The importance of an additional water bridge in making the exchange coupling of bis(μ-phenoxo) dinickel(ii)complexes ferromagnetic

Article abstract of DOI:10.1039/c0dt01585g

Two new nickel(ii) complexes [Ni₂L₂(PhCOO) ₂(H₂O)] (1), [Ni₂L₂(PhCH ₂COO)₂(H₂O)] (2) have been synthesized using a tridentate Schiff base ligand, HL (2-[(3-

Full text of DOI:10.1039/c0dt01585g

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PAPER
The importance of an additional water bridge in making the exchange
coupling of bis(l-phenoxo)dinickel(II) complexes ferromagnetic†
a
a
b
a
Rituparna Biswas, Paramita Kar, You Song and Ashutosh Ghosh*
Received 15th November 2010, Accepted 3rd March 2011
DOI: 10.1039/c0dt01585g
Two new nickel(II) complexes [Ni
2
L
2
(PhCOO)
2
(H
2
O)] (1), [Ni
2
L
2
(PhCH
2
COO)
2
(H O)] (2) have been
2
synthesized using a tridentate Schiff base ligand, HL (2-[(3-dimethylamino-propylimino)-methyl]-
phenol) and the carboxylate monoanions, benzoate and phenylacetate, respectively. The complexes have
been characterized by spectral analysis, variable temperature magnetic susceptibility measurement and
crystal structure analysis. The structural analyses reveal that both complexes are dinuclear in which the
2
+
distorted octahedral Ni ions share a face, bridged by one water molecule and two m -phenoxo oxygen
2
atoms. A monodentate benzoate or phenylacetate anion and two nitrogen atoms of the chelating
deprotonated Schiff base (L) complete the hexa-coordination around the metal ion.
Variable-temperature magnetic susceptibility studies indicate the presence of dominant ferromagnetic
-
1
exchange coupling in complexes 1 and 2 with J values of 11.1(2) and 10.9(2) cm respectively. An
attempt has been made to rationalize the observed magneto-structural behavior considering the
importance of the additional water bridge in the present two complexes and also in other similar species.
Introduction
magneto-structural correlations are limited in nickel(II) complexes,
due to fewer numbers of known complexes and to the large
number of structural parameters that affect the superexchange
mechanism in these systems. The major factor controlling the
Dinuclear transition metal complexes and ligands facilitating their
formation have been extensively investigated due to their potential
applications in bioinorganic chemistry, magnetochemistry, mate-
rials chemistry and catalysis. Multidentate ligands containing
N, O donors, especially phenoxo O donor, have been widely used
19
exchange coupling is observed to be the bridging Ni–O–Ni angle.
1
–5
◦
It has been established that for Ni–O–Ni angles close to 90 ,
the magnetic coupling is ferromagnetic. As the Ni–O–Ni angle
6
to form such dinuclear species. In particular, the dinuclear Ni(II)
◦
increases from 90 , the ferromagnetic coupling decreases and
species are of continuing interest mostly because of the occurrence
of a pair of nickel(II) centers in the active site of the hydrolytic en-
zyme urease that catalyzes the hydrolysis of urea to ammonia and
◦
becomes antiferromagnetic at values ca. 93.5–99 , depending upon
the m
2
and m bridging modes of phenoxo oxygen atoms. To
3
design ferromagnetic phenoxo bridged Ni(II) complexes, it is thus
imperative to understand what factors make the bridging angle less
than this critical value. Most of the dinickel(II) complexes involving
7,8
9
carbamate or the ethanolysis of urea to ethyl carbamate. The
10–12
variable temperature magnetic behavior of urease is intriguing
and several synthetic models have addressed ligand environment
and magnetic and electronic properties of the nickel(II) pair.
However, recent interest in the magnetic study of polynuclear
Ni(II) complexes with various bridging groups emerges from their
a Ni
2
O
2
-bridging moiety that are reported in the literature, are
20–22
antiferromagnetically coupled.
bridged polynuclear Ni(II) complexes are reported but in most
cases they contain single phenoxo-bridge along with carboxylate
bridge.
characterized diphenoxo- bridged dinuclear nickel(II) complexes,
only two show ferromagnetic coupling. From a careful look into
the structures of these two complexes, one can find that in both of
them there is a water bridge in addition to the di-phenoxo bridge,
which may be an important factor for the lower bridging angle.
In this paper we report the synthesis, crystal structure and mag-
netic properties of two new di-phenoxo bridged dinickel(II) com-
plexes, [Ni L (PhCOO) (H O)] (1), [Ni L (PhCH COO) (H O)]
Some ferromagnetic phenoxo-
13,14
potential use as molecular magnetic materials.
Among them,
20,23
To the best of our knowledge, among the structurally
the carboxylates and the m-1,1-azido bridged Ni(II) complexes
deserve special mention as they are capable of transmitting ferro-
24
1
5,16
magnetic interactions.
Unlike hydroxo-, alkoxo- and phenoxo-
6a,17,18
bridged Cu(II) dinuclear complexes
structural modeling and
a
Department of Chemistry, University College of Science, University of
Calcutta, 92, A.P.C. Road, Kolkata, 700 009, India. E-mail: ghosh_59@
yahoo.com
2
2
2
2
2
2
2
2
2
b
State Key Laboratory of Coordination Chemistry, Nanjing National Labo-
(
2) using a tridentate Schiff base ligand, 2-[(3-dimethylamino-
ratory of Microstructures School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing, 210093, P. R. China
propylimino)-methyl]-phenol (HL) and the carboxylate ion, ben-
zoate (for 1) or phenylacetate (for 2). Both compounds contain
an additional bridging water to form a face shared bi-octahedron.
