Influence of excess base and solvents on hydrogen bond and proton transfer in complexes between dichloroacetic acid and substituted pyridines

101

Journalof Molecular Liquids, 33 (1987) 101-117 ElsevierScience PublishersB.V., Amsterdam- Printedin The Netherlands

INFLUENCE OF EXCESS BASE AND SOLVENTS ON HYDROGEN BOND AND PHOTON TRANSFER IN COIUSLEXESBETWEEN DICHLOROACETIC ACID AND SUBSTITUTEDPYRIDINES

PIOTR BARCZYNSKI, ZOFIA DEGA-SZAFRAN and tiIRO%AW SZAFRAN Department

of

Chemistry,

A. hclickiewicz University, 60780

Poznafi (Poland) (Received

1 July

1986)

AEQTRACT IR end 'H Nl6R spectra are reported for mixtures of dichloroacetic l:l,

acid

and pyridines

with

various

acid-base

patios

(2:1,

and 1:4) in benzene and dichloromethane. Both the

continuous absorption in IR spectra and the chemical shift of hydrogen bonded protons are affected when an excess of base is added to the equimolar mixture. The results can be explained by the incomplete reactions (e.g, 3-CN, 4-CN-, and 3-Br-pyridines), forcation of B+H(A'***HA) complex (e.g. Pyridine and its methylderivatives) and homoconjugation (e.g. 4-NLie2-pyridine). The intensity variation of the continuous absorption and the chemical shift with solvent is very similar to that predicted by theory, and may provide evidence that the dipole of the hydrogen bond interacts with the reaction field from the environment. The centre of gravity, VH,

snd the chemical shift, 4,

is correlated with pKa and

discussed with respect to the proton transfer reaction.

0167-‘7322/87/$03.50

0 1987 ElsevierScience PublishersB.V.

102

INTRODUCTION It is generally accepted that the equimolar mixtures of pyridines and acetic acids can be described by the following equilibria (1) - (3) [l-18]: AH + B n AHB Free

+

A-H-*-B

ions

_

A-. . .&ES+

(1) (2)

(Am), e

H-bonded ion pairs

t_

Triple ions

(3)

Recently [19] we have shown that, in the case of the equimolar mixture of 2,4,6_trimethylpyridine and dichloroacetic acid, additionally the 2:l complex, involving two molecules of acid, is formed (equilibrium 4). B+H(A- .*.HA) + B

#

2 A-O.&B+

(4)

In this paper we extend the investigation to other pyridines to get further evidence of the equilibrium (4). EXPERIAGENTAL Liquid pyridines were stored for at least a week over calcium chloride, then distilled and stored over molecular sieves (42). Solid pyridines were recrystallized and stored over P205. Dichloroacetic acid was refluxed over P205 (ca 5% by weight) and distilled. Complexes were prepared by adding stoichiometric amounts of dichloroacetic acid to a diethylether solution of pyridines and cooled in a C02-acetone bath. The resulting precipitates were recrystallized from diethylether and dried over P 0 2 5'

103

The melting points were: 3-CN, 14OC; 4-CN, H, 5O-51°C; 3-Le, 10°C; 4-Me, 75-76OC; ‘&5-h2,

39-40°C;

2,4-bk2,

68-70°C,

2-Lie-4-O&,

29-31°C;

83-84OCi

4-Niue2,

68-69OC;

3,5-Mez,

2,6-l&2, T36-137’C.

3-Br,

18OCi

57-58OCi 28OC;

