Synthesis, crystal structures, and magnetic properties of tetranuclear nickel(II) and copper(II) complexes with tridentate Schiff bases

Article abstract of DOI:10.1007/s11243-012-9661-9

Two cubane-type tetranuclear nickel(II) and copper(II) complexes, [Ni ₄(L¹)₄(CH₃OH)₄] (1) and [Cu₄(L²)₄]·H₂O (2), where L¹ and L² are

Full text of DOI:10.1007/s11243-012-9661-9

Transition Met Chem (2013) 38:63–68
DOI 10.1007/s11243-012-9661-9
Synthesis, crystal structures, and magnetic properties
of tetranuclear nickel(II) and copper(II) complexes with tridentate
Schiff bases
•
Shao-Song Qian Yue Zhao Miao-Miao Zhen
•
•
•
Cheng-Lu Zhang Zhong-Lu You Hai-Liang Zhu
•
Received: 20 September 2012 / Accepted: 15 October 2012 / Published online: 29 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Two cubane-type tetranuclear nickel(II) and cop-
per(II) complexes, [Ni4(L1)4(CH3OH)4] (1) and [Cu4(L2)4]Á
H2O (2), where L1 and L2 are the dianionic forms of the tri-
dentate Schiff bases 4-nitro-2-[(2-hydroxyethylimino)methyl]
phenol (H2L1) and 5-methoxy-2-[(2-hydroxyethylimino)
methyl]phenol (H2L2), respectively, have been synthesized and
characterized by physicochemical methods and single-crystal
X-ray diffraction. The magnetic properties of the complexes
show the presence of ferromagnetic interactions for complex 1
and antiferromagnetic interactions for complex 2, mediated by
hydroxyl bridges.
tridentate ligands, and a number of their bi-, tri-, tetra-, and
polynuclear complexes have been reported [1, 11–14].
However, only one chlorido-bridged dinuclear copper(II)
complex derived from the tridentate Schiff base 4-nitro-2-
[(2-hydroxyethylimino)methyl]phenol (H2L1; Scheme 1)
has been reported [15], and no complexes derived from the
tridentate Schiff base 5-methoxy-2-[(2-hydroxyethylimi-
no)methyl]phenol (H2L2; Scheme 1) have been reported so
far. As part of a detailed investigation into the coordination
chemistry of such Schiff bases, in this paper, two novel
cubane-type tetranuclear nickel(II) and copper(II) com-
plexes, [Ni4(L1)4(CH3OH)4] (1) and [Cu4(L2)4]ÁH2O (2),
are reported. The magnetic properties of the complexes
have been studied.
Introduction
Considerable attention has been focused on polynuclear
nickel and copper complexes due to their interesting
structures [1–3], as well as applications in magnetochem-
istry [4–7]. The most common method to construct poly-
nuclear complexes is the use of suitable bridging groups as
O2N
N
N
OH
OH
OH
H2L1
MeO
OH
H2L2
secondary ligands, including N3 , NCS–, N(CN)2–, dicar-
–
boxylate, 4,4’-bipy etc. [8–10]. In addition, choosing
ligands containing potential inner-bridging groups is also a
preferred way to construct polynuclear complexes. Schiff
bases derived from salicylaldehyde and 2-aminoethanol
and their derivatives represent an interesting class of
Scheme 1 The Schiff bases
Experimental
Materials and methods
5-Nitrosalicylaldehyde, 4-methoxysalicylaldehyde, and
2-aminoethanol were purchased from Aldrich and used as
received. All other reagents were of analytical grade. The
Schiff bases H2L1 and H2L2 were prepared according to the
method described in the literature [15]. Elemental analyses
(C, H, and N) were performed using a Perkin-Elmer 240
elemental analyzer. IR spectra were recorded on a JASCO
FT/IR-480 PLUS Fourier transform spectrophotometer
S.-S. Qian Á H.-L. Zhu (&)
School of Life Sciences, Shandong University of Technology,
ZiBo 255049, People’s Republic of China
e-mail: hailiang_zhu@163.com
Y. Zhao Á M.-M. Zhen Á C.-L. Zhang Á Z.-L. You (&)
Department of Chemistry and Chemical Engineering, Liaoning
Normal University, Dalian 116029, People’s Republic of China
e-mail: youzhonglu@yahoo.com.cn
123

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Transition Met Chem (2013) 38:63–68
with pressed KBr pellets in the range 200–4,000 cm-1
.
1
˚
Table 2 Selected bond lengths/A and angles/° for H2L and the
complexes
Temperature-dependent magnetic measurements were
carried out on a Quantum Design SQUID MPMS XL-5
magnetometer with an applied field of 1 K Oe, and dia-
magnetic corrections were made with Pascal’s constants.
