Chromium(III) Complexes with Some Optically Active

Chromium(III) Complexes with Some Optically Active cr-Hydroxy Acids M. S. El-Shahawi* and A. A. El-Bmdary C h e m istry D e p a rtm e n t, F a c u lty o f Science a t D a m ia tta , M a n so u ra U n iv e rsity , D a m ia tta , E g y p t Z. N a tu rfo rs c h . 4 8 b , 2 8 2 - 2 8 6 (1993); received S e p te m b e r 9 /N o v e m b e r 23, 1992 C h ro m iu m C o m p le x e s, a - H y d r o x y A cid s, IR S p e c tra , C D S pectra C o m p le x e s o b ta in e d fro m C rC l3P y 3 w ith som e o p tic a lly active a -h y d ro x y acids h a v e been c h a ra c te riz e d by e le m en tal a n aly sis, m ag n e tic su sc e p tib ility , v ib ra tio n a l (IR ), e le ctro n ic a n d c irc u la r d ic h ro ism (C D ) sp e c tra . T h e m a g n e tic su sce p tib ility d a ta a re close to th e sp in -o n ly v alue fo r a d 3 c h ro m iu m (III) ion in o c ta h e d ra l o r d isto rte d o c ta h e d ra l sy m m etry . T h re e ( C r - C l ) v ib ra tio n a l m o d e s in th e re g io n 4 1 0 - 2 9 0 c m “ 1 a re observed fo r som e o f th e c o m p le x es in d ic a tin g C 2l. local sy m m e try o f lig a n d a to m s a r o u n d the c h ro m iu m (III). In th e e le c tro n ic sp e c tra tw o p e a k s a re o b se rv e d in th e ra n g e 1 6 9 4 9 - 18018 a n d 2 2 9 8 6 - 2 4 5 7 0 c m '1. T h e y a re assig n ed to d - d tra n s itio n s in p se u d o o c ta h e d ra l sy m m e try . T he stru c tu re o f th e c o m p le x es is likely to be facial since s tro n g a n d well d efin ed C o tto n effects are o b se rv ed . T h e p a ra m e te rs (D B. /?3S) fo r th e c o m p le x es p ro v id e re a ss u ra n c e t h a t p y rid in e n itro g e n is p reserv ed .

Introduction

The chemistry of chromium is of considerable interest [1], Chrom ium (III) is considered to be es sential to mammals for the maintenance of glu cose, lipid and protein metabolism, but chromium(VI) is reported to be toxic [2]. The carcinogen icity of chromium(VI) is considered in terms of the uptake/reduction model [3]. Little work has been carried out on the nonaqueous preparations of chrom ium (III) com pounds with naturally occurring ligands [4, 5], However, for aqueous media a large num ber of chromium (III) complexes with a-hydroxy acids have been reported [6-11]. The coordinating properties of the title ligands and their coordina tion com pounds seem to be implied in the thera peutic activity displayed by drugs o f transition metal ions [12, 13]. The present investigation deals with the characterization of the complexes of chromium(III) with some optically active a-hydroxy acids in methanol.

used without further purification. The complex C rC l 3Py 3 was prepared by the method of Taft et al. [14] and was stored in a vacuum desiccator over CaCl2. Preparation o f the complexes

To a solution of CrCl 3Py 3 (lm m ol) in dry methanol, the appropriate weights of the a-hydroxy acid are added to obtain 1:1 and 1:3 chromium(III) ligand molar ratios. The mixture was then refluxed with constant stirring. A num ber of complexes appeared to be formed in less than one hour, however, all solutions were re fluxed for 4 h. The solutions were then reduced in volume cooled to room tem perature, and 50 cm 3 ether were added. Crystalline solids separated out. They were washed with ether and finally dried in a desiccator. The solid complexes were then redis solved in methanol and the absorption spectra were measured at room tem perature. Chrom ium was determined as oxide by the method reported [15]. Physical measurements

Experimental Reagents and materials

D-tartaric (Tar), L-malic (Mai), L-mandelic (M an) and L-lactic (Lac) acids were o f reagent grade. BDH pyridine (Py) and m ethanol were * R e p rin t re q u e sts to D r. M . E l-S h ah a w i.