2,4,6-h3,

4-Nhe2-pyri-

dine forms also a complex with the 2:l acid:base ratio, m.p. 80°C (from EtOAc). Solvents were purified by standard methods and stored over molecular sieves. All solutions were prepared and all' transfers of nonaqueous solutions were made in a dry box. Ii3spectra were recorded on a Perkin-Elmer 580 spectrophotometer using cells with KBr windows. The centre of gravity of the graphically reconstructed bands were obtained as qH =JA(Y)\ldL)/j-A(g)dQ,

where A = loglo(Tbl/T), Tbl and T

are the transmittance of the base line and complex, respectively. The compensation of solvent in the 4000-1600 cm-' region and the broad absorption of acid in the 1600-400 cm-' region were used as a base line (for the details see ref. 181, 'H NNiR spectra were measured on a Tesla ES 467 spectrometer. All chemical shifts were taken downfield relative to the resonance of the solvents and then converted into &values by adding 7.17

and

5.3,

respectively, to benzene and dichloro-

methane solutions. RESULTS AND DISCUSSION The dimerization constant of dichloroacetic acid in benzene at 25'C is KD = 27.1 1 mol-' [20]. The IR spectrum of CHC12COOH as a 0,2 ki solution in benzene is shown in Figure la. The bands at 1779 and 1745 cm-' are assigned to the C=O stretching frequencies of the monomer and associated species, respectively,

104

LO 20a

60 :

60.

t" ._ E t

LO-

1

20 b

3500

3000

2500

2000

MOO

1600

1600

1200

1000

800

600

cm-’

Figure

1. Infrared spectra of dichloroacetic acid in benzene

and its

miXtm?S

with a) 3-bromopyridine in benzene, b) 4-methyl-

pyridine in benzene, c) 4-N-dimethylaminopyridine in dichloromethane;

.-.... dichloroacetic acid; mixtures with acid-base

mol ratio: (2:t) --

- - ; (1:l) -

; (1:4) -.-.-.

6%

1700 1660

1 730 1 735 1 1

2,6-Ik2

?-Me -4 -0?,1e

2,4,64Ie

7

CIICl,COO’N(Ru)4

~-NT&Z?

2,4-Xe

1775sh

1

1700

1 735

3,5-?&e

3

1725

?,S-Me2

1725

1 735 1 742

4-?Ie

2

1732

1 743

3 -Me

1775s.h

3460

1775s.h

1738

1775sh

3460

1 746

1775sh

3460

H

1745

CH2C12

1746

1 745

C

1 746

3405

1775sh

1779

-

3 C=O ( bonded)

acid

1745

3-3r

)

dichloroacetic

CH2C12

(free

of

1745

3405

4-C1J

1779

c6-D.5

3 C=O

I:1

CB2C12

mixtures

1745

3460

CII2C12

and

1:4

1 745

3405

3-CNL

C6D6

COOS)

‘6%

1: 1 and

C D 6 6’.

of

3 OH (free

in

3405

pyridines

IR bands

CHC1,COO~

Substituent

substituted

C!mrncteristic

1:4

with

1730

1730

1735

1740

1740

1740

1740

1740

1740

1740

1670

1680

1700

1715

1720

1725

1728

171-o

1742

1738

CH2C12

3 C=O (bonded)

M

I %YH6

f

I

I

0.2

106

while the bands at 3405 and 3000 cm-' have been assigned the corresponding hydroxyl stretching modes. The monomeric acid is also observed in the 2:l and 1:l mixtures of dichloroacetic acid with 3-CN-, 4-CN-, and 3-Br-pyridines, and is absent in the 1:4 mixtures (Figure ?a, Table I). Spectra of all investigated mixtures of dichloroacetic acid with 3-CN-, 4-CN-, and 3-Br-Pyridines show the 3C=O band at the same frequency as in acid. However, the band is broader and less intenae.

I

I

I

I

-. ._ .L

_

I

I

.. \

. ‘f \, * 1.

:

1’

i. . * \

I

‘:

I

.’ .,

.

..I .I

!, : ,’ :!‘Ii:

I

5-L

I

1600 1700

I

1600 1700

I

1800 1700

1 1% 1600 1700

cm-'

Figure 2.

Infrared spectra of tetra-n-butylammoniu

acetate f-..-.--,

dichloro-

in CH2C12), dichloroacetic acid (-------,

in CH2C12) and its mixtures with pyridines: a> 4-cyano-pyridine, b) pyridine, c) 2,5_dimethylpyridine, d) 2,4,6-trimethyl-

pyridine. Kixtures with acid-base mol ratio: (1:l) . . . . . . , (l:?)

in CH2C12 -

in benzene

, (1:4) in CH2C12 ----.

