IR and PMR studies of proton transfer in complexes of 4-R-pyridine N-oxides with trifluoroacetic acid in solutions

71

Advances in Molecular Relaxation and Interaction Processes, 15 (1979) 71-79 0 ElsevierScientificPublishingCompsny,Amsterdam-PrintedinTheNetherlands

IR AND PMR STUDIES OF PROTON TRANSFER TRIFLUOROACETIC

B. BRYCKI,

IN COMPLEXES

OF 4-R-PYRIDINE

N-OXIDES

WITH

ACID IN SOLUTIONS

2. DEGA-SZAFRAN

Institute

of Chemistry,

(Received

18 September

and M. SZAFRAN

A. Mickiewicz

University,

60780 Poznan

(Poland)

of complexes

of 4-R-pyridine

1978)

ABSTRACT Infrared

and proton magnetic

N-oxides

with trifluoroacetic

benzene,

chlorobenzene,

correlated proton

with

transfer

resonance

spectra

acid have been investigated

1,2-dichloroethane

ApKa and interpreted

in terms of proton

is found at ApKa = 1.68 kO.05.

ence on 50% proton

transfer.

Solvent

in rigorously

and chloroform.

transfer

The solvents

effect on chemical

dry

The results

were

effect.

50%

used have no influ-

shift is discussed.

INTRODUCTION Interactions

between

have been discussed

Brijnsted acids

such systems must involve between

the neutral

a bewildering

species

of proton

from acid to base

A-H...B


Stable hydrogen Clusters

bonded

(1) strong hydrogen

solvents

These tell us that

bonded

the ion pairs produced

complexes

by the transfer

(1)

complexes

appear due to stabilization are formed.

but often difficult

by solvent molecules.

The structure

to demonstrate.

bonds are made and broken,

species

of these complexes

However,

and charges

work of Barrow et al [7-131 attacked

in complexes

by infrared

of nitrogen

spectroscopy.

studies of PMR and UV spectroscopy, tric polarization, Several

[l-6].

in equation

are created,

and solvated.

The pioneering

vent media

(B) in organic

' A-...H-B+

are easy to postulate

bonded

and reviews

array of hydrogen

and between

of two and more molecules

neutralized

(AH) and bases

in several monographs

This spectroscopic conductance,

and several other experimental

types of structures

other and with the monomeric

are specified acid

the nature

bases with oxygen

of hydrogen

acids in nonpolar work is supported

colligative techniques

properties,

by

dielec-

[1,4-6,14,15].

which may be in equilibrium

(AH) and base

sol-

with each

(B) over wide concentration

range

The simplest

(l-l(r6M).

(HA)~, B...HA,

B...HA...HA,

We restricted

B+H(H2A3)-.

Strong evidence equivalent are present

our consideration

solvents

of higher

Furthermore,

are:

(AHA)-B+,

to the monovalent

(BHA)2_l,, [5,16].

in all organic

of these species

(B+HB)A-,

acid and base. aggregates,

with

traces of water which

created other complications

[17,18], i.e.

of hydrates.

Undoubtedly,

the number

stoichiometry,

nature

ture data indicate to base

defined

B+H...A-...HA,

may also be cited for the presence

stoichiometries

formation

and most clearly

B+H...A-,

that proton

(nitrogen bases)

Mainly

a particularly

from acid

solutions

useful

depends

techniques

35C1 NQR

technique

(carboxylic

on

Litera-

acids and phenols)

and the solid state has been

the following

[19-241, IR [7-13,25-291,

Recently

in solution

of acid, base and solvent used.

transfer

in organic

studied most frequently. dielectric

and type of species presented

and concentration

[30], PMR

were applied: [14,29,31].

for investigating

proton

transfer

seems to be PMR spectroscopy. We are now concerned

to examine

to oxygen bases in four organic N-oxides

exhibit

an unusual

and strong hydrogen

bond

the proton

solvents.

tendency

transfer

from trifluoroacetic

acid

The oxygen bases such as 4-R-pyridine

to form stable complexes

with very short

[32-431.

