Microwave synthesis, crystal structure and magnetic behavior of a schiff base trinuclear nickel cluster

Article abstract of DOI:10.1002/zaac.200801402

The reaction of Ni(ClO₄)₂.6H₂O with 2-hydroxybenzaldehyde and an aqueous solution of methylamine in acetonitrile/ MeOH under microwave irradiation and controlled temperature/ pressure leads to formation of trinuclear clust

Full text of DOI:10.1002/zaac.200801402

ARTICLE
DOI: 10.1002/zaac.200801402
Microwave Synthesis, Crystal Structure and Magnetic Behavior of a Schiff
Base Trinuclear Nickel Cluster
Shu-Hua Zhang,*[a] Ming-Feng Tang,[b] and Cheng-Min Ge[a]
Keywords: Cluster compounds; Magnetic properties; Microwave chemistry; Nickel; N,O ligands
Abstract. The reaction of Ni(ClO4)2 ·6H2O with 2-hydroxybenzal-
dehyde and an aqueous solution of methylamine in acetonitrile/
MeOH under microwave irradiation and controlled temperature/
pressure leads to formation of trinuclear cluster [Ni3(mimp)5-
(CH3CN)]ClO4 (1; mimp ϭ 2-methyliminomethylphenolate anion)
in only 29 min and also results in higher yields in contrast to other
synthesis methods. Complex 1 displays dominant ferromagnetic
interactions through µ3-O (oxidophenyl) and µ2-O (oxidophenyl)
binding modes.
extremely attractive method for synthesizing polynuclear
clusters, has also been introduced into solvothermal meth-
ods. It provides a clean, cheap, and convenient method of
heating that can result not only in higher yields and shorter
reaction times but also in the formation of completely new
products [11Ϫ16]. Indeed, here we demonstrate the micro-
wave-assisted synthesis of a trinickel complex that cannot
be synthesized under ambient or solvothermal conditions.
Introduction
In the past decade, much attention has been given to the
design and synthesis of polymetallic clusters since the dis-
covery of single-molecule magnets (SMMs) [1Ϫ5]. The
search for key ligands is important for advancing these in-
vestigations. Recently, the evergreen Schiff base ligands have
been widely used in the synthesis of magnetic molecular
clusters [6Ϫ9]. Previous research showed that metal com-
plexes constructed by derivatives of the sap2Ϫ ligand
(sap2Ϫ ϭ 2-salicylideneamino-1-propanolate anion) are
usually polynuclear clusters with favored ferromagnetic
coupling through µ3-O bridges and tend to be characterized
by SMM behavior. Here we chose the analogous ligand
mimp (mimp ϭ 2-methyliminomethylphenolate anion) to
serve as a chelating/bridging ligand to bring metal ions into
new type of polymetallic cluster. The vast majority of para-
magnetic cluster compounds were produced by “conven-
tional” techniques involving mixed metal ions and ligands
at a temperature limited by the boiling point of a common
solvent at atmospheric pressure. New preparative routes for
the synthesis of molecular clusters under nonambient con-
ditions have been developed. Recently, higher-temperature
solvothermal methods to synthesize clusters were explored
[1Ϫ5, 10]. More recently, microwave heating, as a new and
Results and Discussion
Crystal Structure of [Ni3(mimp)5(CH3CN)]ClO4 (1)
Complex 1 crystallizes in the monoclinic space group
P21/n (Figure 1). The core of the molecule contains
three NiII ions arranged in an equilateral triangle bridged
by two central µ3-O [oxidophenyl; Ni1ϪO1ϪNi2 87.13(5),
Ni1ϪO1ϪNi3
Ni1ϪO4ϪNi2
83.79(5),
87.16(6),
Ni2ϪO1ϪNi3
Ni1ϪO4ϪNi3
88.73(5),
87.53(6),
˚
Ni2ϪO4ϪNi3 88.09(6)°] with an Ni···Ni distance of 2.9 A,
which is shorter than that of [Ni3(N3)3(O2CMe)3(py)5]
(py ϭ pyridine) [12]. Three nickel ions are further connec-
ted via three µ2-O [oxidophenyl; O2, O3, O5; Ni2ϪO2ϪNi3
86.32(5), Ni1ϪO3ϪNi2 86.31(6), Ni1ϪO5ϪNi3 86.66(5)°].
