Indian Journal of Chemistry Vol. 39A, October 2000, pp. 1044-1049

Synthesis, characterisation and complexation reaction of ( a~S)-pyridine-2,6-dicarboxylato/iminodiacetato (tetraethylenepentamine) cobalt(III)

in aqueous medium-A comparative study

Nigamananda Dast, Amitabh Dash & Prakash Mohanty*

Department of Chemistry, Utkal University, Bhubaneswar 751 004, Orissa, Indi a

Received 25 June 1999; revised 12 January 2000

Synthesis of (af3S)-(tetren)Co(idaHidipicH2)3

+ (where tetren: tetraethylenepentamine) and kineti cs of their complexation reaction with Ni(ll ) leading to formation of binuclear complexes, 1 and 2, have been studied by stopped-flow/ spectrophotometric method. Both N-protonated and NH-deprotonated forms of ida complex are involved in the formation of (tetren)Co(ida)Ni3

+ while only NH-deprotonated form of the complex is involved in the formation of (tetren)Co(dipic)Ni3+.

The rate data for the formation of binuclear complexes are consistent with IJ mechanism. As expected, (tetren)Co(di pic)Ni3+

(where benzene ring forms a part of 5-membered chelate ring) is - 150 times less stable than its ida analogue (which also forms a 5-membered chelate ring but without benzene ring).

The kinetics of reversible formation of binuclear species of Ni(OH2)6

2+ with unused donor functions of

0-bonded (with Com) glycinate 1, pyridine-213··

carboxylate2, histidinate3, iminodiacetate4 and nitrilotriacetate5 have been reported. These studies also provided the opportunity to examjne the complexing abilities towards Ni(ll) when amino acids are already bound to the cobalt(lll) centre. Pyridine-2,6-dicarboxylic acid and iminodiacetic acids are potentially tridentate ligands. The available denticity of these ligands, however, may be reduced to two by prior coordination to an: inert metal such as cobalt(lll). As a result both the complexes form five membered ring (Structures 1 and 2) on complexation with Ni(ll) using the unused donor si tes of aminoacidates. Also it is known that the metal complexes with five membered ring, where benzene ring forms a part of it, are less stable than those without benzene ring6

. The present study is designed to verify the above fact by comparing the di ssociation of resulted binuclear complexes (1 and 2) and also to compare the complexing abilities of amino acid ligands towards Ni(II).

Materials and Methods Both the complexes [Co(tetren)idaH2](CI04)3 and

[Co(tetren)dipic H2](CI04) 3 were prepared for the first time by the following method: (a~S)­

[(tetren)CoCI](CI04)72 (4 g, 10 mmol) in H20 (ca 20

toept. of Chemistry, Godda College, Godda 814 133, Bihar, India

cm3) was treated with NaOH (0.4 g, I 0 mmol) to produce the corresponding hydroxo complex. The free chloride ions were removed as AgCI by addition of a stoichiometric amount of Ag20 to the acidified solution of hydroxo complex. Pyridine-2, 6-dicarboxylic acid (8.4 g, 50 mmol) or imjnodiacetic acid (6.6 g, 50 mmol) suspended in HiO (ca . 40 cm3) was treated with NaOH (2 g) and the mixture heated gently to give a clear solution. This solution was then added to the chloride free aquo complex and the final pH was adjusted to ca. 3.0 by the addition of dil. HCI04 or NaOH. The resulting solution was heated over water bath till the volume was reduced to ca. I 0 cm3. It was then cooled and treated with a few drops of cold 70% HCI04. After further cooling at ca. 5°C, crystals of corresponding complexes, which were formed, were separated by filtration and washed thoroughly with ethanol and diethyl ether. The crude products were recrystallised twice from acidified hot water.

