Copper(I) complexes with Schiff base and triphenylphosphine or …shodhganga.inflibnet.ac.in/bitstream/10603/4358/9/09_chapter 3.pdf · a solution of cis-1,2-bis(diphenylphosphino)ethane

Chapter III

Copper(I) complexes with Schiff base and triphenylphosphine or cis-1,2-bis (diphenylphosphino)ethane

Chapter III Copper(I) complexes....................

3.1. Introduction:

Monovalent copper (d10) chemistry has drawn special attention because of its

instability, unusual structural features, utility in solar energy, supramolecular devices,

catalytic activity in photoredox reaction and biological relevance of high potential

copper complexes [1-6]. Due to favourable soft acid-soft base interaction, the

chemistry of closed-shell d10 metal ion is largely based upon coordination to ligands

such as various N, S, P and halide donor ligands. Synthesis of copper(I) complexes

are of great interest because the diversity of products resulting from similar

methodology. The steric, electronic and conformational effects imparted by the

coordinated ligands play an essential part in stabilizing the copper(I) center and

improving the chemical and physical properties of the cuprous complexes which are

important in practical applications. As copper(I) is an unstable oxidation state; the

complexes with N, S, P potentially donor ligands have been extensively studied due

to their wide variation in structural motifs and rich photophysical properties [7-10],

however, only few complexes with O-donor ligands are synthesized and structurally

characterized [11,12]. Chemistry of Schiff base copper(I) complexes has been

intensively investigated in recent years owing to their coordination behavior and

diverse applications which can be correlated to the structural property of Schiff base

and their metal complexes [13-16]. The variation in the structural aspects can be

related with experimental conditions of their synthesis, nature of donor atoms, the

structure of ligands and also metal-ligand interaction.

The work presented in this section deals with the investigation of structural aspects

of copper(I) complexes derived by the reaction of copper(I) salts CuCl,

[Cu(MeCN)4NO3], [Cu(MeCN)4]ClO4 and [Cu(MeCN)4]BF4 with Schiff base ligands

2-phenyl-3(benzylamino)-1,2-dihydroquinazolin-4(3H)-one (L1), 2(4'-methoxyphenyl)-

55


3(4''-methoxybenzylamino)-1,2-dihydroquinazolin-4(3H)-one (L2) and 2-(4'-nitrophenyl)-

3(4''-nitrobenzylamino)-1,2-dihydroquinazolin-4(3H)-one (L3) in presence of triphenyl-

phosphine (PPh3) or cis-1,2-bis(diphenylphosphino)ethane (dppe). The coordination

behavior of these ligands towards copper(I) was investigated by microanalysis, IR, UV-

visible, 1H NMR and X-ray crystallography studies. The electrochemical behavior of

all the complexes have been also studied.

3.2. Experimental:

3.2.1. Synthesis of copper(I) chloride complexes (1a-6a)

[Cu(L1-3)(PPh3)2]Cl complexes (1a-3a):

To a solution of CuCl (1 mmol, 0.098 g) in 10 ml acetonitrile a solution of two

equivalent of triphenylphosphine (2 mmol, 0.524 g) was added. The reaction mixture

was stirred for 30 min at room temperature under nitrogen atmosphere and allowed to

evaporate slowly. The crystalline product [Cu(MeCN)2(PPh3)2]Cl (1 mmol, 0.705 g)

obtained was subsequently added to a stirring solution of Schiff base ligand L (1

mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g, L3) in 10 ml dichloromethane. The mixture

was stirred at room temperature for 2h and the solution was evaporated to small

volume under vacuum. The yellow coloured complexes were developed by diffusion

of diethyl ether into the solution.

[Cu(L1-3)(dppe)]Cl complexes (4a-6a):

To a solution of Schiff base ligand L (1 mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g, L3)

a solution of cis-1,2-bis(diphenylphosphino)ethane (1 mmol, 0.397 g) and CuCl (1

mmol, 0.098 g) in 10 ml dichloromethane was added. The reaction mixture was

stirred under nitrogen atmosphere at room temperature for 2h and the solution was

evaporated to small volume under vacuum. The pale yellow coloured complex was

developed by diffusion of diethyl ether into the solution.

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3.2.2. Synthesis of copper(I)nitrate complexes (1b-6b):

[Cu(L1-3)(PPh3)2]NO3 complexes (1b-3b):

To a solution of [Cu(MeCN)4]NO3 (1 mmol, 0.291 g) in 10 ml acetonitrile a solution

of two equivalent of triphenylphosphine (2 mmol, 0.524 g) was added. The reaction

mixture was stirred for 30 min at room temperature under nitrogen atmosphere and

allowed to evaporate slowly. The crystalline product [Cu(MeCN)2(PPh3)2]NO3 (1 mmol,

0.631 g) obtained was subsequently added to a stirring solution of Schiff base ligand

L (1 mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g, L3). The mixture was stirred at room

temperature for 2h and the solution was evaporated to small volume under vacuum.

The brownish coloured complex was developed by diffusion of diethyl ether into the

solution.

[Cu(L1-3)(dppe)]NO3 complexes (4b-6b):

To a solution of Schiff base ligand L (1 mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g,

L3) a solution of cis-1,2-bis(diphenylphosphino)ethane (1 mmol, 0.398 g) and

[Cu(MeCN)4]NO3 (1 mmol, 291 g) in 10 ml dichloromethane was added. The reaction

mixture was stirred under nitrogen atmosphere at room temperature for 2h and then

the solution was evaporated to small volume under vacuum. The light brown coloured

complex was developed by diffusion of diethyl ether into the solution.

3.2.3. Synthesis of copper(I) perchlorate complexes (1c-6c):

[Cu(L1-3)(PPh3)2]ClO4 (1c-3c):

To a solution of [Cu(MeCN)4]ClO4 (1 mmol, 0.327 g) in 10 ml acetonitrile a

solution of two equivalent of triphenylphosphine (2 mmol, 0.524 g) was added. The

reaction mixture was stirred for 30 min at room temperature under nitrogen

atmosphere and allowed to evaporate slowly. The crystalline product

[Cu(MeCN)2(PPh3)2]ClO4 (1 mmol, 0.769 g) obtained was subsequently added to a

57


stirring solution of Schiff base ligand L (1 mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g,

L3). The mixture was stirred at room temperature for 2h and the solution was

evaporated to small volume under vacuum. The greenish yellow coloured complexes

were developed by diffusion of diethyl ether into the solution.

[Cu(L1-3)(dppe)]ClO4 (4c-6c):


L3) a solution of cis-1,2-bis(diphenylphosphino)ethane (1 mmol, 0.398 g) and

[Cu(MeCN)4]ClO4 (1 mmol, 0.327 g) in 10 ml dichloromethane was added. The

reaction mixture was stirred under nitrogen atmosphere at room temperature for 2h

and then the solution was evaporated to small volume under vacuum. The greenish

yellow coloured complex was developed by diffusion of diethyl ether into the solution.

