- Email: [email protected]
Potentiometric and 13C NMR study of the interaction between boric acid and pyrogallol (1,2,3-trihydroxybenzene)
Poiyhedron Vol. 9, No. 6, pp. 789-793, Printed in Great Britain
1990 0
0277-5387/90 $3.00+.00 1990 Pergamon Press plc
POTENTIOMETRIC AND 13C NMR STUDY OF THE INTERACTION BETWEEN BORIC ACID AND PYROGALLOL (1,2,3-TRIHYDROXYBENZENE) GUIDO CRISPONI* Dipartimento
and VALERIA NURCHI
di Chimica e Tecnologie Inorganiche e Metallorganiche, 09124 Cagliari, Italy
via Ospedale 72,
and MARIA LUISA GANADU and GIUSEPPE Dipartimento
LURINU
di Chimica, via Vienna 2,071OO Sassari, Italy
(Received 3 1 August 1989 ; accepted 9 October 1989)
Abstract-The boric acid-pyrogallol system has been studied in aqueous solution by means of potentiometric methods. Evidence is given for two adducts of 1: 1 and 1: 2 stoichiometry. An analysis of 13CNMR spectra of the 1: 2 adduct is presented.
In this paper we present the characterization of the boric acid-pyrogallol system by potentiometric and 13C NMR methods. This study is part of general research on the interactions between buffer systems and the medium in which they act ; in fact in many cases the importance of these interactions is neglected, even though they can completely distort the results of the systems studied. For example, borate buffer is often used in the study of the enzyme polyphenoloxidase (PPO),’ but it inhibits the reaction because all substrates of the reactions catalysed by PPO, i.e. L-Dopa, catechol, pyrogallol, chlorogenie, caffeic and gallic acid, strongly interact at pH 8.5 with the buffer itself. The reaction of boric acid with diols has been extensively studied both for analytical purposes2-4 (boric acid is transformed into a relatively strong acid by addition of an organic compound containing two or more hydroxy groups and, therefore, can be easily titrated with a strong base) and for structural studies5*6 (boric acid, interacting only with compounds with two hydroxy groups in the cis position or two phenoxy groups in the ortho position, distinguishes between cis- and transdihydroxy compounds and between ortho and meta or para compounds).
* Author to whom correspondence should be addressed.
The existence of only a 1: 1 complex, or both 1: 1 and 1: 2 complexes in such systems has been examined by different authors.“’ In this study we have attempted to characterize reliably the boric acid-pyrogallol system by potentiometric and NMR methods. EXPERIMENTAL NMR studies 13C NMR spectra were determined in D,O solution with a CFT-20 Varian Spectrometer at a probe temperature of ca 37”C, under an argon atmosphere. Chemical shifts are expressed in ppm relative to internal TMS. Typical Fourier transform conditions under which the ’ 'C results were obtained are as follows: spectral width 6000 Hz, data points 8 K, pulse repetition times 0.8-4.0 s, number of accumulated transients lO,O~O,OOO. Potentiometric
studies
The titrations were performed on a pH M 84 Radiometer pH meter, equipped with a Dosimat 655 Metrohm automatic titrator, at an ionic strength of 0.1 M (KNO,) and at 25°C. The standard potential of the electrode and the slope was determined daily by titrations of chloridic acid with
G. CRISPONI
790
300
NaOH, at an ionic strength of 0.1 M (KNO,). All titrations using pyrogallol were made under a nitrogen atmosphere to prevent decomposition. The boric acid and pyrogallol concentrations used in the various titrations were in the range 0.003-0.018 M and their ratio varied from 2 : 1 to 1: 6.
__ c.II
et al.
%
0
1
A ---_
k-_
Calculations
‘\ \
Potentiometric data for the electrode standardization were analysed by the Magec program. lo The ionization constants of boric acid and pyrogallol, and the formation constants of their adducts were calculated by the PSEQUAD program, ” which we slightly modified to present a graphic display of experimental and calculated data. The calculations were performed on a PS II/50 IBM personal computer.
e--o
\B
-300
l_
2
1
\ ‘\c %i
Boric acid, pyrogallol, isopropanol and sec-butano1 were purchased from Fluka ; D20 and NaOD from Merck.
