Formation of a dinuclear and a trinuclear Ni(II) complex on slight variation of experimental conditions: Structural analysis and magnetic properties

Article abstract of DOI:10.1016/j.poly.2016.09.050

A diphenoxido bridged dinuclear Ni(II) complex [Ni₂L₂(NO₂)₂] (1) and a μ₂and μ₃-phenoxido, μ₃-hydroxido, and μ₂-nitrito (1κO:2κN) bridged trinuclear Ni(II) complex [Ni

Full text of DOI:10.1016/j.poly.2016.09.050

Accepted Manuscript
Formation of a dinuclear and a trinuclear Ni(II) complex on slight variation of
experimental conditions: structural analysis and magnetic properties
Apurba Biswas, Michael G.B. Drew, Carlos J. Gómez-García, Ashutosh Ghosh
PII:
S0277-5387(16)30480-6
DOI:
http://dx.doi.org/10.1016/j.poly.2016.09.050
POLY 12238
Reference:
Polyhedron
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Revised Date:
Accepted Date:
16 August 2016
20 September 2016
24 September 2016
Please cite this article as: A. Biswas, M.G.B. Drew, C.J. Gómez-García, A. Ghosh, Formation of a dinuclear and a
trinuclear Ni(II) complex on slight variation of experimental conditions: structural analysis and magnetic properties,
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.09.050
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Formation of a dinuclear and a trinuclear Ni(II) complex on slight variation
of experimental conditions: structural analysis and magnetic properties
Apurba Biswas,a* Michael G. B. Drew,b Carlos J. Gómez-Garcíac and Ashutosh Ghosh*a
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkata-700 009, India; e-mail: apurbacu@yahoo.co.in (A. Biswas),
ghosh_59@yahoo.com (A. Ghosh).
b School of Chemistry, The University of Reading, P.O BOX 224, Whiteknights, Reading RG
66AD, U.K.
cInstituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/Catedrático José Beltrán, 2.
46980 Paterna, Valencia, Spain; e-mail: carlos.gomez@uv.es
Abstract
A diphenoxido bridged dinuclear Ni(II) complex [Ni2L2(NO2)2] (1) and a P and P -phenoxido,
2
3
P -hydroxido,
and
[Ni3L3(OH)(NO2)]·ClO4 (2), have been synthesized using a tridentate reduced Schiff base ligand
HL (HL 2-[(2-dimethylamino-ethylamino)-methyl]-phenol). Both complexes were
P -QLWULWRꢀ ꢁꢂțOꢃꢄțN)
bridged
trinuclear
Ni(II)
complex
3
2
=
characterized by X-ray structure determination and variable-temperature magnetic susceptibility
measurements. In both complexes the nickel atoms are six-co-ordinate with a distorted
octahedral environment. The interesting feature of the trimeric complex 2 is that the three
mononuclear units are assembled around a P -hydroxido ion in such a way that the three
3
phenoxido groups and one nitrito group form a cyclic bridge that markedly modify the magnetic
properties. Complex 1 presents a moderate antiferromagnetic Ni-Ni coupling (J = -39 cm-1)
through the phenoxido bridges with a wide Ni-O-Ni bond angle of 101.77(6)º whereas the lower
Ni-O-Ni bond angles, in the range 86.66(6)-96.07(7)º, lead to a moderate ferromagnetic coupling
(J = + 18.2 cm-1) in the trimeric complex 2 with a weak antiferromagnetic contribution
(-¶ = -0.51 cm-1) via the 1NO:2NN nitrito bridge.
1

Keywords: Nickel(II); Reduced Schiff base; Nitrite; Crystal structure; Magnetic properties.
1. Introduction
Nickel(II) complexes with different bridging ligands have received considerable attention
over the past two decades due to their potential applications not only in magnetochemistry but
also in the field of catalysis [1-8]. The single atom of oxido/phenoxido/water as well as ȝ -azido
1,1
bridges are very efficient in mediating the magnetic coupling between the Ni(II) ions [9-16]. The
major factor controlling the exchange coupling is the bridging Ni-X-Ni angle (X = O or N).
Mononegative tridentate NNO donor Schiff base ligands derived from diamines and
salicylaldehyde are extremely useful for building oxido or phenoxido bridged Ni(II) compounds
along with various bridging anionic coligands [5-8]. Monodentate (e.g. thiocyanate, water, etc.)
[7,17] as well as chelating bidentate coligands (e.g. nitrite, nitrate, etc.) [16,18] usually form di-
ȝ -phenoxido bridged complexes with large Ni-O-Ni bridging angles. In contrast, when water
2
[13-16], syn±syn carboxylato [19,20] or bidentate nitrito [21,22] act as additional bridge, along
with the phenoxido bridges, the Ni-O-Ni bridging angles become lower. A literature survey
reveals that the majority of the di-ȝ -phenoxido bridged Ni(II) complexes are antiferromagnetic
2
with average Ni-O-Ni bridging angles in the range 97-101º [16,18]. In a few compounds, the
bridging angles are lower than the critical one (ca. 98º) and consequently they present a
ferromagnetic coupling [13-16]. Recently our group has reported a Ni3 core motif with three
different types of bridging, namely phenoxido (P and P ), hydroxido (P ), and P -Cl, P -NCO,
2
3
3
2
1,1
-
or P -N3 in which the Ni(II) ions are ferromagnetically coupled [3]. The nitrite anion, NO2 , in
1,1
another very versatile bridging ligand that attracts much attention in coordination chemistry. This
interest is mainly due to the large variety of coordinating modes that it may present either as
terminal or as bridging ligand, leading to interesting architectures [8,22]. Moreover, the strength
and nature of the magnetic interactions mediated by their different bridging modes vary widely.
For example, the O/N-bridging mode (1NO:2NN) usually shows moderate antiferromagnetic
interactions whereas the O/O¶-bridging mode (1NO:2N2¶) leads to ferromagnetic coupling. In
this context, it would be interesting to investigate the formation of Ni(II) complexes with
tridentate NNO-donor reduced Schiff bases and nitrite ligands by varying the experimental
conditions.
2

