-- With 2 Figures

(1)

Infrared Evidence for the Transmission of Electronic Effects Through a Metal Atom

in a Series of New Cadmium Complexes

By J. M. HAIGH, M. A. VANDAM and DAVID A. THORNTON

--

With 2 Figures Summary

A series of novel cadmium complexes has been synthesized from the reaction of cadmium chloride, bromide and iodide with primary aromatic amines. The complexes are either mononuclear or polynuclear according to the nature of the halide and amine employed. A possible mechanism for their formation is proposed. The N - H stretching and bending frequencies are linearly related to the electronegativity of the co-ordinated halogen, indicating that the electron withdrawing capacity of the halogen is transmitted through the cadmium atom.

Inhaltsiibersicht

Eine Reihe neuer Cadmiumkomplexverbindungen wurde durch Reaktion von Cad- miumchlorid, -bromid und -jodid mit primaren aromatischen Aminen hergestellt. Die Komplexe sind, abhangig von dem Charakter des eingesetzten Halogenids und Amins, einkernig oder mehrkernig. Ein moglicher Bildungsmechanismus wird vorgeschlagen. Die NH-Valenz- und Deformationsfrequenzen nehmen mit steigender Elektronegativitat des Halogenliganden linear zu, was eine Weiterleitung des Elektronenzugs des Halogens iiber das Cadmiumatom anzeigt.

In marked contrast to the reaction of primary aromatic amines with the chlorides of zinc(II) and mercury(II), which form only tetrahedral mononuclear complexesl), we have obtained evidence that the cadmium halides undergo co-ordination with similar amines to form either mononuclear or polynuclear complexes.

The complexes were prepared by reaction of the amine with the cadmium halide in the mole ratio 2.2: 1 in absolute ethanol. Under these conditions mercury and zinc chlorides yield complexes of formula [MOI2(amine)2] whereas

1) J. M. HAIGH, M. A. VANDAM and D. A. THORNTON, J. South African chern. Inst.

(in press); Inorg. nuclear chern. Letters 3, 7 (1967).

(2)

J. M. HAIGH, M. A. VANDAM and D. A. THORNTON,Series of New Cadmium Complexes 95 the present results show that the amines react with cadmium halides var-"

iously in the mole ratios 2: 1, 1: 1, 2: 3, 1: 2 or 2: 5. These ratios correspond with values of n in the formulation I for the complexes, of 0, 1, 2, 3 and 4 respectively.

x (

X

)

L L)Cd (X)Cd >X

I

(L = amine; X = CI,Br, I)

Such diversity in composition of the products resulting from reaction of cadmium halides with tertiary phosphine and arsine ligands, leading to at

"' least three classes of compounds: [OdX2L2], [(OdX2)2L2] and [(OdX2)2L3], is a well established feature2) of the co-ordination chemistry of cadmium.

X-ray structural investigation of the compound II has furthermore shown that is has the trans-symmetric configuration shown and a tetrahedral distribution of ligands about each cadmium atom as expected for cadmium(II) with its dlO outer electron configuration.

Our formulation I is made on this basis.

Br

)

B (C2Ha)3P Cd( r)Cd

(

p(C2Ha)3

Br Br

II

There is evidence for dissociation of the complexos I in ethanolic solution in agreement with similar observations2) on the behaviour of compounds of type II in ethano1. Such dissociation implies that attempted molecular weight determination in solution is valueless: we find that the values obtained are low, irreproducible and dBpend on the concentration employed.

Furthermore, the RAST method for molecular weights is unsuccessful in view of the insolubility of the complexes in appropriate materials.

No attempts have previously been made to suggest a mechanism by which cadmium halides yield a diverse range of co-ordinated products. An interesting observation in the course of the present work leads us to propose a possible mechanism. The complexes formed from cadmium bromide and cadmium iodide are considerably more soluble in ethanol than those derived from the chloride; furthermore, whatever the halide used, the solubility of the complexes is greatfJr in pure ethanol (e. g. during recrystallization) than in the alcoholic reaction medium. It is therefore possible that, in pure ethanol particulary, molecular cleavage occurs with the relative ease 01 < Br < I.

