -4.4 -5.0 -6.1 -6.4 -6.9 -7.0 -7.0 -8.0
19.9 20.4 20.9 20.1 20.3 20.8 20.8
21.5 18.4 22.3 22.6 23.4
20.'2 19.6 19.1 19.7 18.6 20.6 20.8 18.7 20.0 19.4 19.8 20.4 20.6 20.4 20.9 20.8 20.4 21.0 20.5 20.1 20.1 20.9 19.4 19.5 19.8 20.2 19.2 19.8 19.4 19.6 19.2 19.2 20.2 21.2 19.4 19.8 19.1 18.4 19.5 19.9 19.5 20.2 18.9 21.2 21.6 21.0 21.6 21.1 21.9 21.6 21.4 20.4 22.2
19.'4 22.3 20.6 21.9 21.6 23.4 23.4 19.5 20.2 22.1 22.2 22.3 23.7 23.5 23.4 23.7 23.0 22.6
7.'8 9.7 8.5 7.5 9.9 9.2 8.7 8.3 8.4 9.1 8.7 7.2 9.2 8.3 8.7 8.9 8.6 8.0
0:i 0.4 0 0 0 0 0.6 0 0 0.2 1.2 0 0 0 0 0 1.0 0
8.5 6.8 7.6 7.7 4.8 5.1 5.9 7.9 7.6 5.7 6.1 6.6 3.3 6.7 6.8 6.3 6.2 8.5
23.5 21.5 24.1 20.3
9.3 8.6 9.4 7.1
0 1.2 1.5 2.4
4 . 6 31.1 5.1 4 . 5 38.9 6.5 12.3
23.9 23.7 24.7 23.2 26.3 29.2 22.6 21.3 26.0 24.1 23.2 23.0 26.7 20.0 28.3 21.6 20.6 22.2 20.4 25.0
9.2 9.8 10.3 9.4 10.6 8.6 9.8 8.3 9.6 8.2 9.4 7.2 12.0 8.6 11.6 6.3 6.3 7.6 8.6 8.2
1.2 1.7 0 1.8 0 0 1.1 1.1 2.8 3.6 1.5 1.3 1.5 1.1 1.6 1.6 2.0 0 0.4 1.4
4.4 29.0 4.0 35.8 6.8 3.6 30.1 7.0 7.6 6.9 7.3 3.3 5.0 3.6 3.6 3.9 5.1 6.5 2.2 6.6 5.5 5.3 3.0 1.9 1.4
*JZS
9.0 7.2 8.8 8.2 9.0
0 1.0 0 0 0
TJ26
9.4 1.0 10.1 7.4 . . . 8 . 5 13.0 9 . 2 14.0
. . . . . . . . . . . .
. . . . . . . . . . . . ...
J13
Xa
9.5 3.5
7.0 3.0 4.0 2.7 2.7 3.1 . . . . . . 3.0 . . . . . . 3.2
63.2
0 . 9 1.1
,.. 43.6 2.0 0
10.33.6 2 . 5 2.1 3.4 2.3 3.8 3.0
38.7 51.0 5.0 0
... 0 0 0
1.5 1.7 1.3 1.3 0 1.1 0.9 0.9
1.8 1.3 1.4 1.9 0 1.8 1.7 1.6
.....................
. . . . . . . . . . . .
...
...
...
21.0
...
21.1 21.5 20.2 25.2
... ... ... ... 4.4 2:s -3.4 . . . . . . ... ......
...... 4.5 4.6 5.1 6.0
2.3 -3.9 2.7 -3.3 2.3 -4.6 -5.5 0
1.1 0 0.6 0.6 1.6 0 3.0 1 . 7 1 . 0 0.8 1.7 0
6.6 2.6 1.3 0
Sourcec
A A A A A A A GH A AJ A A J A A A AC AJ J A A A A A A A A A A A J A J J A AJ A A A A J E J CD D A D ACD CDF A AC E A A A D A D A AC A
Chemical shifts of 0-,p-, and m-fluorines in parts per million (ppm). The $J scale is based here on C6F6= 163.0 ppm upfield from CFC13. The coupling constants are in cycles per second (cps). Numbering is clockwise around the ring with the substituent at 1 and the o-fluorines a t 2 and 6. The signs are based on the result of ref 26 and 27a in combination with results from this work, The value 0 is given for unre-
5
u
3
was apsolved coupling constants. The resolution was generally approximately 0.2 cps. Due to line overlap the resolution limit for JIZ proximately 2 cps. The symbol , , . . means that analysis was not attempted. This is generally found for coupling constants derived from the meta spectrum which was sometimes broadened (e.g., for compounds with two or three pentafluorophenyl groups, spectra were usually broadened, probably by inter-ring coupling). The couplings to phosphorus in the various phosphines are arranged under columns Jn,J13, J14. The data have been compiled from the following sources: A, this work; C, ref Couplings to other substituent atoms are not presented. 8; D, ref 7; E, ref 20; F, ref 17; G, ref 22, H , ref 19; J, ref 18.
