Infrared spectra of N-aryl salicylaldimine complexes substituted in both aryl rings

J. inorg, nucl. Chem., ~973, Vol. 35. pp. 2319-2327.

Pergamon Press.

Printed in Great Britain

INFRARED SPECTRA OF N-ARYL SALICYLALDIMINE COMPLEXES SUBSTITUTED IN BOTH ARYL RINGS G. C. PERCY Department of Chemistry, University of Cape Town, South Africa and D. A. T H O R N T O N Department of Physiology and Medical Biochemistry, University of Cape Town, South Africa

(Received 28 August 1972) A b s t r a c t - T h e i.r. spectra of eight ~SN-labelled complexes of N-p-tolylsalicylaldimines with substituents in the 3- and 5-positions of the salicylaldimine ring yield assignments of metal-ligand stretching frequencies (uM-L) and certain ligand vibrations. The assignments are supported by observing the effects on the spectra of metal ion substitution and substitution in the salicylaldimine and N-aryl rings. The uM-N values of complexes with common ligand substituents are metal ion dependent in the order Co < Cu > Zn expected from crystal field theory. Substituent-induced shifts in uM-N are related to the resonance polar effects of salicylaldimine substituents and to the inductive effects of N-aryl substituents. INTRODUCTION

IN A PREVIOUS paper[l] we derived assignments of metal-ligand stretching frequencies in the i.r. spectra of N-aryi salicylaldimine chelates by observing the effects of isotopic labelling, metal ion substitution and iigand substitution on the spectra. In this paper a combination of these techniques is applied to the complexes (I; M = Co, Cu, Zn) in which substituents are present in both the N-aryi and salicylaldimine rings. R

x

(1) EXPERIMENTAL The complexes (Table 1) were prepared by the general methods described ([1] and references cited therein) for N-arylsalicylaldimine chelates using the appropriately substituted salicylaldehyde in place of salicylaldehyde itself. The ~SN-labelled compounds were derived from l~N-p-toluidine of 96-7 atom-% isotopic purity supplied by Prochem Ltd. I.R. spectra were determined on Nujol or (between 1500 and 1300cm -1) on hexachlorobutadiene mulls on a Beckman IR-12 spectrophotometer calibrated against carbon dioxide and water vapour. Replicate spectra of the 15N-labelled compounds and their unlabelled analogues yielded a reproducibility of better than 0.4 cm -~ in all quoted frequencies. I. G . C . Percy and D. A. Thornton, J. inorg, nucl. Chem. 34, 3357 (1972). 2319

2320

G . C . PERCY and D. A. T H O R N T O N Table 1. Analytical data for salicylaldimine complexes (formula I)

M

R

R'

X

N isotope

Cu

H

C1

CH3

15N

Cu

Br

Br

CH3

15N

Cu

C1

C1

CHa

15N

Cu

OCH3

H

CHa

15N

Co

Br

Br

CH3

15N

Co

OCH3

H

CH3

15N

Zn

Br

Br

CH3

15N

Zn

OCHz

H

CH~

15N

Cu

H

C1

CH3

14N

Cu

CI

C1

CH3

14N

Co

Br

Br

CH3

14N

Cu

Br

Br

CH,~

14N

Zn

Br

Br

CH.~

14N

Co

Br

Br

H

14N

Cu

Br

Br

H

~4N

Zn

Br

Br

H

~4N

Co

Br

Br

CO2C2H.~

~4N

Cu

Br

Br

CO2C2H5

14N

Zn

Br

Br

CO2C2H~

14N

Co

Br

Br

I

14N

Cu

Br

Br

I

14N

Zn

Br

Br

I

~4N

Co

Br

Br

F

14N

Cu

Br

Br

F

~4N

%C %H %N Calc./Found 60"6 59"5 41 "9 42'2 53 "9 53"4 66"0 65 "6 42'2 42"0 66"5 66"8 41 "8 42"1 65"8 65"7 60"8 60"9 54" 1 54"0 42"3 42'1 42"0 42"0 42"0 41 "8 40"7 40"9 40"5 40"6 40"4 40'3 42"2 42"2 42'0 42"0 41 "9 41"8 30"6 30"9 30"5 30'7 30 "4 30'3 38"9 38"9 38'7 38'6