Here we report the syntheses, crystal structures and magnetic properties of a dinuclear
neutral Ni(II) complex [Ni2L2(NO2)2] (1) and trinuclear Ni(II) compound
a
[Ni3L3(OH)(NO2)]·ClO4 (2) formed with the NNO-donor reduced Schiff base ligand 2-[(2-
dimethylamino-ethylamino)-methyl]-phenol (HL) and nitrito as co-ligand. Compounds 1 and 2
are obtained respectively in absence and presence of triethylamine in the reaction mixture.
Complex 1 presents antiferromagnetic Ni-Ni coupling via the diphenoxido bridge whereas
complex 2 presents a predominant ferromagnetic coupling via three different types of bridges: (i)
P and P -phenoxido, (ii) P -hydroxido and (iii) P -QLWULWRꢀꢁꢂțOꢃꢄțN). Although a search in the
2
3
3
2
CCDC database (updated Feb. 2016) shows that there are two examples of linear trinuclear
Ni(II) complexes with ꢂțOꢃꢄțN-nitrito bridges [23-25], there is no cyclic Ni3 complex and,
therefore, compound 2 is the first cyclic Ni3 complex with such DꢀꢂțOꢃꢄțN-nitrito bridge.
2. Experimental Section
The reagents and solvents used were of commercially available reagent quality.
Caution! Perchlorate salts of metal complexes are potentially explosive; and caution should be
exercised when dealing with such derivatives.
2.1. Synthesis of the reduced Schiff base ligand (HL) 2-[(2-dimethylamino-ethylamino)-methyl]-
phenol
The Schiff base ligand was synthesized by refluxing a solution of salicylaldehyde (0.52
mL, 5 mmol) and N,N-dimethylethylenediamine (0.54 mL, 5 mmol) in methanol (30 mL) for one
hour [26]. The solution was cooled to 0 ºC and solid sodium borohydride (210 mg, 6 mmol) was
added slowly to this methanolic solution with stirring. After completion of the addition, the
resulting solution was acidified with concentrated HCl (5 mL) and then evaporated to dryness
[27,28]. The reduced Schiff base ligand HL was extracted from the solid mass with methanol and
this methanol solution (ca. 20 mL) was used for the preparation of the complexes.
2.2. Synthesis of the complex [Ni2L2(NO2)2] (1)
3

An extracted methanol solution of HL as prepared above was added to a solution of
Ni(ClO4)2·6H2O (1.828 g, 5 mmol) in methanol (20 mL) and an aqueous solution (1 mL) of
sodium nitrite (1.035 g, 15 mmol) was added to this mixture with stirring. The mixture was
stirred for 1 h and filtered. The filtrate was kept undisturbed at room temperature. Light green
crystals of 1 suitable for X-ray diffraction were collected after 1 day on slow evaporation of the
solvent.
Yield: 1.117 g (75 %), Anal. calc. for C22H34N6Ni2O6: C, 44.34; H, 5.75; N, 14.10 %.
found: C, 44.04; H, 5.84; N, 13.66%. IR (KBr): Q(N-H), 3283 cm-1, Q(C-N), 1595 cm-1ꢅꢀȜ
max
(nm), (methanol), 650.
2.3. Synthesis of the complex [Ni3L3(OH)(NO2)]·ClO4 (2)
A methanol solution of HL (20 mL), prepared using the same quantity of reactant as
stated above, was added to a solution of Ni(ClO4)2·6H2O (1.828 g, 5 mmol) in methanol (20
mL). An aqueous solution (1 mL) of sodium nitrite (0.115 g, 1.67 mmol) was added to this
mixture with stirring. Triethylamine (0.7 mL, 5 mmol) was added drop wise to this solution with
constant stirring. The mixture was stirred for 1 h and filtered. The filtrate was kept undisturbed at
room temperature. The resultant solution on keeping overnight at room temperature yielded deep
green single crystals suitable for X-ray analysis.
Yield: 1.652 g (72 %), Anal. calc. for C33H52ClN7Ni3O10: C, 43.16; H, 5.71; N, 10.68 %.
found: C, 42.98; H, 5.85; N, 10.52%. IR (KBr): Q(N-H), 3276 cm-1, Q(C-N), 1596 cm-1ꢅꢀȜ
max
(nm), (methanol), 638.
2.4. Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 240C elemental
analyzer. IR spectra in KBr pellets (4500-500 cm-1) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer. Electronic spectra in methanol (1200-350 nm) were recorded in a
Hitachi U-3501 spectrophotometer. The DC magnetic susceptibility measurements were carried
out in the temperature range 2-300 K with an applied magnetic field of 0.1 T on polycrystalline
samples of compounds 1 and 2 (with masses of 18.54 and 49.65 mg, respectively) with a
4

