B U L L E T I N D E L ' A C A D É M I E P O L O N A I S E D E S S C I E N C E S S é r i e des sciences chimiques V o l u m e X V I I I , No. 7, 1970

ORGANIC CHEMISTRY

D-Mannitol Hexanitrate as Electron Acceptor in Charge Transfer Complexes

by

T. URBAŃSKI, В. H E T N A R S K I and W. POŁUDNIKIEWICZ

Presented ny T. URBAŃSKI on March 2, 1970

As shown in our former papers [1, 2] nitric esters possess the property of electron acceptors and hexanitrates of hexahydroxylic alcohols seem to be relatively strong electron acceptors when tetramethyl-/>-phenylenediamine was used as an electron donor. Among the nitrate in question D-mannitol hexanitrate seems to be parti­cularly interesting, in agreement with former findings of one of us [3].

The present paper is dedicated to a more detailed examination of charge transfer (CT) complexes formed by D-mannitol hexanitrate (MHN) and tetramethyl-^--phenylenediamine (TMPD). As pointed out before [1,2], when dissolved in 1,2--dichloroethane, dichloromethane, or benzene, both components give high-intensity violet colour produced by two absorption bands: at 570 and 620 nm, characteristic of the T M P D cation [1,2,4].

Both bands disappear with time and a new band at near 450 nm becomes evident (Fig. 1). Measurements based on the method of continuous changes [5] indicate that in the violet solutions two moles of T M P D originally reacted with one mole of M H N . Next, after the 450 nm band was established, the donor/acceptor complex was formed with the molar ratio 1:1. Fig. 2 gives Job curve taken 24 hours after the preparation of the solution, i.e. when the original bands at 470 and 620 nm practically disappeared.

It should be pointed out that the change of the colour and, therefore, the for­mation of a new yellow (absorption at 435—450 nm) complex occurred only in solvents with a relatively low dielectric constant. When M H N and T M P D were dissolved in methanol the original violet colour proved to be relatively stable and the solution decolorised only after several days; no yellow complex was produced there.

To examine the nature of the molecular additive compounds, experiments were carried out in two-phase liquid system composed of one polar and another non--polar solvent, e.g. acetonitriles and н-heptane. After и-heptane was added to the violet solution of M H N and T M P D in acetonitrile, л-heptane layer almost imme­diately turned yellow (absorption maximum at 435—450 nm). The intensity of the band increased with time.

[405]

406 T. U r b a ń s k i et al.

The same was observed when methanol and 1,2-dichloroethane were used as a polar and a low-polarity solvent, respectively.

We postulated that the yellow compound is a charge transfer complex and cal­culated equilibrium constants by means of the graphic method based on a modified equation of Benesi—-Hildebrand applied for equimolar concentrations of com­ponents [9] (Fig. 3).

С 1 1 (1)

Di

1.0,

as

0.6

0.4

I ID CK-E

700 A f n m )

Fig. 1. Change with time of electronic spectra of M H N and T M P D in 1,2-dichloroethane at 23°C. Concentration of each of the com­ponents, с = 0.002 mol/I ; after mixing, 2 — after 1

24 hours

1 — immediately hour, 3 — after

Fig. 2. Job curve of the M H N — T M P D complex in 1,2 dichlorocthane after 24hrs.; D — Diffe­rence between the absorption of solution of the complex and absorption of the components measured separately, С — concentration of T M P D in molar fractions of the sum of M H N plus

T M P D

where : С — concentration of components, in mol/1. CK— equilibrium constant of the formation of the complex, in 1/mol. £ — molar extinction coefficient, in l/cm mol,

D — absorbance of the solution at /.=450 nm. The values of cK in dichloromethane and 1,2-dichloroethane at different tem­

peratures are collected in Table I.

T A B L E

Solvent Dielectric К2з° лэр«

constant К2з° лэр«

C H 2 C 1 2 9.08 17.3 13.4 12.0 C1CH 2 CH 2 C1 10.36 59.3 33.3 27.6

The figures in the Table indicate that the formation constant of the complex is lowered with temperature — the feature typical of the CT complexes.

u-Mannitol Hexanitrate as Electron Acceptor in CT Complexes 407

The increase of the dielectric constants increases the values of CK. This indicates a close connection between the concentration of the T M P D cation (formed imme­diately after the solutions of T M P D and M H N were brought together) and the concentration of the complex absorbing at 435—450 nm. When a solvent with high dielectric constant is added the complex dissociates into ions. This was proved by the decrease of the electric resistance in /г-heptane and in mixtures of /г-heptane with 1,2-dichloroethane and methanol (Fig. 4).

