Structure and Magnetic Properties of an Oxalic Acid Bridged Dinuclear Copper(II) Complex

Article abstract of DOI:10.1515/znb-1998-1108

[LCu{μ-(OH)₂(C₂O₂)}CuL](ClO ₄)₂, (HL = N,N-dimethyl N-propylsalicylald-imine) was synthesised and its crystal structure was determined. C₂₆H₃₆Cl₂Cu₂N_4<

Full text of DOI:10.1515/znb-1998-1108

Structure and Magnetic Properties of an Oxalic Acid Bridged
Dinuclear Copper(II) Complex
Manas Kumar Sahaa, Sutapa Sena, Parimal Kundu+,a, Tarakranjan Guptaa,
Volker Gramlichb, Samiran Mitraa>*
a Department of Chemistry, Jadavpur University, Calcutta 700 032, India
b Laboratorium für Kristallographie ETH, Eidgenössische Technische Hochschule Zürich,
CH-8092 Zürich, Switzerland
Z. Naturforsch. 53 b, 1281-1284 (1998); received April 29, 1998
Oxalic Acid Bridge, Dimeric Complex, X-Ray Data, Antiferromagnetic Coupling
[LCu{ji-(0 H)2(C20 2)}CuL](C1 0 4 )2, (HL = N,N-dimethyl N-propylsalicylald-imine) was
synthesised and its crystal structure was determined. C26H36CI2CU2N4O14, triclinic space group
Pi with a = 9.288(9), b = 10.016(11), c = 10.09(2) Ä and a = 101.05(11), ß = 108.22(10),
7
= 110.22(10)°, V = 787(2) Ä3, Z = 2. Two copper(II) ions in a distorted square-planar
coordination are bridged by an oxalic acid molecule to form dinuclear units. The copper(II)
centres are separated by 5.2 A and antiferromagnetically coupled (J = -478 cm“ 1), which
follows from temperature-dependent magnetic susceptibility measurements in the range 12 to
300 K.
Introduction
of copper(II) complexes with an oxalic acid bridge
having a Schiff base as terminal ligand.
The most extensively studied dinuclear metal
compounds by far are dinuclear copper(II) com-
plexes, where super-exchange interactions occur
between two local doublet states of copper(II)
ions [1,2]. The plasticity of the coordination sphere
of copper(II) allows such a metal ion to adopt var-
ious stereochemistries and consequently different
orientations of the resulting magnetic orbitals of the
bridge [3].
Experimental
Preparation
The ligand HL (N,N-dimethyl N-propylsalicylald-
imine) was synthesised by refluxing 10 mmol of N,N-
dimethyl 1,3-diaminopropane with 10 mmol of salicy-
laldehyde in 30 ml of methanol. The resulting mixture
gave an orange solution containing the liquid ligand (HL)
and this was used as the starting material. 10 mmol
of Cu(C1 0 4 )2 -6 H20 was added in solid to a vigorously
stirred 30 ml methanol solution of HL (10 mmol). An
yellow-green coloured solid appeared after 5 min and was
recrystallised from 1:1 methanol-water yielding yellow-
green crystals. The elemental analysis suggests the com-
position [(HL)2Cu](C104)2. 10 mmol of this solid was
dissolved in 100 ml of water and treated with 5 mmol
of an aqueous solution (5 ml) of oxalic acid. On stand-
ing at room temperature for 3 d black green crystals
were obtained. The crystals were thoroughly washed with
methanol and chloroform and air dried.
The influence of the different substituent on the
superexchange mechanism of dinuclear copper(II)
oxalato complexes were studied [3,4]. The topology
regarding the planarity of the two magnetic orbitals
with the bridging plane and the delocalization of the
magnetic orbitals towards the bis bidentate bridges
gives the two key factors influencing the exchange
interaction [2]. A good number of dinuclear oxalato
bridged complexes were investigated [3 - 13], but
to our knowledge an oxalic acid bridged copper(II)
dimer is still unknown. The present work provides
the result of an X-ray structure analysis and of a
temperature-dependent susceptibility measurement
Analysis for C26H16CI2CU2N4O14
Calcd C 37.7 H 4.3 N 6.8 Cu 15.4 C104 31.8 %,
Found C 37.9 H 4.2 N 6.5 Cu 15.6 C104 31.5 %.
Susceptibility measurements
* Reprint requests to Dr. S. Mitra.
Magnetic susceptibility measurements of the powdered
sample were performed on a Faraday-type magnetometer
+ Present address: Department of Chemistry,
Ramananda College. Bishnupur, India.
0932-0776/98/1100-1281 $ 06.00 (c) 1998 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
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1282
M. K. Saha et al. ■An O xalic Acid Bridged Dinuclear Copper(II) Com plex
Table I. Crystal data and structure refinement of [LCu{/x-
Table II. Selected bond lengths (A) and bond angles (°)
(0H)2(C20 2)}CuL](C104)2.
in [LCu{^-(0H)2(C202)}CuL](C104)2.
