Synthesis, structure and magnetic properties of a series of cubane-like clusters derived from Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2014.03.040

One new hexanuclear Cu(II) complex [Cu₄(L¹)₄Na₂(ClO₄)(H₂O)](ClO₄)(H₂O)_0.5 (1) and two new tetranuclear Ni(II) complexes [Ni₄(L²)₄

Full text of DOI:10.1016/j.poly.2014.03.040

Polyhedron xxx (2014) xxx–xxx
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Polyhedron
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Synthesis, structure and magnetic properties of a series of cubane-like
clusters derived from Schiff base ligands
⇑
Xiaoting Qin, Shuai Ding, Xuebin Xu, Rui Wang, Ye Song, Yanqin Wang, Chun-fang Du, Zhi-liang Liu
Key Lab of Nanoscience and Nanotechnology, College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, PR China
a r t i c l e i n f o
a b s t r a c t
One new hexanuclear Cu(II) complex [Cu4(L1)4Na2(ClO4)(H2O)](ClO4)(H2O)0.5 (1) and two new tetranu-
clear Ni(II) complexes [Ni4(L2)4(CH3OH)4] (2), [Ni4(o-vanillinate)4(
3-N3)4(CH3OH)4] (3) have been syn-
thesized and structurally characterized. All the three complexes 1, 2, and 3 have been characterized by
elemental analysis, FTIR, and single crystal X-ray diffraction studies. Single crystal X-ray diffraction stud-
ies reveal that all three complexes consist of a cubane core in which the four Cu(II)/Ni(II) centers in 1 and
Article history:
Received 25 January 2014
Accepted 22 March 2014
Available online xxxx
l
Keywords:
Compounds
Cubane
Schiff base
Magnetic properties
Crystal structures
2 are linked by l3-alkoxo bridges and Ni(II) ions in 3 are linked by azido bridge in end-on fashion. The
skeleton structures of the cubane cores are all distorted from the idealized cube and most show a further
distortion. The variable-temperature (2–300 K) magnetic susceptibilities and magnetizations of the three
compounds were measured. Variable-temperature magnetic susceptibility data exhibit dominant antifer-
romagnetic interaction in complex 1 while ferromagnetic interactions in complexes 2 and 3.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
[27–30]. M4O4 units show interesting magnetic exchange properties
and may under certain circumstances act as single molecular mag-
To design coordination complexes of copper(II) and nickel(II) is
now ubiquitous in the field of condensed matter physics and mate-
rial chemistry [1–4]. In the family of above mentioned complexes,
cubane-like clusters are amazing metalloclusters that have
received great attention during the past decade, for the potential
bioinorganic modeling, catalysis and hydrogenases [5,6]. Several
nets [31]. The diverse structures and interesting properties observed
in these compounds attract us to do some further research about
these interesting ligands. Indeed, ligand H2L1 has previously been
utilized in the assembly of the mononuclear zinc complex and the
binuclear nickel complex [32,33]. Ligand H2L2 has been explored
to construct a mononuclear titanium complex [34] and a dinuclear
nickel complex [35]. Ligand H2L3 has been reported to construct
3d transition metal clusters with various core structures, such as
Co1, Ni2, Cu4, Ni4 [36–41]. A Cambridge Structural Database search
reveals that so far few of cubane-like clusters, however, using these
Schiff base ligands, have been known and little comparative
discussed. In order to enrich the family of cubane-like clusters, we
have successfully obtained three new cubane-like clusters, namely,
[Cu4(L1)4Na2(ClO4)(H2O)](ClO4)(H2O)0.5 (1), [Ni4(L2)4(CH3OH)4] (2),
cubane-like Cu(II)/Ni(II) complexes containing
l3-O bridges have
been extensively studied, mainly stimulated by their significant
contribution to the field of molecular magnetism [7–19]. In terms
of Ni(II) cubane complexes bridged by end-on bridging azides
which possess similar [Ni4(g1
,l3-N3)4] cubane cores, only three
examples have been reported [20–23].
Recently, a series of Schiff base ligands as building blocks have
been used to synthesize the cubane-like 3d transition metal clusters
and most of them provide alkoxo-bridges between metallic centers.
The ligands containing three and four potential donor groups,
such as 2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol
(H2L1), 2-((1-hydroxybutan-2-ylimino)methyl)phenol (H2L2) and
2-((2-hydroxyethylimino)methyl)-6-methoxyphenol (H2L3, see
Scheme 1), consequently, were considered as excellent organic
blocking ligands because of their various coordination modes, such
as M4 arrays, which exhibit a large diversity of structure types:
square planar, [24] face-to-face, [25] cyclic [26] and cubane type
[Ni4(o-vanillinate)4(
magnetic susceptibility data exhibit antiferromagnetic interaction
in complex while dominant ferromagnetic interactions in
l3-N3)4(CH3OH)4] (3). Variable-temperature
1
complexes 2 and 3 exist in the cubane core, and magnetostructural
correlations were discussed furthermore.
2. Material and methods
2.1. Materials
⇑
All reagents and solvents were of high-purity analytical grade
and used as received from commercial supplies without further
Corresponding author. Tel.: +86 471 4995414; fax: +86 471 4992147.
E-mail address: cezlliu@imu.edu.cn (Z.-l. Liu).
http://dx.doi.org/10.1016/j.poly.2014.03.040
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
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2
X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
was measured in the temperature range of 2–300 K using a
Quantum Design MPMS XL-5 SQUID magnetometer equipped with
5 T. According to Pascal’s constants, the diamagnetic corrections
were made and magnetic data were corrected for diamagnetic
contributions of the sample holder.
