Synthesis, crystal structure and magnetic properties of a mononuclear and a ferromagnetically coupled dinuclear nickel(II) complex derived from a hexadentate Schiff base ligand

Article abstract of DOI:10.1016/j.ica.2005.05.011

Reactions with Ni^II of a hexadentate Schiff base ligand, L, prepared from the reaction of 2-benzoylpyridine with N,N′-bis-(3- aminopropyl) ethylenediamine in the 2:1 molar ratio are reported. Mixing of this ligand with Ni(NO₃)₂ affords the mononuclear complex [NiL](NO₃)₂ (1), whereas the presence of NaN₃ in the reaction system leads to the formation of the dinuclear complex [Ni ₂(L)(N₃)₄] (2). Both adducts have been characterized by X-ray crystallography revealing distorted octahedral NiN ₆ coordination environments around the Ni^II centers. The dinuclear complex contains terminal and end-on bridging azido ligands and displays Ni-N-Ni bridge angles of 101.1(2)° and 101.6(2)° and an intramolecular Ni?Ni separation of 3.330(1)A?. The μ-N3- bridges mediate ferromagnetic magnetic exchange interactions between the Ni^II centers of 2, leading to an S = 2 ground state. Fitting of bulk magnetization data provided the coupling constant J = +20.96 cm³ K mol^-1 (in the H = -2JS₁ ? S₂ convention for the Heisenberg Spin Hamiltonian), as well as the parameters g = 2.17, D = 0.69 cm^-1 and TIP = 622 × 10^-6 cm³ mol^-1.

Full text of DOI:10.1016/j.ica.2005.05.011

Inorganica Chimica Acta 358 (2005) 3362–3368
www.elsevier.com/locate/ica
Synthesis, crystal structure and magnetic properties of a
mononuclear and a ferromagnetically coupled dinuclear
nickel(II) complex derived from a hexadentate Schiff base ligand
a
Sushama Sain , Soumitra Bid , A. Usman , Hoong-Kun Fun , Guillem Aromı
a
b
b
c,
*
´
,
d
Xavier Solans , Swapan K. Chandra
,a,1
*
a
Department of Chemistry, The University of Burdwan, Burdwan – 713104, India
X-ray Crystallography Unit, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
b
c
´ `
Departament de Quımica Inorganica, Facultat de Quımica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain
d
´
´ `
Departament de Cristal lografia i Mineralogia, Universitat de Barcelona, Martı i Franques s/n, 08028 Barcelona, Spain
Received 3 February 2005; received in revised form 21 April 2005; accepted 11 May 2005
Available online 16 June 2005
Abstract
Reactions with NiII of a hexadentate Schiff base ligand, L, prepared from the reaction of 2-benzoylpyridine with N,N0-bis-(3-
aminopropyl) ethylenediamine in the 2:1 molar ratio are reported. Mixing of this ligand with Ni(NO3)2 affords the mononuclear
complex [NiL](NO3)2 (1), whereas the presence of NaN3 in the reaction system leads to the formation of the dinuclear complex
[Ni2(L)(N3)4] (2). Both adducts have been characterized by X-ray crystallography revealing distorted octahedral NiN6 coordination
environments around the NiII centers. The dinuclear complex contains terminal and end-on bridging azido ligands and displays Ni–
ꢁ
˚
N–Ni bridge angles of 101.1(2)ꢀ and 101.6(2)ꢀ and an intramolecular Niꢀ ꢀ ꢀNi separation of 3.330(1)A. The l-N3 bridges mediate
ferromagnetic magnetic exchange interactions between the NiII centers of 2, leading to an S = 2 ground state. Fitting of bulk mag-
netization data provided the coupling constant J = +20.96 cm3 K molꢁ1 (in the H = ꢁ2JS1 Æ S2 convention for the Heisenberg Spin
Hamiltonian), as well as the parameters g = 2.17, D = 0.69 cmꢁ1 and TIP = 622 · 10ꢁ6 cm3 molꢁ1
ꢁ 2005 Elsevier B.V. All rights reserved.
.
