Synthesis, spectroscopic and thermal studies of transition metal complexes derived from benzil and diethylenetriamine

Article abstract of DOI:10.1016/j.saa.2006.11.026

A macrocyclic ligand, bdta (where bdta = 3,6,9,12,15,18-hexaaza-1,2,10,11-tetraphenyl-2,9,11,18-tetraenecyclododecane) has been prepared by cyclocondensation of benzil with diethylenetriamine which efficiently encapsulates transition as well as pseudo-transition metal ions leading to the formation of M(bdta)Cl₂ type complexes [where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)]. The analytical, spectroscopic and magnetic moment data suggests an octahedral geometry for all the complexes. EPR spectra of Mn(II) and Cu(II) show considerable exchange interaction in the complex. They are non-conducting in DMSO. The TGA profile of the ligand and its complexes are identical and consists of two discreet stages. The voltammogram of Cu-complex exhibits a quasi-reversible one-electron transfer wave for Cu(II)/Cu(I) couple.

Full text of DOI:10.1016/j.saa.2006.11.026

Spectrochimica Acta Part A 68 (2007) 269–274
Synthesis, spectroscopic and thermal studies of transition metal
complexes derived from benzil and diethylenetriamine
Sadaf Khan, Shahab A.A. Nami, K.S. Siddiqi∗
Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
Received 22 March 2006; received in revised form 17 November 2006; accepted 18 November 2006
Abstract
A macrocyclic ligand, bdta (where bdta = 3,6,9,12,15,18-hexaaza-1,2,10,11-tetraphenyl-2,9,11,18-tetraenecyclododecane) has been prepared by
cyclocondensation of benzil with diethylenetriamine which efficiently encapsulates transition as well as pseudo-transition metal ions leading to the
formation of M(bdta)Cl2 type complexes [where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)]. The analytical, spectroscopic
and magnetic moment data suggests an octahedral geometry for all the complexes. EPR spectra of Mn(II) and Cu(II) show considerable exchange
interaction in the complex. They are non-conducting in DMSO. The TGA profile of the ligand and its complexes are identical and consists of two
discreet stages. The voltammogram of Cu-complex exhibits a quasi-reversible one-electron transfer wave for Cu(II)/Cu(I) couple.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hexaazamacrocycle; Transition metal ions; EPR; IR; Electronic studies
1. Introduction
also retard reaction by coordination to a starting material pro-
viding a protective influence. This phenomenon is the hallmark
of metal template reactions [8].
In this paper, we report the synthesis and characterization of
transition metal complexes with a macrocyclic ligand derived
from the condensation of benzil and diethylenetriamine.
In the last few years a great deal of research has been aimed to
design macrocyclic complexes highly selective and sensitive to
metal ions [1]. Among the various synthetic strategies proposed,
the template condensation is one of the most highlighted. Metal
template condensation often provides selective routes towards
products that are not obtainable in the absence of metal ion [2].
The number and relative position of the donor atoms and the
cavity size in the macrocyclic ligands confer these molecules
with special reactivity [3]. For a particular diketone and diene,
the control of reaction conditions and the ratio of the reactants
permit to design the condensation product [4].
Ethanedionediphenyl, benzil is one of the most basic ␣-
dicarbonyl. Benzil and its related compounds have been
extensively used as biologically active complexing agents and
analytical reagents [5]. It is employed as a selective diketone to
obtain macrocycles of desired cavity size and significant reac-
tivity [6]. It is observed that different products are obtained on
varying the solvent, pH, temperature and nature of metal ion
used [7]. Metal ions can enhance a reaction, by stabilization of a
transition state or product by appropriate coordination. They can
2. Experimental
Hydrated metal chlorides (Merck), benzil, diethylenetri-
amine (Ranbaxy) were used as received. Methanol was distilled
before use. Elemental analyses (C, H and N) were carried
out with a Carlo Erba EA-1108 analyzer. The metal contents
were estimated by complexometric titration [9]. IR spectra
(4000–400 cm−1) were recorded on a RXI FT-IR spectrometer
as KBr disc while the 600–200 cm−1 range was scanned with
CsI on a Nexus FT-IR Thermo Nicolet, Madison Wisconsin. The
conductivity measurements were carried out on a CM-82T Elico
conductivity bridge in DMSO. Magnetic susceptibility measure-
ments were done with a 155 Allied Research vibrating sample
magnetometer at room temperature. TGA was performed with a
Perkin-Elmer (Pyris Diamond) thermal analyzer under nitrogen
atmosphere using alumina powder as reference. The weight of
the sample was between 8 and 12 mg and the heating rate was
maintained at 10 ◦C/min. EPR spectrum of Cu(II) and Mn(II)-
complexes were recorded on a RE-2X Jeol EPR, spectrometer
∗
Corresponding author. Tel.: +91 9837284930.