Present address: C h e m istry D e p a rtm e n t, F a c u lty o f Sci ence. U A E U n iv e rsity . A I - A I N 17551 U n ite d A ra b E m ira te s. V erlag d e r Z e itsc h rift fü r N a tu rfo rs c h u n g , D -W -7 4 0 0 T ü b in g e n 0 9 3 2 - 0 7 7 6 /9 3 /0 3 0 0 -0 2 8 2 /$ 01.0 0 /0

The IR, UV-visible and circular dichroism spec tra were measured from KBr disks with a Perkin Elmer 457 spectrometer, a Varian 634 S spectrom eter and an instrum ent described elsewhere [16], respectively. Magnetic measurements were made on a Johnson M atthey magnetic balance. Silica gel TLC plates (Merck 60 F 254) 10x20 cm were em ployed. Results and Discussion

The prepared complexes are listed in Table I to gether with their elemental analyses as well as with

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M . S. E l- S h a h a w i- A . A. E l-B in d a ry • C h ro m iu m (III) C o m p le x e s w ith S o m e O p tic a lly A ctiv e a -H y d ro x y A cids

283

T a b le I. A n a ly tic al d a ta ; ro o m te m p e ra tu re m a g n e tic m o m e n ts //(B . M .) a n d p h y sical p ro p e rtie s o f th e com plexes. Com pound

P e rc en tag e c a lc u la te d (fo u n d ) Cr C H

N

C o lo u r

Veff ^ß,n

M .p . [ C]

Cl

C r (T a r )3P y 3

7.0 (6.8)

43.7 (43.9)

4.0 (3.7)

5.7 (5.4)

-

p u rp le

3.9

189

C r(M a l)3P y 3

7.6 (7.3)

47.0 (47.4)

4.4 (4.2)

6.1 (5.9)

-

violet

4.05

142

C r (M a n )3P y 3

7.0 (6.8)

63.0 (62.7)

4.9 (5.1)

5.7 (5.5)

-

g reen

4.0

170

C r(L a c )3P y 3

9.4 (9.6)

49.6 (50.0)

5.4 (5.7)

7.6 (7.8)

-

g reen violet

3.87

140

C r 2(T a r)2P y 4

14.6 (14.4)

47.2 (46.8)

3.4 (3-2)

7.9 (8.1)

-

p u rp le

3.4

187

C r2(M a n )2P y 4C l2

13.1 (12.8)

54.6 (54.1)

4.0 (3.9)

7.0 (7.2)

9.0 (8.7)

g reen

3.42

197

C r2(L a c )2P y 4C l2

15.6 (15.9)

46.8 (47.0)

4.1 (4.3)

8.4 (8.1)

15.6 (15.7)

g reen

3.45

160

C r2(M a l)2P y 4C l2

13.7 (13.9)

44.4 (44.6)

3.7 (3.5)

7.3 (7.6)

9.3 (9.6)