Quantum Design MPMS-XL-5 SQUID susceptometer. The susceptibility data were corrected for
the sample holders previously measured using the same conditions and for the diamagnetic
contributions of the salt as deduced by using Pascal´s constant tables (F = -321.9x10-6 and -
dia
499.4x10-6 emu.mol-1, respectively) [29].
2.5. Crystal Data Collection and Refinement
Crystal data for the two crystals are given in Table 1. 3545 and 8853 independent
reflection data were collected for 1 and 2 ZLWKꢀ0R.ĮꢀUDGLDWLRQ. Data for 1 were measured at 150
K using an Oxford Diffraction X-Calibur CCD System while data for 2 were measured at 293 K
using a Bruker SMART diffractometer. The crystal of 1 was positioned at 50 mm from the CCD
and 321 frames were measured with counting times of 10 s. Data analysis was carried out with
the CrysAlis program [30]. Both structures were solved using direct methods with the Shelxs97
program [31]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The
hydrogen atoms were included in geometric positions and given thermal parameters equivalent to
1.2 times those of the atom to which they were attached. Absorption corrections for 1 were
carried out using the ABSPACK program [32]. The structures were refined using Shelxl97 [31]
on F2 to R1 0.0315, 0.0348; wR2 0.0655, 0.1086 for 2834, 7423 data with I > ꢄıꢁI).
3. Results and Discussion
3.1. Synthesis of the Complexes
The condensation of N,N-dimethylethylenediamine in 1:1 molar ratio with
salicylaldehyde afforded the Schiff base, 2-[(2-dimethylamino-ethylimino)-methyl]-phenol
which on reduction with sodium borohydride readily produced the reduced Schiff base, HL
(Scheme 1). HL on reaction with nickel(II) perchlorate in 1:1 molar ratios with presence of
sodium nitrite (excess) yielded the diphenoxido bridged compound 1. However, the elemental
analytical data, X-ray powder diffraction patterns and magnetic data suggested that the
crystalline compound was not pure. We therefore recrystallized the compound from methanol
and got few single crystals of another compound along with the crystalline compound of 1. X-ray
diffraction studies of this crystal revealed that it is a cyclic trinuclear species (complex 2; scheme
1). When triethylamine was added to the reaction mixture, the proportion of complex 2 was
5

found to increase considerably. We succeeded to synthesize complex 2 in pure form by
decreasing the proportion of nitrite ion i.e. by adding a methanolic solution of Ni(ClO4)2·6H2O to
a methanolic solution of the reduced Schiff base ligand (HL) followed by the addition of an
aqueous methanolic solution of NaNO2 in a 3:3:1 molar ratio. Triethylamine was added to
deprotonate the reduced Schiff base ligand and also to make the solution slightly alkaline
(pHꢀ|ꢀ8). In a slightly alkaline medium, the hydroxido ion bridges three metal ions. On the other
hand, compound 1 can not be completely freed from complex 2 even after repeated
recrystallizations. So the magnetic study of 1 had to be performed on a sample that contains a
slight amount of compound 2 as impurity.
6

HC
N
N
OH
NaBH4
H2C
NH
N
OH
(HL)
1) Ni(ClO4)2.6H2O
1) Ni(ClO4)2.6H2O
2) NaNO2
3) Et3N
2) NaNO2
(HL:Ni2+:NO2 = 1:1:3)
-
-
(HL:Ni2+:NO2 = 3:3:1)
O
N
NH
N
N
O
N
O
H
O
N
NH
NH
Ni
Ni
Ni
O
O
O
O
O
O
O
Ni
N
Ni
O
N
NH
N
NH
1
2
Scheme 1. Formation of the complexes.
3.2. IR and Electronic Spectra
A moderately strong, sharp peak due to the N-H stretching vibration, at 3283, and 3276
cm-1 for complexes 1 and 2 respectively, confirms that the imine group of the Schiff base is
reduced. This reduction is also very clearly indicated by the absence of the strong band due to the
imine vibration which appears in the region 1620-1650 cm-1 for the complexes of the
7

