Structure of donor-acceptor complexes—I

J. inorg, nucl, Chem., 1966. Vol. 28. pp. 1225 to 1235. Pergamon Press Ltd. Printed in NorthernIrehmd

STRUCTURE

OF D O N O R - A C C E P T O R

COMPLEXES--I

COMPLEXES OF LEWIS ACIDS WITH AMIDES RAM CHAND PAUL, B. R. SREENATHANand S. L. CHADHA Department of Chemistry, Panjab University, Chandigarh-3, India (Received 9 September 1964; in revisedform 31 July 1965) ~--Complexes of the Lewis acids, sulphur trioxide, antimony (V) chloride, tin (IV) chloride, titanium (IV) chloride and aluminium (III) chloride with formamide, acetamide, benT~mide, acetanilide and benzanilide have been isolated. Their infra-red spectra and dipole moment in chloroform and conductances in molten state or in solution in nitrobenzenehave been studied for elucidating their structure. It has been indicated by infra-red studies that the structure of amide is greatly due to the coordination of the Lewis acid to the carbonyl oxygen of the amide and with strong Lewis acid, the carbonyl group loses its double bond character. Conductance of the complexes at low concentrations in nitrobenzeneand their high dipole moments indicate the ionic nature of the complexes.

COMPLEXESof Lewis acids with oxy-bases such as alcoholsJ I'~> ketones, ~s'.~ carboxylic acids, and amides <9-I~) have been prepared and some of their properties have been reported. Much of the attention, however, has been confined to the coordination of the Lewis acid and practically no attention has been paid to the effect of co-ordination and the strength of the Lewis acids on the structure of the ligands. G r . ~ et al. <9~have studied the effect of the Lewis acids on the amide molecule and have tried to assess the relative donor properties of nitrogen and oxygen atoms by an infra-red study of a few complexes of amides. PAUL et al. a4> have already indicated the co-ordination of Lewis acids to the oxygen atom in the case of dimethylformamide complexes and have reported that the carbonyl absorption band of the amide is absent in its complexes with sulphur trioxide and antimony (V) chloride. The structure which may have the maximum contribution to the resonance structures of the complex CHs H~N

e/

I \ I Lo O

CHs

(0 ~ A. A. BABUS~gJ~,Izvest. Acad. Nauk. FIz. 22, 1131-5 (1958). ~s~p. DwJ~ and R. A. OGo, Nature, Lond. 180, 1114 (1957). cs~B. P. $usz and P. ~ 3 o w , Heir. Chim. Acta 41, 1332 (1958). ~4~B. P. Susz and A. LACHAVAN~,Heir. Chim. Acta 41, 634--6.0958). ~5~A. Ro~NtI~q and W. LOEWENSTA~,Bet. Chem. Dtsch Ges. 35, 11160902). ~6~p. ~ , Lieb~s, Ann. 376, 285 (1910). ~7~R. C. PAULand K. C. MAllOWS, Z. anorg, al~. Chem. 321, 56 0963). cs~R. C. PAUL,D. SINOHand K. C. MAI~OT~, J. Indian Chem. Soe. 41, 541-45 (1964). ~'~ W. GER~.~, M. F. L~P~RT, H. PYSZOI~and J. W. WAta~S, J. Chem. Soc., 2144 (1960). ~0~ R. B. I~LA~D, S. Mxzusm~, C. CURRA~ and J. V. QUAO~U~o, J. Am. chem. Soc. 79, ~575 (1957). ~tx~Q. C. ~ and W. C. F~.J~L~US,Z~ anorg, aUg. Chem. 221, 83 (1934). ~:~ J. AP.CtI~mAULTand R. R.w~'r, Canad. J. Chem. 36, 1461 (1958). ~a~ A. CL~.RVIJ~LDand E. J. MALKreWICH,J. inorg, nucL Chem. 25, 237-~t0 (1963). ~*~ R. C. PAUL,SUDHASHARDAand B. R. S~SATHAS, Indian J. Chem. 2, 97 (1964). 1225