The coordination of the three nickel ions is completed by
five nitrogen atoms of five mimp ligands and one terminal
acetonitrile molecule. All three nickel ions are six-coordi-
nate and adopt distorted octahedral geometries with cis and
trans angles in the ranges 72.6Ϫ105.0° and 150.6Ϫ169.1°,
respectively, but the coordination environment of the Ni1
ion is different from those of Ni2 and Ni3. The Ni2 and
Ni3 atoms are coordinated by four oxygen atoms and two
nitrogen atoms from four different mimp ligands, while Ni1
is coordinated by four oxygen atoms and one nitrogen atom
from four different mimp ligands and one nitrogen atom
from the terminal acetonitrile molecule. All three nickel
ions are in the 2ϩ oxidation state, as evidenced by bond
* Prof. Dr. S.-H. Zhang
Fax: ϩ86-773-5896-671
E-Mail: zsh720108@163.com
[a] Key Laboratory of Nonferrous Metal Materials and Processing
Technology
Department of Material and Chemical Engineering
Guilin University of Technology, Ministry of Education
Guilin, 541004, P. R. China
[b] Department of Foreign Languages
Guilin University of Technology
Guilin, 541004, P. R. China
Supporting information for this article is available on the
WWW under www.zaac.wiley-vch.de or from the author.
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Z. Anorg. Allg. Chem. 2009, 635, 1442Ϫ1446

A Schiff Base Trinuclear Nickel Cluster
valence sum calculations, charge-balance considerations, For 1, at room temperature, spinϪorbital coupling of NiII
and the presence of typical bond lengths for NiII. Recently, ions gives rise to a χMT product of 4.56 cm3 K molϪ1
,
which is much higher than the spin-only value of 3.0
cm3 K molϪ1 for three noninteracting high-spin NiII ions
assuming g ϭ 2.0. The χMT value of 1 at a temperature of
300 K lies in the range of other trinuclear nickel clusters,
which have χMT values from 2.75 to 5.31 cm3 K molϪ1 [12,
24Ϫ28]. With decreasing T, χMT of 1 smoothly increases to
8.11 cm3 K molϪ1 at ca. 2.8 K, and then smoothly falls to
8.02 cm3 K molϪ1 at 2 K (Figure 2). Similar magnetic be-
havior was observed for [Ni3(N3)3(O2CMe)3(py)5] [12].
it was shown that intermolecular CϪH···ClϪM hydrogen
bonds provide a pathway for intermolecular magnetic ex-
change interaction [17]. This type of hydrogen bonds is not
been observed in 1. On the other hand, there are no signifi-
cant intercluster interactions except for very weak CϪH···O
hydrogen bonds (C36···O6i 3.410(4) A, C42···O6ii
˚
˚
3.194(4) A, symmetry code: i) x, y, 1 ϩ z; ii) 3/2 Ϫ x, 1/2
ϩ y, 3/2 Ϫ z; see Figure S1, Supporting Information). The
intermolecular magnetic exchange interactions would be
anticipated to be the weakest among them. Therefore, inter-
molecular magnetic exchange interactions propagated by
this pathway may be negligible.
Figure 2. Plot of χM and χMT versus T for complex 1, the solid
lines correspond to the best theoretical fits.
The temperature dependence of the reciprocal suscepti-
bility χϪM1 above 50 K follows the CurieϪWeiss law [χM
ϭ
C/(T Ϫ θ)] with a Weiss constant of θ ϭ 5.63 K and Curie
Figure 1. Structure of 1. Some of the hydrogen atoms were omitted.
constant of C ϭ 4.55 cm3 K molϪ1 (Figure S2, Supporting
To the best of our knowledge, two coordination modes Information). The larger positive Weiss constant indicates
of the 2-methyliminomethylphenolate ligand have been dominant intramolecular ferromagnetic interaction be-
reported (Scheme 1b and c) [18Ϫ23]; its unusual tetraden- tween adjacent NiII ions through the two kinds of oxygen
tate µ3:η1:η3 coordination mode in 1 (Scheme 1a) has not
exchange bridges (Figure 1). The low-temperature maxi-
appeared before.
mum is indicative of an S ϭ 3 ground state (with g ϭ 2.30).