Keywords: Nickel; Magnetism; Ferromagnetic coupling; Azide ligands
1. Introduction
of various transition metal ions into molecular species.
Such ligands span from simple ꢁmono- or didentate ions
(Clꢁ, HOꢁ, ROꢁ, N3ꢁ, RCOO , etc.) [1,2], all the way
to very elaborate multidentate molecules [3,4]. Very
often, judicious or serendipitous combinations of more
than one species lying on opposite sides of that spectrum
are used [5]. The use of the azido ion has been very
beneficial in this respect [6], thanks to the versatility of
its bridging modes and because it has shown the ability
to facilitate efficiently the magnetic exchange between
the spin carriers that it links. The most common bridg-
ing modes of N3ꢁ are (Scheme 1) the end-on ðg1 : l-N3ꢁÞ
[7–9] and the end-to-end ðg1;3 : l-N3ꢁÞ fashion [10–12],
One of the areas that most have stimulated the
synthesis of polynuclear coordination complexes is that
of molecular magnetism. Many synthetic strategies have
been developed to this end, and all them have in
common the use of bridging ligands for the assembly
*
Corresponding author. Tel.: +34 934021271; fax: +34 934907725.
´
E-mail addresses: guillem.aromi@qi.ub.es (G. Aromı), dr_swapan@
sify.com (S.K. Chandra).
1
Present address: Department of Chemistry, Visva Bharati, Santi-
niketan 731 235, India.
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.05.011

S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
3363
N
N
N
N
N N N
N N N
H
N
M
M
M
M
C
N
N
N
N
N
C
N
N
H
end-to-end
Scheme 1.
end-on
L
Scheme 3.
which generally lead to ferro- and antiferromagnetic
interactions, respectively.
mains unchanged. Both complexes have been crystallo-
graphically characterized and their magnetic properties
are discussed.
Some of us have recently reported on the reactivity
toward NiII of a hexadentate Shiff base-type ligand
(Scheme 2), L1, where the central tetradentate part is
based on a triethylenetetraamine moiety [13]. It was
found that the reaction of L1 with Ni(ClO4)2 lead to
the mononuclear complex [NiL1](ClO4)2, whereas the
presence of NaN3 in the reaction system conduced to
the formation of the dinuclear complex [Ni2L2(N3)4],
where L2 is a hexadentate ligand related to L1, resulting
from in situ transformation of the latter (Scheme 2). The
same reaction using Ni(NO3)2 produced the same neu-
tral dinuclear compound. We report here the analogous
reactivity of a very similar hexadentate ligand, (L,
Scheme 3) with a central moiety originating from the
N,N0-bis-(3-aminopropyl)ethylenediamine group. The
additional methylene groups within the backbone of L
provide the ligand with more flexibility, potentially
inducing to changes of reactivity with respect to L1. In
the absence of other co-ligands, a mononuclear com-
plex, [NiL](NO3)2 Æ H2O (1 Æ H2O), is isolated from the
reaction between L and Ni(NO3)2. The addition of
NaN3 to the mixture, however, leads to the formation
of the dinuclear bis(l-azido) complex [Ni2(L)-
(N3)4] Æ CH3CN (2 Æ CH3CN), where the ligand L re-
2. Experimental
2.1. Synthesis
N,N0-bis-(3-aminopropyl)ethylenediamine, 2-benzoyl
pyridine and sodium azide were purchased from Lancas-
ter Chemical Company Inc. and were used as received.
All other solvents and chemicals were of analytical grade.
2.1.1. Preparation of ligand
N,N0-bis-(3-aminopropyl)ethylenediamine
(0.87 g,
5 mmol) and 2-benzoylpyridine (1.83 g, 10 mmol) were
refluxed in dehydrated ethanol (15 mL) for 15 h.The
hexadentate ligand (L) was isolated after evaporating
the solvent and was recrystallized from water/ethanol.
The product was isolated as a brown wax after drying
in vacuum over P4O10. The yield was 2.05 g (ꢂ81%).