1
Ni1-O3
1.987(2)
2.027(2)
2.131(2)
92.1(1)
83.5(1)
165.3(1)
94.8(1)
79.4(1)
95.1(1)
91.8(1)
165.0(1)
Ni1-N2
1.991(3)
2.037(2)
2.193(2)
171.5(1)
100.6(1)
84.8(1)
Ni1-O4
Ni1-O4A
Ni1-O4B
Ni1-O5
Synthesis of [Ni4(L1)4(CH3OH)4] (1)
O3-Ni1-N2
N2-Ni1-O4
N2-Ni1-O4A
O3-Ni1-O4B
O4-Ni1-O4B
O3-Ni1-O5
O4-Ni1-O5
O4B-Ni1-O5
2
O3-Ni1-O4
O3-Ni1-O4A
O4-Ni1-O4A
N2-Ni1-O4B
O4A-Ni1-O4B
N2-Ni1-O5
O4A-Ni1-O5
H2L1 (0.3 mmol, 63.0 mg), NaOH (0.6 mmol, 25.0 mg),
and NiCl2Á6H2O (0.3 mmol, 71.3 mg) were mixed in
methanol (15 mL). The mixture was stirred at room tem-
perature for 10 min and then transferred to a 25-mL stainless
steel bomb, which was sealed, heated at 150 °C for 12 h, and
cooled gradually to room temperature. Green block-shaped
single crystals of the complex were collected by filtration,
washed three times with methanol, and dried in air. Yield:
37 % based on H2L1. Calc. for C40H48N8Ni4O20: C, 40.2; H,
4.0; N, 9.4. Found: C, 40.0; H, 4.1; N, 9.3 %. Characteristic
IR data (cm-1): 3,272 (m), 1,617 (s), 1,581 (w), 1,523 (m),
1,507 (m), 1,445 (m), 1,392 (w), 1,329 (m), 1,296 (w), 1,221
(m), 1,173 (m), 1,153 (m), 1,072 (m), 837 (w), 631 (w), 535
(w), 501 (w), 437 (w), 410 (w).
104.3(1)
82.2(1)
86.6(1)
84.9(1)
Cu1-O1
1.896(5)
1.930(6)
2.615(6)
1.914(4)
1.949(5)
1.907(5)
1.954(4)
2.404(5)
1.914(6)
1.986(4)
93.8(2)
170.2(3)
84.3(2)
107.9(2)
69.0(2)
94.9(2)
170.7(3)
86.3(2)
110.7(2)
70.9(2)
93.5(2)
170.7(3)
83.8(2)
106.6(2)
76.7(2)
94.1(3)
85.0(3)
161.7(2)
93.8(2)
84.4(2)
Cu1-O2
1.953(5)
1.949(4)
1.886(5)
1.935(6)
2.723(6)
1.936(6)
1.988(5)
1.900(5)
1.952(5)
2.331(5)
93.3(2)
Cu1-N1
Cu1-O5
Cu1-O8
Cu2-O4
Cu2-O8
Cu2-N2
Cu2-O5
Cu2-O11
Cu3-O7
Cu3-N3
Cu3-O11
Cu3-O8
Synthesis of [Cu4(L2)4]ÁH2O (2)
Cu3-O2
Cu4-O10
Cu4-N4
Cu4-O11
H2L2 (0.1 mmol, 19.5 mg), Et3N (0.3 mmol, 30.3 mg), and
Cu(NO3)2Á3H2O (0.1 mmol, 24.2 mg) were mixed in
methanol (15 mL). The mixture was stirred at room tem-
perature for 30 min to give a deep blue solution. Upon
standing at room temperature, blue block-shaped crystals
of the complex, suitable for X-ray crystal structural
determination, were formed. The crystals were isolated
by filtration, washed three times with cold MeOH, and
dried in air. Yield: 51 % on the basis of H2L2. Calc. for
C40H46Cu4N4O13: C, 46.0; H, 4.4; N, 5.4. Found: C, 45.8;
H, 4.5; N, 5.3 %. Characteristic IR data (cm-1): 3,450 (m),
1,613 (s), 1,532 (m), 1,434 (m), 1,395 (w), 1,303 (w), 1,216
(m), 1,172 (m), 1,140 (m), 1,055 (m), 846 (w), 644 (w),
565 (w), 527 (w), 468 (w), 421 (w).
