Mechanochemical control of synthesis and structures of aqua-bridged binuclear nickel(II) benzoate complexes

Article abstract of DOI:10.1002/ejic.200600667

Synthetic and structural aspects of a few binuclear nickel(II) complexes, having two bridging carboxylate groups and aqua groups are reported. Intramolecular O-H...O hydrogen bonding interactions between the bridging aqua group and the monodentate benzoate groups stabilises the bridging structures. Aqua-bridged nickel(II) benzoate complex is stable irrespective of solvent system and also can include solvent molecules such as toluene and benzene. A mixed chloro, aqua and 4-chloro-benzoato-bridged Ni^II binuclear complex was synthesised and structurally characterised. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600667

FULL PAPER
DOI: 10.1002/ejic.200600667
Mechanochemical Control of Synthesis and Structures of Aqua-Bridged
Binuclear Nickel(II) Benzoate Complexes
Anirban Karmakar,[a] Rupam J. Sarma,[a] and Jubaraj B. Baruah*[a]
Keywords: Nickel / Aqua-bridged complexes / Solvation / Hydrogen bonds
Synthetic and structural aspects of a few binuclear nickel(II)
complexes, having two bridging carboxylate groups and
aqua groups are reported. Intramolecular O–H···O hydrogen
bonding interactions between the bridging aqua group and
the monodentate benzoate groups stabilises the bridging
structures. Aqua-bridged nickel(II) benzoate complex is
stable irrespective of solvent system and also can include sol-
vent molecules such as toluene and benzene. A mixed
chloro, aqua and 4-chloro-benzoato-bridged NiII binuclear
complex was synthesised and structurally characterised.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
Results and Discussion
We have observed that the reaction of nickel(II) chloride
hexahydrate, and sodium benzoate in 1:2 stoichiometry
(Scheme 1) in the solid-state followed by addition of pyri-
dine gave the nickel(II) benzoate complex, 2a. The synthetic
procedure is important because the analogous reaction in
solution results in the formation of a mononuclear spe-
cies.[6] The complex is soluble in chloroform, benzene or
toluene and could subsequently be crystallised from ben-
zene in P21/c space group as light blue blocks. The crystal
structure of 2a shows that each of the nickel centres have
octahedral geometry involving two bridging benzoate
groups (Figure 1), two coordinated pyridine molecules and
a monodentate benzoate apart from a bridging aqua group;
The crystal structures of the active sites of binuclear
nickel(II) metallo-enzymes have shown that the metal
centres are bridged by carboxylates as well as water or hy-
droxide ions.[1] The binuclear metal active site of the urease
isolated from Klebseilla aerogenes, has aqua-bridged binu-
clear nickel(II) centres with pseudo-octahedral and square-
pyramid geometry around the nickel ions.[2] So the design
of binuclear aqua-bridged nickel(II) complexes is important
in biological modelling.[3] Study of the structure and prop-
erties of binuclear transition-metal complexes containing
bridging groups such as aqua groups[3] are expected to in-
crease our understanding of the crucial factors that deter-
mine the catalytic activity of metalloenzymes.[4] We recently
described the synthesis and catalytic activity of a binuclear
nickel(II) benzoate complex that contains an aqua group in
the bridging position.[5] We also communicated that the so-
lid state and solution synthetic route results in a variation
of the structure of nickel carboxylate complexes.[5] In the
biological environment the reactions of substrates takes
place in confined media, so, it is necessary to understand
how different types of motifs can be developed from the
complexes that have close structural resemblance to bio-
logical active sites. It is also necessary to understand the
stability of such complexes in the presence of various neu-
tral molecules in the lattice. With these objectives in mind
we investigated multiple component solid-state synthesis of
binuclear nickel(II) carboxylate complexes and the struc-
tural aspects of aqua-bridged nickel complexes in the pres-
ence of different neutral molecules in the lattice.
[a] Department of Chemistry, Indian Institute of Technology,
Guwahati, 781039, India
E-mail: juba@iitg.ernet.in
Scheme 1.