Anal. Calc. for C32H36N6 (L): C, 76.12; H, 7.18; N,
16.60. Found: C, 76.02; H, 7.05; N, 16.48%. IR
(cmꢁ1): 1617, 1591, 1453, 1329, 993, 800, 697.
2.1.2. Preparation of [Ni(L)](NO3)2 (1)
To an acetonitrile solution (5 cm3) of Ni(NO3)2 Æ 6-
H2O (0.146 g, 0.5 mmol) was added ligand L (0.252 g,
0.5 mmol) dissolved in acetonitrile (10 mL) over a peri-
od of 10 min. The resulting dark red solution was then
filtered and left for slow evaporation. After 3–4 days
deep red crystals of 1 Æ H2O were obtained (0.15 g,
ꢂ42% yield) which were suitable for X-ray crystallogra-
phy. These were collected by filtration and dried in vac-
uum. Anal. Calc. for C32H36N8O6Ni (1): C, 54.60; H,
5.16; N, 15.93, Ni, 8.06. Found: C, 54.38; H, 5.01; N,
15.72; Ni, 7.98%. IR (cmꢁ1): 1617, 1592, 1384, 1020,
945, 800, 778,751, 707, 684.
N
H
N
N
N
C
C
N
N
H
L1
N
N
N
C
N
2.1.3. Preparation of [Ni2(L)(N3)4] (2)
To an acetonitrile solution (5 cm3) of Ni(NO3)2 Æ 6-
H2O (0.146 g, 0.5 mmol) was added ligand L (0.126 g,
0.25 mmol) of in acetonitrile (10 mL) over a period of
10 min. To the resulting brown solution, an aqueous
solution (2 mL) of sodium azide (0.065 g, 1.0 mmol)
N
C
N
L2
Scheme 2.

3364
S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
was added slowly. The solution was then filtered and the
insoluble yellow precipitate (proven by IR not to be
NaNO3) was discarded. The filtrate was left for slow
evaporation and after 3–4 days deep brown crystals of
2 Æ CH3CN were obtained (0.06 g, ꢂ30% yield). These
were collected by filtration and dried in vacuum. Anal.
Calc. for C32H36N18Ni2: C, 48.64; H, 4.59; N, 31.91,
Ni, 14.85. Found: C, 48.49; H, 4.41; N, 31.76; Ni,
14.62%. IR (cmꢁ1): 2071, 2039, 1617, 1591, 1442,
1332, 1078, 1037, 795, 702.
detector equipped with a graphite monochromated Mo
Ka radiation source. Intensity data were collected in
the x ꢁ 2h scan mode. The data were corrected for Lor-
entz, polarization and absorption effects, the latter using
SADABS. The non hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included but not re-
fined. Both structures were solved with SHELXS97 and
refined with SHELXL97 computer programs [14,15]. The
crystal data and data collection details are collected in
Table 1.
Caution. Azido complexes are potentially explosive
especially in presence of organic ligands. Therefore,
these compounds must be handled with care and pre-
pared only in small amounts.
3. Results and discussion
3.1. Synthesis
2.2. Physical measurements
The hexadentate ligand was made adopting a proce-
dure modified from that for the related ligand L1 [13].
Thus, L could be easily obtained via the condensation
taking place by refluxing N,N0-bis-(3-aminoethyl)-
ethylenediamine and 2-benzoylpyridine in the 1:2 molar
ratio in dehydrated ethanol. The reaction of L with one
equivalent of Ni(NO3)2 Æ 6H2O in aqueous acetonitrile
produced a dark red colored crystalline complex of
composition [NiL](NO3)2 Æ H2O (1 Æ H2O). Thus, the
flexibility of the ligand allows itself to wrap around
the metallic center and saturate with its six donor atoms
Elemental analyses for carbon, hydrogen and nitro-
gen were performed using a Perkin–Elmer 2400II ele-
mental analyzer. Nickel contents were determined
gravimetrically as the nickel dimethyl glyoximate com-
plex. Variable temperature magnetic susceptibility data
were obtained with a Quantum Design MPMS5
SQUID spectrophotometer. Pascalꢀs constants were uti-
lized to estimate diamagnetic corrections to the molar
paramagnetic susceptibility. IR spectra (4000–
400 cmꢁ1) were taken at 298 K using a JASCO FT/
IR-420 spectrometer.