3.2.4. Synthesis of copper(I)tetrafluoroborate complexes (1d-6d):

[Cu(L1-3)(PPh3)2]BF4 (1d-3d):

To a solution of [Cu(MeCN)4]BF4 (1 mmol, 0.314 g) in 10 ml acetonitrile was

added a solution of two equivalent of triphenylphosphine (2 mmol, 0.524 g). The

reaction mixture was stirred for 30 min at room temperature under nitrogen

atmosphere and allowed to evaporate slowly. The crystalline product

[Cu(MeCN)2(PPh3)2]BF4 (1 mmol, 0.756 g) obtained was subsequently added to a

stirring solution of Schiff base ligands L (1 mmol, 0.325 g, L1; 0.387 g, L2; 0.417 g,

L3). The mixture was stirred at room temperature for 2h and then the solution was

evaporated to small volume under vacuum. The light orange coloured complexes were

developed by diffusion of diethyl ether into the solution.

[Cu(L1-3)(dppe)]BF4 (4d-6d):


L3) was added a solution of cis-1,2-bis(diphenylphosphino)ethane (1 mmol, 0.397 g)

58


and [Cu(MeCN)2(PPh3)2]BF4 (1 mmol, 0.756 g) in 10 ml dichloromethane. The

reaction mixture was stirred under nitrogen atmosphere at room temperature for 2h

and then the solution was evaporated to small volume under vacuum. The light orange

coloured complex was developed by diffusion of diethyl ether into the solution.

3.3. Results and discussion:

3.3.1. Synthesis:

The Schiff base ligands, 2-phenyl-3-(benzylamino)-1,2-dihydroquinazolin-4(3H)-

one (L1), 2-(4'-methoxyphenyl)-3-(4''-methoxybenzylamino)-1,2-dihydroquinazolin-

4(3H)-one (L2) and 2-(4'-nitrophenyl)-3-(4''-nitrobenzylamino)-1,2-dihydroquinazolin-

4(3H)-one (L3) were prepared by the condensation of benzaldehyde, p-anisaldehyde

and p-nitrobenzaldehyde with 2-aminobenzoylhydrazide in 2:1 molar ratio in ethanol.

The copper(I) complexes of the type [Cu(L1-3)(PPh3)2]X was synthesized by the

reaction of one equivalent of copper(I) salts {CuCl, [Cu(MeCN)4]NO3, [Cu(MeCN)4]

ClO4, [Cu(MeCN)4]BF4} and two equivalents of triphenylphosphine followed by the

addition of one equivalent of Schiff base ligand L1-3 in dichloromethane. However,

the complexes of the type [Cu(L1-3)(dppe)]X were prepared by the reaction of one

equivalent of copper(I) salts {CuCl, [Cu(MeCN)4]NO3, [Cu(MeCN)4]ClO4, [Cu(MeCN)4]

BF4} and one equivalents of Schiff base ligand L1-3 followed by the addition of one

equivalent of cis-1,2-bis (diphenylphosphino)ethane in dichloromethane. The generalized

equations for the reaction leading to the formation of the complexes are:

[Cu(MeCN)4]X +N2 atm.

2 PPh3 R.T.

R.T.

N2 atm

[Cu(MeCN)2(PPh3)2]X

[Cu(MeCN)2(PPh3)2]X [Cu(L)(PPh3)2]X+ L

[Cu(MeCN)4]X +N2 atm.

R.T.[Cu(L)(dppe)]Xdppe L+

Scheme 1: Synthesis of copper(I) complexes

59


Where L = 2-phenyl-3(benzylamino)-1,2-dihydroquinazolin-4(3H)-one (L1), 2-(4'-

methoxyphenyl)-3(4''-methoxybenzylamino)-1,2-dihydroquinazolin-4(3H)-one (L2),

2-(4'-nitrophenyl)-3(4''-nitrobenzylamino)-1,2-dihydroquinazolin-4(3H)-one (L3);

PPh3 = triphenylphosphine, dppe = cis-1,2-bis(diphenylphosphino)ethane; X = Cl-,

NO3-, ClO4

-, BF4-. All the complexes were characterized on the basis of elemental

analysis, IR, UV-visible and 1H NMR spectral studies. The representative complex of

the series [Cu(L1)(PPh3)2]BF4 (1d) was characterized by X-ray single crystallography.

The electrochemical behaviors of the complexes have been also studied.

3.3.2. Physical properties:

The Schiff base ligands L1-3 used for the synthesis of copper(I) complexes contain

several potential donor sites and is capable to coordinate with metal ion in neutral or

anionic form. The reaction of Schiff base ligands L1-3 with copper(I) salts like CuCl,

[Cu(MeCN)4]NO3, [Cu(MeCN)4]ClO4 and [Cu(MeCN)4]BF4 in presence of triphenyl-

phosphine or cis-1,2-bis(diphenylphosphino)ethane form stable solid complexes. All

these complexes are non-hygroscopic, air stable and decomposed below 225°C. These

air-stable complexes are soluble in common organic solvents such as ethanol,

methanol, chloroform, dichloromethane, acetonitrile, tetrahydrofuran etc. giving

respective colour to the solution. The colour, percentage yield, microanalysis,

M.P./decomposition temperature of all the complexes is summarized in Table 3.1–3.4.

The results of elemental analysis (C, H and N) of all the copper(I) complexes indicate

that their stoichiometric and physical properties are in accordance with the proposed

molecular formulae. The elemental analysis also confirmed the existence of Cl-, NO3-,

ClO4- and BF4

- anion in the respective complexes. At room temperature all the complexes

are diamagnetic which is characteristic of the presence of copper(I) (d10).

60

Table 3.1: Analytical and physico-chemical data of copper(I) chloride complexes (1a-6a)

Analytical data % found (calcd.) Complex M. F.

Yield

%

M. P. 0C C H N

[Cu(L1)(PPh3)2 ]Cl (1a) C57H47N3OP2ClCu 75 158 71.71 (71.99) 4.82 ( 4.98) 4.28 (4.42)

[Cu(L2)(PPh3)2 ]Cl (2a) C59H51N3O3P2ClCu 69 165 69.87 (70.09) 4.95 (5.08) 4.01 (4.16)

[Cu(L3)(PPh3)2 ]Cl (3a) C57H45N5O5P2ClCu 66 173 65.26 (65.43) 4.14 (4.31) 6.62 (6.81)

[Cu(L1)(dppe)]Cl (4a) C47H41N3OP2ClCu 74 153 68.28 (68.44) 4.86 (5.01) 4.89 (5.09)

[Cu(L2)(dppe)]Cl (5a) C49H45N3O3P2ClCu 76 160 66.34 (66.51) 4.96 (5.13) 4.59 (4.75)

[Cu(L3)(dppe)]Cl (6a) C47H39N5O5P2ClCu 70 157 61.56 (61.71) 4.14 (4.30) 7.48 (7.66)