†
CCDC reference numbers 800753 and 800754. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt01585g
5
324 | Dalton Trans., 2011, 40, 5324–5331
This journal is © The Royal Society of Chemistry 2011

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Face-sharing systems are of special importance in understanding
Table 1 Crystallographic data for complexes 1 and 2
2
2
2
overlapping magnetic orbitals, since both dx -y and dz orbitals
25
1
2
are involved in bridging. The observed moderately strong
ferromagnetic coupling in both complexes has been rationalized
with the help of the low bridging angles.
Formula
M
Crystal System
C
788.17
38
H
46
N
4
Ni
2
O
7
40 50 4 2 7
C H N Ni O
816.22
Monoclinic
P21/c
8.4851(11)
21.865(3)
20.453(3)
90
98.113(2)
90
3756.6(9)
4
Triclinic
¯
Space Group
P1
Experimental
a/A˚
10.351(3)
18.545(5)
22.502(6)
68.265(7)
87.141(8)
83.820(7)
b/A˚
c/A
˚
Materials
◦
a ( )
◦
The reagents and solvents used were of commercially available
reagent quality, unless otherwise stated.
b ( )
◦
g ( )
V/A˚
3
3988.8(19)
4
1.359
0.997
1720
0.042
43562
15762
11199
0.0596, 0.2177,
296
Z
Synthesis of the Schiff-base ligand 2-[(3-dimethylamino-
propylimino)-methyl]-phenol (HL). The Schiff base was prepared
by the condensation of salicylaldehyde (1.05 mL, 10 mmol)
and N,N-dimethyl-1,3-propanediamine (1.26 mL, 10 mmol) in
methanol (10 mL) under reflux for an hour. The Schiff base ligands
were not isolated and the yellow colored methanolic solution was
used directly for complex formation.
3
D
c
/g cm-
m/mm
F(000)
R(int)
1.394
1.055
1656
0.044
-1
Total Reflections
Unique reflections
I > 2s(I)
25199
6241
4745
0.0352, 0.0920
293
R
1
, wR
2
T/K
Synthesis of [Ni
1.828 g, 5 mmol), dissolved in 10 mL of methanol, was added
2
L
2
(PhCOO)
2
(H
2
O)] (1). Ni(ClO
4
)
2
·6H
2
O
(
Crystallographic Data Collection and Refinement
to a methanolic solution (10 mL) of the Schiff base (HL) (5 mmol)
with constant stirring. After ca. 15 min a methanolic solution of
benzoic acid, PhCOOH (0.610 g, 5 mmol) was added with slow
stirring followed by addition of triethylamine (0.70 mL, 5 mmol).
Slow evaporation of the resulting green solution gave a dark green
microcrystalline compound. The green solid was then filtered
and washed with diethyl ether and dissolved in CH CN. X-ray
quality deep-green single crystals of compound 1 were obtained
by slow evaporation of the acetonitrile solution. (Yield: 1.57 g;
8
7
3
n
Suitable single crystals of each complexes were mounted on
a Bruker SMART diffractometer equipped with a graphite
˚
monochromator and Mo-Ka (l = 0.71073 A) radiation. The
structures were solved using Patterson method by using the
SHELXS97. Subsequent difference Fourier synthesis and least-
square refinement revealed the positions of the remaining non
hydrogen atoms. Non-hydrogen atoms were refined with indepen-
dent anisotropic displacement parameters. Hydrogen atoms were
placed in idealized positions and their displacement parameters
were fixed to be 1.2 times larger than those of the attached
non-hydrogen atom. Successful convergence was indicated by the
maximum shift/error of 0.001 for the last cycle of the least squares
refinement. All calculations were carried out using SHELXS
3
0%). Anal. Calcd. For C38
H
46
N
4
Ni
2
O : C, 57.91; H, 5.88; N,
.11. Found: C, 57.82; H, 5.69; N, 7.06. IR (KBr pellet, cm ):
438(broad) n(OH), 1634 n(C N), 1544 nas(C O), and 1466
7
-
1
s
(C O). lmax(CH
3
CN), 645 and 1046 nm.
Synthesis of [Ni
2
L
2
(PhCH COO) (H O)] (2). The procedure
2
2
2
2
6
27
28
29
9
7, SHELXL 97, PLATON 99, ORTEP-32 and WinGX
was the same as that for complex 1, except that a methanolic
solution of phenyl acetic acid, PhCH COOH (0.680 g, 5 mmol)
was added instead of benzoic acid. Needle-shaped deep-green
X-ray quality single crystals of compound 2 were obtained by
slow evaporation of the acetonitrile solution. (Yield: 1.53 g;
30
systemVer-1.64. Data collection and structure refinement param-
eters and crystallographic data for the two complexes are given in
Table 1.
