Synthesis and characterization of Pb(II) complexes of Schiff bases derived from 3-methyl-4-fluoroacetophenone and amino acids

Article abstract of DOI:10.1007/s11164-010-0216-4

A series of Pb(II) complexes of the type Pb(L)₂ have been synthesized with fluorinated Schiff bases derived from 3-methyl-4- fluoroacetophenone and amino acids (viz phenylalanine, alanine, tryptophan, valine, isoleucine, and glycine). These complexes are insoluble in common organic solvents but soluble in DMF and DMSO. The measured molar conductance values in DMF indicate that the complexes are non-electrolytes. On the basis of analytical and spectral (IR, UV-visible, and ¹H, ¹³C and ¹⁹F NMR) studies, it has been concluded that all the metal complexes have square planar geometry in which the ligand is coordinated to the metal ion through the azomethine nitrogen atom and the carboxylate oxygen atom via deprotonation.

Full text of DOI:10.1007/s11164-010-0216-4

Res Chem Intermed (2010) 36:1037–1047
DOI 10.1007/s11164-010-0216-4
Synthesis and characterization of Pb(II) complexes
of Schiff bases derived from 3-methyl-
4-fluoroacetophenone and amino acids
•
Har Lal Singh S. S. Chauhan H. Sachedva
•
Received: 21 August 2010 / Accepted: 4 November 2010 / Published online: 23 November 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A series of Pb(II) complexes of the type Pb(L)2 have been synthesized
with fluorinated Schiff bases derived from 3-methyl-4-fluoroacetophenone and
amino acids (viz phenylalanine, alanine, tryptophan, valine, isoleucine, and
glycine). These complexes are insoluble in common organic solvents but soluble in
DMF and DMSO. The measured molar conductance values in DMF indicate that
the complexes are non-electrolytes. On the basis of analytical and spectral (IR,
1
UV–visible, and H, 13C and 19F NMR) studies, it has been concluded that all the
metal complexes have square planar geometry in which the ligand is coordinated to
the metal ion through the azomethine nitrogen atom and the carboxylate oxygen
atom via deprotonation.
Keywords Fluorinated Schiff bases Á Lead(II) complexes Á Spectral studies
Introduction
Metal carboxylates have been the subject of extensive investigations because of
their remarkable structural diversity [1–3] and substantial biological activity, for
examples as antimicrobial and antitumour agents [4–6]. In recent years, more
and more investigations have focussed on the synthesis of metal carboxylates of
functionalized carboxylic acids with additional oxygen [7, 8], sulfur [8, 9], or
nitrogen [10–12] donor groups.
Amino acids, which constitute a very important class of biomolecules, can act as
potential oxygen and nitrogen donor ligands. It has been found that they utilize their
functional groups as fully as possible in metal coordination [12]. Therefore, several
H. L. Singh (&) Á S. S. Chauhan Á H. Sachedva
Department of Chemistry, Faculty of Engineering and Technology, Mody Institute of Technology
and Science, Lakshmangarh, Sikar, Rajasthan, India
e-mail: hlsingh9@rediffmail.com; harlalsingh@hotmail.com
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H. L. Singh et al.
metal derivatives of different amino acids have been reported [13, 14] and some of
these have been found to have significant biological activity, for example,
tricyclohexyltin alaninate has been found to be active as a fungicide and bactericide
for seeds and plants [15]. It has been reported that metal complexes of amino acid
Schiff bases with transition metals have anticarcinogenic [16], antimicrobial [17]
and antitumour [18] activity. Because its small size, fluorine has been used as a
replacement for hydrogen in many biologically active molecules, including amino
acids [19]. Once introduced, the strong carbon fluorine bond is particularly resistant
to metabolic transformations, and the electronegativity of fluorine can have a
substantial effect on the basicity or acidity of neighboring groups and on the
electron distribution, and can change the overall reactivity and stability of a
molecule [20]. Incorporation of fluorine into heterocycles is known to affect the
course of their reactions and their biological activity [21]. It has been observed that
introduction of a fluorine atom into heterocycles may act as a pharmocophore,
enhancing the pharmacological properties of compounds compared with their non-
fluorinated analogues [22].