Eur. J. Inorg. Chem. 2006, 4673–4678
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A. Karmakar, R. J. Sarma, J. B. Baruah
FULL PAPER
Figure 1. Crystal structure of 2a with included benzene molecules (thermal ellipsoids drawn to 30% probability).
the Ni(1)···Ni(2) distance in this complex is 3.506(3) Å. The 2.104(9) Å, which is slightly longer than that in 2a, while
monodentate benzoate groups are intramolecularly the Ni(1)–O(4)–Ni(1Ј) bond angle is 115.25(8)°. This com-
hydrogen-bonded to the bridging aqua group through plex, 2b, is a polymorph of 2a and is characterised by a
O(9)–H···O(2) [2.565(3) Å, 166(3)°] and O(9)–H···O(8) similar binuclear nickel(II) core except for the presence of
[2.572(3) Å, 170(3)°] interactions. The Ni(1)–O(9) and a lattice water molecule that is intermolecularly hydrogen-
Ni(2)–O(9) bond lengths in this complex are 2.093(2) and bonded to the monodentate benzoate group through O(6)–
2.094(2) Å while the corresponding Ni(1)–O(9)–Ni(2) bond H···O(2) [2.910(2) Å, 164(3)°] interactions. Solid-state FT-
angle is 113.7(8)°.
IR spectroscopy of this complex shows broad signals at
There is a distorted octahedral geometry around each of 3537 and 3478 cm–1 arising from intramolecular and inter-
the nickel centres. The compound has an absorption maxi- molecularly hydrogen bonded O–H stretching vibrations. In
3
3
mum at 645 nm due to a A2 to T1 transition arising from methanol, the UV/Visible spectrum of 2b shows an absorp-
d8-electronic configuration in a weak field. The assembly tion band at 648 nm that may be assigned to the 3A2 to 3T1
is stabilised by weak aromatic C–H···π interactions (dC···π transition.
3.87 Å) involving the C60–H of the benzene molecule as the
Addition of pyridine to a mixture of nickel(II) chloride
donor and the aromatic ring of the benzoate groups of the hexahydrate, 1-naphthoic acid and potassium hydroxide in
molecule as the acceptor. In this case, the self-assembly of toluene/methanol (1:1 v/v) gave a blue solution, from which
the binuclear nickel(II) benzoate complex leads to the in- the corresponding aqua-bridged binuclear complex, [Ni2-
clusion of two benzene molecules in the crystal lattice that (H2O)(O2CNp)4(Py)4] (2c), was obtained (where Np is
are stabilised by weak intermolecular C–H···π interactions. naphthyl). The crystal structure of the complex is shown in
The complex [Ni2(H2O)(O2CPh)4(Py)4] when crystallised Figure 3 (a), which shows that the bridging aqua group is
from toluene results in the formation of a polymorph that hydrogen bonded to the monodentate naphthoate group
contains included benzoic acid and toluene molecules and through O(9)–H···O(8) [2.566(4) Å, 169(5)°] and O(9)–
we have reported this structure in the past.[5] When this H···O(2) [2.533(4) Å, 162(6)°] interactions. The structure of
complex was dissolved in ethanol and layered over water the complex shows that the two nickel(II) centres are
the complex [Ni2(H2O)(O2CPh)4(Py)4]H2O (2b) was crystal- bridged by a water molecule and two naphthoate groups
lised; the crystals were found to have crystallised in the or- such that the Ni(1)–O(9)–Ni(2) bond angle is 117.78(9)°,
thorhombic Pbcn space group. The structure of this com- with a Ni(1)···Ni(2) distance of 3.593(7) Å.
plex is shown in Figure 2. Intra-molecular O(4)–H···O(2)
In a similar manner to the one discussed above, a corre-
[2.573(17) Å, 166(3)°] hydrogen bonding interactions are sponding binuclear nickel(II) complex of 2-nitrobenzoic
also observed between the bridging aqua group and the acid was also prepared and could be crystallised from tolu-
monodentate benzoate group as shown in Figure 2. It is ene as blue-green plates in the C2/c space group. Crystal
observed that the Ni(1)–O(4) bond length in 2b is structure analysis shows that the corresponding aqua bridge
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Aqua-Bridged Binuclear Nickel(II) Benzoate Complexes
FULL PAPER
Figure 2. The structure of 2b containing a hydrogen-bonded water molecule; symmetry for equivalent atoms [ –x, y, –z + 1/2] (thermal
ellipsoids drawn to 30% probability).
in the binuclear nickel(II) complex with 2-nitrobenzoic acid Ni(1)–O(9) bond length is 2.089(9) Å with the Ni(1)–O(9)–
2d is structurally similar to that of 2c (Figure 3, b). It is Ni(2) bond angle being 117.05(8)°.