To cite an example, if the original structure of the complex precipitated from the reaction medium is supposed to be as in III, cleavage at one of the

--

2) R. c. EVANS, F. G. MANN, H. S. PEISER and D. PURDIE, J. chem. Soc. [London]

1940, 1209.

(3)

bridges on recrystallization would lead to the existence of two species, IV and V, in solution:

X X X /L X X X L

"'Cd/ "'Cd

<

^"'Cd/ ^--- ^"'Cd/

⁺ /

^Cd/ ^"'Cd/

L/ "'-X/ .X/ "'X L/ X "'x/ "'X '

III IV V

On reprecipitation, the following reactions are possible:

IV + V --- III

X X L

IV + IV --- _L/\Cd(

"

X"'/ ^Cd("X VI

X X X X L

V

+

V ---

_V

"'Cd/"'X/ "'Cd/"'X

)

^Cd/"'X/ ^"'Cd

<

X VII

The least soluble product would normally be obtained. Thus, although recrystallization leads to the recovery of material, the composition of the recrystallized product is not necessarily the same as that obtained from the original reaction medium. Dissociation in solution precluded confirmation of this mechanism by molecular weight studies but in one instance it was shown by halogen analysis that the molecular species obtained before and after recrystallization were different. Our classification of the complexes according to the value of n is based on microana1ysis of the recrystallized products.

Cadmium exhibits other fundamental differences from mercury and zinc in its complexing behaviour. Cadmium chloride is far more versatile, forming stable complexes with amines which fail to react with the chlorides of zinc and mercury. Furthermore, cadmium bromide and iodide frequently form complexes where these halides of zinc and mercury form no stable product 1).

The limit of stability for cadmium chloride is reached when the HAMMET'l'- a-function of the reacting amine exceeds

+

1.0 and for cadmium iodide when ---....

the a-function is more positive than -0.2. Thus m-nitroaniline (a

=

+0.71) forms a complex with cadmium chloride while p-nitroaniline (a

= +

1.27) fails to react, and neither of these amines will co-ordinate with cadmium bromide or cadmium iodide. The cadmium chloride m-nitroaniline complex is furthermore so unstable that attempted recrystallization leads to reversion into the reactants. These observations lead to a clearcut stability order for the halo complexes of Cl > Br > 1.

--

Infrared Spectra

The frequencies for the complexes containing co-ordinated chlorine (asKBr discs) are listed in Table 1. The assigments are presented in accordance with

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J. M. HAIGH, M. A. VANDAM and D. A. THORNTON,Series of New Cadmium Complexes 97

the evidence previously proposed for simiJar complexes of mercury and zinc 1).

One of the principal objects of the present study was to ascertain whether the frequency data would shed light on the bonding in these complexes.

Cadmium lies in that region of the periodic classification incorporating metals which are borderline cases with respect to the predominance of a- or n-bonding in their complexes3). It has been proposed that in complexes of zinc and mercury with primary aromatic amines1) n-bonding participates, particulary in the case of the mercurly complexes. In the event of a significant degree of n-bonding, the electron shifts illustrated in VIII would lead to an increase in the N -H and O-N stretching frequencies. In Table 2 the relevant data for those vibrations most subject to the influence of n-bond- iug are c?mpared for corresponding complexes of zinc, cadmium and mercury. The results indicate that n-bonding probably has greatest significance

CI H

_~0 ~~

^Cl^I ^H

^/V ~

_VIII

^- ^'\

_R

for the cadmium complexes. In the case of the chloro-bridged complexes this would be assisted by the donor action of the third chlorine atom which is absent from the mercury and zinc complexes.

The formation of chloro-, bromo- and iodo-complexes of cadmium with the ligands 0-, m- and p-toluidine enabled the transmission of electronic effects through the cadmium atom to be demonstrated. The relevant fre-

quencies for these nine complexes are listed in Table 3.