p u rp le

3.4

176

other physical and chemical properties. The ele mental analyses indicate that the complexes ob tained from the 1:3 ratio of reagents have the formula [C r(L -H ) 3Py3], where L = Tar, Mai, M an or Lac. The complexes formed from the 1:1 ratio have the formula [Cr2( L - 2 H ) 2Py 4Cl2] except for D -tartaric acid which forms the complex C r(T a r-3 H ) 2Py4. The complexes are purple or green violet in colour and have fairly low melting points (< 200 °C). The magnetic susceptibility data (3.4-4.05 B.M .) are close to the spin-only value for a d 3 chromium(III) ion in octahedral or slightly distorted octahedral ligand field with a 4A 2 ground state. The higher values of the magnetic susceptibilities observed for the complexes C r(L -H ) 3Py 3 suggest a mixing of 4A 2 with the terms derived from the excited state 4T2. The mix ing is possibly due to the difference in the field strength of the environment [17]. The lower values of the magnetic moment for the complexes pre pared in 1:1 ratio suggest the form ation of dimeric structures [ 10], The significant IR frequencies o f the complexes with relevant bands of the free ligands and their probable assignments are given in Table II. The strong broad bands in the ranges 3290-3460 and 3020-3100 cm -1 observed in the hydroxy acids are tentatively assigned to OH stretching vibrations, because there are no bands in these ranges in

DL-amino-rc-butyric acid which has no hydroxyl group [18, 19]. The largest bathochromic shift (3 5 -9 0 c m '1) for the OH group upon coordina tion is expected for the complexes prepared in 1:1 ratio [18, 19]. In these complexes, the oxygen atom of the hydroxyl group may participate in the for m ation of a C r - O bond, and the hydrogen bond which is present in the free ligand may break or be come weaker for these compounds. However, for the complexes prepared in 1:3 ratio, the position of the absorption bands corresponding to the OH stretching vibration mode agree with the absorp tion bands of the free ligand, showing that the oxygen atom of the OH group does not participate in the form ation o f C r-O . Displacements o f the C O O - symmetric stretches by 2 0 -2 5 cm -1 to lower wavenumbers and of the antisymmetric stretches by 2 0 -6 0 cm " 1 to higher wavenumbers are observed, showing that the car boxylate groups of the hydroxy acids are probably involved in the coordination [19], Three (C r-C l) vibrational modes in the region 410-290 cm -1 for the complexes [Cr2(L - 2 H ) 2Py 4Cl2], L = Mai, M an or Lac are observed, indicating C2v local symmetry of ligand atoms around the chromium rather than C3v which would allow two bands [20]. C r - N stretching vibrations were also observed in the range 365-310 cm ' 1 for most of the prepared complexes.


284

M . S. E l- S h a h a w i- A . A. E l-B in d a ry • C h r o m iu m (II I) C o m p le x e s w ith S om e O p tic a lly A ctive g -H y d ro x y A cids

T a b le 11. S ig n ifican t IR freq u en cies (cm ') o f the co m p le x es, w ith re le v a n t b a n d s o f free lig a n d in b ra c k e ts, a n d b a n d assig n m en ts. Com pound

VasCOO-

vOH

vsC O O "

Av

v (C r-N )

v ( C r - C l)

C r ( T a r ) 3P y 3

3450 (3450)

1620 (1590)

13 5 0 ,1 3 6 0 (1380)

270

370, 320, 300

-

C r (M a l)3P y 3

3455 (3460)

1625 (1595)

1360 (1400)

265

3 6 0 ,3 1 6 , 290

—

C r ( M a n ) 3P y 3

3460 (3460)

1650 (1610)

1350 (1390)

300

3 7 0 .3 1 6 , 290

C r(L a c )3P y 3

3295 (3290)

1640 (1620)

1360 (1380)

240

3 7 5 ,3 1 0 , 295

—

C r 2( T a r)2P y 4

3470

1650

1360

295

360, 340, 300

—

C r ,(M a l) 2P y4C l2

3480

1625

1350

275

3 8 0 ,3 3 5 , 295

370, 330, 285

C r ,( M a n ) 2Py4C l2

3485

1630

1355

275

365, 340, 316

2 9 0 ,3 1 0 , 290

C r 2(L a c )2P y 4C l2

3340

1645

1355

290

360, 340, 310

370, 320, 295

gesting symmetry reduction of these com pounds m. The spectra of the complexes formed in 1:1 ratio are different from those prepared in 1:3 ratio con firming the form ation of different types of struc tures involving more extensive replacement of the initial ligands by hydroxy acid ligands. Bonding of the tartrate ligand to chromium (III) could be ei ther through two carboxyl groups; or one hydroxy and one carboxyl group [22 ], or dimers could be formed by tartrate bridges [ 10]. The param eters D q, B, and ß 35 have been calcu lated by Tanabe-Sugano procedures [21, 23] and are summarized in Table III. The Dq values of the complexes are close to that of the chloride ion (D q

