Chelating behavior of 14-membered Schiff base macrocycles and their transition metal chelates

Article abstract of DOI:10.1081/SIM-120027322

The new Schiff base 14-membered macrocycles, 1,5,8,12- tetraazacyclotetradeca-6,7,13,14-tetraaminoacetic acid-5,7,12,14-tetraene (L¹), 1,5,8,12-tetraazacyclotetradeca-6,7,13,14-tetraaminophenyl-5,7, 12,14-tetraene (L²) and 1,5,8,12-t

Full text of DOI:10.1081/SIM-120027322

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Chelating Behavior of 14‐Membered Schiff Base
Macrocycles and Their Transition Metal Chelates
N. Nishat a , K. S. Siddiqi b , Shahab A. A. Nami b & Ahmad Umar b
a Department of Chemistry , Jamia Millia Islamia , New Delhi, India
b Department of Chemistry , Aligarh Muslim University , Aligarh, 202002, India
Published online: 16 Nov 2010.
To cite this article: N. Nishat , K. S. Siddiqi , Shahab A. A. Nami & Ahmad Umar (2004) Chelating Behavior of
14‐Membered Schiff Base Macrocycles and Their Transition Metal Chelates, Synthesis and Reactivity in Inorganic and Metal-
Organic Chemistry, 34:1, 145-161, DOI: 10.1081/SIM-120027322
To link to this article: http://dx.doi.org/10.1081/SIM-120027322
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 1, pp. 145–161, 2004
Chelating Behavior of 14-Membered Schiff
Base Macrocycles and Their Transition
Metal Chelates
2
N. Nishat,1 K. S. Siddiqi,2, Shahab A. A. Nami,
*
and Ahmad Umar2
1Department of Chemistry, Jamia Millia Islamia, New Delhi, India
2Department of Chemistry, Aligarh Muslim University,
Aligarh, India
ABSTRACT
The new Schiff base 14-membered macrocycles, 1,5,8,12-tetraazacyclo-
tetradeca-6,7,13,14-tetraaminoacetic acid-5,7,12,14-tetraene (L1),
1,5,8,12-tetraazacyclotetradeca-6,7,13,14-tetraaminophenyl-5,7,12,14-
tetraene (L2) and 1,5,8,12-tetraazacyclotetradeca-6,7,13,14-tetraamino-
pyridyl-5,7,12,14-tetraene (L3), have been synthesized by the reaction
of N,N-dicarboxymethyloxamide, N,N-diphenyloxamide or N,N-
dipyridyloxamide with 1,3-diaminopropane. Their complexes with
Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) ions have been
prepared and characterized by elemental analyses, IR, 1H NMR and
electronic spectra, magnetic moment and conductivity measurements.
*
Correspondence: K. S. Siddiqi, Department of Chemistry, Aligarh Muslim University,
Aligarh 202002, India; E-mail: ks_siddiqi@yahoo.co.in.
145
DOI: 10.1081/SIM-120027322
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Nishat et al.
The complexes of the divalent metal ions are non-ionic while those of the
trivalent metal ions appear to be 1:1 electrolytes. On the basis of these
studies an octahedral geometry is proposed for all of the complexes.
Key Words: 14-Membered macrocycles; Schiff base; Synthesis; Metal
chelates.
INTRODUCTION
The design and synthesis of macrocycles and their metal chelates is a
subject of current research interest.[1–6] A great deal of interest has been
directed to metal complexes with functionalized macrocycles that have
pendant functional groups.[7–9] The chemistry of macrocycles has aroused
increasing interest in recent years due to their use as models for protein metal
binding sites in metalloproteins, as therapeutic reagents in chelate therapy for
the treatment of metal intoxication, in catalysis and in the treatment of
cancer.[10–12]
In this paper we report the synthesis and characterization of the 14-
membered N4 macrocycles L1, L2 and L3 and their complexes with Cr(III),
Mn(II), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II) ions.
RESULTS AND DISCUSSION
The ligands were synthesized according to the scheme in Fig. 1. The
complexes were synthesized by heating over a water bath for 6 h a 1:1 mixture
of hydrated metal chloride dissolved in methanol and ligand in ethanol and an
aqueous buffer of pH 4, which afforded a precipitate. It was washed with
ethanol and dried over CaCl2. The molar conductances of one millimolar
solutions in DMSO showed that the complexes of the divalent metal ions are
non-electrolytes while those of the trivalent metal ions appear to be 1:1
electrolytes[13] (Tables 1–3).
MCl2 þ L ꢀ! [MLCl2]
MCl3 þ L ꢀ! [MLCl2]Cl
where L ¼ L1, L2 and L3.