This paper deals with the synthesis and characterization of lead complexes in
order to study the mode of bonding of fluorinated ligands (Fig. 1). The ligands have
C=N and COOH as the main donating groups. In this paper we examine the mode of
bonding of these sites to the central metal ion and whether this takes place through
nitrogen or oxygen, or both, donor atoms to confirm the bidentate nature of the
ligands in these complexes. The structures of the ligands are shown in Fig. 1.
Experimental
All chemicals used in this work, viz, 3-methyl-4-fluoroacetophenone, amino acids,
and metal acetates, were of analytical grade. Solvents used were dried and purified
by standard methods and moisture was excluded from glass apparatus by use of
CaCl2 drying tubes. Melting points were determined in an open glass capillary and
were uncorrected.
Preparation of the ligands
The ligands were synthesized by condensation of 3-methyl-4-fluoroacetophenone
(0.601 g, 3.94 mmol) with the amino acids (phenylalanine alanine, tryptophan,
isoleucine, valine, and glycine; 0.805–0.296 g, 3.94 mmol) in 1:1 molar ratio, using
methanol (120 mL) as the reaction medium, under reflux for 6–8 h. After cooling at
room temperature the solid products were obtained. The excess solvent was
removed on a rotary evaporator. The residue was further dried and then purified by
recrystallization from the same solvent.
L1H was prepared by reacting 3-methyl-4-fluoroacetophenone with glycine,
colour, yellow; yield, 70%; mp, 230 °C (d) and elemental analysis (%), calcd. for
C11H12FNO2: C, 63.15; H, 5.78; N, 6.69; found, C: 63.10; H, 5.71; N, 6.61;
molecular weight: found, 201.63, calcd. 209.22. 1H NMR (DMSO-d6, d ppm): 11.28
123

Synthesis and characterization of Pb(II) complexes
1039
R
N
O
Fig. 1 Structures of the ligands
H3C
C
OH
H3C
F
1a-f
R
Abbreviation
L1H
a
b
c
H
L2H
L3H
CH3
CH(CH3)2
L4H
L5H
d
CH(CH3)CH2CH3
CH2
e
f
L6H
CH2
NH
(s, 1H, COOH), 4.20 (s, 2H, –CH2–), 1.91 (s, 3H, –C(CH3)=N–), 2.15 (s, 3H,
Ph–CH3), 6.99–7.20 (m, 3H, aromatic); 13C NMR (DMSO, d ppm): 175.9 (COOH),
63.6 (–CH2–), 165.2 (C=N), 17.1 (–CH3), 14.2 (Ph–CH3), 137.2, 128.1, 119.9,
164.1, 121.8, 128.4 (Aromatic carbons); 19F NMR (DMSO-d6, d ppm): -76.1;
UV–visible (kmax, nm): 390, 214, 265; Infrared (KBr, cm-1): m(C=N), 1625;
m(COOH), 3100–2750; m(C=O), 1715.
L2H was prepared by reacting 3-methyl-4-fluoroacetophenone with valine, colour,
light pink; yield, 78%; mp, 140 °C (d) and elemental analysis (%), calcd. for
C14H18FNO2: C, 66.91; H, 7.22; N, 5.57; found, C: 66.76; H, 7.10; N, 5.49; molecular
1
weight: found, 240.09, calcd. 251.29. H NMR (DMSO-d6, d ppm): 11.48 (s, 1H,
COOH), 4.08 (d, 1H, NCH–C), 1.86 (s, 3H, –C(CH3)=N–), 2.15 (s, 3H, Ph–CH3), 1.07
(d, 3H, –C–CH3), 2.39 (m, 1H, –CH–), 7.02–7.15 (m, 3H, aromatic); 13C NMR
(DMSO, d ppm): 177.2 (COOH), 71.3 (N–CH–), 161.1 (C=N), 17.4 (–CH3), 15.2
(Ph–CH3),33.2(–CH–), 18.6(–CH3), 136.7,128.3,120.1,164.3,122.2, 129.6(aromatic
carbons); 19F NMR (DMSO-d6, d ppm): -75.9; UV–visible (kmax, nm): 395, 205, 265;
Infrared (KBr, cm-1): m(C=N), 1630; m(COOH), 3090–2695; m(C=O), 1710.