281 Table In. The Differences Between Chemical Shifts of Nine from Their CBF5-X Compounds Measured at -10 mol Extrapolated Value a t Infinite Dilution. X
ortho
para
meta
CN CF3 I Br H C1
0.07 0.08 0.08 0.06 0.06 0.01 0.03 0.01 0
0.18 0.02 0.03 0.01 -0.03 0 -0.05 0.32 -0.10
0.08 -0.03 0 0 -0.02 0 0.05 0.04 -0.08
‘3%
OH NH?
a Negative values indicate a downfield shift with increased dilution. Differences are given in parts per million (ppm). The value at infinite dilution was extrapolated from the chemical in benzene to which shifts measured at -20, 10, 5 , and 2.5 mol 1 wlw hexafluorobenzene had been added.
0
8 -
6 4 -
2n
”.
0 -
m
7
-2
-
-4
-
-6
-
-8-
-1Ol1
136
‘
I
I40 144
CHEMICAL
p - , and m-fluorines in different ways, there seems no other way of accommodating all results measured under different conditions than by the gross assumptions outlined above. Coupling constants, which are less sensitive to concentration changes, were not adjusted for differences in the conditions of measurement. In the statistical analysis, the mean values of chemical shifts and coupling constants were used when more than one source is cited. A close examination of the results in Table I1 shows that o-fluorine chemical shifts (9,) fall in the range 117164 ppm, m-fluorine chemical shifts (&J in the range 156-169 ppm, and para chemical shifts (c$~) in the range 136-176 ppm. These changes are illustrated in Figure 4 for 12 common organic substituents. The ortho coupling constants, J23 and J34,varied little with substituent. The mean value for J23 was 22.7 cps; that for J3awas 20.9 cps. N o trends were discerned for the only para coupling constant, J Z j ,which ranged from 4 to 12 cps. Of the three meta coupling constants Jz4, J N , and J 3 5 ,the first two showed obvious correlations with the nature of the substituent. J Z 46s. &. It was found that JZ4formed a straight line relationship with C$p (Figure 5). Since this relationship is the principal concern of this paper, the results of Table I1 are tabulated in order of decreasing .Ill values. The straight line in Figure 5 can be represented algebraically by JZ4= -0.4534, 71.95, as found by a computerized least-square method. The root-meansquare deviation for a plot of all 61 substituents was 0.69 cps. The goodness of fit of the data to a straight line, as defined by the square of the correlation coefficient ( Y , ~ ) ~for , the plot of 61 substituents, was 0.974.24
+
For the very wide range of substituents measured or plotted from data of other workers, only two points (CN.BCl3, CN.BFa) existed outside 2.2 standard deviations. These two points, however, formed a good (24) We report values of (rx,.)2, which range from 0 to 1 as a completely random scatter of points approaches a perfect straight line, in preference to root-mean-square values, which can be misleading. For example, comparing eq 1 and 2 in Table VI, we find that the points represented by eq 1 approximate much more closely to a straight line (Le., higher ( r x y ) 2value) than do the points represented by eq 2; the root-mean’-square deviation for eq 1 is considerably larger than for eq 2, however.
I
148
I
’
152
t
156 160
I
164
SHIFT OF PARA F L U O R I N E
“
168
‘
172
1
176
180
( 4 p ) ppm ,
Figure 5. The relationship of the coupling constant between the p-fluorines ( J 2 4 ) t o the chemical shift of the pfluorine (.&) for all 61 pentafluorophenyl derivatives. The point representing fluorine as substituent is interpolated from the chemical shift of hexafluorobenzene (163 ppm). 0- and
straight line when linked with the two similar substituents, CN and CN.BBra. Values for (CeF5),P and (CeF&PO were measured in this work, and established that the previously reported parameters, 2 5 inconsistent with the relation proposed here, were incorrect. For groups of similar substituents (as summarized in Table IV) such as the phosphines, benzonitriles, halobenzenes, and the phenoxy derivatives of group IV organometallic compounds, the root-mean-square deviation was close to the experimental error (0.2 cps). Table IV. Equations Relating Jla with the Chemical Shifts of the p- and m-FluorinesQ
J24
=
All compounds (61) Phosphines (28) Halogens (3) Nitriles (4) OMRa’(6)
-0.453 -0.390 -0.375 -0.359 -0.396
All compounds (61) Phosphines (28) Halogens (3) Nitriles (4) OM&’ (6)
- I . 47 -0.86 -0.82 -1.27 -0.72
J24
All compounds (61)
524
=
=
a&
+71.98 c
0.974 0.932 0.995 0.996 0.984
0.69 0.42 0.04 0.08 0.16
c 239.3 142.8 133.4 206.8 114.2
0.84 0.67 0.98 0.96 0.66
1.7 0.9 0.1 0.3 0.7
+ + 40.69
0.98
0.65
62.98 59.41 57.30 61.89 a+,
-0.5246, 0.2626,
+
a Figures in parentheses refer to the number of points from which the equation is derived. RMS is the root-mean-square deviation; (rxy)2 is the square of the correlation coefficient. Pentafluorophenoxy derivatives in which M = Si, Ge, Sn, Pb and R = CeHs or CH,.