4"0 4"0 2'5 2"5 3 '2 3"2 5"2 5 '0 2"5 2"4 5"2 5"3 2"5 2"5 5"1 5"1 4"0 4"0 3"2 3"3 2"5 2"6 2"5 2"5 2'5 2"4 2' 1 2"2 2" I 1"9 2"1 1"9 2"6 2"6 2"6 2"5 2'6 2"6 1"4 1"4 1"4 1"4 1"3 I "4 1"7 1'7 1'7 1"9

5'4 4'8 3"7 3"3 4"8 4"1 5"5 5"0 3"8 3"4 5'5 5"2 3"7 3"5 5"5 5"1 5"1 5"0 4"5 4"5 3'5 3'2 3'5 3'2 3"5 3"4 3"6 3"6 3"6 3"2 3"6 3'3 3" 1 3' 1 3'0 3"0 3"0 3"1 2"7 2"6 2"7 3"0 2"7 2'9 3'5 3"2 3"5 3"4

2321

I.R. spectra of N-aryl salicylaldimine complexes

Table 1 (Cont.) N M

R

R'

X

isotope

Zn

Br

Br

F

'4N

Co

Br

Br

OC6H~

14N

Cu

Br

Br

OC6H5

14N

Zn

Br

Br

OC6H.~

14N

Co

OCH3

H

CH3

14N

Cu

OCH3

H

CH3

14N

Zn

OCH3

H

CHa

'4N

Co

OCH3

H

Br

~4N

Cu

OCHa

H

Br

~4N

Zn

OCH3

H

Br

'4N

%C

%H

%N

Calc./Found 38-6 38-3 48-0 47.8 47.7 47,4 47,7 47.8 66.8 66-2 66.2 66.3 66.0 66.0 50-2 49.9 49.9 49.9 49.8 50.2

1.7 1-8 2.5 2-5 2.5 2.5 2.5 2-6 5.2 5.2 5.2 5-2 5,2 5.2 3'3 3-0 3.3 3.4 3.3 3.6

3-5 3.4 2.9 3.0 2.9 3"0 2-9 3,0 5.2 5.3 5.1 5.2 5.1 5,2 4.2 4.1 4.1 3-9 4-1 4.2

RESULTS A N D DISCUSSION

In the following discussion, shifts in the infrared bands induced by four different effects will be referred to. For brevity, these bands will be designated: ~SN-sensitive (shifted by 15N-labelling), M-sensitive (shifted by metal ion substitution), R-sensitive (shifted by the substituents R and R' in the 3- and 5positions of the salicylaldimine aryl ring) and X-sensitive (shifted by the substituents X in the N-aryl ring).

Effect of '~N-labelling Figure 1 depicts the spectra of the eight complexes with a common N-aryl substituent (X = CH3) which were investigated by the 15N-labelling technique. Frequencies and assignments are given in Table 2. The three stretching vibrations expected to exhibit maximum 15N-sensitivity are uC=N, u C - N and uM-N. Each of these vibrations is subject to the probability of coupling, which will have the effect of diminishing the magnitude of the 15N-induced shifts and increasing the number of bands which may be assigned to any one mode. Thus a theoretical '~Ninduced shift o f - 40 cm -1 is expected [2] for a hypothetically uncoupled uC--N band whereas none of the shifts observed here exceeds - 10 cm -1. ~5N-Labelling generally shifts both of the two strong bands within the range 1570-1620 cm -~ towards lower frequency, suggesting their assignment to coupled uC=N. The band of lower frequency is always the more 'SN-sensitive and is 2. G. Karabatsos, J. org. Chem. 25, 315 (1960).

