Chapter V

=================================================

Synthesis, Characterization and Pulse Radiolysis

of Cobalt (III) Complexes of 2-Picolinate and

Polypyridyl Ligands

=================================================

131

Chapter V

Synthesis, Characterization and Pulse Radiolysis of

Cobalt(III) Complexes of 2-Picolinate and Polypyridyl

Ligands

5.1 Introduction

Radiolysis of metal complexes in solution provides valuable information on

how ligands influence the reactivity of transition metal ions. The LOMI (low

oxidation state metal ions) process for the chemical decontamination of nuclear

power reactor system consists of low oxidation state of transition metal ions

complexed with organic ligands such as formic acid, picolinic acid etc. The

dissolution of metal oxides is of considerable importance in the descaling of high-

temperature water-cooled plants. Many procedures currently available use strong

acids or chelating agents in the dissolution of the oxide.1 Blesa and co-workers have

investigated the dissolution of Fe3O4 using thioglycolate2 and oxalate3 and in the

latter reaction have indicated the presence of a heterogeneous electron-transfer

pathway. Zabin and Taube4 noted the importance of electron transfer in the

dissolution of metal oxides by Cr2+, and tris(picolinato)vanadate(II), V(pic)3−, has

been introduced5 as a powerful descaling agent capable of dissolving Fe2O3, Fe3O4,

and NiFe2O4.6 During the decontamination, Fe3+ ion in the Fe2O3 matrix gets reduced

to Fe2+ by low oxidation state of transition metal ion such as V2+ and thus dissolution

of Fe2O3 takes place. In decontamination process the ligands and their metal

complexes are exposed to ionizing radiation and undergo radiolytic degradation.7,8 2-

Pyridine carboxylic acid commonly called as picolinic acid (Scheme 1) is an

important constituent of decontaminant formulations 9-11 used for reducing radiation

fields in the coolant water circuits of nuclear reactors. Radiation chemical studies of

reactions of various primary radiolytic species such as e−aq, •H atom and •OH radicals

generated using pulse radiolysis with picolinic acid have been carried out,12-16 while

those on transation metal complexes have not been studied in detail.

132

Chapter V

Thus a systematic radiation chemical investigation of transition metal

complexes containing picolinic acid and other polydentate ligands will elucidate the

mechanisms of chemical decontamination. Polypyridyl ligands are another important

class of chelating ligands with a capacity to reduce metal ions due to their inherent

strong π-bonding character, which stabilizes the lower oxidation state of the metal

ions as a result of back electron transfer and also forms water soluble complexes.

Thus as a part of a programme to study the effect of ionizing radiation on mixed

ligand polypyridyl transition metal complexes,17-20 the rationale for the choice of 2-

picolinate is that it is an excellent π-acceptor ligand and also it forms stable

complexes with various transition metal ions.

In this chapter the synthesis of mixed ligand complexes of the type

[Co(NN)2(pic)]Cl2, where pic = picolinate; NN = 2,2'-bipyridine (bpy) (10) and 1,10-

phenanthroline (phen) (11) (Scheme 1) and characterization by elemental analysis,

IR, UV-Visible, NMR, ESI-MS spectroscopy and single crystal X-ray diffraction

method are reported. Co(III)/Co(II) reduction potentials have been determined by

cyclic voltammetry and the concomitant spectral changes measured by

spectroelectrochemistry during the reduction. The reactions of primary radiolytic

species such as one electron reducing (e−aq) and one electron oxidizing (•OH) radicals

generated using pulse radiolysis with these metal complexes are studied by optical

absorption technique.

N NLL

N N

O

N O=

pic bpy phen

Scheme 1: Structures of ligands used in this study

133

Chapter V

5.2 Experimental section

The instrumental techniques used for characterization of complexes such as

elemental analysis, UV−Visible, IR and NMR spectroscopic method X-ray

crystallography, and pulse radiolysis technique have been described in Chapter II.