Empirical formula
Formula weight
Temperature
CnHisClCuNiOv
413.28
293(2)K
Cu-0(1)
Cu-0(2)
Cu-Cu#
1.867(4)A
1.997(5)A
5.20 A
Cu-N(8)
Cu-0(3)
1.947(5) A
1.991(4) A
Wavelength
Crystal system
Space group
Unit cell dimensions
0.71073 Ä
Triclinic
PI
a = 9.288(9) A
b= 10.016(11) A
c = 10.09(2) A
q = 101.05(11)°
[3= 108.22(10)°
0(1)-Cu-N(8)
N(8)-Cu-0(3)
N(8)-Cu-0(2)
95.5(2)
94.1(2)
173.07(14) 0(3)-Cu-0(2)
170.37(14)
86.9(2)
83.5(2)
0(l)-C u-0(3)
0(l)-C u-0(2)
126.1(4)
116.3(6)
111.4(3)
0(3)#-C-0(2)
0(2)-C-C#
C-0(2)-Cu
0(3)#-C-C#
C(l)-0(1)-Cu
C-0(3)-C u
117.6(6)
128.1(3)
110.9(3)
Symmetry transformations used to generate equivalent
atoms: # -x, -y, -z.
Volume
787(2) A3
Z
2
effects. The structure was solved by direct methods using
the SHELXTL PLUS [16] system and refined by afull ma-
trix least squares calculation. Selected bond lengths and
angles of the title compound are summarized in Table II.
Density(calculated)
Absorption coefficient
F(000)
Crystal size
9 Range for data collection
Index range
1.744 mg/nr
1.597 mm“ 1
424
0.5x0.3x0.3 mm
2.26 to 20.03°
-7 < h < 6 , -9 < k < 9,
0 < / <9
Discussion
Reflection collected
Independent reflections
Absorption correction
Max. and min transmission 0.6784 and 0.6156
Refinement method
1168
X-ray structure
1168 [/?(int) = 0.0000]
Integration
The molecule is made up of centrosymmetric
binuclear cations and uncoordinated C10 4 ~ anions.
Fig. 1 gives a perspective view of the cation and
Fig. 2 represents the crystal packing.
The copper(II) ions are bridged by an oxalic acid
molecule. Each copper(II) ion in a distorted square
planar coordination is surrounded by three oxygen
and one nitrogen atoms. The Cu-O(L) and Cu-N(8)
bond lengths are 1.867 and 1.947 A respectively.
Full-matrix least-squares
on F2
1168/0/223
1.056
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [ I > 2<r(I)] R1 =0.0282, wR2 = 0.0637
R indices (all data)
Extinction coefficient
R1 =0.0282, wR2 = 0.0637
0.005(4)
0.481 and -0.326 e.A~3
Largest diff. peak and hole
consisting of a microbalance, equipped with an helium
continuous flow cryostat. The applied magnetic field was
~
1.2 T. The experimental susceptibility data were cor-
rected for the underlying diamagnetism [14].
X-ray structure determination
A deep green crystal having 0.5 x 0.3 x 0.3 mm3 di-
mension was put on a Syntex P2i diffractometer, which
was then used for the determination of lattice parame-
ters, space group and data collection [15] using graphite
monochromatised MoKQradiation. The lattice constants
were determined by least squares refinements of the an-
gular setting of 25 reflections near 9 value 10°. The crys-
tallographic data are listed in Table I. The stability of the
crystal was checked by measuring standard reflections at
fixed intervals during the data collection. However, no
significant loss of intensity was noted for the crystal. The
data were always corrected for Lorentz and polarisation
Fig. 1. Perspective view of the cation of
[LCu{a/-(0H)2(C20 2)}CuL](C104)2.
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M. K. Saha et al. • An O xalic A cid Bridged Dinuclear Copper(II) Com plex
1283
Table III. Some magnetic and structrual data for similar
compounds.
] Ref.
1
J [cm
[°]
Complexes
hM[A] a [ ° ]
7
4.8
4.4
0
5.3
4.7
8.4 -385
3.2 -385
8.8 -349
-402
[3, 17]
(tmen)2Cu2(ox) 0.18
[13, 17]
[11,17]
[8, 17]
0.10
(bipy)2Cu2(ox)
(bzpm)2Cu2(ox) 0.19
0.24
13.9
(mpz)2Cu2(ox)
-418
This work
3.9
0.06
(L)2Cu2(oxH2)
tmen = trtramethylethylene diamine, bipy = 2,2'-bi-
pyridine, bzpm = bromozepam, mpz = mepirizole, ox =
oxalato anion, 0 XH2 = oxalic acid.