corresponding unreduced Schiff bases [8]. The absorption bands at 1268, 1203, and 872 cm-1 for
1 and at 1287, 1238, and 875 cm-1 for 2 are WHQWDWLYHO\ꢀ DVVLJQHGꢀ WRꢀ Ȟ (NO2ꢆꢇꢀ Ȟ (NO2), and
s
as
įꢁ12 ), respectively [33]. Complex 2 shows a single band at 3575 cm-1 indicating the presence
2
of a hydroxido group. The strong single bands at 1096 cm-1 for complex 2 indicate the presence
of anionic perchlorate groups.
The electronic spectra of these two compounds were recorded in methanol solution. The
electronic spectra show absorption bands at 650 and 1026 nm (for 1), and 638 and 1006 nm (for
3
3
3
3
2), which can be assigned to the spin-allowed transitions T1g(F)m A2g and T2gm A2g
respectively [34]. Moreover, the intense UV absorption bands observed at 359, and 352 nm for
complexes 1 and 2, respectively, have been assigned to the result of mixing of L o M charge-
3
transfer with 3T1g(P) m A2g transitions.
3.3. Crystal structures of the complexes 1 and 2
The crystal structure of 1 consists of di-ȝ -phenoxido-bridged centrosymmetric dimeric
2
units of formula [Ni2L2(NO2)2]. The structure is shown in Fig. 1 together with the atomic
numbering scheme. Dimensions in the metal coordination spheres of 1 are given in Table 2.
8

Fig. 1. The structure of the centro-symmetric complex 1 with ellipsoids at 20% probability.
The nickel atoms are six-co-ordinate with a distorted octahedral environment, the
distortions mainly arise from the small bite angle of 57.47(5)º of the chelating nitrite ligand. The
metal is also bonded to the tridentate ligand L in a fac configuration through O(11) at 2.0470(12)
Å, N(19) at 2.0632(14) Å and N(22) at 2.0962(15) Å. In addition the metal atom is bonded to
O(11)a (a = 1-x, -y, 1-z) from the second ligand L at 2.0680(12) Å. The bond lengths to the
bidentate nitrite are very similar (2.1807(13) and 2.1847(13) Å). The four donor atoms O(11),
O(31), O(33), and N(22) describe the basal plane of the Ni(1) atom. The deviations of these
coordinating atoms from the least-square mean plane through them are -0.022(1), 0.036(2),
-0.037(1), and 0.024(2) Å. The displacement of the Ni(1) atom from the same plane towards the
axially coordinated O(11)a atom is 0.091(1) Å. The two Ni atoms are separated by 3.193(1) Å
and the Ni(1)ꢁO(11)ꢁNi(1)a angle is 101.77(6)º. The proton on N(19) forms two hydrogen bonds
to O(33) and N(32) with dimensions (Table 3) N···O = 3.168(2) Å, H···O = 2.31 Å, NꢁH···Ob
(b = 1-x, -y, 2-z) = 157º and N···N = 3.338(2) Å, H···N = 2.61 Å and NꢁH···Nb (b = 1-x, -y, 2-
z) = 138º, respectively, resulting in a 1D supramolecular structure (Fig. 2). There are also two
intramolecular hydrogen bonds C(13)ꢁH(13)···O(33)a (a
=
1-x, -y, 1-z) and
C(24)ꢁH(24C)···O(31)a (a = 1-x, -y, 1-z) with dimensions C···O = 3.188(2) Å, H···O = 2.31 Å,
CꢁH···Oa = 158º and C···O = 3.067(2) Å, H···O = 2.38 Å and CꢁH···Oa = 128º, respectively.
Fig. 2. H-bond network giving rise to the 1D supra-molecular structure in compound 1. hydrogen
atoms except H13, H19 and H24C have been omitted for clarity.
9

The asymmetric unit of complex 2 consists of a discrete [Ni3L3(NO2)(OH)]+ cation, and a
non-coordinated perchlorate anion required to balance the charge. The perspective view of the
trinuclear structure of 2 is shown in Fig. 3.
Fig. 3. a) The structure of the [Ni3L3(NO2)(OH)]+ cation in 2 with ellipsoids at 20 % probability;
b) Face and edge-sharing octahedra presentation of compound 2.
10

In the cation of compound 2 each nickel is six-coordinate with a slightly distorted
octahedral environment and is bonded to the three donor atoms of a ligand L in a fac
configuration. Bond lengths and angles around the metal centres are given in Table 4 and are
similar to those in complex 1. It should be noted that, as in 1, the bonds to the secondary amine
nitrogen atom (in the range 2.034(2)-2.076(2) Å) are shorter than those to the tertiary one (in the
range 2.108(2)-2.157(2) Å). All three metal atoms are bonded to two oxygen atoms at varying
distances; a central hydroxide anion O(74) at 2.093(2), 2.049(2), 2.007(2) Å and O(51) from a
ligand L at 2.095(2), 2.349(2), 2.046(2) Å from Ni(1), Ni(2) and Ni(3), respectively. These
variations are presumably due to the differing atoms that complete the coordination spheres of
the metal atoms. Thus Ni(1) and Ni(3) are bridged by a 1NN:2NO-nitrite anion with distances
Ni(1)-O(72) 2.068(2) Å and Ni(3)-N(71) 2.146(2) Å while Ni(2) forms a bond to phenoxido
O(11) atom at 2.053(2) Å connecting Ni(2) with Ni(1). The distances between the Ni atoms
(Ni(1)···Ni(2) = 2.887(1) Å, Ni(1)···Ni(3) = 3.050(1) Å and Ni(2)···Ni(3) = 2.873(1) Å) indicate
that complex 2 can be described as an isosceles triangle where the distance between the Ni atoms
bridged by the nitrite is significantly longer than the other two. All three distances are
considerably shorter than the Ni···Ni distance of 3.193(1) Å in the dimeric complex 1. The two
P bridges, O(51) and O(74), are located at 1.311(2) and 1.149(2) Å from the Ni3 plane,
3
respectively. The geometry around Ni(1) can be described as a tetragonally elongated
octahedron with two large axial bonds from the µ3-hydroxido O(74) and the amino nitrogen
N(22) in trans. The four donor atoms O(11), O(51), O(72) and N(19) form the basal plane of
Ni(1). The r.m.s. deviation of the four basal donor atoms from their mean plane around Ni(1) is
0.0035 Å. The deviation of the Ni(1) atom from the same plane is 0.145(1) Å. In contrast, the
geometry around Ni(2) and Ni(3) can be described as a tetragonally compressed octahedron with
two short axial bonds from the µ3-hydroxido O(74) and the amino nitrogen trans to it (N(39) and
N(59) in Ni(2) and Ni(3), respectively). The four donor atoms O(11), O(51), O(31), N(42) and
O(31), O(51), N(71), N(62) form the basal planes of Ni(2) and Ni(3) respectively. The r.m.s.
deviation of the four basal donor atoms from their mean plane around Ni(2) and Ni(3) are
0.0284, 0.0694 Å respectively. The deviations of the Ni(2) and Ni(3) atoms from the same plane
are 0.135(1) and 0.053(1) Å, respectively. The central hydroxide O(74) forms a hydrogen bond
to a perchlorate oxygen atom O(81) with O···O = 3.022(4) Å, H···O = 2.05 and OꢁH···O = 174º
11