5.2.1 Synthesis of the complexes

5.2.1. a [Co(bpy)2pic]Cl2 (10)

The complex was synthesized by refluxing the [Co(bpy)2Cl2]Cl (0.1 g, 0.20

mM) and picolinic acid (0.0256 g, 0.20 mM) in water and methanol (1:1, v/v) for

10−12 h, during the reaction the color changes from violet to orange. The reaction

mixture was evaporated and dried. The product was purified by column

chromatography. Yield: 0.074 g, 63 %. Elemental anal. (%) for C26 H20 N5 O2 Co Cl2,

Calcd: C, 55.8; H, 3.5; N, 12.4; Found C,55.6 ; H,3.6 ; N,12.6 ; IR (KBr pellet,

νmax/cm-1): 1684s (C=O), 1614m (C=C, C=N), 1327m (C–O), 3086s (=C–H), 700s

(M–N), 764s (M–O). λmax (H2O)/nm: 478, 316, 306 and 216 (ε/dm3 mol-1 cm-1 71,

17670, 20960 and 76220. MS (ESI-MS, CH3CN: H2O, 90: 10) m/z 528

[Co(bpy)2(pic)]+Cl, 493 [Co(bpy)2(pic)]2+, 337 [Co(bpy)(pic)]2+. Cyclic

voltammogram of complex in acetonitrile at a scan rate 100 mVs-1, E1/2/V: 1.02 and

0.30.

5.2.1.b [Co(phen)2pic]Cl2 (11)

The complex was synthesized by refluxing the [Co(phen)2Cl2]Cl (0.1 g, 0.19

mM) and picolinic acid (0.023 g) in water and methanol (1:1, v/v) for 10−12 h, during

the reaction the color changes from violet to orange. The reaction mixture was

evaporated and dried. Then the product was purified by column chromatography.

Yield: 0.098 g, 84 %. Elemental anal. (%) for C30 H20 N5 O2 Co Cl2, Calcd: C, 58.9; H,

3.3; N, 11.4; Found C,58.8 ; H, 3.3; N,11.6 ; IR (KBr pellet, νmax/cm-1): 1685s (C=O),

1616w (C=C, C=N), 1334m (C–O), 3086m (=C–H), 719m (M–N), 774s (M–O). λmax

(H2O)/nm: 482, 274, 216 and 204 (ε/dm3 mol-1 cm-1 82, 45300, 84410 and 88800. MS

134

Chapter V

(ESI-MS, CH3CN: H2O, 90: 10) m/z 576 [Co(phen)2(pic)]+Cl, 541 [Co(phen)2(pic)]2+,

361 [Co(phen)(pic)]2+, 181 [Co(pic)]2+. Cyclic voltammogram of complex in

acetonitrile at a scan rate 100 mVs-1, E1/2/V: 0.20 and 0.89.

5.3 Result and discussion

5.3.1 Synthesis and characterization

The precursor complexes [Co(III)(NN)2Cl2]Cl, where NN= 2,2'-bipyridine

(bpy) and 1,10–phenanthroline (phen) were synthesized in the laboratory by chlorine

oxidation method and characterized using standard techniques like elemental analysis,

UV−Visible, IR and NMR spectroscopic method (Chapter IV). The mixed ligand

complexes of the type [Co(NN)2(pic)]Cl2 were synthesized by refluxing the precursor

complex [Co(III)(NN)2Cl2]Cl (1 mmol) and picolinic acid (1 mmol) in water and

methanol (1:1, v/v) (Scheme 2) for 10−12 h, during which color of the solution

changes from violet to orange then solution evaporated to dryness. Solid product

purified by column chromatography, characterized by elemental analysis,

UV−Visible, IR, NMR, ESI-MS spectroscopy and single crystal X-ray diffraction

method. The elemental analyses of these complexes correlated well with the

calculated values and the stoichiometries of the above complexes are confirmed by

elemental analysis.

Sche

+ +2

meth

abso

me 2. Synthetic scheme of mixed ligand cobalt(III) complexes.

N

N

Cl

Cl

NCo

N

N

N

O

N

NCo

N

IIIIIIN O

Water : Methanol (1:1)

Reflux for 10-12 h

NN = bpy or phen NO = pic

+

Absorption spectrum of the mixed ligand cobalt(III) complexes 10 and 11 in

anol is given in Figure 1 and 2. The complexes 10 and 11 exhibit weak

rption bands in the range 425−575 nm in visible region due to d-d transition and

135

Chapter V

ligand based π−π* transition occurs in the UV region of 210−325 nm. The molar

extinction coefficient (εmax) values for the complexes are of the order of 102 dm3 mol-