Fig. 3. Experimental (■) and calculated (— ) temperature
dependence of \ mT for
[LCu{At-(0H)2(C202)}CuL](C104)2.
Magnetic properties
Magnetic susceptibility measurement for a pow-
dered sample of the complex was performed by the
Faraday method in the temperature range 12-300
K. Experimental data were corrected for the un-
derlying diamagnetism. The data were fitted to the
Bleaney-Bowers expression
(1 —p) + Ng2d2 p + Na ,
Ngzß z 2ex
=
\
m
Fig. 2. Crystal packing of
[LCu{^-(0H)2(C202)}C uL](C104)2.
kT 1 + 3ex
2kT
where x = J/kT, using the isotropic exchange
Hamiltonian (H = -JS 1S2) for two interacting
S = V2 centres. J is the singlet-triplet energy
gap arising from the intramolecular interaction
and p is the molar fraction of monomeric impu-
rity. NQ(6010_6cm3/mol for each copper atom) is
the temperature-independent paramagnetism. The
least-squares fit of the data displayed in Fig. 3 leads
to J = -478 cm- 1, g = 2.11 and p - 1.20%.
The bond lengths of the copper atom to the oxygen
atoms of the oxalic acid molecule are close to 1.99
A. The C-O bond lengths of the oxalic acid bridge
range from 1.250(6) to 1.252(6) A and do not dif-
fer significantly. The length of the C- C bond in
C2Ü2(OH)2 is of a typical single bond 1.522 A. The
distance between the two copper centres is 5.2 A
which is within the range of other reported similar
dinuclear copper(II) complexes [5-8].
All the oxalato-bridged dinuclear copper(II)
complexes with copper atoms in a tetragonal en-
vironment (/. e., square planar, square pyramidal or
Three distortion parameters (i) displacement of
the metal atoms out of the equatorial plane (Iim),
(ii) the twist angle (a), (iii) the angle between the pseudo octahedral) with a centrosymmetric struc-
oxalato plane and the equatorial plane (7 ) are impor- ture, so far have been found to be antiferromag-
tant for discussing the magnetic properties [17]. For netic [3, 5, 17]. It is a remarkable feature of their
magnetic behaviour that the magnitude of the cou-
the complex [LCu|/i-(OH)2(C2C>2)}CuL](ClC)4)2,
hjvi is only ± 0.06 A, a = 4.7(5)° and 7 = 3.9(3)°. pling constant appears to be only slightly sensitive
These values are in good agreement with other re- to the structural differences, with values of J be-
ported values of similar dinuclear oxalato bridged tween -330 and -402 cm-1 [17]. Related di- and
complexes (Table III).
trinuclear oxamato and oxamidato-bridged com-
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1284
M. K. Saha et al. ■An Oxalic Acid Bridged Dinuclear Copper(II) Com plex
plexes have also been synthesised and appear to
behave similarly [17 - 18]. The present complex
by oxygen atoms have a formal negative charge
and this hinders the migration of positive holes
[LCu{//-(0H)2(C20 2)}CuL](C104)2 shows a quite through the 7r-bond system and weakens antifer-
larger value of J = -^4-78 cm“ 1 though it has the
same range of distortions found in other related
compounds (Table III).
romagnetic interaction between copper atoms [20].
This formal negative charge has been removed in
this complex [LCu{/7-(0 H)2(C20 2)}CuL](C10 4)2,
where two copper(II) ions are neutrally bridged by
an oxalic acid molecule. Owing to this effect, the
magnetic interaction is presumed to operate more
Besides the structural factors, the magnitude of
the superexchange interaction depends to a great
extent on the electronic structure of the bridging
ligand [2-4], When paired copper atoms are sep-
arated from each other by three diamagnetic atoms
(for example Cu-O-C-O-Cu links), two paths are
conceivable for the migration of a positive hole,
one via 7r-electronic clouds and the other via a-
bonding electron orbitals of the O-C-O bonds [19].
The 7r-path can give rise to a strong antiferromag-
netic coupling whereas the cr-path leads to ferro-
magnetic interaction [19]. In the clusters of oxalato
bridged copper(II) dimers the positive holes can
migrate through the 7r-bond system of the Cu-O-
C-O-Cu links, giving rise to antiferromagnetism in
agreement with the sign of the exchange integrals.
The oxalato group coordinated to copper(II) ions
strongly in [LCu{/i-(0 H)2(C20 2)}CuL](C10 4 )2,
than in oxalato-bridged complexes.
Crystallographic data for the structure reported in
this paper has been deposited with the Cambridge
Crystallographic Data Centre. Copies of the data
CCDC no. 102092, can be obtained free of charge on
application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: int.code +(1223)
336-033, e-mail: heale@ccdc.cam.ac.uk).
Acknowledgements
This work was supported by UGC and CSIR New
Delhi. M. K. Saha is grateful to the UGC New Delhi for
financial assistance.
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