61

Table 3.2: Analytical and physico-chemical data of copper(I) nitrate complexes (1b-6b)


Yield

%

M. P. 0C C H N

[Cu(L1)(PPh3)2]NO3 (1b) C57H47N4O4P2Cu 65 150 69.89 (70.04) 4.68 (4.85) 5.58 (5.73)

[Cu(L2)(PPh3)2]NO3 (2b) C59H51N4O6P2Cu 67 153 68.12 (68.30) 4.78 (4.95) 5.23 (5.40)

[Cu(L3)(PPh3)2]NO3 (3b) C57H45N6O8P2Cu 63 165 63.95 ( 64.13) 4.08 (4.25) 7.72 (7.87)

[Cu(L1)(dppe)]NO3 (4b) C47H41N4O6P2Cu 61 173 66.13 (66.31) 4.66 (4.85) 6.43 (6.58)

[Cu(L2)(dppe)]NO3 (5b) C49H45N4O6 P2Cu 61 168 64.38 ( 64.57) 4.82 ( 4.98) 5.98 (6.15)

[Cu(L3)(dppe)]NO3(6b) C47H39N6O8P2Cu 70 162 59.79 (59.97) 4.01 (4.18) 8.75 (8.93)

62

Table 3.3 : Analytical and physico-chemical data of copper(I) perchlorate complexes (1c-6c)


Yield

%

M. P. 0C C H N

[Cu(L1)(PPh3)2]ClO4 (1c) C57H47N3O5P2ClCu 65 156 67.27 (67.45) 4.49 (4.67) 4.02 (4.14)



[Cu(L1)(dppe)]ClO4 (4c) C47H41N3O5P2ClCu 61 177 63.35 (63.51) 4.48 (4.65) 4.57 (4.73)

[Cu(L2)(dppe)]ClO4 (5c) C49H45N3O7P2ClCu 61 162 61.85 (62.03) 4.62 (4.78), 4.26 (4.43)

[Cu(L3)(dppe)]ClO4 (6c) C47H39N5O9P2ClCu 70 166 57.49 (57.67) 3.84 (4.02) 6.97 (7.16)

63

Table 3.4 : Analytical and physico-chemical data of copper(I) tetrafluoroborate complexes (1d-6d)


Yield

%

M. P. 0C C H N

[Cu(L1)(PPh3)2]BF4 (1d) C57H47N3OP2F4BCu 65 169 68.11 ( 68.30) 4.56 (4.73) 4.01 (4.19)

[Cu(L2)(PPh3)2]BF4 (2d) C59H51N3O3P2CF4Bu 67 157 66.53 (66.39) 4.62 (4.80) 3.78 (4.00)

[Cu(L3)(PPh3)2]BF4 (3d) C57H45N5O5P2F4BCu 63 162 62.53 ( 62.68) 3.97 (4.15) 6.23 (6.41)

[Cu(L1)(dppe)]BF4 (4d) C47H41N3OP2F4BCu 61 155 64.25 (64.43) 4.55 (4.72) 4.63 (4.80)

[Cu(L2)(dppe)]BF4 (5d) C49H45N3O3P2F4BCu 61 173 62.70 ( 62.86) 4.67 (4.84) 4.31 (4.49)

[Cu(L3)(dppe)]BF4 (6d) C47H39N5O5P2F4BCu 70 157 58.25 (58.43) 3.86 (4.07) 7.08 (7.25)

64

Chapter III Copper(I) complexes……..

3.3.3. IR spectra:

The IR spectroscopy is a powerful technique and is quite useful in determining the

coordination mode of ligands in the complexes. On critically examining the position

and direction of shifts in the frequencies of the ligands in complexes as compared to

their positions in free State, the mode of coordination can be suggested for all the

investigated complexes. The IR spectra of the ligands L1-3 and their copper(I)

complexes are found to be quite complex as they in general exhibit large number of

bands of varying intensities. However, an attempt has been made to identify and

assign several structurally important bands to draw fruitful inference about the nature

of bonding in these complexes. This is done on the basis of comparing them with the

reported positions of similar bands in the spectra of related compounds. Some of the

important IR bands in the spectra of copper(I) complexes and their assignments are

summarized in Table 3.5-3.8. The typical IR spectra of the representative complexes

are given in Figs. 3.1-3.16.

All the Schiff base ligands (L1-3) investigated in this work have carbonyl group

(C=O) as a prominent functional group. It is expected that this group can donate lone

pair of electron to the metal atom during coordination. In the uncomplexed Schiff

base ligands L1-3, a medium strong band observed at around 1660 cm-1. This band may

be due υ(C=O) vibrations of the quinazoline ring [17]. The υ(C=O) band is generally

found at 1680 cm-1. The observed shift of υ(C=O) frequency in the ligand L1-3 to

lower region might be due to the presence of intramolecular hydrogen bonding

between oxygen of the C=O entity and the hydrogen of the azomethine group [18]. In

the spectra of the copper(I) complexes, this υ(C=O) band shifted to lower frequency

and appeared at 1611-1627 cm-1 in complexes 1a-d, 2a-d and 3a-d and 1614-1625

cm-1 in 4a-d, 5a-d and 6a-d provides strong evidence for involvement of carbonyl

65


oxygen in coordination with copper(I) metal ion via breakdown of the intramolecular

hydrogen bond [19]. This view is also supported by the presence of a new band at

~480 cm-1 in the spectra of all the complexes due to Cu-O stretching vibrations [20].

Many researchers working on Schiff base chemistry have given prime importance

to the position of azomethine υ(C=N) stretching vibrations. It is difficult to identify

this band due to considerable changes in its environment and additionally, if the

ligands have -C=C- linkage causing the overlapping of frequencies. It has been

reported that any absorption band in the region 1620-1645 cm-1 can be assigned to

azomethine group [21]. The IR spectra of all the Schiff base ligands L1-3 exhibit a

strong band in the region 1610-1628 cm-1 which are assignable to υ(C=N) vibrations.

In the complexes under study this band is shifted to slightly lower frequency region

viz. 1580-1587 cm-1 in 1a-d, 2a-d and 3a-d and 1581-1587 cm-1 in 4a-d, 5a-d and

6a-d. The lowering in the position of these bands suggests linkage between donor

nitrogen atoms of azomethine group with metal ion. Analogous observations have

also made by many authors [22-26].

The IR spectra of Schiff base ligands L1-3 exhibit medium intensity band at around

3283 cm-1 corresponds to υ(NH) of quinazoline ring. In the spectra of all the copper(I)

complexes this band is observed at 3281-3292 cm-1 in 1a-d, 2a-d and 3a-d and 3269-

3290 cm-1 in 4a-d, 5a-d and 6a-d ruling out the possibility of deprotonation of the NH

group of quinazoline and suggests the noninvolvement of NH nitrogen in coordination

with the metal ion [27].