1 cm-1 in visible region and for the ligands of the order of 104 dm3 mol-1 cm-1 in UV

region. The most characteristic difference between the IR spectra of mixed ligand

complexes of cobalt is the C=O stretch at 1685 cm-1 and M−O stretch at 764−774cm-1

are present. The 1H−NMR spectra of mixed ligand complexes indicate that a

picolinate binds to metal ion and complex of the type [Co(NN)2(pic)]Cl2 formed

which is further confirmed by single crystal X-ray structure analysis. The positive ion

detection mode ESI-MS spectra of these complexes were determined by 100%

relative intensity ion m/z characteristic of [Co(pic)(NN)2]+Cl ion.

200 300 4000.0

0.1

0.2

0.3

0.4

0.5

0.6Complex 10

O.D

.

Wavelength (nm)400 500 600 700

0.00

0.02

0.04

0.06

0.08

0.10Complex 10

O.D

.

Wavelength (nm) Figure 1. UV-Visible absorption spectra of the complex 10 recorded in water.

200 300 4000.0

0.1

0.2

0.3

0.4

0.5

0.6Complex 11

O.D

.

Wavelength (nm)

400 500 600 7000.00

0.02

0.04

0.06

0.08

0.10Complex 11

O.D

.

Wavelength (nm)

Figure 2. UV-Visible absorption spectra of the complex 11 recorded in water.

136

Chapter V

5.3.2 Crystal structures

5.3.2.a Crystal structure of [Co(bpy)2pic](ClO4)2 (10)

The single crystal of [Co(bpy)2pic](ClO4)2 were grown by converting chloride

salt of complex 10 to its perchlorate salt. The crystal was gown from mixture of

water-methanol solvent by slow evaporation method. The ORTEP and packing

diagram of complex 10 is given in Figure 3. The crystallographic data are

summarized in Table-1. Table-2 lists of bond lengths, bond angles parameters

respectively. The picolinate ligand co-ordinates to cobalt atom through N and O

atoms, thus forming five membered chelate ring and bpy ligand coordinates through

two N atoms, forming five membered chelate ring. The central cobalt(III) atom

exhibits a distorted octahedral coordination geometry (formed by one picolinate and

two 2,2'-bipyridine ligand) as the bite angles between picolinate and bpy chelate rings

which are less than 90°. The bond lengths of picolinate ligand are Co(1)–O(1)

1.879(3) and Co(1)–N(1) 1.924(3) Å, which are consistent with cobalt(III)

complexes.21,22 The Co–N bond lengths of 2,2'-bipyridine ligand are similar to values

reported in literature.23

A B

Figure 3. A) ORTEP and B) packing diagram of complex 10.

137

Chapter V

Table 1. Crystal parameters and structure refinement data for complex 10

Complex 10

Empirical formula C26 H20 Cl2 Co N5 O10

Formula weight 692.30

Temperature 293(2) K

Wavelength 0.71073 Å

Crystal system triclinic

Space group P-1

Unit cell dimensions a = 9.8914(6) Å,

b = 11.8349(8) Å

c = 13.1997(8) Å

α = 83.041(5)°

β = 71.928(4)°

γ = 69.079(5)°

Volume 1372.04(15) Å3

Z 2

Density (calculated) 1.676 Mg/m3

Absorption coefficient 0.889 mm-1

F(000) 704

Crystal size 0.3 × 0.2 × 0.2 mm3

Theta range for data collection 1.62 to 25.35°

Index ranges -11<=h<=11, -14<=k<=14, -15<=l<=15

Reflections collected 13394

Independent reflections 4989 [R(int) = 0.0377]

Completeness to theta = 25.35° 99.5 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 4989 / 9 / 390

138

Chapter V

Goodness-of-fit on F2 1.108

Final R indices [I>2sigma(I)] R1 = 0.0688, wR2 = 0.1873

R indices (all data) R1 = 0.0806, wR2 = 0.1958

Largest diff. peak and hole 0.894 and -0.571 e.Å-3

Table 2. Bond lengths [Å] and angles [°] for complex 10

Co(1)−O(1) 1.879(3) Co(1)−N(11) 1.930(4)

Co(1)−N(1) 1.924(3) Co(1)−N(12) 1.925(4)

Co(1)−N(21) 1.926(4) Co(1)−N(22) 1.933(4)