(Table 3). Two other hydrogen bonds are found from the aliphatic nitrogen atoms to the
perchlorate oxygen atoms: N(19)-H···O(83)c (c = x, -1+y, z) and N(39)-H···O(84) with N···O =
3.358(6) and 3.269(4) Å, H···O = 2.50 and 2.46 Å and N-H···O = 157 and 149º, respectively.
N(59)-H forms no hydrogen bonds. In addition, an oxygen atom of nitrite group, O73, forms an
intermolecular hydrogen bond C(58)-H(58A)···O(73)d (d = 1-x, -y, 1-z) with dimensions C···O
= 3.180(4) Å, H···O = 2.55 Å and CꢁH···O = 123º resulting in a 2D-supramolecular polymeric
structure (Fig. 4). There is also an intra-molecular hydrogen bond C(63)-H(63B)···O(73) with
dimensions C···O = 3.075(5) Å, H···O = 2.39 Å and C-H···O = 128º.
Fig. 4. Hydrogen bonded polymeric structure of compound 2 (all hydrogen atoms except H19,
H39, H74, H58A and H63B are omitted for clarity).
Trinuclear nickel(II) complexes with a Ni3 core motif similar to that of the present
complexes have been reported by our group with three different types of bridging, namely
phenoxido (P and P ), hydroxido (P ), and P -Cl, P -NCO, or P -N3 [3]. Besides these, there
2
3
3
2
1,1
1,1
are also few Ni3 complexes with bridging oxygen atoms of phenoxido [35], amino alcohol [36],
and carbohydrate systems [37-39]. A similar oxido-bridged core has also been found in two
heterometallic clusters Ni3McO4 (Mc = K and Na) [40]. However, to our knowledge, complex 2 is
the first example of a complex where an oxygen and a nitrogen atom acts as two separate
bridging atoms along with the oxido bridges, making the bridging system heteroatomic.
12

3.4. Magnetic Properties
The thermal variation of the product of the molar magnetic susceptibility per Ni dimer
times the temperature (F T) of compound 1 shows a room temperature value of ca. 2.5 cm3 K
m
mol-1, the expected value for two isolated Ni(II) ions with a g value of ca. 2.2 (Fig. 5). When the
sample is cooled, the F T product shows a progressive decrease down to ca. 10 K and a more
m
abrupt decrease at lower temperatures to reach a value of ca. 1.0 cm3 K mol-1 at 2 K. This
behaviour suggests the presence of predominant antiferromagnetic Ni-Ni exchange interactions
in the dimer. Since the structure of compound 1 shows the presence of Ni dimers, we have used a
simple S = 1 dimer model to fit the magnetic properties of this compound. This simple model
reproduces very satisfactorily the magnetic properties of compound 1 with g = 2.085,
J = -39 cm-1 and an impurity of 14.7 % of a S = 1 trimer complex (R = 0.99854 and chi-square =
0.021327, solid line in Fig. 5). Note that this impurity can also be observed as weak peaks at low
T values in the powder X-ray diffractogram. Note that this impurity represent ca. 1/7 of the total
contribution and, therefore, the real coupling value should be stronger (i.e., the module of J
should be above 39 cm-1).
3.0
2.5
2.0
1.5
1.0
0.5
0
50 100 150 200 250 300
T (K)
Fig. 5. Thermal variation of the F T product per Ni dimer for compound 1. Red line is the
m
best fit to the S = 1 dimer model (see text)
13