The electronic spectra of the complexes are sum marized in Table III, and representative spectra are given in Fig. 1. Two peaks are observed in the ranges 16949-18018 and 22986-24570 cm “ 1 and are assigned to 4A2g —» 4T2g and 4A2g —> 4T lg(f} d - d transitions, respectively, in the octahedral and pseudooctahedral symmetry [21]. A nother strong peak is observed in the range 29 411 — 32258 cm - 1 for some of the complexes and is assigned to 4A2g —> 4T lg(p) d - d transitions. The spectra of the complexes C r 2(M al) 2Py4Cl2; Cr 2(Lac)2Py 4Cl2 and Cr(Lac) 3Py 3 reveal that the spin forbidden transition at 22988-24390 cm “ 1 gives higher absorption coefficients than the spin allowed band at 16948-17 794 cm -1 (Fig. 1) sug

T a b le III. E le ctro n ic sp e c tra (cm ') a n d c irc u la r d ic h o rism (n m ) d a ta o f th e c o m p le x es w ith lig a n d field p a ra m e te rs in m e th a n o l. Complex C r(Tar),Pyi Cr(M al),Py, Cr(Lac),Py, Cr(M an),Py, Cr,(Tar),Py4 Cr,(M al),Py,Cl, Cr,(Lac),Py,Cl, C r2(M an)2Py2Cl,

__» -»T A lg(0

4A2g- 4T 2g vi

4A

17921 18018 17094 17 544 17794 17921 16949 17391

23 809 24570 23 255 23 809 24390 24096 22986 23255

v2

2g

4A2g _»4tA lg(p)

B

Ä5

CD (nm)

559 638 597 606 645 592 584 560

0.61 0.70 0.65 0.66 0.70 0.65 0.64 0.61

44 5 (-), 490( + ), 570( + ), 590(+), 620(+, sh) 384(+), 440( + ), 486( + ), 550(-), 594( + ), 640(-), 654(-) 4 0 0 (-),4 7 0 (-),5 3 0 (-), 580( + ), 630(-) 290( + ), 4 0 0 (-), 5 26(-). 590( + ), 655(-), 670(-) 410( —), 520( + ), 550( + ), 590( + ), 650(-), 640(-), 654(-) 386( + ), 440( +), 480( - ) , 550( +), 590( - ) , 640( - ) , 650(-) 405( —), 475( —), 5 34(-), 580(+), 630(-) 300( + ) , 410( + ) , 545( + ) , 595( - . w), 650( - )

V3 -

34482 29411 30303 31250

1792 1802 1709 1754 1779 1792 1695 1739

* (B fo r th e C r(III) io n is 918 cm ')•


M . S. E l- S h a h a w i- A . A . E l-B in d a ry • C h ro m iu m (III) C o m p le x e s w ith S om e O p tic a lly A ctiv e a -H y d ro x y A cid s

Wavelength ( n m )

Fig. la . E le c tro n ic s p e c tra of th e C r2(L a c )2P y 4C l2, -----------; C r 2(L a c )3P y 3, C r2(M a l)2P y 4C l2, --------in m e th a n o l.

co m p lex es, ------- a n d

Wavelength ( n m )

F ig. 1 b. C irc u la r d ic h ro is m sp e c tra o f th e co m p lex es, C r2(T a r) 2P y 4, ----------- a n d C r(T a r)3P y 3, -------in m e th a nol.