2
7
6
3
n
5%). Anal. Calcd. For C40
.86. Found: C, 58.80; H, 6.09; N, 6.76. IR (KBr pellet, cm ):
423(broad) n(OH), 1632 n(C N), 1580 nas(C O), and 1458
H
50
N
4
Ni
2
O : C, 58.86; H, 6.17; N,
7
Results and discussion
-
1
Formation of the complexes
s
(C O). lmax(CH CN), 643 and 1043 nm.
3
The complexes were synthesized simply by allowing the Schiff base
to react with Ni(ClO
4
)
2
·6H
2
O in a methanol solution followed
Physical Measurements
by the addition of benzoic acid (for 1) or phenyl acetic acid
(for 2) in 1 : 1 : 1 molar ratios. Equimolar amount of triethyl
amine was added for deprotonation of the respective carboxylic
acid. The crystal field stabilization energy of the Schiff base
ligands those are derived from the 1,3-propanediamine derivatives
and salicylaldehyde is usually lower due to the longer M–N
bond distances compared to their ethylenediamine analogue and
consequently hexa-coordinated Ni(II) is stabilized.
negative tridentate Schiff base ligand along with a benzoate or
phenylacetate anion balance the charge of the complex species
but the metal ion remains coordinately unsaturated. In such a
situation, the phenoxo oxygen and or the carboxylate group can
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin–Elmer 240C elemental analyzer. IR
spectra in KBr (4500–500 cm ) were recorded using a Perkin–
Elmer RXI FT-IR spectrophotometer. Electronic spectra (1400–
-
1
350 nm) in CH
3
CN (for 1 and 2) were recorded in a Hitachi U-
31
3
501spectrophotometer. The magnetization data were recorded
20,32
on a Quantum Design MPMS-XL7 SQUID magnetometer with
an external magnetic field of 2000 Oe in the temperature ranges
of 1.8–300 K. The experimental magnetic susceptibility data are
corrected for the diamagnetism estimated from Pascal’s tables and
sample holder calibration.
The mono-
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Fig. 1 (a) Perspective view of the complex 1 (25% thermal probability ellipsoids). Methyl groups of amine nitrogen atoms (N(23) and N(38)) are omitted
for clarity. The intramolecular hydrogen bonds are shown as dashed lines. (b) The face sharing bi-octahedron structure of compound 1.
◦
Table 2 Selected bond lengths (A
˚
) and bond angles ( ) for complex 1
exploit their well known bridging properties to complete the hexa-
coordination in Ni(II) to result in polynuclear complexes. The
solvent molecules have also been found to be coordinated in some
Ni(1)–O(11)
Ni(1)–O(26)
Ni(1)–O(3)
Ni(1)–O(41)
Ni(1)–N(19)
Ni(1)–N(23)
2.011(2)
2.175(2)
2.096(2)
2.033(2)
2.003(2)
2.161(3)
91.48(9)
100.61(8)
167.51(7)
89.83(8)
99.53(9)
90.01(7)
174.74(8)
80.21(7)
91.60(7)
85.83(7)
76.48(7)
96.17(9)
85.71(10)
78.96(7)
127.96(18)
86.53(7)
86.03(7)
171.29(8)
98.27(8)
Ni(2)–O(11)
Ni(2)–O(26)
Ni(2)–O(3)
Ni(2)–O(51)
Ni(2)–N(34)
Ni(2)–N(38)
2.163(2)
2.017(2)
2.108(2)
2.059(2)
1.998(3)
2.146(3)
91.52(9)
99.51(8)
167.76(7)
90.64(8)
97.99(9)
90.10(7)
174.38(9)
76.58(7)
92.22(7)
79.71(7)
95.76(9)
87.48(10)
79.14(7)
123.54(17)
170.38(9)
97.82(9)
33
cases. In the present complexes, a solvent water molecule bridges
the two Ni(II) along with phenoxo oxygen while carboxylate ion
remains as monodentate. A strong H-bond between the uncoor-
dinated oxygen atom of the carboxylate and the bridged water
molecule may have important role in stabilizing the structures.
Therefore, it is very difficult to predict the structure of such self-
assembled species as various other parameters such as the steric
and electronic requirements of the Schiff base ligand, different
coordination modes of the anion, weak non-covalent forces etc
play important role in stabilization of the resulting structure in the
solid state.