0-h
\ .
‘1,
-u)o
RESULTS AND DISCUSSION
-\--
q;
/OH
HO,;/"\ HO, ,,/"
=
1
2
-_;-- - _ ---_-__=
3
4
Fig. 1. The titration curves (EMF vs the ratio [base added]/ [total pyrogallol] for curve A or [base added]/[total boric acid] for curves B, C and D) are reported for pure pyrogallol 0.003 M (curve A), pure boric acid 0.003 M (curve B) and for the mixtures boric acid 0.022 M-pyrogall01 0.066 M (curve C) and boric acid 0.0033 M-pyrogall01 0.0099 M (curve D).
The potentiometric titrations of H,B03, pyrogall01 and of their mixtures in different ratios allow the evaluation of the ionization constants of the two reagents (reported in Table 1) and the proposal of the equilibria :
R
5
300-
Materials
H,BOa +
3
BaSe
+ HaO+
'OH I
(1) /0,-/O\
HBBO, + \
II
The relative formation constants are also reported in Table 1 and are in good agreement with those of Antikainen and Katila. lz Some differences between our results and the literature values13 for the boric acid ionization constant are noteworthy. Table 1. pK values for the ionization of boric acid and pyrogallol, and log of formation constants for the reactions in Scheme 1
Boric acid Pyrogallol Adducts
+ H30+ + Hz0
R\o/B\o./R
‘OH
K,
K,
9.07kO.03 8.83 + 0.03 -6.4kO.01
10.81 kO.04 -2.8kO.l
The titration curves of the mixtures were tentatively fitted with a simple 1 : 1 complexation
model, but the fit was significantly poorer than that obtained with the scheme above. The mixture of boric acid and isopropanol or secbutanol were also examined under the same conditions ; their titration curves do not show any significant difference to those of free boric acid. This was expected on the basis of the behaviour of boric acid with simple alcohols, but is also permits the exclusion of medium effects for alcohol concentration on H3B03 titration. The titration curves in Fig. 1 and the distribution functions in Fig. 2 require some comments : (1) the titration
graphs
of the pure compounds,
791
Interaction between boric acid and pyrogallol (b)
/A
i
I
’ ,*-\ ‘I ‘\
, /c -__
\a
en
PH
lA
(f)
(d)
I/ I I
C, I
/‘P,
/
\
‘\
//
0
‘l/C
9”--
:
/I ./I
;
\
‘?\
5
PH
Fig. 2. The ratios [free pyrogallol]/[total pyrogallol] (curves A), [l : 2 complex]/[total pyrogallol] (curves B) and [ 1 : 1 complex]/[total pyrogallol] (curves C) are reported as a function of pH, calculated with the equilibrium constants reported in Table 1, for the solutions : (a) Boric acid (B.A.) 0.0033 M, pyrogallol (Pyr) 0.0099 M ; (b) B.A. 0.0033 M, Pyr 0.0033 M ; (c) B.A. 0.013 M, Pyr 0.0066 M ; (d) B.A. 0.022 M, Pyr 0.066 M; (e) B.A. 0.066 M, Pyr 0.066 M; (f) B.A. 0.13 M, Pyr 0.066 M. having similar pK, values, look very different because of the second ionization constant of pyrogallol ; (2) opposite to the behaviour with other diols, the addition of pyrogallol eliminates the inflection in the boric acid curve rather than emphasize it. This is ascribed to two reasons :
(4 the interaction of boric acid with pyrogallol is weaker than with other diols ; (b) pyrogallol exhibits an acid behaviour which is different from the other diols, and this leads to a flattening of the titration curve when an excess of pyrogallol is titrated. The formation of adducts with pyrogallol, therefore, cannot be exploited for the analytical determination of boric acid. The complexes, as can be clearly seen from the distribution curves in Fig. 2, have a sensible weight at pH values above 5. In particular, the 1 : 2 complex, whose existence
has been significantly demonstrated by the potentiometric analysis, exists alone at a concentration of 0.07 mol dmp3 and pH 7.5. A study by 13C NMR spectroscopy has therefore been possible. The 13C NMR spectrum of the pure pyrogallol, shown in Fig. 3(a), exhibits four peaks: from offresonance experiments the two bands at 146.61 and 133.57 ppm were attributed to the three quaternary carbon atoms C(l), C(2) and C(3) ; in particular, from symmetry considerations, the more intense band at 146.61 to C(1) and C(3) carbon atoms, and that at 133.57 to C(2). By analogous considerations the band at 121.67 can be attributed to C(5) and that at 109.69 to the two equivalent C(6) and C(4) carbon atoms. By addition of boric acid, in the reported amounts, six new narrow peaks appear well distinct from the four of pure pyrogallol [see Fig. 3(b) and Table 21. These peaks can be attributed to the complexed form (II) of pyrogallol, in slow exchange with the free form, at the probe temperature of
792
G. CRISPONI
-
et al.