107

(Figure la and 2a). The observed change in shape is undoubtedly caused by overlap with the absorption of several species and cont$nuous absorption. These results indicate that in a 1:1 mixture of dichloroacetic acid with 3-CN-,

4-CN-,

and 3-b-

pyridines a simple molecular complex (A-H***B) is in equilibrium with a monomeric and associated (dimeric) acid. From the dielectric measurements [4] one can estimate fraction of 'A-*e-H-B+is very low (less than 5%)

that the and it is

difficult to recognize it in TT( spectra. II-I the literature the 2:f complex @-HA-m)

is postulated [3,7]. If such specie8

is formed in our ca8e, then it8 absorption is similar to the associated acid. Furthermore, as Figure la shows, when an exce88 of base is added to a 1:l mixture the formation of a simple complex AHB becomes more complete. Figures ?b-c illustrate the interaction of dichloroacetic acid with more basic pyridines. The lack of absorption at 3405 cm-' indicates that the acid is completely bonded even in the 2:7 acid-base mixture. Spectra of the 2:? mixtures of dichloroacetic acid with pyridine and its more basic derivatives show broad absorption in the 2550

-1 region. This cm

absorption belongs to the B+H(A-***HA) species since the intensity of this absorption increases with basicity of pyridines. Another characteristic feature is a strong

SC=0

-1 and the absorption in the 1660-1610 cm-1 band at 1745 cm region attributed to

Va8COO- and the ring stretching vibration8

in protonated pyridines. The intensity of the former band slightly decreases, but the latter increases when pyridine becomes more basic. The bands at 1640 and 1660 cm-' should be ascribed to the ring stretching vibrations and to

ua,coo-

108

in the 1:l hydrogen bonded ion pair (A-***H-B+), respectively [1,2]. The band at 1610

cmc'

belong to the

\3asCOO- vibration

in the 2:l homoconjugate ion pair (B+H***A-***HA) since it is -1 to the lower frequency relative to that in shifted ca 45 cm tetraalkylammonium salt. Such lowering is caused by addition of the second acid molecule to the ionic pair [21]. The band -t can be ascribed to the 3C=O vibration in the at 1745 cm dimer, as well as to the molecular complex (A-H..=B) and to the second acid molecule in B+H+..A- **OHA. When the concentration of pyridines increases a systematic decrease in -1 region is observed. intensity of the band in the 2500 cm Simultaneously, the intensity of the absorption in the regions 2000 and 900-400 cm-' increases (Figure lb). Further changes are observed in the carbonyl-carboxy region. This region is, however, less useful because of the strong interference from an excess of pyridines (Figure 2b). To shift the equilibria almost entirely to the 1:l complex represented as protometric equilibrium (5) requires a three-fold excess of base. A-H.**B

+

A-.O.H_S

(5)

4-NUe2-pyridine shows some differences (Figure lc). As shown in Figure lc, the addition of 4-Nke2-pyridine to the 1:l acid-base mixture caused an increase in the intensity of -1 regions. Since two such the bands in the 2500 and 2000 cm bands are observed in the homoconjugated ion

(B+H***B) [22]

this result is most easily compatible with the formation of the

(D+Hl*.B)A- complex. Another possibility, the interaction

of acid with the lone pair of the IWe

group, is not the case.

Both 'H NIV&measurements and the electronic characteristics

109

of 4-NMe2-pyridine determined by m0 calculations at a semiempirical level, indicate that both protonation and methylation should occur at heterocyclic nitrogen [23]. Our results demonstrate that many possible species are in equilibrium in solutions and that the maintenance of a given stoichiometry in no way guarantees the quantitative formation of a complex species corresponding to that composition. Dichloroacetic acid shows a relative large tendency to form 2:l complexes with pyridines. Arnett and Chawla [7 ] have

shown

that the formation of 2:l trifluoroacetic acid - pyridines complexes are by far the more exotermic process. Our previous results [lg ] demonstrated that trifluoroacetic acid forms the 1:l complex when mixed with a stoichiometric amount of