O(11)–Ni(1)–N(19)
O(11)–Ni(1)–N(23)
O(11)–Ni(1)–O(41)
O(41)–Ni(1)–N(23)
O(26)–Ni(1)–N(19)
O(26)–Ni(1)–O(41)
O(26)–Ni(1)–N(23)
O(3)–Ni(1)–O(11)
O(3)–Ni(1)–O(41)
Ni(1)–(O3)–Ni(2)
O(3)–Ni(1)–O(26)
O(41)–Ni(1)–N(19)
N(19)–Ni(1)–N(23)
O(11)–Ni(1)–O(26)
Ni(1)– O(11)–C(12)
Ni(1)– O(11)–Ni(2)
Ni(1)– O(26)–Ni(2)
O(3)– Ni(1)–N(19)
O(3)– Ni(1)–N(23)
O(26)–Ni(2)–N(34)
O(26)–Ni(2)–N(38)
O(26)–Ni(2)–O(51)
O(51)–Ni(2)–N(38)
O(11)–Ni(2)–N(34)
O(11)–Ni(2)–O(51)
O(11)–Ni(2)–N(38)
O(3)–Ni(2)–O(11)
O(3)–Ni(2)–O(51)
O(3)–Ni(2)–O(26)
O(51)–Ni(2)–N(34)
N(34)–Ni(2)–N(38)
O(11)–Ni(2)–O(26)
Ni(2)– O(11)–C(12)
O(3)– Ni(2)–N(34)
O(3)– Ni(2)–N(38)
IR and UV-VIS spectra of complexes. In the IR spectra of
complexes 1 and 2, a strong and sharp band due to azomethine
-
1
-1
n(C N) appears at 1634 cm and 1632 cm . The appearance
-
1
of a broad band near 3400 cm indicates the presence of water
molecule in both of them. The IR spectral bands in the 1300–
1
1
650 cm- region are difficult to assign due to the appearance
of several absorption bands both from the Schiff base and the
carboxylate ligands. Nevertheless, by comparing the IR spectra
of the Ni(II) complexes of the same ligand but with other anions
numbering scheme. Selected bond lengths and angles are sum-
marized in Table 2. The crystal structure of 1 consists of a
-
1
(
azide and halides), the strong bands at 1544 and 1580 cm are
likely to be due to the antisymmetric stretching mode of the
discrete dinuclear unit of formula [Ni
dinuclear unit is formed by two independent nickel atoms, labeled
Ni(1) and Ni(2), bridged by one water molecule O(3) and two m
2
L
2
(PhCOO)
2
(H O)]. The
2
-
1
carboxylate group, whereas the bands at 1460 and 1458 cm may
be attributed to the symmetric stretching modes of the carboxylate
ligands in complexes 1 and 2 respectively. The absorption bands
in the electronic spectra, taken in acetonitrile solution are very
similar for the two complexes. Both of them exhibit a distinct band
at 645 and 643 nm for 1 and 2 respectively which can be assigned
2
-
phenoxo oxygen atoms O(11) and O(26). Each of the two metal
centers, Ni(1) and Ni(2) present distorted octahedral environment,
being coordinated to the deprotonated chelating Schiff base ligand
through the secondary amine nitrogen atoms N(23), N(38), the
imine nitrogen atoms N(19), N(34) and the phenoxo oxygen atoms
O(11) and O(26) respectively in facial configuration with usual
3
3
to the spin-allowed d-d transition T1g(F)← A2g. Another weaker
broad band which is centered at 1046 and 1043 nm for 1 and 2
respectively and is well-separated from the first band is assignable
2
0,24a,34
bond distances (Table 2).
The carboxylate oxygen atoms
3
3
to the transition T2g← A2g. These values are in agreement with
O(41) and O(51) of the terminally coordinated monodentate
benzoate ions, the oxygen atom O(3) of the water molecule
and the bridging phenoxo atoms O(26) and O(11) complete the
hexacoordination of Ni(1) and Ni(2) respectively. Thus two nickel
the literature values for octahedral Ni(II) compounds.
Description of structure of [Ni
2
L
2
(PhCOO)
2
(H O)] (1). The
2
structure of 1 is shown in Fig. 1a together with the atomic
5
326 | Dalton Trans., 2011, 40, 5324–5331
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◦
Table 3 Selected bond lengths (A
˚
) and bond angles ( ) for complex 2
atoms are linked through a triple oxygen bridge to form a face
shared bi-octahedron (Fig. 1b), leading to a Ni–Ni distance
Bond
A
B
˚
of 2.862 A, which is slightly shorter to those found in other
similar dinuclear nickel(II) complexes having a diphenoxo bridge
Ni(1)–O(11)
Ni(1)–O(26)
Ni(1)–O(3)
Ni(1)–O(41)
Ni(1)–N(19)
Ni(1)–N(23)
Ni(2)–O(11)
Ni(2)–O(26)
2.014(3)
2.159(4)
2.096(3)
2.018(4)
2.004(4)
2.148(6)
2.203(3)
2.013(3)
2.024(3)
2.156(3)
2.107(3)
2.052(4)
2.002(4)
2.132(5)
2.211(3)
2.006(3)
19,21,35,36
only.
