Synthesis and spectroscopic characterization of bivalent lead macrocyclic complexes

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SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF
BIVALENT LEAD MACROCYCLIC COMPLEXES
Anil Bansal a , Sudhir Kumar b & R. V. Singh c
a Department of Chemistry , University of Rajasthan , Jaipur, 302004, India
b Malviya Regional Engineering College , Civil Engineering, Jaipur, 302017, India
c Department of Chemistry , University of Rajasthan , Jaipur, 302004, India
Published online: 15 Feb 2007.
To cite this article: Anil Bansal , Sudhir Kumar & R. V. Singh (2001) SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION
OF BIVALENT LEAD MACROCYCLIC COMPLEXES, Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 31:6,
1085-1095, DOI: 10.1081/SIM-100105263
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 31(6), 1085±1095 (2001)
SYNTHESIS AND SPECTROSCOPIC
CHARACTERIZATION OF BIVALENT
LEAD MACROCYCLIC COMPLEXES
Anil Bansal,1 Sudhir Kumar,2 and R. V. Singh*,1
1Department of Chemistry, University of Rajasthan,
Jaipur-302004, India
2CivilEngineering, Mavl iya RegionalEngineering
College, Jaipur-302017, India
ABSTRACT
Lead(II) macrocyclic complexes of some 12- and 13-mem-
bered tetraazamacrocycles have been synthesized by the
template process using bis(benzil)ethylenediamine as pre-
cursor. The change in ring size has been achieved solely by
varying the number of methylene carbon that link adjacent
benzilnitrogen atoms. Bis(benzi)lethyelnediamine (L) reacts
with PbCl2 or Pb(OAc)2 and primary diamines, i.e., 1,2-dia-
minoethane (1,2-en) or 1,3-diaminopropane (1,3-pn) in a 1:1:1
molar ratio in methanol to give several solid complexes of the
generalcomposition [Pb(L)X
]
(L ˆ macrocyclic ligand,
2
X ˆ Clor OAc). The precursor and the metalcomplexes of the
macrocycles have been characterized by elemental analyses,
*Corresponding author. E-mail: kudiwal@datainfosys.net
1085
Copyright # 2001 by MarcelDekker, Inc.
www.dekker.com

1086
BANSAL, KUMAR, AND SINGH
molecular weight determinations, molar conductances, IR, 1H
NMR, 13C NMR, electronic and mass spectral studies. On the
basis of the IR and NMR spectraldata the invovlement of
amido nitrogen in coordination to the centralmetalion has
been con®rmed. An octahedralgeometry around the metal
ion is suggested for the complexes. Based on molecular
weights and conductivity measurements, their monomeric and
non-electrolytic nature has been con®rmed.
INTRODUCTION
The coordination chemistry of lead(II) with macrocyclic ligands has
been investigated in the past decade, although not extensively. The coor-
dination chemistry of macrocyclic ligands is a fascinating area which has
attracted the attention of many inorganic chemists. Macrocyclic ligands
form stable complexes with lanthanides and they serve as a springboard to
explore the coordination chemistry of these metal ions1. Tetraazamacro-
cycles and their metal complexes have been studied extensively in connection
with their resemblance to naturally occuring porphyrins. Saturated macro-
cycles with various numbers of ring members have been synthesized reg-
ularly. These compounds have produced interesting information concerning
both the stabilities and structures of their metal complexes2.
The novelapproach to macrocyclic synthesis, therefore, overcomes the
usualilmitations of the more traditionaltempalte routes where the metal
tends to be ``locked'' into the macrocycle with demetallation often resulting
in decomposition3. The in¯uence of variation of the structuralfeature on
metal complexation, namely the length of the alkyl bridge linking the benzil
nitrogen atoms, was investigated4. Increasing the length of this bridge is
expected to increase the ``lopsidedness'' of the N4-donor set. In this way, it
was intended not only to increase the available macrocyclic cavity size, but
also to introduce increasing ``lopsidedness'' into the larger members of this
series. The use of metal ions as templates in such reactions has led to the
synthesis of many metal complexes of macrocyclic ligands5. These ligands
display a number of features of chemical interest. They are relatively rigid in
6
nature and thus impose a speci®c coordination geometry on the metalion .
The catalytic activity of vitamin B12 is in part due to the presence of an
unsymmetricaland ¯exibel equatorialilgand system
7. An iron(III) macro-
cyclic complex as a model system for catalase has been reported8. This
report shows that iron(III) macrocyclic system can function as substitute for
catalase. A large number of enzymes requiring two or more metal ions for

