Synthesis, crystal structure and magnetic properties of a new ferromagnetic nickel(II) dimer derived from a hexadentate Schiff base ligand

Article abstract of DOI:10.1016/S0277-5387(02)01336-0

A new end-on bis(μ-azido) nickel(II) complex of the formula [Ni₂(L¹)(N₃)₄] (1) (L¹ = hexadentate Schiff base ligand made from the reaction of 2 moles of 2-benzoyl pyridine and 1 mole of triethylenetetramine) has been prepared and characterized by single crystal X-ray diffraction studies. Each nickel center is coordinated in a distorted octahedral (NiN₆) fashion consisting of three nitrogen atoms from the hexadentate ligand moiety, two nitrogen atoms from two end-on azido ligands and the last one from another terminal azido ion. The variable temperature magnetic susceptibility of compound 1 shows that the nickel centers are ferromagnetically coupled. The Ni-N-Ni bridge angles are very similar [98.36(8)° and 97.66(9)°] and the Ni···Ni separation is 3.155(1) A?. The magnetic exchange coupling constant J = 21.8 cm^-1 is consistent with the structure and the value has been compared with some similar reported compounds.

Full text of DOI:10.1016/S0277-5387(02)01336-0

Polyhedron 22 (2003) 257Á262
/
www.elsevier.com/locate/poly
Synthesis, crystal structure and magnetic properties of a new
ferromagnetic nickel(II) dimer derived from a hexadentate Schiff base
ligand
Soma Deoghoria a, Sushama Sain a, Monica Soler b, W.T. Wong c, George Christou b,
Sujit K. Bera a, Swapan K. Chandra a,ꢀ
a Department of Chemistry, The University of Burdwan, Burdwan 713104, India
b Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
c Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Received 27 May 2002; accepted 9 October 2002
Abstract
A new end-on bis(m-azido) nickel(II) complex of the formula [Ni2(L1)(N3)4] (1) (L1ꢁ
/hexadentate Schiff base ligand made from
the reaction of 2 moles of 2-benzoyl pyridine and 1 mole of triethylenetetramine) has been prepared and characterized by single
crystal X-ray diffraction studies. Each nickel center is coordinated in a distorted octahedral (NiN6) fashion consisting of three
nitrogen atoms from the hexadentate ligand moiety, two nitrogen atoms from two end-on azido ligands and the last one from
another terminal azido ion. The variable temperature magnetic susceptibility of compound 1 shows that the nickel centers are
ferromagnetically coupled. The NiÃ
/
NÃ
/
Ni bridge angles are very similar [98.36(8)8 and 97.66(9)8] and the Ni
/
Á Á Á
/
Ni separation is
21.8 cmꢂ1 is consistent with the structure and the value has been
˚
3.155(1) A. The magnetic exchange coupling constant Jꢁ
/
compared with some similar reported compounds.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Binuclear nickel(II) complex; Crystal structure; Magnetic properties; End-on azido ligand complex; Terminal azido ion complex;
Hexadentate ligand
1. Introduction
meric compounds [1Á
complexes can act as either end-on (m1,1-N3) [1Á
/
10,13]. The azide ligand in such
4] or
/
Studies on the coordination behavior of transition
metal complexes along with their structures and mag-
netic properties have received considerable attention in
end-to-end (m1,3-N3) [5,6] bridges (Chart 1). The most
important aspect of these bridged complexes is the
variety of their magnetic exchange interactions. The
end-on bridging mode exerts ferromagnetic exchange
interactions while that of end-to-end propagates anti-
recent years [1Á10]. Common strategies to prepare such
/
complexes using a polynucleating ligand and an unused
arm or donor site of a mononuclear complex have been
recognized [11,12]. Another way of their preparation is
by utilization of flexidentate behavior of a multidentate
ligand and judicial choice of bridging ligands. Among
the bridging ligands pseudohalide ions especially the
azido ion has long been known for its versatile bridging
coordination modes that generate dimeric and poly-
ꢀ Corresponding author. Tel.:
530452
E-mail address: dr_swapan@sify.com (S.K. Chandra).
ꢃ
/
91-342-558545; fax:
ꢃ/91-342-
Chart 1. Possible coordination modes for azido ligands.
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 3 3 6 - 0

258
S. Deoghoria et al. / Polyhedron 22 (2003) 257Á262
/
ferromagnetic exchange between paramagnetic centers
[14Á18].
preparation and handling of these compounds deserve
special attention.