This pattern is compatible with moderate ferromagnetic
coupling [29Ϫ31]. The sudden decrease in χMT at low tem-
perature is assigned to zero-field splitting in the ground
state, Zeeman effects, or intermolecular antiferromagnetic
interactions.
Further evidence of ferromagnetic coupling between NiII
ions can be observed in variable-field magnetization curves
(Figure S3, Supporting Information). At low fields from 0
to 1 T the magnetization of complex 1 sharply increases.
Above 1 T the magnetization slowly increases and saturates
at 6.40 NµB at 5 T.
Alternating-current susceptibility measurements on 1
were carried out in the range 2Ϫ10 K at 50 and 499 Hz
(Figure 3). The absence of out-of-phase ac signals above
Scheme 1. Coordination modes of the 2-methyliminomethyl-
phenolate ligand.
Magnetic Properties
The magnetic susceptibility of 1 was measured on a
crushed single-crystalline sample. The dc susceptibility of 1
was measured under an applied field of 1 kOe at 2 to 300 K. 2 K indicates that 1 does not behave as an SMM. A possible
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S.-H. Zhang, M.-F. Tang, C.-M. Ge
ARTICLE
photometer in the 4000Ϫ400 cmϪ1 region by using KBr pellets.
Variable-temperature magnetic susceptibility measurements were
carried out with a SQUID MPMS XL7 magnetometer in the tem-
perature range 2Ϫ300 K in a magnetic field of 1000 Oe. The molar
magnetic susceptibility was corrected for the sample holder and
diamagnetic contributions of all constituent atoms by using
Pascal’s constants. The crystal structure was determined by single-
crystal X-ray diffraction, and SHELXS-97 and SHELXL-97
software was used for structure solution and refinement corre-
spondingly.
explanation for this behavior of 1 could come from the in-
termolecular magnetic interactions propagated by this path-
way, which may be negligible.
Preparation of [Ni3(mimp)5(CH3CN)]ClO4 (1): 2-Hydroxybenzal-
dehyde (0.244 g, 2 mmol), aqueous methylamine solution (0.5 mL,
25 %), triethylamine (0.6 mL), Ni(ClO4)2 ·6H2O (0.361 g, 1 mmol),
methanol (15 mL), and acetonitrile (15 mL) were placed in a 60 mL
Teflon-lined autoclave, which was then inserted into the cavity of
a microwave reactor. The reaction mixture was maintained at T ϭ
120 °C, power ϭ 300 W, and p ϭ 7.2 atm for a total of 29 min,
followed by cooling (ca. 120 min). The resulting solution was then
filtered. Green blocklike crystals suitable for X-ray structure analy-
sis were grown at room temperature by slow evaporation for 6 d.
The crystals were collected by filtration, washed with methanol and
dried in air. Phase-pure crystals of 1 were obtained by manual sep-
aration. Yield: 87 % (based on Ni). C42H43N6ClO9Ni3: C, 51.09;
H, 4.39; N, 8.51 %. Found: C, 51.02; H, 4.45; N, 8.47 %. Major IR
absorptions: 1638 νs(CϭN) [34], 1038, 950 cmϪ1 ν(ClO4Ϫ).
Figure 3. In-phase and out-of-phase ac susceptibility of 1.
Examination of the bond lengths and angles between the
NiII centers in 1 for magnetostructural correlations revealed
an obvious trend based on structural parameters. When the
values reported for the different NiII clusters are compared,
some general observations can be made. Three intracluster
Ni···Ni interactions are nearly equilateral with the exchange
angles in the range from 83.79 to 87.73°, and tend to yield
orthogonality for Ni···Ni ferromagnetic superexchange
pathways [32, 33]. Employing the isotropic spin Hamilton-
ian in Equation (1) and χM in Equation (2), allowed us to
satisfactorily model the data with the parameters J ϭ
ϩ5.18 cmϪ1 and g ϭ 2.30 (Figure 2).