O(1)−Co(1)−N(1) 84.83(15) N(12)−Co(1)−N(11) 83.40(17)

O(1)−Co(1)−N(12) 92.63(16) N(21)−Co(1)−N(11) 91.16(15)

N(1)−Co(1)−N(12) 88.29(15) O(1)−Co(1)−N(22) 87.42(15)

O(1)−Co(1)−N(21) 88.89(14) N(1)−Co(1)−N(22) 93.78(15)

N(1)−Co(1)−N(21) 173.21(15) N(12)−Co(1)−N(22) 177.92(15)

N(12)−Co(1)−N(21) 94.57(16) N(21)−Co(1)−N(22) 83.35(16)

O(1)−Co(1)−N(11) 176.02(15) N(11)−Co(1)−N(22) 96.54(16)

N(1)−Co(1)−N(11) 95.29(16)

5.3.2.b Crystal structure of [Co(phen)2pic](ClO4)2 (11)

The single crystal of [Co(phen)2pic](ClO4)2 was also grown by similar as that

of complex 10. The ORTEP and packing diagram of complex 11 is given in Figure 4.

The crystallographic data are summarized in Table-3. Table-4 lists of bond lengths,

bond angles parameters respectively. The picolinate co-ordinates to cobalt atom

through N and O atoms, thus forming five membered chelate ring and phen ligand

coordinates through two N atoms, forming five membered chelate ring. The central

cobalt(III) atom exhibits a distorted octahedral coordination geometry formed by one

picolinate and two 1,10-phenanthroline ligand. The bond lengths of picolinate ligand

values is Co(1)–O(1) 1.8910(18) and Co(1)–N(1) 1.944(2) Å, which is consistent

139

Chapter V

with cobalt(III) complexes.21,22 The Co–N bond lengths of 1,10-phenanthroline ligand

are similar to values reported in literature.23

BA

Figure 4. A) ORTEP and B) packing diagram of complex 11.

Table 3. Crystal parameters and structure refinement data for complex 11

Complex 11

Empirical formula C30 H20 Cl2 Co N5 O10

Formula weight 740.34

Temperature 170(2) K

Wavelength 0.71073 Å

Crystal system monoclinic

Space group P21/c

Unit cell dimensions a = 12.8001(10) Å

b = 17.7995(11) Å

c = 12.9515(11) Å

α = 90°

β = 93.212(10)°

γ = 90°

140

Chapter V

Volume 2946.2(4) Å3

Z 4

Density (calculated) 1.669 Mg/m3

Absorption coefficient 0.835 mm-1

F(000) 1504

Crystal size 0.3 × 0.3 × 0.2 mm3

Theta range for data collection 2.78 to 27.96°

Index ranges -16<=h<=15, -23<=k<=23, -17<=l<=17

Reflections collected 20395

Independent reflections 6997 [R(int) = 0.0505]

Completeness to theta = 25.35° 98.9 %

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 6997 / 0 / 452

Goodness-of-fit on F2 0.989

Final R indices [I>2sigma(I)] R1 = 0.0484, wR2 = 0.1115

R indices (all data) R1 = 0.0756, wR2 = 0.1217

Largest diff. peak and hole 0.496 and -0.567 e.Å-3

Table 4. Bond lengths [Å] and angles [°] for complex 11

Co(1)−O(1) 1.8910(18) Co(1)−N(1) 1.944(2)

Co(1)−N(21) 1.918(2) Co(1)−N(2) 1.949(2)

Co(1)−N(11) 1.943(2) Co(1)−N(12) 1.954(2)

O(1)−Co(1)−N(21) 85.13(8) N(11)−Co(1)−N(2) 179.66(9)

O(1)−Co(1)−N(11) 90.85(9) N(1)−Co(1)−N(2) 84.52(10)

N(21)−Co(1)−N(11) 88.41(9) O(1)−Co(1)−N(12) 175.17(9)

O(1)−Co(1)−N(1) 88.89(8) N(21)−Co(1)−N(12) 94.82(9)

N(21)−Co(1)−N(1) 172.98(9) N(11)−Co(1)−N(12) 84.32(9)

141

Chapter V

N(11)−Co(1)−N(1) 95.36(10) N(1)−Co(1)−N(12) 91.45(9)