Microanalysis (C, H and N) were carried out at CDRI, Lucknow. Anal. Calcd. for [Co(tetren)idaH2] (CI04)3: Co, 8.67; C, 21.18; N, 12.35; H, 4.41%. Found: Co, 8.51; C, 20.95 ; N, 12.31; H, 4.85%, for [Co(tetren)dipic H2] (CI04)3: Co, 8.26; C, 25.21; N, 11.76; H, 4.06%. Found: Co, 8.15; C, 25.02; N, 11.55; H, 4.29%. FT-IR in KBr phase (4000-400 cm- 1

) and 1H NMR in 0 20 medium were recorded using JASCO FT­IR, 5300 and Bruker WM-400 FT-NMR spectrometers, respectively. UV-visible measurements were made on a Perkin-Elmer (Model Lambda 20) recording

DAS eta/.: STUDIES ON COBAL T(lll) COMPLEXES 1045

spectrophotometer using 10 mm matched quartz cells. Measurements of acid dissociation constants of the unbound carboxylate/ amine function and the stability constants of the binuclear complexes, (tetren)­Co(ida/dipic)Ni3+ were carried out potentiometrically at 25°C, I= 0 .3 mol dm-3

. The pH measurements were made with an Elico (Model LI 120) pH meter, equipped with a glass combination electrode (Model CL 51). The meter was calibrated using NBS buffers. The concentration of [W] (= aH+/fH+) were calculated from the pH data using calculated values of fH+ by the Davies equation8

.

Kinetics The formation of the binuclear complexes,

(tetren)Co(ida/dipic)Ni3+ was studied under pseudo­first order conditions at pH = 5.7 ± 0.03 (for dipic complex) and 5.7 to 6.9 (for ida complex) using a fully automated Hi-Tech 5F 51 (UK) stopped-flow

(1)

spectrophotometer. 2-(N-morpholino)-ethanesulphonic acid (MES) was used to maintain the pH of the reaction mixtures. The formation (absorbance increased with time) of the binuclear complexes were followed at 280-290 nm. For dissociation, the pre-formed binuclear species (at pH ca. 5.0) were acidified and kinetics of their decay followed . All other kinetic details have been described in our earlier works4

·5

. The rate constants reported were average of atleast seven determinations and the error quoted is the standard deviation . All calculation were made by an IBM PC using appropriate least squares programs.

Results and Discussion Elemental analysis C, H, N and Co data collected in

Table I confirmed the proposed formulae for cobalt(III) complexes. As expected, the UV-visible spectra exhibited absorption maxima at - 345 and 495 nm (see Table 2). The IR spectra clearly indicate the

3+

( 2)

Table !-Analytical data of the cobalt (Ill ) complexes

Complex" Found (Calcd) % Co c N H

[Co(tetren) ida H2] (Cl04)3 8.51 (8.67) 20.95 (2 1.18) 12.31 ( 12.35) 4.85 (4.41)

[Co(tetren) dipic H2] (CI04)3 8.15 (8 .26) 25 .02 (25.2 1) 11.55 (11.76) 4.29 (4.06)

Table 2-UV -visible spectral data, acid dissociation and stability (log KN;) constants of some (amino carboxylato)cobalt(III) complexes"

Complex" "-max (nm) Emax pKd 1 pKd2 logKN;

(dm3mol- 1cm - 1)

(NH 3) 5Co(pycl-1)3+" 350,505 63.0, 76.0 4.10±0.11 4.64±0.151

(NH3) 5Co(idaH2)3+ 340,500 60.9, 66.7 2.30 8.15±0.04 5.65±0.03

(NH3) 5Co(ntaH2i•d 350,500 56.6, 69.3

(tctren)Co(pycH)3+ 349,494 174, 223 3.98±0.11 4.72±0.13

(tetren)Co(idaH2)3+ 346, 493 217 , 225 -2.32 8.27 5.60±0.12

(3.01) (9.6) (8.19)g

(tetren)Co(dipicH2)'+ 345 , 493 219, 228 <2.0 3.8 4.68±0.12 (2. 13±0. 1 )c (4.51±0.03)" (6.95)h

" At 25°C, I= 0.3 mol dm _,(except otherwise mentioned) , values in the parentheses are for free aminocarboxylic acids. " pyc, pyridine-2-carboxylate; ida, iminodiacetate dianion; nta, nitrilotriacetate trianion.

c at pH -4.3; d at pH - 2.0; c I= 0.5 mol dm - 3; rat 30°C; gat 20°C; hat 20°C; I= 0.1 mol dm- 3

Ref.

