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Spectroscopic evidence for medium controlled hydrogen bond inhibition of resonance delocalization of charge in complexes of tetrabutylammonium fluoride with phenols
Spectrochimica Acta, Vol. 42A, No. 7, pp. 815-819, 1986.
0584-8539/86 $3.00 + 0.00 Pergamon Journals Ltd.
Printed in Great Britain.
Spectroscopic evidence for medium controlled hydrogen bond inhibition of resonance delocalization of charge in complexes of tetrabutylammonium fluoride with phenols JAMES H. CLARK,* DAVID G. CORK and JULIE A. TINSDALE Department of Chemistry, University of York, Heslington, York YO 1 5DD, U.K.
(Received 6 June 1985; infinal form 27 January 1986; accepted 29 January 1986) Abstract--Infrared spectra of tetrabutylammonium fluoride complexes of both 4-cyanophenol and methyl4-hydroxybenzoate but not 3-cyanophenoi reveal the presence of two distinct types of hydrogen bond corresponding to charge localized and partially charge delocalized forms. The latter only occur in polar aprotic protophilic solvents at low temperatures. The activation energies for the interconversion of the two forms reflect the relative abilities of the two groups to delocalize charge through resonance. The chemical shifts of the aromatic protons of the complexes are also medium dependent although the precise nature of this medium dependence is less easily understood.
INTRODUCTION We recently reported preliminary results from the i.r. investigation of the hydrogen bonded complex tetrabutylammonium fluoride (TBAF}-4-cyanophenol in solution 11]. We observed a medium dependent shift of the fundamental C-=N stretching band v (C-=N) for this complex and interpreted our results in terms of the first spectroscopically observed example of hydrogen bond inhibition of resonance delocalization of charge in solution. It seems that both delocalized and localized forms o f the complex can exist in solution. In view of the importance of this observation to the understanding of the nature and reactivity of hydrogen bonded complexes [-2-5], we have extended our investigations to include hydrogen bonded complexes of 3cyanophenol and 4-methyoxybenzoate to see if the observed effects are indeed due to inhibition of resonance. Further to this, variable temperature i.r. spectroscopic studies should enable us to see if we can interconvert the two forms of the complexes and determine the appropriate activation energies. We also report for the first time the observation that the chemical shifts of the aromatic protons in these complexes are sensitive to their environment.
EXPERIMENTAL
Infrared spectra were recorded on a Perkin-Elmer 683 ratio recording spectrophotometer interfaced to a dedicated 64K computer for spectra processing. Solution phase spectra were all run using 0.1 mm pathlength NaCI cells. For the variable temperature work, the cells were electrically heated and the temperature was monitored using a thermocouple in direct contact with the solution. NMR spectra were recorded on both Bruker WP90 (90 MHz) and Jeol FX90Q (90 MHz) spectrometers.
Preparation of complexes and salts All of the complex preparations were based on the same general method. Tetrabutylammonium fluoride trihydrate (prepared by careful drying of an equimolar mixture of the hydroxide and HF) was added in equimolar quantities to a
solution of the phenol in acetonitrile (typically ca 20 g solvent/g phenol). The resulting solution was then slowly evaporated on a rotary evaporator at 20°C. The resulting white solid was then washed with cold acetonitrile and/or dry diethyl ether, recrystallized from tetrahydrofuran and dried at 20°C and 1 mm Hg for ca 24 h. The dried complexes undergo slow decomposition on standing and are best used fresh. tH NMR spectroscopy was routinely used to confirm the formulae of the complexes (via peak integration). The i.r. spectra of the complexes showed no evidence for bifluoride formation. The tetrabutylammonium phenoxide salts were prepared by adding solutions of the phenols in acetonitrile to equimolar amounts of tetrabutylammonium hydroxide (40 aqueous solution). Evaporation at 40°C on a rotary evaporator followed by recrystallization from tetrahydrofuran and drying at 20°C and 1 mm Hg for ca 24 h gave white crystals of the salts. 1H NMR spectroscopy was used to confirm the formulae of the salts.
