Synthesis and characterization of two acetato-bridged dinuclear copper(II) complexes with 4-bromo-2-((4 or 6-methylpyridin-2-ylimino)methyl)phenol as ligand

Article abstract of DOI:10.1080/00958970903330294

The synthesis and characterization of two new acetato-bridged dinuclear copper(II) complexes are described. Both compounds have the general formula [Cu(L)(μ-O₂C-CH₃)]₂, in which L =4-bromo-2-((4-methylpyridin-2-ylimino)methyl)phenol or 4-bromo-2-((6-methylpyridin-2-ylimino)methyl)phenol. The title compounds consist of dinuclear units with bridging acetato groups and a ligand linked to each copper via the phenol oxygen and nitrogen. Both compounds were synthesized in a one-step reaction and characterized by elemental analysis, Fourier transform infrared (FTIR), electron spin resonance (ESR), and electronic spectra and by room temperature magnetic moments. The compounds exhibit antiferromagnetic interactions at room temperature. UV-Vis spectra show four absorptions attributed to d-d transitions of copper, ligand → metal charge transfer and π → π* or n → π* transitions of ligand. The FTIR spectra indicate a Cu₂O₄C₂ ring vibration. Both complexes show room temperature magnetic moments of about 1.6 B.M. per copper. The X-band ESR studies indicate a weak half-field band, characteristic of the Cu(II)-Cu(II) dimer, observed at 1552 and 1558 G for the complexes, strongly suggesting that the hyperfine structure arises from a spin triplet species. The spectra of frozen samples in DMSO or DMF at liquid nitrogen temperature show a typical Δm = 1 transition.

Full text of DOI:10.1080/00958970903330294

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Synthesis and characterization of two
acetato-bridged dinuclear copper(II)
complexes with 4-bromo-2-((4 or 6-
methylpyridin-2-ylimino)methyl)phenol
as ligand
Farshideh Hasanvand a , Ali Hoseinzadeh a , Javad Zolgharnein a
Saeid Amani a
a Department of Chemistry, Arak University, Dr Beheshti Ave.,
Arak 38156, Iran
&
Published online: 12 Oct 2009.
To cite this article: Farshideh Hasanvand , Ali Hoseinzadeh , Javad Zolgharnein & Saeid Amani
(2010): Synthesis and characterization of two acetato-bridged dinuclear copper(II) complexes
with 4-bromo-2-((4 or 6-methylpyridin-2-ylimino)methyl)phenol as ligand, Journal of Coordination
Chemistry, 63:2, 346-352
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Journal of Coordination Chemistry
Vol. 63, No. 2, 20 January 2010, 346–352
Synthesis and characterization of two acetato-bridged
dinuclear copper(II) complexes with 4-bromo-2-((4 or
6-methylpyridin-2-ylimino)methyl)phenol as ligand
FARSHIDEH HASANVAND, ALI HOSEINZADEH,
JAVAD ZOLGHARNEIN and SAEID AMANI*
Department of Chemistry, Arak University, Dr Beheshti Ave., Arak 38156, Iran
(Received 26 July 2009; in final form 27 July 2009)
The synthesis and characterization of two new acetato-bridged dinuclear copper(II)
complexes are described. Both compounds have the general formula [Cu(L)(m-O2C–CH3)]2,
in which L ¼ 4-bromo-2-((4-methylpyridin-2-ylimino)methyl)phenol or 4-bromo-2-((6-methyl-
pyridin-2-ylimino)methyl)phenol. The title compounds consist of dinuclear units with bridging
acetato groups and a ligand linked to each copper via the phenol oxygen and nitrogen. Both
compounds were synthesized in a one-step reaction and characterized by elemental analysis,
Fourier transform infrared (FTIR), electron spin resonance (ESR), and electronic spectra and
by room temperature magnetic moments. The compounds exhibit antiferromagnetic interac-
tions at room temperature. UV-Vis spectra show four absorptions attributed to d–d transitions
of copper, ligand ! metal charge transfer and ꢀ ! ꢀ* or n ! ꢀ* transitions of ligand. The
FTIR spectra indicate a Cu2O4C2 ring vibration. Both complexes show room temperature
magnetic moments of about 1.6 B.M. per copper. The X-band ESR studies indicate a weak
half-field band, characteristic of the Cu(II)–Cu(II) dimer, observed at 1552 and 1558 G for the
complexes, strongly suggesting that the hyperfine structure arises from a spin triplet species.
The spectra of frozen samples in DMSO or DMF at liquid nitrogen temperature show a typical
Dm ¼ 1 transition.
Keywords: Dinuclear copper(II) complexes; Spin–spin interaction; Diacetato-bridged
1. Introduction
A fascinating field of research is the study of polynuclear complexes. Magnetic
properties of a polynuclear complex of paramagnetic metal ions are, in general, different
from the sum of the magnetic properties of each metal ion fragment separately [1].
Bleaney and Bowers [2] demonstrated that the magnetic behavior of copper(II) acetate
monohydrate was due to the dinuclear nature and magnetic exchange interaction. Since
then, exchange coupled polynuclear systems have seen exponential increase in
publications [3–8]. Such interaction between metal centers lies at the center of widely
separated areas, such as physics of magnetic materials and the role of polynuclear
reaction sites in biological [9–12] and catalytic processes [13–16]. Dinuclear copper(II)
*Corresponding author. Emails: s-amani@araku.ac.ir; amani1952@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903330294