2322

G . C . PERCY and D. A. T H O R N T O N

M

R

R'

Cu

H

CL

Cu

C(

Cl

Co

Br

Br

X

-~'

Br

Br

Zn

Br

Br

Co

~;I-I 3

H

Cu

DCH3

H

Zn

OCH3

H

Z# ' '

CH3

Tt

A Cu

=

uVJL ,a^

~' '--

# I , ~ ^ A ~ }&'..A.~

'--

~.~

"¢ -_

~

~

1600

1400

1200

I000

-~

800

600

.~

,

400

cm-I Fig. 1. Infrared spectra of N-p-tolylsalicylaldimine complexes (formula I, X = CHa). Figures in parentheses are ~SN-induced shifts exceeding 2 cm-L

therefore the vibrationally purer vC=N band. In some spectra an intermediate shoulder occurs near 1605 cm -1. Its position and insensitivity to labelling suggest its assignment to the aromatic ring stretching frequency. In each spectrum, a band within the range 1370-1410 cm -1 exhibits a 15N-induced shift of between - 5 and - 8 cm -1. This band, masked by Nujol absorption but observed in the hexachlorobutadiene mull spectra, is assigned to the exocyclic vC-N mode. 15NSensitive bands near 780 and 880 cm -1 probably originate from the C = N - C bending mode. Generally, only one of these bands is significantly shifted. The vC-O assignments are made on the basis of those previously proposed[l, 3] for salicylaldimine complexes. Below 600 c m - 1 we expect to observe the metal-ligand stretching frequencies (vM-L). In salicylaldimine complexes both M - O and M - N bonds are present and coupling between vM-O and vM-N will undoubtedly occur. Thus it is not surprising to find as many as five bands below 600 cm -~ which are shifted towards lower frequencies by lSN-labelling. These occur as a group of two to four bands within the range 490-550 cm -~ and a single band below 450 cm-k In general, the ~SN-sensitivity of these bands increases with decreasing frequency, suggesting assignment of the band below 450 cm -1 as the vibrationally purest vM-N band. 3. J. E. Kovacic, S p e c t r o c h i m .

Acta

23A, 183 (1967).

I.R. spectra of N-aryl salicylaldimine complexes

2323

II

I

i

I

i

I

I

I

I

I

~

~

~

I

I

~

i

I

I

i

I

i

~

i

I

I

i

I

;<

g ...Q

#

%

T E

~

",'i'

~

+

,,@

~ -

b

~

~

~

I

t

I

- ~

I

- ~

I

~

I

7

I

I

77

I

~.o [-

E

2324

G.C.

P E R C Y and D. A. T H O R N T O N

On this basis, the band nearest 550 cm -x will originate in v M - O with sufficient coupling from v M - N to render it just sensitive to the heavier isotope. This argument does not preclude the existence of a less-coupled v M - O band in which the ~,M-N contribution is small enough to yield a 15N-induced shift outside the limits of detection. Such a band, if it exists, could only be reliably assigned by a corresponding 1sO-labelling study. The frequencies of the bands assigned above on the basis of their 15N-sensitivities are generally similar to those previously proposed[l] for unsubstituted salicylaldimine complexes (R = R' ----H). Furthermore, the vibrationally purest v M - O bands observed in the present study occur within the range of assigned v M - O in the spectra of salicylaldehyde complexes with similar R and R' substituents [4]. Effect o f metal ion substitution Consistent with structural evidence [5] which points to tetrahedral coordination in Co(II) and Zn(II) chelates of N-arylsalicylaldimines, complexes of these ions with common ligand substituents yield practically identical band patterns. The spectra of the Cu(II) complexes have different band patterns in accordance with their unique structures (generally square planar with some tendency towards pseudo-tetrahedral symmetry). Crystal field theory predicts [6, 7] a stability order Co < Cu > Zn for complexes of these ions with identical ligand substituents. It is now well established[7, 8] that metal-ligand stretching frequencies follow the stability order predicted by crystal field theory. Hence, any band correctly assigned to v M - L on the basis of its 15N-sensitivity is expected to be M-sensitive in the order Co < Cu > Zn. The present study includes two sets of 15N-labelled complexes of these ions with common ligand substituents (X = CH3, R = R' = Br and X = CH3, R = OCH3, R' = H). In both sets, the bands assigned to v M - L on the basis of their 15N-sensitivity are also M-sensitive in the expected order Co < Cu > Zn (Fig. 1). The magnitudes of both its ~SN- and M-sensitivity establish the band near 400 cm -~ as the vibrationally purest v M - N band in the 3,5-dibromo- and 3methoxy-N-p-tolylsalicylaldimine complexes (X = CH3). This band is observed (Fig. 1) to lie on the low frequency side of a blank spectral region centred near 450 cm -~. In extending the metal ion substitution study to complexes with different N-aryl substituents (X) it is observed (Fig. 2) that the corresponding band is M-sensitive in the order Co < Cu > Zn for each set of three complexes of these ions with common X-substituent. Ligand vibrations commonly exhibit M-sensitivities which are either in the same or opposite direction to those exhibited by metal-ligand vibrations [7, 8]. In the present study it is observed (Fig. 2 and Table 3) that the M-sensitivity of the principal vC=N band in the complexes with common ligand substituents is generally Co < Cu > Zn. Thus, any change in the bond order of the M - N bond 4. 5. 6. 7. 8.