The IR spectra of copper(I) complexes 1a–d, 2a-d and 3a-d show four bands at

around 1480, 1434, 692, 517 cm–1. These bands can be assigned to symmetric (υs) and

asymmetric (υas) stretching vibration modes of phenyl group of PPh3 ligand [28, 29].

66


The presence of these bands in the complexes is indicative of the involvement of

phosphorus of PPh3 group in coordination with copper(I) atom. The spectra of

copper(I) complexes 4a–d, 5a-d and 6a-d exhibited the expected bands due to the

dppe ligand at ca 1435, 1165, 742, 692, and 516 cm-1 [30-32]. The shape and intensity

of the vibrational absorption peak changes obviously in the range of 1000-1500 cm-1.

The P-Ph absorption, at about 1090-1100 cm-1, show an increase in frequency and

intensity, which is characteristic of P-metal coordination [33].

The coordinated nitrate group shows six absorption bands 1505, 1031, 1307, 816,

750 and 695 cm-1 which are assigned to υ4, υ2, υ1, υ6, υ3 and v5 vibrations, respectively.

The magnitude (Δυ) between υ4- υ1, and υ3- υ5 lies between 198-210 cm-1 and 55-61

cm-1, respectively, indicating the coordination of nitrate group in bidentate fashion

[34]. According to Massoud et al. [35] nitrato anion exhibit very strong band at about

1501 cm-1 and 1383 cm-1 corresponds to the υas(NO3-) and υs(NO3

-), respectively. The

strong band observed at ca 1520 and 1374 cm-1 in the spectra of the complexes 1b-6b

assigned for noncoordinated NO3- ion in the complexes.

Hathaway and Underhill have theoretically demonstrated that the infrared spectrum

of the complexes containing the perchlorate group is unique due to several possible

coordination modes of the group with metal ion [36]. They have shown that, as the

perchlorate ion becomes involved in covalent bonding, its symmetry is reduced from

Td to C3v or C2v depending upon whether one or two of its oxygen atoms are involved

in bonding. Two infrared-active bands with Td symmetry are observed near 1100 cm-1

(asymmetric stretch) and 625 cm-1 (asymmetric bend) are split into two components

in C3v symmetry and into three components in C2v symmetry. Thus, one should be

able to distinguish the mode of coordination of the perchlorate ion in complexes. In

67


the present perchlorate complexes (1c-6c), the band at ca 1090 cm–1(υ3) and another

band at ca 625 cm–1 (υ4) is devoid of any splitting suggesting that the ClO4- ion is not

coordinated to copper atom [37-39].

According to Wolfgang Beck and Karl Heinz Sunkel the coordination of highly

symmetric anion BF4- (Td) to the metal center leads to the significant lowering of

symmetry [40]. This results in characteristic splitting of B-F stretching vibrations. The

υ(BF4) vibrations are especially sensitive to change in neighborhood of BF4 anion.

Four υ(BF) bands are expected for Cs symmetry of M-F-BF3 group. Sometimes three

bands are visible at 1105, 1070, 1030 cm-1. Splitting of υ(BF4) band may also be

observed without coordination of the anion to a metal center. In present

tetrafluoroborate complexes (1d-6d) a broad band observed at ca 1094 cm-1

corresponds to presence of BF4- anion in the complexes [41-43].

It is observed that the intensity of ligand bands appearing in the region 400–600

cm-1 often interfere with the metal-ligand band. Thus the assignment of bands of

various υ(M-N) and υ(M-O) vibrations in this region becomes complicated. However

the assignment of the bands to various modes has been made by comparing the

spectra of complexes with those of ligands. Nakamoto [44] has reported that no band

in the structure of Schiff base complexes can be assigned to υ(M-N) vibrations

because of strong coupling between various modes. There are many authors [45-48]

who have assigned metal-nitrogen band in the region 400–600 cm-1. The weak to

medium intensity band observed in the region 513–618 cm-1 in the spectra of all

complexes (1a-d, 2a-d, 3a-d, 4a-d, 5a-d and 6a-d) under study can be attributed to

υ(M-N) vibrations. Taking into consideration the observations of other authors

towards υ(M-O) assignment, the medium intensity band appearing in the region 456–

68

Table 3.5: Infrared spectral data of copper(I) chloride complexes (1a-6a) (cm-1)

Complex υ(NH) υ(C=O) υ(C=N) υ(PPh3) υ(dppe) υ(Cu-N) υ(Cu-O)

1a 3285 1622 1583 1480, 1434, 695, 517 - 513 456

2a 3281 1618 1580 1478,1432, 692, 515 - 525 463

3a 3284 1611 1587 1481, 1435, 694, 516 - 532 454

4a 3286 1614 1584 - 1450, 1172, 744, 694, 511 519 476

5a 3287 1625 1587 - 1456, 1178, 746, 695, 516 523 465

6a 3286 1620 1582 - 1452, 1176, 744, 694, 514 533 468

69

Table 3.6: Infrared spectral data of copper(I) nitrate complexes (1b-6b) (cm-1)

Complex υ(NH) υ(C=O) υ(C=N) υ(PPh3) υ(dppe) υ(NO3) υ(Cu-N) υ(Cu-O)

1b 3285 1622 1585 1480, 1434, 695, 517 - 1509,1387 524 458

2b 3284 1611 1586 1481, 1435, 694, 516 - 1521, 1364 515 464

3b 3287 1627 1583 1480, 1434, 695, 517 - 1518,1376 523 476

4b 3289 1625 1585 - 1436, 1172, 746, 698, 510 1521,1374 527 462

5b 3285 1624 1587 - 1435, 1172, 744, 694, 516 1531,1362, 532 454

6b 3281 1623 1584 - 1434, 1172, 742, 695, 517 1538, 1382 516 474

70

Table 3.7: Infrared spectral data of copper(I) perchlorate complexes (1c-6c) (cm-1)

Complex υ(NH) υ(C=O) υ(C=N) υ(PPh3) υ(dppe) υ( ClO4) υ(Cu-N) υ(Cu-O)

1c 3285 1622 1583 1480, 1434, 695, 517 - 1094, 623 518 462

2c 3292 1625 1587 1479, 1435, 695, 515 - 1093, 624 513 468

3c 3289 1625 1585 1482,1431, 692,519 - 1095,628 532 464

4c 3269 1622 1584 - 1434, 1172, 744, 694, 516 1095, 622 515 458

5c 3289 1625 1585 - 1436, 1170, 748, 698, 510 1094, 623 523 474

6c 3285 1611 1581 - 1435, 1168, 745, 696, 515 1092, 625 527 466

71

Table 3.8: Infrared spectral data of copper(I) tetrafluoroborate complexes (1d-6d) (cm-1)

Complex Υ(NH) υ(C=O) υ(C=N) υ(PPh3) υ(dppe) υ(BF4) υ(Cu-N) υ(Cu-O)