N
N
N
OH
OH
OH
OH
OH
OH
H2L2
O
O
H2L1
H2L3
2.4. X-ray crystallography
Scheme 1. The structure of the Schiff base ligands (H2L1, H2L2 and H2L3).
Single crystals of the complexes were selected and mounted on
a Bruker ApexII CCD diffractometer with graphite-monochromated
Mo K
a
radiation (k = 0.71073 Å), operating in
x
ꢁ2h scanning mode
purification. Ethanolamine, 2-amin-1-butanol, salicylaldehyde,
o-vanillin, Cu(ClO4)2ꢀ6H2O, Ni(ClO4)2ꢀ6H2O and all the solvents
were purchased from Aladdin and Alfa Aesar, respectively. The
Schiff base ligands H2L1, H2L2 and H2L3 were synthesized according
to literature procedures [33,37,42].
using suitable crystals for data collection. Lorentz-polarization
correction was applied to the data. The structure was solved by
direct methods (SHELX-97) and refined by full-matrix least-squares
procedures on F2 using SHELX-97 [43]. Hydrogen atoms were added
theoretically and refined with riding model position parameters
and fixed isotropic thermal parameters. Experimental details for
the structural determinations are summarized in Table 1.
2.2. Synthesis of complex
2.2.1. [Cu4(L1)4Na2(ClO4)(H2O)](ClO4)(H2O)0.5 (1)
3. Results and discussion
Cu(ClO4)2ꢀ6H2O (0.37 g, 1 mmol) was added to a solution of
H2L1 (1 mmol) and NaOH (0.08 g, 2 mmol) in 15 ml methanol/
dichloromethane (1:2 v/v), the resulting filtered solution was left
unperturbed to allow a slow evaporation of the solvent. After
3 days, dark blue block-shaped crystals, suitable for X-ray diffrac-
tion analysis, were obtained. Yield: 0.056 g (16%). Anal. Calc. for
3.1. Syntheses and IR spectra of the complexes
The ligands were synthesized by condensation of 2-hydroxy-
benzaldehyde, or its derivative o-vanillin, with corresponding
amino alcohol under refluxing in the methanol solution. The
resulting orange yellow solution containing the required product
was used without further purification. Suitable crystals of hexanu-
clear complex [Cu4(L1) 4Na2(ClO4)(H2O)](ClO4)(H2O)0.5 (1) and tet-
ranuclear complex [Ni4(L2)4(CH3OH)4] (2) were synthesized from
corresponding Schiff base ligands (H2L1, H2L2) and M(ClO4)2ꢀ6H2O
(M = Cu2+, Ni2+) in methanol/dichloromethane (1:2 v/v) under
basic conditions to promote deprotonation and subsequent coordi-
nation of the phenoxyl and alkoxyl groups. For the reaction of
Ni(ClO4)2ꢀ6H2O and H2L3, slow diffusion in a H-tube tube at room
temperature of a methanolic solution of Ni(ClO4)2ꢀ6H2O into a
methanolic solution of H2L3 and NaN3 resulted in light green
block-shaped crystals of general formula [Ni4(o-vanillinate)4
C
48H63Cu4N4O21.5Cl2Na2: C, 40.89; H, 4.43; N, 3.97. Found: C,
40.66; H, 4.34; N, 3.85%. IR (KBr, cmꢁ1): 3418 (w), 2918 (m),
2850 (w), 1634 (s), 1550 (m), 1443 (s), 1306 (w), 1242 (w), 1170
(s), 1104 (w), 1014 (w), 975 (w), 848 (m), 785 (s), 622 (w).
2.2.2. [Ni4(L2)4(CH3OH)4] (2)
Ni(ClO4)2ꢀ6H2O (0.366 g, 1 mmol) was added to a solution of
H2L2 (1 mmol) and Et3N (0.202 g, 2 mmol) in 15 ml methanol/
dichloromethane (1:2 v/v), the resulting filtered solution was left
unperturbed to allow a slow evaporation of the solvent. After
3 days, green block-shaped crystals, suitable for X-ray diffraction
analysis, were obtained. Yield: 0.08 g (28%). Anal. Calc. for C48H68
Ni4N4O12: C, 51.11; H, 6.07; N, 4.98. Found: C, 51.50; H, 6.38; N,
4.37%. IR (KBr, cmꢁ1): 3641 (w), 2931 (m), 2859 (w), 1632 (s),
1599 (m), 1447 (s), 1391 (w), 1336 (m), 1187 (m), 1061 (m), 942
(w), 907 (m), 758 (s), 651 (w), 521 (m).
(
g1
,l3-N3)4(CH3OH)4]. However, it is noteworthy that, according
to X-ray crystallographic analysis, o-vanillin coordinates to nickel
ion instead of ligand H2L3. This may be attributed to the Schiff base
ligand H2L3 decomposed during the assemble process.
The structures of 1–3 were characterized by infrared (IR)
spectra, which showed valuable information about the counter
ions in their respective coordination environments. The peak at
1634 cmꢁ1 for 1, 1632 cmꢁ1 for 2 and 1670 cmꢁ1 for 3 can be rea-
sonably attributed to stretching vibrations of imino C@N and C@O
groups, respectively. In the IR spectra of 1, the strong peak at
1170 cmꢁ1 should be ascribed to the presence of perchlorate anion.