BIVALENT LEAD MACROCYCLIC COMPLEXES
1087
activity have been discovered and many of these appear to owe their cata-
lytic activity to ®nely balanced metal-metal interactions9.
In this paper, we report the synthesis and characterization of new
lead(II) complexes of twelve- and thirteen-membered tetraazamacrocycles
from the reaction of bis(benzil)ethylenediamine with aliphatic diamines,
which show the marked e€ect of any build-up of ligand strain on the
coordination geometry of the centralmeta.l
EXPERIMENTAL
Materials and Methods
Melting points were determined in capillary tubes and are uncorrected.
All solvents used were of high purity and were distilled in the laboratory before
use. Moisture was excluded from the glass apparatus using CaCl2 drying
tubes. PbCl2, Pb(OOCCH3)2 (BDH) and the primary diamines, i.e., 1,2-di-
aminoethane and 1,3-diaminopropane were used as obtained from E. Merck.
Conductivity measurements were made with a Systronic model305
conductivity bridge in dry dimethylformamide. Molecular weights were
determined by the Rast camphor method. IR spectra of the solid samples
were recorded as KBr discs on a Nicolet Megna FTIR 550 spectrophotometer
1
in the range 4000±200 cmÀ1. H NMR spectra were recorded on a JeolFX
90Q spectrometer in DMSO-d6 using TMS as the internalstandard. Che-
micalshifts are reported in ppm units. 13C NMR spectra were recorded in
methanolusing TMS as the internalreference. Eelctronic spectra were
recorded on a Hitachi U-2000 spectrophotometer. Nitrogen and chlorine
were determined by Kjeldahl's or Volhard's method, respectively. Carbon
and hydrogen analyses were performed at the Central Drug Research Insti-
tute, Lucknow, India. Lead was determined gravimetrically as lead sulphate.
Synthesis of Bis(benzil)ethylenediamine
In a 100 mL short-necked round bottom ¯ask, an ethanolic solution of
benzil(5 g, 24 mmo)l was added to a soultion of 1,2-diaminoethane
(0.80 mL, 12 mmol) in 15 mL of ethanol and the reaction mixture was
re¯uxed for 8 h. After reducing the solvent to 1=2 of its originalvolume on a
ratio head, the solution was cooled and the reddish yellow crystalline
ꢀ
compound thus obtained was recrystallized from ethanol, m.p., 86789 C;
yield, 3.7341 g (70%).

1088
BANSAL, KUMAR, AND SINGH
Synthesis of the Complexes
The reaction was carried out in 1:1:1 molar ratio by combining bis-
(benzil)ethylenediamine (0.7818 g, 1.75 mmol or 0.7557 g, 1.70 mmol), 1,2-
diaminoethane (0.1057 g, 1.75 mmolor 0.1022 g, 1.70 mmo)l and PbCl or
2
Pb(OAc)2 (0.4891 g, 1.75 mmolor 0.6449 g, 1.70 mmol) in methanol(25 mL).
The mixture was heated under re¯ux for 576 h. The solution was cooled and
transferred to an evaporating dish and set aside overnight at room tem-
perature. The solid product that separated out was collected, washed with hot
water, then with cold methanol and dried under vacuo. The complexes were
recrystallized from a 1:1 solution of methanol and benzene; yields, 52764%.
RESULTS AND DISCUSSION
The template condensation of bis(benzil)ethylenediamine and 1,2-dia-
minoethaneor1,3-diaminopropaneinthepresenceofPbCl2 orPb(OOCCH3)2
produced a new series of 12- and 13-membered tetraazamacrocyclic metal
complexes of the types Pb(L1)X2 and Pb(L2)X2 as shown in Eq. (1).