Table 1
Crystal data for 1 Æ H2O and 2 Æ CH3CN
2.3. Crystal structure determinations and refinement
A crystal of 1 Æ H2O was mounted on an area detec-
tor CCD diffractometer equipped with a graphite
monochromated Mo Ka radiation source. This crystal
was twinned with a twin law mirror plane perpendicu-
lar to the c-axis. The molar fraction of each twin com-
ponent was 0.275(14) and 0.725(14), respectively, and
the results of the major component are given. Intensity
data were collected in the x ꢁ 2h scan mode. Lorentz
and polarization corrections were made. The non-
hydrogen atoms were refined anisotropically. The en-
hanced thermal motion of atoms C9, Cꢁ10, C11, as well
as that of the O atoms of the NO3 ions is likely
caused by some type of disorder that could not be re-
solved. The aliphatic and aromatic H-atoms have been
calculated and refined using a riding model, whereas
the amine H-atoms where localized through difference
synthesis.The H-atoms of H2O molecules could not
1 Æ H2O
2 Æ CH3CN
Formula
Ni2C64H76N16O14
705.41
Pca21 (no. 29)
Ni2C34H39N19
831.26
P21/c (no. 14)
Formula weight
Space group
Unit cell dimensions
˚
a (A)
23.3761(12)
9.6785(5)
28.9032(15)
90
12.1292(16)
10.8317(14)
31.739(4)
90
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
90
90
112.241(4)
90
3
˚
V (A )
6539.2(6)
8
1.433
3859.6(9)
4
1.431
Z
Dcalc (g cmꢁ3
F(000)
)
2944
0.20 · 0.25 · 0.30
0.654
1728
0.42 · 0.34 · 0.18
1.029
Crystal size (mm)
l (Mo Ka) (mmꢁ1
)
˚
k (A)
Temperature (K)
0.71073
100(2)
56.7
0.71073
293(2)
56.64
2hmax (ꢀ)
Reflections collected
ꢁ
be localized. Two of the NO3 groups are involved
44129
Independent reflections 12036 (Rint = 0.0604) 9447 (Rint = 0.0308)
23246
in H-bonds with amine hydrogen atoms from both
complex cations, respectively, forming 1-D networks
along the crystallographic b direction (not shown).
The other two NO3 anions are located at interstitial
positions.
Reflections observed
[I>2r(I)]
Goodness-of-fit
R; Rw
10365
9447
1.036
0.0550; 0.1283
0.649 and ꢁ0.325
1.101
0.0947; 0.2360
0.939 and ꢁ1.256
Largest difference
peak and hole
Data collection on a suitable crystal of 2 Æ CH3CN
were performed with a Siemens SMART CCD area
ꢁ3
˚
(e A
)

S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
3365
all the positions of the common octahedral coordination
geometry of NiII. This reactivity is analogous to that of
the previously reported similar ligand L1, which features
only ethylene groups within its backbone. Of especial
interest was the reactivity of L within the same reaction
system in the presence of N3ꢁ. A specific question to an-
swer was whether L would undergo in situ transforma-
tion as observed with L1 in the same conditions. On
the contrary, mixing Ni(NO3)2 Æ 6H2O, L and NaN3 in
aqueous acetonitrile medium using the 2:1:4 molar ratio
afforded a deep brown product in form of crystals suit-
able for X-ray crystallography, identified as [Ni2L-
(N3)4] Æ CH3CN (2 Æ CH3CN). Thus, the addition of
ꢁ
N3 allows the combined presence of a multidentate li-
gand and a small bridging group within a dinuclear spe-
cies, with the metals featuring the same octahedral NiN6
environment as in 1. The above transformations are
summarized in Eqs. (1) and (2).
Fig. 1. Labeled ORTEP representation at 50% probability level of the
crystallographically independent complex cations of [NiL](NO3)2 (1).
For clarity, the nitrate anions, crystallized water and hydrogen atoms
are not shown.
