88 (1969) RECUEIL 1361

541.852/.854: 541.863/.865: 543.94

ELECTROCHEMICAL REDUCTION OF AZA AROMATICS. Part II Protonation products of di-aza aromatics

BY

D. VAN DER MEER Chemical Physics Laboratory, Twente University of Technology,

Enschede (The Netherlands)

The formation of proton products plays an essential part in the explanation of the reduction mechanism of the di-aza aromatics. in dimethylformamide (DMF). By recording cyclic polarograms of the di-aza aromatics and com- paring them with those obtained from the protonated products of the di-aza aromatics it is sometimes possible to identify the proton products which are formed in the reduction process. From the experimental evidence and from calculations by means of the HMO method it can be shown that protonation of the mononegative ions is the first step in the dimerization of quinazoline, phthalazine and pyrimidine.

Introduction In Part I of this series 1 it was shown that the protonation of the mono-

negative ions gave a qualitative explanation of the reduction process of the di-aza aromatics in DMF. Assuming that water is the dominant proton donor, the following scheme for the reduction process can be given:

first wave: R + e Z R - R- + Hz0 + RH + OH- RH + e * RH-

R- + e z$ R2- second wave: R2- + 2HzO + RHz + 2OH-

By means of cyclic triangular voltammetry the reduction process can be investigated and also the reoxidation of the reduced species 2 . Moreover, the products formed in the reaction steps following the reduction process can be studied 3-5.

D. vun der Meer and D. Feil, Rec. Trav. Chim. 87, 746 (1968). R. S. Nicholson and I . Shain, Anal. Chem. 36, 706 (1964). R. Dietz and M . E. Peover, Trans. Faraday SOC. 62, 3525 (1966). Z. Galus, H . Y. Lee and R. N . Adams, J. Electroanal. Chem. 5, 17 (1963). Z. Galus and R. N. Adams, J. Phys. Chem. 67, 862 (1963).

1362 D. van der Meer

By adding acid to the solutions of the aza aromatics we obtain mono- and dihydro cations of the aza aromatics. From a comparison of the cyclic voltammograms of these compounds with the pattern of the anodic branch of the cyclic voltammograms of the neutral di-aza compounds, it is possible to identify the protonated products formed according to the above scheme.

Experimental The polarographic measurements were made with DMF as solvent and tetraethyl-

ammonium perchlorate as supporting electrolyte. The three-electrode polarographic cell was a water-jacketed one, thermostatted at 25.0 f 0.01 "C. The polarograms and cyclic voltammograms were obtained with a Beckman Electroscan TM 30. The po- tentials were measured against a Ag/AgCl electrode with a potential of - 0.04 volt vs. the Kumar-Pantony electrode 6.

To keep the circumstances as constant as possible, the polarographic cell was placed in a glove-box. The latter was dried by recirculating air over Molecular Sieve Linde A5 and its moisture level was 50-100 ppm, measured with a hygrometer (Panametrics model 1000). If no water is added to the solution, the water concentration is rather difficult to measure, but the upper limit certainly does not exceed 0.02 % (about 10 mmoles litre-1).

To investigate the influence of mercury on the El,2 values of the di-aza aromatics, the El,, values are not only obtained using a dropping mercury electrode @ME, regulated droptime 1.94 s), but are also determined using a rotating platinum electrode (RPE).

The platinum electrode consisted of 0.5 mm diameter platinum wire sealed into the axis of a thick-walled glass capillary, ground flat; used as a rotating electrode, its speed of rotation was 600 r.p.m.

The cyclic voltammograms were recorded using a DME (droptime about 20 s, meas- ured in open circuit) and also using a stationary platinum electrode (the same platinum electrode without rotation).

The results obtained using the DME are given in Table I. The scan range was from - 0.20 volt vs. the Ag/AgCl electrode to about 0.20 volt beyond the half-wave potential of the first wave. The cathodic peak currents (ip,.J given in Table I are obtained with the stationary platinum electrode, because this electrode has the advantage of a constant surface. The values of ip,c/l/v were measured with scan rates (v) of 0.100, 0.200 and 0.500 volt s-l. Only the values for a scan rate of 0.100 volt s-l and 0.500 volt s-l, are reported, because the 0.200 volt s-1 values do not show anything new.