EXPERIMENTAL Pyridine

N-oxide

(m.p. 220-222') ing pyridines was prepared

(b.p. 137-140"/15

-pyridine

by the nitration

N-oxide

was prepared

N-oxide

[44].

form-carbon were:

were obtained

acid.

Trifluoroacetic standard use.

methods.

Benzene,

anhydrous

N-oxides

acid was distilled Chloroform

chlorobenzene

a dead-stop

titration end point,

N-oxide

from 4-nitro-

(m.p. 225-226'

Other complexes

from P2O5.

the anhydrous

(5:l) and adding from chloropoints

were prepared

Solvents were purified

titration

by

acid.

through alumina

immediately

were passed

sieves, and used within

dec.)

[45].

The melting

obtained.

in trifluroacetic

using a Radiometer

(m.p. 161') (m.p. 167'

were recrystallised

and 1,2-dichloroethane

stored over Linde 4A molecular Karl Fischer

were prepared

tetrachloride

analyses

was passed

N-oxide

4-Chloro-

form by dissolving

precipitates

and correct

of the correspond-

and dimethylamine

of chloroform-carbon

The resulting

tetrachloride,

various

[443.

N-oxides

in a crystalline

4-Me- m.p. 34", 4-NMe2- m.p. 81-83".

dissolving

4-Nitropyridine

4-N,N-dimethylaminopyridine N-oxide

(m.p. 184'), and 4-cyano-

by oxidation

N-oxide

(m.p. 130') -pyridine

in a small volume

trifluoroacetic

[44].

of pyridine

from 4-chloropyridine

Two complexes

mm Hg), 4-methyl-

were prepared

with ~30% Hz02 in AcOH

dec.) and 4-phenoxypyridine

N-oxides

by

prior to

through

alumina,

2 weeks of purification.

assembly,

showed that the samples of solvents

type TTAl/KF,

used generally

with

73 contained

All solutions

less than 0.001% water.

of nonaqueous

solutions

were made

and complexes

were determined

in dry box.

by weighing

were prepared

Concentrations

out the solutes,

and all transfers of N-oxides,

then making

acid

them up

to known volume. The IR spectra were recorded sodium chloride

cells

All chemical

Model 580 spectrometer

with

(0.17 mm and 0.15 M).

PMR spectra were measured ?2"C.

on a Perkin-Elmer

at 60 MHz on a Varian EM 360 spectrometer

shifts were reported

solvent

and then converted

RESULTS

AND DISCUSSION

downfield

relative

at 24

to the resonance

of

to 6 values.

IR spectra The spectra similar. ties.

of the complexes

investigated

in benzene and chloroform,

Most of the bands show the same frequencies

In the other bands the maximum

difference

and very similar

are very intensi-

is up to 2 cm-l in frequency

and 5% in intensity. All the spectra towards

lower frequencies

note the absence indicates

TABLE

investigated

contain

with increasing

of any distinct

the absence

the carbonyl

band, which

ApK, (Table 1).

band in the 1700-1650

of unperturbated+

carboxylate

shifts slightly

It is important

to

cm-' region which

ions

(Fig. 1)

tt .

1

The vC=O frequencies acid

in complexes

of 4-R-pyridine

N-oxides

with trifluoroacetic

vc=o R

AeK,*

NO2 CN Cl H Me OPh NMe2 * ApK

-1.93 -1.40 0.13 0.56 1.06 2.44 3.65

a

= pK

B+H

The spectra

benzene

chloroform

1777 1778 1772 1765 1762 1759 1746

1777 1776 1771 1763 1762 1757 1746

- pKAH; pKBtH values

of complexes

broad and complex

were taken from ref. 46, pKAH = 0.23.

of the weakest N-oxides

bands in the 3000-2450

cm-l

(R = NO2 and CN) exhibit

(bands A and B, [32]) and 2000-

+ In acid salts of type A the short hydrogen bond strongly perturbated structure of carboxylate anion and only carbonyl band is observed e.g. potassium hydrogen bis(trifluoroacetate) 148,571. tt Two strong bands (1650 and 1550 cm-') (Fig. lc) are attributed to the ring stretching vibrations. Similar strong bands are observed in the spectrum of a free base.