3

4a

5

4b

This work

This work

1046 fNDIAN 1 CHEM, SEC. A, OCTOBER 2000

presence of both free and coordinated carboxylates in both the complexes. The peaks at- 628 and 1086 cm- 1

indicated the presence of ionic perchlorate. The purity of these complexes was further evident from 1 H NMR spectra. For dipic complex, the broad peaks in the region 8(ppm) = 8.2 and 7.6 are attributed to two sets of pyridine ring protons. The multiplet bands in the region 8(ppm) = 2.6 to 4 .6 are ass igned to -CHrCHr protons of tetren. The signals at 8(ppm) = 4.8 to 5.8 are attributed to -NH (ring) -NHI-NH2 protons of tetren. For ida complex , the signal at 8(ppm) = 2.6 is attributed to -CH2 group of ida, while the peaks in the range 8(ppm) = 2.8 to 3.4 and 5.6 to 7.5 are attributed to -CHrCH2 and -NHINH2 protons of tetren, respectively.

The acid dissociation constants of both the complexes, defined by Eqs. ( I) and (2), are attributed to the proton di ssociation from free -COOH and - NH of ida or dipic, respectively.

. . . I+ Kd 1

(tetren)Co(JdaH 2/d1p1cH2) ~

(tetren)Co(idaH/dipicH/+ +H+ . . . ( I )

KJ,

(tetren)Co(idaH/dipicH/+ ~ (tetren)Co( idaH/dipict +H+ ... (2)

KN,

(tetren)Co(idaldipict +Ni2+ ~ (tetren)Co(idald ipic)Ni 3

+ ... (3)

The stability constant (log KN;) of the I: I complex is defined as Eq. (3) . The values of ac id dissociation constants of I: I complexes, calcu lated using the pH

titration data in the appropriate range by Irving and Rossotti method9

, are also presented in Table 2. The values compare well with those reported for similar complexes. The lower Kd values relat ive to those for free iminodiacetic and pyridine-2,6-dicarboxylic acids indicated the effect of coordination of one carboxylate group to cobalt(III).

The interaction of Ni(II) with (tetren)Co(idaH2)3+

leading to the formation of (tetren)Co(ida)Ni3+ is evident from the instantaneous change in the UV-vis spectra shown in Fig. I; a similar observation was also made with pyridine-2, 6-dicarboxylate complex. Further, the spectral scans for the mixture of Ni(II) with both the complexes at pH.:: ca. 5.7, 0.01 :S [Ni2+], mol dm-3 :S 0.04 indicated that the formation of binuclear complexes is essentially complete at the lowest concentration of the metal ion used .

The reversible formation of binuclear species, (tetren)Co(idaldipic)Ni 3+, follows simple first order kinetics till completion . The pseudo-first order rate constant (kobs) are presented in Tables 3 and 4. The kobs versus [Ni 2+] plots are linear with practically zero intercepts (for ida complex) but with a definite

Table 4-Rate data for reversible fo rmation of binuclear species between Ni (OH 2)/+ and (tetren)Co(dipicH2).1+ at 25°C, pH= 5.70 ±

0.03; 1=03 , !Complex]= 1.5x !O- ·' mol dm- 1; A= 290nm

102 1Ni2• ]. kl)hS 1 o" k2 k.2

(mol dm-·') (s- 1) (dmJ mol- 1 s- 1) (s - ~)

1.0 5.02 ± 0.40 2.34 ± 0.04 2.56 ± 0.09

1.5 5.99 ± 0.38

2.0 7. 18±0.20

3.0 9.61 ±0.63

4.0 11 .96 ± 0.50

Table 3- Rate data for reversible formation of the binuclear complex between Ni(OH2)/+ and (tetren)Co(idaH2)3+

at 25°C, I= 0.3, [Complex] = 1.5x I 0- 3 mol dm - 3; A= 290 nm

I 02 [Ni 2+j pHav 5.7 ± 0.03 6.0 1 ±0.02 6.30 ± 0.01 6.60 ± 0.02 6.85 ± 0.03

(mol dm- 3) k, l,,(s- 1)