observed that solvent molecules are not included in the
So far in the above reactions we have obtained systemati-
crystal lattice of this complex apparently due to the steric cally aqua-bridged complexes, starting from nickel(II) chlo-
crowding caused by the nitro substituent on the aromatic ride as the inorganic salt. However, in the reaction of 4-
carboxylic acid molecules. As described earlier, each chlorobenzoic acid under analogous conditions a chloro-
nickel(II) centre in complex 2d is coordinated to the mono- bridged complex [Ni2(H2O)(Cl)(O2CAr)3(Py)4] (2e) was
dentate benzoate groups and two pyridine molecules, apart formed. The crystal structure of this complex revealed the
from the two bridging benzoate and bridging aqua group, presence of an aqua bridge (Figure 4) and a benzoate
which gives a six-coordinate octahedral geometry. The aqua bridge unlike complexes 2a–2d, along with a chloride bridg-
molecule in this complex is intramolecularly hydrogen ing group between the two nickel centres. In this case the
bonded to the monodentate benzoate through O(9)–H··· presence of three monodentate 4-chlorobenzoate groups led
O(2) [2.571(16) Å, 167(2)°] interactions; in this case the to the binuclear tribridged [Ni2(H2O)(Cl)(O2CAr)3(Py)4]
Figure 3. Crystal structures of (a) 2c and (b) 2d showing the intramolecular hydrogen bonding interactions between the bridging aqua
molecule and the monodentate carboxylate groups (thermal ellipsoids drawn to 30% probability).
Eur. J. Inorg. Chem. 2006, 4673–4678
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Karmakar, R. J. Sarma, J. B. Baruah
FULL PAPER
Figure 4. Crystal structure of 2e (thermal ellipsoids drawn to 30% probability).
complex, (2e) as shown in Figure 4. The complex also has the carbonyl closer, making the Ni–Oaqua–Ni bond angle
distorted octahedral geometry around the nickel centres smaller than in 2a. In the case of complex 2e the bond angle
3
and has an electronic absorption at 633 nm due to the A2 is the smallest in comparison to 2a–2d because both the
3
to T1 transition.
chloro and aqua groups bridge it.
It is observed that in complex 2e, the bridging aqua
molecule is intramolecularly hydrogen bonded to the car-
boxylate units of the monodentate 4-chlorobenzoate groups
via O(7)–H···O(2) [2.593(3) Å, 165(3)°] and O(7)–H···O(6)
[2.561(3) Å, 165(4)°] interactions. The corresponding Ni(1)–
O(7) and Ni(2)–O(7) bond lengths are 2.126(2) and
2.121(2) Å respectively while the Ni(1)–O(7)–Ni(2) bond
angle is found to be 101.18(9)°. Although the Ni(1)···Ni(2)
distance is 3.281(2) Å it is substantially shorter than 2a or
2c (Table 1). The reported values for Ni···Ni distance of sep-
aration in aqua-bridged complexes ranges from 3.497–
3.676 Å.[7] However, the effect of the substituent on the aro-
matic ring as well as solvation on the Ni–Oaqua–Ni angle
becomes prominent in the complexes 2c and 2d, which are
close to the reported bond angles.[7] When the Ni–Oaqua–Ni
angles among 2a and 2b are compared it is found that 2a
Ͻ 2b. This happens because the hydrogen bonding interac-
tion of the bridging water molecules in each case is dif-
ferent. In the case of complex 2b, the hydrogen bonding
interaction involving the interstitial water molecules draws
Conclusion
In summary, we have described the synthetic and struc-
tural aspects of a few binuclear nickel(II) complexes, having
two bridging carboxylate groups and aqua groups. Intra-
molecular O–H···O hydrogen bonding interactions between
the bridging aqua group and the monodentate benzoate
groups are observed in these complexes which add to the
stability of the bridging structure. From this structural
study on aqua-bridged structures we could show the struc-
tural implications of solvent molecules in different types of
co-crystals. Aqua-bridged nickel(II) benzoate complexes are
stable irrespective of solvent system and also can include
solvent molecules such as toluene and benzene. A mixed
chloro, aqua and 4-chloro-benzoato-bridged NiII binuclear
complex is formed from the reaction of 4-chlorobenzoate
with nickel(II) chloride in the presence of pyridine. This
suggests that a substituent on the aromatic ring has a role
in deciding the mechanochemical process during the solid-
state synthesis.
Table 1. Bond lengths (in Å) and angles (in °).
2a
2b
2c
2d
2e
Experimental Section
Ni(1)–Oaqua
Ni(2)–Oaqua
2.094(2)
2.094(2)[a] 2.143(2)
Ni(1)–Oaqua–Ni(2) 113.7(8)
Ni(1)···Ni(2) 3.506(5)
2.139(2)
2.088(2) 2.089(9)
2.108(2) 2.089(9)[a] 2.121(2)
2.126(2)
The reagents used were procured from commercial sources (Sigma–
Aldrich or Fluka) and used as received unless otherwise stated.
Solvents such as benzene, toluene, methanol and acetonitrile were
distilled and used.