The two phenoxo bridging angles, Ni(1)–O(11)–Ni(2)
◦
◦
and Ni(1)–O(26)–Ni(2) are 86.53(7) and 86.03(7) respectively,
and the water bridge angle, Ni(1)–O(3)–Ni(2) is 85.83(7) . Each
◦
bridging phenoxo oxygen atom is asymmetrically bound, with one
˚
Ni–O bond slightly longer [Ni(1)–O(26) 2.175(2) A; Ni(2)–O(11)
˚
˚
2
2
.163(2) A] than the other [Ni(1)–O(11) 2.011(2) A; Ni(2)–O(26)
Ni(2)–O(3)
Ni(2)–O(51)
Ni(2)–N(34)
2.084(4)
2.010(4)
2.014(4)
2.069(4)
2.015(3)
1.999(4)
˚
.017(2) A]. The bridging water molecule is also asymmetrically
˚
bonded between the two metal atoms [Ni(1)–O(3) 2.096(2) A;
˚
Ni(2)–N(38)
2.151(4)
2.163(4)
Ni(2)–O(3) 2.108(2) A]. These distances are comparable to those
37
O(11)–Ni(1)–N(19)
O(11)–Ni(1)–N(23)
O(11)–Ni(1)–O(41)
O(41)–Ni(1)–N(23)
O(26)–Ni(1)–N(19)
O(26)–Ni(1)–O(41)
O(26)–Ni(1)–N(23)
O(3)–Ni(1)–O(11)
O(3)–Ni(1)–O(41)
Ni(1)–(O3)–Ni(2)
O(3)–Ni(1)–O(26)
O(41)–Ni(1)–N(19)
N(19)–Ni(1)–N(23)
O(11)–Ni(1)–O(26)
Ni(1)–O(11)–C(12)
Ni(1)–O(11)–Ni(2)
O(3)–Ni(1)–N(19)
O(3)–Ni(1)–N(23)
Ni(1)–O(26)–Ni(2)
O(26)–Ni(2)–N(34)
O(26)–Ni(2)–N(38)
O(26)–Ni(2)–O(51)
O(51)–Ni(2)–N(38)
O(11)–Ni(2)–N(34)
O(11)–Ni(2)–O(51)
O(11)–Ni(2)–N(38)
O(3)–Ni(2)–O(11)
O(3)–Ni(2)–O(51)
O(3)–Ni(2)–O(26)
O(51)–Ni(2)–N(34)
N(34)–Ni(2)–N(38)
O(11)–Ni(2)–O(26)
Ni(2)–O(11)–C(12)
O(3)–Ni(2)–N(34)
O(3)–Ni(2)–N(38)
90.83(15)
102.89(16)
166.29(13)
88.30(17)
96.87(15)
88.68(14)
176.02(17)
79.20(13)
91.11(13)
86.59(14)
76.94(13)
97.89(15)
86.12(18)
79.75(13)
127.9(3)
85.51(12)
169.01(16)
100.52(17)
86.73(13)
90.28(15)
101.23(15)
167.51(16)
89.43(17)
100.25(16)
89.87(15)
172.41(19)
75.30(13)
91.83(15)
80.49(14)
96.77(16)
87.3(2)
86.59(14)
102.46(16)
165.09(15)
90.67(17)
98.59(15)
86.85(14)
173.86(16)
78.69(13)
92.61(14)
87.04(13)
76.37(13)
96.36(16)
87.26(17)
79.38(13)
126.8(3)
85.42(12)
169.48(15)
98.15(15)
87.32(13)
90.46(15)
101.55(14)
167.57(15)
89.15(16)
101.82(15)
90.03(14)
171.56(18)
75.42(13)
92.06(14)
80.61(14)
96.53(16)
86.6(2)
found in some aqua-bridged dinuclear Ni(II) complexes, in the
range 2.09–2.25 A reported for a few other known examples of
water bridges in Ni(II) structures.
˚
24,38
The sets of four donor atoms
O(11), N(19), O(41), O(3) describe the basal plane of Ni(1) and
the deviations of these coordinating atoms from the least-square
mean plane through them are -0.060(2), 0.053(3), -0.051(2),
˚ ˚
.058(2) A, respectively. Ni(1) atom is displaced 0.114(1) A from
0
the same plane in the direction of N(23) atom. The basal bond
lengths around the Ni(1) atom are in the range of 2.003(2)–
˚ ˚
.096(2) A. The apical bond lengths are Ni(1)–O(26) 2.175(2) A
and Ni(1)–N(23) 2.161(3) A, the bond angle O(26)–Ni(1)–N(23)
being 174.74(8) . Similarly in the coordination sphere of Ni(2)
the basal plane consists of O(3), O(26), O(51) and N(34) with
bond lengths in the range of 1.998(3)–2.108(2) A. The apical bond
lengths are Ni(2)–O(11) 2.163(2) A and Ni(2)–N(38) 2.146(3) A,
the bond angle O(11)–Ni(2)–N(38) being 174.38(9) . Deviations
of the coordinating atoms O(3), O(26), O(51), N(34) from the
least-square mean plane through them are -0.045(2), 0.046(2),
2
˚
◦
˚
˚
˚
◦
˚
0
.039(2), -0.040(3) A, respectively, and that of Ni(2) atom from
˚
the same plane is -0.126(1) A in the direction of N(38) atom.
The two hydrogen atoms H(3A) and H(3B) of the bridging water
molecule participate in strong intramolecular hydrogen bonding
to the oxygen atoms O(43) and O(53) respectively of the terminally
coordinated benzoate anions within the dimeric unit with D ◊ ◊ ◊ A
˚
˚
78.71(12)
118.1(3)
170.34(15)
97.17(18)
78.46(12)
119.8(3)
171.00(15)
96.22(18)
bond distances of 2.574(3) A and 2.554(3) A. (Table 4)
Description of structure of [Ni (PhCH COO) (H O)] (2).