O(1)−Co(1)−N(2) 88.83(9) N(2)−Co(1)−N(12) 95.99(9)

N(21)−Co(1)−N(2) 91.68(9)

5.3.3. Electrochemical and spectroelectrochemical studies

The reduction potential of the complexes have been measured in 0.1 M TEAP

in acetonitrile solutions. The cyclic voltammogram of complexes 10 and 11 are given

in Figure 5. The E1/2 values vs. Ag/AgCl for the Co(III)/Co(II) redox couple in the

complexes 10 and 11 are +0.30 (∆E = 71 mV) and +0.20 V (∆E = 62 mV) and the

E1/2 values vs. Ag/AgCl for the L/L•− redox couple in the complexes 10 and 11 are

+1.01 (∆E = 205 mV) and +0.89 V(∆E = 158 mV). These two quasi reversible

processes are assigned as ligand centered reduction and metal center reduction in both

the complexes. The reduction potential values for the [Co(NH3)6]3+, [Co(bpy)3]3+,

[Co(phen)3]3+, [Co(H2O)6]3+, [Co(ox)3]3−, [Co(edta)]− and [Co(CN)6]3− are +0.10,

+0.31, +0.38, +1.84, +0.57, +0.6 and –0.83 V.24-26 A comparison of the E1/2 values of

complex 10 with [Co(bpy)3]3+ indicate there is slight change in reduction potential.

The spectra obtained after electrolysis indicates that the one electron reduced species

is stable in the timescale of the spectroelectrochemical experiment, i.e about 30

minutes suggesting high stability of the electrogenerated species.

1.5 1.2-0.000015

-0.000010

-0.000005

0.000000

0.000005

0.000010

Potent

Cur

rent

/ A

0.000007

0.000014

B

Figure 5. Cyclic volTEAP acetonitrile so

A

0.9 0.6 0.3 0.0

ial / V vs. Ag/AgCl

1.2 1.0 0.8 0.6 0.4 0.2 0.0-0.000021

-0.000014

-0.000007

0.000000

Potential / V vs. Ag/AgCl

Cur

rent

/ A

tammogram of 1mmol complexes 10 (A) and 11 (B) in 0.1 M lution at a scan rate 100 mVs-1.

142

Chapter V

The spectroelectrochemical behavior for complexes 10 and 11 were

investigated using an in situ spectroelectrochemical technique. The convenient

applied potential values for the in situ spectroelectrochemical experiment were

determined for each process by recording cyclic voltammograms of the complexes.

For the reduction of complex 10 a constant potential +0.15 V was applied and spectra

were recorded during the bulk electrolysis. The UV-Visible spectral changes for

complex 10 during the electrolysis in acetonitrile solution containing 0.1 M TEAP is

given in Figure 6. The spectrum show a sharp peak at 306 nm shifted to 294 nm. The

peak at 232 nm disappears and the new peak at 246 nm appears. The absorption band

centered at 319 and 480 nm also decreases. The reduction of complex 10 at +0.15 V

is reversible in the CV time scale; however, controlled potential bulk electrolysis

studies demonstrate that the one electron reduced species is stable for

spectroelectrochemical analysis.

For the reduction of complex 11 a constant potential +0.1 V was applied and

spectra were recorded during electrolysis. The UV-Visible spectral changes for

complex 11 during the electrolysis in acetonitrile solution containing 0.1 M TEAP are

given in Figure 7. The spectrum exhibits an increase in absorbance of peak at 270 nm

with blue shift at 266 nm and decrease in absorbance at 244 nm and the band at 480

nm decreases during electrolysis. The spectrum shows an isobestic point at 294 nm.

However, controlled potentional bulk electrolysis studies demonstrate that the one

electron oxidized species is stable for spectroelectrochemical analysis.

The spectroelectrochemical studies indicate reduction of cobalt(III) to

cobalt(II) complexes as evidenced by the peak positions and intensity. Similar

changes in the intensity of the visible absorption were described.27-29

143

Chapter V

200 250 300 350 4000.00

0.25

0.50

0.75

1.00

1.25

O.D

.

Wavelength (nm)400 450 500 550

0.00

0.03

0.06

0.09

0.12

0.15

O.D

.

Wavelength (nm)

A B

Figure 6. A and B- UV-Visible spectral changes of complex 10 during the reduction in CH3CN solution containing 0.1 M TEAP.