111.87(11) 117.78(9) 117.05(8) 101.18(9)
3.554(3)
3.593(7) 3.563(6)
3.281(2)
[a] Symmetry equivalence Ni(1) and Ni(2) in 2a and 2d.
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Eur. J. Inorg. Chem. 2006, 4673–4678

Aqua-Bridged Binuclear Nickel(II) Benzoate Complexes
FULL PAPER
Table 2. Crystal data and refinement parameters for 2a–e.
2a
2b
2c
2d
2e
Formula
C57H51N4Ni2O9
1053.40
monoclinic
P21/c
C48H44N4Ni2O10
954.29
orthorhombic
Pbcn
C64H50N4Ni2O9
1136.50
monoclinic
P21/c
C48H38N8Ni2O17
1116.28
monoclinic
C2/c
C41H34Cl4N4Ni2O7
953.94
monoclinic
P21/c
Mol. weight
Crystal system
Space group
Temperature [K]
Wavelength [Å]
a [Å]
b [Å]
c [Å]
α [°]
296
296
296
296
296
0.71073
10.7917(2)
38.3193(7)
12.5877(2)
90
0.71073
14.2706(6)
21.2846(8)
15.2229(6)
90
0.71073
10.490(4)
20.917(9)
25.180(11)
90
0.71073
15.9027(5)
13.5204(5)
23.6140(10)
90
0.71073
12.9935(5)
14.3654(5)
22.6612(8)
90
β [°]
91.3750(1)
90
5203.90(16)
4
1.345
0.784
90
90
4623.9(3)
4
1.371
0.876
none
1984
96.16(3)
90
5493.0(4)
4
1.374
0.748
99.906(2)
90
5001.6(3)
4
1.482
0.834
97.762(3)
90
4191.1(3)
4
1.512
1.207
γ [°]
V [Å3]
Z
Density [Mg m–3]
Abs. coeff. [mm–1]
Abs. correction
F(000)
none
2196
none
2360
none
2296
none
1952
Total no. of reflections
Reflections, IϾ2σ(I)
Max. 2θ [°]
Ranges (h, k, l)
50748
12870
28.33
–11 Յ h Յ 14
–51 Յ k Յ 45
–16 Յ l Յ 16
99.1
34247
5768
28.32
–13 Յ h Յ 19
–28 Յ k Յ 28
–16 Յ l Յ 20
99.9
39758
12863
28.28
–13 Յ h Յ 13
–27 Յ k Յ 27
–27 Յ l Յ 33
94.4
21611
6153
28.33
–16 Յ h Յ 21
–17 Յ k Յ 16
–31 Յ l Յ 31
98.9
41439
10403
28.32
–16 Յ h Յ 17
–19 Յ k Յ 19
–30 Յ l Յ 30
99.7
Complete to 2θ (%)
Refinement method
full-matrix least
full-matrix least
squares on F2
5768/0/298
1.021
full-matrix least
squares on F2
12863/0/720
0.948
full-matrix least
squares on F2
6153/0/343
0.993
full-matrix least
squares on F2
10403/0/531
0.974
squares on F2
Data/restraints/parameters 12870/0/657
Goof (F2)
1.016
R indices [I Ͼ 2σ(I)]
R indices (all data)
0.0417
0.0761
0.0332
0.0556
0.0472
0.1025
0.0319
0.0452
0.0451
0.1146
The X-ray data were collected at 296 K with Mo-Kα radiation (λ
= 0.71073 Å) with a Bruker Nonius SMART CCD diffractometer
equipped with a graphite monochromator (Table 2). The SMART
software was used for data collection and also for indexing the
reflections and determining the unit cell parameters; the collected
data were integrated using SAINT software. The structures were
solved by direct methods and refined by full-matrix least-squares
calculations using the SHELXTL software.[8] All the non-H atoms
were refined in the anisotropic approximation against F2 of all re-
flections. The H-atoms, except those attached to N, O and F were
placed at their calculated positions and refined in the isotropic
approximation; those attached to heteroatoms (N, O and F) were
located in the difference Fourier maps, and refined with isotropic
displacement coefficients.