Conclusions

We are suggesting the diagram for our reactions as follows:

C H , O N 0 2

I ( C H O N 0 2 ) 4 + 2

I C H 2 O N 0 2

C H 3 СНз

N I

I N

/ " \ СНз СНз

fast C H , O N O , " I

( C H O N O , ) 4

I C H , O N 0 2

20

+ 2

(A)

C H 3 ^ ^ С Н з

N

N / * \ CHc N C H 4 _

" С Н з ^ ^ Н Н з "

Ń I

N

с н У \ : h 3

С Н з ^ ^ С Н з

Ń I

slow

+

N / " \ СНз с н 3

СНз СНз \ /

N II

/ ч

N

_ С Н 3 ^ \ т Н 3 .

2©

(В)

C H 2 O N 0 2 • I

( C H O N O , ) 4

I C H 2 O N 0 2

20

+

(С)

" С Н з ^ с н 3

N

N

С Н з ^ \ : Н 3 .

29

C H 2 O N 0 2 "

( ΠO N 0 2 ) 4

I C H 2 O N 0 2

29

С Н з ^ C H 3 "

N

N

СНз СНз.

2©

(D)

When M H N and T M P D are brought together in a solvent a fast reaction leading to the violet salt (A) occurs (see the diagram). It was formed by two T M P D cations and one M H N anion. Tn a low-polarity solvents (dichloromethane, 1,2-dichloro-

408 T. U r b a ń s k i et al.

Q*103{Q)

Fig. 3. Graphie presentation of figures from Eq. (I) Fig. 4. Change of resistance of the for the complex M H N / T M P D (л = 450 nm) in 1,2 solution of M H N in n heptane/1,2 dichloroethane; d — absorption of the reaction mixture dichloroethane against concentra­

tion of methanol added

ethane) the system undergoes disproportionation (B) (a slow reaction confirmed by TLC) and a new yellow complex (C, D) is formed. Here the ratio of compo­nents used was 1:1.

It was found that the formation of the complex (C, D) can be accelerated by the addition of an excess M H N .

It should be pointed out that in the earlier work of one of us [3] the molar ratio of the acceptor (MHN) to the aromatic donor was found to be 1:2 as in salts (A). The high ionisation potential of the formerly used donors and probably the experi­mental conditions (no solvent and a relatively high temperature) favoured the for­mation of only one system of (A) type.

The divalent T M P D cation present in (C) is most likely stabilized by a weak anion of mannitol hexanitrate. This is contrary to the existing statement [10] of its low stability due to the presence of the strong perchlorate anion.

However, we were unable to isolate the solid complex as it readily decomposes.

I n f r a r e d s p e c t r a

We were able to confirm the presence of divalent T M P D cation (in В, B, and D system) by infrared spectroscopy. M H N and T M P D were dissolved in the equi-molar proportion in 1,2-dichloroethane, or benzene, or acetonitrile and the spectra (Fig. 5) were taken 24 hours later.

Band at 1620 c m - '

The most important feature of the spectrum of the complex is a strong band at 1620 c m - 1 absent in the spectra of components. The band should be assigned to the C = N bond. According to the literature [8] the band at 1630 c m - 1 in the spectrum of ^^A^'^Vtetramethyl-^-quinonediimonium diperchlorate in potassium bromide should be assigned to this bond.

Bands at 1540 and 1640 c m - 1 and shoulder at 1520 c m " 1

The bands should be assigned to the selected vibrations of C = C in the quinoid ring.

D-Mannitol Hexanitrate as Electron Acceptor in CT Complexes 409

Bands of the O-nitro groups

The band at 1680 c m - 1 , which is one of three О—N0 2 bands present in M H N 2 6 , was subjected to a bathochromic shifting by 20 c m - 1 in our complex. Such shifting is not unusual in CT complexes [10].