L3H was prepared by reacting 3-methyl-4-fluoroacetophenone with alanine,
colour, brown; yield, 72%; mp, 176 °C (d) and elemental analysis (%), calcd. for
123

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H. L. Singh et al.
C12H14FNO2: C, 64.56; H, 6.32; N, 6.27; found, C: 64.36; H, 6.25; N, 6.18;
molecular weight: found, 215.87, calcd. 223.24. 1H NMR (DMSO-d6, d ppm):
11.68 (s, 1H, COOH), 4.56 (q, 1H, NCH–C), 1.89 (s, 3H, –C(CH3)=N–), 2.24
(S, 3H, Ph–CH3), 1.48 (d, 3H, –CH3), 6.98–7.36 (m, 8H, aromatic); 13C NMR
(DMSO, d ppm): 175.7 (COOH), 64.5 (N–CH–), 163.1 (C=N), 17.3 (–CH3), 14.8
(Ph–CH3), 18.6 (–CH2–Ph), 136.4, 127.3, 120.1, 164.1, 120.1, 128.9 (aromatic
carbons); 19F NMR (DMSO-d6, d ppm): -76.7; UV–visible (kmax, nm): 392, 208,
264; Infrared (KBr, cm-1): m(C=N), 1620; m(COOH), 3070–2590; m(C=O), 1620.
L4H was prepared by reacting 3-methyl-4-fluoroacetophenone with isoleucine:
colour, light yellow; yield, 68%; mp, 244 °C (d) and elemental analysis (%), calcd.
for C15H20FNO2: C, 67.90; H, 7.60; N, 5.28; found, C: 67.82; H, 7.48; N, 5.22;
molecular weight: found, 255.58, calcd. 265.32. 1H NMR (DMSO-d6, d ppm): 11.59
(s, 1H, COOH), 4.51 (d, 1H, NCH–C), 1.56 (q, 2H, –CH2–), 1.88 (s, 3H,
–C(CH3)=N–), 2.18 (s, 3H, Ph–CH3), 1.27 (d, 3H, –C–CH3), 0.98 (m, 1H, –C–CH–C),
7.00–7.19 (m, 3H, aromatic); 13C NMR (DMSO, d ppm): 178.2 (COOH), 66.7
(–CH2–), 162.1 (C=N), 16.3 (–CH3), 14.6 (Ph–CH3), 36.5 (–CH2–Ph), 136.9, 128.4,
119.1, 164.2, 122.2, 129.8,138.8, 128.4, 129.7, 116.7 (aromatic carbons); 19F NMR
(DMSO-d6, d ppm): -76.2; UV–visible (kmax, nm): 390, 210, 260; Infrared
(KBr, cm-1): m(C=N), 1628; m(COOH), 3100–2680; m(C=O), 1724.
L5H was prepared by reacting 3-methyl-4-fluoroacetophenone with phenylala-
nine: colour, Pink; yield, 75%; mp, 220 °C (d) and elemental analysis (%) calcd. for
C18H18FNO2: C, 72.22; H, 6.06; N, 4.68; found, C: 72.01; H, 6.00; N, 4.60; molecular
1
weight: found, 286.44, calcd. 299.34. H NMR (DMSO-d6, d ppm): 11.52 (s, 1H,
COOH), 4.21 (t, 1H, NCH–C), 2.98 (d, 2H, –CH2–), 1.82 (s, 3H, –C(CH3)=N–), 2.09
(s, 3H, Ph–CH3), 6.98–7.50 (m, 8H, aromatic); 13C NMR (DMSO, d ppm): 178.2
(COOH), 66.7 (–CH2–), 162.1 (C=N), 16.3 (–CH3), 14.6 (Ph–CH3), 36.5 (–CH2–Ph),
136.9, 128.4, 119.1, 164.2, 122.2, 129.8,138.8, 128.4, 129.7, 116.7 (aromatic
carbons); 19F NMR (DMSO-d6, d ppm): -76.3; UV–visible (kmax, nm): 390, 210,
260; Infrared (KBr, cm-1): m(C=N), 1622; m(COOH), 3080–2640; m(C=O), 1722.