The EPts and E p , e values correspond to the anodic and cathodic peak potential, respectively, of the first reduction step.

The extra Ep,B values correspond to additional anodic peaks visible on the anodic branch, and due to consecutive reactions.

The influence of protons on the reduction was investigated in the presence of 0.78, 1.56, 74 and 185 mmoles of perchloric acid litre-1. Two stock solutions of perchloric acid in DMF were made: 0.930 and 0.0195 mole litre-l. With these stock solutions the dilute acid solutions were obtained. The values obtained with 74 mmoles litre-l are given in Table I1 and are determined using the DME.

6 G. P . Kumar and D. A . Pantony, "Polarography 1964", Vol. 2, pp. 1061-1075, Ed. by Graham J. Hills, MacMillan, London-Melbourne, 1966.

Electrochemical reduction of aza aromatics. Part II 88 (1969) RECUEIL 1363

Discussion of the results The following aspects will be discussed :

a. The results obtained with cyclic voltammetry.

b. The reduction of the cations of the protonated di-aza aromatics and a

c. Identification of protonated products by analysis of the anodic

d. Some remarks about the reduction mechanism.

correlation of the Ella values with the results of Hiickel calculations.

branch of the cyclic voltammograms.

a. The results obtained with cyclic voltammetry As can be seen from Table I the Ep,c-El/z value for all compounds is

about 0.060 volt, whereas the Ep,c-Ep,a value is about 0.120 volt for compounds which show an anodic peak at a scan rate of 0.100 volt s-1. Sometimes the Ep,a can be obtained only at scan rates of 0.500 volt s-1

(the influence of the scan rate is discussed below). The accuracy of these values is not so good because the peaks are ill-defined. These values are placed in parentheses.

For a reversible one-electron transfer we expect a difference of 0.057/na (ref. 2; about 0.060 volt) between EPIC and Ep,a. As we have measured these values with a recorder there is always the possibility of a shift in Ep values due to the slowness of the recorder. Moreover, there can be a shift in these values owing to the uncompensated i . R drop, even with the three- electrode assembly 7 .

For anthracene the same differences between the peak voltages and the Ella values are found, especially when a small amount of water is added (see Table I). Hoijtink et al. 8 have shown that the first reduction step for anthracene is reversible in the polarographic sense, so we assume from this similarity that the first electron transfer for the di-aza compounds is also reversible. The second electron transfer is in all cases irreversible (no anodic current is obtained) which is possibly due to a fast protonation reaction (see for aromatic hydrocarbons ref. 3).

To explain the results for the ip,c/l/v values we consider the reaction:

e k e R R- --f RH RH-

[]I PI [31

R. 5’. Nicholson, Anal. Chem. 37, 667 (1965). A. C. Aten, C. Biithker and G. J . Hoijtink, Trans. Faraday SOC. 55, 324 (1959).

1364 D. van der Meer

As long as k/v is small, the reversible electron transfer of reaction [l] is not much influenced by consecutive reactions 9. According to the for- mula 2 , iP,&v = const x C x D1/a (C = concentration, D = diffusion coefficient), the value of ip,c/vv is independent of the scan rate in the absence of consecutive reactions.

If the klv value is high we can expect a decrease in ip,c/vv when the scan rate is increased 9 . In this case much of the reduced product will be proton- ated, so the Ep,a value will be difficult to measure.

This picture offers qualitatively a good explanation of the results. We may conclude that the k values for pyridazine, pyrazine and cinnoline are quite high, so the small amount of water present in the solvent is sufficient to observe a decrease of ip,c/vv when increasing the scan rate.

A more quantitative evaluation of these protonation rate constants with the aid of classical polarography will be published shortly.

For quinazoline and pyrimidine no anodic peaks are observed, whereas the ip ,c / lv value remains practically constant. A possible explanation for the reduction of these compounds is given in Section d .

The Ell, values obtained using the RPE are almost the same (maximum difference 0.02 volt) as those obtained with the DME, so no indication is obtained of a specific effect of the DME.

b. The reduction of the cations of the protonated di-aza aromatics and a correlation of the E,, values with the results of Hiickel calculations

Recording polarograms using the DME, a reduction wave appears less negative than the first reduction wave, when small amounts of acid are present (1 to 2 mmoles litre-I), whereas the diffusion current of the original reduction wave diminishes. The new reduction wave is rather smoothed out and shows several irregularities. This wave is possibly due to reduction of RH+ products, complicated by hydrogen evolution and absorption phenomena 10. As this wave is ill-defined, no half-wave potentials for the products can be given.