Preparation of Ni(mimp)2 [35]: A solution of 2-hydroxybenzal-
dehyde (0.244 g, 2 mmol), aqueous methylamine solution (0.5 mL,
25 %), and triethylamine (0.6 mL) in methanol (15 mL) was added
slowly to a solution of Ni(ClO4)2 ·6H2O (0.361 g, 1 mmol) in aceto-
nitrile (15 mL). The mixture was stirred and heated to reflux for
2 h at 353 K. The solution was filtered and the filtrate left to stand
at room temperature. Red sheet crystals suitable for X-ray diffrac-
tion were obtained in 54 % yield (based on Ni). C16H16N2O2Ni: C,
58.77; H, 4.93; N, 8.56 %. Found: C, 58.71; H, 5.05; N, 8.52 %. The
unit cell parameters of the red sheet crystals are the same as in
reference [35].
(1)
X-ray crystallography: A green block crystal of complex 1 having
approximate dimensions 0.318 ϫ 0.282 ϫ 0.262 mm was selected
and mounted on a glass fiber. All measurements were made on a
Bruker Smart 1000 CCD diffractometer with graphite-monochro-
(2)
˚
mated Mo-Kα radiation (λ ϭ 0.71073 A). A total of 7615 reflec-
tions were collected by a ωϪϕ scan technique at 293(2) K in the
range of 2.41 Յ θ Յ 25.10° with index ranges of Ϫ20 Յ h Յ 20,
Ϫ14 Յ k Յ 14, Ϫ24 Յ l Յ 24, of which 6294 were independent
(Rint ϭ 0.0251). Empirical absorption corrections by SADABS
were carried out. The structure was solved by direct methods with
the SHELXS-97 program [36] and refined with SHELXL-97 [37] by
Conclusions
In summary, we have synthesized a triangular nickel clus-
ter with the composition [Ni3(mimp)5(CH3CN)]ClO4 (1) full-matrix least-squares techniques on F2. All non-hydrogen atoms
were refined anisotropically, while all hydrogen atoms were added
in calculated positions, included in the stage of refinement with
where mimp the deprotonated form of 2-methylimino-
methylphenol, which prevents effective intermolecular
interaction. Magnetic measurements show that 1 displays
dominant ferromagnetic interactions.
isotropic thermal parameters U(H)
ϭ
1.2Ueq(C) [U(H)
ϭ
1.5Ueq(CMe)]. Final 0.0274 and wR
R
ϭ
ϭ
0.0753
(R ϭ ΣʈFoΗ Ϫ ΗFcʈ/ΣΗFoΗ, wR2 ϭ [Σw(Fo ϪFc ) /Σw(Fo2)2]1/2, w ϭ
2
2 2
2
2
2
1/[σ2(Fo ) ϩ 0.0397P ϩ 1.0351P], where P ϭ (Fo ϩ 2Fc )/3). A
summary of crystallographic data is given in Table 1. Selected bond
lengths and angles are listed in Table 2. Atomic coordinates are
contained in Table S2 (Supporting Information). CCDC-713023
Experimental Section
General: All reagents were commercially available and used without
further purification. Microwave synthesis was performed with a contains the supplementary crystallographic data for this
WX-4000 microwave digestion system. Elemental analyses for C, paper. These data can be obtained free of charge from The Cam-
H, N were performed on an Elemental Vario-EL CHNS elemental bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
analyzer. IR spectra were recorded on Bio-Rad FTS-7 spectro- data_request/cif.
1444
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 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2009, 1442Ϫ1446

A Schiff Base Trinuclear Nickel Cluster
clusters (Table S1), and atomic coordinates (ϫ 104) and equivalent
isotropic displacement parameters (A ϫ 10 ) for 1 (Table S2).
Table 1. Crystal data and structure refinement for 1.
2
3
˚
Formula
C42H43Ni3O9N6Cl
987.34
monoclinic
P21/n (No. 14)
17.1676(16)
12.2894(12)
20.855(2)
Formula weight/g molϪ1
Crystal system
Space group
Acknowledgement
˚
a/A
˚
b/A
This project was supported by Guangxi Key Laboratory for Advance
Materials and New Preparation Technology (No: 0842003-25) and
the Young Science Foundation of Guangxi Province of China (No:
0832085).