Acetato-bridged dinuclear copper(II)
347
complexes have relatively simple single-ion properties facilitating interpretation of
magnetic data. The simplest bridging systems use monoatomic bridges, such as hydroxo
[17], alkoxo [18], chloro [19], bromo [20], fluoro [21], and azido [22]. Despite a vast
number of experimental and theoretical studies, our understanding of the magnitude of
the copper(II)–copper(II) coupling is still imperfect. In the present study, two dinuclear
copper(II) complexes are reported with 4-bromo-2-((4-methylpyridin-2-ylimino)-
methyl)phenol or 4-bromo-2-(6-methylpyridin-2-ylimino)methyl)phenol (hereafter
abbreviated L1 and L2, respectively), [Cu2(L1)2(O2C–CH3)2] or [Cu2(L2)2(O2C–CH3)2].
2. Experimental
2.1. Chemicals
All chemicals were of reagent grade quality purchased from Merck Chemical Company
and used as received.
2.2. Physical measurements
The C, H, and N determinations were made in the Research Institute of Petroleum
Industry of Iran. Cu determination was carried out on a Perkin-Elmer 2380 Atomic
Absorption spectrophotometer at 324.7 nm. Electronic spectra were recorded on a
Perkin-Elmer Lambda 900 spectrophotometer. Fourier transform infrared (FTIR)
spectra were made from 4000–250 cmꢀ1 using a Galaxy 5000 spectrophotometer.
Spectra were calibrated using polystyrene bands at 3028, 1601, and 1208 cmꢀ1. X-band
electron paramagnetic resonance spectra were recorded on powder and frozen solutions
at room and at liquid nitrogen temperatures in DMF or DMSO on an IBM electron
spin resonance (ESR) spectrometer using DPPH (g ¼ 2.0036) as a standard. A Johnson
Matthey Alfa products magnetic susceptibility balance was used to measure the room
temperature magnetic moments.
2.3. Preparation of the ligands
The ligands were prepared according to the following procedures.
2.3.1. 4-Bromo-2-((4-methypyridin-2-ylimino)methyl)phenol (L1). 10 mmol 2-amino-
4-methylpyridine in 15 mL methanol was added to 10 mmol 4-bromo-2-hydroxyben-
zaldeyhyde in 25 mL CH3OH. By the addition of five drops concentrated H2SO4 and 2 h
reflux, yellow precipitate separated was washed with hot methanol and air dried. Yield
ca ꢁ 96%. Elemental analysis for C13H11N2BrO: Found (%): C, 52.65; H, 3.74; N, 9.87.
1
Calcd (%): C, 53.36; H, 3.81; N, 9.62. H NMR (CDCl3, ppm): ꢁ 2.28(s, 3Hmet.), 6.95–
8.40(m, 6Haro.), 9.47(s, 1Himin.), 13.05(s, 1Hphen.) (scheme 1).
2.3.2. 4-Bromo-2-((6-methypyridin-2-ylimino)methyl)phenol (L2). This compound was
obtained by similar method as described for L1. Yield ca ꢁ 95%. Elemental analysis for