If more acid (CH+ = 74 mmoles litre-l) is added to the solution, new well-defined waves appear (see Table 11). The diffusion currents of these waves for pyridazine, pyrazine, cinnoline and phthalazine increase when more acid (up to 185 m o l e s litre-1) is added and sometimes the Ell, values shift (maximum 0.05 volt) towards less negative values. For pyrimidine no good polarograms could be obtained.

R. S. Nicholson and 1. Shain, Anal. Chern. 37, 178 (1965). lo J. Heyrovsk.9 and J . KGfa, Grundlagen der Polarografie, p. 383 et seq., Akademie-

Verlag, Berlin, 1965.

Electrochetnical reduction of aza aromatics. Part I I 88 (1969) RECUEIL 1365

These well-defined waves obtained in the presence of 74 or 185 mmoles of perchloric acid litre-1 may certainly be ascribed to a reduction of the dihydro cations (RH$+). Assuming the diffusion coefficient of RHz2+ to be the same as the diffusion coefficient .of the neutral di-aza molecules, it may be concluded that the reduction of dihydroquinazoline, dihydro- quinoxaline and dihydrophenazine is a two-electron transfer.

The cyclic voltammograms obtained at the stationary platinum electrode clearly show anodic peaks for these compounds, whereas these peaks have disappeared using the mercury electrode.

From the dihydroquinoxaline and dihydrophenazine monopositive ions well resolved ESR spectra were obtained by Barton and Fraenkel l1. The monopositive ions were obtained by electrochemical reduction of the di- hydro cations at a mercury pool electrode. As these results are not in agreement with the two-electron transfer as indicated above, it needs further attention.

The Ell, values obtained with the RPE are not well-defined for phenazine, cinnoline and quinoxaline because of the lack of a diffusion plateau. These Ell, values may be approximated by taking the mean value of Ep,c and Ep,a as obtained using the stationary platinum electrode. For the above compounds the Ell, values obtained in this manner are always slightly (maximum 0.05 volt) more negative than the corresponding values at the mercury electrode. A pronounced effect of mercury on the Ell, values is not observed.

As the existence of monopositive ions of a number of these compounds has been established by means of ESR, we assume that the waves correspond to a reversible one-electron transfer followed by a chemical reaction. In such cases the electron transfer seems to be irreversible. Although a cor- relation of these values with quantumchemical calculations appears to be rather speculative, we see from Fig. 1 that a reasonable correlation is obtained.

The root for the lowest vacant orbital of RH$+ (designated by x) is taken as a measure for the electron affinity and the calculation is performed by taking the Coulomb integral of the protonated nitrogen atoms as a m = ac + 2.0 PCC (see ref. 12). The value of the resonance integral is taken as / ~ C N = BNN = PCC. The straight line is given by:

El,, == 1 . 5 2 ~ - 0.61 with a correlation coefficient of 0.890. Only the value for 1, haphthyridine is rather far removed from the straight line.

l1 B. L. Barton and C. K. Fraenkel, J. Chem. Phys. 41, 1455 (1964). l2 A. Streitwieser, Jr., “Molecular Orbital Theory”, H VII, John Wiley and Sons, Inc.,

New York, 1962.

1366 D. van der Meer

Excluding the value for 1,5-naphthyridine the folIowing relation is obtained (Fig. 1):

Ell, = 1 . 7 1 ~ - 0.66 with a correlation coefficient of 0.974.

X

I -0,500

Fig. 1. El,, values in presence of 74 mmoles litre-' of HClO4 vs. x.

We have not attempted to get a better fit of the data, as the qualitative trend is not much altered by changing the value of QNH.

c. IdentiJkation of protonated products by analysis of the anodic branch of the cyclic voltammograms

As protonation is expected to play an important part, the voltammograms of the compounds after addition of water were taken as well.

From Table I we can see that all compounds (except pyrimidine) show anodic peaks at less negative potentials than the peak due to the oxidation of the mononegative ions. Fig. 2 shows the cyclic voltammogram of 1 , 5-

Electrochemical reduction of aza aromatics. Part II 88 (1969) RECUEIL 1367

naphthyridine without addition of water and Fig. 3 the cyclic voltammo- gram after addition of water.