The moderate antiferromagnetic coupling found in compound 1 is not surprising since the
two Ni-O-Ni bond angles (101.77(5)º) are well above 98º, which is the approximate cross point
between ferro- and antiferromangetic exchange coupling in double oxido bridged Ni(II)
complexes [41-47]. Note that the abrupt decrease observed a very low temperatures is not
reproduced by this simple model and must be due to the ZFS of the trimeric Ni(II) impurity.
The thermal variation of the product of the molar magnetic susceptibility per Ni trimer
times the temperature (F T) for compound 2 shows a room temperature value of ca. 3.7 cm3 K
m
mol-1, close to the expected value for three isolated Ni(II) ions with a g value of ca. 2.2 (Fig. 6).
When the sample is cooled, the F T product increases and reaches a maximum of ca. 6.85 cm3 K
m
mol-1 at ca. 6 K. Below this temperature F T sharply decreases to reach a value of ca. 5.6 cm3 K
m
mol-1 at 2 K. This behaviour suggests the presence of predominant ferromagnetic Ni-Ni
exchange interactions in the trimer. The sharp decrease at low temperatures has to be attributed
to the ZFS of the resulting S = 3 ground spin state in the cluster.
7.0
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
6.5
6.0
5.5
5.0
4.5
0
10 20 30 40 50
0
50 100 150 200 250 300
T (K)
Fig. 6. Thermal variation of the F T product per Ni trimer for compound 2. Red line is the best
m
fit to the S = 1 trimer model (see text).
Given the similarities in the structural parameters (bond distances and angles) between the
two double oxido bridges connecting Ni(1)-Ni(2) and Ni(2)-Ni(3), we can assume, in a first
approach, that the structure of compound 2 can be described as an isosceles triangle (see above).
14

Accordingly, we have fit the magnetic properties of compound 2 with a S = 1 isosceles triangular
model with two different exchange interactions (J12 = J23 = J connecting Ni(1)-Ni(2) and
Ni(2)-Ni(3) and J13 = -¶ connecting Ni(1)-Ni(3), Fig. 7) by using the equation derived for a S = 1
isosceles triangle with the Kambé method [48]. This model reproduces very satisfactorily the
magnetic properties of 2 above the maximum with g = 2.146, J = + 18.2 cm-1 and -¶= -0.51 cm-1
(R = 0.99994 and chi-square = 0.021432, solid line in Fig. 6).
The moderate ferromagnetic exchange coupling found for J agrees with the expected one
from the Ni-O-Ni bond angles (88.37(6)º and 90.14(7)º for Ni(1)-O-Ni(2) and 90.16(7)º and
86.66(6)º for Ni(2)-O-Ni(3), all well below 98º). On the other side, although the Ni(1)-O-Ni(3)
bond angles (94.86(7)º and 96.07(7)º) are significantly larger, they are still below 98º and,
therefore, they should lead to a weak (but ferromagnetic) -¶ coupling. The weak
antiferromagnetic -¶ coupling constant found for this Ni(1)-Ni(3) interaction may be attributed to
the contribution of the extra 1NO:2NN-NO2 bridge connecting Ni(1) and Ni(3), which is well
known to produce moderate antiferromagnetic couplings [49-53]. Furthermore, the three
structurally and magnetically characterized Ni(II) complexes with a similar triple bridge (double
oxido and 1NO:2NN-NO2) show similar Ni-O-Ni bond angles and weak antiferromagntic
couplings (Table 5), as observed in compound 2.
Fig. 7. View of the Ni3 isosceles triangular complex in 2 showing the magnetic exchange
scheme.
15

Table 5. Structural and magnetic parameters of all the known Ni(II) complexes with double
oxido and single 1NO:2NN NO2 bridges.
Compound
J (cm-1) reference
Ni-O-Ni (q)
EQUIBIQ 95.02 / 95.32 -11.45
EQIBOW 93.86 / 96.18 -10.66
22
22
8
HOZXOK 96.12 / 95.12
96.21 / 94.85
-3.2
-0.51
this work
2
Conclusions
The tridentate reduced Schiff base ligand HL (2-[(2-dimethylamino-ethylamino)-methyl]-
phenol) on reaction with nickel(II) perchlorate in 1:1 molar ratios in presence of sodium nitrite
(excess) yielded a diphenoxido bridged Ni(II) compound 1. The presence of triethylamine with a
reduced proportion of nitrite ion in the above solution, yielded a triangular Ni3 compound (2) in
which the Ni(II) ions are triply bridged with the help of P - and P -phenoxido, P -hydroxido, and
2
3
3
P -QLWULWRꢀ ꢁꢂțOꢃꢄțN). The magnetic study revealed that compound 1 presents a moderate
2
antiferromagnetic Ni-Ni coupling (J = -39 cm-1) as expected from the large Ni-O-Ni bridging
angle (101.77(5)º). In contrast, compound 2 presents a predominant ferromagnetic coupling
(J = 18.2 cm-1) along the two equivalent sides of the isosceles triangle, in agreement with the
small Ni-O-Ni bond angles (in the range 86.66(6)º-90.16(7)º). The Ni-Ni coupling along the third
side of the triangle shows a very weak antiferromagnetic coupling (-¶ = -0.51 cm-1) as a result of
larger Ni-O-Ni bond angles (94.86(7)º and 96.07(7)º) and specially of the presence of a
1NO:2NN-NO2 bridge that cancels the weak ferromagnetic coupling expected from the Ni-O-Ni
bond angles.
Acknowledgements
We are thankful to the Department of Science and Technology (DST), New Delhi, India, for
financial support (SR/S1/IC/0034/2012). We thank DST FIST for financial support for the X-ray
diffraction facility. We also thank the British Engineering and Physical Sciences Research
16