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147
Figure 1. Synthesis of the ligands.
IR Spectra
The relevant IR bands and their assignments are listed in Tables 4–6.
The two n(C55O) bands appearing at 1710 and 1640 cm21 in N,N-
dicarboxymethyloxamide are lost and n(C55N) (1670 cm21) appears, as a
consequence of its reaction with 1,3-diaminopropane forming a macrocyclic
Schiff base (Fig. 1). In the present work we have observed only one band in
the 1670–1690 cm21 in the free Schiff base macrocycles which undergoes a
shift to lower wave number after chelation. This decrease in n(C55N) in the
complexes has been attributed to the coordination of the metal ion via
nitrogen atoms.

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Nishat et al.

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Nishat et al.

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Table 4. Important IR bands (cm21) of L1 and its metal chelates.
Compounds n(NH) n(C55N) ns(COOH) nas(COOH) n(M–N) n(M–Cl)
L1
3,350 s
1,680 s
1,655 s
1,410 w
1,410 w
1,410 w
1,410 w
1,410 w
1,410 w
1,410 w
1,410 w
1,615 w
1,615 w
1,615 w
1,615 w
1,615 w
1,615 w
1,615 w
1,615 w
—
—
[CrL1Cl2]Cl 3,350 s
[MnL1Cl2]
3,350 s
[FeL1Cl2]Cl 3,350 s
490 m
460 m
470 m
480 m
470 m
470 m
490 m
330 w
350 w
340 w
342 w
350 w
345 w
370 w
1,640 m
1,660 m
1,670 m
1,665 m
[CoL1Cl2]
[NiL2Cl2]
[CuL2Cl2]
[ZnL1Cl2]
3,350 s
3,350 s
3,350 s 1,665–m
3,350 s 1,665 m
The complexes exhibited new bands in the far-IR spectral region. The
chromium(III) complexes show vibrational modes in the 490–510 cm21 range
which are assigned to (Cr–N) stretches. The other metal ions exhibit M–N
stretches in the 420–495 cm21 range. The n(M–Cl) band has been found to
appear in the 310–370 cm21 range. It is apparent from the spectra that both the
nitrogen and chlorine are bonded to the metal ion.
In the case of metal complexes of L1 nitrogen and oxygen of the pendant
groups are not coordinated and hence their absorption bands remain
unchanged throughout (Table 4). Similarly in the case of L2 and L3 the
pendant arms have not been found to be involved in coordination which is
evident from their spectra (Tables 5 and 6).
1H NMR Spectra
The ligands L1, L2 and L3 show a triplet for the CH2 protons at d 2.7–2.9,
which shows a downfield shift in the complexes as given in Table 7.
Table 5. Important IR bands (cm21) of L2 and its metal chelates.
Compounds
n(NH)
n(C55N) n(C55C)
n(C–H)
n(M–N) n(M–Cl)
L2
3,230 s
3,230 s
3,230 s
3,230 s
3,230 s
3,230 s
3,230 s
3,230 s
1,670 s
1,645 m
1,650 m
1,655 m
1,630 m
1,625 m
1,620 m
1,665 m
1,580 w
1,580 w
1,580 w
1,580 w
1,570 w
1,575 w
1,580 w
1,570 w
2,940 w
2,955 w
2,950 w
2,960 m
2,980 m
2,960 m
2,960 m
2,980 m
—
—
[CrL2Cl2]Cl
[MnL2Cl2]
[FeL2Cl2]Cl
[CoL2Cl2]
[NiL2Cl2]
[CuL2Cl2]
[ZnL2Cl2]
495 m
475 m
420 m
480 m
475 m
485 m
485 m
330 w
345 w
340 w
350 w
340 w
310 w
320 w