348
F. Hasanvand et al.
CH3
H
Br
N
N
HO
Scheme 1. Structure of ligand L1.
H
Br
H3C
N
N
HO
Scheme 2. Structure of ligand L2.
CH3
Br
CH3
N
O
O
O
N
Cu
Cu
O
N
O
O
N
CH3
CH3
Br
Scheme 3. Proposed structure for 1.
C13H11N2BrO: Found (%): C, 54.58; H, 3.91; N, 9.50. Calcd (%): C, 53.36;
H, 3.81; N, 9.62. 1H NMR (CDCl3, ppm): ꢁ 2.66(s, 3Hmet.), 6.92–7.78(m, 6Haro.),
9.40(s, 1Himin.), 13.10(s, 1Hphen.) (scheme 2).
2.4. Preparation of the complexes
Coordination compounds were prepared according to the following general procedure.
2.4.1. [Cu2(L1)2(k-O2C–CH3)2] (1). 2 mmol ligand L1 and 2 mmol triethylamine were
dissolved in 25 mL methanol; 2 mmol Cu(O2C–CH3)2 ꢂ H2O was also dissolved in 50 mL
CH3OH. The Cu(II) salt solution was then added slowly to the ligand solution. After
4 h reflux, the brown precipitate separated and was washed with hot methanol and
THF. Yield ca ꢁ 55%. Elemental analysis for C30H26Br2O6N4Cu2: Found (%): C,
44.13; H, 3.44; N, 6.37; Cu, 15.73. Calcd (%): C, 43.65; H, 3.18; N, 6.79; Cu, 15.40
(scheme 3).

Acetato-bridged dinuclear copper(II)
349
CH3
CH3
Br
O
O
Cu
O
O
Cu
O
N
N
N
N
O
CH3
CH3
Scheme 4. Proposed structure for 2.
Br
Table 1. Spectroscopic data for 1 and 2.
Absorption
UV-Vis
(nm)
IR(Cu–O)
(Cu–N)
ESR
solution
in DMF
IR (acetate)
(cmꢀ1
ESR
half-field
ESR
powder
mCu (R.T.)
B.M.
Complex
)
(cmꢀ1
)
1
667, 418,
295, 270
1556, 1415
549, 464,
349, 279
gh ¼ 4.36
gh ¼ 4.04
g ¼ 2.06
g ¼ 2.06
g ¼ 2.06
1.58
1.63
k
k
A ꢁ 165
A
n? ꢁ 15
2
671, 413,
292, 277
1527, 1381
534, 439,
354, 283
g ¼ 2.07
k
A ꢁ 160
k
A
N? ꢁ 15
2.4.2. [Cu2(L2)2(k-O2C–CH3)2] (2). This complex was obtained by similar method as
reported for 2. Yield ca ꢁ 45%. Elemental analysis for C30H26Br2O6N4Cu2: Found (%):
C, 44.28; H, 2.97; N, 6.61; Cu, 16.22. Calcd (%): C, 43.65; H, 3.18; N, 6.79; Cu, 15.40
(scheme 4).
3. Results and discussion
3.1. Electronic spectra
Spectroscopic data for all compounds are presented in table 1. Electronic spectra of both
complexes were obtained from a solid sample using diffuse reflectance and are very
similar, illustrating similar geometry. The compounds show a broad band at 667 nm for
1 and 671 for 2 due to ligand field transition for a CuN2O3 chromophore [23, 24].
An absorption band at 418 nm for 1 and 413 nm for 2 are assigned to charge transfer from
oxygen to dx2ꢀy2 orbital of copper(II) [23, 25]. The last two absorptions at 295 and 275 nm
for both 1 and 2 are associated with ꢀ ! ꢀ* or n ! ꢀ* transitions of the ligand [26].
3.2. Infrared spectra
The infrared spectra of acetate and Cu2N2O2 are of particular interest [27–34]. All
bands present in the free ligand are also observed in the spectra of the complexes.