BIVALENT LEAD MACROCYCLIC COMPLEXES
1089
The resulting metal derivatives are coloured solids which are stable at
room temperature and are non-hygroscopic. The monomeric nature of these
complexes is con®rmed by molecular weight determinations. All the com-
plexes are slightly soluble in methanol or ethanol but soluble in DMF and
DMSO. The molar conductance of 10À3 M solutions for all the complexes in
anhydrous DMF is in the range 15720 ohmÀ1 cm2 molÀ1 indicating their
non-ionic nature. The analytical results of the isolated solid complexes, their
colours and melting points are given in Table I.
The mode of bonding in these macrocyclic complexes is proposed on
1
the basis of infrared, electronic, H NMR, 13C NMR and mass spectral
evidence.
IR Spectra
The infrared spectra of the ligand C30H24N2O2 and the metalcom-
plexes show the absence of uncondensed functional groups (NH2 and CˆO),
stretching modes of starting materialand the appearance of bands char-
acteristic of the imine group10. The bands characteristic of the benzilmoiety
appeared in all the complexes at 147771498 cmÀ1 (nasymC6H5) and
137371392 cmÀ1 (nasymC6H5). The major changes observed in the IR spectra
of the macrocyclic complexes are the absence of stretching and deformation
vibrations of NH2 groups, indicating their deprotonation and the appear-
ance of strong bands due to coordinated n(CˆN) vibrations11 in the range
1626±1605 cmÀ1. Strong and sharp bands for C-H stretching and bending
vibrations appear at ca. 2810 and 1404 cmÀ1, respectively10. In the IR
spectra nsym(OCO) appears at 1370 cmÀ1 and nasym(OCO) together with the
imine n(CˆN) as an intense broad absorption band in the region
160571626 cmÀ1. Such widely spaced acetate bands are consistent with the
monodentate coordination proposed in the metalcompelxes Pb(L 1)(OAc)2
and Pb(L2)(OAc)2. The presence of new bands in the spectra of the metal
complexes in the far-IR region at 4307475 cmÀ1 due to the n(Pb-N) vibra-
12
tion supports the coordination of the imine nitrogen to the metalion
.
Electronic Spectra
The electronic spectra of bis(benzil)ethylenediamine and its derivatives
have been recorded on a Hitachi U-2000 spectrophotometer. The absorp-
tion maximum at 400 nm (e ˆ 148 dm3 molÀ1 cmÀ1) in the case of bis(benzil)-
ethylenediamine may be assigned to the n-p* transition of the azomethine

1090
BANSAL, KUMAR, AND SINGH

BIVALENT LEAD MACROCYCLIC COMPLEXES
1091
Table II. 1H NMR SpectralData ( d, ppm) of the Precursor and the MetalCom-
plexes of the Macrocycles
Aromatic Protons
-NCH2 -NCH2
Compound
(bs)
(t)
OOCCH3
2
3
4
C30H24N2O2 (L)
[Pb(L1)Cl2]
3.12
3.52
7
3.61
7
7
7
3.46
7
3.57
7
7
7
1.32
8.00 (d) 7.36 (dd) 7.65 (d)
8.11 (d) 7.32 (dd) 7.58 (d)
8.02 (d) 7.28 (dd) 7.49 (d)
8.24 (d) 7.31 (dd) 7.86 (d)
8.35 (d) 7.38 (dd) 7.60 (d)
[Pb(L2)Cl2]
[Pb(L1)(OAc)2]
[Pb(L2)(OAc)2]
1.18
group. The shift of this band (ca. 15 nm) in the spectra of the complexes
indicated the coordination of the nitrogen to the metalatom 13,14. In addi-
tion to this band, the spectra of the metalcompelxes exhibit bands around
275 nm (e ˆ 703 dm3 molÀ1 cmÀ1) and 326 nm (e ˆ 1366 dm3 molÀ1 cmÀ1) due
to p-p* electronic transitions. However, the position of these bands remains
almost unchanged on complexation.
1H NMR Spectra
The bonding pattern in the resulting complexes has been further
substantiated by the proton magnetic resonance spectra of the precursor and
the metal complexes of the macrocycles (Table II). The 1H NMR spectra of
the complexes do not show any signal corresponding to primary amino
protons. This suggested that the proposed macrocyclic skeleton has been
formed. A singlet observed at d 3.5273.61 ppm in the complexes [Pb(L1)Cl2]
and [Pb(L1)(OAc)2] and a triplet observed at d 3.4673.57 ppm in [Pb(L2)Cl2]
and [Pb(L2)(OAc)2] complexes may be assigned to methylene protons
adjacent to the nitrogen atoms, respectively. The shift of the signals towards
lower ®eld is an indication of the coordination of the macrocycles. In case of
unsymmetrical chelates, a multiplet in the region d 2.1972.24 ppm is
assigned as the middle methylene protons of the 1,3-diaminopropane
moiety. The multiplet of aromatic protons was observed at d 7.2878.35 ppm
in the spectra of the precursor and the metal complexes of the macrocycles.
1
The H NMR spectra of the complexes with acetate groups in DMSO-d6
show a proton resonance in the region d 1.1871.32 ppm and the 13C NMR
spectra in DMSO show an acetate-methylresonance at 20.3 721.7 ppm.
A carboxylate resonance at 169.27170.6 ppm is consistent with the structure
of the complexes [Pb(L1)(OAc)2] and [Pb(L2)(OAc)2] shown in Figure 1.