E-mail address: khwajas siddiqi@yahoo.co.in (K.S. Siddiqi).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.026

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S. Khan et al. / Spectrochimica Acta Part A 68 (2007) 269–274
Scheme 1. Preparation of the ligand bdta.
Scheme 2. Synthesis of the complex M(bdta)Cl2 where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II).
fitted with 100 KHz field modulation. Cyclic voltammetry was
performed on a 0.1 mM DMSO solution. The potential was
scanned at a rate of 50 mV/s. The potentiometer was a Princeton
Applied Research model 263A attached to a P.C. using Elec-
trochemistry Powersuite Software. The working electrode was
a glass electrode, the counter electrode was a platinum wire and
the reference electrode consisted of Ag, AgCl/KCl (saturated).
the metathetical procedure gives poor yield the template method
was adopted (Scheme 2).
2.3. Synthesis of Zn(bdta)Cl2
Since in the case of Zn and Cd very low yields were obtained,
metal template procedure was adopted. To a hot ethanolic solu-
tion (20 ml) of benzil (2 mmol, 0.42 g) neat diethylenetriamine
(2 mmol, 0.22 ml) was added dropwise with continuous stirring.
An ethanolic solution (15 ml) of ZnCl2 (2 mmol, 0.26 g) was
added dropwise to the above mixture over a period of 30 min
with continuous stirring and no precipitation was observed till
an hour. The reaction mixture was then refluxed on a water bath
for about five hours until the appearance of a crystalline product.
It was decanted, washed with ethanol, diethyl ether and dried in
vacuo over CaCl2 (Scheme 3).
2.1. Synthesis of macrocycle (bdta)
A mixture of benzil (4.75 mmol, 1.0 g), and diethylenetri-
amine (4.75 mmol, 0.51 ml) in 50 ml hot ethanol was refluxed
for about 4 h. It was left at 0 ◦C which afforded a crystalline
product. It was decanted, washed with cold diethyl ether and
dried over CaCl2 in vacuo (Scheme 1).
2.2. Synthesis of complexes
2.4. Synthesis of Cd(bdta)Cl2
A hot ethanolic solution of the macrocycle and the metal
dichloride in 1:1 molar ratio was refluxed for about 5 h and left
overnight at room temperature, which afforded a product. It was
filtered, washed with cold diethyl ether and dried in vacuo. Since
A similar procedure was followed for Cd-complex except that
theCdCl2 (2 mmol, 0.4 g)wasmadesolubleinminimumamount
of DMSO (8 ml). A light yellow complex was obtained after 6 h
Scheme 3. Template synthesis of the complex M(bdta)Cl2 where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II).