Cu4-O2
Cu4-O5
O1-Cu1-N1
N1-Cu1-O5
N1-Cu1-O2
O8-Cu1-O1
O8-Cu1-O5
O4-Cu2-O8
O8-Cu2-N2
O8-Cu2-O5
O11-Cu2-O4
O11-Cu2-O8
O7-Cu3-N3
N3-Cu3-O11
N3-Cu3-O8
O7-Cu3-O2
O11-Cu3-O2
O10-Cu4-N4
N4-Cu4-O11
N4-Cu4-O2
O10-Cu4-O5
O11-Cu4-O5
O1-Cu1-O5
O1-Cu1-O2
O5-Cu1-O2
O8-Cu1-O2
O8-Cu1-N1
O4-Cu2-N2
O4-Cu2-O5
N2-Cu2-O5
O11-Cu2-O5
O11-Cu2-N2
O7-Cu3-O11
O7-Cu3-O8
O11-Cu3-O8
N3-Cu3-O2
O8-Cu3-O2
O10-Cu4-O11
O10-Cu4-O2
O11-Cu4-O2
N4-Cu4-O5
O2-Cu4-O5
176.3(2)
88.2(2)
75.7(2)
115.0(2)
94.4(2)
174.9(2)
84.4(2)
74.4(2)
105.7(2)
92.7(2)
173.0(2)
89.2(2)
107.9(2)
80.4(2)
X-ray crystallography
177.3(2)
93.9(2)
Diffraction intensities for H2L1 and the complexes were
collected at 298(2) K using a Bruker APEX II CCD area
87.7(2)
˚
detector with Mo Ka radiation (k = 0.71073 A). The
118.3(2)
77.5(2)
collected data were reduced using the SAINT program
[16], and empirical absorption corrections were performed
using the SADABS program [17]. The structures were
solved by direct methods and refined against F2 by full-
matrix least-squares methods using the SHELXTL package
[18]. All of the non-hydrogen atoms were refined aniso-
tropically. The H1 hydrogen atom in the ligand and H5
atom in complex 1 have been found in difference Fourier
maps and their coordinates were refined freely. The
remaining hydrogen atoms were placed in calculated
positions and constrained to ride on their parent atoms. The
lattice water molecule in 2 is disordered; hence the
hydrogen atoms were not added. The crystallographic data
123

Transition Met Chem (2013) 38:63–68
65
Table 1 Crystallographic data for H2L1 and the complexes
Compounds
H2L1
1
2
Empirical formula
Formula weight
Temperature/K
C9H10N2O4
210.2
C40H48N8Ni4O20 C40H46Cu4N4O13
1,195.7
298(2)
1,045.0
298(2)
298(2)
˚
Wavelength/A
0.71073
0.71073
0.71073
Triclinic
Crystal system
Space group
Orthorhombic Tetragonal
ꢀ
P 1
Pbca
I41/a
˚
a/A
8.666(1)
9.690(2)
23.498(4)
90
13.649(2)
13.649(2)
25.867(4)
90
12.783(1)
13.231(2)
14.263(3)
75.458(2)
85.104(3)
78.534(2)
2,286.8(6)
2
˚
b/A
˚
c/A
a/°
b/°
c/°
Fig. 1 A perspective view of the molecular structure of H2L1 with
the atom-labeling scheme. Thermal ellipsoids are drawn at the 30 %
probability level. Intramolecular O–HÁÁÁN hydrogen bond is shown as
a dashed line
90
90
90
90
3
˚
V/A
1,973.1(5)
8
4,819.1(13)
4
Z
l/mm-1
Dc/g cm-3
0.113
1.415
10,269
2,044
1,561
1.625
1.648
5,561
2,224
1,730
1.899
in methanol at a concentration of 10-3 M are 35 and 26
X-1 cm2 mol-1, respectively, indicating that both are non-
electrolytes [19].
1.518
Reflections collected
Unique reflections
16,992
9,660
Observed reflections
4,358
X-ray crystal structures
[I C 2r(I)]
F(000)
880
2,464
1,068
The molecular structure of H2L1 is shown in Fig. 1. There
is an intramolecular O–HÁÁÁN hydrogen bond in the com-
pound, giving rise to an S(6) ring motif [20]. The dihedral
angle between the nitro plane and the benzene ring is
6.4(3)°, while the torsion angle N2-C8-C9-O2 is 66.0(3)°.
In the crystal structure of the compound, molecules are
linked through intermolecular O–HÁÁÁO hydrogen bonds to
form layers parallel to the ab plane (Fig. 2).