The frequency data indicate that the majority of the vibrations are sensitive to the nature of the halogen and in most cases there is a significant increase in frequency in the order 01 < Br < I. This increase can readily be attributed to the relative electronegativities of the halogens. A plot of halogen electronegativity against the symmetric and asymmetric N -H stretching frequencies (Fig. 1) shows that a linear relationship exists. A similar relationship has previously been demonstrated for a series of amine complexes of platinum (II) and paHadium(II) but for these compounds the N-H stretching frequency varied against its prediction on the basis of relative

3) S. AHRLAND, J. CHATTand N. R. DAVIES, Quart. Rev. (chern. Soc., London) 12,265 (1958).

7 z. anorg. aJlg. Chemie. Bd. 355.

(5)

Table 1

Infrared spectral data for cadmium complexes I; X = Cl (cm-l)*) No.

I

aniline

I beny.ylamine I p-tolnidine I p-anisidine ^I

p.chloro-

I

p-oromo-

1m-toluidine

aniline aniline

1 3322 s 3328 s 3311 s 3311 s 3311 m 3338 s 3318 s

2 3247 s 3252 m 3247 s 3236 s 3242 m 3257 s 3247 s

3 3135 vw 3125 sh 313(; m 3135 w 3145 w 3125w

4 3049 w 3030 w 3030 w 3017 w 3030 m

5 2950 vw 2929 m 2933 mb 2924 w 2933 w 2924 m

6 2857 vw 2857 vw 2841 s 2855 vw 2857 vw

7 1610 s 1600 sh 1602 m 1597 m 1603 m 1610 sh 1612 s

8 1585 m 1563 s 1572 w 1575 s 1575 m 1592 s 1592 s

9 1565 s

10 1497 s 1493 m 1513 s 1515 s 1493 s 1488 s 1492 s

11

12 1441 m 1431 w 1449 m 1470 m /"

13 1471 m 1430 m 1437 m 1425 w 1425 m

14 1361 vw

15 1342 vw

16

I

1299 s

17 1256 s

18 1238 s 1205 w 1235 s 1232 s 1235 s I 1245 s I ^{1260 s}

19 1172 w

20 1150 vw

I

1175 s

I

1167 w

,I

1175w

21 I ^{1167 s}

22

23 1136 vw

24 1099 s

25 1096 w 1098 s 1098 s

I

1C87 sh

26 1087 sh

27 28

29 1058 sh

I

1066 m

30 1044 s 1056 sh 1073 s

31 1030 s 32

I

1030 s 1033 s 1028 sh

I

1010s

33 I ^{1050 s}

34

351

1015 w

36 965m 990 s

I I I I

971 sb

I

998w

37 933m 917 s

38 39

I I

816 s

I

818 s

I

816 s

40 797 mb 804 s 804m 806 sh 776 s

41 745 s 738 s

42

I

I 734s

43

I

^730w

44

45 685 s 691 s

1

700w I 697 w 700m

\ 688 s

46 I I

667w

47 640 vw 645 sb

*) Intensity abbreviations: s = strong; m = moderate; w = weak; vw = very weak;

halogen electronegativity4). This was explained qualitatively by supposing that dative n-bonding from metal to halogen increases so rapidly along the

4) J. CHATT, L. A. DUNCANSON,B. L. SHAW and L. M. VENANZI, Discuss. Faraday Soc.

26, 131 (1958).

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J. M. HAIGH, M. A. VANDAM and D. A. THORNTON,Series of New Cadmium Complexes 99

m-chloro-

!

m-nitro-

I o-touidine I

o-anisidine

I

o-chloro-

I mean ^I

assignment

aniline aniline aniline frequency

3317 m 3333 w 3330 m 3342 s 3317 vw 3323 v N -H; asym

3257 m 3268 sh 3252 m 3263 s 3247 vw 3251 v N -H; gym.

3135 vw 3096 vw 3135 w 3130 vN -H...Cl intermol.

3067 w 3067 vw 3030 w 3030 vw 3039 vC-H; aromatic

2924 vw 2920 vw 2915 vw 2937 m 2929 vC-H; aliphatic

2857 vw 2845 w 2853 do.