RESULTS AND DISCUSSION The i.r. spectra of the solid complexes T B A F - 4 cyanophenol (I), TBAF-3-cyanophenoi (II) and TBAF-methyl-4-hydroxybenzoate (III) all show broad structured bands in the 3000-1600 c m - 1 region of the spectra characteristics of strong hydrogen bonding [2] (Fig. 1). Also notable are the strong and quite sharp bands due to v(C=N) (complexes I and II) and v(C=O) (complex III). We used these bands to monitor the degree of resonance delocalization of charge using the corresponding band positions for the parents and anions as reference values. Significantly, whereas the v ( C - N ) band for I occurs in between those of the parent phenol and its anion, the v(C=N) band for II occurs very close to that of its parent phenol and the v(C=O) band for III occurs at a higher frequency than those of either parent or anion. The unusual shift to higher energies in I H is undoubtedly a result of loss of intermolecular H-bonding in the pure phenol ( C = O . . . H - O ) which would result from the formation of strong F . . . H - O bonds. The difference in behaviour of I and II may well be due to the relative 815
816
JAMES H. CLARKet al.
~
\ 3500
3000
111
I
I
I
I
2500
2000
1800
1600
Wave number Fig. 1. Infrared spectra of the hydrogen bonded complexes of tetrabutylammonium fluoride and 4cyanophenol (I), 3-cyanophenol (II) and methyl-4-hydroxybenzoate (Ill) (volatef oil mulls). All spectra show an increase in absorption in the 3000-1600 cm -t region compared to the parent phenols. abilities of 3- and 4-cyano groups to compete for charge through resonance effects. The most revealing results come from the solution phase i.r. spectra which are summarized in Table 1. Both I and I l l show medium controlled H-bond inhibition of resonance delocalization of charge. Polar -protic protophilic solvents (high ~r* an fl value solvents on the Taft scale ['6]) appear to favour the partially delocalized form of the H-bond in both cases. All other solvents favour the fully H-bond localized form of the H-bond. The positions of the v(C-_-N) and v(C=O) bands in the parent phenols and their anions
are much less medium dependent. The position of the v ( C - N ) band in the H-bonded complex II is almost medium independent and remains at a value very close to that for its parent indicative of charge localization and strongly supporting the hypothesis that with the other species we are indeed observing an inhibition of resonance effect. Clearly the O H F H - b o n d is acting as a powerful electron sink since even the normally powerful C N group can only successfully compete for charge in highly polar, protophilic environments which can assist resonance delocalization of charge through non-specific medium effects.
Table 1. Shifts (with respect to the parent phenols in can- t) for the v(C ---N) a n d v(C--O) bands in the H-bonded complexes and anions of 4-cyanophenol, 3-cyanophenol and methyl-4-hydroxybenzoate Solvent (in order of incre~ing n* value) CHCIa (n* = 0.58; ~ = O) MeaCO (n* = 0.71; ~ -- 0.48) MeCN (Tt* = 0.75; fl = 0.31) CH2CI2 (n* ffi 0.81; fl = O)
Me,NCHO (Tt* = 0.88; fl -- 0.69) (~H~(CH~)2COI~IMe 0t* ffi 0.92;/3 -- 0.77) Me2SO (n* = 1.00; fl = 0.76) Solid
4-Cyanophenolt Av(C-=N)[[ Complex Anion
3-Cyanophenol:~
Methyl-4-hydroxybenzoate§
A~(C--N)U
A~(C=O)II
Complex
Anion
Complex
Anion 39
4
30
2
12
4
3
33
--
--
3
4
35
2
12
1
33
3
34
--
--
4
35
16
34
14
--
12
30
14 22
34 48
12 14¶
29 10t~"
2 .
12 .
2 3
. 11 12
.
~'The parent phenol values are: 2226 (CHCI~, Me2CO and MeCN); 2225 (CH2C12); 2221 (Me2NCHO) and 2220 ((~H2(CH2)2CONMe and Me2SO). :~The parent phenol values are" 2239 (CHCI 3 and M e C N ) and 2232 (Me2NCHO and Me2SO). § The parent phenol values are: 1716 (CHCIa); 1711 (Me2CO and Me2NCHO); 1715 (MeCN and (~H2 (CH2)2COI~IMe); 1716 (CH2C!2); 1710 (Me2SO)and 1680 (solid). HAll values are to + I cm- t. ¶Shift to higher wavenumbers (see text). ttThis value is deceptively small due to the low value for the parent in the solid state (see text).