1226

R~

CHAND PAUL, B. R. SREENATHANand S. L. C~AOrtA

has been indicated as (i) where the double bond character of the carbonyl group is absent. With weaker Lewis acids, however, there is a shift in the earbonyl bond frequeacy. To make a detailed study of the behaviour of various Lewis acids on the donor molecule, their complexes with some amides have been isolated and their physical properties such as dipole moment, conductance and infra-red absorption spectra are being reported here. EXPERIMENTAL Atmospheric moisture was as far as possible excluded throughout. Forrnamide and acetamide were purified by a procedure adopted by PAULet aL,~1s,16~ benzamide, benzanilide and acetanilide were purified as described by Hra~RON~m. Antimony (V) chloride (B.D.H., L.R.) was used as such. Sulphur trioxide was prepared by distilling fuming sulphuric acid (A.R.) through a short column in all glass apparatus over phosphorus (V) oxide (b.p. 44.6°/747 ram). Tin (IV) chloride was prepared and purified as done by PAULet aL ~'~ Titanium (IV) chloride was used after purification as described earlier. ~18~Anhydrous aluminium chloride was sublimed, in an atmosphere of dry chlorine, before use. Carbon tetrachloride, methylene chloride, benzene, ether, chloroform and nitrobenzene were purified by standard methods. ~

Preparation of complexes Complexes of sulphur trioxide with acetamide, benzamide, acetanilide and benzanilide. Sulphur trioxide (4 g) was taken in a 50 ml standard joint (B-14) two necked flask, to one of which a side tube containing the well powdered amide was attached where as the other was provided with a silica gel guard tube. The flask was cooled in an ice-bath and small quantities of the amide were added to the flask intermittently. The contents were warmed and recooled after each addition. The process was repeated till a quantity of amide slightly smaller than equimolar amount of sulphur trioxide had been added. To the viscous mass, in all the eases, dry carbon tetrachloride was added and it was refluxed and the complex was washed with carbon tetrachloride to remove excess of sulphur trioxide. The complex was, then, dried by subjecting it to vacuum for several hours. Attempts to further purify the complexes by sublimation under reduced pressure decomposed the products. Attempts to crystallize them were also not successful. Sulphur trioxide-formamide complex. Sulphur trioxide (4 g) was directly distilled into cold formamide (2 g) taken in 50 ml standard joint flask. The viscous liquid thus formed, was washed several times with carbon tetrachloride and finally subjected to vacuum to remove excess of the solvent. Complexes of SbC15, SnCI4 and TIC14 respectively with two moles of acetanilide. Acetanilide was dissolved in dry benzene and cooled in an ice-bath and an equimolar proportion of respective Lewis acid was added dropwise. Complexes were slightly soluble. Complexes of SbCI5 with three moles of acetanilide and SnCI~ and TiCI4 with four moles of acetanilide. These were obtained by treating the filtrates after separation of the above mentioned complexes, i.e., SbCls.2 acetanilide, SnC14.2 acetanilide and TiCI,.2 acetanilide complexes respectively with dry carbon tetrachloride. The filtrates left over still contained unreacted Lewis acids. These complexes were also obtained by adding acetanilide to the respective Lewis acid solution in benzene and refluxing the contents on water bath for I hr. Complexes of SbC15, SnC14with 2 formamide and 2 acetamide respectively and SnCI,.4 acetamide. These were prepared by similar procedure as adopted by PAULet al.~t6,TM Complexes of SbCIs, SnC14 and TiCI~ with benzamide and benzanilide respectively. Respective amides were dissolved in benzene and then added in small amounts to a well cooled Lewis acid in an equi-molar proportion dissolved in carbon tetrachloride. txs~R. C. PAUL, K. C. MALrIOTRAand O. C. VAIOYA(Unpublished work). ~*~ R. C. PAULand RAn~VERDEV, Indian d. Chem. In press. taT~I. HEILBRONand H. M. Bur,mtmY, Dictionary of Organic Compounds, p. 6 and 238 Oxford Univ. Press, New York (1953). ~xs~KAm.A GOYAL,R. C. PAULand S. S. SANDI-IU,,L Chem. Soc. 322-325 (1959). Cl,~A. WEIssBtrmtOEI~and E. S. PROSKAVmt,Organic Solvents, Vol. 8. Interscience, New York (1955).