for CrClg- is 1318 cm -1) and provide reassurance that pyridine nitrogen rather than chlorine binding is preserved [24, 25], The ligand field strengths of the complexes can be arranges in the order: C r(M al) 3Py 3 > C r 2(M al) 2Py4Cl2 = C r(Tar) 3Py 3 > Cr 2(Tar)2Py4 > C r(M an) 3Py 3 > Cr 2(M an) 2Py4Cl2 > Cr(Lac) 3Py 3 > C r 2(Lac) 2Py4Cl2. The same infor m ation regarding the coordinating capacity of the «-hydroxy acids arranges in the above order is also confirmed from the values of ß 35 for these com plexes. The ß 35 values are slightly higher compared to the range observed for C rN 3X 3 chromophores (ß i5: 0.58-0.65) [26] or CrS 6 systems (/?35: 0 .4 4 -

285

0.45). Hence the chelates appear to involve C rN 30 3 and/or C rN 20 2Cl2 chromophores. The ß values for most of the 1:3 complexes are higher than those of the 1:1 complexes. This is probably due to the fact that not all chlorine ligands in CrCl 3Py 3 have been replaced by the a-hydroxyacid ligand in the 1:1 ratio, or it could be due to config urational dissymmetry and vicinal effects of the coordinated a-hydroxy acid [ 10]. The results of the CD spectra are summarized in Table III, and representative spectra are given in Fig. lb. The observed C otton effects confirm that the a-hydroxy acids are complexed with Chromium(III) ion to produce optically active octa hedral chromium (III) complexes. The complexes are likely to be fac in terms of «-hydroxy acid carboxylate and/or hydroxy groups since strong and well defined C otton effects are observed [19, 21], The CD spectra of D -tartaric or L-malic acid com plexes have greater strengths than those prepared from L-lactic or L-mandelic acid. This is possibly due the ability of tartaric or malic acid to coordi nate as tridentate species through the oxygens of carboxyl and prim ary hydroxyl groups, while L-lactic or L-mandelic acid can only function as bidentate complexing agents. The different signs o f the C otton effects of Cr(Tar) 3Py3, [445(-), 490( + ), 570( + ), 590( + ), 620( +) nm] and Cr(M al) 3Py 3 [384( + ), 440(+), 486( + ), 550(-), 594( + ), 640(-), 6 5 4 (-) nm] in methanol suggest similar complexes but with li gands of opposite configuration. On the other hand, the signs o f the C otton effects of Cr(Lac) 3Py 3 [400(-), 4 7 0 (-), 530(-), 580(+) and 630(-) nm] C r(M an) 3Py 3 [290(+), 4 0 0 (-), 526(-), 590(+), 655(-), 6 7 0 (-) nm] are analogous, sug gesting form ation o f similar complex species. It is not clear from the CD curves whether the C otton effects arise from one species or a mixture of different complex species in methanol. There fore, TLC using ethanol-water 8 :3 (v/v) was used, and it confirmed only one complex species is pre sent in solution. Hence, the observed C otton ef fects [360-380( + ), 4 3 9 -4 6 0 (-) nm] in the spin forbidden transition are assigned to 2E(2E), 2A 2(2T Ig) and 2E(2T lg) in octahedral symmetry. The C otton effects observed in the range 510650 (nm) arise from the splitting of the spin al lowed 4A2g —> 4T2g to 4A(T2g) and 4E(4T 2g) transi tions. The band observed at 285-290( + ) nm could


286

M . S. E l- S h a h a w i- A . A. E l-B in d a ry • C h ro m iu m (III) C o m p le x e s w ith S om e O p tic a lly A c tiv e g -H y d ro x y A cids

arise from the charge transfer peak observed at 260-270 nm in the UV-spectra of most chromium (III) complexes [16, 17].