Charge delocalization in TBAF complexes On heating a solution of I in a solvent such as dimethylsulphoxide which favours delocalization of charge at ambient temperatures, the characteristic v(C=N) band at 2206 cm- 1 is replaced by a new band at 2218cm -1. The position of the new band is consistent with a localized form of the complex. At temperatures in the range 30-60°C, the change is measurably slow enabling rates of conversion to be measured at a series of temperatures. Concentrations were taken to be directly proportional to the intensities of the v(C=N) bands at 2218 and 2206 c m - 1 and in this way rates of conversion measured with respect to an increase in intensity of the 2218 cm-1 band and those measured with respect to a decrease in intensity of the 2206 cm-~ band were in good agreement. Heating a solution of I11 in dimethylsulphoxide resulted in a similar effect with the characteristic v(C=O) band at 1696 c m - ~ being slowly replaced by a new band at 1708 cm-~ which we again interpret as a delocalized form-localized form of the complex conversion. Activation energies for the interconversion of the two forms of the two complexes were determined by plotting In k against 1/T which gave reasonable straight lines (Fig. 2). The calculated activation energies are Eact (I) = ca 20 kJ mol- 1 and Eact (III) = ca 6 kJ mot- 1 (using complex concentrations of 0.4 M in both cases). The relative values of the activation energies presumably reflect the relative abilities of the CN and CO2CH3 groups to compete for charge through resonance effects. No changes are observed on heating solutions of the
817
complexes in other types of aprotic solvents (other than eventual complex breakdown). Infrared monitoring of solutions of the complexes in dimethylsulphoxide at concentrations significantly lower than 0.4 M showed much more rapid conversion of delocalized to localized forms of the complexes. At 0.1 M, for example, solutions of the complexes showed bands characteristic of both forms at 25°C (we were unable to measure activation energies for the interconversion of the localized and delocalized forms at concentrations of less than 0.4M due to partial crystallization of solutions at sub-ambient temperatures). It would appear that the presence of the complexes themselves encourages resonance delocalization presumably through an increase in "medium polarity". Our conclusions from the i.r. spectroscopic studies on the nature of the hydrogen bonded complexes can be summarized by the equilibria shown below. In an attempt to learn more about the interaction of the medium with the hydrogen bonded complexes we turned our attention to the use of NMR for the study of these systems. The hydroxyl protons of the Hbonded complexes experience a large shift to lower fields in all types of aprotic solvents consistent with the presence of strong H-bonding. Typical shifts with respect to the parent phenols are 4-5 ppm for I and II in solvents including chloroform (H-D exchange occurs in deuteriochloroform solutions), acetone and dimethylsulphoxide and shifts of up to 7 ppm for IlL The aryl proton chemical shifts of the H-bonded
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JAMESH. CLARKet al.
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Fig. 2. Plots of In k against 1/ T for the interconversion of the two forms of the complexes I ( x ) and III (O) (see text). complexes I and III and those of their anions are all medium dependent. Inspection of the values for the complexes (Table 2) reveal that the solvents do not fall into two distinct classes as they do for the i.r. shifts. In all cases the chemical shifts of the aryl protons of the complexes fall in between those of their respective parent phenols and anions. In an attempt to correlate the observed aryl proton chemical shifts with Taft's solvatochromic parameters [6], we plotted the chemical shifts of the protons ortho to the H-bond (A6ntcoH)) for both I and I I I against the medium polarizability parameter (n*). We also plotted the C(2)-H shifts for the anions of the phenols of I and III against n* for comparison (Figs 3 and 4). For both complexes, and especially I, the proton chemical shifts do not correlate well with it* (R = 0.51 for I and 0.80 for III). The chemical shift-x* correlations for the
Fig 3. Plots of C(2)-H chemical shifts (with respect to the parent phenol) against medium polarizability (~*) for the Hbonded complex I ( x ) and its phenoxide anion (O). Chemical shifts are accurate to + 0.02 p.p.m.