S. Khan et al. / Spectrochimica Acta Part A 68 (2007) 269–274
271
Table 1
Analytical data and physical properties of the complexes
Compounds
Colour
mp (◦C)
Yield (%)
Molar conductance
(ꢀ−1 mol−1 cm2)
Analysis, (%) found (calculated)
C
H
N
M
–
bdta
Yellow
Light-yellow
Brown
Dark-brown
Red
Green
Red
Light-yellow
Yellow
148
162
184
254
235
152
230
206
218
55
62
67
70
68
55
48
72
70
13.1
32.5
17.2
31.7
10.8
8.7
9.2
14.7
21.6
77.50 (77.94)
62.92 (63.53)
62.88 (63.44)
63.01 (63.16)
62.99 (63.18)
62.04 (62.73)
61.89 (62.57)
57.65 (58.50)
51.83 (52.30)
6.58 (6.90)
5.55 (5.64)
5.54 (5.60)
5.50 (5.59)
5.25 (5.59)
5.50 (5.55)
5.23 (5.54)
5.42 (5.18)
5.12 (4.60)
14.61 (15.15)
12.28 (12.40)
12.27 (12.33)
12.22 (12.27)
12.25 (12.28)
12.20 (12.19)
12.05 (12.16)
11.28 (11.38)
10.09 (10.17)
Mn(bdta)Cl2
Fe(bdta)Cl2
Co(bdta)Cl2
Ni(bdta)Cl2
Cu(bdta)Cl2
Zn(bdta)Cl2
Cd(bdta)Cl2
Hg(bdta)Cl
8.26 (8.07)
8.34 (8.19)
8.34 (8.60)
8.84 (8.57)
9.45 (9.22)
9.66 (9.46)
15.48 (15.22)
24.51 (24.20)
of refluxing. The resulting precipitate was filtered, washed with
ethanol, diethyl ether and dried in vacuo over CaCl2 (Scheme 3).
noconsiderablechangeinν(N–H)wasobservedonmovingfrom
free to the complexed ligand it is suggested that the ν(NH) is
not involved in coordination. The spectrum also shows that the
absorption due to CH2 group (2940 cm−1) does not shift in the
case of complexes, hence they are insensitive to coordination.
The sharp and distinct bands present in the far IR region,
418–448 and 320–346 cm−1 provide a compelling evidence
for the presence of metal–nitrogen and metal–chlorine bonds,
respectively, in the complexes, which were absent in the spec-
trum of the macrocyclic ligand. However, ν(M–N) does not
follow any regular trend [12].
3. Results and discussion
The compounds correspond to the composition M(bdta)Cl2,
where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)
and Hg(II), and bdta = 3,6,9,12,15,18-hexaaza-1,2,10,11-tetra-
phenyl-2,9,11,18-tetraenecyclododecane. The physico-chemi-
cal properties of the ligand and its complexes are summarized in
Table 1. They are thermally stable and decompose between 148
and 254 ◦C. They are insoluble in common organic solvents but
soluble in hot methanol, DMSO and DMF.
3.2.1. EPR spectra
The EPR spectrum of the polycrystalline complexes
Cu(bdta)Cl2 and Mn(bdta)Cl2 showed strong signals at room
3.1. Molar conductance
temperature. The axial symmetry parameter, i.e. G (g − g ) has
ꢀ
⊥
been calculated. The complexes do not show hyperfine splitting.
According to Hathaway et al. the G value measures the exchange
interaction between metal centers in polycrystalline solids [13].
In case G > 4, the exchange interaction is negligible but if G < 4,
considerable exchange interaction occurs in the complexes. The
EPR spectrum of the Cu-complex shows a broad signal with
G average at 2.002, which is consistent with an axially elon-
gated octahedral geometry. The broadening of this signal might
be due to dipolar interactions, indicating lowered site symme-
try suggesting that the unpaired electron resides mainly in the
dx2−y2 orbital. This indicates a considerable exchange interac-
tion in the complex [14]. However, the solution spectra of the
Cu-complex in frozen DMSO (77 K) exhibit hyperfine interac-
tion attributable to the coordination of four imine nitrogen. The
trend g (2.102) > g (2.001) has been observed in accordance
The low molar conductance values of 1 mM solution of
the complexes measured in DMSO indicated them to be non-
electrolytes [10].
3.2. IR spectra
The important IR spectral bands are listed in Table 2. The
disappearance of a band at 1720 cm−1 (C O) and an appearance
of a strong band at 1640 cm−1 (C N) in the ligand confirms an
effective Schiff base condensation. The band due to ν(C N)
observed at 1640 cm−1 in the case of the ligand has shown a
negative shift of 15–20 cm−1 in the complexes. This suggests the
coordination of azomethine group with the metal ion [11]. Since
ꢀ
⊥
Table 2
with the criterion of Kivelson and Neiman implying the presence
of unpaired electron in localized dx2−y2 orbital for the Cu(II) ion
characteristic of the axial symmetry [15].