1d 3285 1611 1581 1481, 1435, 690, 517 - 1094 519 462

2d 3269 1622 1584 1480, 1434, 695, 517 - 1095 523 459

3d 3289 1625 1585 1479, 1435, 695, 517 - 1098 528 456

4d 3290 1614 1584 - 1435, 1167, 744, 694, 516 1093 533 468

5d 3285 1620 1583 - 1431,1165, 742, 692, 513 1089 517 454

6d 3287 1616 1586 - 1434, 1172, 746, 696, 515 1096 513 456

72


Fig. 3.1: IR spectrum of [Cu(L1)(PPh3)2]Cl (1a)

Fig. 3.2: IR spectrum of [Cu(L2)(PPh3)2]NO3 (1b)

73


Fig. 3.3: IR spectrum of [Cu(L1)(PPh3)2]ClO4 (1c)

Fig. 3.4: IR spectrum of [Cu(L1)(PPh3)2]BF4 (1d)

74


Fig. 3.5: IR spectrum of [Cu(L2)(PPh3)2]NO3 (2b)


75




76



Fig. 3.10: IR spectrum of [Cu(L1)(dppe)]NO3 (4b)

77



Fig. 3.12: IR spectrum of [Cu(L2)(dppe)]Cl (5a)

78



Fig. 3.14: IR spectrum of [Cu(L2)(dppe)]ClO4 (5c)

79


Fig. 3.15: IR spectrum of [Cu(L3)(dppe)]ClO4 (6c)

Fig. 3.16: IR spectrum of [Cu(L3)(dppe)]BF4 (6d)

80


521 cm-1 in the spectra of these complexes can be ascribed to υ(M-O) vibrations. It

may be noted that these bands are not present in the spectra of constituent ligands.

These assignments are based on the assumption that, since oxygen is more

electronegative than nitrogen, the M-O bond tends to be more ionic than the M-N

bond; consequently M-O vibrations are expected to appear at lower frequencies

relative to M-N stretching vibrations [49].

3.3.4. Electronic spectra:

The electronic absorption spectra in UV-visible region can furnish information on

various transitions incorporated in the metal ligand cluster. Moreover, a wealth of

information about the geometry and electronic structure of the complexes can also be

obtained from the electronic spectra. The electronic absorption spectra of Schiff base

ligands and corresponding complexes were recorded in dichloromethane (10-4 M) in

the range 800-200 nm. The representative spectra of the complexes are displayed in

the Figs. 3.17-3.22 and their spectral data are given in Table 3.9 and 3.10.

The electronic absorption spectra of Schiff base ligands (L1-3) are characterized by

three bands in the UV-visible region. The bands between 280-290 and 280-290 nm

undoubtly originate from the perturbed local excitation of phenyl group. However,

another band observed between 310-326 nm may be due to the n→π* transition

within azomethine group. These ligand bands are expected to undergo substantial

changes on coordination with the metal ion.

3.3.4.1 Complexes 1a-d, 2a-d and 3a-d:

The electronic absorption spectra of the copper(I) complexes in dichloromethane

feature a two absorption bands at 256-272 and 283-290 nm in 1a-d, 266-268 and 282-

289 nm in 2a-d and 265-272 and 287-293 nm in 3a-d. These bands can be assigned to

81


π→π* and n→π* transitions of coordinated ligands. Another broad band with true

maxima at 342-350 nm, 346-350 nm and 346-352 nm is observed in the complexes

1a-d, 2a-d and 3a-d, respectively. The intensity and position of these bands are

consistent which being assigned as ligand centered π→π* or metal to ligand charge

transfer (MLCT) transition [50, 51]. All the complexes are diamagnetic and no d-d

transition is expected due to d10 configuration.

Table 3.9: Electronic spectral data of copper(I) complexes with PPh3 ligand

Complexes UV-Vis (CH2Cl2) λmax (nm)(ε x103, M-1 cm-1)

1a 266 (15.2), 286 (14.2), 382 (5.3)

1b 272 (16.2), 283 (15.3), 385 (6.5)

1c 265 (18.2), 286 (15.5), 384 (7.6)

1d 256 (19.6), 290 (16.0), 390 (9.8)

2a 268 (15.8), 282 (9.05), 386 (5.3)

2b 264 (15.2), 284 (11.0), 388 (6.2)

2c 265 (16.8), 282 (11.0), 348 (6.8)

2d 266 (18.0), 286 (11.5), 410 (7.6)

3a 272 (15.7), 289 (11.8), 389 (7.5)

3b 266 (17.5), 291 (13.0), 386 (8.1)

3c 268 (17.4), 287 (14.2), 388 (8.7)

3d 265 (19.5), 293 (15.1), 392 (9.8)

82


Fig. 3.17: Electronic spectra of copper(I) complexes (1a-d)


83


Fig.3.19: Electronic spectra of copper(I) complexes (3a-d)

3.3.4.2 Complexes 4a-d, 5a-d and 6a-d:

In the complexes 4a-d, 5a-d and 6a-d, the visible range of their electronic spectra

is dominated by metal to ligand charge transfer (MLCT) transition which is a

characteristic feature of the copper(I) complexes when bonded with conjugated

organic chromophores. The absorption spectra of the complexes 1c-3c in dichloro-

methane feature a band with maxima at 342 nm. The complexes 5a-d shows a band at

346-3.60 nm. However, the complexes 6a-d shows a band at 342-348 nm. This band

is assigned to ligand-originating intra-ligand transition together with some metal-

ligand charge transfer (MLCT) character. In high energy region the complexes show

two absorption bands at 268-272 and 288-294 nm in 4a-d, 265-269 and 284-298 nm

in 5a-d and 263-271 and 285-297 nm in 6a-d which are assigned to π→π* and n→π*

transitions of the coordinated ligands. All the copper(I) complexes are diamagnetic

therefore no d-d transitions are observed due to d10 configuration.

84


Table 3.10: Electronic spectral data of copper(I) complexes with dppe ligand

Complexes UV-Vis (CH2Cl2) λmax (nm)(ε x103, M-1 cm-1)

4a 269 (15.8), 288 (14.8), 386 (6.7)

4b 268 (15.2), 296 (13.9), 389 (5.9)

4c 270 (16.7), 289 (15.2), 395 (8.2)

4d 272 (18.4) 294 (16.2), 415 (8.7)

5a 269 (15.1), 284 (13.9), 386 (5.2)

5b 268 (14.3), 296 (13.1), 387 (5.1)

5c 265 (15.9), 286 (14.5), 390 (6.9)

5d 268 (17.5), 297 (14.9), 426 (7.8)

6a 271 (16.3), 288 (15.2), 388 (7.8)

6b 263 (15.9), 285 (14.5), 359 (6.5)

6c 265 (17.5), 292 (16.3), 394 (9.6)

6d 268 (18.5), 297 (16.8), 414 (9.9)


85




86


3.3.5. 1H NMR spectra:

The 1H NMR spectra is powerful tool for investigating the nuclear structure of

molecule distinguishing the proton in similar functional group and also furnishes the

information of steric effect in bonding. The interpretation of 1H NMR spectra for

predicting the structure of unknown compound depends on line position, intensities

and the precise nature of spin multiplets. The peaks are assigned on the basis of

splitting of resonance signals and confirmed by reported literature. The 1H NMR

spectra of all the copper(I) complexes are recorded in CDCl3. The representative 1H

NMR spectra of the complexes are given in the Figs. 3.23-3.40 and their peak

assignments are summarized in Table 3.11-3.13.