74

1800 cm-l (band C, 1321) regions (Fig. la). The higher frequency absorption shows fine structure and four bands near 2870-2820, 2720, 2540 and 2460 cm-' can be distinguished. The bands A, B and C are believed to arise from the 26OH, &OH + yOH, and 2yOH overtone-combinations,respectively. As the ApKa increases (R = Cl) the absorption in the 3000-1800 cm-l region decreases and simultaneously a broad absorption appears near 1050 cm-1 (band D, [31]) (Fig. lb).

b

c

Fig. 1. IR spectra of complexes of 4-R-pyridine N-oxide with trifluoroacetic acid in benzene. Absorption in the regions 1820-1800, 1490-1450 and 1050-1020 R = N02, - - - R * CN; cm-" are drawn from chloroform solutions (a) R=NMep. R = Cl, - - - R - Me; (c) - - - R = OPh, (b) In the complexes of 4-Me and 4-OPh-pyridineN-oxides the absorption in the 3000-1800 cm-l region becomes very weak but band D moves to 800 cm-' region and becomes much broader, implying a large increase of integrated intensity

(Fig. lb and lc). At the extreme end of the series is the complex of 4-NMe_7pyridine N-oxide. Here, band D is again shifted to higher frequency and its intensity decreases. Simultaneously the absorption in the 3000-1800 cm-' region becomes again stronger (Fig. lc). The spectra demonstrate that with increasing of ApK, the hydrogen bond shortening at first and then again lengthening. The shortest hydrogen bond in these series exhibits in the complexes of 4-Me- and C-OPh-pyridineN-oxides. Both spectra are of type (ii) 1471. This suggests that the O...0 distance should be close to 2.4 i [48,49].

PMK spectra A plot of the observed chemical shift, of the hydrogen bonded protons, 6, against ApK, consists of two intersecting straight lines (Fig. 2). The similar correlation between 6 and ApKa has been observed previously for complexes of triethylamine,N,N-dimethylaniline [14] and pyridines [29] with acetic acids.

wm 19 -

16 -

-2 -1

0

1

2

3

L ApK,

Fig. 2. 6 of hydrogen bonded protons plotted against ApK, for complexes of 4-Rpyridine N-oxides with trifluoroaceticacid (0.3 M) in rigorous dry (1) benzene, (2) chlorobenzene, (3) 1,2-dichloroethane,and (4) chloroform.

76

It has been shown that the point of intersection corresponds to equimolar equilibrium of the molecular complex and hydrogen bonded ion pair [14,29,31] (eq. 1). The observed gradual upfield shift with decrease of ApK, on the first line is caused by the molecular complexes, whereas the gradual upfield shift with increase of ApKa on the second line is attributed to the hydrogen bonded ion pairs (Fig. 2). It is widely accepted that the position of a proton resonance signal is shifted toward lower applied fields when the hydrogen atom takes part in a hydrogen bond [SO]. The magnitude of the shift can be taken as a qualitative measure of the hydrogen bond strength [50]. The very large down-field shift observed for some of our complexes e.g. when R = Me and OPh, is suggestive of the formationof a very strong hydrogen bond. Table 3 summarizes the available data on very strong hydrogen bonds. It is seen from Table 3 that our complexes are slightly weaker. The calculated chemical shifts for equimolar equilibrium between molecular complexes and hydrogen bonded ion pairs (Table 2) are of the same order of magnitude in relation to data in Table 3.

Thus it can be expected that the O...H...O dis-

tance for a complex with ApKa = 1.68 should be comparable to that in hydrogen bis(trifluoroacetate)ion.