2
L
2
2
2
2
The molecular structure of complex 2 is very similar to that of
complex 1. The only difference between the two structures is that
the terminally coordinated carboxylate group is phenyl acetate
in 2 in stead of benzoate in complex 1. The asymmetric unit
of 2 contains two independent Ni(II) dimers denoted as A and
B. The structures of the two dimers are very similar to each
other, except for a slight difference in the bond parameters, so
we limit the description to only one dimer 2B which is shown in
Fig. 2 together with the atomic numbering scheme. The structural
description of 2A is given in Supporting Information.† Selected
bond lengths and angles both are summarized in Table 3. Like
complex 1, the dinuclear structure of complex 2 consists of a
face sharing bi-octahedron having triple oxo bridge. All the donor
atoms in the coordination spheres of the Ni centers are bonded
in similar fashion as that of 1. The two Ni atoms are separated
those in 1. Moreover we note that in 1 the phenoxo oxygen atoms
and the water molecule bridge the two Ni centers asymmetrically
˚
which is maintained in complex 2 also [Ni(1B)–O(26B) 2.156(3) A;
˚
˚
Ni(2B)–O(11B) 2.211(3) A, Ni(1B)–O(11B) 2.024(3) A; Ni(2B)–
˚
˚
O(26B) 2.006(3) A] and [Ni(1B)–O(3B) 2.107(3) A; Ni(2B)–O(3B)
˚
2.069(4) A]. It is apparent that the basal planes around the metal
centers consist of three oxygen atoms and one imine nitrogen atom
as that of 1. The deviations of the four basal donor atoms O(3B),
O(11B), O(41B), N(19B) of Ni(1B) from their mean planes are
˚
0.073(3), -0.076(3), -0.063(4), 0.066(4) A. Similarly the deviations
of the four basal donor atoms O(3B), O(26B), O(51B), N(34B) of
Ni(2B) from their mean planes are 0.075(3), -0.077(3), -0.066(4),
˚
0.068(4) A. The two metal atoms Ni(1B) and Ni(2B) deviate from
˚
˚
˚
by 2.876 A with two phenoxo bridging angles, Ni(1B)–O(11B)–
the mean plane by 0.151(1) A and 0.106(1) A in the direction
of N(23B) and N(38B) atoms. The Ni–O and Ni–N distances
◦
◦
◦
Ni(2B) of 85.42(12) and Ni(1B)–O(26B)–Ni(2B) of 87.32(13)
˚ ˚
and the water bridge angle Ni(1B)–O(3B)–Ni(2B) of 87.04(13) .
The Ni–Ni distance and the bridging angles slightly differ from
in the basal planes are in the ranges 2.002(4) A-2.024(3) A and
˚
˚
1.999(4) A-2.069(4) A respectively. The apical bond lengths are
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 5324–5331 | 5327

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◦
Table 4 Hydrogen bonding distances ( A˚ ) and angles ( ) for the complex 1 and 2
.
. .
Compound
D–H
A
D–H
D ◊ ◊ ◊ A
A ◊ ◊ ◊ H
∠D–H ◊ ◊ ◊ A
1
O3–H3A ◊ ◊ ◊ O43
0.97
0.97
0.97
0.97
0.97
0.97
2.574(3)
2.554(3)
2.520(6)
2.582(5)
2.526(5)
2.573(6)
1.76
1.74
1.69
1.78
1.72
1.73
139
139
141
138
139
142
O3–H3B ◊ ◊ ◊ O53
2
2
B
A
O3B–H3B1 ◊ ◊ ◊ O43B
O3B–H3B2 ◊ ◊ ◊ O53B
O3A–H3A1 ◊ ◊ ◊ O53A
O3A–H3A2 ◊ ◊ ◊ O43A
Fig. 2 Perspective view of the complex 2B (25% thermal probability ellipsoids). Methyl groups of amine nitrogen atoms (N(23B) and N(38B)) are
omitted for clarity. The intramolecular hydrogen bonds are shown as dashed lines.
˚
˚
Ni(1B)–O(26B) 2.156(3) A and Ni(1B)–N(23B) 2.132(5) A and
˚
˚
Ni(2B)–O(11B) 2.211(3) A and Ni(2B)–N(38B) 2.163(4) A. The
◦
bond angles are O(26B)–Ni(1B)–N(23B) 173.86(16) and O(11B)–
Ni(2B)–N(38B) 171.56(18) . All these bond distances and angles
◦
show only minor variations from those in 1. In 2 also a strong
intradimer H-bonding is observed between the non-coordinating
oxygen atoms of phenylacetate ligand and bridging water with
˚
˚
D ◊ ◊ ◊ A bond distances of 2.520(6) A and 2.582(5) A which are
very close to those of complex 1. (Table 4)
Magnetic properties
The variable-temperature magnetic properties of 1 and 2 in the
forms of c
respectively (c
As can be seen, the two compounds show room temperature
M
T vs. T plots are illustrated in Fig. 3 and Fig. 4
is the molar susceptibility for two Ni(II) ions).
Fig. 3 Plot of c T vs. T for complex 1. The solid line is generated from
the best-fit magnetic parameters.
M
M
-
1
-1
c
M
T values of ca, 2.514 and 2.560 cm mol K which are slightly
the first step, we have fitted the magnetic properties of compounds
higher than the expected value for two non-interacting Ni(II) S =
1
and 2 to a simple S = 1 dimer model (the Hamiltonian is always
-
1
-1
1
ions (the expected spin only value is 2.0 cm mol K). When the
temperature is lowered compounds 1 and 2 show very similar
behavior: c T increases gradually to reach a maximum value
written as H = -JS
1
S
2
)
2
2
2J / kT
6J / kT
M
2Ng b
e
+5e
c =
×
(
1)
-
1
-1
of 3.431 and 3.410 cm mol K at 9 K and 16 K respectively
2J / kT
6J / kT
kT
1+3e
+5e
followed by a pronounced drop to reach values of 2.952 and
-
1
-1
In the second step, we have used the molecular field approxi-
mation expression to reproduce the interdimer antiferromagnetic
2
.527 cm mol K respectively at 1.8 K.