Fig. 5. Infra red absorption spectra of solutions in ben­zene; A — M H N ; Б — T M P D ; С — M H N + T M P D (mole ratio 1:1) (solid line) Tetramethyl-/?-quinonedi-

imonium diperchlorate broken line

e so £ in

J 60

40

20

80

60

40

20

Ja

•

\

— з — з

i iw У V li \ У V

--

•-л

v s-

ш r _ .

a *Л -

и

r _ .

a *Л -

\

f — и j - f

1 \ ШЮ0 1600 Ш B0O Wavenumber

Another band in the complexes has the frequency 1360 cm- 1 . The band is not present in the components. It occurs in the region of inorganic nitrate ion ( _ O N 0 2 ) [11]. We suggest that the band in question should be assigned to O N 0 2 groups in M H N negatively charged under the influence of the electron donor of a low ioni­sation potential.

Experimental

The solvents: benzene, methanol, n-heptane used were pure for spectroscopy grade. 1,2-dichloroethane was purified as described in our previous paper [1]. Acetonitrile was purified by repeated refiuxing over P 2 0 5 and distilling [15]. D-Mannitol hexanitrate was prepared and purified as in our previous paper [1]. Electronic spectra were examined on a Unicam SP. 500 spectrophotometer with thermostated

cells of 0.2 cm thickness of the liquid layer and concentration с = 0.002 mol/1. Infrared spectra were measured on a Unicam SP. 200 spectrophotometer in an NaCl cell.

The thickness of the layer was 0.2 mm. Concentration of the substances (in 1,2-dichloroethane, benzene, and acetonitrile) was 0.05—0.02 mol/1.

In order to have a detailed examination of the bands, a Hilger H-800 spectrophotometer was also used. The frequencies reported in our present work were determined on a Hilger H-800 appa­ratus.

410 T. U r b a ń s k i et al.

Electric resistance

н-Heptane (10 ml) was added to a solution of M H N and T M P D in acetonitrile 20 ml con­centration of each of the components was 0.05 mol/1.

The mixture was shaken for 15 min., н-heptane layer was separated, diluted with 1,2-dichloro-ethane in proportion 1:1 (by vol.). Different quantities of methanol were added to the heptane — dichloroethade solution. The conductivity was measured by means of a "Eureka" 37/62 conduc-tometer.

D E P A R T M E N T O F O R G A N I C C H E M I S T R Y , T E C H N I C A L UNIVERSITY, W A R S A W , K O S Z Y K O W A 75 ( K A T E D R A T E C H N O L O G I I O R G A N I C Z N E J II, W A R S Z A W A )

INSTITUTE OF O R G A N I C C H E M I S T R Y , POLISH A C A D E M Y O F SCIENCES, W A R S A W 42, K A S P R Z A ­K A 44/52

(INSTYTUT CHEMII O R G A N I C Z N E J PAN)

R E F E R E N C E S

[1] В. He tna r sk i , W. P o ł u d n i k i e w i c z , T. U r b a ń s k i , Tetrahedron Lett., 1970, 3. [2] , , Bull. Acad. Polon. Sci. Sér. Sci.

Chim., [see preceeding paper in this issue], [3] T. U r b a ń s k i , Roczniki Chem., 13 (1933), 399; 14 (1934), 925; 15 (1935), 191 ; 16 (1936),

359; 17 (1937), 474. [4] А. С. A l b r e c h t , W. T. S impson , J. A m . Chem. Soc , 77 (1955), 4454. [5] P. Job, Compt. Rend., 180 (1925), 928; Ann. Chim. Phys., (10) 9 (1928), 113. [6] G . Br iegleb , Elekronen-Donator-Acceptor-Komplexe, Springer, Berlin, 1961. [7] S. Dahne , Z . Chem., 3 (1963), 191. [8] F . R i t s c h l , Spectrochim. Acta 23A (1967), 655. [9] T. U r b a ń s k i , M . W i t a n o w s k i , Trans. Faraday Soc , 59 (1963), 1047.

[10] J. Rose , Molecular complexes, Pergamon Press, Oxford, 1967. [11] A . D . Cross , Introduction to practical infrared spectroscopy, Butterworth, London, 1967. [12] A . Weissberger , E . Proskauer , D . R i d d i c k , E . Toops Jr., Organic solvents, Intersci.,

N . Y . , 1955.