Structure of donor-acceptor complexes--I

1227

SbCls.benzamide To a solution of benzamide (5 g) in benzene, antimony (V) chloride (15 g) was added and the mixture was refiuxed for 1 hr. SnCl,.4 formamide To a cold solution of SnCI, (5 g) in benzene, formarnide (8 g) was added dropwise. Complexes of aluminium (III) chloride with benzamide and acetanilide. To a well cooled solution of aluminium (III) chloride (4 g) in benzene, amide (6 g) was added and the mixture was refluxed for about one hour. All the addition complexes formed, as mentioned under their preparations, were filtered, repeatedly washed with dry carbon tetrachloride and kept in vacuum dessicator for 4-8 hr. The physical properties such as colour, melting point, conductance, dipole moment and molecular weights of the complexes along with the analytical data are given in Table 1. Nitrogen content of the complexes was estimated by micro analytical methods. Chlorine was estimated by Volhard's method. ~N~ Sulphur was estimated as barium sulphate. Tin, titanium and aluminium as described in the literature, c2~j Molecular weights of these complexes soluble in CHsCI, were determined in that solvent. The molecular weights of the complexes insoluble in such solvents could not be determined. Conductance of complexes in rdtrobenzene was determined by measuring the resistances of the solutions in nitrobenzene by using a precision measuring bridge as described by PAULet al. ~4~ Cells of cell constants 0.3 and 0.8 were employed. Dielectric constant of the complexes were determined in dilute solutions in chloroform by means of Heterodyne Beat method. ~u~ A gold plated cell with lead capacity equal to 8.53 pF was used. All the measurements were carried out at 22 ± 0.1°C. Refractive index measurements of the dilute solutions of complexes in chloroform were carried out using Abbe's refractometer at 22 ± 0.1°C. Density of the solutions was measured by using the specific gravity bottle. Dipole moments of the complexes have been calculated with the help of Hedestrand formula as modified by LE F~Vl~ and Vn~ ~u~. Infra-red spectra of the complexes have been recorded at different concentrations ranging from 10-a to 10-m molar in chloroform solutions using Beckman double beam I.R.5 spectrophotometer, RESULTS

Dipole moments of the complexes in chloroform solutions have been calculated and the values are given in Table 1. The specific conductance of the complexes were determined at the same concentration range in nitrobenzene. F r o m the plots of the specific conductance vs. molar concentration, specific conductance of all the complexes at 8 × 10-3 mole was extrapolated and values so obtained are also given in Table 1. The infra-red absorption bands of pure amides and their complexes in dilute solutions in chloroform have been given in wave numbers in Tables 2a and 2f. Spectra of the complexes were unaltered in the 10-4-10 -z molar concentration range. The values of molecular weight, obtained by ebullioscopic method, for those complexes which are soluble in CHzC12 are also tabulated in Table 1. DISCUSSION Infra-red spectra o f amides

Infra-red spectral frequencies of pure formamide, acetamide, benzamide, acetanilide and benzanilide in dilute solutions in chloroform are tabulated (Tables 2a to 2f). It is rather difficult to discuss with confidence the ~ N - - H stretching frequencies in pure amides due to the possibility of the formation of hydrogen bonds in their solutions in chloroform. It has also been indicated that due to dipole interaction also, the cm~I. A. VOGEL,A Text-book of Quantitative Inorganic Analysis; p. 366 Longmans, London (1961). c,~) I. A. VOOEL,A Text-book of Quantitative Inorganic Analysis; p. 366 Longmans, London (1961). ~n~ E, F. T~IAN, J. M. PATrr, Electronic Measurements, p. 210 McGraw Hill, New York (1956). ~u~ C. W. N. COM]'~ and S. WALteR, Trans. Faraday Soc. 52, 193 (1956).

SofAcetanilide SBC1¢2 Acetanilide SbC!¢3 Acetanilide

SnC142 B e n z a m i d e TiCl,.2 B e n z a m i d e A1Cls.Benzamide

SBCI¢2 Acetamide SNC1¢4 A c e t a m i d e SOs-Benzamide SBC1¢2 Benzamide SbCleBenzamide

SnCI¢2 F o r m a m i d e SNCI4-4 F o r m a m i d e SOeAcetamide

SBC1¢2 F o r m a m i d e

SO,-Formamide

Complexes

White White Brown Grey Dirty green White Yellow Light brown Brown Green Deep green

brown

Brown viscous Mass Dirty white White White Reddish

Colour

m.p.