I
I
140
100 PP*
Fig. 3. 13C NMR spectra of pyrogallol from aqueous solutions at pH 7 of (a) pure pyrogallol 0.07 M, (b) pyrogallolO.07 M-boric acid 0.02 M and (c) pyrogallolO.07 M-boric acid 0.14 M.
37°C. Their assignment can be made on the basis of shielding effects for the complexation with boric acid ; it can, in fact, bind only to two hydroxy groups in the ortho position, according to the structure : OH
The peaks at 151.77 and 138.06 ppm can therefore be attributed to the C(3) and C(2) atoms, for the high-field shifts of 5.16 and 4.49 ppm from the bands of the free pyrogallol ; these shifts have been
assigned to a deshielding effect caused by binding with boric acid. The C(1) and C(4) atoms, which are one bond apart from C(2) and C(3), exhibit a downfield shift of -5.52 and - 5.21 ppm from the peaks of pure pyrogallol, according to the values of transmission of substituent effects along an aromatic molecule. ’ 4 C(5) and C(6) show a low shift of - 0.89 and 0.63 ppm, respectively. The slight high-field shift of C(6), opposite to the expected downfield shift of C(5), can be tentatively explained in terms of resonance effects with the neighbouring OH group on the C( 1) atom. These values are in a very good agreement with those reported by Pasdeloup and Brisson’ for the analogous 1: 2 complex of boric acid with cat-
Table 2. “C chemical shifts (in ppm downfield from TMS) of the species present in the solution of boric acid and pyrogallol at pD 7.5
Free pyrogallol
1 : 2 Complex A6
C(l), C(3)
C(2)
C(5)
C(4), C(6)
146.41
133.57
121.67
109.69
C(3)
C(1)
C(2)
C(5)
C(6)
C(4)
151.77 + 5.36
141.09 - 5.32
138.06 +4.49
120.78 -0.89
110.32 +0.63
104.48 -5.21
Interaction
793
between boric acid and pyrogallol
Fig. 4, where the two-fold axis of symmetry contains a chiral centre of stereoisomery.’ ‘j Acknowledgement-This M.P.I.
work has been supported
by
REFERENCES
Fig. 4. The enantiomers of the 1 : 2 complex, showing the two possible relative positions of the -OH groups.