/
We describe here the synthesis of a new bis(m-azido)
nickel(II) complex of the formula [Ni2(L1)(N3)4] (1)
(L1ꢁ
hexadentate ligand), where two azido ions act in
/
2.4. Physical measurements
an end-on bridging mode and other two act as terminal
donors. The single crystal X-ray structure with its
magnetic properties are also discussed.
Elemental analysis for carbon, hydrogen and nitrogen
were performed using a Perkin Elmer 2400II elemental
analyzer. The nickel content was determined gravime-
trically as the nickel dimethyl glyoximate complex.
Variable temperature magnetic susceptibility data were
obtained with a Quantum Design MPMS5 SQUID
spectrophotometer. Pascal’s constants were utilized to
estimate diamagnetic correction, this value was sub-
tracted from the experimental susceptibility data to give
the molar magnetic susceptibility (xM). IR spectra
2. Experimental
2.1. Materials
Triethylenetetramine, 2-benzoyl pyridine and sodium
azide were purchased from the Lancaster Chemical
Company Inc. and were used as received. All other
solvents and chemicals were of analytical grade.
(4000Á
400 cmꢂ1) were taken at 298 K using a JASCO
FT/IR-420 spectrometer.
/
2.5. Crystal structure determination and refinement
2.2. Synthesis of ligand
A suitable crystal of 1 was mounted on a Enraf-
Nonius CAD4 diffractometer equipped with graphite
monochromated Mo Ka radiation. The unit cell para-
meters were determined by least-squares refinement of
setting angles of 25 reflections. Intensity data were
The hexadentate ligand was made adopting a proce-
dure modified from that for a similar type of ligand [19].
Triethylenetetramine (0.73 g, 5 mmol) and 2-benzoyl
pyridine (1.83 g, 10 mmol) were refluxed in 15 cm3
dehydrated alcohol for 4 h. The hexadentate ligand was
isolated after evaporating the solvent and purified from
alcohol. The product was isolated as a brown semisolid
mass after drying in a vacuum over P4O10: yield 2.07 g
collected in the v Á2u scan mode. The data were
/
corrected for Lorentz and polarization effects [20]. The
structure was solved by direct methods and expanded by
Fourier techniques. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were included
but not refined. All calculations were performed using
teXsan [21] crystallographic software package of Mole-
cular Structure Corporation. The crystal data and data
collection details are shown in Table 1.
(Â87%). Anal. Found: C, 75.92; H, 6.91; N, 17.29.
/
Calc. For C30H32N6: C, 75.61; H, 6.77; N, 17.63%.
3. Results and discussion
2.3. Preparation of [Ni2(L1)(N3)4] (1)
3.1. Synthesis
To an acetonitrile solution (5 cm3) of Ni(NO3)2×
(0.146 g, 0.5 mmol) was added 0.119 g (0.25 mmol) of
/
6H2O
The ligand (L) was synthesized by refluxing triethyl-
enetetramine and 2-benzoyl pyridine in a 1:2 mole ratio
in dehydrated alcohol. It reacted with nickel(II) nitrate/
perchlorate hexahydrate and sodium azide in the ratio
ligand (L) in 15 cm3 acetonitrile over a period of 15 min.
To the resulting greenishÁred solution, an aqueous
/
solution (0.065 g, 1.0 mmol, 2 cm3 H2O) of sodium
azide was added slowly. The solution was then filtered,
insoluble precipitate was discarded and the filtrate was
NiII:L:N3ꢂ ꢁ
2:1:4 in aqueous acetonitrile medium af-
/
fording red crystalline products suitable for X-ray
crystallographic work and having composition
[Ni2L1(N3)4] (1), where L1ꢁ
changed hexadentate ligand
left for slow evaporation and after 3Á/4 days deep red
needle crystals of 1 were obtained (0.023 g, Â
/12%).
/
Anal. Found: C, 48.86; H, 4.16; N, 31.76; Ni, 14.62.
Calc. For C32H34N18Ni2: C, 48.77; H, 4.35; N, 31.99; Ni,
14.89%.
Caution: Azido complexes are potentially explosive
especially in the presence of organic ligands. Therefore,
L in the transformation process. It is also interesting to
note that the reaction of L and nickel(II) perchlorate
hexahydrate using a mole ratio of 1:1 in aqueous
acetonitrile produced a dark colored crystalline complex
of composition [NiL](ClO4)2 (2) [22].