2.2.3. [Ni4(o-vanillinate)4(l3-N3)4(CH3OH)4] (3)
Ni(ClO4)2ꢀ6H2O (0.366 g, 1 mmol) was readily dissolved in
15 ml methanol, resulting in a green solution. Schiff base ligand
H2L3 (1 mmol) and NaN3 (0.13 g, 2 mmol) were dissolved in
15 ml methanol, resulting in a yellow solution. In a 50 ml H-shaped
tube, the filtered green solution and filtered yellow solution were
added dropwise to each branch of the tube, respectively. Additional
10 ml methanol was distributed and layered upon the mixture to
allow a slow diffuse of the solutes. After 2 months, light green
block-shaped crystals, suitable for X-ray diffraction analysis, were
obtained. Yield: 0.11 g (35%). Anal. Calc. for C36H44N12Ni4O16: C,
38.08; H, 3.91; N, 14.80. Found: C, 38.12; H, 3.93; N, 14.83%. IR
(KBr, cmꢁ1): 3632 (w), 2924 (w), 2861(w), 2087 (s), 1670(s),
1577(s), 1447 (s), 1338 (m), 1236 (w), 1089 (w), 942 (m), 847
(w), 748 (m).
For complex 3, the IR spectra exhibit strong abortion at 2087 cmꢁ1
,
which is due to the vibration of Nꢁ3 anion.
3.2. Structure description
3.2.1. Crystal structure of [Cu4(L1) Na2(ClO4)(H2O)](ClO4)(H2O)0.5 (1)
4
X-ray single crystal diffraction analysis reveals that complex 1
ꢀ
crystallizes in the triclinic space group P1. Each of the unit cells
comprises of one cationic complex unit [Cu4(L1)4Na2(ClO4)(H2O)]+,
one isolated ClOꢁ4 counterion and half a crystallization water. Per-
spective views of the cationic complex unit are depicted in Fig. 1.
Some selected bond lengths and angles are presented in Table 2.
The structure of the cationic complex [Cu4(L1)4Na2(ClO4)(H2O)]+
in complex 1 can be described as hexanuclear skeleton (Cu4O4Na2
O4), around which the four tetradentate Schiff base ligands (L1)2ꢁ
are oriented mutually staggered. The four deprotonated butanol
2.3. Physical measurements
Fourier transform infrared (FTIR) spectra (KBr disk) were mea-
sured with a Vertex 70 FTIR on a spectrophotometer (4000–
400 cmꢁ1). Elemental analyses for C, H and N were obtained from
a Perkin-Elmer 2400 elemental analyzer. Magnetic susceptibility
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X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
3
Table 1
Crystallographic data and structure refinement for complexes 1–3.
1
2
3
Empirical Formula
Formula weight
T (K)
C
48H63Cl2Cu4N4Na2O21.5
C48H68Ni4N4O12
1127.90
296(2)
C36H44N12Ni4O16
1135.67
296(2)
1410.10
293(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
monoclinic
P21/c
19.917(10)
15.218(8)
16.904(9)
90.00
90.46(10)
90.00
5123.4(5)
4
tetragonal
I41/a
22.347(19)
22.347(19)
9.536(17)
90.00
90.00
90.00
4761.9(10)
4
ꢀ
11.716(2)
14.681(3)
17.556(3)
95.61(3)
90.91(3)
101.42(3)
2943.8(9)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (g cm–3)
Absorption coefficient (mmꢁ1
F (000)
1.591
1.609
1444
1.462
1.510
2368
1.584
1.636
2336.0
)
h Range (°)
1.59 < h < 25
30470
10296
1.02 < h < 25.02
26038
9019
3.64 < h < 56.68
14528
2967
Reflections collected
Unique reflections
Rint
0.0543
1.063
0.0489
1.031
0.0328
Goodness-of-fit (GOF) on F2
1.072
Final R indices [I > 2
r(I)]
R
a = 0.0618
R
a = 0.0402,
R
a = 0.0461
wRb = 0.1654
R = 0.0717
wRb = 0.0863
R = 0.0652
wRb = 0.1313
R = 0.0781
R indices (all data)
wR = 0.1741
1.34 and ꢁ0.86
wR = 0.0980
0.723 and ꢁ0.098
wR = 0.1538
0.72 and ꢁ0.67
Largest residuals (e Åꢁ3
)
a
R =
wR = [
R
||F0| ꢁ |Fc||/
R|F0|.
b
R
w(F20 ꢁ F2c)2/
R .
(wF40)]1/2
Table 2
Selected bond distances (Å) and angles (°) for complex 1.
Bond lengths (Å)
Cu1–N4
Cu2–N1
Cu1–O3
Cu1–O5
Cu1–O6
Cu1–O9
Cu2–N1
Cu2–O6
Cu2–O8
Cu2–O9
Cu2–O12
1.919(4)
1.931(4)
1.991(3)
1.909(3)
1.995(3)
2.322(3)
1.931(4)
1.995(3)
1.907(3)
1.997(3)
2.288(3)
Cu3–N2
Cu4–N3
Cu3–O3
Cu3–O9
Cu3–O11
Cu3–O12
Cu4–N3
Cu4–O2
Cu4–O3
Cu4–O6
Cu4–O12
1.932(5)
1.931(4)
2.310(3)
1.986(3)
1.904(4)
1.972(3)
1.931(4)
1.896(3)
1.974(3)
2.264(3)
2.001(3)
Bond angles (°)
O5–Cu1–N4
O5–Cu1–O3
N4–Cu1–O6
O3–Cu1–O6
O5–Cu1–O9
N4–Cu1–O9
O3–Cu1–O9
O6–Cu1–O9
O8–Cu2–N1
O8–Cu2–O6
N1–Cu2–O9
O6–Cu2–O9
O8–Cu2–O12
N1–Cu2–O12
O6–Cu2–O12
O9–Cu2–O12
94.63(16)
98.05(14)
84.12(15)
85.66(13)
97.35(13)
120.21(16)
80.62(13)
77.85(12)
93.77(17)
96.22(14)
84.61(16)
86.08(13)
101.32(14)
118.79(16)
80.32(13)
77.67(13)
O11–Cu3–N2
N2–Cu3–O12
O11–Cu3–O9
O12–Cu3–O9
O11–Cu3–O3
N2–Cu3–O3
O12–Cu3–O3
O9–Cu3–O3
O2–Cu4–N3
N3–Cu4–O3
O2–Cu4–O12
O3–Cu4–O12
O2–Cu4–O6
N3–Cu4–O6
O3–Cu4–O6
O12–Cu4–O6
94.72(19)
85.00(18)
94.49(15)
85.87(14)
101.03(13)
114.21(17)
78.80(13)
81.03(13)
93.91(16)
85.94(16)
94.95(14)
86.79(13)
97.43(13)
120.46(16)
79.17(13)
80.79(13)
Fig. 1. Perspective views of the cationic complex unit [Cu4(L1)4Na2(ClO4)(H2O)]+.