echol, and the shifts of C(2) and C(3) agree with the statement’ ’ that all C-OH carbon atoms are
shifted about 6 ppm downfield in the carbohydrate residues of nucleosides. At higher boric acid concentrations the spectrum of pure pyrogallol completely disappears. At the concentrations used, also with an excess of boric acid, we were not able to detect the resonance of 1: 1 adducts, but this could also be expected on the basis of the distribution curves. The spectra of the 1: 2 complex indicate that the boron forms equivalent bonds with the two pyrogallol moieties in the adduct. In fact no splitting of the signals is observed which implies symmetry of the molecule. The enantiomers of the adduct, recognized as centrally chiral, are shown in
1. G. G. Pinna, persona1 communication. 2. Mellor’s Comprehensive Treatise on Inorganic and Theoretical Chemistry, Supplement, Vol. V-Boron, Part A-Boron-Oxygen Compounds. Longmans, London (1980). 3. N. H. Greenwood and A. Eamshaw, Chemistry of the Elements. Pergamon Press, Oxford (1984). 4. I. M. Kolthoff, E. B. Sandell, E. J. Meehan and S. Bruckenstein, Quantitative Chemical Analysis. Macmillan, New York (1973). 5. W. G. Henderson, M. J. Howe, G. R. Kennedy and E. F. Mooney, Carbohydrate Res. 1973,28, 1. 6. G. R. Kennedy and M. J. How, Carbohydrate Res. 1973, 28, 13. 7. R. Pizer and L. Babcock, Znorg. Chem. 1977, 16, 1677. 8. G. L. Roy, A. L. Laferriere and J. 0. Edwards, J. Znorg. Nucl. Chem. 1957,4, 106. 9. M. Pasdeloup and C. Brisson, Org. Magn. Res. 1981, 16, 167. 10. P. M. May, D. R. Williams, P. V. Linder and R. G. Torrington, Talanta 1982,29f, 249. 11. L. Zekany and I. Nagypal, Computational Methods for the Determination of Formation Constants (Edited by D. J. Leggett), Chap. 11. Plenum Press, New York (1985). 12. P. J. Antikainen and R. Katila, Suomen Kemistilehti B 1971,44,256. 13. E. Hogfeldt, Stability Constants of Metal-Zon Complexes, Part A Compiled for ZUPAC, Pergamon Press, Oxford (1982) ; D. D. Perrin, Stability Constants of Metal-Zon Complexes, Part B. Pergamon Press, Oxford (1979). 14. F. W. Wehrly and T. Wirthlin, Interpretation of Carbon- I3 NMR Spectra. Heyden, London (1976). 15. E. Breitmaier and W. Voelter, ‘%-NMR Spectroscopy, Vol. 5, p. 269. Verlag Chemie, Weinheim (1978). 16. B. Testa, Principles of Organic Stereochemistry (Edited by P. Gassman). Marcel Dekker, New York (1979).
1990 0
0277-5387/90 $3.00+.00 1990 Pergamon Press plc
POTENTIOMETRIC AND 13C NMR STUDY OF THE INTERACTION BETWEEN BORIC ACID AND PYROGALLOL (1,2,3-TRIHYDROXYBENZENE) GUIDO CRISPONI* Dipartimento
and VALERIA NURCHI
di Chimica e Tecnologie Inorganiche e Metallorganiche, 09124 Cagliari, Italy
via Ospedale 72,
and MARIA LUISA GANADU and GIUSEPPE Dipartimento
LURINU
di Chimica, via Vienna 2,071OO Sassari, Italy
(Received 3 1 August 1989 ; accepted 9 October 1989)
Abstract-The boric acid-pyrogallol system has been studied in aqueous solution by means of potentiometric methods. Evidence is given for two adducts of 1: 1 and 1: 2 stoichiometry. An analysis of 13CNMR spectra of the 1: 2 adduct is presented.
In this paper we present the characterization of the boric acid-pyrogallol system by potentiometric and 13C NMR methods. This study is part of general research on the interactions between buffer systems and the medium in which they act ; in fact in many cases the importance of these interactions is neglected, even though they can completely distort the results of the systems studied. For example, borate buffer is often used in the study of the enzyme polyphenoloxidase (PPO),’ but it inhibits the reaction because all substrates of the reactions catalysed by PPO, i.e. L-Dopa, catechol, pyrogallol, chlorogenie, caffeic and gallic acid, strongly interact at pH 8.5 with the buffer itself. The reaction of boric acid with diols has been extensively studied both for analytical purposes2-4 (boric acid is transformed into a relatively strong acid by addition of an organic compound containing two or more hydroxy groups and, therefore, can be easily titrated with a strong base) and for structural studies5*6 (boric acid, interacting only with compounds with two hydroxy groups in the cis position or two phenoxy groups in the ortho position, distinguishes between cis- and transdihydroxy compounds and between ortho and meta or para compounds).
