Binuclear transition metal complexes of unsymmetrical tetradentate Schiff base ligands

Article abstract of DOI:10.1081/SIM-120021934

From the condensation of the Schiff base (HMBP-PHEN) prepared from 5-methyl-2-hydroxybenzophenone (HMBP) and o-phenylenediamine (PHEN) in the 1:1 mole ratio with substituted salicylaldehydes, a series of homo-binuclear metal complexes with non-symmetrical tetradentate Schiff base ligands have been synthesized by the template method. These new types of binuclear metal complexes have been characterized by elemental analyses, IR, UV, FAB and EPR spectroscopy and magnetic properties.

Full text of DOI:10.1081/SIM-120021934

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Synthesis and Reactivity in Inorganic and Metal-Organic
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Binuclear Transition Metal Complexes of Unsymmetrical
Tetradentate Schiff Base Ligands
Hua Han a , Wen‐Juan Ruan a , Xiao‐Jing Zhao a , Guo‐Hang Hu a & Zhi‐Ang Zhu a
a Department of Chemistry , Nankai University , Tianjin, 300071, P.R. China
Published online: 15 Feb 2007.
To cite this article: Hua Han , Wen‐Juan Ruan , Xiao‐Jing Zhao , Guo‐Hang Hu & Zhi‐Ang Zhu (2003) Binuclear Transition
Metal Complexes of Unsymmetrical Tetradentate Schiff Base Ligands, Synthesis and Reactivity in Inorganic and Metal-Organic
Chemistry, 33:6, 1011-1023
To link to this article: http://dx.doi.org/10.1081/SIM-120021934
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 6, pp. 1011–1023, 2003
Binuclear Transition Metal Complexes of
Unsymmetrical Tetradentate Schiff Base Ligands
*
Hua Han, Wen-Juan Ruan, Xiao-Jing Zhao,
Guo-Hang Hu, and Zhi-Ang Zhu
Department of Chemistry, Nankai University, Tianjin, P.R. China
ABSTRACT
From the condensation of the Schiff base (HMBP–PHEN) prepared
from 5-methyl-2-hydroxybenzophenone (HMBP) and o-phenylenedia-
mine (PHEN) in the 1:1 mole ratio with substituted salicylaldehydes, a
series of homo-binuclear metal complexes with non-symmetrical
tetradentate Schiff base ligands have been synthesized by the template
method. These new types of binuclear metal complexes have been
characterized by elemental analyses, IR, UV, FAB and EPR spectro-
scopy and magnetic properties.
Key Words: Binuclear transition metal complexes; Unsymmetrical
tetradente Schiff base; EPR spectroscopy; Magnetic properties.
*
Correspondence: Wen-Juan Ruan, Department of Chemistry, Nankai University,
Tianjin 300071, P.R. China; Fax: 0086-22-23502458; E-mail: wjruan@nankai.edu.cn.
1011
DOI: 10.1081/SIM-120021934
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

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Han et al.
INTRODUCTION
Binuclear metal complexes play essential roles in biological systems by
mimicking the active sites of some metalloenzymes.[1] They are also
important for understanding the mutual influences of two metal centers on
the electronic, magnetic, and electrochemical properties of such bimetallic
cores.[2] There is current interest in binuclear ligands and their transition-
metal complexes with emphasis being placed on structure correlation and
on bioinorganic modeling studies. Robson[2] first reported the synthesis of a
binuclear macrocycles comprised of two moles of substituted salicylalde-
hydes and two moles of propane-1,3-diamine. This type of metal complex
is called a Robson-type complex.[3]
Recently, the chemistry of Robson-type complexes has been expanded
by increasing the number of donor atoms in the ligands and by varying the
substituents. In this way, both symmetric and asymmetric binuclear metal
complexes can be obtained. These phenol-based binuclear macrocycles[4,5]
have a ‘‘Salen’’-like N2O2 metal-binding site sharing both phenolic oxy-
gens. Recent studies have indicated that the metal ions are often found in
chemically or geometrically distinct environments in binuclear transition-
metal biosites. Thus, the design of asymmetric ligands or asymmetric bi-
nuclear cores is of particular importance. Many investigations focus on the
macrocyclic complexes with different metal centers; however, such com-
plexes are not included in this study.
Figure 1. Structure of the asymmetrical ligands (1) and (2).