Hydrogen atoms are omitted for clarity.
oxygens each acts as l3-O bridge connecting four neighboring cop-
per(II) ions to form a [Cu4O4] core exhibiting a cubic array of alter-
nating copper and oxygen atoms occupying the corners of the cube.
While on the two sides of the [Cu4O4] cubane, the two deproto-
nated phenol oxygens each bridge to Na+ cations with the methoxy
groups providing additional chelation of the Na+ cations, which are
further ligated by a water molecule or a perchlorate anion.
Within the [Cu4O4] cubane core, all copper(II) centers are five-
coordinate with a NO4 donor set from the Schiff base ligands. The
basal plane of the square-pyramid is consisted of the phenolate
oxygen atom, the imine nitrogen atom of the salicylidene fragment
atoms show longer Cu–O bond lengths range from 2.264 to
2.322 Å. The elongation of the Cu–O axial bonds is due to a
pseudo-Jahn–Teller distortion of the d9 copper(II) center.
and two
l3-alkoxide oxygen atoms, while the apical position is
occupied by another
l
3-alkoxide oxygen atom. Cu1, Cu2, Cu3 and
3.2.2. Crystal structure of [Ni4(L2)4(CH3OH)4] (2)
The X-ray single crystal diffraction study reveals that complex 2
crystallizes in the monoclinic space group P21/c. The molecular
structure of complex 2 is shown in Fig. 2, selected bond lengths
Cu4 are deviated from the corresponding mean planes range from
0.119 to 0.433 Å, towards the apical ligand atom. The Cu–O and
Cu–N bond lengths in the equatorial plane range from 1.896 to
2.001 Å and from 1.919 to 1.932 Å (see Table 2). The apical oxygen
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X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
Table 3
and angles are summarized in Table 3. The structure of complex 2
Selected bond distances (Å) and angles (°) for complex 2.
consists of a distorted [Ni4O4] cubane core with alternating nickel
ions and oxygen atoms occupying the corners of the cube. The l3
-
Bond lengths (Å)
Ni1–O6
Ni1–N3
Ni1–O3
Ni1–O4
Ni1–O1
Ni1–O10
Ni2–N4
Ni2–O4
Ni2–O2
Ni2–O1
Ni2–O5
Ni2–O9
1.982(2)
1.983(3)
2.027(2)
2.016(2)
2.129(2)
2.184(3)
1.985(3)
2.130(2)
2.051(2)
2.019(2)
1.987(2)
2.146(3)
Ni3–N2
Ni3–O12
Ni3–O2
Ni3–O3
Ni3–O1
Ni3–O8
Ni4–O2
Ni4–N1
Ni4–O3
Ni4–O4
Ni4–O7
Ni4–O11
1.989(3)
2.157(3)
2.129(2)
2.208(2)
2.043(2)
1.992(2)
2.020(3)
1.994(3)
2.115(2)
2.042(2)
1.980(2)
2.176(3)
oxygen bridges among the cubane come from the deprotonated
hydroxy oxygen atoms of Schiff base ligands (L2)2ꢁ, which were
coordinated to the central metal ions in a similar tri-chelating fash-
ion with one hydroxy oxygen atom, one phenoxy oxygen atom and
one imino nitrogen atom. The peripheral ligation for Ni(II) ions in
the cubane are completed by four terminal methanol ligands,
which participate in intramolecular hydrogen bonding interactions
(see Fig. S1. Oꢀ ꢀ ꢀO distances: 2.636–2.714 Å, Oꢀ ꢀ ꢀH-O angles:
165.163–169.393°) with the phenol oxygen atoms of the (L2)2ꢁ
ligands, along four of the six faces of the cubane. As a result, these
four faces exhibit shorter Niꢀ ꢀ ꢀNi separations and smaller Ni–O–Ni
angles.