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Nishat et al.
Table 6. Important IR bands (cm21) of L3 and its metal chelates.
Pyridine
ring
vibration n(M–N) n(M–Cl)
Compounds n(N–H)
n(C55N)
n(C55C)
L3
3,250 s
3,250 s
3,250 s
3,250 s
3,250 s
3,250 s
3,250 s
3,250 s
1,675 m
1,660 m
1,640 m
1,655 m
1,650 m
1,650 m
1,650 m
1,650 m
1,580 w
1,580 w
1,580 w
1,580 w
1,580 w
1,580 w
1,580 w
1,580 w
1,615 m
1,625 m
1,625 m
1,620 m
1,615 m
1,625 m
1,620 m
1,625 m
—
—
[CrL3Cl2]Cl
[MnL3Cl2]
[FeL3Cl2]Cl
[CoL3Cl2]
[NiL3Cl2]
[CuL3Cl2]
[ZnL3Cl2]
510 m
495 m
470 m
480 m
495 m
465 m
480 m
335 w
340 w
350 w
340 w
330 w
350 w
350 w
The pendant group containing the moiety CH2CO2H (L1) exhibits a combined
singlet for all of the methylene protons which is not influenced after
metallation.[14] The phenyl proton signal in N,N-diphenyloxamide (L2)
exhibits multiplets at d 6.5–6.8 while L3 shows multiplets due to pyridyl
protons at d 6.8–7.2. Since these pendant groups, both in the free and chelated
macrocycles, did not show any change in the position of their proton signals,
it is ascertained that they are not involved in coordination.
Electronic Spectra and Magnetic Moments
The magnetic moment values and electronic spectral bands are given in
Tables 8–10. Three characteristic bands are anticipated for octahedral Ni(II)
ion in the high-spin state. In the present case three bands observed at 23,255–
25,000 cm21, 16,000–17,241 cm21 and 12,048–13,157 cm21 have been
3
3
3
assigned to T2g(F) 3A2g(F), T1g(F) 2A2g(F) and T1g(P) 3A2g(F)
transitions, respectively. The relatively low magnetic moment values and the
electronic spectral bands suggest a distorted octahedral geometry for the
nickel(II) ion (Fig. 2).
In all Co(II) complexes three bands have been observed. The slightly low
magnetic moment value for the complex of L1 (4.4 B.M.) is attributed to the
thermal depopulation of an excited quartet spin state which may be tentatively
ascribed to the increasing size of the pendant groups. However, the electronic
bands and the pink colour of the complexes are consistent with an octahedral
environment around Co(II) ion.
The magnetic moments for the Cr(III), Fe(III) and Mn(II) complexes
correspond to spin-free octahedral structures for the metal ion.[15,16]

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153
d
d
d
d
d
d

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Nishat et al.
b

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155
b

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Nishat et al.
b

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157
Figure 2. General structure for the M(II) complexes (M(II) ¼ MN(II), Co(II), Ni(II),
Cu(II) and Zn(II)).
The Cu(II) complexeswith all ofthe three ligands showa chargetransferand
2
a broad band in the 13,000–16,000cm21 region assigned to the T2g 2Eg
transition which is characteristic of an octahedral geometry for the Cu(II) ion.
The magnetic moment values also support the above geometry for the Cu(II) ion.
In the case of small macrocycles, the field strength of the ligand is affected
by the ring size, especially in N4 ligands.[17] The ring which fits a given metal ion
produces a normal ligand field but when the size of the metal ion is relatively
Figure 3. General structure for the M(III) complexes (M(III) ¼ Cr(III) and Fe(III)).