TABLE 2 Correlation of chemical shift (8, ppm) of hydrogen bonded proton in 4-R-pyridine N-oxides complexes with trifluoroaceticacid with ApKa 50% proton transfer

6 = f(ApKa)

Solvent

h ApK, d 1.68*

ApK, & 1.68

ApK a

1.62 20.11

Benzene

2.27

6 = 17.9130 + 1.3559 ApKa

Chlorobenzene

5.61

6 = 17.6378 + 1.3397 ApK,

6 = 21.5335 - 0.8740 ApK a 6 = 21.7507 - 1.0331 ApKa

1,2-Dichloroethane 10.37 6 = 17.4417 + 1.3609 ApK, Chloroform

4.70

6 = 16.9287 + 1.4622 ApKa

6

1.73

19.96

6 = 21.8091 - 1.2537 ApK,

1.67

19.71

6 = 21.6851 - 1.3033 ApK,

1.72 19.44

average 1.685 kO.051 * Correlation coefficient varies from 0.996 to 0.999.

Effect of solvent The different chemical shift values calculated for the equimolar equilibrium of equation (1) given in Table 2 deserve further comment. It is important to note that the crossing point in the investigated complexes is roughly independent of solvent used.

In weaker complexes, e.g. nitrogen bases - oxygen acids,

77

TABLE 3 Hydrogen bond distances and chemical shifts of hydrogen bonded protons (ppm) of some complexes O..H..O length in, 6 crystal (A) ppm

Complex

1. Sodium hydrogen maleate

2. Potassium hydrogen maleate 3. Tetrabutylammoniumhydrogen maleate 4. Potassium hydrogen phtalate 5. Tetrabutylammoniumhydrogen phtalate 6. Tetrabutylammoniumhydrogen furan-3,4-dicarboxylateion 7. Potassium hydrogen bis (trifluoroacetate) a. Cesium hydrogen bis (trifluoroacetate) 9. 4-MethylpyridineN-oxide. trifluoroaceticacid

10. 4-PhenoxypyridineN-oxide. trifluoroaceticacid

aRef.54.

Solvent

Ref.

19.95 20.32 DMSO 20.5 CH2C12

51 52 53

20.07 DMSO 21.0 a2c12

51 53

20.3

CH2C12

53

19.7

net liquid 54

19.8

net liquid 54

19.33 19.10 18.75 18.43 19.40 19.23 18.75 18.51

C6H6 C6H5Cl ClCH2CH2Cl CHC13 C6H6 C6H5Cl ClCH2CH2Cl CHC13

this this this this this this this this

work work work work work work work work

b Ref.48.

crossing point strongly depends on solvent permittivity 114,551; this point shifts to lower ApKa values with increasing of solvent permittivity. This is caused by shift the equilibrium (1) toward ion pair [eq. (l)] with increasing of solvent permittivity [21]. The data collected in Table 2 strongly suggests that the solventuseddoesnotchange the equilibrium (1). This conclusion is in agreement with IR spectra. The observed chemical shifts depend on the solvent used. On increasing solvent permittivity the signal shifts to higher fields. The solvent shift is usually interpreted by the solvent anisotropy contribution, the Van der Waals term, the electric field (reaction field) dependence, the bulk susceptibilitycontribution, and the specific solute-solventand solute-solute interaction (see for example ref. 55, 56). Complexes of heterocyclic N-oxides with halogenoacetic acids form dimers [eq. (2)] and hydrates req. (311 [38,39,41,43]. Kl \ 2(BHA) -@HA)2 K2 BHA + H20%-

(2) \ BHA.H20

(3)

78

Formation of dimers shifts signal to lower field, but hydrates to upper field. In our measurements the concentration of water is very low; it is 2.7 x 10m3 times lower in comparison to complex concentration. Whole contribution from hydrates to chemical shift is less than 0.01 ppm. The effect of dimers on chemical shifts is large. Recently it has been shown 1161 that Kl decreases with increasing of SOlV. solv. This suggests that the observed upfield shifts with increasing E E . is in part caused by dimers. In aprotic solvents linear trend is observed solv. between 6 and E (correlationcoefficient varies from 0.9 to 0.999). The largest upfield shift is in chloroform. This is probably caused by an additional interaction of chloroform with the complex through hydrogen bond:

//O”‘HCC13 CF3C, O-H+NO

3

v= R

ACKNOWLEDGEMENTS This work is supported by the Polish Academy of Sciences (MR-1.9.4.4.3).

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