1.0 0.24 ± 0.03 0.42 ± 0.05 0.70 ± 0.03 0.9 1 ± 0.03 1.43 ± 0.06

1.5 0.32 ± 0.02 0.63 ± 0.02 0.99 ± 0.03 1.53 ± 0.07 2.30 ± 0.1 2

2.0 0.40 ± 0.02 0.75 ± 0.03 1.27 ± 0.08 1.9 1 ± 0.07 3.29 ±0. 19

2.5 0.57 ± 0.03 1.05 ± 0.03 1.59 ± 0.07 2.56 ± 0. 16 4.00 ±0.21

3.0 0.82 ± 0.04 1.32 ± 0.08 1.86 ± 0.08 3. 14± 0.23 4.63 ± 0.3 1

4.0 0.96 ± 0.05 1.58 ± 0.10 2.52 ± 0. 11 4.08 ± 0.27 6.52 ± 0.30

k1 (dm' mol- 1 s-1) 26.8 ± 3.1 40.3 ± 2.7 60.4 ± 1.0 106 ± 3 166 ± 5

k, (s- 1) - 0.08 ± 0.06 0.02 ±0.06 0.08 ± 0.02 --0.12 ± 0.07 --0. 18 ± 0.12

k1 (dm3 mol- 1 s- 1) 15.4±0.9 10- 3 k2 (d m3 mol- 1 s- 1) = 5.70 ± 0.28

DAS eta/.: STUDIES ON COBAL T(lll) COMPLEXES 1047

2.0 ,.--...,.,---------,

"' u c

"' 1 0 LJ '-0

"' LJ

<l

300 400

.A, nm

Fig. !-Spectral evidence of the interaction between Ni 2+ and

(tetren)(Co(idaH2)3+. ( I) [Ni2+] = 2xl 0--:! mol dm - 3

, (2) [Complex] =

2.2x l0- 3 mol dm- 3 and (3) [Complex] = 2.2xl04 mol dm- 3 +

[Ni2+] = 2x 1 0-2 mol dm - 3. All spectra were recorded at pH 5.7.

intercept for dipic complex (see Fig. 2). The inverse [W] dependence of the gradient of such plots at different pH values is attributed to the involvement of N-protonated and NH-deprotonated forms of the ida complex (pKd2 = 8.27, 25°C) . Considering the above facts the possible reaction pathways are given in Scheme I .

k,

(tetren)Co(idaH)2+ +Ni2+ ~ (tetren)Co(ida)Ni3+ +H+ k_,

k,

(tetren)Co(ida) ++Ni2+ ..,.-::_ ( tetren)Co(ida)Ni3+

k_2

Scheme 1

Accordingly kobs takes the form of Eq. 4 and kr and k, are given by Eqs 5 and 6, respectively.

.. . (4)

7 . 0~--------------,

6 .0

s.o

) . 0

2 .0

t . O

(N i 2•) ltiOI dlft-:3

Fig. 2-Piots of k.,hs versus [Ni2+] for the formation of

(tetren)Co(ida)Ni3+ at 25°C (I= 0.3 mol dm- 3

) and pH= 5.71 (1), 6.01(2), 6.30(3), 6.60(4), 6.85(5).

. .. (5)

... (6)

However, only NH-deprotonated form of the dipic

complex (pKd2 = 3.8, 25°C) exist under the experimental conditions (pH ;;:::: 5.7) . Also the gradient of kobs versus [Ni2+] plots in the pH range 5.7 - 6.9 for dipic complex are practically constant. Hence, the rate equation for dipic complex is reduced to Eq. (7).

... (7)

The k1 and k2 values, obtained from the slope and intercept of kt{[W] + Kd2) versus [H+] , using the known value of Kd2 are presented in Table 2. However, k2 and k_2 values for dipic complex are directly derived from kobs versus [Ni2+] plots. A comparison of k1 and k2

values showed that Ni 2+ reacts with (tetren)Co(idat at a faster rate (at least 370 times) than with (tetren)Co(idaH)2+. This rate difference may be ascribed to the difference in the electrostatic interaction between the reactants. This is further evident from the formation rate constants listed in Table 5, which shows that the reactivities of different cobalt(III) substrate with Ni2+ are strongly dependent on the charge of the ligand.