NiðNO3Þ þ L ! ½NiLꢃðNO3Þ
ð1Þ
ð2Þ
2
2
2NiðNO3Þ þ L þ 4NaN3 ! ½Ni2LðN3Þ ꢃ þ 4NaNO3
2
4
3.2. IR spectroscopy
of observed N–Ni–N angles are 166.8(1)–172.7(1)ꢀ
(trans) and 78.4ꢁ(1)–96.3(1)ꢀ (cis) degrees, respectively.
Two of the NO3 groups are involved in H-bonds with
amine hydrogen atoms from both complex cations,
respectively, forming 1-D networks along the crystallo-
The IR spectrum of 2 exhibits two broad bands
corresponding to the asymmetric stretching vibrations
of the ligand N3 at 2071 and 2039 cmꢁ1, respectively,
ꢁ
which are similar to those previously reported [9,16].
The stretching vibrations of the C@N bond of the
ꢁ
graphic b direction (not shown). The other two NO3
anions are located at interstitial positions. The shortest
Schiff base L are located at 1617 and 1591 cmꢁ1
respectively.
,
˚
Niꢀ ꢀ ꢀNi distance within the lattice is 8.243 A.
[Ni2(L)(N3)4] Æ CH3CN (2). The molecular structure
of 2 is shown in Fig. 2 in form of an ORTEP represen-
tation. Table 3 includes a list of selected interatomic dis-
tances and bond angles. Complex 2 is a neutral
dinuclear molecule of NiII where the metals are chelated
and connected by one hexadentate ligand, L, and further
3.3. Description of the structures
[Ni(L)](NO3)2 Æ H2O (1). An ORTEP representation
of the independent complex cations of 1 is shown in
Fig. 1, whereas selected bond distances and angles are
listed in Table 2. The crystal structure of 1 Æ H2O con-
sists of two independent, but structurally equivalent,
cationic molecules per asymmetric unit, with two non-
ꢁ
bridged by two end-on l-N3 groups. The latter are
responsible for holding both metals in close proximity
˚
to each other (3.3299(13) A. The distorted octahedral
environment around each NiII center is completed by
ꢁ
coordinated NO3 anions per molecule located in be-
tween these. The complex cation of Ni(1) displays C2
ꢁ
one terminal monodentate N3 ligand. The central
˚
molecular symmetry within a tolerance of 0.5 A, which
[Ni2(l-N)2] moiety of the complex lies on an almost per-
fect plane, the molecule showing a virtual C2 axis of
symmetry passing through the center of that plane and
the midpoint of the ethylene group of L. The Ni–N–
Ni bridging angles within this plane are 101.1(2)ꢀ and
101.6(2)ꢀ, respectively. The bridging azido ligands are
significantly bent with respect to the planar core of the
dimer (as gauged by the angles N8–N7–N13 and
N14–N13–N7 of 147.18(5)ꢀ and 146.2(5)ꢀ, respectively)
and point opposite from the side where the backbone
of the multidentate ligand L is located. The coordina-
tion Ni–N distances in 2 range from 2.075(6) to
is not the case for the other complex. This difference is
caused by small deviations in torsion angles, both mol-
ecules presenting otherwise essentially the same geome-
try. Each molecule features one NiII center surrounded
and chelated by one hexadentate ligand L which satu-
rates the six positions of a distorted octahedral NiN6