Cardinal IR bands and their assignment
Complex
ν(NH)
ν(CH2)
ν(C N)
ν(M–N)
ν(M–Cl)
bdta
3287s
3293s
3293s
3287s
3287s
3287s
3293s
3287s
3287s
2940m
2940m
2940m
2940m
2940m
2940m
2940m
2940m
2940m
1640s
1620s
1618s
1623s
1624s
1627s
1618s
1622s
1627s
–
–
In a similar fashion, a single unresolved signal observed in
the Mn complex suggests an exchange interaction between the
Mn(II) centers. In the present case, the average G value is found
to be 2.008, which can be corroborated with an octahedral
environment. Thus, the EPR spectrum supports the binding of
the ligand with four potential nitrogen donor atoms to the metal
ion in an octahedral environment [16]. The solution spectra of
Mn(II)-complex at liquid nitrogen temperature (77 K) and that
Mn(bdta)Cl2
Fe(bdta)Cl2
Co(bdta)Cl2
Ni(bdta)Cl2
Cu(bdta)Cl2
Zn(bdta)Cl2
Cd(bdta)Cl2
Hg(bdta)Cl2
448m
424m
432m
418m
424m
436s
341s
342s
344s
346s
328s
331s
339s
324s
432m
424m

272
S. Khan et al. / Spectrochimica Acta Part A 68 (2007) 269–274
of polycrystalline sample has similar features with Gav = 2.007
further implying octahedral geometry for Mn(II) ion
[17].
According to the literature data, the principal feature of octa-
hedral nickel(II) complexes is the presence of three well-defined
bands [23]. The universally accepted ground term for octahe-
3
dral Ni(II) ions is A2g(F). We have also observed three weak
intensity bands at 9008, 15,402 and 22,560 cm−1 correspond-
3.3. Electronic spectra and magnetic moment
3
3
3
3
ing to A2g(F) to T2g(F), T1g(F) and T1g(P) transitions,
respectively. The magnetic moment value (2.94 B.M.) further
supports an octahedral environment around the Ni(II) ion [24]
(Table 4).
The room temperature magnetic moment of Mn(II) complex
(5.8 B.M.) is close to the spin only value for an octahedral Mn(II)
ion corresponding to five unpaired electrons [18]. This pale-
yellow complex in DMSO exhibits three d–d absorption with
low molar extinction coefficients. The series of bands observed
are weak and narrow stretching over the entire visible region,
Generally the octahedral Cu(II) complexes exhibit three tran-
sitions in the UV-range with poor resolution. However, we
have observed only two d–d transitions [25]. A small unre-
solved peak was obtained at 11,123 cm−1 and a pronounced
6
which are assigned from sextet ground term, A1g to quar-
2
2
band at 15,423 cm−1. These may be assigned to A1g ← B1g
4
4
4
tet excited terms T1g(P), T2g(G) and T1g(G), respectively
[19]. On the basis of magnetic moment and spectral assign-
ments an octahedral geometry has been proposed for the Mn(II)
ion [20]. The various ligand field parameters are calculated
for the Mn(II) complex. The value of Dq has been calculated
from Orgel energy level diagram using ν2/ν1 ratio and the
value comes out to be 1576 cm−1. The nephelauxetic parame-
and 2Eg ← B1g transitions, respectively. The magnetic moment
2
(1.87 B.M.) fits well in the region 1.73–2.20 B.M. reported for
the octahedral Cu(II) complexes [26].
3.4. Thermal studies (TGA/DSC)
ter, β is readily obtained using the relation β = Bcomplex/Bfree ion
,
The weight loss corresponding to different stages of pyroly-
sis were calculated and compared with those of the expelled
groups. The thermogram of the ligand consists of two well-
defined stages. The first decomposition stage starts at 140 ◦C
and continues until 320 ◦C corresponding to the degradation of
four peripheral phenyl groups constituting about 55% of the
total weight loss (calcd. 55.6%). The initial decomposition of
the exocyclic rings is a common behaviour encountered in such
complexes [27]. Since the decomposition started above 140 ◦C,
the presence of any solvent/water molecule may be ruled out [28]
which is also evident from the elemental analysis. The second
step runs over 320–600 ◦C and is consistent with the degradation
of the remaining part of the molecule. Above 600 ◦C the TGA
curve shows no change even when the temperature was raised
to 850 ◦C. The DSC plot exhibits well-defined endothermic
and exothermic peaks. The first transition is a sharp endotherm
while the second step is exothermic in nature. However, a small
exothermic hump is obtained in the case of complexes owing to
the conversion of metal to its elemental form [29].
where Bfree ion is 1199 cm−1. The value of β indicates that
the covalent character of metal ligand sigma bond is very low
(Table 3).