L6H was prepared by reacting 3-methyl-4-fluoroacetophenone with tryptophan:
colour, brownish gray; yield, 82%; mp, 254 °C (d) and elemental analysis (%),
calcd. for C20H19FN2O2: C, 70.99; H, 5.66; N, 8.28; found, C: 70.85; H, 5.54; N,
8.02; molecular weight: found, 328.20, calcd. 338.37. 1H NMR (DMSO-d6, d ppm):
11.54 (s, 1H, COOH), 4.49 (t, 1H, NCH–C), 3.17 (d, 2H, –CH2–) 1.93 (s, 3H,
–C(CH3)=N–), 2.18 (s, 3H, Ph–CH3), 6.98-7.55 (m, 8H, aromatic); 13C NMR
(DMSO, d ppm): 178.2 (COOH), 66.7 (–CH2–), 162.1 (C=N), 16.3 (–CH3), 14.6
(Ph–CH3), 36.5 (–CH2–Ph), 136.9, 128.4, 119.1, 164.2, 122.2, 129.8,138.8, 128.4,
129.7, 116.7 (aromatic carbons); 19F NMR (DMSO-d6, d ppm): -76.4; UV–visible
(kmax, nm): 388, 210, 262; Infrared (KBr, cm-1): m(C=N), 1625; m(COOH),
3095–2710; m(C=O), 1715.
Synthesis of lead(II) complexes
The complexes were prepared under anhydrous conditions by slow addition of a
dry, hot methanol solution of lead(II) acetate in 1:2 molar ratio to a solution of the
123

Synthesis and characterization of Pb(II) complexes
1041
Schiff base in hot absolute methanol. The mixture was heated under reflux with
constant stirring for 30 min, giving a clear solution; heating under reflux was then
continued for 4–5 h. The acetic acid librated in the reaction was removed
azeotropically with solvent. Excess solvent was removed under reduced pressure
and the compound was dried under vacuum at 45 5 °C after repeated washing with
dry cyclohexane. The compounds were purified by recrystallization from methanol.
The purity of the compounds was checked by TLC using silica gel G as adsorbent.
Pb(L1)2 was prepared by reacting lead(II) acetate with ligand (L1H); colour,
cream; yield, 65%; and elemental analysis (%), calcd. for PbC22H22F2N2O4: Pb,
33.22; C, 42.37; H, 3.56; N, 4.49; found: Pb, 33.10; C, 42.49; H, 3.69; N, 4.42;
molecular weight: found, 608.85, calcd. 623.62. Molar conductance (DMF, 10-3
X-1, mol-1, cm2): 21.6; 1H NMR (DMSO-d6, d ppm): 4.22 (s, 2H, –CH2–),
2.04 (s, 3H, –C(CH3)=N–), 2.13 (s, 3H, Ph–CH3), 6.96–7.28 (m, 3H, aromatic); 13
,
C
NMR (DMSO, d ppm): 166.4 (COO), 64.1 (–CH2–), 154.9 (C=N), 17.2 (–CH3),
14.4 (Ph–CH3), 136.8, 128.7, 120.9, 164.1, 121.2, 128.7 (aromatic carbons); 19F
NMR (DMSO-d6, d ppm): -72.1; UV–visible (kmax, nm): 380, 215, 268, 365;
Infrared (KBr, cm-1): m(C=N), 1610; m(C=O), 1712; masym(COO), 1555; msym(COO),
1310; m(Pb / N), 535; m(Pb–O), 465.