Binuclear Transition Metal Complexes
1013
The physical and structural properties of the Cu(II) and Ni(II)
complexes[6,7] of the asymmetrical ligands (1) and (2) in Figure 1 have
been reported. In this study, we have synthesized homo-binuclear com-
plexes M2L (3)–(8) (M = Cu(II), Ni(II), Co(II), Mn(II)) by the template
condensation of substituted salicylaldehydes with HMBP–PHEN in the
presence of metal ions.
EXPERIMENTAL SECTION
Materials
All the reagents were of analytical grade. Substituted salicylaldehydes,[5]
5-methyl-2-hydroxybenzophenone (HMBP)[8] and HMBP–PHEN[3] were
prepared according to the literature methods.
Synthesis of M2L
To a mixture of HMBP–PHEN (1 mmol) in absolute methanol (10
mL) and the desired metal percholorate salt (1 mmol) in absolute methanol
(10 mL) was added 2,6-diformal-4-methylphenol or 2,6-diformal-4-tert-
butylphenol (0.5 mmol). This mixture was refluxed for 2 h. Then the
solution was allowed to cool to room temperature, and the product was
collected by filtration, washed with methanol and dried under vacuum.
Elemental analyses (C, H, N) were obtained on a Perkin–Elemer 240
elemental analyzer. Mass spectra were measured on a VGZAB-HS
spectrometer using Fast Atom Bombardment (FAB) technique. The IR
spectra of solid samples were recorded in the range of 4000–400 cm–1 on a
Bio-Rad 135 FT-IR spectrophotometer using KBr pellets. The UV-Vis
spectra were measured on a Beckman DU-8B spectrophotometer in the
range of 190–900 nm. ESR spectra were measured on a Bruker EMX 100
ESR spectrometer and variable-temperature magnetic susceptibilities on a
LDJ-9600 VSM magnetometer.
RESULTS AND DISCUSSION
When 5-methyl-2-hydroxybenzophenone reacts in methanol with 1,2-
diaminobenzene in the presence of piperidine, Schiff base condensation
occurs at only one of the two available amino groups, even in the
presence of a large excess of the ketone (Figures 2 and 3). Without

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Han et al.
Figure 2. Synthesis of complexes.
Figure 3. Structure of the homo-binuclear metal complexes M2L (3)–(8).

Binuclear Transition Metal Complexes
1015

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Han et al.
isolation of the free ligand, the metal complexes were obtained by
template reaction.
The elemental analyses of the complexes are listed in Table 1.
Shown in Figure 4 are the positive ion FAB mass spectra of the metal
complexes (3) and (4). In the FAB mass spectrum of the metal complex (3),
the intense peak centered at m/z 856 corresponds to the ion that has lost
one proton in addition to one perchlorate anion. The intense peak centered
at m/z 847 corresponds to ion that has lost one proton and two perchlorate
anions from metal complex (4) (Table 2).
Figure 4. The FAB mass spectrum of the metal complexes (3) and (4).