1605 s

I

1623 s

I

1603 sh 1595 s 1616 s 1606 v C=C (1); aromatic

1575 sh 1587 g 1546 m 1576 6 N -H; scissor

1565 v C=C (2); aromatic

1485 s I 1517 s I 1494 s I 1498 s I 1489 s 1498 v CC (3); aromatic

1451s 1451

1486 m

I

1461 g

I

14618

I

1457 6 C-H; aliphatic

1449 m I 14498h 1431 s 144Q v C=C (4); aromatic

361 _{6 C}

-

^{H ;CH.} ^scissor

1342 do.

1302 w 1297 vw

I

1305 s 1301

1255 s 1256 v C -0 -C; ether

1235 s 1266 s I 1236 8 I 1216 s 1258 m 1238 vC-N

1172 P C - H; monosubst.

1172 P C-H; l,4-disubst.

1166 w I I 1167 P C-H; l,3-disubs1.

1193m

I

^{1180 w} ^{1157 m} ¹¹⁷⁷ PC-H; 1,2-disubst.

1136

1099 P C-H; monosubBt.

1095 P C - H; l,4-di8Ub8t.

J 087 sh 1089 w I

I

1088 P C-H; l,3-di8ub8t.

1080 m 1091 8

I

10758b 1082 PC-H; l,2-disub8t.

1043 w 1048 III 1046

1062

1058 P C - H; l,4-di8ubst.

1030 P C - H; mono8ub8t.

1034 P C-H; l,4-di8Ubst.

10248

I

1037 P C-H; l,3-disub8t.

1012 8 10228 1017 P C-^H; l,2-di8Ubst.

1015

994m 997 w 9828 985 NH. bend

8708 933 8h 913

865m 827 s 843w ,8,45. yC-H

8148b 82Gw 817 NH. bend (rock)

7698 790w 792

742 y C-H; monosub8t.

734 y C- H; l,4-di8ubs\.

7348 732 y C-H; l,3-di8Ubst.

7508 741 s 7458 745 y C-H; l,2-disubst.

6788 711 8 703 vw 6788 693

668 sh 666m 667

643 sh = shoulder; b = broad.

series 01 < Br < I that the total electron drift from metal to halogen increases in that order also, i. e. against the normal trends of electronegativity.

It is therefore apparent that in the case of the cadmium complexes, the electronic effect of the halogens is that to be expected from their relative 7*

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Table 2

Mean frequencies of vibrations subject to n- bonding effects in chloro complexes of cadmium, mercury and zinc

Table 3

Infrared spectral data for chloro-, bromo- and iodo-complexes of cadmium

".-

*) The assignment is obtained by referring the figure in this column to the appropriate row in Table 1.

electronegativities and in this respect cadmium differs in its behaviour from palladium and platinum.

A linear relationship also exists between the N -H scissoring frequency and the electronegativity of the halogen (Fig. 2). The slopes are similar to those of Fig. 1 lending support to the assignment of the frequencies in the range 1598-1546 cm-1 to a N -H deformation mode. With many other vibrations which would be expected to be less sensitive to the nature of the

vibration

I ^cadmiummean frequencies(cm-')

I

^mercury')

I

^zinc')

. N -H; asym. 3323 3309

I

³²⁸⁵

.N-H; sym. 3251 3239

I

3232 /j N -H; scissor 1576 1574 1474

.

^C-N ¹²³⁸ ¹²²¹ ¹²²⁰

*) Data from reference 1).

p-toluidine

I

m-toluidine

I

0-toluidine assignment

Cl

I

^Br

I

^I ^Cl

I

^Br

I

^I ^Cl

I

^Br I ^I ^')