350
F. Hasanvand et al.
Figure 1. Frozen solution (77 K) ESR X-band for complex 1 in DMF.
*Denotes (diphenylpicrylhydrazine) DPPH marker (g ¼ 2.0036).
Based on data from earlier reports [35, 36], we assign bands at 549 and 464 cmꢀ1 and at
534 and 439 cmꢀ1 (for 1 and 2, respectively) to Cu–O vibrations. The bands at 349 and
279 cmꢀ1 and at 354 and 283 cmꢀ1 (for 1 and 2, respectively) are assigned to Cu–N
vibrations: however, these vibrations are likely coupled Cu–N and Cu–O vibrations.
These data indicate L1 is closer to copper(II) than L2 due to the steric effect of methyl
ortho to nitrogen in L2. The bridging acetate in 1 has higher Cu–O vibration energy in
2 consistent with shorter Cu–Cu distance in 1 than in 2.
3.3. Magnetic moment and ESR
Both complexes are dimeric with room temperature magnetic moments of 1.58 and 1.63
B.M. per copper for 1 and 2, respectively, low for a d9 configuration from spin–spin
interaction through the bridging ligands [37]. The spin–spin interaction in 1 is higher
than in 2, in agreement with data from IR spectra [8]. To provide further evidence for
antiferromagnetic interaction, X-band powder and frozen solution ESR spectra were
obtained at liquid nitrogen and at room temperatures. In the solid state, the spectra
were similar and ESR silent with only a weak signal due to monomeric impurity
(g ¼ 2.06). Frozen solution ESR spectra of both compounds is typical for a dinuclear
copper(II) complex. A weak Dm ¼ 2 transition for both compounds confirms Cu ꢂ ꢂ ꢂ Cu
magnetic exchange interaction at half-field (1552 G for 1 and 1558 G for 2), which is
characteristic of a dimer [2, 38, 39].
The g values of 2.06 and 2.07 (for 1 and 2, respectively) with A ꢁ 162 G is in
k
k
agreement with a dx2ꢀy2 ground state and a square-pyramidal geometry [40–43]. The AN
value of ꢁ15 G and the presence of seven peaks for nitrogen superhyperfine structure
for the complexes agrees with two N donors per copper(II) (see figure 1 and
Supplementary material).
4. Conclusion
Two new bis-(m-alkoxo)-bridged dinuclear copper(II) complexes with 4-bromo-2-((4-
methylpyridine-2-ylimino)methyl)phenol or 4-bromo-2-((6-methylpyridine-2-ylimino)-
methyl)phenol as the ligands have been made in a one-step synthesis and

Acetato-bridged dinuclear copper(II)
351
spectroscopically characterized. Both complexes show subnormal magnetic moments
between 1.58 and 1.63 B.M. per copper at room temperature, suggesting an
antiferromagnetic spin-exchange interaction within each molecule. Data from IR
spectroscopy show that L1 is closer to copper(II) than L2 due to the steric effect of
methyl; the IR for bridging acetate is in agreement with the magnetic moments. The
X-band ESR spectra of 1 and 2 with g ¼ 2.06 and A ꢁ 163 G are typical of dimeric
k
k
square-pyramidal copper(II) complexes.
Acknowledgements
The authors would like to thank the Research Council of Arak University for financial
support of this research. The authors also appreciate the Chemistry Institute and
DAAD Organization in Chemnitz University in Germany for the services, facilities, and
physical measurements.
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