Pb(L2)2 was prepared by reacting lead(II) acetate with ligand (L2H): colour, light
yellow; yield, 76%; mp, 158–159 °C and elemental analysis (%), calcd. for
PbC28H34F2N2O4: Pb, 29.27; C, 47.52; H, 5.84; N, 3.96; found; Pb, 29.36; C, 47.32;
H, 5.73; N, 3.77; molecular weight: found, 692.50, calcd. 707.76. Molar
conductance (DMF, 10-3, X-1, mol-1, cm2): 18.15; 1H NMR (DMSO-d6,
d ppm): 4.09 (d, 1H, NCH–C), 1.98 (s, 3H, -C(CH3)=N–), 2.17 (s, 3H, Ph–CH3),
1.09 (d, 3H, –C–CH3), 2.45 (m, 1H, –CH–), 6.99–7.26 (m, 3H, aromatic); 13C NMR
(DMSO, d ppm): 167.5 (COO), 71.3 (N–CH–), 152.8 (C=N), 17.6 (–CH3), 15.1
(Ph–CH3), 33.4 (–CH–), 18.5 (–CH3), 136.6, 128.5, 120.3, 164.1, 121.9, 129.6
(aromatic carbons); 19F NMR (DMSO-d6, d ppm): -72.4; UV–visible (kmax, nm):
385, 208, 260, 360; Infrared (KBr, cm-1): m(C=N), 1615; m(C=O), 1712;
m
asym(COO), 1535; msym(COO), 1306; m(Pb / N), 530; m(Pb–O), 455.
Pb(L3)2 was prepared by reacting lead(II) acetate with ligand (L3H): colour, light
brown; yield, 73%; and elemental analysis (%), calcd. for PbC24H26F2N2O4: Pb,
31.79; C, 44.23; H, 4.02; N, 4.30; found: Pb, 31.58, C, 44.15; H, 4.22; N, 4.18;
molecular weight: found, 645.08, calcd. 651.66. Molar conductance (DMF, 10-3
,
1
X-1, mol-1, cm2): 17.55; H NMR (DMSO-d6, d ppm): 4.66 (q, 1H, N–CH–C),
1.99 (s, 3H, –C(CH3)=N–), 2.20 (s, 3H, Ph–CH3), 1.52 (d, 3H, –C–CH3), 6.96-7.30
(m, 3H, aromatic); 13C NMR (DMSO, d ppm): 165.7 (COO), 64.5 (N–CH–), 156.4
(C=N), 17.5 (–CH3), 15.2 (Ph–CH3), 18.7 (–CH2–Ar), 136.5, 127.1, 120.4, 164.2,
121.1, 128.6 (aromatic carbons); 19F NMR (DMSO-d6, d ppm): -72.6; UV–visible
(kmax, nm): 378, 210, 260, 360; Infrared (KBr, cm-1): m(C=N), 1608; m(C=O),
1715; masym(COO), 1545; msym(COO), 1300; m(Pb / N), 530; m(Pb–O), 460.
Pb(L4)2 was prepared by reacting lead(II) acetate with ligand (L4H): colour,
white; yield, 70%; mp, 232 °C and elemental analysis (%), calcd. for
PbC30H38F2N2O4: Pb, 28.16; C, 48.97; H, 5.20; N, 3.81; found: Pb, 28.02; C,
48.71; H, 5.02; N, 3.72; molecular weight: found, 726.17, calcd. 735.82. Molar
1
conductance (DMF, 10-3, X-1, mol-1, cm2): 16.39; H NMR (DMSO-d6, d ppm):
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H. L. Singh et al.
4.55 (d, 1H, NCH–C), 1.55 (q, 2H, –CH2–), 1.97 (s, 3H, –C(CH3)=N–), 2.20 (s, 3H,
Ph–CH3), 1.28 (d, 3H, –C–CH3), 1.08 (m, 1H, –C–CH–C), 7.00–7.25 (m, 3H,
aromatic); 13C NMR (DMSO, d ppm): 167.6 (COO), 65.9 (–CH2–), 155.3 (C=N),
16.2 (–CH3), 15.2 (Ph–CH3), 36.8 (–CH2–Ph), 136.9, 128.6, 119.7, 163.8, 123.1,
129.1,136.8, 128.5, 129.2, 118.2 (aromatic carbons); 19F NMR (DMSO-d6, d ppm):
-70.3; UV–visible (kmax, nm): 382, 210, 266, 366; Infrared (KBr, cm-1): m(C=N),
1610; m(C=O), 1720; masym(COO), 1540; msym(COO), 1310; m(Pb / N), 550;
m(Pb–O), 450.