G. C. Percy and D. A. Thornton, J. inorg, nucl. Chem. To be published. R. H. Holm and G. W. Everett, Prog. lnorg. Chem. 7, 83 (1966). R. D. Hancock and D. A. Thornton, J. S.Afr. Chem. Inst. 23, 71 (1970). J. M. Haigh, R. D. Hancock, L. G. Hulett and D. A. Thornton, J. Molec. Struc. 4, 369 (1969). R. D. Hancock and D. A. Thornton, J. Molec. Struc. 4, 361 (1969); 6, 441 (1970).

I.R. spectra of N-aryl salicylaldimine complexes

co

Cu

CH 3

Cu

H

_oos~ 0

[ ~, a ~ ] ~

~k,

A#L~

I ~A~o

co

~h/~

cu :o,c#,+o.~s

~

~_

I ,/~/~ ~ ~

z,

I

•/v, ~ I

~A~,~.~I

h A~

~

2325

I..~^

/

IAAAAAA. l mmA A #IA A#m,A It4 ~

co Cu

[~

F ,o.r, f ~

co

1~

~A

M

I

,~/,/ I , ~ ~

r

~

1

] I

kA~.l

Cu 0CBH511"07 '5 ~

t'/~,A^I AAAtbqA I

Zn

A,,~,A a~AAAI

co Cu

~

,~ ~

CH 3 -0"05

Zn

co

Cu

H

0

Br

+O-"t:3

Zn

C-

~

I

^t

A ~ ,,~

A I

JMI, A

A I

•

^I

IA~^ A A A]

lN~/laA,I, ^ A ,^I A,,vlA

Zn

AAAAA

AIAAA 1650 1550 600

500

400

I ^l 300

cm-I

Fig. 2. Infrared spectra (1650-1550 and 600-250 cm ') of 3,5-dibromo- and 3-methoxysalicylaldimine complexes showing M- and X-sensitive bands. Shaded peaks: principal vC=N bands; solid peaks: v M - N .

imposed by the crystal field effect of the metal ion is transmitted to affect the C=N bonds similarly. A similar relationship between v M - O and v C - O has been observed [8] in metal/3-ketoenolates. The bands assigned to v C - N and vC-O have M-sensitivities Co > Cu < Zn, i.e. the opposite of v M - N and vC=N. Thus, where metal ion substitution leads to stabilization of the M - N and C=N bonds, this is accomplished at the expense of the C - N and C - O bonding.