S. Deoghoria et al. / Polyhedron 22 (2003) 257Á
/262
259
Table 1
The crystal data and data collection summary for 1
3.3. Crystal structure of [Ni2(L1)(N3)4] (1)
The ORTEP drawing of 1 is shown in Fig. 1 and the
selected bond distances and angles are listed in Table 2.
Each nickel atom is located in a distorted octahedral
coordination environment surrounded by six nitrogen
atoms, three from the hexadentate ligand moiety, two
from end-on m2-bridges azido groups and the last one
Empirical formula
Formula weight
Space group
C32H34N18Ni2
788.14
P21/c (no. 14)
15.724(1)
8.9170(7)
25.497(1)
90
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
V (A )
Z
from the one terminal azido ion. The Ni(1)
/
Á Á Á
/
Ni(2)
104.28(1)
90
˚
distance is 3.155(1) A. The NiÃ
range from 2.061(2) to 2.130(2) A; NiÃ
distances are the same [Ni(1)ÃN(13), 2.070(3) and
N(16), 2.070(3) A] and other NiÃN distances
(from six nitrogen atoms of the hexadentate ligand
/
N(bridging) distances
3
˚
˚
3464.5(4)
4
1.511
/N(terminal)
/
Dcalc (g cmꢂ3
F(000)
Crystal size (mm)
)
˚
Ni(2)Ã
/
/
1632
0.22ꢄ0.20ꢄ0.12
1.141
m (Mo Ka) (mmꢂ1
l (A)
)
˚
moiety) vary between 2.005(2) and 2.271(2) A. The NiÃ
N bond distances mostly match those observed in
analogous NiN6 chromophores [1Á6], but there are
two slightly longer distances [Ni(1)ÃN(1), 2.2420(18)
N(4), 2.271(2) A] possibly due to structural
constraints imposed by the diazacyclohexane ring of the
ligand. The bridging angles are very similar [Ni(1)Ã
N(7)ÃNi(2), 98.36(8)8 and Ni(1)ÃN(10)ÃNi(2),
97.66(9)8]. The azido ligands (terminal and end-on) are
very close to linear with an average Na ÃNb ÃNg angle of
178.6(5)8. The Na ÃNb bond lengths [1.172(4)Á1.201(4)
A] are a little longer than those of Nb ÃNg bonds
1.166(4) A], where Na indicates the nitrogen
/
˚
0.71069
273
55.1
Temperature (K)
2umax (8)
Reflections collected
Independent reflections
Reflections observed [I ꢀ1.5s(I)]
Goodness-of-fit
/
/
21 470
˚
and Ni(2)Ã
/
8309 (Rintꢁ0.022)
5336
0.95
/
R; Rw
Largest difference peak and hole (e A
0.037; 0.036
0.40 and ꢂ0.22
/
/
/
ꢂ3
˚
)
/
/
/
/
˚
/
˚
[1.141(5)Á
/
atom coordinated to nickel(II) centers. These variations
in bond distances have also been observed for azido ions
bonded in a terminal or m2-1,1-bridging fashion [1Á
/6].
The rearrangement of L to L1 occurred in the
presence of both nickel salt and azido ion. The poor
yield of 1 coupled with other experimental evidences
have indicated that most probably the origin of the extra
3.4. Magnetic properties
A variable-temperature magnetic susceptibility study
was performed on polycrystalline samples of 1. The
magnetic susceptibility (xM) of this compound was
Á Á ÁCH2Ã/CH2/
/
Á Á Á fragment present in L1 compared to L
examined in a 10.0 kG field in the 2.0Á300.0 K range.
/
was the ligand itself. The azido ion in the presence of
The meff per Ni2 molecule slowly increases from 5.00mB
at 300 K to 5.49mB at 30 K and then decreases to 4.35mB
at 2 K. These correspond to xMT values of 3.11, 3.74
and 2.37 cm3 K molꢂ1 at these three temperatures (Fig.
2). The xMT vs. T plot is indicative of ferromagnetic
coupling. The decrease of the magnetic moment at low
temperature is attributed to a combination of zero field
splitting of the ground state and intermolecular ex-
change interactions. Ginsberg [26] pointed out that these
two parameters are very strongly dependent on each
other. It is thus difficult to calculate these values with
reasonable accuracy. These two parameters are also
important at low temperature where experimental un-
certainties are high [2a]. That is why neglecting D as a
fitting parameter, the data were fit using the theoretical
nickel undergoes nucleophilic attack on one Á Á Á
CH2
Á Á Á part of L followed by hydrolysis and breakdown
of the ligand to generate some diketone [23] like
derivatives (
NHÃ
/
CH2Ã
/
/
). This diketone can again react
CH2ÃNH
with the Á Á Á
/
/
CH2Ã
/
/
/
Á Á Á part of L to produce
the dimminium ion [24], which is further reduced by an
azido ion and subsequently forms L1.