Rint
0.0258
1.079
0.0282
1.006
0.0538
1.054
Goodness of fit on F2
R1 [I C 2r(I)]
wR2 [I C 2r(I)]
R1 (all data)
wR2 (all data)
0.0458
0.1340
0.0601
0.1460
0.0333
0.0918
0.0471
0.0982
0.0597
0.1548
0.1615
0.2071
The molecular structure of complex 1 is shown in Fig. 3.
This complex possesses a [Ni4(l3-O)4] cubane core with
twofold rotation axis symmetry. The NiÁÁÁNi distances are
for the complex are summarized in Table 1. Selected bond
lengths and angles are summarized in Table 2.
˚
3.046(1) and 3.119(1) A. Each Ni atom is octahedrally
coordinated, with the three donor atoms of the Schiff base
ligand and a hydroxyl O atom of a different Schiff base
ligand defining the equatorial plane, plus the hydroxyl O
atom of another Schiff base ligand and one methanol O
atom occupying the two axial positions. Hence, the Schiff
base ligand acts not only as a chelating but also as a
bridging ligand, whereby the hydroxyl atoms in the ligands
act as l3-O bridges connecting three neighboring Ni atoms.
The Ni atom deviates from the least-squares plane defined
Results and discussion
The Schiff bases H2L1 and H2L2 were readily prepared by
the reactions of 2-aminoethanol with 5-nitrosalicylalde-
hyde and 4-methoxysalicylaldehyde, respectively, in
methanol. Single crystals of H2L1 suitable for X-ray dif-
fraction were obtained by slow evaporation of a methanolic
solution of the compound. Only micro-crystals of H2L2
could be obtained from common organic solvents. The
nickel complex was prepared by the reaction of H2L1,
sodium hydroxide, and nickel chloride in methanol under
solvothermal conditions, while the copper complex was
prepared by the reaction of H2L2, Et3N, and copper nitrate
in methanol under ambient conditions. The Schiff bases
and their complexes are stable in air and soluble in com-
mon polar organic solvents such as DMSO, DMF, metha-
nol, ethanol, and acetonitrile, but insoluble in water. The
molar conductance values for complexes 1 and 2 measured
˚
by the four equatorial donor atoms by 0.011(2) A. Dis-
tortion of the octahedral coordination can be observed from
the coordinate bond angles, which range from 82.2(1) to
104.3(1)° for the perpendicular angles and from 165.0(1) to
171.5(1)° for the trans angles. The coordinate bond lengths
in the complex are comparable to those observed in other
Schiff base nickel(II) complexes with octahedral coordi-
nation [21]. The six-coordinate Ni atoms give rise to the
cubane structure. Four Ni atoms are bridged by oxygen
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Transition Met Chem (2013) 38:63–68
atoms of the Schiff base ligands to form approximately
cubic arrays of alternating Ni and O atoms. The torsion
angle N2-C8-C9-O4 is 45.4(3)°. There are four intramo-
lecular O–HÁÁÁO hydrogen bonds among the methanol
ligands and the phenolate groups.
to those observed in Schiff base copper(II) complexes with
octahedral coordination [24, 25]. The five-coordinate Cu
atoms give rise to the cubane structure, which is similar to
1. The Cu atoms are bridged by oxygen atoms of the Schiff
base ligands to form approximately cubic arrays of alter-
nating Cu and O atoms.
The molecular structure of complex 2 is shown in Fig. 4.
The complex possesses a [Cu4(l3-O)4] cubane core. The
˚
CuÁÁÁCu distances are in the range of 3.012(1)–3.489(1) A.
Spectroscopic and magnetic properties
Each Cu atom has square pyramidal coordination, with the
three donor atoms of the Schiff base ligand and the
hydroxyl O atom of a different Schiff base ligand defining
the basal plane, plus one hydroxyl O atom of another Schiff
base ligand in the apical position. Thus, this Schiff base
also acts not only as a chelating but also as a bridging
ligand, whereby the hydroxyl atoms in the ligands act as
l3-O bridges connecting three neighboring Cu atoms. The
Cu1, Cu2, Cu3, and Cu4 atoms deviate from the least-
squares planes defined by the corresponding four basal
donor atoms by 0.083(2), 0.039(2), 0.111(2), and 0.120(2)
The IR spectra of the Schiff bases and their complexes pro-
vide information about the metal–ligand bonding. The weak
and broad absorptions in the region 3,250–3,450 cm–1 are
assigned to the O–H groups. The strong absorption bands at
1,656 cm–1 for H2L1 and 1,645 cm–1 for H2L2 are assigned
to the azomethine groups, m(C=N). These bands are shifted to
lower wavenumbers in the spectra of the complexes
(1,617 cm–1 for 1 and 1,613 cm–1 for 2), attributed to coor-
dination of the azomethine nitrogen atoms to the metal
atoms. The strong absorption band of the Ar–O bond for free
H2L at 1,245 cm–1 is located at lower wavenumbers for the
complexes (1,207 cm–1 for 1 and 1,216 cm–1 for 2). Weak
absorption bands indicative of the M–O and M–N bonds are
observed in the region 300–650 cm–1.