conjugate phenoxide ions are somewhat better (R = 0.84 for I and 0.91 for IlI). Close inspection of the medium-dependency of the'aryl proton chemical shifts of the complexes reveals that the H-bond electron donor (proton acceptor) power of the solvent (~ [6]) also appears to play an important role. Thus a ~olvent such as dichloromethane (high n* but zero ~) generally causes especially low aryl proton chemical shifts for the complex in comparison to those for the phenoxide ions (Table 2). We tested this observation by plotting A6H(COH)against n* + aft using values of a in the range 0-2. Optimum correlations for both I and III occurred with values for a in the range 0.5-1.0 IRmax (I) -- 0.82 and Rmax (III) = 0.91]. These results
Table 2. Proton NMR chemical shiftst (with respect to the parent phenols in p.p.m.) for the aromatic CH protons in the H-bonded complexes and anions of 4-cyanophenol and methyl-4-hydroxybenzoate Solvent (in order of increasing 7t* value) CHCIs 0t* = 0.58; fl -- O) Me2CO (n* = 0.71; fl = 0A8) MeCN (n* = 0.75; fl = 0.31) CH2CI2 (~* = 0.81; i~ = O) Me2NCHO (it* = 0.88; /] = 0.69) (~H2(CH~)2CON (CH3) (n* = 0.92; fl = 0.77) Me2SO (It* = 1.00; fl = 0.76)
Methyl-4-hydroxybenzoate
4-Cyanophenol At~H(COH ) Complex Anion
/~k~H{CN) Complex Anion
/~k~H(OH) Complex Anion
/~k~H(COaCH3) Complex Anion
0.27
0.47
0.26
0.35
0.08
0.47
0.11
0.20
0.38
0.70
0.35
0.52
0.27
0.58
0.18
0.30
0.30
0.68
0.26
0A2
0.31
0.58
0.25
0.39
0.26
0.62
0.24
0.44
0.21
0.61
0.22
0.33
0.45
0.72
0.46
0.61
0.34
0.80
0.22
0.36
0.46
0.89
0.39
0.75
0.43
0.84
0.26
0.41
0.43
0.88
0.35
0.60
0.38
0.86
0.28
0.42
tAll values to +0.02 p.p.m.
Charge delocalization in TBAF complexes 0.9 0
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hydrogen bonded complexes of fluoride with phenols containing powerful electron withdrawing groups substituted at the 4-position (CN and CO2CH3). This effect is medium dependent so that in the more powerful polar aprotic protophilic solvents the substituent groups can compete with the hydrogen bond for the charge. In other types of aprotic solvent the charge appears to be heavily localized at the hydrogen bond. The activation energy for the partially delocalized-localized conversion is dependent on the nature of the substituent group. Correlations of proton N M R data with solvent parameters are less simple than those based on i.r. data although medium protophilicity (high fl value solvents) is again observed to play an important role.
I
0:9
1.0
Medium polarizability (~*) Fig. 4. Plots of C(2)-H chemical shifts (with respect to the parent phenol) against medium polarizability (7~*)for the Hbonded complex III( x ) and its phenoxide ion (O). Chemical shifts are accurate to + 0.02 p.p.m. suggest that for the H-bonded complexes in solution both specific solvation and non-specific solvation play important roles. CONCLUSIONS Hydrogen bond inhibition of resonance delocalization of charge can be observed in solutions of
Acknowledgements--We are grateful to the SERC for the award of an earmarked studentship and to BDH Chemicals for supporting this work. We are also grateful to various colleagues including E. M. GOODMANfor helpful discussions.
REFERENCES
[1] J. H. CLARK and D. G. CORK, J. chem. Soc. Chem. Commun. 1014 (1984). [2] J. EMSLEY,Chem. Soc. Rev. 91 (1980). [3] J. W. LARSONand T. B. MCMAHON,J. Am. chem. Soc. 105, 2944 (1983). [4] M. J. K A M L E T , C . D I C K I N S O N , T. GRANSTADand R. W. TAFT, J. org. Chem. 47, 4971 (1982). [5] J. H. CLARK,Chem. Rev. 80, 429 (1980). [6] M. J. KAMLET,J.-L. M. ABBOUD,M. H. ABRAHAMand R. W. TAFT,J. org. Chem. 48, 2877 (1983).