If the complex formation studied proceeds through an Eigen-Wilkins type 10

, basically dissociative interchange (/d), then k' 1 (= k/Kos) or k'2 (= kz/K'os)

(where Kos and K' os are the outer-sphere association

1048 rNDIAN J CHEM, SEC. A, OCTOBER 2000

Table 5-Comparison of rate parameters for the reversible formation of binuclear species between nickel(II ) and (tetren)Co(idaHzfdipicH2)

3+ with related systems at 25°C

Reacting species -] /(mol dm · ) /y(dm3 mol- 1 s-1) k., (s-1) Refs.

(NH3)5Co(pyc/• 0.3 0.057 ± 0.002 2

(NH3) 5Co(gly)2' 0.3 270 ± 37 0.10 ± 0.007

(tetren)5Co(glyH)2+ 0.3 271 ±41 0.14 ± 0.015

(tetren)5Co(pyci--J)'+ 0.3 --6.7 ± 2.9 4b

(tetren)5Co(pyd' 0.3 607 ± 34 (7.88 ±0.10) X 10-:1 4b

(NI--1 3) 5Co(idaHi• 0.3 9.7 ± 1.9 2.11 ±0.009 4a

(NH3) 5Co(ida)' 0.3 (3. 11 ± 0.08) x I 03 -3.33 ± 6.2 4a

(N H1)4Co(ida)' 0.3 (4.9 ± o.3) x 1 o' 1.9 ± 0.6 4a

(tetren)Co(dipic)' 0.3 234 ±4 4.83 ± 0.09 This work

(tetren)Co(idaHi• 0.3 15.4 ± 0.9 2.48 ± 0.13 This work

(tetren)Co(ida)' 0 .3 (5.70 ± 0.28) x 1 o3 0.03 ± 0.02 This work

Table 6--Rate data for the dissociation of (tetren)Co(ida/ dipic)Ni3+

at 25°C, [Complex] = 1.5x I 0- 3, [Ni 2+] = 1.5x l 0--:~ , I = 0.3 mol

dm - 3; A.= 290 nm

IO[W]

(mol dm- 3)

2.0

5.0

8.0

1.2

1.6

2.0

2.5

k_1 (dm3 mol- 1 s- 1)

- I k.2 (s )

(tetren)Co(ida)Ni3+

I 0 k.,hs• S - I

0.4 1 ± 0 OJ

0.98 ± 0.04

1.48 ± 0.06

2.52 ± 0.10

3.46 ± 0.2

4.39 ± 0.28

6.26 ± 0.68

2.48 ± 0.1 3

0.03 ± 0.02

( tetren )Co( di pic )N i3+ - I

k tlh."o ! s

4.83 ± 0.32

4.64 ± 0.38

4.61 ±0.14

4.68±0. 13

(4.69 ± 0.08)"

a Average value calcd . from rate data at different [H+]

constants for the diffusion-controlled precursor complexes { Ni(OH2) 6

2•, (tetren)Co(idaH)2+} and { Ni(OH2) 6

2+, (tetren)Co(ida)'} or { Ni(OH2)62+, (tetren)Co(dipic)'), respectively (not shown in Scheme I) should be comparable to the water exchange rate constants ofNi(OH2)6

2+ (kex = 3.2x104 s- 1, 25°C) 11

.

Using Fuoss'+9s equation 12 and assuming sA to be closest approach between 2+/2+ and 2+/ I+ charged

reactants, the Kos and K' os values at 25°C were found to be 0.04 7 and 0.07 dm3 mol- 1

. The k' 1 ( = 3.27x I 02

s- 1) and k'2 (= 8. 14x l04 s- 1

) value for ida complex are derived using the above Kos values which are comparable (atleast k'2 values) with the above cited water exchange rate constant of Ni(OH2) 6

2+ (kex = 3.2x l04 s- 1

, 25°C). The k'2 value (3.342x l03 s- 1) for

dipic complex is also comparable (within a factor of

10) with kex value of Ni(OH2)62+. Based on these

comparisons it appears that H20 dissociation from Ni(OH2)l+ largely controls the rate-determining step at least in the formation of (tetren)Co(ida)Ni3+ via k2 path. However, in case of dipic complex, the ring closing i.e. the formation of Ni-N bond with release of H20 from Ni 2+ centre is more likely to be the rate-determining step (see also di scussion under dissoc iation).