Council (EPSRC) and the University of Reading for funds for the X-Calibur system. We also
thank the Generalitat Valenciana (projects PrometeoII/2014/076 and ISIC) and the Spanish
MINECO (project CTQ2014-52758-P) for financial support.
Appendix A. Supplementary material
CCDC 1487011 & 1487012 contain the supplementary crystallographic data for 1, and 2
respectively. These data can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit @ccdc.cam.ac.uk.
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19

Table 1. Crystal data and structure refinement of complexes 1 and 2.
Compound
Formula
M
1
C22H34N6Ni2O6
595.93
2
C33H52ClN7Ni3O10
918.40
Crystal System
Space Group
a/Å
Triclinic
Triclinic
P
1
P
1
8.0145(8)
9.0640(9)
9.4177(8)
69.182(9)
84.316(7)
85.916(8)
635.85(11)
1
11.2220(6)
11.3231(6)
16.0412(8)
87.325(2)
85.559(2)
76.300(2)
1973.54(18)
2
b/Å
c/Å
D/q
E/q
J/q
V/Å3
Z
Dc/g cm-3
µ/mm-1
F (000)
R(int)
1.556
1.545
1.530
1.547
312
960
0.020
0.025
Total Reflections
Unique reflections
I > 2V(I)
R1, wR2
Temp (K)
4490
23149
3545
8853
2834
7423
0.0315, 0.0674
150
0.0348, 0.1161
296
20

Table 2. Bond distances (Å) and angles (q) in the metal coordination spheres for complex 1.
Atoms
distance
Atoms
angle
Ni(1)-O(11)
Ni(1)-N(19)
Ni(1)-O(11)a
Ni(1)-N(22)
Ni(1)-O(33)
Ni(1)-O(31)
2.0470(12)
2.0632(14)
2.0680(12)
2.0962(15)
2.1807(13)
2.1847(13)
O(11)-Ni(1)-N(19)
O(11)-Ni(1)-O(11)a
N(19)-Ni(1)-O(11)a
O(11)-Ni(1)-N(22)
N(19)-Ni(1)-N(22)
N(22)-Ni(1)-O(11)a
O(11)-Ni(1)-O(33)
N(19)-Ni(1)-O(33)
O(33)-Ni(1)-O(11)a
N(22)-Ni(1)-O(33)
O(11)-Ni(1)-O(31)
N(19)-Ni(1)-O(31)
O(31)-Ni(1)-O(11)a
N(22)-Ni(1)-O(31)
O(33)-Ni(1)-O(31)
Ni(1)-O(11)-Ni(1)a
90.73(5)
78.23(5)
168.19(5)
105.40(5)
84.41(6)
94.42(5)
158.89(5)
83.92(5)
107.89(5)
94.42(5)
102.32(5)
90.44(5)
95.94(5)
151.85(5)
57.47(5)
101.77(6)
a = 1-x, -y, 1-z
Table 3. Hydrogen bonding distances (Å) and angles (q) for complexes 1 and 2. D = donor atom,
A = acceptor atom.
Complex D±H···A
D-H (Å) A···H (Å) D···A (Å)
‘D-H-A (q)
157
N(19)ꢁH(19)···O(33)b
0.91
0.91
0.93
0.96
0.91
0.91
0.98
0.97
0.96
2.31
2.61
2.31
2.38
2.50
2.46
2.05
2.55
2.39
3.168(2)
3.338(2)
3.188(2)
3.067(2)
3.358(6)
3.269(4)
3.022(4)
3.180(4)
3.075(5)
1
N(19)ꢁH(19)···N(32)b
C(13)ꢁH(13)···O(33)a
C(24)ꢁH(24C)···O(31)a
N(19)ꢁH(19)···O(83)c
N(39)ꢁH(39)···O(84)
O(74)ꢁH(74)···O(81)c
C(58)ꢁH(58A)···O(73)d
C(63)ꢁH(63B)···O(73)
138
158
128
157
2
149
174
123
128
Symmetry code: a = 1-x, -y, 1-z; b = 1-x, -y, 2-z; c = x, -1+y, z; d = 1-x, -y, 1-z.
21