This behavior is typical of a system exhibiting dominant
20,39
coupling.
intramolecular ferromagnetic exchange coupling and at lower
temperature region; the drop of c T is due to ZFS of ground
M
c
cM
=
(2)
state (S = 2) or due to possible interactions between the dimers.
Because the evaluation of all the possible contributions at low
temperatures in a simultaneous way is difficult and little reliable,
we have proceeded in the following ways for both complexes. In
2
2
1
−(2zj'/ Ng b )c
where j¢ is the interdimer exchange interaction and cdim is the
expression for an S = 1 dimer (eqn (1)). For both compounds,
5
328 | Dalton Trans., 2011, 40, 5324–5331
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netic behavior has been found only in seven m
2
-phenoxo bridged
24,38,43,44
complexes
which have also been structurally characterized.
Among them in two complexes, the two Ni(II) ions are linked
by a carboxylate (acetate or benzoate) and a phenoxo bridge.
The ferromagnetic behavior of these compounds has been ten-
tatively ascribed to the 5-fold coordination of one of the Ni(II)
ions. However, the authors’ caution “Since it is hard to prove
the conservation of the dinuclear structure as resulting from
X-ray structure analysis under the experimental conditions of the
magnetic measurements, we abstain from further explanations”
should be noted as ferromagnetic coupling through such wide
◦
Ni–O–Ni angles that has been found (128.2 and 129.1 ) in
these compounds is unsupported by any other example. The
bridging mode between the two Ni(II) of three other ferromagnetic
compounds (two of which are dinuclear and the third one is
hexanuclear) is the same as the present two complexes, i.e. a triple
bridge is formed by the oxygen atoms of two phenoxo groups and
a molecule of water. The rest two compounds are heterometallic
Fig. 4 Plot of c T vs. T for complex 2. The solid line is generated from
the best-fit magnetic parameters.
M
this model reproduces satisfactorily the magnetic data in the
-
+
+
cubic clusters of [Ni
3-methoxy-benzylidene)-amino]-ethanesulfonate). Selected pa-
3
(OH)(L) ] and Na or K (L = 2-[(2-hydroxy-
45
3
whole temperature range with the following parameters J =
-
1
-1
1
1.1(2) cm , g = 2.176(3), zj¢ = -0.057(2) cm with R = 4.25 ¥
-
5
rameters relevant to the magnetic properties of both m phenoxo
10
{R is the agreement factor defined as: R = R[(c
M
T)exp.
-1
–
2
2
2
and water bridge complexes have been shown in Table 5. The
coupling in the complexes may be interpreted in terms of two
(
c
M
T)calcd.] /R(c
M
T)exp. } for complex 1 and J = 10.9(2) cm , g =
-
1
-
5
2
.195(2), zj¢ = -0.126(2) cm with R = 2.52 ¥ 10 for complex 2.
different bridges, the phenoxo and the H O (Fig. 6).
2
The inclusion of a ZFS does not improve significantly the fit
In the present two complexes, both type of bridging angles (phe-
noxo and water) are comparable but in two of the other reported
complexes, the phenoxo bridging angle(s) is considerably greater
than that of water bridge. For di- and tetranuclear nickel(II)
complexes magneto-structural correlations have been put forward.
These correlations indicate Ni–O–Ni border line angles of about
because the interdimer magnetic coupling and the D parameter are
very closely related and their independent contributions cannot
be easily accounted for. This result indicates that, besides the
intradimer coupling, the ZFS and the interdimer coupling are
present but their correct evaluation is not possible given their
46
47
20,40
close relation as was also found in various Ni(II) complexes.
The fit of the isothermal magnetization measurements at 1.8 K
Fig. 5) to a S = 2 Brillouin function yielded g = 2.20 (for 1) and
.23 (for 2). The results confirm a ferromagnetic coupling within
9
3.5 and 99.08 for the corresponding m
2
-O and m -O bridged di-
3
andtetranuclear systems, respectivelybelow whichaferromagnetic
(
2
interaction is observed. In all five compounds (Table 5), all three
◦
bridging angles are well below the critical value of 93.5 and hence
the dimers with S = 2 ground state.
Dinuclear nickel(II) complexes in which one or two phe-
ferromagnetic exchange through the accidental orthogonality of
the orbitals with the unpaired electrons (magnetic orbitals) is
expected. In this context, the role of the additional water bridge
noxo groups act as bridges are generally found to be
19,22,41,42
antiferromagnetic.
To the best of our knowledge, ferromag-
Fig. 5 Isothermal magnetizations for compounds 1 and 2 at 1.8 K.