0584-8539/86 $3.00 + 0.00 Pergamon Journals Ltd.
Printed in Great Britain.
Spectroscopic evidence for medium controlled hydrogen bond inhibition of resonance delocalization of charge in complexes of tetrabutylammonium fluoride with phenols JAMES H. CLARK,* DAVID G. CORK and JULIE A. TINSDALE Department of Chemistry, University of York, Heslington, York YO 1 5DD, U.K.
(Received 6 June 1985; infinal form 27 January 1986; accepted 29 January 1986) Abstract--Infrared spectra of tetrabutylammonium fluoride complexes of both 4-cyanophenol and methyl4-hydroxybenzoate but not 3-cyanophenoi reveal the presence of two distinct types of hydrogen bond corresponding to charge localized and partially charge delocalized forms. The latter only occur in polar aprotic protophilic solvents at low temperatures. The activation energies for the interconversion of the two forms reflect the relative abilities of the two groups to delocalize charge through resonance. The chemical shifts of the aromatic protons of the complexes are also medium dependent although the precise nature of this medium dependence is less easily understood.
INTRODUCTION We recently reported preliminary results from the i.r. investigation of the hydrogen bonded complex tetrabutylammonium fluoride (TBAF}-4-cyanophenol in solution 11]. We observed a medium dependent shift of the fundamental C-=N stretching band v (C-=N) for this complex and interpreted our results in terms of the first spectroscopically observed example of hydrogen bond inhibition of resonance delocalization of charge in solution. It seems that both delocalized and localized forms o f the complex can exist in solution. In view of the importance of this observation to the understanding of the nature and reactivity of hydrogen bonded complexes [-2-5], we have extended our investigations to include hydrogen bonded complexes of 3cyanophenol and 4-methyoxybenzoate to see if the observed effects are indeed due to inhibition of resonance. Further to this, variable temperature i.r. spectroscopic studies should enable us to see if we can interconvert the two forms of the complexes and determine the appropriate activation energies. We also report for the first time the observation that the chemical shifts of the aromatic protons in these complexes are sensitive to their environment.
EXPERIMENTAL
Infrared spectra were recorded on a Perkin-Elmer 683 ratio recording spectrophotometer interfaced to a dedicated 64K computer for spectra processing. Solution phase spectra were all run using 0.1 mm pathlength NaCI cells. For the variable temperature work, the cells were electrically heated and the temperature was monitored using a thermocouple in direct contact with the solution. NMR spectra were recorded on both Bruker WP90 (90 MHz) and Jeol FX90Q (90 MHz) spectrometers.
Preparation of complexes and salts All of the complex preparations were based on the same general method. Tetrabutylammonium fluoride trihydrate (prepared by careful drying of an equimolar mixture of the hydroxide and HF) was added in equimolar quantities to a
solution of the phenol in acetonitrile (typically ca 20 g solvent/g phenol). The resulting solution was then slowly evaporated on a rotary evaporator at 20°C. The resulting white solid was then washed with cold acetonitrile and/or dry diethyl ether, recrystallized from tetrahydrofuran and dried at 20°C and 1 mm Hg for ca 24 h. The dried complexes undergo slow decomposition on standing and are best used fresh. tH NMR spectroscopy was routinely used to confirm the formulae of the complexes (via peak integration). The i.r. spectra of the complexes showed no evidence for bifluoride formation. The tetrabutylammonium phenoxide salts were prepared by adding solutions of the phenols in acetonitrile to equimolar amounts of tetrabutylammonium hydroxide (40 aqueous solution). Evaporation at 40°C on a rotary evaporator followed by recrystallization from tetrahydrofuran and drying at 20°C and 1 mm Hg for ca 24 h gave white crystals of the salts. 1H NMR spectroscopy was used to confirm the formulae of the salts.