3.2. IR spectroscopy
The IR spectrum of 1 exhibits two broad bands
corresponding to the asymmetric stretching vibrations
of N3 [nas(N3)] at 2061 and 2022 cmꢂ1, which are similar
to those reported [1a,3b]. The stretching vibrations of
xM vs. T expression for a dimer of two Sꢁ
ions. Least-squares fitting of the experimental data to
this expression gave Jꢁ 2.28 and uꢁ
21.8 cmꢂ1, gꢁ
1.10, where u is the CurieÁWeiss constant. The fit is
/1 nickel(II)
the CÄ
/
N bond of the Schiff base are at 1629 and 1581
cmꢂ1 and 1637 and 1592 cmꢂ1 for L and compound 1
respectively [25].
/
/
/
ꢂ
/
/

260
S. Deoghoria et al. / Polyhedron 22 (2003) 257Á262
/
Fig. 1. ORTEP drawing of [Ni2(L1)(N3)4] (1) with atom labelling scheme. For clarity the hydrogen atoms are not shown.
Table 2
˚
Selected bond distances (A) and bond angles (8) for complex 1
Bond distances
Ni(1)ÃN(1)
Ni(1)ÃN(3)
Ni(1)ÃN(10)
Ni(2)ÃN(4)
Ni(2)ÃN(6)
Ni(2)ÃN(10)
N(7)ÃN(8)
N(10)ÃN(11)
N(13)ÃN(14)
N(16)ÃN(17)
2.2420(18) Ni(1)ÃN(2)
2.0126(19)
2.061(2)
2.070(3)
2.005(2)
2.108(2)
2.070(3)
1.143(5)
1.155(4)
1.166(4)
1.141(5)
2.099(2)
2.130(2)
2.271(2)
2.102(2)
2.061(2)
1.189(4)
1.201(4)
1.181(4)
1.172(4)
Ni(1)ÃN(7)
Ni(1)ÃN(13)
Ni(2)ÃN(5)
Ni(2)ÃN(7)
Ni(2)ÃN(16)
N(8)ÃN(9)
N(11)ÃN(12)
N(14)ÃN(15)
N(17)ÃN(18)
Bond angles
Fig. 2. Plot of xMT vs. T for 1. Solid line shows the best fit as
indicated in the text.
N(1)ÃNi(1)ÃN(2)
N(1)ÃNi(1)ÃN(7)
N(1)ÃNi(1)ÃN(13)
N(2)ÃNi(1)ÃN(7)
N(2)ÃNi(1)ÃN(13)
N(3)ÃNi(1)ÃN(10)
N(7)ÃNi(1)ÃN(10)
81.02(7) N(1)ÃNi(1)ÃN(3)
101.86(8) N(1)ÃNi(1)ÃN(10)
91.17(9) N(2)ÃNi(1)ÃN(3)
171.25(9) N(2)ÃNi(1)ÃN(10)
95.37(9) N(3)ÃNi(1)ÃN(7)
87.94(9) N(3)ÃNi(1)ÃN(13)
78.63(8) N(7)ÃNi(1)ÃN(13)
158.51(7)
88.34(7)
78.07(8)
93.28(7)
98.13(9)
95.65(10)
91.84(10)
80.90(9)
88.87(9)
91.14(10)
95.00(9)
93.93(10)
101.43(9)
79.14(8)
97.66(9)
pling of end-on azido-bridged binuclear complexes of
Cu(II), Ni(II) and Mn(II), Ruiz et al. [15] applied hybrid
density functional methods on some model complexes.