Table 4. Bond distances (Å) and angles (q) in the metal coordination spheres for complex 2.
Atoms
distance Atoms
angle
Atoms
angle
Ni(1)±O(11) 2.025(2)
93.82(8)
169.48(7)
92.52(8)
83.28(7)
171.49(8)
89.25(7)
82.46(7)
99.36(8)
88.23(7)
72.36(7)
103.34(8)
83.92(9)
85.67(9)
104.52(8)
173.20(8)
83.96(6)
154.92(7)
82.89(6)
92.80(7)
171.84(8)
97.27(7)
102.50(8)
103.58(7)
101.31(7)
84.41(8)
78.69(6)
68.01(6)
76.60(6)
104.04(7)
171.44(7)
166.45(8)
75.17(6)
92.78(8)
105.83(9)
86.42(10)
177.94(9)
89.08(7)
95.90(9)
84.04(8)
94.15(9)
80.99(6)
91.42(8)
82.33(6)
99.58(8)
164.82(8)
90.14(7)
88.37(6)
86.66(6)
90.16(7)
94.86(7)
96.07(7)
O(11)ꢁNi(1)ꢁN(19)
O(11)ꢁNi(1)ꢁO(72)
N(19)ꢁNi(1)ꢁO(72)
O(11)ꢁNi(1)ꢁO(51)
N(19)ꢁNi(1)ꢁO(51)
O(72)ꢁNi(1)ꢁO(51)
O(11)ꢁNi(1)ꢁO(74)
N(19)ꢁNi(1)ꢁO(74)
O(72)ꢁNi(1)ꢁO(74)
O(51)ꢁNi(1)ꢁO(74)
O(11)ꢁNi(1)ꢁN(22)
N(19)ꢁNi(1)ꢁN(22)
O(72)ꢁNi(1)ꢁN(22)
O(51)ꢁNi(1)ꢁN(22)
O(74)ꢁNi(1)ꢁN(22)
O(31)ꢁNi(2)ꢁO(74)
O(31)ꢁNi(2)ꢁO(11)
O(74)ꢁNi(2)ꢁO(11)
O(31)ꢁNi(2)ꢁN(39)
O(74)ꢁNi(2)ꢁN(39)
O(11)ꢁNi(2)ꢁN(39)
O(31)ꢁNi(2)ꢁN(42)
O(74)ꢁNi(2)ꢁN(42)
O(11)ꢁNi(2)ꢁN(42)
N(39)ꢁNi(2)ꢁN(42)
O(31)ꢁNi(2)ꢁO(51)
O(74)ꢁNi(2)ꢁO(51)
O(11)ꢁNi(2)ꢁO(51)
N(39)ꢁNi(2)ꢁO(51)
N(42)ꢁNi(2)ꢁO(51)
O(74)ꢁNi(3)ꢁN(59)
O(74)ꢁNi(3)ꢁO(51)
N(59)ꢁNi(3)ꢁO(51)
O(74)ꢁNi(3)ꢁN(62)
N(59)ꢁNi(3)ꢁN(62)
O(51)ꢁNi(3)ꢁN(62)
O(74)ꢁNi(3)ꢁN(71)
N(59)ꢁNi(3)ꢁN(71)
O(51)ꢁNi(3)ꢁN(71)
N(62)ꢁNi(3)ꢁN(71)
O(74)ꢁNi(3)ꢁO(31)
N(59)ꢁNi(3)ꢁO(31)
O(51)ꢁNi(3)ꢁO(31)
N(62)ꢁNi(3)ꢁO(31)
N(71)ꢁNi(3)ꢁO(31)
Ni(1)ꢁO(11)ꢁNi(2)
Ni(1)ꢁO(74)ꢁNi(2)
Ni(2)ꢁO(31)ꢁNi(3)
Ni(2)ꢁO(74)ꢁNi(3)
Ni(1)ꢁO(51)ꢁNi(3)
Ni(1)ꢁO(74)ꢁNi(3)
2.057(2)
2.068(2)
2.095(2)
2.093(2)
2.157(2)
2.012(2)
2.049(2)
2.053(2)
2.076(2)
2.108(2)
2.349(2)
2.007(2)
2.034(2)
2.046(2)
2.131(2)
2.146(2)
2.171(2)
Ni(1)ꢁN(19)
Ni(1)ꢁO(72)
Ni(1)ꢁO(51)
Ni(1)ꢁO(74)
Ni(1)ꢁN(22)
Ni(2)ꢁO(31)
Ni(2)ꢁO(74)
Ni(2)ꢁO(11)
Ni(2)ꢁN(39)
Ni(2)ꢁN(42)
Ni(2)ꢁO(51)
Ni(3)ꢁO(74)
Ni(3)ꢁN(59)
Ni(3)ꢁO(51)
Ni(3)ꢁN(62)
Ni(3)ꢁN(71)
Ni(3)ꢁO(31)
22

Graphical abstract - Pictogram
Formation of a dinuclear and a trinuclear Ni(II) complex on slight variation
of experimental conditions: structural analysis and magnetic properties
Apurba Biswas, Michael G. B. Drew, Carlos J. Gómez-García and Ashutosh Ghosh
23

Graphical abstract - Synopsis
Formation of a dinuclear and a trinuclear Ni(II) complex on slight variation
of experimental conditions: structural analysis and magnetic properties
Apurba Biswas, Michael G. B. Drew, Carlos J. Gómez-García and Ashutosh Ghosh
A diphenoxido bridged antiferromagnetically coupled dinuclear Ni(II) complex [Ni2L2(NO2)2]
(1)
and a P and P -phenoxido, P -hydroxido, and P -QLWULWRꢀ ꢁꢂțOꢃꢄțN) bridged
2 3 3 2
ferromagnetically coupled trinuclear Ni(II) complex [Ni3L3(OH)(NO2)]·ClO4 (2), have been
synthesized using a tridentate reduced Schiff base ligand and varying the reaction conditions.
24