˚
c/A
β/°
103.1380(10)
4284.8(7)
3
˚
V/A
Z
4
ρcalcd/g cmϪ3
1.531
1.429
µ/mmϪ1
F(000)
Crystal size/mm
θ range/°
No. of measured reflns
No. of unique reflns
No. of observed [I > 2σ(I)) reflns
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Factors of weighting scheme*
2040
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0.318 ϫ 0.282 ϫ 0.262
2.41 Յ θ Յ 25.10
31119
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Ni1ϪN4
Ni1ϪN6
Ni1ϪO3
Ni2ϪN3
Ni2ϪO1
Ni2ϪO4
Ni3ϪN2
Ni3ϪO2
Ni3ϪO4
2.021(2)
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Ni1ϪO1
Ni1ϪO5
Ni2ϪN1
Ni2ϪO3
Ni2ϪO2
Ni3ϪN5
Ni3ϪO5
Ni3ϪO1
2.0235(15)
2.1123(14)
2.1358(215)
2.016(2)
2.0502(15)
2.2690(15)
2.0124(19)
2.0388(15)
2.1780(14)
2.1288(15)
2.010(2)
2.0354(14)
2.1224(15)
1.9862(19)
2.0332(15)
2.1179(15)
O4ϪNi1ϪN6
O4ϪNi1ϪO1
N4ϪNi1ϪO4
N4ϪNi1ϪO3
N6ϪNi1ϪO3
N4ϪNi1ϪO5
N6ϪNi1ϪO5
O3ϪNi1ϪO5
N3ϪNi2ϪO1
N3ϪNi2ϪO3
O1ϪNi2ϪO3
N1ϪNi2ϪO4
O3ϪNi2ϪO4
N1ϪNi2ϪO2
O3ϪNi2ϪO2
N2ϪNi3ϪO2
N5ϪNi3ϪO2
N5ϪNi3ϪO5
N2ϪNi3ϪO4
O2ϪNi3ϪO4
N2ϪNi3ϪO1
O2ϪNi3ϪO1
O4ϪNi3ϪO1
169.05(7)
75.86(6)
92.03(7)
101.27(7)
98.11(7)
97.61(7)
98.03(7)
152.85(6)
167.29(7)
93.28(7)
80.39(6)
167.40(7)
79.28(6)
99.84(7)
150.60(6)
94.45(7)
104.24(7)
92.86(7)
171.38(7)
82.59(6)
98.94(7)
79.05(6)
72.58(5)
N4ϪNi1ϪO1
N6ϪNi1ϪO1
N4ϪNi1ϪN6
O4ϪNi1ϪO3
O1ϪNi1ϪO3
O4ϪNi1ϪO5
O1ϪNi1ϪO5
N3ϪNi2ϪN1
N1ϪNi2ϪO1
N1ϪNi2ϪO3
N3ϪNi2ϪO4
O1ϪNi2ϪO4
N3ϪNi2ϪO2
O1ϪNi2ϪO2
O4ϪNi2ϪO2
N2ϪNi3ϪN5
N2ϪNi3ϪO5
O2ϪNi3ϪO5
N5ϪNi3ϪO4
O5ϪNi3ϪO4
N5ϪNi3ϪO1
O5ϪNi3ϪO1
167.89(7)
93.19(7)
98.92(8)
79.72(6)
76.88(6)
80.28(6)
80.56(5)
99.92(8)
92.02(7)
99.35(7)
92.67(7)
75.39(6)
105.07(7)
76.88(6)
77.13(6)
92.35(8)
100.15(7)
157.02(6)
96.22(7)
80.37(6)
168.00(7)
81.22(6)
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Z. Anorg. Allg. Chem. 2009, 1442Ϫ1446
 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1445

S.-H. Zhang, M.-F. Tang, C.-M. Ge
ARTICLE
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Received: January 16, 2009
Published Online: May 11, 2009
1446
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