Dissociation of binuclear complexes The acid dependent rate data for the dissociation of

Ni 2+ from (tetren)Co(ida)Ni3+ are presented in Table 6. This observed acid dependence on the dissociation can be reconciled with the participation of both protonated and deprotonated forms of cobalt(Ill) substrate in the formation of (tetren)Co(ida)Ni3+, thus supporting the proposed mechanism. In contrast, the dissociation of (tetren)Co(dipic)Ni3+ is acid independent in the range [H+] = 0.02 - 0.20 mol dm-3 (see Table 5). A similar observation had been made in the dissociation N5Co(Pyc)Ni3+ [N5 = (NH3) 5 or tetren]2

.4b_ The observed acid independence might be attributed to the following two reasons: First, the chelate ring opening by Ni-N bond breaking may be rate-limiting and according to microscopic reversibility, Ni-N bond making, would be rate-limiting step in the formation of binuclear complex. Secondly the pyridine ring of pyridine-2,6-dicarboxylate might play some specific role for this different behaviour from the corresponding ida complex whose dissociation is acid catalysed. The ida complex also formed a 5-membered chelate ring in the binuclear species (see Structures l and 2).

The spontaneous (k_1) and acid-catalysed dissociation (k_2) rate constants for (tetren)Co(ida)Ni3+

DAS et al.: STUDIES ON COBAL T(III) COMPLEXES 1049

derived from kobs (i.e. kr) versus [Ir] plot are also presented in Table 6. As the dissociation of (tetren)Co­(dipic)Ni3+ is acid independent, the average kobs value at different [Ir] is taken · as the spontaneous dissociation rate constant (k.2). The k.2 value for (tetren)Co(dipic)Ni3+ is- 150 times higher than that of (tetren)Co(ida)Ni3+. This provides further evidence to the fact that 5 membered chelate, where benzene ring forms a part of the ring (as in dipic complex) is less stable than that of same without benzene ring (as in ida complex). A comparison of k.2 values for both complexes with related systems (see Table 5) indicated that Ni2+ is not bound to ida or dipic moiety through free carboxylate end in an unidentate fashion, but that a chelate structure involving also the unbound nitrogen of the pyridine ring is more likely.

Acknowledgement The authors thank Prof. A C Dash, Utkal University,

Bhubaneswar, for providing the stopped-flow spectrophotometer.

References I Das N N & Dash A C, Indian J Chern, 31 A ( 1992) 678.

2 Das N N & Dash A C, Indian J Chern, 32A (1993) 531.

3 Dash A C, Nanda R K & Das R, Indian J Chern, 30A (1991) 1001.

4 (a) Das N N & Das R, Trans met Chern, 20 ( 1995) 463.

(b) Das N N, Trans met Chern, 23 (1998) 455.

(c) Dash A C, Acharya AN & Das S P, Trans met Chern, 23 (1998) 175.

5 Das R, Das N N & Dash A C, J chem Soc, Dalton Trans, (1995) 3627.

6 Margerum D W, Caley G R, Weatherbums DC & Pagenkopf G K, Coordination chemistry, edited by A E Martell (ACS monograph, 174, Washington DC,) 1978, Vol 2, ChI , P 96.

7 HouseD A & Gamer C S, lnorg Chern, 5 ( 1966) 2097.

8 Davies C W, J chem Soc, (1938) 2093 ..

9 Irving H M & Rossotti H S, J chem Soc, ( 1954) 2904.

I 0 Wilkins R G & Eigen M, Adv chem Ser, 49 ( 1965) 55.

II Wilkins R G, Kinetics & mechanism of reactions of transition metal complexes, 2nd Edn (VCH, New York) 199 I, p 202.

12 Fuoss R M, JAm chem Soc, 80 (1958) 5059.