Dalton Trans., 2011, 40, 5324–5331 | 5329
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Table 5 Magnetic and structural parameters of compounds 1 and 2 and of other known Ni(II) complexes presenting both m
2
phenoxo and water bridge
J value
Ni–m
2
phenoxo–Ni angles and
distances
2
Ni–mH O–Ni angles
and distances
Magnetic coupling in cm-
1
Ni ◊ ◊ ◊ Ni distances
Complex
1
◦
◦
˚
◦
˚
[
[
Ni
2
2
(H
2
L )
2
(H
2
O)]·H
2
O
Ferromagnetic
Ferromagnetic
9.9
3.1
91.7(2) , 91.2(3) 1.983(5) A,
81.0(2) 2.253(7) A,
2.913
2
.088(6) A˚ , 2.076(5) A˚ , 1.987(6) A˚ 2.232(7) A˚
2
◦
◦
˚
82.2(4) 2.240(11) A 2.953
˚
◦
Ni
(L )
2
(H
2
O)(NCS)
2
]·3H
2
O
91.5(4) , 91.5(4) 1.990(10) A,
2
.131(9) A˚
3
◦
˚
˚
◦
˚
[
[
Ni
Ni
6
(HL )
4
-H
2
O(C
5
H
5
N)
4
] _ 2DMF Ferromagnetic
5.14
11.1
87.25(18) , 2.032(6) A, 2.266(5) A 86.7(2) 2.164(4) A
2.970
85.83(7) 2.096(2) A, 2.862
.175(2) A˚ , 2.163(2) A˚ , 2.017(2) A˚ 2.108(2) A˚
4
86.53(7) ,86.03(7) 2.011(2) A,
˚
◦
◦
◦
˚
2
(L )
2
(PhCOO)
2
(H
2
O)]
Ferromagnetic
2
4
85.42(12) , 87.32(13) 2.024(3) A, 87.04(13) 2.107(3) A, 2.876
˚ ˚
◦
◦
◦
[
Ni
2
(L )
2
(PhCH
2
COO)
2
(H
2
O)]
Ferromagnetic
10.9
2
.156(3) A˚ , 2.211(3) A˚ , 2.006(3) A˚ 2.069(4) A˚
1
2
3
H
2
L = N,N-bis(3,4-dimethyl-2-hydroxybenzyl)-N¢,N¢-dimethylethylenediamine, L = N-(3-aminopropyl)salicylaldimine, HL = N-(3-t-butylbenzoyl)-5-
4
bromosalicylhydrazide, L = 2-[(3-dimethylamino-propylimino)-methyl]-phenol.
Fig. 6 Ni(II) environments in compounds 1 and 2 with the two different bridging (phenoxo and water) angles (deg).
in making the exchange coupling ferromagnetic deserves special
mention. Comparison of the structural parameters of these triple
bridged compounds with that of the solely diphenoxo bridged
species reveals that the Ni–Ni distance decreases due to the
additional water bridge. As a consequence the average Ni–O–
Ni bridging angle becomes lower and in general shorter is the
Ni–Ni distances, lower is the average Ni–O–Ni angles. However,
Table 5 shows that there are deviations from this linear relationship
because of the variation in the bond distances. It is obvious that
longer bond distances lower the bridging angle further and this
explains the difference in bridging angles between the two types of
bridges in the first two compounds of Table 5. In general, it may be
assumed that any additional bridge other than water should also
bring down the Ni–O(phenoxo)–Ni angle to a value lower than the
critical one in diphenoxo bridged complexes to make the exchange
coupling ferromagnetic. The presence of overall ferromagnetic
Ni(II) ions. An unusual feature of the structures is that the
carboxylate ions, which are well known for their various bridging
modes, remain monodentate in both complexes whereas the water
molecule acts as a bridging ligand. The strong H-bond between
the bridging water and the uncoordinated oxygen atom of the
carboxylate group seems to stabilize the resulted structure. The
majority of the reported di-phenoxo-bridged dinickel complexes
enforce a planarity geometry about the bridging oxygen donor
and result in strong antiferromagnetic interactions (-20 > J >
-
1
-100 cm ) as the bridging angle is higher than the critical value.
On the contrary, the face sharing octahedron created by triple oxo-
bridges (two phenoxo and one water) between the Ni(II) ions like
the present complexes are very rare. The magnetic study of these
complexes reveals that such bridges favor moderate ferromagnetic
coupling between the Ni(II) ions as the additional water bridge
brings the Ni(II) ion closer in the dinuclear entity, causing the
◦
interaction in a recently reported trinuclear isosceles Ni
3
core,
average bridging angle to be lower than the critical value of 93.5 .
44
bridged by m
3
-alcoholate O3 oxygen atoms is consistent with this
proposition.
Acknowledgements
We thank CSIR, Government of India [Junior Research Fel-
lowship to R.B and P.K Sanction no.09/028(0746)/2009-EMR-I
and 09/028(0733)/2008-EMR-I]. Crystallography was performed
at the DST-FIST, India-funded Single Crystal Diffractometer
Facility at the Department of Chemistry, University of Calcutta.
We also thank “National Natural Science Foundation of China
(20631030 and 20771057)”. We are thankful to Dr P. S. Mukherjee,
IISc, India, for discussion on magnetic results.
Conclusions
The NNO donor Schiff base ligand, derived from the condensation
of N,N-dimethyl-1,3-propanediamine and salicylaldehyde has
been used to synthesize two neutral dinuclear complexes with
Ni(II) having benzoate or phenylacetate ion as coligand. In both
structures, in addition to the di-phenoxo bridges from the Schiff
base ligand, a water molecule acts as a bridge between the two
5
330 | Dalton Trans., 2011, 40, 5324–5331
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