-196 210

228 148 192

62 61 -53 84

250 250 --

250

--

(*C)

-29.9 24-6

27.1 31.0 41"1

41-0 27-0 -31.25 40-8

--

31-3

38-95

44-2

--

Chlorine

14"8" 20.2 16-8

21 "9 11-14 9"9

28.3 22"2 14"07" 21.32 28"9

32.76 26"2 22"3*

30-9

25"5"

Metal

Found (%)

6"2 4.72 --

5-42 ---

6-5 10"5 6.02 4.9 3-1

7-57 11"5 9.25

7-08

11-05

Nitrogen

-31.0 25-0

27"9 32.15 41"8

42.4 28-9 -32.7 42-14

40.45 32"2 --

45"45

--

14"88" 21.0 17-3

22.2 11.34 10-6

29;2 23"9 14.54" 22-58 30"0

33.9 26"9 23.0*

31.35

26-4"

6-5 4.9 --

5'56 ---

6.7 11 "2 6.36 5,18 3.34

7.98 119 10.07

7.2

11.2

Nitrogen

R e q u i r e d (~o)

Chlorine Metal

TABLE 1

3"4 x 10 -4 2-13 x 10 -5

11.73 7.75 7.598 12.12

2.5 X 10 -4 2-26 X 10 -4

1"55 x 1 0 -6 1"56 × 10 -5

3"3 x 10 -4 3"12 x 10 -4 2"8 X 10 -*

13.12 12.608 12"81 8"636 8.774

13"27 12"69 12"33

--

3"32 x 10 -4

12"19

542 686

536

136

569 704

541

139

Sp. conductance Dipole ~ - ~ n -1 Mol. weight m o m e n t in n i t r o b e n z e n e at in m e t h y l e n e (Debye) 8 x 10-' molar chloride at concentration. 22 4- 0 - 1 C at 22 4- 0 - 1 ° C F o u n d R e q .

(3

ixa

Orange White Brown Dirty green

White Deep yellow

TiCI,'4 Acetanilide Alas.Acetanilide SOs.Benzanilide SBC15.2 Benzanilide

SNC14"2 Bcnzanilide TiCI~'2 Benzanilide

150 -Dccomp. above 190°C High High -Decamp. above 180°C High High

Note: * Percentage of sulphur.

White White Yellow

SnCI¢2 At~tanilide SnClc4 Acetanih'de TiCI4-2 Acetanilide

20-9 23"6

19.2 58-0 -24.2

25.9 16-9 31-56

4.89 3"5

4.25

10-88" 14"8

16"02 --

59

- -

11.55" 16.16

18-16

21 "67 24-31

- -

--

22"4 14-85 10"91

-25-5

'4

20.0

7-5

--

26"75 17"72 32"27

5"08 -6-02

20.9 14"1 10.2

4"79

5"05 4-04

- -

7"88

6'36

5 "27

8"244 8"292

12"94 12"76

- -

7"784

8.22 7.905

1"49 x 10-5 1.8 x 10-5

2"32 x 10 -4

3.58 × 10 -4

1"21 x 10 -5

1"22 × 10 -6

1-4 x I0 -~

1-4 × I0 -~ 1-29 × 10 -5

642 568

518 776 533

584

658

803

[

0

e~

&

3430

3398

3420

1680

Benzanilide

x

x

x

1690

Acetanilide

x

Acetamide

1668

Benzamide

x

1105 (w)

1675

Acetamide

x

Formamide

1705

x

x

3390~h)

x

3310 (sh)

3450

x

3435

SOs'IB

x

3535

Formamide

Benzanilide

Acetanilide

Benzamide

x

3410

3545

Acetamide

x~

v.w.

3515 3410

SOa.IB*

Formamide

in CHCla

Pure amide

x

x

x, x**

w, w**

3398

x

3415

x

3395

v.w.