Bond angles (°)
O6–Ni1–N3
N3–Ni1–O4
O6–Ni1–O3
O4–Ni1–O3
O6–Ni1–O1
N3–Ni1–O1
O4–Ni1–O1
O3–Ni1–O1
O6–Ni1–O10
N3–Ni1–O10
O4–Ni1–O10
O3–Ni1–O10
N4–Ni1–O5
N4–Ni1–O1
O5–Ni1–O1
O1–Ni1–O1
N4–Ni1–O4
O5–Ni1–O4
O1–Ni1–O4
O1–Ni1–O4
N4–Ni1–O9
O5–Ni1–O9
O1–Ni1–O9
O1–Ni1–O9
94.50(11)
83.52(11)
98.24(10)
84.51(10)
94.38(10)
102.49(10)
78.98(9)
N1–Ni3–O8
N1–Ni3–O3
O8–Ni3–O1
O3–Ni3–O1
N1–Ni3–O1
O8–Ni3–O1
O3–Ni3–O1
O1–Ni3–O1
N1–Ni3–O11
O8–Ni3–O11
O3–Ni3–O11
O1–Ni3–O11
O7–Ni4–N1
N1–Ni4–O1
O7–Ni4–O4
O1–Ni4–O4
O7–Ni4–O3
N1–Ni4–O3
O1–Ni4–O3
O4–Ni4–O3
O7–Ni4–O11
N1–Ni4–O11
O1–Ni4–O11
O4–Ni4–O11
93.77(11)
83.26(11)
99.95(9)
All nickel(II) centers are six-coordinate with a NO5 donor set
from the Schiff base ligands and methanol. The basal plane of the
octahedron is constructed by the phenoxy oxygen atom, the imino
84.01(9)
103.21(11)
94.44(10)
78.76(10)
82.43(9)
89.77(11)
95.78(11)
91.98(11)
82.33(10)
93.65(11)
83.79(11)
98.27(10)
85.58(9)
nitrogen atom, and two
l3-hydroxy oxygen atoms while the apical
positions are occupied by another
l3-hydroxy oxygen atom and
81.87(9)
methanol oxygen atom. In addition, complex 2 appears to possess
a slight apical elongation of the Ni(II) coordination octahedron,
which occurs along the O–Ni–O vector involving the methanol
and hydroxy oxygen atoms, with Ni–O (alkoxo) bonds on this vec-
tor of 2.115–2.184 Å versus 1.980–2.027 Å for the equatorial
bonds. Several NiI4I complexes with similar cubane structures have
been reported previously, including examples with analogous intra-
molecular hydrogen-bonding interactions; although, an apical elon-
gation is not evident in theses complexes [41,44,45].
96.55(10)
86.79(11)
90.64(10)
86.51(10)
93.48(11)
83.48(10)
99.20(10)
84.98(9)
102.66(10)
92.19(10)
78.88(9)
91.98(10)
104.71(11)
79.28(9)
82.57(9)
81.67(9)
3.2.3. Crystal structure of [Ni4(o-vanillinate)4(g1
,l3-N3)4(CH3OH)4] (3)
88.85(11)
98.98(11)
90.89(10)
83.65(10)
99.75(11)
86.28(11)
89.86(10)
85.09(10)
The X-ray single crystal diffraction study shows that complex 3
crystallizes in the tetragonal space group I41/a. Each of the unit
cells for complex 3 consists of a neutral discrete cubane [Ni4(o-
vanillinate)4(g1
,l3-N3)4(CH3OH)4]. Some selected bond lengths
and angles for complex 3 are presented in Table 4. As shown in
Fig. 3, there are four nickel(II) ions bridged by four azides in end-
on (EO) fashion to afford a [Ni4(g1
,l3-N3)4] cubane structure. Four
O3 from o-vanillinate and two bridging azide nitrogen atoms N1
and N1A are coordinated in the basal plane of Ni1, with bond
lengths varying in a range of 1.982(3)–2.067(3) Å. The axial posi-
tions are occupied by another bridging azide nitrogen N1B and
methanol oxygen O4 with large bond lengths of Ni1–
O4 = 2.085(3) Å and Ni1–N1B = 2.107(3) Å, which lies within the
typical axial bond lengths range for octahedron nickel(II)
nickel(II) centers symmetrically related by a crystallographic four-
fold axis occupy alternate cubane vertices and each of them is com-
pleted by one methanol molecule and one deprotonated o-vanillin
decomposed from the Schiff base ligand H2L3 under the reaction
condition. The nickel(II) center has a slightly distorted octahedron
coordination geometry. The phenoxy oxygen O2, carbonyl oxygen
Table 4
Selected bond distances (Å) and angles (°) for complex 3.
Bond lengths (Å)
Ni1–N1A
Ni1–N1B
Ni1–N1
Ni1–O3
2.107(3)
2.067(3)
2.067(3)
2.012(3)
Ni1–O2
Ni1–O4
N1–Ni1A
N1–Ni1C
1.982(3)
2.085(3)
2.107(3)
2.067(3)
Bond angles (°)
N1–Ni1–N1A
N1–Ni1–N1B
N11–Ni1–N1B
N1–Ni1–O4
N11–Ni1–O4
O3–Ni1–N1B
O3–Ni1–N1
O3–Ni1–N1A
O3–Ni1–O4
82.51(11)
81.33(11)
81.54(11)
89.46(11)
90.58(11)
97.45(12)
92.50(11)
175.00(11)
89.69(13)
171.63(11)
90.90(10)
O2–Ni1–N1A
O2–Ni1–O3
O2–Ni1–O4
O4–Ni1–N1B
Ni13–N1–Ni1B
Ni1–N1–Ni1B
Ni1–N1–Ni1C
N2–N1–Ni1
93.34(10)
91.57(10)
97.87(12)
168.55(11)
96.70(11)
98.27(11)
97.97(11)
120.5(2)
N2–N1–Ni1C
N2–N1–Ni1B
119.0(2)
119.4(2)
O2–Ni1–N1
O2–Ni1–N1B
Symmetry transformations used to generate equivalent atoms:
4 ꢁ X,9/4 ꢁ Z; B ꢁX,1/2 ꢁ Y,+Z; C 1/4 ꢁ Y,1/4 + X,9/4 ꢁ Z.