3311 s 3318 s 3328 m 3318 s 332C s 3322 s 3330 s 3338 m 3348 w 1

3247 s 3252 m 3257 s 3247 s 3250 s 3252 m 3252 m 3257 m 3263 s 2

3135w 3140w 3125 w 3125 w 3125 vw 3135 w 3135 vw 3165 sh 3

3030 w 3026 m 3021 w 3030 m 3030 w 3030 w 3030 w 3035 w 3035 vw 4

2929 m 2929 m 2924 m 2924 m 2924 m 2924 w 2915 vw 2924w 2933 m 5

2857 VW' 2861 w 2865 vw 2857vw 2857 vw 2857 vw 2857 vw 2861 w 6

1602 m 1616 m 1616 m 1612s 1615s 1617 s 1603 sh 1613 sh 1618 sh 7

1572 w 1579 w 1586 s 1592 s 1595 s 1598 s 1587 s 1591 s 1595 s 8

1513 s 1516 s 1516 s 1492 s 1493 s 1495 s 1494 s 1496 s 1497 s 10

1431 w 1439 w 1443 w 1470 m 1470 w 1472 w 1461 s 1468 s 1472 s 12

1235 s 1239 s 1239 s 1260 s 1261 s 1262 s 1236 s 1244 s 1235 s 18

1150 vw 1156 vw 1159 vw 11678 1170 m 1172 m 1193 m 1195 m 1196 w 20, 21, 22 1096 w 1099 w 1099 w 1087 sh 1087 sh 1089 sh 1080 m 1083 m 1085 m 25, 26, 27

1044 s 1044 8 1027 s 1050 s 1048 s 1032s 1037 sh 30,33

1030 s 1030 s 1011 s 1024 s 32,34

998w 1000 w 1001 w 982 s 983 s 36

917 s 918 s 919 s 933 sh 935w 938w 37

887 s 887 s 887 s

830 sh 831 sh 827 s 831 s 833 s 38

804 s 806 s 8()7 s 776 s 775 s 774 s 750 s 752s 757 s 40,44

736w 736m 735m 730w 729w 729w 741 sh 742 sh 742 sh

700 w 701m 702m 688 s 687 s

I

^{689 s} ^{711 s} ^{713 s} ^{711 s}

667 w 667m

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J. M. HAIGH, M. A. VANDAM and D. A. THORNTON,Series of New Cadmium Complexes 101

halogen than those involving the N -H bond, there is a small increase in the order expected on the basis of the capacity for electron withdrawal of the halogen atom. There is no apparent correlation between the HAMMETT-

3.0f-CI

2.8f-Br --.

;;;-

"-

""

~_".

'"

..

~<..

~

<;:j 2.5f- 1

Fig. 1.

Variation of asymmetric and symmetric stretching frequencies with electronegativity of co-ordinated halogen

3250

\x. .-p-tol.⁰

-

^m-/ol.

\,1,1

\

^x

^\

3270v 3320

1/N-Hosym.

3350 1/N-Hsym.

Frequencyem-1

a-function of the co-ordinated amine and the N -H stretching frequencies of the complexes; it is somewhat remarkable that a vibration such as the N -H stre.tch should be more dependent on the nature of the halogen at a point separated from the aromatic nucleus by a metal atom than on the nature of a substituent attached to the ring itself.

...

Fig. 2. Variation of N - H bending frequency with electronegativity of co-ordinated halogen

JO'r-C{

2.8'r-Br

;;;

~ c§"

~'"

"

<;:j

~

251-1

7570 7500

N-H bending frequency cm-1

(9)

Experimental

The complexes were prepared by the addition of the primary amine (0.022 mole) to the- cadmium halide (0.010 mole) in absolute ethanol. Precipitation of the complexes occurred immediately or after shaking. The compounds were dried at 68°jO.1 mm for 24 hr prior to analysis and the determination of the infrared spectra. None of the compounds melted or decomposed below 350°. The spectra were determined as previously described 1).

[Od2Ol4(aniline)2]: from ethanol (n= 1). (Found: 025.5; H 2.8; N 4.9; 0125.4. Oalcd.

for C12H140d2Ol4N2:026.0; H 2.55; N 5.1; 0125.6%).

[Od012(benzylamine)2]: from methanol (n= 0). (Found: N 7.1; 0117.4. Oalcd. for C14H1SOd012N2:N7.05; 0117.8%). -

[Od2Ol4(o-toluidine)2]: from water (n = 1). (Found: N 5.2; 0124.3. Oalcd. for C14H1SOd2Ol4N2:N 4.8; 0124.4%).

[Od2Br4(o-toluidine)2]:.fromethanol (n = 1). (Found: N 3.0; Br 41.95. Oalcd. for C14H1SBr40d2N2:N 3.7; Br 42.1%).