The 1H NMR spectra of the free Schiff base ligands L1-3 shows a singlet due to

azomethine protons at δ 8.70-8.84 ppm. The aromatic protons appear as multiplets in

the region 6.80-7.12 ppm. The signal due to N-H proton of quinazoline ring is

observed as a doublet at δ 7.95-7.97 ppm in all the ligands. However, the resonance

due to OCH3 protons in L2 is appeared as a singlet at δ 3.81 ppm.

3.3.5.1. Complexes 1a-d and 4a-d:

The 1H NMR spectra of the complexes 1a-d and 4a-d shows that the aromatic

protons of the coordinated PPh3 and dppe ligand overlap to some extent with those of

the aromatic protons of the ligand L1. In the spectra of these complexes the aromatic

region consists of multiplets in the range δ 6.58-8.71 ppm (1a-d) and δ 6.58-8.72 ppm

(4a-d) due to aromatic protons of phosphine ligand and phenyl ring protons of the

Schiff base ligand L1 [52]. Moreover, the azomethine protons of the free ligand L1 is

shifted to downfield region and observed at δ 9.05-9.28 ppm in 1a-d and δ 9.15-9.26

ppm in 4a-d on coordination. The downfield shift of the azomethine protons relative

to the free ligands L1 can be attributed to the deshielding effect resulting from the

87


coordination of the ligand L1 to copper(I) [53]. The multiplet due to NH proton

remains unperturbed at δ ~7.98 ppm in the complexes. The spectra of the complexes

4a-d shows the resonances of methylene proton due to dppe group at δ 2.56-2.65 ppm

[54, 55].

Table 3.11: 1H NMR spectral data of copper(I) complexes with L1

Complex δ (s, HC=N) δ (m, Ar-H) δ (m, NH) δ (s, CH2)

1a 9.20 6.63-8.71 7.96 -

1b 9.05 6.58-7.90 7.95 -

1c 9.17 6.71-7.91 7.94 -

1d 9.16 6.65-7.91 7.95 -

4a 9.22 6.98-8.65 7.97 2.65

4b 9.15 6.94-8.68 7.94 2.58

4c 9.20 6.58-8.72 7.98 2.56

4d 9.18 6.94-8.62 7.99 2.56

3.3.5.2. Complexes 2a-d and 5a-d

The 1H NMR spectra of the copper(I) complexes 2a-d and 5a-d shows the

azomethine proton as a singlet at δ 9.19-9.28 and 9.22-9.28 ppm, respectively. This

azomethine signal shifted to downfield region as compared to the corresponding free

ligand L2 suggesting deshielding of azomethine proton due to coordination of the

azomethine nitrogen. In the spectra of these complexes the resonance of aromatic

protons of the coordinated PPh3 and dppe ligand overlaps to some extent with those of

the aromatic protons of the ligand L2. The aromatic region of the complexes 2a-d and

5a-d consists of several coupled multiplets in the range δ 6.92-8.68 and δ 6.92-8.70

ppm due to the aromatic protons of PPh3 or dppe ligand as well as phenyl protons of

88


the Schiff base ligand L2. The NH proton of the quinazoline ring appeared as multiplet

at δ ~7.97 ppm in the complexes. The singlet observed at δ 3.79-3.85 ppm in the

spectra of 2a-d and δ 3.79-3.87 ppm in 5a-d is assigned to the resonances of methoxy

group of Schiff base L2 [56]. However, the spectra of the complexes 5a-d shows a

broad singlet at δ 2.56-2.68 ppm corresponds to the methylene protons of the dppe ligand.


Complex δ (s,HC=N) δ (m, Ar-H) δ (m, NH) δ (s, CH2) δ (s, OCH3)

2a 9.28 6.95-8.68 7.97 - 3.83

2b 9.22 6.93-8.62 7.98 - 3.79

2c 9.23 6.92-8.58 7.96 - 3.85

2d 9.19 6.97-8.52 7.99 - 3.81

5a 9.26 6.96-8.70 7.98 2.68 3.87

5b 9.25 6.94-8.68 7.96 2.63 3.82

5c 9.22 6.97-8.68 7.95 2.62 3.79

5d 9.28 6.95-8.69 7.96 2.65 3.86

3.3.5.3. Complexes 3a-d and 6a-d

The 1H NMR spectra of complexes 3a-d and 6a-d shows a singlet at δ 9.19-9.26 ppm

corresponds to azomethine proton of ligand L3. The downfield shift of this

azomethine signal relative to the free ligand L3 can be attributed to the deshielding

effect resulting from the coordination of the ligand L3 to copper(I). The 1H NMR

spectra of the complexes shows that the aromatic protons of the coordinated PPh3 and

dppe ligand overlap to some extent with those of the aromatic protons of the ligand

L3. However, the spectra of the complexes 3a-d and 6a-d shows a broad multiplets in

the range δ 6.92-8.60 ppm and δ 6.92-8.68 ppm, respectively due to aromatic protons

of triphenylphosphine or diphenylphosphinoethane and phenyl rings of the Schiff base

89


ligand. The multiplet due to NH proton remains unperturbed at δ ~7.98 ppm in the

complexes. The spectra of the complexes 6a-d show broad singlet at δ 2.59-2.66 ppm

corresponds to methylene protons of the dppe ligand.


Complex δ (s,HC=N) δ (m, Ar-H) δ (m, NH) δ (s, CH2)

3a 9.20 6.99-8.60 7.98 -

3b 9.26 6.94-8.59 7.96 -

3c 9.19 6.92-8.56 7.99 -

3d 9.22 6.93-8.64 7.95 -

6a 9.19 7.05-8.68 7.97 2.63

6b 9.24 6.97-8.60 7.96 2.66

6c 9.26 6.92-8.60 7.98 2.64

6d 9.23 6.99-8.61 7.95 2.59

Fig. 3.23: 1H NMR spectrum of [Cu(L1)(PPh3)2]NO3 (1b)

90


Fig. 3.24: 1H NMR spectrum of [Cu(L1)(PPh3)2]ClO4 (1c)

Fig. 3.25: 1H NMR spectrum of [Cu(L1)(PPh3)2]BF4 (1d)

91


Fig. 3.26: 1H NMR spectrum of [Cu(L2)(PPh3)2]Cl (2a)


92


Fig. 3.28: 1H NMR spectrum of [Cu(L2)(PPh3)2]BF4 (2d)

Fig. 3.29: 1H NMR spectrum of [Cu(L3)(PPh3)2]Cl (3a)

