Novel N4 Macrocycles and Their Transition Metal Chelates

Article abstract of DOI:10.1081/SIM-120026552

Four novel N₄-type macrocyclic chelating agents L¹, L², L³, L⁴ with pendant groups and their metal chelates with some transition metal ions have been synthesized. Electrical conductance of the Cr(III)

Full text of DOI:10.1081/SIM-120026552

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 10, pp. 1835–1855, 2003
Novel N4 Macrocycles and Their
Transition Metal Chelates
2
1
K. S. Siddiqi,1, N. Nishat, and Fouzia Rafat
*
1Department of Chemistry, Aligarh Muslim University, Aligarh, India
2Department of Chemistry, Jamia Millia Islamia, New Delhi, India
ABSTRACT
Four novel N4-type macrocyclic chelating agents L1, L2, L3, L4 with
pendant groups and their metal chelates with some transition metal ions
have been synthesized. Electrical conductance of the Cr(III) and Fe(III)
chelates indicated them to be 1:1 electrolytes whilst those of divalent
metal ions are non-electrolytes in DMSO. Spectroscopic evidence
suggests that all of the complexes are six-coordinate and the pendant
groups are not involved in coordination.
*
Correspondence: K. S. Siddiqi, Department of Chemistry, Aligarh Muslim
University, Aligarh 202002, India; E-mail: ks_siddiqi@yahoo.co.in.
1835
DOI: 10.1081/SIM-120026552
0094-5714 (Print); 1532-2440 (Online)
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Copyright D 2003 by Marcel Dekker, Inc.

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INTRODUCTION
The coordination properties of macrocycle bearing pendant groups have
attracted great deal of attention.[1–4] More recently, the crystal and mole-
cular structure of [3-(4-pyridiniumcarbonyl)-1,3,5,8,12,-pentaazacyclotetra-
decane]-nickel(II) was described[5] in which pyridine acts as pendant group.
A metallocyclam subunit has been appended to pyridine through a metal
template procedure which involves the condensation of amides of 3- and 4-
pyridine carboxylic acid with formaldehyde and the nickel(II) complex
with the open-chain tetraamine. These metallocyclam units act as building
blocks of supramolecular system and act as two-electron redox systems. In
the recent past, a 36-membered macrocycle with four ethylenediamine
entities has been reported.[6]
The synthesis of transition metal complexes with macrocycles
containing a variety of functional groups on the periphery has been
reported.[7] These groups have been termed superstructures. Such com-
pounds are of particular importance in view of their use in the treatment of
malignant tumors.[8] Increasing interest has been shown in the design of
new macrocycles with predetermined guest complexation.[9,10] In view of
these observations it was considered worthwhile to investigate, in detail, the
synthesis of four tetradentate macrocycles and their chelates with transition
metal ions.
RESULTS AND DISCUSSION
A mixture of 4-aminoantipyrine, acetylacetone and 1,3-diaminopropane
or 1,6-diaminohexane was refluxed. It was cooled to zero °C and an excess
of dilute HCl was added to cause precipitation of L1 and L2 as shown in
Figure 1.
For the synthesis of L3 and L4, a mixture of 4-aminoantipyrine, 1,3-
diaminopropane or ethylenediamine was refluxed for 20 h. It was cooled to
zero °C and precipitated with dilute HCl. This product was dissolved in
ethanol and refluxed with benzoylacetic acid. Addition of dilute HCl
afforded the ligands L3 and L4 (Figure 2).
Metal complexes were synthesized directly by the addition of hydrated
metal chloride solutions to the ligand solution in the same solvent. Since
the yield of the complexes was too poor (10 to 15%), the in situ method
was employed. In this method the ligand was not isolated. The metal
chloride was added to the solution of the ligand under preparation.
Elemental analyses of the complexes correspond to the compositions
MLCI2 and M’LCI3 where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) and

Novel N4 Macrocycles
1837
Figure 1. Synthesis of ligands L1 and L2.
M’ = Cr(III) and Fe(III). The molar conductances measured in DMSO (15–
42 ohm–1 cm2 mole–1) indicated that the divalent metal complexes are non
electrolytes[10] while those of M(III) ions are 1:1 electrolytes (Table 1).
IR Spectra
It has been observed by many workers[11,12] that in Schiff base
macrocyclic complexes the C =N stretching frequency appears in the 1620–
1660 cm–1 region. A comparison of the IR spectra of the free macrocycles

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Siddiqi, Nishat, and Rafat
Figure 2. Synthesis of ligands L3 and L4.
with those of their complexes indicated a substantial shift in n(C=N) and
n(C–N) (Table 2). The presence of n(NH) in L3 and L4 and their com-
plexes has been supported by the appearance of a band in the 3080–3230
cm–1 region. It is interesting to note that n(COOH) consistently appears at
1430 cm–1 in both the free (L3 and L4) and chelated macrocycles which
implies that the pendant group is not coordinated in any case although
macrocycles having pendant donor groups that are capable of coordinating
with the central metal ions are known.[13] Some ‘scorpiand’ type molecules
are also reported having H2N–CH2 ‘tails’ to a tetraamine macrocycle.[9]