Binuclear Transition Metal Complexes
1017
Table 2. The major peaks in the FAB mass spectrum of the metal complexes (3)
and (4).a
No.
Compound
M/e
F.W. (g/mol)
(3)
(4)
[Cu2L1]ClO4
[Ni2L1](ClO4)2
856.4
847.5
856.43
847.21
aL1: R = CH3 in Figure 3.
IR Spectra
In the region 3200–3400 cm–1, the infrared spectrum of HMBP–
PHEN exhibits two bands, a sharp and intense band at 3365 cm–1, and a
much weaker band at 3241 cm–1. These two bands are assigned to the
primary amine stretches.[9] They are absent in the spectrum of the metal
complexes where the amine group has been lost due to the condensation
with salicylaldehyde.
An intense sharp band at 3457 cm–1 in the spectrum of HMBP–PHEN
is tentatively assigned to the O–H stretching vibrations, which are
observed in the infrared spectrum of the metal complexes. The lack of
sharp bands at 3365 and 3457 cm–1 in the spectra of the metal complexes
Table 3. Important IR absorption bandsa (cm–1).
Compound
n(OH)
n(NH)
n(C=N)
n(C–O)
HMBP-PHEN
3457 s
3365 s
3241 w
–
1624 s
1292 m
(3)
(4)
(5)
(6)
(7)
(8)
[Cu2L1]ClO4
[Ni2L1](ClO4)2
[Co2L1](ClO4)3
[Cu2L2]ClO4
[Ni2L2]ClO4ꢀH2O
Mn2L2
3440 b
3422 b
3416 b
3441 b
3422 b
3405 b
1620 s
1542 s
1628 s
1545 s
1648 s
1560 s
1622 s
1544 s
1622 s
1541 s
1609 s
1569 s
1330 m
1230 s
1326 m
1239 s
1380 m
1237 s
1334 m
1232 s
1335 m
1243 s
1335 m
1231 s
–
–
–
–
–
as = strong, m = medium, w = weak, b = broad; L1: R = CH3; L2: R = t-Bu in Figure 3.

1018
Han et al.
provides clear evidence for the loss of the amine and hydroxy protons[10] of
HMBP–PHEN.
The band at 1624 cm–1 in the spectrum of HMBP–PHEN is assigned
to n(C=N). However, the metal complexes show two bands in the regions
of 1620–1645 and 1539–1569 cm–1, which indicate the existence of an
asymmetric environment for the metal ion (Table 3).
Furthermore, a comparison between the IR spectra of HMBP–PHEN
and the metal complexes, shows that n(C–O) at 1292 cm–1 is split into two
bands, one at ꢁ 1360 cm–1 and the other at ꢁ 1240 cm–1. This splitting is
probably due to presence of the new C–O bond and the formation of a C–
O–M bond. Strong bands at 1100 and 623 cm–1 in the spectrum of the
perchlorate salt are due to that anion.[10]
Electronic Spectra
The electronic spectra of the complexes (3)–(8) were measured in
CH2Cl2 and the peaks are given in Table 4.
All of the complexes show shoulders or bands in the range of 227–280
nm, which are assigned to the p–p* transition associated with the azo-
methine group.[11] The band occurring between 320 and 350 nm is assigned
as a n–p* transition involving molecular orbitals of the C=N chromophore
and the benzene rings. A band at 420–480 nm is tentatively attributed to
the charge-transfer band from the filled P orbital of the phenolic oxygen to
a vacant metal orbital.[12] Although these transitions are relatively weak,
such assignments and absorption coefficients in similar environments are
well documented.[13]
The dicopper(II) complexes exhibit a broad band at ꢁ 700 nm. This
band can be assigned to the d–d transition. But a broad band in the near-UV
region is usually assigned to an intraligand transition of the C=N fragment
Table 4. Electronic spectral bands in CH2Cl2.
Complexa
l
max/nm (lg e/mol–1ꢀdm3ꢀcm–1
)
(3)
(4)
(5)
(6)
(7)
(8)
[Cu2L1]ClO4
[Ni2L1](ClO4)2
[Co2L1](ClO4)3
[Cu2L2]ClO4
[Ni2L2]ClO4ꢀH2O
Mn2L2
700 (2.53) 424 (4.33) 320 (sh) 230 (4.87)
595 (2.95) 520 (3.54) 450 (4.08) 332 (4.46) 255 (4.75)
780 (2.11) 710 (2.31) 462 (3.67) 282 (4.57) 229 (4.74)
708 (2.28) 423 (4.45) 320 (sh) 227 (4.81)
550 (3.58) 453 (3.79) 339 (4.13) 252 (4.42) 230 (4.65)
480 (3.80) 345 (4.28) 270 (4.58) 230 (4.74)
aL1: R = CH3; L2: R = t-Bu in Figure 3.