coordination environment through the formation of
four fused chelate rings. Of these, two are planar, conju-
gated five-membered rings, whereas the other two com-
prise aliphatic carbons and are formed by six atoms that
are far from lying in a plane. The coordination Ni–N
˚
˚
2.159(6) A, whereas the N–Ni–N angles are in the ranges
distances span 2.079(3)–2.111(3) A, whereas the ranges

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S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
Table 2
Table 3
˚
˚
Selected interatomic distances (A) and bond angles (ꢀ) for 1 Æ H2O
Selected interatomic distances (A) and bond angles (ꢀ) for 2 Æ MeCN
Ni(1)–N(2)
Ni(1)–N(4)
2.085(3)
Ni(1)–N(1)
Ni(1)–N(6)
2.118(7)
2.091(6)
2.144(6)
2.124(7)
2.124(7)
2.079(6)
2.117(7)
2.152(6)
2.085(7)
2.075(6)
2.159(6)
2.150(6)
3.3299(13)
2.092(3)
2.101(3)
2.079(3)
Ni(1)–N(5)
Ni(2)–N(11)
Ni(1)–N(13)
Ni(2)–N(2)
Ni(2)–N(14)
Ni(2)–N(15)
2.088(3)
2.098(3)
Ni(2)–N(4)
Ni(2)–N(10)
Ni(1)–N(3)
Ni(1)–N(6)
2.087(4)
2.093(3)
Ni(1)–N(5)
Ni(1)–N(7)
Ni(1)–N(1)
Ni(2)–N(13)
2.111(3)
2.085(3)
Ni(1)–N(16)
Ni(2)–N(3)
Ni(2)–N(12)
Ni(2)–N(16)
2.098(3)
2.106(3)
Ni(2)–N(7)
Ni(2)–N(13)
N(2)–Ni(1)–N(3)
N(3)–Ni(1)–N(4)
N(3)–Ni(1)–N(6)
N(2)–Ni(1)–N(5)
N(4)–Ni(1)–N(5)
N(2)–Ni(1)–N(1)
N(4)–Ni(1)–N(1)
N(2)–Ni(1)–N(4)
N(2)–Ni(1)–N(6)
N(4)–Ni(1)–N(6)
N(3)–Ni(1)–N(5)
N(6)–Ni(1)–N(5)
N(3)–Ni(1)–N(1)
N(6)–Ni(1)–N(1)
N(5)–Ni(1)–N(1)
N(11)–Ni(2)–N(14)
N(11)–Ni(2)–N(12)
N(14)–Ni(2)–N(12)
N(13)–Ni(2)–N(15)
N(12)–Ni(2)–N(15)
N(13)–Ni(2)–N(16)
N(12)–Ni(2)–N(16)
N(11)–Ni(2)–N(13)
N(13)–Ni(2)–N(14)
N(13)–Ni(2)–N(12)
N(11)–Ni(2)–N(15)
N(14)–Ni(2)–N(15)
N(11)–Ni(2)–N(16)
N(14)–Ni(2)–N(16)
N(15)–Ni(2)–N(16)
93.51(14)
84.43(15)
93.52(14)
167.37(14)
93.96(13)
78.45(13)
93.92(13)
95.38(13)
92.29(13)
172.17(13)
95.85(13)
78.70(13)
171.62(14)
89.17(13)
92.45(13)
91.38(13)
78.78(13)
94.83(12)
96.32(13)
166.80(13)
95.24(14)
92.48(12)
171.49(13)
84.70(14)
93.99(14)
91.49(12)
94.31(13)
89.62(13)
172.67(13)
78.41(12)
Ni(1)ꢀ ꢀ ꢀNi(2)
N(1)–Ni(1)–N(5)
N(1)–Ni(1)–N(7)
N(1)–Ni(1)–N(16)
N(5)–Ni(1)–N(7)
N(5)–Ni(1)–N(16)
N(6)–Ni(1)–N(13)
N(7)–Ni(1)–N(13)
N(13)–Ni(1)–N(16)
N(2)–Ni(2)–N(4)
N(2)–Ni(2)–N(10)
N(3)–Ni(2)–N(4)
N(3)–Ni(2)–N(10)
N(4)–Ni(2)–N(7)
N(4)–Ni(2)–N(13)
N(7)–Ni(2)–N(13)
Ni(1)–N(7)–Ni(2)
N(1)–Ni(1)–N(6)
N(1)–Ni(1)–N(13)
N(5)–Ni(1)–N(6)
N(5)–Ni(1)–N(13)
N(6)–Ni(1)–N(7)
N(6)–Ni(1)–N(16)
N(7)–Ni(1)–N(16)
N(2)–Ni(2)–N(3)
N(2)–Ni(2)–N(7)
N(2)–Ni(2)–N(13)
N(3)–Ni(2)–N(7)
N(3)–Ni(2)–N(13)
N(4)–Ni(2)–N(10)
N(7)–Ni(2)–N(10)
N(10)–Ni(2)–N(13)
Ni(1)–N(13)–Ni(2)
169.7(2)
92.0(2)
89.6(3)
94.0(2)
85.5(3)
167.7(2)
78.7(2)
93.8(3)
170.2(2)
89.3(3)
94.0(2)
95.4(2)
91.9(2)
93.8(2)
78.4(2)
101.1(2)
77.6(2)
97.5(2)
94.1(2)
91.9(2)
90.1(2)
97.4(3)
172.5(3)
77.9(2)
97.0(2)
92.0(2)
169.7(2)
92.8(2)
85.9(3)
93.4(2)
171.8(2)
101.7(2)
77.6(2)–97.5(2)ꢀ and 167.7(2)–172.5(3)ꢀ for pairs of N
atoms in cis and trans positions, respectively.