* Author to whom correspondence should be addressed.
The existence of only a 1: 1 complex, or both 1: 1 and 1: 2 complexes in such systems has been examined by different authors.“’ In this study we have attempted to characterize reliably the boric acid-pyrogallol system by potentiometric and NMR methods. EXPERIMENTAL NMR studies 13C NMR spectra were determined in D,O solution with a CFT-20 Varian Spectrometer at a probe temperature of ca 37”C, under an argon atmosphere. Chemical shifts are expressed in ppm relative to internal TMS. Typical Fourier transform conditions under which the ’ 'C results were obtained are as follows: spectral width 6000 Hz, data points 8 K, pulse repetition times 0.8-4.0 s, number of accumulated transients lO,O~O,OOO. Potentiometric
studies
The titrations were performed on a pH M 84 Radiometer pH meter, equipped with a Dosimat 655 Metrohm automatic titrator, at an ionic strength of 0.1 M (KNO,) and at 25°C. The standard potential of the electrode and the slope was determined daily by titrations of chloridic acid with
G. CRISPONI
790
300
NaOH, at an ionic strength of 0.1 M (KNO,). All titrations using pyrogallol were made under a nitrogen atmosphere to prevent decomposition. The boric acid and pyrogallol concentrations used in the various titrations were in the range 0.003-0.018 M and their ratio varied from 2 : 1 to 1: 6.
__ c.II
et al.
%
0
1
A ---_
k-_
Calculations
‘\ \
Potentiometric data for the electrode standardization were analysed by the Magec program. lo The ionization constants of boric acid and pyrogallol, and the formation constants of their adducts were calculated by the PSEQUAD program, ” which we slightly modified to present a graphic display of experimental and calculated data. The calculations were performed on a PS II/50 IBM personal computer.
e--o
\B
-300
l_
2
1
\ ‘\c %i
Boric acid, pyrogallol, isopropanol and sec-butano1 were purchased from Fluka ; D20 and NaOD from Merck.
0-h
\ .
‘1,
-u)o
RESULTS AND DISCUSSION
-\--
q;
/OH
HO,;/"\ HO, ,,/"
=
1
2
-_;-- - _ ---_-__=
3
4
Fig. 1. The titration curves (EMF vs the ratio [base added]/ [total pyrogallol] for curve A or [base added]/[total boric acid] for curves B, C and D) are reported for pure pyrogallol 0.003 M (curve A), pure boric acid 0.003 M (curve B) and for the mixtures boric acid 0.022 M-pyrogall01 0.066 M (curve C) and boric acid 0.0033 M-pyrogall01 0.0099 M (curve D).
The potentiometric titrations of H,B03, pyrogall01 and of their mixtures in different ratios allow the evaluation of the ionization constants of the two reagents (reported in Table 1) and the proposal of the equilibria :
R
5
300-
Materials
H,BOa +
3
BaSe
+ HaO+
'OH I
(1) /0,-/O\
HBBO, + \
II
The relative formation constants are also reported in Table 1 and are in good agreement with those of Antikainen and Katila. lz Some differences between our results and the literature values13 for the boric acid ionization constant are noteworthy. Table 1. pK values for the ionization of boric acid and pyrogallol, and log of formation constants for the reactions in Scheme 1
Boric acid Pyrogallol Adducts
+ H30+ + Hz0
R\o/B\o./R
‘OH
K,
K,
9.07kO.03 8.83 + 0.03 -6.4kO.01
10.81 kO.04 -2.8kO.l
The titration curves of the mixtures were tentatively fitted with a simple 1 : 1 complexation