1092
BANSAL, KUMAR, AND SINGH
Figure 1. Suggested structures of the complexes.
The 13C NMR spectraldata of the precursor and the metalcompel xes
of the macrocycles (Table III) also support the formation of these complexes
and the formulation of the macrocyclic ring structure. Similar synthetic
work in case of metal complexes have also been reported by other
workers15717
.
Mass Spectra
The FAB mass spectrum of one of the lead complexes, [Pb(L1)(OAc)2],
was recorded on a JeolSX 102/DA-6000 mass spectrometer =data system
using argon=xenon (6 KV, 10 mA) as the FAB gas. The accelerating voltage
was 10 kV and the spectrum was recorded at room temperature (34ꢀC). m-
Nitrobenzylaclohol(NBA) was used as matrix. In the mass spectrum, the
molecular ion peak of the complex [Pb(C32H28N4)(OOCCH3)2] appeared at
‡
‡
‡
m=z 794 [M] , 796 (M‡2] and 798 [M‡4] . Some other peaks appeared at
‡
m=z 469, 588, 616, 679, 716 and 766 corresponding to the [C32H28N4] ,
[Pb(C18H18N2)(OOCCH3)2] , [Pb(C18H18N4)(OOCCH3)2] , [Pb(C30H24N2)
‡
‡
‡
‡
(OOCCH3)] ,
[Pb(C26H23N4)-(OOCCH3)2]
and
[Pb(C30H26N4)-
‡
(OOCCH3)2] species, respectively, which resulted from the loss of the
Pb(OOCCH3)2, C14H10N2, C14H10, C4H7N2O2, C6H5 and C2H4 fragments,
respectively, from the parent compound. Two peaks at m=z 676 and 735 are
due to the loss of two and one acetate ions, respectively. Lead occurs in
severalisotopic forms and out of these, four having the mass numbers 204,
206, 207 and 208 are important. The last isotope is, however, the most
abundant one (52.3%). In the mass spectra, a particular ion consisting of
lead shows the expected combination of peaks corresponding to the various
isotopes of lead. The molecular ion peak of [Pb(C32H28N4)(OOCCH3)2]

BIVALENT LEAD MACROCYCLIC COMPLEXES
1093

1094
BANSAL, KUMAR, AND SINGH
is obtained at m=z 794 (calcd. on the basis of 208Pb) and peaks at 793, 792
and 790 are also observed having the expected abundances.
ACKNOWLEDGMENT
The authors are thankfulto the Government of India, Ministry of
Science and Technology, New Delhi, India, for ®nancial assistance through
Grant No. SP=S1=F06=96.
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Received December 17, 1999
Accepted May 13, 2001
Referee I: K. Moedritzer
Referee II: A. Nazarenko