RESULTS AND DISCUSSION The i.r. spectra of the solid complexes T B A F - 4 cyanophenol (I), TBAF-3-cyanophenoi (II) and TBAF-methyl-4-hydroxybenzoate (III) all show broad structured bands in the 3000-1600 c m - 1 region of the spectra characteristics of strong hydrogen bonding [2] (Fig. 1). Also notable are the strong and quite sharp bands due to v(C=N) (complexes I and II) and v(C=O) (complex III). We used these bands to monitor the degree of resonance delocalization of charge using the corresponding band positions for the parents and anions as reference values. Significantly, whereas the v ( C - N ) band for I occurs in between those of the parent phenol and its anion, the v(C=N) band for II occurs very close to that of its parent phenol and the v(C=O) band for III occurs at a higher frequency than those of either parent or anion. The unusual shift to higher energies in I H is undoubtedly a result of loss of intermolecular H-bonding in the pure phenol ( C = O . . . H - O ) which would result from the formation of strong F . . . H - O bonds. The difference in behaviour of I and II may well be due to the relative 815
816
JAMES H. CLARKet al.
~
\ 3500
3000
111
I
I
I
I
2500
2000
1800
1600
Wave number Fig. 1. Infrared spectra of the hydrogen bonded complexes of tetrabutylammonium fluoride and 4cyanophenol (I), 3-cyanophenol (II) and methyl-4-hydroxybenzoate (Ill) (volatef oil mulls). All spectra show an increase in absorption in the 3000-1600 cm -t region compared to the parent phenols. abilities of 3- and 4-cyano groups to compete for charge through resonance effects. The most revealing results come from the solution phase i.r. spectra which are summarized in Table 1. Both I and I l l show medium controlled H-bond inhibition of resonance delocalization of charge. Polar -protic protophilic solvents (high ~r* an fl value solvents on the Taft scale ['6]) appear to favour the partially delocalized form of the H-bond in both cases. All other solvents favour the fully H-bond localized form of the H-bond. The positions of the v(C-_-N) and v(C=O) bands in the parent phenols and their anions
are much less medium dependent. The position of the v ( C - N ) band in the H-bonded complex II is almost medium independent and remains at a value very close to that for its parent indicative of charge localization and strongly supporting the hypothesis that with the other species we are indeed observing an inhibition of resonance effect. Clearly the O H F H - b o n d is acting as a powerful electron sink since even the normally powerful C N group can only successfully compete for charge in highly polar, protophilic environments which can assist resonance delocalization of charge through non-specific medium effects.
Table 1. Shifts (with respect to the parent phenols in can- t) for the v(C ---N) a n d v(C--O) bands in the H-bonded complexes and anions of 4-cyanophenol, 3-cyanophenol and methyl-4-hydroxybenzoate Solvent (in order of incre~ing n* value) CHCIa (n* = 0.58; ~ = O) MeaCO (n* = 0.71; ~ -- 0.48) MeCN (Tt* = 0.75; fl = 0.31) CH2CI2 (n* ffi 0.81; fl = O)
Me,NCHO (Tt* = 0.88; fl -- 0.69) (~H~(CH~)2COI~IMe 0t* ffi 0.92;/3 -- 0.77) Me2SO (n* = 1.00; fl = 0.76) Solid
4-Cyanophenolt Av(C-=N)[[ Complex Anion
3-Cyanophenol:~
Methyl-4-hydroxybenzoate§
A~(C--N)U
A~(C=O)II
Complex
Anion
Complex
Anion 39
4
30
2
12
4
3
33
--
--
3
4
35
2
12
1
33
3
34
--
--
4
35
16
34
14
--
12
30
14 22
34 48
12 14¶
29 10t~"
2 .
12 .
2 3
. 11 12
.
~'The parent phenol values are: 2226 (CHCI~, Me2CO and MeCN); 2225 (CH2C12); 2221 (Me2NCHO) and 2220 ((~H2(CH2)2CONMe and Me2SO). :~The parent phenol values are" 2239 (CHCI 3 and M e C N ) and 2232 (Me2NCHO and Me2SO). § The parent phenol values are: 1716 (CHCIa); 1711 (Me2CO and Me2NCHO); 1715 (MeCN and (~H2 (CH2)2COI~IMe); 1716 (CH2C!2); 1710 (Me2SO)and 1680 (solid). HAll values are to + I cm- t. ¶Shift to higher wavenumbers (see text). ttThis value is deceptively small due to the low value for the parent in the solid state (see text).