SbCI6.2B

3280 (w)

x

3300(w)

x

3400

x

--

3400

w

SnCII'2B

VIBRATIONAL FREQUENCIES ( c m - x )

(C) ~ C - - O ~

--

--

1658

1685"*

x

--

--

SbCIdlB

ABSORPTIONFREQUENCIES

x 1650

1676

X~X**

x

1634

x

1685,

x

x

x 1665

--

x 1670

x 1676

1700

x

--

--

(b) > C = O FREQUENCIES(AMIDE I BAND)

--

--

3400

x

--

--t

SbCI6.1B

(a) N--H

--

1658

1686

--

1645

1670

1674

1700

x

x

x

x

--

3415

x

3400

w

SnCI,.4B

TABLE 2.--INFRA-RED ABSORPTIONFREQUENCIES

1648

x

1649

x

1637

x

--

--

x

x

x

x

3400

x

--

--

TiCla2B

1112

--

SbC15"2B

--

1649

1685

--

--

--

x

x

x

x

--

--

--

TiCI4"4B

--

1650

x

1627

x

--

--

x

x

x

x

3402

x

--

--

AICIa'IB

t")

.r-

.~

z

.~

.~

U

~v

z

~J

~_~

1575 1610 1615

1398 1385 1425 1370 1395

1335

1335

1370

1315

1325

Benzamide Acetanilide Benzanilide

Formamide Acetamide Benzamide Acetanilide BenzaniHde

Formamide

Acetamidc

Benzamide

Acetanilide

Benzaullide

X

x

x x

x x

X

x

X

x

1395 (w) 1380 1420 1370 1390

1585 1615 1604

1590(w) 1600

if) C

1398

-1428 1370

1398 (w)

1410 x 1365

X

1375 x,x** 1320, 1 3 2 0 "

- -

X

x ---

X

1362

X

1330

X

X 1375 --

X

- -

1338 X 1340

N STRETCHING FREQUENCIES

1392

--

- -

1430 --

1398 (w)

DEFORMATION FREQUENCIES

1610

1610 (e) C - - H

1588 -1570 1605

1590 (w) 1602 1580 1613, 1 6 1 3 " *

DEFORMATION FREQUENCIES

__

- -

1109

1385 1430 1365, 1 3 7 0 " *

- -

- -

--1580 (w) --

(d) N - - H

~: x t h e f r e q u e n c y w a s a b s e n t . ** m e a n s t h e c o m p l e x e s w i t h t h r e e m o l e c u l e s o f a m i d e . v.w., Very weak. w, Weak. sh, S h o u l d e r . B r e p r e s e n t s a n a m i d e .

* B represents an amide. t -- indicates that the complex was not isolated.

1595 1595

Acetanilide Benzanilide

Formamide Acetamide

X

X

l102(w)

Benzamide

--

1365

1320

--

1375 1338 1340

1338

--

1385 -1370

1390(w)

--

1590 1605 -1606

1372

X

1410 --

X

--

--

1395

-1428 --

--

1615

--1575 --

1120

(w),

1086

1128

1110

--

1369

1320

--

--

--

--

--1320

--

- -

---1615

(w)**

--

1370

1416 x

X

--

--

--

-1428 1370

--

- -

--1578 1620

p.b

T.

e~

o

1232

RAM CRAND PAUl,, B. R. SRm~NATRANand S. L. CRADrIA

amides are associated, cu~ There are two frequencies corresponding to N - - H stretching represented by bands at or near 3500 cm-1 and 3400 cm-1. The lower one (around 3400cm -1) probably corresponds to the ( > N - - H . . . O = C ) hydrogen bonded > N - - H or to the antisymmetric > N - - H frequencies. A comparison of amide (I) band, which is mainly due to the >C~----O absorption frequencies in various amides shows that this frequency is successively lowered from formamide (1705 cm-1) to benzamide (1668 cm-x through acetamide (1675 cm-1). The lowering of >C---------Ofrequency in acetamide is due to the fact that methyl group acts as an electron donor. Phenyl group on carbon has also similar effect on >C-----O frequency. On comparing the >C~----O absorption frequencies of acetanilide and benzanilide with acetamide and benzamide respectively, it can be easily followed that the phenyl group on nitrogen is definitely acting as an electron acceptor as the >C--------Ofrequency is increased. The amide II band is the result of strong absorption mainly due to the - - N H bending or deformation frequencies, The substitution of methyl or phenyl group on carbonyl carbon has negligible effect on - - N - - H deformation frequencies, where as the secondary amides, acetanilide and benzanilide have slightly higher absorption frequencies. Similar effects may be expected if Lewis acids were to be co-ordinated at the nitrogen atom. The C--H deformation frequencies are not much affected, except in the case of benzamide where the value is the highest which might be due to the C--H deformation in the phenyl group. The amide III band could not be clearly observed on the apparatus used in the present work. The C - - N stretching frequencies (amide IV band) in acetamide and formamide are same showing that the substitution of methyl group on earbonyl carbon has practically no effect on C - - N frequency. In benzamide, the C - - N frequency is increased. The phenyl group, however, has been indicated an electron donor as is shown in >C----O frequency decrease. Therefore, it is difficult to say anything with absolute certainty regarding the increase of C - - N stretching frequency in benzamide. The fact that phenyl group on nitrogen is electron accepting is further confirmed by the lowering of C - - N stretching frequency in acetanilide and benzanilide.