A
ꢁ1/4 + Y,1/
Fig. 2. Perspective views of the molecule structure of complex 2. Hydrogen atoms
are omitted for clarity.
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X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
5
Fig. 4.
vMT (circles) vs. T plot for complex 1. The solid line is the best-fit line to the
experimental data, see text for fitting parameters.
Fig. 3. Perspective views of the molecule structure of 3. Hydrogen atoms are
omitted for clarity.
complex 1, the analysis of the structural features within the tetra-
nuclear Cu4O4 cubane core (see Fig. 5) shows that the bridging sys-
tem between the Cu(II) ions is quite asymmetric. As may be
anticipated for five-coordinate Cu(II) ions, there are short Cu–O
bond lengths corresponding to the equatorial plane and long Cu–
O bonds in the axial position. This asymmetry is also obvious from
the Cuꢀ ꢀ ꢀCu separations with the Cu1ꢀ ꢀ ꢀCu2, Cu1ꢀ ꢀ ꢀCu4, Cu3ꢀ ꢀ ꢀCu2
and Cu3ꢀ ꢀ ꢀCu4 distances being significantly shorter [3.0703–
3.1098 Å] than the Cu1ꢀ ꢀ ꢀCu3 and Cu2ꢀ ꢀ ꢀCu4 distances [3.2599–
3.2785 Å] (see Table S1). Therefore, complex 1 clearly belongs to
(4 + 2) category and the magnetic data have been analyzed consid-
ering two different magnetic coupling parameters such as J1 for the
shorter distances of Cuꢀ ꢀ ꢀCu interactions and J2 for the longer dis-
tances of Cuꢀ ꢀ ꢀCu interactions (see Fig. 5). The magnetic behavior of
such cubane-like cluster can be described by the Hamiltonian
given in Eq. (1):
complexes [29,46,47]. The complex 3 reported herein can be com-
pared to those earlier reported similar nickel(II) cubane complexes
bridged by end-on doubly bridging azides [20–23] and possessing
similar [Ni4(g1
,l3-N3)4] cubane cores. In complex 3 cubane core,
the Ni–N bond lengths are in a range from 2.067 to 2.107 Å, which
are slightly shorter compare to those complexes Ni–N distances.
And the N–Ni–N0 angles cover a range of 81.33(1)–82.51(1)°, the
Ni–N–Ni0 angles are in a range of 96.70(1)–98.27(1)°. All the bridg-
ing angles are comparable to those previously reported for the
[Ni4(g1
,l3-N3)4] cubane complexes.
3.3. Magnetic properties
Magneto-structural correlations are important in understand-
ing the magnetic coupling between the central metallic ions med-
iated by bridges and in designing novel molecular magnetic
materials. The magnetic property of complex 1 was studied to eval-
uate the magnetic interactions between the four paramagnetic
centers. Variable temperature magnetic susceptibility measure-
ments from 2 to 300 K under a constant magnetic field of 5 kOe
were carried out. The magnetic properties of the complex 1 as
^
^ ^
^ ^
^ ^
^ ^
^ ^
^ ^
H ¼ ꢁ2J1ðS1S2 þ S1S4 þ S2S3 þ S3S4Þ ꢁ 2J2ðS1S3 þ S2S4Þ
ð1Þ
v
MT against T plots are shown in Fig. 4. Data were corrected for gel-
atine capsule sample holder as well as for diamagnetic contribu-
tions. As shown in Fig. 4, the room-temperature value of MT
(1.38 cm3 K molꢁ1) is lower than that expected for four uncorre-
lated spins with S = 1/2 (
MT = 1.5 cm3 K molꢁ1 assuming g = 2),
and MT decreases slowly upon lowering of the temperature. This
v
v
v
result indicates that there exists weak antiferromagnetic coupling
interaction in the complex.
Based upon the structural features within the tetranuclear
Cu4O4 cubane core, the magneto-structural correlation of com-
pound 1 may be drawn successfully and it was analyzed as follows:
magneto-structural correlations for the hydroxo- and l-alkoxo-
bridged Cu4O4 cubane complexes were proposed earlier [48].
According to literature [49], there exists three major types of
Cu4O4 cubane structures if they are classified according to the dis-
tribution of long Cuꢀ ꢀ ꢀCu distances: (i) complexes with two short
and four long Cuꢀ ꢀ ꢀCu distances, call them (2 + 4) type; (ii) com-
plexes with four short and two long Cuꢀ ꢀ ꢀCu distances, and may
be labeled as (4 + 2) type; and (iii) complexes where all six Cuꢀ ꢀ ꢀCu
distances are similar, and they will be termed (6 + 0) type. For
Fig. 5. Core representations of complex 1 showing the coupling scheme used in the
magnetic model.
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6
X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
which leads to Eq. (2):
2Ng2b2
2expðJ2=kTÞ þ exp½ð2J2 ꢁ J1Þ=kTꢃ þ 5exp½ð2J2 þ J1Þ=kTꢃ
v
¼
þ Na
ð2Þ
kT 1 þ 6expðJ2=kTÞ þ exp½ð2J2 ꢁ 2J1Þ=kTꢃ þ 3exp½ð2J2 ꢁ J1Þ=kTꢃ þ 5exp½ð2J2 þ J1Þ=kTꢃ
N = 240 ꢂ 10ꢁ6 cm3 molꢁ1
temperature region may be attributed to zero-field splitting factor
or weak antiferromagnetic exchange among the nickel clusters.