3.4. Magnetic properties
The magnetic properties of 1 and 2 were evaluated by
means of bulk magnetization measurements. The value
of the product vMT (vM is the molar paramagnetic sus-
ceptibility) at 296 K (1.04 cm3 K molꢁ1) for a powdered
sample 1 is close to the spin-only value expected for a
divalent nickel(II) compound comprising a 3A2 elec-
tronic state with g = 2. Variable temperature magnetic
susceptibility measurements were performed on a poly-
crystalline sample of 2 in the 2–300 K temperature
range, under a constant magnetic field of 7 kG. The
Fig. 2. Labeled ORTEP representation at 50% probability level of the
complex [Ni2(L)(N3)4] (2). For clarity, the nitrate anions, crystallized
acetonitrile and hydrogen atoms are not shown.

S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
3367
3.5
3.0
2.5
2.0
1.5
in the 1.8–6.8 K temperature range at various constant
magnetic fields. The resulting isofield M/NlB versus H/
T plots (Fig. 4) do not superimpose, consistent with
the presence of zero field splitting in this molecule.
The results observed from this study are not unexpected.
It is now firmly established that azido ligands bridging
open-shell transition metals in the end-on fashion medi-
ates, almost unvariably, ferromagnetic super-exchange
between the spins of the metals. For the cases of CuII
[19] and MnII [20], correlations between the strength
of the coupling, J, and certain structural parameters
have been identified. In the case of NiII, however, de-
spite the numerous examples reported hitherto [13], it
has not been possible to establish such relationship. This
issue has been the object of a theoretical study in form of
hybrid density functional (DFT) calculations on models
consisting of bis-azido bridged dinuclear complexes [21].
For the case of NiII, a parabolic relationship between
the coupling constant and the Ni–N–Ni bridging angle,
h, was established for a range of energetically attainable
values of h. The coupling was predicted to be ferromag-
netic within the entire range, and maximum for the most
stable configuration (h ꢄ 104ꢀ). The ferromagnetic nat-
ure of the coupling has been corroborated by the exper-
imental findings from real complexes [7–9,16,22–25].
However, no specific correlation could be identified
and the magnitude of the coupling is in all cases approx-
imately half the predicted values. Complex 2 represents
a new addition to the list of ferromagnetic complexes of
this category. Finding a correlation between the strength
of the coupling remains an unsolved challenge which
will require consideration of more than one structural
parameter at once.
T
M
0
100
200
300
T [K]
Fig. 3. Plot of experimental vMT vs. T. The solid line is a fit to the
experimental data (see text for details).