Pb(L5)2 was prepared by reacting lead(II) acetate with ligand (L5H): colour,
cream; yield, 69%; and elemental analysis (%) calcd. for PbC36H34F2N2O4: Pb,
25.77; C, 53.79; H, 4.26; N, 3.48; found: Pb, 25.60; C, 53.68; H, 4.18; N, 3.38;
molecular weight: found, 791.89, calcd. 803.86. Molar conductance (DMF, 10-3
,
X-1, mol-1, cm2): 18.75; 1H NMR (DMSO-d6, d ppm): 4.24 (t, 1H, NCH–C), 2.92
(d, 2H, –CH2–), 1.96 (s, 3H, –C(CH3)=N–), 2.19 (s, 3H, Ph–CH3), 6.94–7.56
(m, 8H, aromatic); 13C NMR (DMSO, d ppm): 169.3 (COO), 66.6 (–CH2–), 153.7
(C=N), 16.6 (–CH3), 14.9 (Ph–CH3), 35.5 (–CH2–Ph), 135.1, 128.6, 120.1, 163.9,
124.3, 128.8,136.7, 128.4, 129.3, 117.5 (aromatic carbons); 19F NMR (DMSO-d6, d
ppm): -71.9; UV–visible (kmax, nm): 380, 210, 260, 368; Infrared (KBr, cm-1):
m(C=N), 1605; m(C=O), 1725; masym(COO), 1545; msym(COO), 1305; m(Pb / N),
540; m(Pb–O), 450.
Pb(L6)2 was prepared by reacting lead(II) acetate with ligand (L6H), Colour,
yellow; yield, 81%; mp, 222 °C and elemental analysis (%), calcd. for
PbC40H36F2N4O4: Pb, 23.49; C, 54.58; H, 4.11; N, 6.35; found; Pb, 23.40; C,
55.14; H, 4.01; N, 6.19; molecular weight: found, 872.25, calcd. 881.92. Molar
1
conductance (DMF, 10-3, X-1, mol-1, cm2): 18.05; H NMR (DMSO-d6, d ppm):
4.52 (t, 1H, N–CH–C), 3.20 (d, 2H, –CH2–) 20.6 (s, 3H, –C(CH3)=N–), 2.15 (s, 3H,
Ph–CH3), 6.94–7.60 (m, 8H, aromatic); 13C NMR (DMSO, d ppm): 166.1 (COO),
66.3 (–CH2–), 156.7 (C=N), 17.4 (–CH3), 15.6 (Ph–CH3), 37.1 (–CH2–Ph), 135.8,
129.1, 119.7, 164.3, 122.9, 128.3,137.8, 129.1, 129.3, 116.5 (aromatic carbons);
19F NMR (DMSO-d6, d ppm): -71.8; UV–visible (kmax, nm): 375, 215, 260, 363;
Infrared (KBr, cm-1): m(C=N), 1610; m(C=O), 1715; masym(COO), 1550; msym(COO),
1310; m(Pb / N), 555; m(Pb–O), 460.
Analytical methods and spectral measurements
Lead was determined gravimetrically as lead sulfate. Nitrogen was determined by
Kjeldahl’s method. Molar conductance measurements were made in anhydrous
dimethylformamide at 45 5 °C using a Systronics model 305 conductivity bridge.
Molecular weight determinations were carried out by the Rast camphor method.
Electronic spectra were recorded in DMSO on a Thermo spectrophotometer.
Infrared spectra were recorded on a Perkin–Elmer RX1 FT–IR spectrophotometer in
the region 4,000–400 cm-1. 1H NMR and 19F NMR were recorded on a Jeol (model
FX 90Q) using DMSO-d6 as solvent at 89.55 MHz and 84.25 MHz, respectively.
13C NMR was recorded on a 90 MHz Jeol (FX 90Q) NMR spectrometer using dry
123

Synthesis and characterization of Pb(II) complexes
1043
1
DMSO as the solvent at 84.25 MHz. TMS was used as internal reference for H
NMR and 13C NMR and hexafluorobenzene as external reference for 19F NMR.