Binuclear Transition Metal Complexes
1019
overlapping with a copper-phenolic oxygen charge transfer transition.[14]
Sometimes, this band is broader and less intense than that of the symmetric
complexes with Robson-type ligands. It might be suggested that, in our
complexes, the two metal environments are not completely equivalent.
The appearance of a broad, low-intensity band near 520 nm is attri-
buted to a d–d transition of the Ni(II) ion. This band is typical of planar
Ni(II) ions. The absence of absorption above 660 nm also indicates a
square-planar or distorted square-planar geometry around the Ni(II) ion.[15]
The complex Mn2L does not exhibit any bands above 600 nm. The d–
d spectra of the cobalt complexes consist of a broad band at ꢁ 710 nm and
a band at ꢁ 780 nm.[16]
Despite the different salicyladehyde substituents, the spectra of the
complexes with the same metal ions are similar. It might be suggested that
some d–d bands occur as a shoulder on the intense UV band, which can be
expected to obscure changes in the d–d band positions brought about by
small changes in the environment of the metal ions.
ESR Spectroscopy
The room temperature ESR spectrum of powdered samples of the
complex (3) is shown in Figure 5. It should be noted that the solid-state
ESR spectrum shows a half-field signal at g = 4.1, which is evidence for at
least weak interaction between copper(II) centres.[17]
Figure 5. The room temperature ESR spectrum of a powdered sample of the
complex (3).

1020
Han et al.
Figure 6. Temperature dependence of magnetic susceptibilities (wM) and effective
magnetic moments (meff) for complex (3) ([Cu2L1]ClO4) and complex (4)
([Ni2L1](ClO4)2).
Magnetic Properties
The molar magnetic susceptibility of the solid-state powder of complex
(3) in the temperature range 77–300 K is shown in Figure 6. The meff value
of complexes (3) at room temperature is 2.43 mB which is significantly
lower than that of 2.45 mB expected for a compound containing two S = 1/2
(CuII) uncoupled spins. For complex (4), the B.M. value at room
temperature is 3.97 B.M. which is lower than that of 4.00 B.M. expected
for a compound containing two S = 1 (NiII) uncoupled spins. These results
indicate that the binuclear units are not well isolated magnetically. The meff
values of complexes (3) and (4) decrease with decreasing temperature to the
values of 1.75 B.M. and 3.36 B.M. near 77 K. The results suggest that there
is weakly antiferromagnetic interaction between two metal ions of the
binuclear metal complex. Magnetic simulations of complexes (3) and (4)
have been made on the basis of the Heisenberg model resulting in follow-
ing expressions:
À1
wM ¼ 2Ng2b2=KT  ½3 þ expðÀ2J=KTފ þ Na
½For complexð3ފ

Binuclear Transition Metal Complexes
1021
wM ¼ 2Ng2b2=KT  f½expð2J=KTÞ þ 5 expð6J=KTފ=
½1 þ expð2J=KTÞ þ 5 expð6J=KTފg
½For complexð4ފ
where Na = 6.0Â 10–5 cm3ꢀmol–1 and other symbols have their usual
meaning. As indicated by the solid line in Figure 6, the magnetic properties
of complex (3) are well reproduced by the above equation, using
J = À 47.212 cm–1 and g = 2.070. The discrepancy factor is 1.177 Â 10–5
.
For complex (4), the best fit parameters are J = À 17.08 cm–1, g = 1.976
and the discrepancy factor is 5.86 Â 10–4. The results also indicate that
there is a weakly antiferromagnetic interaction between two Cu(II) ions and
two Ni(II) ions, respectively.
ACKNOWLEDGMENT
This work was supported by a grant from the National Natural Science
(No. 20171024, 20271030), Tianjin Natural Science Foundation (No.
023604011) and the Research Fund for the Return of People Study-
ing Abroad.
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Received September 25, 2002
Accepted February 27, 2003
Referee I: E. Sinn
Referee II: R. H. Gayhart