2,4,6_trimethylpyridine; formation of the 2:l complex requires an excess of acid. Figures 2 and 3 show the solvent effect on IH spectra. Variation of the absorption caused by solvent depends on the basicity of pyridines. In the case of weaker pyridines, when the electric permittivity of the solvent increases, a systematic decrease in the intensity of absorption in the 2500 cm-' region is observed, and simultaneously the intensity of absorption in the 1000-400 cm-' region increases (Figure 3a). In the spectra of stronger bases absorption changes on the reverse side on increasing of the electric permittivity (Figure 3~). The observed intensity variation of the continuous absorption is very similar to those observed in complexes between trifluoroacetic acid and pyridines [17] and to those predicted by ab initio [24] and SCF [25] calculations. According to the theoretical calculations the observed variations in absorption

60 -

60 40 20 t I 3600

3000

2500

2000

1800

1600

lLO0

1200

1000

800

600

cm-’

Figure 3.

Infrared spectra of mixtures of dichloroacetic acid

with pyridines, acid-base mol ratio 1:4, a) 3-bromopyridine, b) 3_methylpyridine, c) 2,6_dimethy$pyridine; in benzene -

, in dichloromethane - - - -O

d..

F . .

c

d.. r-

F . .

**. F

. .

-

d..

F . .

d-t-Q KLL-woT. . . . ?l--C

lnmw

c,.

.

.

Ln

0

M

In F

0

w

w

w

w

t-

* d-. W.

.

.

s c

,”. CI

112

as the field increases can be explained either by a change of symmetry in the potential surface or a change in the hydrogen bond length. The observed variation of intensity caused a change of the gH values (Table 2), which is directly related to the hydrogen bond distance [ 261. Figure 2 shows that with an increase of solvent electric permittivity increases the proton transfer. The most instructive means for comparing the proton transfer reaction comes against pKa (Figures 4 and 5).

from plots of gH and &

Two-line correlations between

gH or c& and pKa have been

o&ib / \?A

1300 lLO0 -

0

1500 -

0\

; 1600 E \" 1700 16 1800 -

0

0

0

A

1900 -

L/

2000 2100 -

\

A

0

/IP ~ I

1

12

\~ I

I

3

L

I

6

I

L

I

6

7

0

I

9,

9

10

PKO

Figure 4.

Plot of the centre of gravity (3,)

against aqueous

pKa values of pyridine bases. Acid-Base mol ratio 1:4; n

in benzene, 0

in dichloromethsne.

113

observed previously for several complexes [18]. The point of intersection corresponds to the equimolar equilibrium of the molecular complexes and hydrogen bonded ion pairs (5%

of PT).

The observed gradual upfield shift of 6;1and shift of ?H to higher wavenumber with decreasing pKa values on the first line is caused by the molecular complexes whereas similar changes with increasing pKa values on the second line reflect an increase in the hydrogen bonded ion pairs. Both I& and 'H NIV;R data in dichloromethane PKa

~6

(E=

for 50% PT. In benzene (E=

Figure 5.

8.9)

give a similar value

2.3) the values are

Plot of the chemical shift of hydrogen bonded

protons (8,) against aqueous pKa values of pyridine bases. Acid-base mol ratio ?:4, n

in benzene,

0

in dichloromethane.

114 slightly different; pKa -

6.75 ('H pJb&) and pKa -

6.5 (IX).

Figure 2 confirms this interpretation. Previously [18] we have reported a correlation between &I and 3, for 55 complexes of pyridines with various acids (see Figure 6 in ref. 18). Points for complexes with dichloroacetic acid show the largest scattering. Using values of 5, and d;Idetermined for a 1:4 mixture improve the correlation (Figure 6). The least-squares equations are: line (a)

d;, (TO.241 = 33.174 - 6.501 x IO-' Qh

(6)

r = 0.994, n = 9

20 19 18 17 16 Ers\ol‘'oll312lllO98-

Figure

6.