200 250 300 3500.0

0.3

0.6

0.9

1.2

1.5

O.D

.

Wavelength (nm))

400 450 500 550 6000.0

0.1

0.2

0.3

O.D

.

Wavelength (nm)

B A

Figure 7. A and B- UV-Visible spectral changes of complex 11 during the reduction in CH3CN solution containing 0.1 M TEAP.

5.3.4 Pulse radiolysis studies

In the complexes containing multiple ligands it is interesting to study the site of initial

radical (one electron oxidizing and reducing) attack and the subsequent fate of the

adduct thus formed. Pulse radiolysis is an ideal technique for studying the redox

chemistry of complexes because both oxidizing and reducing radicals can be

selectively produced in known yields, which make the evaluation of kinetics and

measurements of transient absorption spectra easy and convent. Pulse radiolysis is an

144

Chapter V

appropriate technique to study these effects in the microsecond time scale to derive

reaction mechanisms. The pulse radiolysis experiments were carried out at BARC,

Mumbai.

5.3.4.1 Reaction of e–aq with the [Co(bpy)2pic]Cl2 and [Co(phen)2pic]Cl2

The reactions of e–aq with complexes 10 and 11 were carried out in N2

saturated aqueous solution of 2×10-4 M complexes which does not shows any

absorption maxima. The rate constants (k) for the reaction of e–aq with the complexes

1 and 2 at pH 6.8 were determined from the decay traces of hydrated electron at 700

nm from varying the complex concentration from the 0.2×10−4 to 1×10−4 M and the

rate constant are 2.8×1010 and 6.0×1010 dm3 mol-1 s-1. Similar rate constants were

observed for the reaction e–aq with the bpy and phen ligand.

0 20 40 60 80 1000

1

2

3

4

k /1

06 s-1

[Complex 1] (µM)

20 40 60 80 1000

1

2

3

4

k /1

06 s-1

[Complex 2] (µM)

Figure 8 Dependence of k on concentration of A) complex 1 and B) complex 2 in the reaction of e− aq at pH 6.8.

5.3.4.2 Reaction of •OH radicals with [Co(bpy)2pic]Cl2 (10)

The transient absorption spectrum (Figure 9) obtained in the reaction of •OH

radicals with complex 10 after the electron pulse shows absorption maxima at 360 nm

(ε360 = 3.3×103 dm3 mol-1 cm-1) and 390 nm (ε390 = 3.3×103 dm3 mol-1 cm-1). It

indicates that the absorption is due to the formation of OH-adduct of bpy ligand

([Co(pic)2(bpy-OH)•]) in the complex.

145

Chapter V

[Co(bpy)2pic]2+ + •OH → [Co(pic)2(bpy)(bpy-OH)•] 2+ (1)

Similar band was observed in the reaction of •OH radicals with the bpy ligand

was observed in the reaction of •OH radicals with free bpy ligand, •OH radical adds to

nitrogen or carbon atom.30,31 The N-adduct shows absorption peaks at 305 and 365

nm whereas the C-adduct has absorption in 300−400 nm the region. The transient

absorption spectrum obtained in the reaction of •OH radicals with the complex 1 are

assigned to the carbon centered adduct species of bpy ligand. The spectrum does not

show any characteristic peak of OH-adduct of the picolinic acid.

300 400 500 600 7000.0

0.7

1.4

2.1

2.8

3.5

ε (x

103 ) d

m3 m

ol-1 c

m-1

Wavelength (nm)

50 100 150 200 250 30000

2

44

6

88

k /1

05 s-1

[Complex 1] (µM)

Figure 9 Transient absorption spectrum observed in the reaction of •OH radicals with N2O saturated aqueous solution of 1×10−4 mol dm−3 complex 10 at 10(■) µs after the electron pulse. Inset: Dependence of k on concentration of complex 10 in the reaction of •OH radicals at pH 6.8. Dose/ pulse =14.5 Gy.

The rate constant (k) for the reaction of •OH radicals with the complex at pH

6.8 determined from the formation traces of transient species at 360 nm from varying

the complex concentration is 2.0×109 dm3 mol−1 s−1. The reaction of •OH radicals with

bpy 31,32 have maxima at 365 nm with k = 5.6×109 dm3 mol-1 s-1. It can be seen that

146

Chapter V

the rate constant of OH radical with complex has decreased as compared to the bpy

ligand due to the positively charged complex.