1037 (m), 814 (m), 757 (m), 716 (s), 696 (s), 676 (s) cm–1. λmax(me-
thanol) = 645 nm, ε = 18.91 –1 cm–1, 383 nm, ε = 65.6 –1 cm–1
Preparation of 2b: Benzoic acid (0.24 g, 2 mmol), potassium hy-
droxide (0.116 g, 2 mmol) and nickel(II) chloride hexahydrate
(0.238 g, 1 mmol) were finely ground in the mortar and heated at
100 °C for 15 min. The solid mixture was cooled to room tempera-
ture and transferred into a round-bottomed flask. Toluene (20 mL)
was added to the solid mass and the heterogeneous mixture was
stirred at room temperature for 5 min. To this mixture pyridine
(0.158 g, 2 mmol) was added. The residue was filtered off and the
blue filtrate was left undisturbed. From this solution blue crystals
were obtained after 3 d and dried in air. The blue crystals were
further dissolved in ethanol and water (1:1) mixture (5 mL) to ob-
tain crystals of 2b. Yield: 0.553 g, 58% (based on Ni). FT-IR (KBr):
CCDC numbers 614270–614274 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ν = 3537 (w), 3478 (w), 3060 (w), 2065 (w), 1630 (s), 1602 (m),
˜
1572 (s), 1528 (m), 1484 (m), 1447 (s), 1397 (s), 1215 (m), 1152 (w),
1070 (m), 1039 (m), 825 (s), 760 (m), 720 (s), 697 (s), 674 (s) cm–1.
λmax(methanol) = 648 nm, ε = 16.55 –1 cm–1; 389 nm, ε =
36.63 –1 cm–1.
Preparation of 2a: Nickel(II) chloride hexahydrate (0.238 g,
1 mmol) and sodium benzoate (0.28 g, 2 mmol) were finely ground
in a mortar and heated at 100 °C for 15 min. The solid mixture was
cooled to room temperature and transferred into a round-bottomed
flask and benzene (20 mL) was added and the heterogeneous mix-
ture was stirred at room temp. for 5 min followed by the addition
of pyridine (0.158 g, 2 mmol). The blue supernatant liquid was fil-
tered off and was left undisturbed. Blue crystals were collected after
3 d and dried in air. Yield: 0.371 g, 34% (based on Ni). FT-IR
The complexes 2c–2e were prepared from the corresponding acid
in an analogous manner to that for 2b. However, for the crystallisa-
tions different solvents were used.
2c: Crystallised from a toluene and methanol mixture (1:1). Yield:
0.296 g, 26%. IR (KBr): ν = 3423 (w), 3043 (m), 2071 (w), 1626
˜
(s), 1614 (s), 1588 (w), 1525 (w), 1484 (w), 1445 (m), 1410 (s), 1374
(s), 1256 (w), 1215 (m), 1152 (w), 1070 (w), 1039 (w), 891 (w), 861
(m), 787 (s), 697 (s), 655 (m), 628 (w) cm–1. λmax(methanol) 649 nm,
ε = 19.53 –1 cm–1; 388 nm, ε = 55.45 –1 cm–1.
(KBr): ν = 3412 (b), 3065 (w), 3032 (w), 1630 (s), 1602 (m), 1572
˜
(m), 1530 (w), 1481 (w), 1445 (m), 1394 (s), 1215 (m), 1067 (m),
Eur. J. Inorg. Chem. 2006, 4673–4678
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A. Karmakar, R. J. Sarma, J. B. Baruah
FULL PAPER
ham, D. Coucouvanis, Inorg. Chem. 1998, 37, 1232–1241; d)
S.-B. Yu, S. J. Lippard, I. Shweky, A. Bino, Inorg. Chem. 1992,
31, 3502–3504.
2d: Crystallised from toluene. Yield: 0.235 g, 21%. IR (KBr): ν =
˜
3445 (m), 3071 (m), 2071 (w), 1637 (s), 1604 (m), 1555 (w), 1520
(s), 1486 (m), 1448 (s), 1396 (s), 1363 (s), 1303 (w), 1218 (w), 1155
(w), 1071 (w), 858 (w), 824 (m), 781 (m), 736 (s), 701 (s) cm–1.
λmax(acetonitrile) = 638 nm, ε = 12.09 –1 cm–1.
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2e: Crystallised from an ethanol and acetonitrile mixture (2:1).
Yield: 0.134 g, 14%. IR (KBr): ν = 3420 (w), 3065 (w), 1703 (w),
˜
1614 (s), 1566 (w), 1533 (w), 1484 (w), 1445 (m), 1400 (s), 1218
(m), 1166 (m), 1091 (m), 1039 (w), 1012 (w), 850 (w), 820 (m), 772
(m), 697 (m), 631 (w), 532 (s) cm–1. λmax (methanol) 633 nm, ε =
24.42 –1 cm–1; 389 nm, ε = 40.92 –1 cm–1.
Acknowledgments
[5]
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The authors thank the Department of Science and Technology,
New-Delhi, India for supporting the X-ray facility.
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Received: July 17, 2006
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