Plot of the chemical shift of hydrogen bonded

protons CC&) against the centre of gravity (3,) complex absorption in dichloromethane;a. 0

data from this paper.

of the

data from ref. 18,

115 line (b)

$B (+0.26) = 25.350 - 4.927 x 1O-3 i&

(7)

r = 0.990, n = 29 line cc>

s, (20.19) = 23.525 - 2.556 x 1O-3 GH r = 0.941,

(8)

n = 14

From equation (6) and (7) we can estimate the chemical shift of "free" protonated pyridine snd monomeric acid, respectively (see ref. 18). There are not enough data to join line (b) and (cl.

ACKi;OWLEIiGiLZXTS We acknowledge financial support from the Polish Academy of Sciences (project U.I.9.4.4.4).

REZ'ERFXES 1

G. AU. Barrow, J. Am. Chem. Sot., 78 (1956) 5802.

2

G. V. Gusakova, G. S. Denisov, A. L. Smolyansky, and V. m. Schreiber, Dokl. Akad. Nauk SSSB, 193 (1970) 1065.

3

R. Lindemann and G. Zundel, J. Chem. Sot., Faraday Trans. 2, 68 (1972) 979; 73 (1977) 788,

4

L. Sobczyk and Z. Pawelka, J. Chem. Sot., Faraday Trans. 1, 70 (1974) 832.

5

E. E. Odinokov, A. A. Uashkovsky, V. P. Glazunov, A. V. Iogansen, and B. V. tiassadin, Spectrochim. Acta, 32A (1976) 1355.

6

V. P. Glazunov, A. A. mashkovsky, and S. E. Gdinokov, Spectroscopy Letters, 9 (1976) 391.

7

E. Y. Arnett and B. Chawla, J. Am. Chem. Sot., 100 (1978) 217.

8

S. K. mehta and B. Chawla, Electrochim. Acta, 27 (1982) 9.

9

B. Chawla and S. K. tiehta, J. Phys. Chem., 88 (1984) 2650.

116 10

Z. Dega-Szafran, Adv. i?101. Relax. Interact. Processes, 18 (1980) 61.

11

Z. Dega-Szafran and E. Dulewicz, Adv. mol. Relax. Interact. Processes, 21 (1981) 207-

12

Z.

Dega-Szafrm

and E. Lulewicz, Org. magn. hes., 16 (1981)

214. 13

Z. Dega-Szafran and IL Szafran, J. Chem. Sot., Perkin Trans. 2, (1982) 195.

14

Z. Dega-Szafran and IL Szafran, J. iisl. Liquids, 25 (1983) 109.

15

Z. Dega-Szafran and E. Dulewicz, J. Chem. Sot., Perkin Trans.

2, (1983) 345. 16

Z. Bega-Szafrm (1984)

17

and I& Szafran, J. Chem.

SOC.,

Cl-m.

coma,

1470,

Z. Dega-Szafran, E. Dulewicz, and L

Szafran, J. U-m.

SOC.,

Perkin Trans. 2, (1984) 1997. 18

P. Barczyfiski, Z. Dega-Szafran, and L. Szafran, J. CA-m. so:., Perkin Trans. 2, (1985) 765.

19

P. Barczyfiski, Z. Dega-Szafran, and L

Szafran, J. Chem.

sec., Perkin Trans. 2, submitted. 20

J. Steigman and W. Cronkright, Spectrochim. Acta, 26A (1970) 1805.

21

G. V. Guaakova, G. S. Denisov, and A. L. Smolyansky, Opt. Spektrosk., 32 (1972) 922.

22

B. Brzezinski and G. Zundel, J. Chem. Sot., Faraday Trans. 2, 72 (1976) 2127.

23

G. Barbieri, R. Benassi, R. Grandi, U. L;.Pagnoni, and F. Taddei, Org. magn. Res., 12 (1979) 159.

24

A. Janoschek, E. G. Weidemann, and G. Zundel, J. Chem. Faraday Trans. 2, 69 (1973) 505.

SOC,,

117

25

A. Hayd, E. G. Weidemsnn, and G. Zundel, J. Chem. Phys., 70 (1979) 86.

26

A. Novak,

Struct. Bonding (Berlin), 18 (1974) 177.