Spectra of complexes Although complex formation affects all the prominent absorption frequencies of the amide, yet the carbonyl absorption and C - - N stretching frequencies are effected the most and therefore are being discussed in greater detail. The pure amides have two - - N - - H stretching frequencies at or near 3500 cm-1 and 3400 cm-x (Table 2a). In the case of complexes of strong Lewis acids viz. sulphur trioxide and antimony (V) chloride, the higher vibrational frequency, (near 3500 cm-1) is absent, and in the case of sulphur trioxide complex with formamide, both these absorption frequencies are absent which indicates either the absorption is weak or shifted to chloroform regions. Probably, the higher N - - H frequency which is affected and can be confirmed by studying the complexes of the secondary amides, acetanilide and benzanilide, which have only one sharp absorption band at higher frequency, and is found to vanish with complex formation (Table 2a). Therefore, the effect of Lewis acids on the amides may be represented as: ¢s4~G. G. CANNON,Mikrochim Acta 555-58 (1955).

Structure of donor-~cceptorcomplexes---I // R--C

O-+L

\

/ N

/

R' ~-+ R--C

\

O--L •

"~/ N

O--L o

/

R' ~---R--C ~

/

H (b) (ii)

R'

N \

\

H (a)

1233

H® (c)

Carbonyl absorption frequency. On complex formation of amides with Lewis acids, the amide (I) band shifts to lower frequency, the magnitude of shift depending upon the strength of the Lewis acid. The order of the strength of Lewis acids has already been examined with respect to dimethylformamide as the base, and they have been assumed to be in the following decreasing order, irrespective of the amide, viz. SO3 > SbC16 > TiCI~ > SnBr4 > SnCI4.c14) Aluminium (III) chloride seems to be almost as strong as TiC14. It can be observed from the Table 2b that in the case of all the amide complexes with sulphur trioxide and antimony (V) chloride, the carbonyl absorption frequency (amide I band), vanishes. This implies that the double bond character of the carbonyl group might have been lost. The mechanism may be represented as in (ii-b). However, as indicated by the structures (ii), the Lewis acids should also have large effect on the C - - N stretching frequencies. In all other complexes of amides, with rest of the Lewis acids, there is a shift in the carbonyl absorption frequency and co-ordination of the Lewis acid through the oxygen atom has been pointed out by others earlier. ~9'1°'n'14~ In the complexes of weaker Lewis acids of the formula MX¢4B, the carbonyl frequency splits up into two bands. The higher carbonyl frequency is almost at the same region as for the pure amide, which may belong to the amide molecule attached through hydrogen bonding as indicated in (iii). The lower frequency is due to the bonded carbonyl group (bonded to Lewis acid). In the case of similar complexes of strong Lewis acids such as antimony (V) chloride, the carbonyl frequency is retained at almost the same region where the pure amide absorbs (iv). The carbonyl absorption that exists represents the hydrogen bonded carbonyl frequency as represented by (iv). A set of new and weak absorption bands that have been observed with the complexes of sulphur trioxide and antimony Of) chloride have been tabulated in Table 2c. These may be possibly assigned to the ~C--O--- vibrational frequencies which are /