[OdI2(o-toluidine)2]: (n = 0). (Found: 144.0. Oalcd. for °14H1SOdI2N2:143,7%).

[Od2Ol4(m-toluidine)2]:from ethanol (n

=

1). (Found: 01 24.1. Oalcd. for °14H1SOd2Ol4N2:

Cl 24.4%).

[OdaBr6(m-toluidine)2]: from ethanol (n = 2). (Found: 017.3; H 2.0; N 3.0; Br 45.9.

Calcd. for °14H1SBr60daN2: 0 16.3; H 1.8; N 2.7; Br 46.5%).

[Od5I1o(m-toluidine)2]: from ethanol (n = 4). (Found: 05.4; H 0.65; N 1.1; 161.4.

Calcd. for °14H1SOd5I1ON2:05.2; H 0.9; N 1.4; 162.0%).

[OdOI2(p-toluidine)2]: from ethanol (n = 0). (Found: 041.7; H 4.7; N 7.2; 0117.8;

Calcd. for °14H1SOd012N2: 042.3; H 4.6; N 7.05; 0117.8%).

[OdBr2(p.toluidine)2]: from ethanol (n = 0). (Found: N 5.3; Br 32.75. Oalcd. for C14H1SBr20dN2: N 5.8; Br 32.9%).

[OdI2(p-toluidine)2]: from ethanol (n = ^0). ^(Found: ^{N 3.8;} ^143.85. ^Oalcd. ^for

C14H1SOdI2N2: N 4.8; 143.7%).

[Od2Ol4(o-anisidine)2]: from methanol (n = 1). (Found: N 4.1; 0123.4. Oalcd. for C14H1SOd2Ol4N202: N 4.6; 0123.1%).

[Od2Ol4(p-anisidine)2]: from ethanol (n= 1). (Found: 0123.1. Oalcd. for °14H1SOd2Ol4N202:

01 23.1%).

[Od4Ols(o.chloroaniline)2]: frolll ethanol (n = 3). (Found: 014.0; H 1.3; N 2.9; ionic 0128.2. Oalcd. for012H120d40110N2: C 14.6; H 1.2;N 2.8; ionic Cl 28.7%).

[Cd2C14(m-chloroaniline)2]: from ethanol (n = 1). (Found: C 23.0; H 1.7; N 4.5; ionic 0122.5. Oalcd. for 012H12Cd2C~N2: 023.2; H 1.9; N 4.5; ionic C122.8%).

[Cd2Ol4(p-chloroaniline)2]: from ethanol (n = 1). (Found: ionic 0123.4. Calcd. for

°12H12Cd2C16N2: ionic 0122.8%).

[Od2Ol4(p-bromoaniline)2]: from ethanol (n = 1). (Found: 0120.1. Oalcd. for C12H12Br2Cd2014N2: 0120.0%).

[Od4Ols(m-nitroaniline)2]: (n = 3). (Found: N 5.1; 0127.8. Calcd. for °12H12Od4OlsN404:

N 5.5; Cl 28.1%).

----

Evidence for change of composition after recrystallization: The complex formed from o-chloroaniline and cadmium chloride analyzed for chlorine as follows:

Before recrystallization: Found: ionic C127.0. Oalcd. for [Oda016(o-chloroaniline)2]

(n = 2), C12H120daOlsN2:ionic 0126.4%.

5) H. 0. PRITCHARDand H. A. SKINNER, Ohem. Reviews 00,767 (1955).

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J. M. HAIGH, M. A. VANDAM and D. A. THORNTOK,Series of Xew Cadmium Complexes 103

After recrystallization: Found: ionic CI28.2. Calcd. for [Cd4CIs(o-chloroaniline)2]

(n = 3), C12H12Cd4Cl1oN2:ionic CI 28.7%.

We thank the South African Council for Scientific and Industrial Research and the Council of Rhodes University for grants towards equipment.

Grahamstown, South Africa, Department of Chemistry, Rhodes Uni- versity.

Bei der Redaktion eingegangen am 2. Januar 1967.