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The far IR spectra of the complexes show intense halogen-sensitive
absorptions in the 255–300 cm–1 region. The bands in the 320–390 cm–1
range are assigned to n(M–N) vibrations. A band at 1070 cm–1 in all the
ligands and complexes is assigned to n(N–N) in pyrazole.
Electronic Spectra and Magnetic Moments
The magnetic moment values and electronic spectral bands are given in
Table 3. Three bands characteristic of octahedral nickel(II) ion in a high-
spin state are anticipated. In the present case the bands observed have been
3
3
3
assigned to T2g(P) 3A2g(F), T1g(F) 3A2g(F) and T1g(P) 3A2g(F)
transitions.[14] The electronic spectral bands and the relatively low magnetic
moments suggest a distorted octahedral geometry for nickel(II) ion.
In all of the cobalt(II) complexes three bands have been observed. The
magnetic moment value of Co(II) complexes with L1, L3 and L4 is very low
which is probably due to an equilibrium between low and high spin states of
Co(II). However, the electronic bands and the pink colour of the complexes
are consistent with an octahedral environment around cobalt(II) ion (Figure 3).
The observed magnetic moment values for chromium(III) complexes are
close to that calculated for a d3 ion, corresponding to an octahedral geometry
(Figure 4).
The magnetic moment for all of the manganese(II) complexes ranges
between 5.98–6.12 B.M. which correspond to a high-spin octahedral
structure for the metal ion.[15]
The iron(III) complexes exhibit three bands. In the high-spin complexes
the theoretical magnetic moment value is 5.91 B.M. We have obtained for
these complexes values in the range for a high-spin octahedral geometry.[16]
The copper(II) complexes show one or two charge transfer bands. The
band exhibited in the 11,235 ꢀ 13,157 cm–1 region is assigned to the
2T2g 2Eg transition which is characteristic of an octahedral copper(II) ion.
The magnetic moment value also supports the above geometry.
EXPERIMENTAL
4-Aminoantipyrine, ethylenediamine, 1,3-diaminopropane, 1.6-diami-
nohexane (Koch Light) ethyl acetoacetate (S.D. Fine chemicals), acetylacz-
etone, benzoylchloride, and hydrated metal chlorides (BDH) were used as
received. Benzoylacetic acid was synthesized and recrystallized by the
literature method.[17] Elemental analyses were carried out with a Carlo Erba
1106 analyser. The metals were determined by complexometric titration[18]

Novel N4 Macrocycles
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using EDTA and CI was determined gravimetrically.[19] The IR spectra
(4000–200 cm–1) were recorded on a 621 Perkin–Elmer grating spec-
trometer as KBr disc. The electronic spectra were recorded in DMSO on a
Carl–Zeiss VSU2P spectrophotometer. Magnetic susceptibility measure-
ments were done with an Allied Research model 155 vibrating sample
magnetometer and the molar conductances were measured at room
temperature using a Systronics 321 conductivity bridge.
Figure 3. Synthesis of complexes of ligands L1, L2, L3 and L4.

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Siddiqi, Nishat, and Rafat
Synthesis of L1 and L2
To a solution of 4-aminoantipyrine (20 mmol, 4.06 g) dissolved in
100 mL of hot ethanol, acetylacetone (10 mmol, 1.03 mL) was added and
the mixture refluxed for half an hour in a round bottom flask. On cooling
this mixture to room temperature, a light-orange precipitate was obtained
Figure 4. Synthesis of complexes of ligands L1, L2, L3 and L4.

Novel N4 Macrocycles
1853
(Figure 1). It was redissolved in THF (100 mL) and 1,6-diaminohexane (10
mmol, 1.16 g) or 1,3-diaminopropane (10 mmol, 0.85 mL) were then added
and the mixture was refluxed for about 10 h. It was then cooled to 0 °C and
an excess of 0.1 molar HCl (20 mL) was added to cause precipitation. The
product was thoroughly washed with diethyl ether and dried in vacuo. Yield
1.27 g for L1 (25%) and 1.48 g for L2 (27%).
Synthesis of L3 and L4
A mixture of 4-aminoantipyrine (20 mmol, 4.06 g) and 1,3-
diaminopropane (10 mmol, 0.85 mL) or ethylenediamine (10 mmol, 0.66
mL) in 100 mL ethanol was refluxed for about 20 h. It was cooled to 0 °C
and acidified with 20 mL of 0.1 molar HCI which afforded a brown
product. It was redissolved in hot ethanol (100 mL) and benzoylacetic acid
(20 mmol, 3.2 g) was added to it .It was further refluxed for about 30 h and
then cooled to 0 °C followed by the addition of 0.1 molar (20 mL) HCl
which yielded a precipitate (Figure 2). It was washed thoroughly with
ethanol and dried in vacuo. Yield 1.81 g for L3 (25%) and 1.7 g for
L4 (23%).
General Methods for the Synthesis of Complexes
Initially we tried to synthesize the complexes by adding the metal salts
to ligand solution, however the yields were poor (10 to 15%). To overcome
this problem, the complexes were synthesized in situ, in which salts were
immediately added to the solution of ligand under preparation (Figures 3
and 4).
Acetylacetone (2 mmol, 0. 20 mL) was added drop-wise to an ethanolic
solution (50 mL) of 4-aminoantipyrine (4 mmol, 0.80 g). After refluxing
this mixture for about half an hour 1,6-diaminohexane (2 mmol, 0.23 g)
was added and further refluxed for 24 h. The mixture was then cooled to
0 °C and hydrated metal chloride (2 mmol) in ethanol (25 mL) was added
which afforded brisk precipitation of coloured complexes. It was filtered,
washed with cold ethanol and dried in vacuo.
ACKNOWLEDGMENT
One of the authors (F.R.) thanks the Council of Scientific and
Industrial Research (CSIR) New Delhi for the award of JRF.

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Siddiqi, Nishat, and Rafat
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Received October 4, 2001
Accepted July 12, 2003
Referee I: K. Moedritzer
Referee II: M. W. Smith

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