model, but the fit was significantly poorer than that obtained with the scheme above. The mixture of boric acid and isopropanol or secbutanol were also examined under the same conditions ; their titration curves do not show any significant difference to those of free boric acid. This was expected on the basis of the behaviour of boric acid with simple alcohols, but is also permits the exclusion of medium effects for alcohol concentration on H3B03 titration. The titration curves in Fig. 1 and the distribution functions in Fig. 2 require some comments : (1) the titration
graphs
of the pure compounds,
791
Interaction between boric acid and pyrogallol (b)
/A
i
I
’ ,*-\ ‘I ‘\
, /c -__
\a
en
PH
lA
(f)
(d)
I/ I I
C, I
/‘P,
/
\
‘\
//
0
‘l/C
9”--
:
/I ./I
;
\
‘?\
5
PH
Fig. 2. The ratios [free pyrogallol]/[total pyrogallol] (curves A), [l : 2 complex]/[total pyrogallol] (curves B) and [ 1 : 1 complex]/[total pyrogallol] (curves C) are reported as a function of pH, calculated with the equilibrium constants reported in Table 1, for the solutions : (a) Boric acid (B.A.) 0.0033 M, pyrogallol (Pyr) 0.0099 M ; (b) B.A. 0.0033 M, Pyr 0.0033 M ; (c) B.A. 0.013 M, Pyr 0.0066 M ; (d) B.A. 0.022 M, Pyr 0.066 M; (e) B.A. 0.066 M, Pyr 0.066 M; (f) B.A. 0.13 M, Pyr 0.066 M. having similar pK, values, look very different because of the second ionization constant of pyrogallol ; (2) opposite to the behaviour with other diols, the addition of pyrogallol eliminates the inflection in the boric acid curve rather than emphasize it. This is ascribed to two reasons :
(4 the interaction of boric acid with pyrogallol is weaker than with other diols ; (b) pyrogallol exhibits an acid behaviour which is different from the other diols, and this leads to a flattening of the titration curve when an excess of pyrogallol is titrated. The formation of adducts with pyrogallol, therefore, cannot be exploited for the analytical determination of boric acid. The complexes, as can be clearly seen from the distribution curves in Fig. 2, have a sensible weight at pH values above 5. In particular, the 1 : 2 complex, whose existence
has been significantly demonstrated by the potentiometric analysis, exists alone at a concentration of 0.07 mol dmp3 and pH 7.5. A study by 13C NMR spectroscopy has therefore been possible. The 13C NMR spectrum of the pure pyrogallol, shown in Fig. 3(a), exhibits four peaks: from offresonance experiments the two bands at 146.61 and 133.57 ppm were attributed to the three quaternary carbon atoms C(l), C(2) and C(3) ; in particular, from symmetry considerations, the more intense band at 146.61 to C(1) and C(3) carbon atoms, and that at 133.57 to C(2). By analogous considerations the band at 121.67 can be attributed to C(5) and that at 109.69 to the two equivalent C(6) and C(4) carbon atoms. By addition of boric acid, in the reported amounts, six new narrow peaks appear well distinct from the four of pure pyrogallol [see Fig. 3(b) and Table 21. These peaks can be attributed to the complexed form (II) of pyrogallol, in slow exchange with the free form, at the probe temperature of
792
G. CRISPONI
-
et al.
I
I
140
100 PP*
Fig. 3. 13C NMR spectra of pyrogallol from aqueous solutions at pH 7 of (a) pure pyrogallol 0.07 M, (b) pyrogallolO.07 M-boric acid 0.02 M and (c) pyrogallolO.07 M-boric acid 0.14 M.