results are shown in Fig. 3 in form of a vMT versus T
plot. The product vMT takes a value of 2.83 cm3 K
molꢁ1 at room temperature, slightly higher than that ex-
pected for two independent NiII centers with g = 2.17
(2.35). It increases gradually with cooling to reach a
smooth maximum of 3.36 cm3 K molꢁ1, at 35 K and de-
crease sharply down to 1.60 cm3 K molꢁ1 at 2 K. The
expected value for a system with an isolated S = 2 spin
ground state and g = 2 is 3.53 cm3 K molꢁ1. This behav-
ior shows that the intramolecular coupling within 2 is
ferromagnetic, thus leading to the quintuplet as a
ground state. The decrease of vMT at low temperature
is very likely to be caused by combination of zero field
splitting of the ground state and intermolecular ex-
change interactions. Ginsberg et al. [17] pointed out that
these two parameters are very strongly dependent of
each other. The experimental data were fit to a theoret-
ical vM = f(T) expression derived from the Van Vleck
equation for a dimer of two S = 1 ions [18], modified
in order to take into account the zero field splitting of
the ground state [18], by introducing a D parameter
(See supplementary material, Eq. S1).The best fit (Fig.
4. Concluding remarks
3, solid line) was obtained for J = 20.96 cmꢁ1
,
Reactions of the flexible hexadentate Shiff-base li-
gand L with NiII has led to the preparation of a mono-
nuclear or a bis-azido bridged dinuclear complex,
D = 0.69 cm1, g = 2.17 and TIP (temperature indepen-
dent paramagnetism) = 622 · 10ꢁ6 cm3 molꢁ1. Reduced
magnetization, M/NlB, measurements were also collected
ꢁ
depending on the presence or not of N3 ligands in
the reaction mixture. The dinuclear complex confirms
the feasibility of using complicated multidentate ligands
for the preparation and study of dinuclear ðl-N3ꢁÞ-
bridged NiII complexes. In this compound, ferromag-
netic super-exchange has been found between the
paramagnetic centers, in a magnitude that is consistent
with previously found values on compounds displaying
analogous structural features.
4
3
2
M/N
1
0
0
10000
20000
30000
Acknowledgements
H/T [G/K]
The project was supported by grants from the Coun-
cil of Scientific and Industrial Research New Delhi
(Grant No. 01(1836)/03/EMR-II, S.K.C) and the
Fig. 4. Isofield reduced magnetization, M/Nb, vs. H/T plots, recorded
at various constant magnetic fields. The lines joining the data points
are a guide to the eye.

3368
S. Sain et al. / Inorganica Chimica Acta 358 (2005) 3362–3368
[6] G.S. Papaefstathiou, A. Escuer, R. Vicente, M. Font-Bardia, X.
Solans, S.P. Perlepes, Chem. Commun. (2001) 2414.
[7] A. Escuer, R. Vicente, S. El Fallah, X. Solans, M. Font-Bardia,
Inorg. Chem. Acta 247 (1996) 85.
Department of Science and Technology, New Delhi
(Grant No. SR/S1/IC-22/2003, S.K.C.), Government
of India, the Malaysian Government and Universiti
Sains Malaysia for research (Grant R&D No. 305/PFI-
ZIK/610961, H.K.F) and the Spanish Ministry of Sci-
[8] A. Escuer, R. Vicente, J. Ribas, X. Solans, Inorg. Chem. 34
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[9] M.G. Barandika, R. Cortes, L. Lezama, M.K. Urtiaga, M.I.
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Appendix A. Supplementary data
[12] T.K. Maji, P.S. Mukherjee, G. Mostafa, T. Mallah, J. Cano-
Boquera, N.R. Chaudhuri, Chem. Commun. (2001) 1012.
[13] S. Deoghoria, S. Sain, M. Soler, W.T. Wong, G. Christou, S.K.
Bera, S.K. Chandra, Polyhedron 22 (2003) 257.
Crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre, with deposi-
tion numbers CCDC 258210 and 258211. Copies of
these data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK,
fax: +44 1223 336 033, e-mail: deposit@ccdc.cam.ac.uk;
or on the web: http://www.ccdc.cam.ac.uk. Supplemen-
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the online version, at doi:10.1016/j.ica.2005.05.011.
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mination, Go¨ttingen, Germany, 1997.
[15] G.M. Sheldrick, SHELXL97 Program for Crystal Structure Deter-
mination, Go¨ttingen, Germany, 1997.
[16] R. Cortes, J.I.R. Delarramendi, L. Lezama, T. Rojo, K. Urtiaga,
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