188

068

176

Fig. 2.

I

Cyclic voltammogram of 1 ,Snaphthyridine without addition of water.

I 1.85

- - E (volts) \ I

Fig. 3. Cyclic voltammogram of 1 , Snaphthyridine after addition of water.

When the scan is reversed before the first wave (R + e 7t R-) starts, no peaks are observed in either case. Without addition of water the extra peaks are only observed when the voltage scan is extended beyond the second wave (R- + e RZ-). After addition of water this phenomenon is already observed when scanning reversal takes pIace before the second wave starts, be it that the extra peaks are better developed after extending the scan beyond the second wave (see Fig. 3).

Tab

le I

--Ella

fir

st

wav

e (v

olt)

1.76

1.

72

1.64

1.

59

1.82

1.

79

1.64

1.

61

1.97

1.

94

2.32

2.

25

2.18

2.

05

2.12

2.

06

1.15

1.

12

1.98

1.

96

Com

poun

d '

1

a

2.27

2.

27

2.60

3.

50

2.26

2.

60

2.40

2.

55

, 2.

45

2.60

2.64

2.

64

3.52

4.

85

3.00

4.

50

1.94

2.

03

2.10

2.

10

1.

quin

azol

ine

(1,3

-dia

za-n

apht

hale

ne)

2. ci

nnol

ine

(1,2

-dia

za-n

apht

haIe

ne)

3. 1,

5-na

phth

yrid

ine

(1 ,S

diaz

a-na

phth

alen

e)

4. qu

inox

alin

e (1

,4-d

iaza

-nap

htha

lene

)

5.

phth

alaz

ineb

(2

,3-d

iaza

-nap

htha

lene

)

6.

pyrim

idin

e (1

,3-d

iazi

ne)

7.

pyrid

azin

e (1

,2-d

iazi

ne)

(1 ,4

-dia

zine

)

(9,lO

-dia

za-a

nthr

acen

e)

8.

qyra

zine

9.

phen

azin

e

10.

anth

race

ne

14.9

14

.9

14.3

18

.2

16.0

17

.1

C

(mm

ole

litre

-1)

<0.0

2 0.

90

<0.0

2 0.

90

<0.0

2 0.

90

1.50

1.

50

1.55

1.

55

1.54

1.

54

1.54

1.

54

1.48

1.

48

1.57

1.

57

1.42

1.

42

1.47

1.

47

1.35

1.

35

1.54

1.

54

16.0

13

.8

14.0

13

.8

16.6

16

.6

18.2

18

.9

17.6

22

.0

13.8

13

.8

11.6

14

.5

<0.0

2 0.

90

<0.0

2 0.

90

<0.0

2 0.

90

<0.0

2 0.

90

t0.0

2 0.

90

<0.0

2 0.

90

<0.0

2 0.

90

Firs

t w

ave

-EP*

c (v

olt)

1.82

1.

78

1.69

1.

63

1.88

1.

85

1.70

1.

67

2.03

2.

01

2.38

2.

31

2.23

2.

11

2.18

2.

12

1.20

1.

18

2.02

2.

01

-

-EP+

(v

olt)

-

-

1.57

(1

.40)

1.76

1.

73

1.59

1.

55

1.92

(1

.82)

-

-

(2.0

7)

-

2.07

-

1.10

1.

07

1.84

1.

91

Extra

0.70

0.

68

1.35

0.

60

-

0.46

1.37

0.

70

-

0.70

-

-

0.65

0.

57

0.70

1.

20

0.90

0.

85

-

0.58

a

ZI =

(id

)I/C

rn2/

3t'le

pA

litre

mm

ole-

1 m

g-'/3

s'

ls;

(id)I

= d

iffus

ion

curr

ent

first

wav

e;

rna/3

t'le =

0.8

28 m

g2/3

s-'/a

, rn

mea

sure

d in

DM

F w

ith o

pen

circ

uit.'

Ex

tra

= - 0.

57 v

olt.

I

-EP

,a

(vol

t)

0.50

0.

45

0.37

0.

35

-

0.31

0.80

0.

25

-

0.38

-

-

-

-

-

0.60

0.80

0.