5.3.4.3 Reaction of •OH radicals with [Co(phen)2pic]Cl2 (11)

The transient absorption spectrum obtained in the reaction of •OH radicals with

complex 11 after the electron pulse is given in Figure 10. The transient spectrum

shows absorption maxima at 340 nm (ε340 = 1.8×103 dm3 mol-1 cm-1) and 460 nm (ε460

= 2.7×103 dm3 mol-1 cm-1) which are due to the formation of OH-adducts of phen

ligand in the complex. A similar spectrum was observed in the reaction of •OH

radicals with [Co(pic)2phen] complex indicating the formation of OH-adduct of the

phen ligand.19

[Co(phen)2pic]2+ + •OH → [Co(pic)2(phen)(phen-OH) •]2+ (2)

The rate constant for the reaction of •OH radicals with the complex 2 at pH

6.8 determined by the formation traces of transient species at 340 and 460 nm from

varying the complex concentration is 4.7×109 dm3 mol-1 s-1. In the reaction of •OH

radicals with phen formation of OH-adduct of phen ligand takes place with rate

constant of 7.0×109 at pH 7 measured by following the product build up at 440 nm.33

The rate constant for the reaction of •OH radicals with picolinic acid at pH 9 was

2×109 dm3 mol-1 s-1.

A summary of spectral and kinetic parameters of the reaction of e−aq, •OH

radicals with the mixed ligand cobalt(III) complexes of picolinic acid and polypyridyl

ligands are tabulated in Table 5.

147

Chapter V

300 400 500 600 7000.0

0.6

1.2

1.8

2.4

3.0

ε (x

103 ) d

m3 m

ol-1 c

m-1

Wavelength (nm)

80 100 120 140 160 180 2000.3

0.6

0.9

1.2

k /1

06 s-1

[Complex 2] (µM)

Figure 10 Transient absorption spectrum observed in the reaction of •OH radicals with N2O saturated aqueous solution of 1×10-4 mol dm-3 complex 11 at 5(■) µs after the electron pulse. Inset: Dependence of k on concentration of complex 11 in the reaction of •OH radicals at pH 6.8. Dose/ pulse =14.1 Gy.

Table 5 Spectral and kinetic parameters of reactions of e−aq and •OH radicals with [Co(bpy)2pic]Cl2 and [Co(phen]2pic]Cl2 at pH 6.8.

Complex Reacting

species

λmax

(nm)

ε (×103)

(dm3 mol-1 cm-1)

k formation

(dm3 mol-1 s-1)

[Co(bpy)2pic]Cl2 e−aq – – 2.8×1010

•OH 360 3.3 2.0×109

390 3.3 -

[Co(phen)2pic]Cl2 e−aq – – 6.0×1010

•OH 340 1.8 4.7×109

460 2.7 -

Picolinic acid e−aq 300 6.9 5.0×109

(pH 9) •OH 285,

345

2.5 2.0×109[a]

a: Reference 12

148

Chapter V

5.4. Conclusion

A complexes of the type [Co(NN)2(pic)]Cl2, where pic = picolinate; NN =

polypyridyl ligand have been synthesized and characterized by various analytical

techniques. The [Co(bpy)2(pic)](ClO4)2 (10) and [Co(phen)2(pic)](ClO4)2 (11)

complexes were crystallized from water: methanol solvent and crystal structures have

been determined by single crystal X-ray diffraction method. Reduction potentials of

complexes have been measured by using cyclic voltammetry and the concomitant

spectral changes measured by spectroelectrochemistry during the reduction indicate

formation of Co(II) complex. The transient absorption spectrum obtained in the

reaction of •OH radicals with complexes indicates formation of OH-adduct of

polypyridyl ligand. The rate constant for the [Co(bpy)2pic]2+ complex is higher than

the bpy ligand is due to the complex is positively charged and lower than

[Co(bpy)3]3+ due to the one less charge. Similarly it is observed in case of

[Co(phen)2pic]2+ complex. The rate constant for the reaction of e–aq with

[Co(bpy)2pic]2+ is lower than the [Co(phen)2pic]2+ complex which is due to the

affinity of electron on the bpy and phen ligand.

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