a
In these expressions, J1 and J2 are the magnetic coupling param-
eters, g is the Landé factor and N, b and k have their usual mean-
However, the maximum of
for a ground state spin of ST = 4 and a Landé factor of g = 2.23
v
MT is slightly below that expected
ings. Least-squares best-fit parameters are J1 = ꢁ23.39 cmꢁ1
,
J2 = ꢁ16.20 cmꢁ1, g = 2.0 and R = 3.24 ꢂ 10ꢁ4 (R is the agreement
obtained by Curie law (v
MT = 0.125 g2ST(ST + 1) = 12.43 cm3
factor defined as R =
R
[(vM obs
)
ꢁ (
v
)
M calc]2/
R
[(
v
)
M obs]2). Noting
K molꢁ1). This feature indicates the presence of antiferromagnetic
coupling within Ni4O4 core. The temperature dependence of the
reciprocal susceptibility (vMꢁ1) above 40 K follows the Curie–Weiss
that J1 and J2 are different, we want to stress here that our attempts
to use a one-J model for simulation of the experimental data failed
miserably.
law (
v
M = C/(T ꢁ h)) with a Weiss constant h = 14.30 K and Curie
Strong dependence of the exchange integral magnitude versus
the Cu–O–Cu angle is well documented in oxo-bridged dimeric
complexes. An angle of 97° marks the border between ferro- and
antiferromagnetic interactions. Ferromagnetic interaction is found
in complexes with smaller Cu–O–Cu angles, and antiferromagnetic
interaction for larger ones [48,50]. Hence, in the case of the
hydroxo-bridged Cu4O4 cubane complexes, an angle of 98.5° marks
the border between ferro- and antiferromagnetic coupling interac-
tions. Alkoxo-/phenoxo-bridged Cu4O4 cubane complexes show a
similar trend: ferromagnetic interaction is found in complexes
with smaller angles, and antiferromagnetic interaction for larger
ones. The larger the Cu–O–Cu angle is, the stronger the antiferro-
magnetic coupling will be [49,51,52]. However, the interaction
within our tetranuclear Cu(II) complex 1 is antiferromagnetic with
the Cu–O–Cu angles in the range of 91.12–102.13°(see Table S1).
For complex 2, variable-temperature (2–300 K) magnetic sus-
ceptibility data at a magnetic field strength of 1 KOe were col-
constant C = 5.17 cm3 K molꢁ1, indicating a dominated intra-cube
ferromagnetic interaction. The analysis of the structural features
within the tetranuclear Ni4O4 cubane core shows that the asym-
metric topology of the cubane core for complex 2 is similar to that
of complex 1 (see Fig. S2). This asymmetry is also obvious from the
Niꢀ ꢀ ꢀNi separations with the Ni1ꢀ ꢀ ꢀNi3, Ni1ꢀ ꢀ ꢀNi4, Ni2ꢀ ꢀ ꢀNi3 and
Ni2ꢀ ꢀ ꢀNi4 distances being significantly shorter [3.0348–3.0544 Å]
than the Ni1ꢀ ꢀ ꢀNi2 and Ni3ꢀ ꢀ ꢀNi4 distances [3.1987–3.2020 Å]
(see Table S2). Therefore the magnetic behavior for complex 2
can be described by the Hamiltonian
^
^ ^
^ ^
^ ^
^ ^
^ ^
^ ^
H ¼ ꢁ2J1ðS1S3 þ S1S4 þ S2S3 þ S2S4Þ ꢁ 2J2ðS1S2 þ S3S4Þ:
The best fitting gave: g = 2.23, J1 = +7.23 cmꢁ1 and J2 =
ꢁ1.84 cmꢁ1 with agreement factor of R =
R
[(vM obs
)
ꢁ (
v )
M calc]2/
R
[(v
)
M obs]2 = 4.15 ꢂ 10ꢁ4. As shown in the insert of Fig. 6, isother-
mal magnetization measurement does not show a saturation value.
This result indicates that there exists no 3D ferromagnetic order in
complex 2.
lected. Plots of
temperature the product of
the spin–only value of 4 cm3 K molꢁ1 based on Ni4 unit assuming
v
MT versus T for 2 are shown in Fig. 6. At room
v
MT is 5.32 cm3 K molꢁ1, higher than
The values of exchange-parameter are comparable to those pre-
viously reported for Ni(II) complexes with similar bridges in Ni4O4
cubane [23,46,53]. It would be desirable to establish a correlation
between the crystal structure and the magnetic properties of these
compounds, and in particular the observation of ferro- or antiferro-
magnetic coupling interactions. The magnetic exchange pathway is
through the alkoxo-ligands which bridge each pair of metals in the
cubane. It is generally accepted that the value of the M-O-M bond
g = 2. With cooling,
vMT gradually increases reaching a maximum
of 10.97 cm3 K molꢁ1 at 7 K then quickly goes down to 9.72 cm3
K molꢁ1 at 2 K, indicating presence of ferromagnetic coupling
within Ni4O4 core. The sharp decrease of
vMT value at very low
Fig. 6.
vMT (circles) vs. T plot for complex 2. The solid line is the best-fit line to the
experimental data, see text for fitting parameters. Inset: M vs. H/T plot at 2 K for
complex 2.
Fig. 7.
vMT (circles) vs. T plot for complex 3. The solid line is the best-fit line to the
experimental data, see text for fitting parameters.
Please cite this article in press as: X. Qin et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.03.040

X. Qin et al. / Polyhedron xxx (2014) xxx–xxx
7
Table 5
Structural parameters and exchange constants for [Ni4(g1
,
l
3-N3)4] cubane complexes.
Complex
(Ni–N)av/Å
(Ni–N–Ni)av/°
J (cmꢁ1
)
g
Ref.