37°C. Their assignment can be made on the basis of shielding effects for the complexation with boric acid ; it can, in fact, bind only to two hydroxy groups in the ortho position, according to the structure : OH
The peaks at 151.77 and 138.06 ppm can therefore be attributed to the C(3) and C(2) atoms, for the high-field shifts of 5.16 and 4.49 ppm from the bands of the free pyrogallol ; these shifts have been
assigned to a deshielding effect caused by binding with boric acid. The C(1) and C(4) atoms, which are one bond apart from C(2) and C(3), exhibit a downfield shift of -5.52 and - 5.21 ppm from the peaks of pure pyrogallol, according to the values of transmission of substituent effects along an aromatic molecule. ’ 4 C(5) and C(6) show a low shift of - 0.89 and 0.63 ppm, respectively. The slight high-field shift of C(6), opposite to the expected downfield shift of C(5), can be tentatively explained in terms of resonance effects with the neighbouring OH group on the C( 1) atom. These values are in a very good agreement with those reported by Pasdeloup and Brisson’ for the analogous 1: 2 complex of boric acid with cat-
Table 2. “C chemical shifts (in ppm downfield from TMS) of the species present in the solution of boric acid and pyrogallol at pD 7.5
Free pyrogallol
1 : 2 Complex A6
C(l), C(3)
C(2)
C(5)
C(4), C(6)
146.41
133.57
121.67
109.69
C(3)
C(1)
C(2)
C(5)
C(6)
C(4)
151.77 + 5.36
141.09 - 5.32
138.06 +4.49
120.78 -0.89
110.32 +0.63
104.48 -5.21
Interaction
793
between boric acid and pyrogallol
Fig. 4, where the two-fold axis of symmetry contains a chiral centre of stereoisomery.’ ‘j Acknowledgement-This M.P.I.
work has been supported
by
REFERENCES
Fig. 4. The enantiomers of the 1 : 2 complex, showing the two possible relative positions of the -OH groups.
echol, and the shifts of C(2) and C(3) agree with the statement’ ’ that all C-OH carbon atoms are
shifted about 6 ppm downfield in the carbohydrate residues of nucleosides. At higher boric acid concentrations the spectrum of pure pyrogallol completely disappears. At the concentrations used, also with an excess of boric acid, we were not able to detect the resonance of 1: 1 adducts, but this could also be expected on the basis of the distribution curves. The spectra of the 1: 2 complex indicate that the boron forms equivalent bonds with the two pyrogallol moieties in the adduct. In fact no splitting of the signals is observed which implies symmetry of the molecule. The enantiomers of the adduct, recognized as centrally chiral, are shown in
1. G. G. Pinna, persona1 communication. 2. Mellor’s Comprehensive Treatise on Inorganic and Theoretical Chemistry, Supplement, Vol. V-Boron, Part A-Boron-Oxygen Compounds. Longmans, London (1980). 3. N. H. Greenwood and A. Eamshaw, Chemistry of the Elements. Pergamon Press, Oxford (1984). 4. I. M. Kolthoff, E. B. Sandell, E. J. Meehan and S. Bruckenstein, Quantitative Chemical Analysis. Macmillan, New York (1973). 5. W. G. Henderson, M. J. Howe, G. R. Kennedy and E. F. Mooney, Carbohydrate Res. 1973,28, 1. 6. G. R. Kennedy and M. J. How, Carbohydrate Res. 1973, 28, 13. 7. R. Pizer and L. Babcock, Znorg. Chem. 1977, 16, 1677. 8. G. L. Roy, A. L. Laferriere and J. 0. Edwards, J. Znorg. Nucl. Chem. 1957,4, 106. 9. M. Pasdeloup and C. Brisson, Org. Magn. Res. 1981, 16, 167. 10. P. M. May, D. R. Williams, P. V. Linder and R. G. Torrington, Talanta 1982,29f, 249. 11. L. Zekany and I. Nagypal, Computational Methods for the Determination of Formation Constants (Edited by D. J. Leggett), Chap. 11. Plenum Press, New York (1985). 12. P. J. Antikainen and R. Katila, Suomen Kemistilehti B 1971,44,256. 13. E. Hogfeldt, Stability Constants of Metal-Zon Complexes, Part A Compiled for ZUPAC, Pergamon Press, Oxford (1982) ; D. D. Perrin, Stability Constants of Metal-Zon Complexes, Part B. Pergamon Press, Oxford (1979). 14. F. W. Wehrly and T. Wirthlin, Interpretation of Carbon- I3 NMR Spectra. Heyden, London (1976). 15. E. Breitmaier and W. Voelter, ‘%-NMR Spectroscopy, Vol. 5, p. 269. Verlag Chemie, Weinheim (1978). 16. B. Testa, Principles of Organic Stereochemistry (Edited by P. Gassman). Marcel Dekker, New York (1979).