20

-

-

' = 0

.100

vo

lt s-

1

14.9

14

.9

16.5

22

.0

16.0

18

.2

16.0

16

.0

16.0

17

.6

17.6

17

.6

23.7

32

.0

20.6

29

.2

13.8

13

.8

14.2

16

.1

I

Electrochemical reduction of aza aromatics, Part I I 88 (1969) RECUEIL 1369

-

-

0.39

The scheme proposed offers an explanation of these facts assuming the anodic peaks are due to an oxidation of RH2 in two steps to RH22f. If we use the Ell, values of the dihydro cations (see Table 11) as an indication where the oxidation of RH2 can be expected, we see that for 1,5-naphthy- ridine two Ell, values are given. The first one corresponds to the reduction R H P + e + RH2+ and the second one corresponds to the reduction R H z + + e Z R H z .

Although the extra anodic peaks do shift considerably on addition of water (see Figs. 2 and 3) they can be assigned to an oxidation of RH2 in two steps to RHz2+. A comparison of Tables I and 11 shows that for pyri- dazine, pyrazine, cinnoline and quinoxaline the extra anodic peaks may also be ascribed to oxidation of protonated products.

-0.2591

+0.1833

-0.1169

Table I1

Polarography and cyclic voltammetry in presence of HClO4 (74 mmoles litre-l)

-

-

- - -

0.60

-0.04

-

No.

-

1.

2.

3.

4.

5.

6.

7.

8.

9.

-

+0.1650

-0.1859

-0.4142

0.0000

0.0000

+0.3379

Compound

quinazoline

cinnoline

1, 5-naphthyridine

quinoxaline

phthalazine

pyrimidine

pyridazine

pyrazine

phenazine

C (mmole li tre-l)

1.50

1.55

1.54 1.54 .

1.54

1.48

1.52

1.42

1.47

1.35

--R/.2

(volt)

1.15

0.35

0.45 0.73 C

0.32

0.86

-

0.78

0.69

0.08

4.40

2.90

2.20 2.07

4.60

1 so -

0.80

0.80

4.06

-EP.c (volt)

1.22

0.41

0.51 0.75

0.39

0.95

-

0.86

0.76

0.15

I

a See Table I .

b III == (id)II/C m2/3t1/e p A litre mmole-l mg-2/3s'/a; (id)Ir = diffusion current second wave; for m2/3t'l@, see Table I .

c Ell, value second wave.

1370 D. van der Meer

The number of electrons involved in these oxidation processes is not established, although the oxidation of the protonated products of cinnoline and quinoxaline seems to proceed in two one-electron steps. We have shown in Section b, that a one-electron reduction of the dihydro cation of quinoxaline is possible, so oxidation in two steps of RH2 to RHz2+ seems quite probable.

For phenazine some extra peaks are obtained before the oxidation of RHz is attained. These peaks are possibly due to oxidation of RH or RH- products. The extra anodic peak at - 0.70 volt for phthalazine is probably due to oxidation of RH2 but the peaks at - 0.57 volt and - 0.35 volt can hardly be ascribed to protonated products.

The potentials of the extra anodic peaks for quinazoline are not in agreement with the potential where the oxidation of the neutral dihydro product would be expected. As there is not the slightest indication that the reduction of quinazoline requires more than one electron and the formation of RH2 takes two electrons, it seems quite improbable that RHz products are formed. For pyrimidine no extra anodic peaks are observed. The reduction of pyrimidine is also a one-electron process, so the absence of oxidation of RH2 is to be expected.

d. Some remarks on the reduction mechanism As the mononegative ions of pyrimidine and quinazoline cannot be

detected (no anodic peak current in the cyclic voltammograms and no ESR spectra) we must conclude that the mononegative ions of these compounds are quite unstable.

A bimolecular deactivation of R- (R- + R- --f inactive product) would show a marked dependence of the Ell, value on concentration 13, which was not found (e.g. for pyrimidine the El,, value is constant within 0.002 volt in a concentration range of 0.25-3.5 m o l e s litre-I), whereas a bi- molecular deactivation would predict a shift of at least 0.020 volt.

So we conclude that the mononegative ions of quinazoline and pyri- midine are deactivated by a monomolecular reaction 14. The most probable deactivation seems to be a protonation of the mononegative ion by DMF, because there is no indication of water influencing the diffusion current. In this reaction RH radicals are formed.