93


Fig. 3.30: 1H NMR spectrum of [Cu(L3)(PPh3)2]NO3 (3b)


94


Fig. 3.32: 1H NMR spectrum of [Cu(L1)(dppe)]Cl (4a)

Fig. 3.33: 1H NMR spectrum of [Cu(L1)(dppe)]ClO4 (4c)

95



Fig. 3.34: 1H NMR spectrum of [Cu(L1)(dppe)]BF4 (4d)

F


96




97




98



3.3.6. X-Ray structure:

The most powerful tool for the characterization of coordination solids is the single

crystal X-ray crystallography. This technique provides an accurate account of the

structure and properties of materials in crystalline state. Additional advanced

analytical and graphical tools associated with this process allows for an in-depth study

of the material chemistry. The X-ray crystallography study of representative copper(I)

complexes of the series [Cu(L)(PPh3)2]BF4 (1d) was carried out on a Nonius MACH-

3 four-circle diffractometer with graphite-monochromatized MoKα radiation and is

presented in Fig. 3.3941. X-ray crystallography data were collected in Table 3.14 and

selected bond lengths and bond angles are given in Table 3.15 and 3.16.

3.3.6.1. Crystal structure of [Cu(L1)(PPh3)2]BF4 (1d):

The crystals of [Cu(L1)(PPh3)2]BF4 (1d) were grown by slow diffusion of diethyl

ether into a solution of complex in dichloromethane and its structure was determined

99


by X-ray crystallography. X-ray analysis revealed that the complex 1d crystallizes in

the triclinic space group P-1. The crystal of complex 1d contains discrete cation

[Cu(L)(PPh3)2]+ and tetrafluoroborate as a counter anion.

The complex 1d is mononuclear and central copper(I) ion exhibit highly distorted

tetrahedral geometry with CuNOP2 coordination. The quinazoline ligand is chelated

to the copper ion in neutral bidentate form through azomethine nitrogen and carbonyl

oxygen forming a five-membered chelation ring. The distorted four-coordinate

geometry of Cu(I) is completed by two triphenylphosphine ligands. The largest

deviation from the ideal tetrahedral geometry is reflected by the restricted bite angles

of the chelating ligands. The intraligand O(1)-Cu(1)-N(1) chelate angle, 76.53 (12)° is

much less than 109.4°. However, the P(2)-Cu(1)-P(1), 127.91(5)° angle have opened

up due to the steric effects from the bulky PPh3 ligand. The average Cu-N and Cu-P

bond distances are 2.123 and 2.246 Ǻ, respectively, and are comparable to those

reported for [Cu(A)(PPh3)2]ClO4 (2.098 and 2.251 Ǻ) [57].

Torsion angles in the chelating ring and quinazoline group are listed in Table 3. 16

The chelating ring Cu(1)-N(1)-N(2)-C(1)-O(1) is nearly planar with sum of three N

atom bond angles is 359.3°. However, some strain in the chelate ring is suggested by

the deviation from the 120° angle about the N atom Cu(1)-N(1)-C(15), 132.1(3)°;

Cu(1)-N(1)-N(2), 110.5(3)° and C(15)-N(1)-N(2), 116.7(4)°.

In the heteroatomic part of quinazoline, the angles N(3)-C(8)-C(9), 111.3(5);

N(3)-C(8)-N(2), 106.2(5) and C(9)-C(8)-N(2), 110.7(5); indicate the sp3 hybridized

state of the carbon atom, and the geometry around C(8) can be viewed in terms of a

distorted tetrahedral geometry. The two N-C (sp2) bond distances [N(2)-C(1),

1.360(6) and N(3)-C(7), 1.370(7)] show double bond character and two N-C (sp3)

bond distances [N(2)-C(8), 1.511(7) and N(3)-C(8), 1.482(7)] show single bond

100


character. The sum of the angles around N(2) and N(3) are 359.7 and 360.0°,

respectively. The benzaldehyde moiety directly linked at C(8) is oriented at an angle

of 83.7(7)° with respect to the quinazoline ring. The quinazoline ring and the

benzaldehyde moiety linked through N(1) and C(15) are trans to each other, thus

showing E-configuration. Further, the tortional angle of N(2)-N(1)-C(15)-C(16) is

175.2(3)° indicating an anti-periplanar arrangement.

Fig. 3.41: X-ray Structure of [Cu (L1)(PPh3)2]BF4 (1d)

101


Table 3.14: Crystal data and structure refinements details for [Cu(L)(PPh3)2]BF4 (1d)

Empirical formula C57H45BcuF4N3OP2

Formula weight 1000.25

Temperature 150(2) K

Wavelength 0.71073 A

Crystal system, space group Triclinic, P -1

Unit cell dimensions a = 12.8591(5) Å alpha = 106.882(5)°

b = 13.7884(10) Å beta = 97.097(4)°

c = 15.8076(7) Å gamma = 109.548(5)°

Volume 2450.5(2) Å3

Z, Calculated density 2, 1.356 Mg/m3

Absorption coefficient 0.570 mm-1

F(000) 1032

Crystal size 0.33 x 0.28 x 0.21 mm

Theta range for data collection 3.35 to 25.00°

Limiting indices -15<=h<=15, -14<=k<=16, -18<=l<=18

Reflections collected / unique 17744 / 8615 [R(int) = 0.0464]

Completeness to theta = 25.00 99.8 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 0.8896 and 0.8341

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 8615 / 0 / 622

Goodness-of-fit on F^2 0.931

Final R indices [I>2sigma(I)] R1 = 0.0563, wR2 = 0.1366

R indices (all data) R1 = 0.0993, wR2 = 0.1465

Largest diff. peak and hole 1.397 and -0.657 e.A-3

102


Table 3.15: Selected bond lengths (Ǻ) and bond angles (°) for [Cu(L)(PPh3)2]BF4 (1d)

Cu(1)-O(1) 2.123(3)

Cu(1)-N(1) 2.123(4)

Cu(1)-P(2) 2.2460(12)

Cu(1)-P(1) 2.2476(11)

N(2)-C(1) 1.360(6)

N(2)-C(8) 1.511(7)

N(3)-C(7) 1.370(7)

N(3)-C(8) 1.482(7)

O(1)-Cu(1)-N(1) 76.53(12)

O(1)-Cu(1)-P(2) 101.86(8)

N(1)-Cu(1)-P(2) 114.77(10)

O(1)-Cu(1)-P(1) 108.41(8)

N(1)-Cu(1)-P(1) 112.93(10)

P(2)-Cu(1)-P(1) 127.91(5)

C(15)-N(1)-N(2) 116.7(4)

C(15)-N(1)-Cu(1) 132.1(3)

N(2)-N(1)-Cu(1) 110.5(3)

N(3)-C(8)-C(9) 111.3(5)

N(3)-C(8)-N(2) 106.2(5)

C(9)-C(8)-N(2) 110.7(5)

C(15)-N(1)-N(2) 116.7(4)

C(15)-N(1)-Cu(1) 132.1(3)