Charge delocalization in TBAF complexes On heating a solution of I in a solvent such as dimethylsulphoxide which favours delocalization of charge at ambient temperatures, the characteristic v(C=N) band at 2206 cm- 1 is replaced by a new band at 2218cm -1. The position of the new band is consistent with a localized form of the complex. At temperatures in the range 30-60°C, the change is measurably slow enabling rates of conversion to be measured at a series of temperatures. Concentrations were taken to be directly proportional to the intensities of the v(C=N) bands at 2218 and 2206 c m - 1 and in this way rates of conversion measured with respect to an increase in intensity of the 2218 cm-1 band and those measured with respect to a decrease in intensity of the 2206 cm-~ band were in good agreement. Heating a solution of I11 in dimethylsulphoxide resulted in a similar effect with the characteristic v(C=O) band at 1696 c m - ~ being slowly replaced by a new band at 1708 cm-~ which we again interpret as a delocalized form-localized form of the complex conversion. Activation energies for the interconversion of the two forms of the two complexes were determined by plotting In k against 1/T which gave reasonable straight lines (Fig. 2). The calculated activation energies are Eact (I) = ca 20 kJ mol- 1 and Eact (III) = ca 6 kJ mot- 1 (using complex concentrations of 0.4 M in both cases). The relative values of the activation energies presumably reflect the relative abilities of the CN and CO2CH3 groups to compete for charge through resonance effects. No changes are observed on heating solutions of the
817
complexes in other types of aprotic solvents (other than eventual complex breakdown). Infrared monitoring of solutions of the complexes in dimethylsulphoxide at concentrations significantly lower than 0.4 M showed much more rapid conversion of delocalized to localized forms of the complexes. At 0.1 M, for example, solutions of the complexes showed bands characteristic of both forms at 25°C (we were unable to measure activation energies for the interconversion of the localized and delocalized forms at concentrations of less than 0.4M due to partial crystallization of solutions at sub-ambient temperatures). It would appear that the presence of the complexes themselves encourages resonance delocalization presumably through an increase in "medium polarity". Our conclusions from the i.r. spectroscopic studies on the nature of the hydrogen bonded complexes can be summarized by the equilibria shown below. In an attempt to learn more about the interaction of the medium with the hydrogen bonded complexes we turned our attention to the use of NMR for the study of these systems. The hydroxyl protons of the Hbonded complexes experience a large shift to lower fields in all types of aprotic solvents consistent with the presence of strong H-bonding. Typical shifts with respect to the parent phenols are 4-5 ppm for I and II in solvents including chloroform (H-D exchange occurs in deuteriochloroform solutions), acetone and dimethylsulphoxide and shifts of up to 7 ppm for IlL The aryl proton chemical shifts of the H-bonded
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818
JAMESH. CLARKet al.
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015
Temperature-l/ K-~x I0 s
016 017 018 0:9
1:0
Medium polarizability (fr*)
Fig. 2. Plots of In k against 1/ T for the interconversion of the two forms of the complexes I ( x ) and III (O) (see text). complexes I and III and those of their anions are all medium dependent. Inspection of the values for the complexes (Table 2) reveal that the solvents do not fall into two distinct classes as they do for the i.r. shifts. In all cases the chemical shifts of the aryl protons of the complexes fall in between those of their respective parent phenols and anions. In an attempt to correlate the observed aryl proton chemical shifts with Taft's solvatochromic parameters [6], we plotted the chemical shifts of the protons ortho to the H-bond (A6ntcoH)) for both I and I I I against the medium polarizability parameter (n*). We also plotted the C(2)-H shifts for the anions of the phenols of I and III against n* for comparison (Figs 3 and 4). For both complexes, and especially I, the proton chemical shifts do not correlate well with it* (R = 0.51 for I and 0.80 for III). The chemical shift-x* correlations for the
Fig 3. Plots of C(2)-H chemical shifts (with respect to the parent phenol) against medium polarizability (~*) for the Hbonded complex I ( x ) and its phenoxide anion (O). Chemical shifts are accurate to + 0.02 p.p.m.