Results and discussion
Several new lead–Schiff base complexes were synthesized by reaction of lead(II)
acetate with the ligands in 1:2 molar ratios using anhydrous benzene and absolute
methanol in 3:1 ratio as solvent. These reactions proceed with the liberation of
acetic acid, which was azeotropically removed, are indicated below (Scheme 1):
The reactions were found to be quite facile and could be completed after reflux
for 5–7 h. All the complexes were intensely coloured solids. They are insoluble in
common organic solvents and only soluble in DMF and DMSO. The chelates were
dissolved in DMF and the molar conductance of 10-3 M solution at 45 °C was
measured. The molar conductance valves of the complexes fall in the range
15–20 X-1 cm2 mol-1 indicating that these chelates are non-electrolytes. The
elemental analysis results (Experimental section) agree with the formulae proposed
for the ligands and also confirmed the Pb(L)2 (Fig. 2) compositionof the Pb(II) chelates.
Pb(N O)2 + 2CH3COOH
Pb(OOCCH3)2 + 2N OH
where N OH represents the donor system of the Schiff bases
Scheme 1 Representative equation illustrating the formation of Pb(II) complexes
O
C
CH3
C
R
O
F
H3C
F
N
Pb
N
CH3
C
O
R
CH3
C
O
R =
,
,
,
H
,
CH3
,
CH(CH3)2
CH(CH3)CH2CH3
CH2
CH2
NH
Fig. 2 Structures of the lead(II) complexes
123

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H. L. Singh et al.
Electronic spectra
The spectra of the ligands and their complexes were recorded in dry DMSO. The
various bands observed were assigned to interligand and charge transfer of n–p*
transitions according to their energies and intensities. A band at *390 nm due to the
[C=N– chromophore in the spectrum of ligand (p–p* transition) shifts to a lower
wavelength and appears at 375 nm in the spectra of the lead complexes. This clearly
indicates the coordination of the azomethine nitrogen to the lead atom. It was found
that the electronic spectra of these complexes have very intense bands in the range
205–215 nm, which may be due to the n–p* transition of the carboxylate
chromophore [23]. The spectra of the complexes also have a few sharp bands in the
region 240–265 nm, with one more band at *362 nm; these may be assigned as
charge transfer bands. It has been reported that the metal is capable of forming
dp–pp* bonds with ligands containing nitrogen as the donor atom.
IR spectra
The important frequencies observed in FT–IR spectra of the compounds with their
assignments are given in the Experimental section. The IR spectra of the complexes
were compared with those of the free ligands in order to determine the coordination
sites that may be involved in chelation. The position and the intensities of these peaks
are expected to be changed on chelation. New peaks and quasi-peaks are also a guide to
chelation. Moreover, comparison of the IR spectra of the ligands with those of their
lead(II) complexes revealed [24] a major shift to lower wave numbers by 10–16 cm-1
in azomethine [24–26] m(C=N) at 1610–1618 cm-1 suggesting involvement of the
azomethine nitrogen with the Pb(II) ion. The spectra of the ligands contain a broad
absorption band in the region 3,100–2,560 cm-1 which is assigned to hydrogen-
bonded m(OH) [27]. This band disappears oncomplexation, suggesting chelation of the
oxygen to the lead atom. The masym(COO) and msym(COO) stretching vibrations
characteristic of the coordinated carboxylate anions were intense bands near 1,545 and
1,300 cm-1, respectively [28]. The observed large separation between masym(COO)
and
msym(COO) stretching vibrations (Dm = masym(COO) - msym(COO) *
245 cm-1) for carboxylate group shows the bidentate nature of the ligands in the
complexes. However, in the case of the free ligands and complexes, a strong band
present at 1,720 5 cm-1 in the free carboxylate region shows that in these
complexes the carboxylate group is bonded to metal in a unidentate manner [29].
These results suggest that the azomethine nitrogen and carboxylate oxygen
groups are involved in coordination with the lead(II) ion in the complexes. Several
new bands appear in the complexes at *430 and *545 cm-1, and are probably due
to m(Pb-O) [30] and m(Pb/N) [30], respectively.
1H NMR spectra
All the protons were found as to be in their expected regions [31]. The conclusions
drawn from these studies lend further support to the mode of bonding discussed in
123

Synthesis and characterization of Pb(II) complexes
1045
their IR spectra. In the spectra of lead(II) complexes, coordination of the ligands via
azomethine nitrogen and carboxylate oxygen was established by the downfield
shifting of these signals in the lead(II) complexes, because of the increased
conjugation and coordination [32]. The number of protons calculated from the
integration curves and those obtained from the values of the expected CHN analyses
agree with each other. It was observed that DMSO had no coordinating effect either
on the spectra of the ligands or on its metal complexes.