To see whether the R- or RH radicals are more susceptible to radical attack we have calculated the electron density of the lowest vacant orbital of the R molecule and of the RH+ ion (Table 111).

13 See ref. 9, p. 367. 14 See ref. 9, p. 365.

Electrochemical reduetion of aza aromatics. Part I1 88 (1969) RECUEIL 1371

T a b l e I11

Comparison of the C atoms with the highest electron density in the lowest unoccupied orbital of the R molecule and the RH+ ion

No.

1.

2.

3.

4.

5.

6.

7.

8.

9.

Compound

quinazoline a

quinazoline b

cinnoline c

cinnoline d

I ,S-naphthyridine

quinoxaline

phthalazine

pyrimidine

pyridazine

pyrazine

phenazine

R molecule

C42 = 0.3152

C42 = 0.3152

C42 = 0.2063

C42 = 0.2063

C B ~ = C ~ ~ = 0.1816

c22=C32 = 0.1245

ci2=c42 = 0.2223

C42=Ce2 = 0.3326

C42=C52 = 0.1865

c22=c32=c52=cs2 = 0.1118

C12=C42=Cs2=Cg2 = 0.0608

RH+ ion

C42 = 0.3596

C42 = 0.4295

C42 = 0.2813

C42 = 0.1230

C42 = 0.2655

Cz2 = 0.2432

Ci2 = 0.3730

C42 = 0.3645

C42 = 0.2541

Cz2=Csa = 0.2082

C32=C72 = 0.0777

* a N 1 = a c -I- 2.08. b a ~ 3 = a c + 2.08. 0 a N 1 = ac + 2.08. d a N 2 = aC + 2.08.

In the calculation of the molecules we have used for the Coulomb integral a N = ac + 0.7 ,!? and for the resonance integral ~ C N = /INN = pcc. The calculation of the RH+ ions is performed with a m = ac + 2.0 p , in which the hydrogen atom is attached to the nitrogen atom with the lowest index and a value of aN = ac + 0.7 f i for the other nitrogen atom. The value of j9 is the same as for the molecules. For cinnoline and quina- zoline both possibilities are given.

The value of QNH = ac + 2.0 ~ C C is rather arbitrary, but the qualitative statements remain unchanged when the value of ai-w is varied. Only the value of the C atom with the highest electron density of the unpaired electron is given.

1372 D. van der Meer, Electrochemical reduction of aza aromatics. Part II

The electron density of the unpaired electron seems to be a good criterion for the participation of the R- and RH radicals in radical reactions. The method is analogous to the one suggested for neutral molecules by Fukui et al. 15, who .use, as index for participation in radical reactions the sum of the electron density of the highest occupied and the lowest unoccupied orbital.

If we use the electron density of the unpaired electron in the R- and RH radicals in this manner, we can deduce from Table I11 that the RH radicals are generally more susceptible to radical reactions than the mononegative ions. Moreover, the electron density of the RH radicals of pyrimidine, quinazoline and phthalazine is high compared with the other compounds, but these compounds also show an irregular behaviour in the anodic scan of the cyclic voltammograms.

Our conclusion that quinazoline and phthalazine undergo fast dimeriza- tion reactions after the uptake of an electron and a proton, is supported by the work of Lund, who investigated the polarographic reduction of quinazoline 16 and phthalazine 17 in alkaline solution, for which he assumes the same reaction mechanism.

The dimerization of pyrimidine after the uptake of a proton and an electron is mentioned in ref. 18.

In a subsequent publication 19 a rather high protonation rate constant for quinazoline and pyrimidine is predicted, which is in agreement with the monomolecular deactivation of the mononegative ions by means of protonation.

Acknowledgement

nuscript. I wish to thank Prof. Dr. D. Feil for criticizing and reviewing the ma-

(Received May dth, 1969).

l5 K. Fukui, T. Yonezawa, C. Nagata and H . Shingu, J. Chern. Phys. 22, 1433 (1954). l6 H . Lund, Acta Chern. Scand. 18, 1984 (1964).

Idem, Discussions Faraday SOC. 45, 193 (1968). l8 3. Janik and P. J. Elving, Chem. Rev. 68, 295 (1968). l9 To be published.