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Nishat et al.
smaller it shows an abnormally large ligand field. In order to reduce strain,
smaller rings get folded during chelation or the metal ions are kept on top of the
ring as in the case of heme.[18] However, the ionic radii of Co(II), Cu(II) and
Zn(II) ions are almost identical (73–75pm). They may match the cavity
provided by the ring,[18] while the Cr(III), Fe(III) and Ni(II) ions, (61.5–69.0pm)
appear to be loosely fitted. Mn(II) is fairly large (83.0 pm) and it may, therefore,
sit on top of the hole of the ring and may undergo appreciable distortion.
EXPERIMENTAL
Synthesis of N,N-Diphenyloxamide and
N,N-Dipyridyloxamide
NaOH (20 mmol, 0.80 g) dissolved in the minimum amount of water was
added to aniline (20 mmol, 1.86 mL) or 2-aminopyridine (20 mmol, 1.88 g) in
ethanol (25 mL). Diethyl oxalate (10 mmol, 1.35 mL) was then added and the
mixture was heated to 60 8C for 4 h with continuous stirring. It was then cooled
to 0 8C and an excess of 10% HCl was added causing immediate precipitation
of a white product which was washed with ethanol and crystallized from a 2:1
mixture of ethanol and an aqueous buffer of pH 4 prepared from a mixture of
8 mL of 0.2 N acetic acid and 2 mL of 0.2 N sodium acetate.[19]
Synthesis of L1, L2 and L3
1,3-Diaminopropane (20 mmol, 1.3 mL) was added drop-wise to N,N-
dicarboxymethyloxamide (20 mmol, 4.08 g) dissolved in the 100 mL ethanol
buffered at pH 4 at ambient temperature and refluxed for about 20 h. This
mixture was cooled to 25 8C and an excess of 10% HCl was added and the
mixture was then left for two days at this temperature, which afforded a white
product. It was recrystallized from a 1:1 mixture of ethanol and aqueous buffer
of pH 4. Yield, 1.6 g (30%).
L2 and L3 were synthesized by the same procedure using N,N-
diphenyloxamide and N,N-dipyridyloxamide in place of N,N-dicarboxy-
methyloxamide. Yields, 1.5 g (24%) and 1.3 g (22%), respectively.
Synthesis of Complexes
A mixture of hydrated metal chloride (1 mmol ) in 20 mL methanol and
the ligand (1 mmol) dissolved in a mixture of 10 mL ethanol and 10 mL of an

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159
aqueous buffer of pH 4 was heated on a water bath for 6 h, which yielded a
precipitate. It was filtered, washed with ethanol and dried in vacuo.
Materials and Methods
Glycine, aniline, 2-aminopyridine, diethyl oxalate, hydrated metal salts
(BDH) and 1,3-diaminopropane (Koch Light) were used as received. N,N-
dicarboxymethyloxamide was synthesized by the literature method.[20]
Elemental analyses were carried out with a Thomas and Coleman
analyzer. The metals were determined by complexometric titration and
chloride was determined gravimetrically.[21] The IR spectra (4000–200 cm21
)
were recorded on a 621 Perkin–Elmer grating spectrophotometer as KBr disc.
The electronic spectra were recorded on a Carl-Zeiss VSU2P spectrometer in
DMSO. Magnetic susceptibility measurements were done with a model 155
Allied Research vibrating sample magnetometer and the molar conductances
were measured at room temperature using a Systronics 321 conductivity
bridge. The NMR spectra were recorded on a Varian A-60 D spectrometer.
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Received February 15, 2002
Accepted September 5, 2003
Referee I: R. F. Jordan
Referee II: M. W. Smith

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