N(10)ÃNi(1)ÃN(13) 171.15(9) N(4)ÃNi(2)ÃN(5)
N(4)ÃNi(2)ÃN(6)
N(4)ÃNi(2)ÃN(10)
N(5)ÃNi(2)ÃN(6)
N(5)ÃNi(2)ÃN(10)
N(6)ÃNi(2)ÃN(7)
N(6)ÃNi(2)ÃN(16)
Ni(1)ÃN(7)ÃNi(2)
N(7)ÃN(8)ÃN(9)
158.81(8) N(4)ÃNi(2)ÃN(7)
98.96(9) N(4)ÃNi(2)ÃN(16)
78.22(9) N(5)ÃNi(2)ÃN(7)
174.15(9) N(5)ÃNi(2)ÃN(16)
89.36(9) N(6)ÃNi(2)ÃN(10)
93.86(10) N(7)ÃNi(2)ÃN(10)
98.36(8) Ni(1)ÃN(10)ÃNi(2)
They have found a correlation between J and the MÃ
/
NÃM bridging angle. For bis(m-azido) end-on nickel(II)
/
complexes the interactions are ferromagnetic for all the
ranges of angles of u with increasing values of u to a
maximum around uꢁ1048. The bond distances between
/
metal and bridging atoms also strongly influence the
value of ferromagnetic coupling which decreases with
178.2(4)
N(10)ÃN(11)ÃN(12) 178.9(3)
N(16)ÃN(17)ÃN(18) 178.5(4)
N(13)ÃN(14)ÃN(15) 178.5(3)
increasing Ni
/
Á Á Á
/
Ni separation and NiÃ
/
N(bridging) dis-
tances. Table 3 compares the structural and magnetic
parameters with other reported bis(m2-1,1-azido)nickel-
(II) complexes. Compound 1 has the slightly shorter
Ni. . .Ni separation as well as the lowest bridge angle
shown as a solid line in Fig. 2. The fit confirms a Sꢁ2
/
ground state for 1. The azido ligands generally propa-
gate ferromagnetic coupling interaction between nick-
el(II) centers bound in a m2-1,1 end-on mode and
antiferromagnetic coupling when bridging in a m2-1,3
end-to-end fashion. In order to study the influence of
structural parameters on the magnetic exchange cou-
[average NiÃ
/
NÃ
/
Ni angle is 98.01(10)8] among this type
of end-on dibridged azido complexes. So, Jꢁ
/
21.8 cmꢂ1
for complex 1 is consistent with its structural para-
meters.

S. Deoghoria et al. / Polyhedron 22 (2003) 257Á
/262
261
Table 3
Structural and magnetic properties of 1 and selected di-m2-1,1-azido bridged nickel(II) complexes
J (cmꢂ1
)
Ref.
˚
Ni (A)
˚
Compound
NiÃNÃNi (8)
Ni/
Á Á Á
/
NiÃN(bridging) (A)
[Ni(terpy)(N3)2]2×2H2O
[{Ni(pepci)(N3)2}2]
101.3
3.276
3.297
3.273
3.434
3.448
3.449
3.274
3.265
3.369
3.155
2.038, 2.198
ꢃ20.1
ꢃ36.3
ꢃ43.9
ꢃ33.8
ꢃ46.7
ꢃ34.3
ꢃ22.8
ꢃ13.6
ꢃ20.9
ꢃ21.8
[1b]
[3b]
[4a]
[4a]
[2]
102.2, 101.0
103.8
104.9
2.107, 2.163, 2.109, 2.128
2.092, 2.067
2.166, 2.167
(m-N3)2[Ni(Me3[12]N3)]2(ClO4)2×2H2O
(m-N3)2[Ni(232-N4)]2(ClO4)2
[{Ni2(Medpt)2(N3)2}(m-(1,1-N3)2]
(m-N3)2[Ni(232-tet)]2(PF6)2
[{Ni(terpy)(N3)}2]×H2O
[Ni2(terpy)2(N3)3(H2O)](ClO4)×H2O
[{Ni(en)2}2(m-N3)2](ClO4)2
[Ni2(L1)(N3)4]
103.7
104.6
101.6
2.217, 2.169
2.177, 2.183
2.039, 2.184
[3a]
[1a]
[1a]
[4b]
102.46, 100.60
104.3
98.36, 97.66
2.068, 2.119, 2.069, 2.175
2.144, 2.123
2.061, 2.108, 2.130, 2.061
Present work
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4. Conclusion
The present study describes the preparation of a
ferromagnetically coupled nickel(II) dinuclear complex
from a flexible hexadentate Schiff base ligand and
judical choice of bridging as well as terminal azido ion
coordination. Complex 1 has a slightly shorter Ni
separation as well as the lowest NiÃNÃNi bridging
angle among the end-on bis(m-azido) nickel(II) com-
plexes. This type of ligand therefore provides an avenue
to get a binuclear compound having interesting mag-
netic properties. Presently, in order to rationalize the
process and to understand the mechanism of the ligand
transformation, further experiments by changing the
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metals are in progress.
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Crystallographic data for the structural analysis have
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Data Centre, CCDC No. 172396 for 1. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
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Acknowledgements
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