˚
A, respectively. For Cu1 and Cu2 coordination, the apical
˚
bonds are much longer than usual values of 1.9–2.4 A [14,
22, 23], indicating weak coordination of O8 and O11. The
distortion of the square pyramidal coordination can be
observed from the coordinate bond angles between the
apical and basal donor atoms, which range from 69.0(2) to
115.0(2)° for Cu1, 70.9(2) to 110.7(2)° for Cu2, 76.7(2)
to 107.9(2)° for Cu3, and 77.5(2) to 118.2(2)° for Cu4.
The coordinate bond lengths in this complex are comparable
The temperature dependence of the magnetic suscepti-
bility of the complexes was measured on polycrystalline
samples between 2 and 300 K under a field of 1 K Oe. For
complex 1, on decreasing the temperature, vT increases
gradually from the room temperature value of 6.005 cm3
Fig. 2 Molecular packing of
H2L viewed along the a axis.
Hydrogen bonds are shown as
thin dashed lines
123

Transition Met Chem (2013) 38:63–68
67
Fig. 4 A perspective view of the molecular structure of 2 with the
atom-labeling scheme. The thermal ellipsoids are drawn at the 30 %
probability level. Water oxygen and hydrogen atoms have been
omitted for clarity
Fig. 3 A perspective view of the molecular structure of 1 with the
atom-labeling scheme. Thermal ellipsoids are drawn at the 30 %
probability level. Intramolecular hydrogen bonds are shown as thin
dashed lines. The hydrogen atoms not related to the hydrogen
bonding have been omitted for clarity
30
25
20
15
10
5
50
40
30
20
10
0
mol-1 K, which is significantly higher than that calculated
for four magnetically interacting high-spin NiII centers,
value of 4.00 cm3 mol-1 K (g = 2, S = 1) (Fig. 5). This is
because the high-spin NiII (S = 1) centers are expected to
exhibit orbital contributions to the magnetic moment,
affording g values that deviate significantly from 2.0 [26].
At room temperature, vT increases with decreasing tem-
perature, reaching a maximum 26.06 cm3 mol-1 K around
3.0 K, and further decreases to 25.650 cm3 mol-1 K at
2 K. This behavior indicates the dominant ferromagnetic
interaction. The magnetic susceptibility data above 50 K
were fitted by the Curie–Weiss law, which gave a Curie
constant of 5.685 cm3 mol-1 K and a Weiss temperature of
9.79 K; the positive value of the Weiss temperature further
confirms the dominant ferromagnetic interactions in the
studied temperature range.
0
50
100 150 200 250 300
Temperature, K
Fig. 5 Thermal variation of magnetic susceptibility, vT versus T
(filled circle) and v-1 versus T (square) of 1 under an applied field
of 1 K Oe in the temperature range of 2–300 K
significantly from 2.0. The value of vT decreases contin-
uously on cooling, reaching 1.012 cm3 mol-1 K at 2.0 K.
This behavior indicates the dominant antiferromagnetic
interaction. The magnetic susceptibility data above 50 K
were fitted by the Curie–Weiss law, which gave a Curie
constant of 1.761 cm3 mol-1 K and a Weiss temperature
of -21.40 K. The negative Weiss temperature further
confirms the dominant antiferromagnetic interactions in the
studied temperature range.
For complex 2, on decreasing the temperature, vT
increases gradually from the room temperature value of
1.659 cm3 mol-1 K, which is significantly higher than
that calculated for four magnetically interacting high-spin
CuII centers, value of 1.50 cm3 mol-1 K (g = 2, S = 1/2)
(Fig. 6). This is because the high-spin CuII (S = 3/2)
centers are expected to exhibit orbital contributions to the
magnetic moment and afford g values that deviate
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Transition Met Chem (2013) 38:63–68
200
160
120
80
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CCDC—856174 for H2L, 856175 for 1, and 890710 for 2
contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge at http://
www.ccdc.cam.ac.uk/const/retrieving.html or from the Cam-
bridge Crystallographic Data Centre (CCDC), 12 Union
Road, Cambridge CB2 1EZ, UK; fax: ?44(0)1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk.
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Acknowledgments This work was financially supported by the
Distinguished Young Scholars Program of Higher Education of Lia-
oning Province (Grant No. LJQ2011114).
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