In the proton magnetic resonance spectra of the ligands, a sharp signal at *d
1.80 ppm is observed, due to –C(CH3)=N–. This moves downfield (*d 0.10 ppm)
in the complexes in comparison with its original position in the ligands, because
of coordination of azomethine nitrogen to the metal atom. The ligands also give
an OH proton signal at d 11.20 0.10 ppm (s) which is absent from the spectra
of the corresponding lead complexes, showing thereby chelation of the ligand
moiety through the deprotonated carboxylic oxygen. The ligands give a complex
multiplet signal in the region d 6.90–7.60 ppm (m) for the aromatic protons
and this remains almost at the same position in the spectra of the metal
complexes.
Schiff bases derived from glycine, alanine, valine, and isoleucine contain three
aromatic protons, as expected. In the spectrum of phenylalanine, the integral of the
aromatic region corresponds to eight protons; five protons on the phenyl ring are
recognizable at ca. 7.2 ppm. With tryptophan, protons on the five-membered rings
of the indole moieties give rise to signals partially overlapped with the aromatic
proton signals. However, signals are still recognizable. Methyl (for glycine,
methylene) protons on the a-carbon of the carboxylic acid moieties appear at
4.1–4.7 ppm. This signal is a singlet for (1), a quartet for (3), and doublet of
doublets for (2) and (4), all of which arise from the non-equivalent methylene
protons in structures (1), (5), and (6).
13C NMR Spectra
13C NMR spectra of the ligands and their corresponding lead complexes were
recorded in dry DMSO. The signals for the carbon atoms attached to the carboxylate
and azomethine groups in the ligands appear at *d 177.2 ppm and *d 162.1 ppm,
respectively. However, in the spectra of the corresponding lead complexes, these
appears at *d 167.5 ppm (due to the carboxylate group) and *d 156.4 ppm (due to
the azomethine group), respectively. The considerable shifts in the positions of these
signals clearly indicate the involvement of these functional groups in bond
formation with the metal.
Although it is also possible that the shifting of the azomethine carbon signal is
because of a change in hybridization of the nitrogen attached to the carboxylate
group, in the light of IR, UV, and 1H NMR spectral studies it seems more plausible
that the shifting of the signals for these carbons because of the involvement of
carboxylate oxygen and azomethine nitrogen in bonding.
123

1046
H. L. Singh et al.
19F NMR Spectra
19F NMR spectra of the ligands and the corresponding lead(II) complexes were
recorded relative to hexafluorobenzene. Ligands give a single fluorine resonance
peak at *d -76.2 ppm whereas the lead(II) complexes show absorption peaks from
d -70.3 to d -72.6 ppm. Similar downfield shifting in fluorine resonance chemical
shift has been reported in the literature [33], and suggests non-involvement of the
fluorine atom in bonding.
Therefore, four-coordinate square-planar geometry may be proposed for the
resulting lead(II) complexes. Thus on the basis of the discussion above, it is clear
that the ligand, by coordinating to the lead atom through the azomethine nitrogen,
behaves as a bidentate ligand. Because all the resulting lead complexes are
monomeric, the structure in Fig. 2 is proposed for the lead(II) complexes.
Conclusion
Complexes 1–6, derivatives of fluorinated Schiff bases, have been successfully
synthesized and characterized. Elemental analysis C, H, N, and Pb results were in
agreement with predicted formulae. Results from infrared and NMR spectroscopy of
the ligands and complexes showed that coordination takes place via the oxygen
atom from the carboxylate group and the nitrogen atom from the azomethine group.
As a result, in the solid and liquid state the lead(II) complexes are in a four-
coordinated environment around the metal atom.
Acknowledgments The authors are thankful to the Dean, Faculty of Engineering and Technology,
Mody Institute of Technology and Science, Deemed University, Lakshmangarh, Sikar, for providing the
necessary facilities. The authors are also grateful to Dr Ankit Gandhi for linguistic corrections.
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