G. C. PERCY and D. A. THORNTON

2326

Table 3. vM-L and vC=N values (cm-~) of salicylaldimine complexes (formula I) X

F

M = Co vC=N vM-N

M=Cu vC=N vM-N

M = Zn vC=N vM-N

3,5-dibromosalicylaldimine complexes (R = R' = Br) -0-05 1612 394 1615 433 1616 1575 1596 1588 1580 H 0 1606 413 1605 422 1607 1581 1590 1587 CO2C2H5 +0-55 1609 417 1600 426 1615 1591 1594 1594 I +0.67 1591 424 1596 426 1597 1573 1572 1575 F +0.71 1612 429 1608 458 1614 1584 1592 1588 OCoH5 + 0.75 1611 434 1612 444 1612 1593 1601 1593 1575 1593 1579 CH3

CHa H Br

3-methoxysalicylaldimine -0"05 1609 1588 0 1606 1583 +0.73 1611 1590 1580'

complexes (R = OC H3, R' = H) 404 1619 406 1619 1596 1594 427 1609 436 1612 1588 1590 434 1609 436 1615 1601 1597 1582 1582

390 397 410 405 424 426

402 425 429

E f f e c t s o f salicylaldimine substitution ( R - s e n s i t i v e b a n d s ) I f the electronic effects of ligand substituents are transmitted to the chelate ring we would anticipate some effect on the v M - L bands. Included in the 15Nlabelling study were complexes with a c o m m o n metal ion, Cu(II), and a c o m m o n N-aryl substituent, X = CH3, but having five different combinations of 3,5salicylaldimine substituents, R and R ' , which are known[9] to have strong *r-electron releasing capacities. A quantitative measure of these capacities is provided by Taft's [9] resonance polar parameter, o-p-o,'. In Table 2 these complexes are arranged in order o f decreasing value o f o-f o-'. It is observed that, except for R = OCH3, v M - N is shifted towards higher frequencies as the combined ,r-electron releasing capacities o f the R and R ' substituents increases. T h e anomalously low value of v M - N for the complex with R = O C H a possibly arises from the steric effect imposed by a bulky substituent ortho to the carbonyl group o f the chelate ring. E f f e c t o f N - a r y l substitution ( X - s e n s i t i v e b a n d s ) The transmission of the electronic effects o f the R and R ' substituents to the chelate ring m a y be facilitated or supressed by the X-substituents in the N - a r y l 9. R. W. Taft, In Steric Effects in Organic Chemistry (Edited by M. S. Newman) p. 591. Wiley, New York (1956).

I.R. spectra of N-aryi salicylaldimine complexes

2327

ring according to the electron withdrawing or releasing capabilities of X. The mechanism of transmission of the electronic effects of X through the molecule may be mesomeric or inductive or may occur by a combination of both processes. Recently, Swain and Lupton[10] have achieved a quantitative separation of the field (largely inductive) and resonance (largely mesomeric) capacities of organic substituents. The values of v M - N for the series of salicylaldimine complexes with common M, R and R' are found to be related to Swain and Lupton's field parameter (F) and not in any obvious way to the resonance capacities of the substituents. Inductively electron withdrawing X-substituents shift v M - N towards higher frequencies. This effect may be observed from the data in Table 3 where the complexes are arranged in order of decreasing values of F (i.e. increasing electron withdrawing capacity of X). Quite apart from their secondary effect on the R-substituents, the electronic effects of X may directly transfer electron density to or from the M - N bonds. In our previous study[l] of N-arylsalicylaldimine complexes with R = R ' = H, only this direct mechanism was possible and v M - N was observed to shift to higher frequencies in relation to the Hammett substituent parameter, trp, which comprises both inductive and resonance contributions. The fact that the inductive effects of the X-substituents assume increased significance in the complexes with R-substituents suggest that in these complexes, the secondary effect of X on the electron releasing capacities of the R-substituents plays an important role in determining the M - N bond order and hence the value of v M - N . The difference between the relative significance of the inductive and mesomeric effects of the X-substituents in the salicylaldimine complexes with and without R-substituents illustrates the generalization[10] that there are not just two or three levels for the relative importance of field vs resonance parameters but rather, a broad continuum, the precise balance between the two effects depending on both the molecular composition and the physical property being determined. thank the University of Cape Town Research Grants Committee and the South African Council for Scientific and Industrial Research for financial assistance.

Acknowledgements-We

10. C. G. Swain and E. C. Lupton, J. am. chem. Soc. 90, 4328 (1968).