[1] K . G . S to lle n w e rk a n d D . B. G ro v e , J. E n v iro n . A n a l. 14, 3 9 6 (1 9 8 5 ). [2] S. L a n g a rd a n d T. N o rs e th , in L. F rib e rg (ed.): H a n d b o o k o n th e T o x ic o lo g y o f M e ta ls, p. 338, E l s e v ie r/N o rth H o lla n d B io m ed ical P ress, A m ste r d a m (1979). [3] K . E. W e tte rh a h n a n d P. H . C o n n e tt, S tru c t. B o n d ing (B erlin) 54, 93 (1983). [4] D . H . B ro w n , W . E. S m ith , M . S. E l-S h a h a w i, a n d M . F. K . W a z ir, In o rg . C h im . A c ta. 124, 125 (1986). [5] M . S. E l-S h a h a w i, Ph. D. T hesis, S tra th c ly d e U n i versity , U . K . (1986). [6] A. J. M c C a ffe ry a n d S. F . M a so n , T ra n s . F a ra d , Soc. 59, 1 (1963). [7] T . B ruer, H elv. C h im . A c ta 46, 2389 (1963). [8] H . B enar, E. V io n a , a n d T . L u p u , R ev; F iz C him . Ser. 5 ,371 (1969). [9] R. E. T a p s c o tt a n d R . L. B elfo rd , In o rg . C h e m . 6, 735 (1967). [10] S. K iz ak i, J. H id a k a , a n d Y. S h im u ra , Bull. C hem . Soc. J p n .4 0 , 2 2 0 7 (1 9 6 7 ). [11] P. Vieles, A . B o n m io l, a n d B. L isso rg u g e s, C. R. A c ad . Sei. P a ris, Ser. C 266, 1482 (1968). [12] A. G e rce ly a n d I. S o v a g o , M e tal Io n s in B iological S ystem s, V ol. 9, p. 77 (1978). [13] D. H . B row n, G . C . M c K in la y , a n d W . E. S m ith , J. C h e m . Soc. D a lto n 1978, 199.

We thank Professors D. H. Brown and W. E. Smith for helpful discussions and the facilities provided for number the CD spectra.

[14] J. C. T a ft a n d M . M . Jo n e s, In o rg . S y n th . 7, 132 (1963). [15] R. C. M a ry a , In d ia n J. C h em . 22 A , 529 (1983). [16] D . H . B row n, G . C. M c K in la y , a n d W . E. S m ith , J. C h em . Soc. D a lto n 1977, 1874. [17] M . F re n i, A. G e rv a s in i, a n d P. R o m iti, S p e c tro chim . A c ta 39 A , 85 (1983). [18] Y. I n o m a ta , T . T a k e u c h i, a n d T . M o riw a k i, Spectro c h im e A c ta 40, 179(1984). [19] Y. In o m a ta , T . T a k e u c h i, a n d T . M o riw a k i, In o rg . C h im . A c ta 68, 186 (1983). [20] R. J. H . C la rk a n d C. S. W illia m s, In o rg . C h e m . 4, 3 5 0 (1 9 6 5 ). [21] A. B. P. L ever, I n o rg a n ic E le c tro n ic S p e c tro s c o p y , E lsevier, N ew Y o rk (1968). [22] S. K a iz a k i, J. H id a k a , a n d Y. S h im u ra , Bull. C h e m . Soc. Jp n . 42, 988 (1969). [23] D. S u tto n , E le c tro n ic S p e c tra o f T ra n s itio n M e ta l C o m p le x e s, M c G ra w -H ill, L o n d o n (1968). [24] J. A . C o p p e r. B. F. A n d e rso n , P. D . P u c k ley , a n d L. F . B lackw ell, In o rg . C h im . A c ta 91, 1 (1984). [25] L. E. G e rd o n a n d H. M . G o ff, In o rg . C h e m . 21, 3847 (1 9 8 2 ). [26] C. P reti a n d G . T o si, C a n . J. C h e m . 53, 2545 (1974).


Chromium(III) Complexes with Some Optically Active

Chromium(III) Complexes with Some Optically Active

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