The sign change was demonstrated by comparing the sign of JZ4 relative to J23 for the two compounds pentafluorobenzoyl chloride and pentafluoroaniline. For pentafluorobenozyl chloride, JZ4and’5 2 3 were of opposite sign, whereas for pentafluoroaniline J z was ~ (25) H. J. Emelbus and J. M. Miller, J . Inorg. Nucl. Chem., 28, 662 (19 66).
Hogben, Graham
1
I9F Chemical Shifts in Pentajluorophenyl Dericatioes
288
-
---ll-OHJ
I
Do
0
0
4 1 1
140
150
160
170
180
9, ( P P d Figure 6. The relationship of the coupling constant between the two o-fluorines, Jue,and the chemical shift of the p-fluorine: &.
of the same sign of J23.Since fz3remained approximately constant at 20 cps for all 61 derivatives, it was assumed that J 2 3 did not change sign. A detailed account of the sign determination has been given in the Experimental Section. These sign designations are consistent with two previous determinations. Snyder and Anderson26found that .JFFmetain hexafluorobenzene, as observed in the nematic liquid crystal, was - - 3 cps and therefore was of the same sign as JFFoYtho, which was assumed to be negative. Our interpolated value of J24 for hexafluorobenzene using Figure 5 is -2.0 cps, the value of J z 4 corresponding to 4p = 163.0. We presume the very different conditions of measurement have caused this discrepancy. A recent determination of relative signs and coupling constants has been made on iodopentafluorobenzene using low-field nmr.”* Basing the signs on .JFFcl”iho as negative, .J26 and J3; were both found to be negative, while the third mefa coupling constant, Jz4,was found to be anomalously positive. Since our results show that the sign change of Jza occurs for substituents with & above 158 ppm, the apparent inconsistency between the sign of Jz4for iodopentafluorobenzene and hexafluorobenzene is resolved.2ih Jz4us. The weaker relation between JZ,, and $m is shown in Table IV. Since the J24values are the same in relationships with and 4p,the equations derived from the same number of points can be compared by their root-mean-square value as well as their ( r x y ) 2 value. The root-mean-square value of the 4, series is in each case more than double that for the q5p series. The last equation in Table IV shows, as expected, that a better correlation exists for a linear combination of & and +m when related to JZ4. The combination of Gp and 4, found to give the best straight line was &, o.50q+lm. J26 cs. &. I n the analysis of the pentafluorophenyl group, the pair of coupling constants J z , and Jgj are indistinguishable. However, the study of iodopenta(26) L. C . Snyder and E. W. Anderson, J . Chern. Phq’s., 42, 3336
CHEMICAL SHIFT OF THE ORTHO FLUORINE
(+e)
PPM
Figure 7. The relationship of the electronegativity of the substit uent atom to the chemical shift of the o-fluorine.
fluorobenzene mentioned yielded the unanibiguous assignment > IJ3il,to which we have adhered. When J 1 6 is plotted against 4,,, a straight-line relationship is found (Figure 6 and eq 1, Table V). Like the J L I - $ pplot, the points with & greater than 170 ppni are incorporated into the straight-line relationship by means of a sign change i n J J 6for these points. No trend was discerned for the Jdjvalues, which were usually less than 3 cps. Table V. Equations Derived by Least-Mean-Square Analysis,a Relating bP with Coupling Constants J2c,J3:, and &, N i t h the Allred-Rochow Electronegativity, E Eq no. i 2 3
4 5
Equation
+
--2.38J26 164.19 5.31JZj + 146.90 = 1.95(Ja; - J z ~ ) 160.08 qp = 2.52(JJj &) 167.35 E = O.O371p,, - 2.68 d,
=
=
+
+ +
(rxy)2
RMS
0.84 0.51 0.88 0.65 0.81
3.4 6.0 3.0 5.1 9.23
Pts 61 61 61 61 75*
(rxs)’ is the square of the correlation coefficient, RMS is the root-mean-square deviation, and Pts is the number of points used in the analysis. Ei The extra 14 points are found from chemical shifts in the literature for C6FjX where X = Mn(CO),, BCL, BFn, BCla-py, BFy-py, B(OH)?. CGHa, B(CI)C6Hs, Ti(CjH5)2CsH:, TI(Br)CsFj, Hg(CeFj), Fe(CO)GHj, Pb(CcHa).i:and NOu: ref 29-32. a
As shown by Figure 6 and the statistical parameters in Table V, the considerable scatter in the Jz6-4pplot reduces its predictive value, in contrast to the Jz4-&, which is sufficiently linear to be applied in the evaluation of substituent effects. The Electronegativity of the Substituent Atom cs. &,. Figure 7 and eq 5, Table V show that a fair straightline relationship exists between & and E, the eiectronegativityz8 of the substituent atom bonded directly to the pentafluorophenyl ring. Additional points, including many C6Fj-metal compounds, have been taken from the literature. 2 y - 3 2 These results are not
(1965).
(28) Using the hllred-Rochow values listed in F. A. Cotton and G . (27) (a) W . B. Moniz and E. Lustig, ibid., 46, (1967). (b) A very Wilkinson, “Advanced Inorganic Chemistry,” 2nd ed, Intcrscicncc recent paper (I