conjugate phenoxide ions are somewhat better (R = 0.84 for I and 0.91 for IlI). Close inspection of the medium-dependency of the'aryl proton chemical shifts of the complexes reveals that the H-bond electron donor (proton acceptor) power of the solvent (~ [6]) also appears to play an important role. Thus a ~olvent such as dichloromethane (high n* but zero ~) generally causes especially low aryl proton chemical shifts for the complex in comparison to those for the phenoxide ions (Table 2). We tested this observation by plotting A6H(COH)against n* + aft using values of a in the range 0-2. Optimum correlations for both I and III occurred with values for a in the range 0.5-1.0 IRmax (I) -- 0.82 and Rmax (III) = 0.91]. These results
Table 2. Proton NMR chemical shiftst (with respect to the parent phenols in p.p.m.) for the aromatic CH protons in the H-bonded complexes and anions of 4-cyanophenol and methyl-4-hydroxybenzoate Solvent (in order of increasing 7t* value) CHCIs 0t* = 0.58; fl -- O) Me2CO (n* = 0.71; fl = 0A8) MeCN (n* = 0.75; fl = 0.31) CH2CI2 (~* = 0.81; i~ = O) Me2NCHO (it* = 0.88; /] = 0.69) (~H2(CH~)2CON (CH3) (n* = 0.92; fl = 0.77) Me2SO (It* = 1.00; fl = 0.76)
Methyl-4-hydroxybenzoate
4-Cyanophenol At~H(COH ) Complex Anion
/~k~H{CN) Complex Anion
/~k~H(OH) Complex Anion
/~k~H(COaCH3) Complex Anion
0.27
0.47
0.26
0.35
0.08
0.47
0.11
0.20
0.38
0.70
0.35
0.52
0.27
0.58
0.18
0.30
0.30
0.68
0.26
0A2
0.31
0.58
0.25
0.39
0.26
0.62
0.24
0.44
0.21
0.61
0.22
0.33
0.45
0.72
0.46
0.61
0.34
0.80
0.22
0.36
0.46
0.89
0.39
0.75
0.43
0.84
0.26
0.41
0.43
0.88
0.35
0.60
0.38
0.86
0.28
0.42
tAll values to +0.02 p.p.m.
Charge delocalization in TBAF complexes 0.9 0
~,
0.8
-~
0.7
0
E
0
0
0
o
0.6 0,. e~
0.5
0
o
0.4 <
x ~ X "x
0.3
XX
0.2
o15
I
0:6
017
0.8
819
hydrogen bonded complexes of fluoride with phenols containing powerful electron withdrawing groups substituted at the 4-position (CN and CO2CH3). This effect is medium dependent so that in the more powerful polar aprotic protophilic solvents the substituent groups can compete with the hydrogen bond for the charge. In other types of aprotic solvent the charge appears to be heavily localized at the hydrogen bond. The activation energy for the partially delocalized-localized conversion is dependent on the nature of the substituent group. Correlations of proton N M R data with solvent parameters are less simple than those based on i.r. data although medium protophilicity (high fl value solvents) is again observed to play an important role.
I
0:9
1.0
Medium polarizability (~*) Fig. 4. Plots of C(2)-H chemical shifts (with respect to the parent phenol) against medium polarizability (7~*)for the Hbonded complex III( x ) and its phenoxide ion (O). Chemical shifts are accurate to + 0.02 p.p.m. suggest that for the H-bonded complexes in solution both specific solvation and non-specific solvation play important roles. CONCLUSIONS Hydrogen bond inhibition of resonance delocalization of charge can be observed in solutions of
Acknowledgements--We are grateful to the SERC for the award of an earmarked studentship and to BDH Chemicals for supporting this work. We are also grateful to various colleagues including E. M. GOODMANfor helpful discussions.
REFERENCES
[1] J. H. CLARK and D. G. CORK, J. chem. Soc. Chem. Commun. 1014 (1984). [2] J. EMSLEY,Chem. Soc. Rev. 91 (1980). [3] J. W. LARSONand T. B. MCMAHON,J. Am. chem. Soc. 105, 2944 (1983). [4] M. J. K A M L E T , C . D I C K I N S O N , T. GRANSTADand R. W. TAFT, J. org. Chem. 47, 4971 (1982). [5] J. H. CLARK,Chem. Rev. 80, 429 (1980). [6] M. J. KAMLET,J.-L. M. ABBOUD,M. H. ABRAHAMand R. W. TAFT,J. org. Chem. 48, 2877 (1983).