The brown colour Fe(II) complex in DMSO is a character-
istic of high-spin Fe(II) ion. Since in the case of octahedral
Fe(II) complexes the intense charge transfer bands obscure d–d
bands [21], a single band has been observed in the present study.
Thus, the broad band centered at 11,268 cm−1 can be effectively
5
5
assigned to Eg ← T2g transition [21]. The spectrum and the
magnetic moment (5.40 B.M.) support an octahedral geometry
for the Fe(II) ion.
The μeff value of (4.49 B.M.) for Co(II) complex is indica-
tive of a high-spin octahedral geometry for the ion [22]. Two
bands of medium intensity observed at 9652 and 18,122 cm−1
correspondto4T2g(F) ← T1g(F)and4A2g(F) ← T1g(F)transi-
tions, respectively [22]. These bands together with the magnetic
moment are attributable to an octahedral environment around
the Co(II) ion.
4
4
Table 3
Magnetic moment values and spectral bands of the complexes
Compounds
Mn(bdta)Cl2
Magnetic moment
(B.M.)
Electronic bands
(cm−1
log ε (l mol−1 cm−1
)
Possible assignments
10 Dq (cm−1
15,760
)
B (cm−1
)
β
)
6
5.80
26,385
21,052
15,527
2.77
2.49
2.32
4T1g(P) ← A1g
477
0.58
6
4T2g(G) ← A1g
6
4T1g(G) ← A1g
5
Fe(bdta)Cl2
Co(bdta)Cl2
Ni(bdta)Cl2
5.40
4.49
2.94
11,268
35,400
3.94
4.56
5Eg ← T2g
–
–
1076
–
–
Charge transfer
4
9,652
18,122
2.39
1.52
4T2g(F) ← T1g(F)
5,380
–
0.96
–
4
4A2g(F) ← T1g(F)
3
9,008
15,402
22,650
3.58
3.10
2.78
3T2g(F) ← A2g(F)
3
3T1g(F) ← A2g(F)
3
3T1g(P) ← A2g(F)
2
Cu(bdta)Cl2
1.87
11,123
15,423
2.69
2.19
2A1g ← B1g
15,960
–
–
2
2Eg ← B1g

S. Khan et al. / Spectrochimica Acta Part A 68 (2007) 269–274
273
Fig. 1. Cyclic voltammogram of the complex [Cu(bdta)]Cl2.
3.5. Electrochemical behaviour
The electrochemical behaviour of the complexes particularly
with tetraaza ligands has been studied in order to examine the
spectral and structural changes associated with electron transfer
processes [30]. In the present work, the Cu(II) complex has been
studied by cyclic voltammetry in the range +1.5 to −1.5 V with
a scan rate of 50 mV/s. The typical quasi-reversible one-electron
transfer wave (Fig. 1) is governed by the equation:
Cu(II)(bdta)Cl2 + e− ⇔ Cu(I)(bdta)Cl2
The voltammogram in the cathodic region displays peak
for Cu(II) → Cu(I) at Epc = 0.640 V while for the anode it was
located at Epa = 0.715 V. However, the region −1.5 to 0.0 V
exhibits peaks due to Cu(I) → Cu(II) at Epc = −0.600 V and
Epa = −0.500 V corresponding to cathodic and anodic poten-
tials, respectively [31]. The ratio of cathodic peak current versus
anodic peak current is close to unity (Ipc/Ipa = 0.886) implying
that the quasi-reversible one-electron transfer process is con-
trolled by diffusion.
Acknowledgement
Authors are thankful to Prof. R.J. Singh, Department of
Physics, AMU, Aligarh for a stimulating discussion of EPR
spectra.
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