N(2)-N(1)-Cu(1) 110.5(3)

C(1)-N(2)-N(1) 116.9(4)

C(1)-N(2)-C(8) 126.1(4)

N(1)- N(2)-C(8) 116.7(4)

103


Table 3.16: Torsion angles for chelating ring

Cu(1)-N(1)-N(2)-C(1) -4.8 (4)

Cu(1)-O(1)-C(1)-N(2) 8.8 (5)

N(1)-N(2)-C(1)-O(1) -2.6(6)

O(1)-Cu(1)- N(1)-N(2) 6.6 (2)

N(1)-Cu(1)-O(1)-C(1) -8.4 (3)

N(3)-C(8)-C(9)-C(10) 83.7(7)

N(2)-N(1)-C(15)-C(16) 175.2(3)

3.3.7. Electrochemical studies:

The electrochemical properties of all the copper(I) complexes (1a-6a, 1b-6b, 1c-6c

and 1d-6d) have been examined cyclic voltammetrically in 10-3 M CH2Cl2 solution

containing 0.05 M n-Bu4NclO4 as supporting electrolyte and redox potentials are

expressed with reference to Ag/AgCl. All the measurements were carried out in the

potential range +1.5 to -1.5 V with scan rate 50 mVs-1. The cyclic voltammogram of

the copper(I) complexes are presented in Figs. 3.40–3.45 and the results are collected

in Table 3.17 and 3.18.

The copper(I) complexes 1a-d, 2a-d and 3a-d undergo a quasireversible oxidation-

reduction reaction. Since, the ligands used in this work are not reversibly oxidized or

reduced in the applied potential range. The redox potentials are assigned to metal

centers only. This response is attributed to the copper(II)/copper(I) couple

[Cu(L)(PPh3)2]2+ + e- [Cu(L)(PPh3)2]+

The cyclic voltammogram of the complexes 1a-d displays a reduction peak at Epc =

0.642-0.676 V due to Cu(II)/Cu(I) with a corresponding oxidation peak at Epa =

104


0.673-0.720 V due to Cu(I)/Cu(II). The complexes 2a-d undergoes a reversible single

electron redox process E1/2 = 0.661-0.692 V (Epc = 0.646-0.685 V and Epa = 0.675-

0.722 V). The complexes 3a-d also display a redox process at E1/2 = 0.655-0.695 V

(Epc = 0.648-0.688 V and Epa = 0.678-0.725 V) corresponding to Cu(II)/Cu(I)

couple. The difference between anodic and cathodic peak potential for complexes

(∆Ep) is in the range 0.028-0.06 mV. All these copper(I) complexes have reversible

character as the separation peak potentials are ≤ 59 mV [58].

Table 3.17: Electrochemical data for copper(I) complexes with PPh3 ligand

Oxidation potential Compound

Epa Epc ∆Ep E1/2

1a 0.706 0.557 149 0.631

1b 0.673 0.542 131 0.608

1c 0.694 0.576 118 0.635

1d 0.720 0.558 162 0.639

2a 0.688 0.540 148 0.614

2b 0.656 0.532 124 0.594

2c 0.676 0.562 144 0.619

2d 0.694 0.542 152 0.618

3a 0.723 0.573 150 0.648

3b 0.698 0.558 140 0.628

3c 0.708 0.588 120 0.648

3d 0.735 0.567 168 0.651

105


For the copper(I) complexes 4a-d, the reduction wave (Epc, 0.646 to 0.695 V)

corresponding to reduction of Cu(II) to Cu(I) is obtained. During the reverse scan the

oxidation of Cu(I) to Cu(II) occurs in the potential range (Epa, 0.678 to 0.722 V). In

the complexes 5a-d the one electron oxidation peak, which is attributed to the Cu(I) to

Cu(II) couple, occurs in the range 0.675 to 0.724 V(Epa) with an associated peak in

the reverse scan at 0.644 to 0.699 V (Epc) was observed corresponding to Cu(II) to

Cu(I). However, the complexes 6a-d displayed redox process at E1/2 = 0.658–0.686 V

the complexes 5a-d the one electron oxidation peak, which is attributed to the Cu(I) to

Cu(II) couple, occurs in the range 0.675 to 0.724 V(Epa) with an associated peak in

the reverse scan at 0.644 to 0.699 V (Epc) was observed corresponding to Cu(II) to

Cu(I). However, the complexes 6a-d displayed redox process at E1/2 = 0.658–0.686 V

(Epc = 0.649-0.698 V and Epa = 0.673 to 0.727 V) corresponding to Cu(II)/Cu(I)

couple. The difference between anodic and cathodic peak potential for copper(I)

complexes (∆Ep) is in the range 0.028-0.06 mV. All the copper(I) complexes have

reversible character as the separation peak potentials are 120-168 mV.

It is found that the redox potential observed in all the copper(I) complexes is

sensitive to the electron donating or electron withdrawing nature of substituents on the

Schiff base ligands. An observable deviation is found for the complexes 3a-d (0.655-

0.695 V) and 6a-d (0.658–0.686 V) containing electron withdrawing group (p-NO2)

on phenyl ring of the Schiff base (L3), where the Cu(II)/Cu(I) couple appears at

higher potential than corresponding 1a-d (0.658-689 V), 2a-d (0.661-0.692), 4a-d

(0.658-0.686 V) and 5a-d (0.653-0.681 V) complexes. In the complexes 2a-d (0.661-

0.692) and 5a-d (0.653-0.681 V) containing electron donating group (p-OCH3) on

phenyl rings of Schiff base (L2), the redox potential is observed at less positive

potential as compared to the complexes 1a-d (0.658-689 V), 3a-d (0.655-0.695 V),

106


4a-d (0.658-0.686 V) and 6a-d (0.658–0.686 V). These results evidently corresponds

to the electron donating effect of p-OCH3 group and the electron withdrawing effect

of p-NO2 group of Schiff base ligands [59, 60].

Table 3.18: Electrochemical data for copper(I) complexes with dppe ligand

Oxidation potential Compound

Epa Epc E1/2 ∆Ep

4a 0.710 0.550 160 0.630

4b 0.678 0.546 132 0.612

4c 0.706 0.595 111 0.650

4d 0.722 0.559 168 0.640

5a 0.682 0.556 126 0.619

5b 0.645 0.544 101 0.594

5c 0.683 0.569 114 0.626

5d 0.684 0.556 128 0.620

6a 0.732 0.585 147 0.658

6b 0.704 0.549 155 0.626

6c 0.715 0.598 117 0.656

6d 0.737 0.556 181 0.646

107


Fig. 3.42: Cyclic voltammogram of copper(I) complexes (1a-d)

Fig. 3.43: Cyclic voltammogram of copper (I) complexes (2a-d)

108



Fig. 3.45: Cyclic voltammogram of copper(I) complexes (4a-d)

109


Fig.3.46: Cyclic voltammogram of copper(I) complexes (5a-d)


110


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