[NH(C2H5)3]8ꢀ[Ni4(dchaa)4(N3)4]2
[Ni4(N3)4(dbm)4(EtOH)4]
[Ni4(o-vanillinate)4(N3)4(CH3OH)4]
2.151
2.132
2.080
97.37
97.29
97.64
+55.8
+11.9
+6.77
2.13
2.05
2.57
[21]
[22,23]
this work
H2dchaa = 3,5-dichloro-2-hydroxy-benzylaminoacetic acid, Hdbm = dibenzoylmethane.
angle is critical in characterizing the exchange and for nickel there
is a consensus that below 98–99°, the interaction will have ferro-
magnetic character, and above this angle it will be antiferromag-
netic. For complex 2, the average mean Ni–O–Ni angle for J1 is
95.47° and for J2 is 101.02° (see Table S2), thus, according to such
studies the coupling is predicted to be ferro- and antiferromagnetic
[54,55]. Furthermore, the ac susceptibility measurements on com-
plex 2 were carried out in the range 2–22 K at 1000 Hz with ac
field. The absence of out-of-phase ac signals above 2 K indicates
that complex 2 does not behave as a SMM (see Fig. S3).
Ni-(l-N) distances (see Table 5). Furthermore, the ac susceptibility
measurements on complex 3 were carried out in the range 2–20 K
at 1000 Hz with ac field. The absence of in-phase and out-of-phase
ac signals above 2 K indicates that complex 3 does not behave as a
SMM (see Fig. S4).
4. Conclusions
In summary, three cubane-type complexes have been synthe-
sized in a facile route utilizing transition metal ions (CuII and NiII)
and Schiff base ligands. Both complexes 1 and 2 have M4O4 cubane
cores and the chelating form of the Schiff base ligands [H2L1 and
Variable-temperature magnetic study on 3 is carried out over
the temperature range of 2.0–300 K and under a constant magnetic
field of 1 kOe. The magnetic property of compound 3 as
vMT
H2L2]. Whereas complex 3 accompany [Ni4(g1
,l3-N3)4] cubane
against T plots is shown in Fig. 7. Data were corrected for gelatine
capsule sample holder as well as for diamagnetic contributions. At
room temperature, magnetic susceptibility data per [Ni4N4] entity
core constructed by o-vanillin which decomposed from Schiff base
ligand H2L3 under reaction condition. X-ray structural analysis
shows the four metal ions and l3-bridging oxygen atoms/EO-azide
for 2 shows a
higher than the spin only value 4 cm3 molꢁ1 K of four non-interac-
tion high spin Ni(II) ions (assuming g = 2). Plots of MT versus T
v
MT value of 7.46 cm3 molꢁ1 K (see Fig. 7) which is
bridges are located at alternating vertices of a cube. Magnetic mea-
surements reveal that the ‘‘4 + 2’’ type of cubane complex 1 shows
moderately strong antiferromagnetic spin exchange interactions
v
increase with decreasing temperature and reach a maximum at
19 K then follow by a rapidly decrease until 2 K. This behavior indi-
cates a characteristic feature of intramolecular ferromagnetic
interactions between metal centers of 3. The sharp decrease of
through four
l3-alkoxo bridge. For complex 2, antiferromagnetic
and ferromagnetic couplings coexist with a non-zero spin ground
state, exhibiting dominant ferromagnetic interactions. In the case
of complex 3, the four EO-azide bridges between Ni(II) atoms
transmit ferromagnetic interactions. This work shows the potential
of Schiff base ligands in designing cubane geometry clusters with
various transition metal ions. Furthermore, this work might be
good examples for understanding the correlations between mag-
netic properties and bridging modes within the cubanes.
vMT value at very low temperature region may be attributed to
zero-field splitting factor or weak antiferromagnetic exchange
among the nickel ions centers. As is shown in the crystallographic
part, the complex 3 is made up of Ni4 entities where the azide
ligand acts as a bridge to afford a cubane system (Fig. 3). The anal-
ysis of the structural features shows that the symmetric topology
of the cubane core for complex 3 indicates the Niꢀ ꢀ ꢀNi distances
in a narrow range of 3.12(6)–3.16(6) Å. Thus, we have tried to fit
the magnetic property of this compound to a cubane model with
the Hamiltonian
Acknowledgements
This work was financially supported by the NSFC (Nos.
21061009, 21361016) and Inner Mongolia Autonomous Region
Fund for Natural Science (2013ZD09).
^
^ ^
^ ^
^ ^
^ ^
^ ^
^ ^
H ¼ ꢁ2JðS1S2 þ S2S3 þ S3S4 þ S1S4 þ S1S3 þ S2S4Þ
where a single coupling constant, J, characterizes exchange across
the six faces of the cubane [29]. A Weiss-like parameter h has been
introduced in consideration of the interactions among the tetranu-
clear clusters in 3. The best fits yielded the parameters
J = 6.77 cmꢁ1, g = 2.57 and h = ꢁ1.6 K. The negative h value reveals
the presence of antiferromagnetic interactions in 3. The high value
of g reflects the spin–orbit coupling effects and orbital degeneracy
of octahedral Ni(II) [56,57].
Appendix A. Supplementary data
CCDC 922835, 922838, 981294 contain the supplementary crys-
tallographic data for compounds 1, 2 and 3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-
ciated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2014.03.040.
To understand the magnetic interactions of compound 3, we
were therefore interested to compare and contrast the magneto-
structural properties of reported [Ni4(g1
,
l
3-N3)4] cubane com-
plexes. Similar magnetic behaviors have been observed for the
[Ni4(g1
3-N3)4] cubane species which display dominant ferromag-
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