due to the loss of the double bond character of carbonyl group (c.f. ~ C - - O - - ) . ' ~ N - - H deformation frequencies. The N - - H deformation frequencies of the amides (Table 2d)are retained as weak bands or only slightly changed in the case of these complexes as compared with those of the pure amides. This fact confirms the coordination of the Lewis acid to the oxygen of the carbonyl atom. This is also in confirmity with the substitution of an electron acceptor like phenyl group on the nitrogen which increases the N - - H deformation frequencies, as in benzanilide and acetanilide. The C--H deformation frequencies (Table 2e) in all the complexes are relatively negligibly affected and it is noteworthy that the Lewis acid, irrespective of its strength, has practically no effect on the C--H deformation frequencies of the amides. C - - N stretching frequencies. A reference to Table 2f indicates that amide IV ,u~ p. j. Sro~z and H. W. THOMPSON,Spect. Chim. Acta 10, 17 (1957). /

RAM CHAND PAUL,B. R. SREENATHANand S. L. CHADHA

1234

R R r

\ R

\

R t

\ /

/

/

X

\ ,/

\

/

X

C==0~M~O----C

/ \

X

N

R

/

R'

~/

X

N

\

/ H--0~C

C-~0----H

R

\

/ N

N

R'

\ H

H

(iii) ®

/

O--SbCI~

R--C

R' N

/ H---O=C

R

\

/ N

R'

\ H

(iv) where R may be H, CH8 or CtHi groups. and R" may be H or C6H5 and M ~ Sn or Ti, X = CI or Br. band is effected like to the amide I band in the formation of complexes, as has been discussed earlier. A close observation o f the Table 2f dearly proves the structure o f the complexes as indicated in structure (iii, iv) derived from the infra-red spectra data of carbonyl absorption frequency. The Lewis acids themselves are either non-conducting or only slightly polar in character. {~ The amides are considerably polar and the polarity differs from one amide to the other amide as indicated by their dipole moments. {27'2s} The values of the dipole moments of these complexes are very much higher than their corresponding amides in the same solvent (Table 1). Thus, it may be possible that the polarity o f t b e amide molecule increases due to complex formation with Lewis acids. It may also be observed from the dipole moment values (Table 1) that the effect of sulphur trioxide and antimony Of) chloride on the polarity of the amide molecule is very much higher as compared to that of any other Lewis acid on the same amide. Thus, it may be concluded that these are much stronger Lewis acids as compared to others. This is in eonfirmity with the infra-red studies of the complexes and the structural effects can be explained as indicated earlier by (ii). Specific conduetances of low melting complexes in molten state, given below, Complex SBC15.2 benzamide SbC15. benzamide TIC14.2 benzamide

Specific conductance (f~ cm -1) 3.6 x I0 q at 60°C 4-9 x 10q at 90°C 6.2 × 10-4 at 160°C

c26~p. WALDEN,Z. anorg, allg. Chem. 25, 209 (1900). ~sT~W. W. BATESand M. E. HOBB8,.jr. Am. chem. So¢. 73, 2125-6 (1951). css~W. J. VAOOHNand G. P. SEARS,Zphys. Chem. 62, 183 (1958).

Structure of donor-acceptorcomplexes--I

1235

and the high values of specific conductances of the complexes in nitrobenzene, as compared with that of pure liquid nitrobenzene (<10 -9 ~-1 cm-1), show the possibility of association of nitrobenzene with the complex resulting in dissociation into ions. From the comparison of conductances of the different Lewis acid complexes, it may be confirmed that sulphur trioxide and antimony (V) chloride complexes are more ionic in nature. However, the conductances of these complexes may be explained as below, considering sulphur trioxide complex as an example: (SOa.ROCNR'H) --~ CeHsNO~ ~ (SOa.RCONR')- -~ (CeHsNOe.H)+ The fact that these complexes are protonic in nature, as represented by the above mechanism of dissociation of complexes in nitrobenzene, is already confirmed by detailed electrochemical~14~and thermochemical studies ~ of Lewis acids and protonic acids in dimethylformamide. Although the spectra and dipole moment have been studied in chloroform, in view of the very low dielectric constant of the medium, the complexes of amides with Lewis acids may be expected to be very slightly polar in solid state. Thus, from the above studies, it is concluded that these Lewis acid complexes are highly polar in character and Lewis acids have a large effect on the structure of the amides. cn~R. C. PAUL,S. C. AHLUWALO,and S. S. PAmL,Indian J. Chem. 3, 300 (1965).