Diamine incorporated compounds derived from polymeric nickel(II) fumarates and oxalates: Crystal structure, spectral and thermal properties of [Ni(en)3](O2C{single bond}CH{double bond, long}CH{single bond}CO2)·3H2O and [Ni(en)3](O2C{single bond}CO2)

Article abstract of DOI:10.1016/j.molstruc.2007.10.012

Lewis-base mediated fragmentation of polymeric nickel(II) fumarate and oxalate are attempted using chelating σ-donor diamines like ethylenediamine (en) and 1,3-diaminopropane (dap) in various conditions which yielded [Ni(en)₃](fum)·3H₂O (1), [Ni(en)₃](ox) (2), [Ni(dap)₂(fum)] (3) and [Ni(dap)(ox)]·2H₂O (4). While 1 and 2 are molecular products each containing octahedral [Ni(en)₃]²⁺ moieties and the anionic dicarboxylate species, 3 and 4 are dap-incorporated polymeric products. The fumarate derivative 1 containing [Ni(en)₃]²⁺ moieties crystallizes in the monoclinic space group C2/c with a = 17.899(4) A?, b = 11.747(2) A?, c = 10.748(2) A?, β = 125.59(3)°, V = 1837.7(6) A?³, Z = 4, while the oxalate analogue 2 is seen to be in the trigonal space group P-31c with a = 8.8770(13) A?, b = 8.8770(13) A?, c = 10.482(2) A?, γ = 120°, V = 715.3(2) A?³, Z = 2. The octahedral [Ni(en)₃] units in both 1 and 2 are seen to be strongly H-bonded to the dicarboxylate moieties through the coordinated en units leading to a three-dimensional network. However, in 1 the water molecules also take part in the H-bonding and contribute to the overall 3D structure. In both 1 and 2 the crystal packing is done with the [Ni(en)₃]²⁺ units with absolute configuration Λ(δδδ) and its mirror conformer with Δ configuration in exactly equal numbers. Spectral (IR and UV-Visible) and magnetic measurements were carried out and some of the ligand-field parameters like D_q, B and β were evaluated for all the four compounds. These values suggest the presence of octahedrally coordinated nickel(II) in all the four complexes. Spectral data suggest that 3 has the two chelating dap moieties and the fumarate coordinated in η¹ form through both its carboxylate moieties while 4 has one chelating dap and the oxalate moiety coordinated in η⁴-bis-chelating form. Though both 1 and 2 are made of the same type of [Ni(en)₃]²⁺ units their thermograms give entirely different thermal features; 1 showing three clearly successive and step-wise dissociation of each en unit while 2 having a combined loss of two en units in the first thermal step. The relevant thermodynamic and kinetic parameters like E_a and ΔS also could be evaluated for various thermal steps for the compounds 1-4 using Coats-Redfern equation.

Full text of DOI:10.1016/j.molstruc.2007.10.012

Available online at www.sciencedirect.com
Journal of Molecular Structure 885 (2008) 36–44
www.elsevier.com/locate/molstruc
Diamine incorporated compounds derived from polymeric
nickel(II) fumarates and oxalates: Crystal structure, spectral
and thermal properties of [Ni(en) ](O CACH@CHACO )Æ3H O
3
2
2
2
and [Ni(en) ](O CACO )
3
2
2
a,
a
b
b,
*
*
M. Padmanabhan , James C. Joseph , Xiaoying Huang , Jing Li
a
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686560, Kerala, India
b
Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 0854, USA
Received 20 July 2007; received in revised form 24 September 2007; accepted 4 October 2007
Available online 23 October 2007
Abstract
Lewis-base mediated fragmentation of polymeric nickel(II) fumarate and oxalate are attempted using chelating r-donor diamines like
ethylenediamine (en) and 1,3-diaminopropane (dap) in various conditions which yielded [Ni(en) ](fum)Æ3H O (1), [Ni(en) ](ox) (2),
Ni(dap) (fum)] (3) and [Ni(dap)(ox)]Æ2H O (4). While 1 and 2 are molecular products each containing octahedral [Ni(en)
and the anionic dicarboxylate species, 3 and 4 are dap-incorporated polymeric products. The fumarate derivative 1 containing [Ni(en)3]
3
2
3
+
2
[
2
2
3
]
moieties
2
+
˚
˚
˚
moieties crystallizes in the monoclinic space group C2/c with a = 17.899(4) A, b = 11.747(2) A, c = 10.748(2) A, b = 125.59(3)ꢀ,
˚
3
˚
V = 1837.7(6) A , Z = 4, while the oxalate analogue 2 is seen to be in the trigonal space group PÀ31c with a = 8.8770(13) A,
˚
˚
˚
3
b = 8.8770(13) A, c = 10.482(2) A, c = 120ꢀ, V = 715.3(2) A , Z = 2. The octahedral [Ni(en) ] units in both 1 and 2 are seen to be strongly
3
H-bonded to the dicarboxylate moieties through the coordinated en units leading to a three-dimensional network. However, in 1 the water
molecules also take part in the H-bonding and contribute to the overall 3D structure. In both 1 and 2 the crystal packing is done with the
2+
[
Ni(en)3] units with absolute configuration K(ddd) and its mirror conformer with D configuration in exactly equal numbers. Spectral (IR
and UV–Visible) and magnetic measurements were carried out and some of the ligand-field parameters like D , B and b were evaluated for
q
all the four compounds. These values suggest the presence of octahedrally coordinated nickel(II) in all the four complexes. Spectral data
1
suggest that 3 has the two chelating dap moieties and the fumarate coordinated in g form through both its carboxylate moieties while 4 has
4
one chelating dap and the oxalate moiety coordinated in g -bis-chelating form. Though both 1 and 2 are made of the same type of
2+
[
Ni(en)
3
]
units their thermograms give entirely different thermal features; 1 showing three clearly successive and step-wise dissociation
of each en unit while 2 having a combined loss of two en units in the first thermal step. The relevant thermodynamic and kinetic parameters
like E and DS also could be evaluated for various thermal steps for the compounds 1–4 using Coats–Redfern equation.
a
ꢁ
2007 Elsevier B.V. All rights reserved.
Keywords: Nickel(II); Fumarate; Oxalate; 1,2-Diaminoethane; 1,3-Diaminopropane; X-ray studies; Spectral properties; Thermal properties
1
. Introduction
of research due to their interesting structural features and
potential applications in the field of molecular sieves,
molecular recognition, sorption, catalysis, non-linear
optics, electrical conductivity and magnetic materials
Metal dicarboxylate (MDC) based organic–inorganic
framework materials have emerged as a fascinating area
[
1–6]. Structural and electronic tuning of MDCs is possible
in a variety of forms which includes incorporating auxiliary
ligands on the coordination sphere of the metal ion [7–21].
Amine incorporated metal carboxylates are usually pre-
pared by the interaction of the Lewis-bases with suitable
*
Corresponding authors. Tel.: +91 481 273 1036; fax: +91 481 273 1002
(
M. Padmanabhan); tel.: +1 732 445 3758; fax: +1 732 445 3312 (J. Li).
E-mail addresses: chemistryfolder@gmail.com (M. Padmanabhan),
jingli@rci.rutgers.edu (J. Li).
0
022-2860/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.10.012

M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
37
metal salt and dicarboxylic acids. But such an approach
often has the disadvantage of yielding mixture of products,
in some of which the anions of the metal salt also get incor-
porated. To avoid such a situation a better strategy would
be to prepare the metal dicarboxylates first and then inter-
act them with the auxiliary ligands. In this work we report
our attempts to interact polymeric Ni(II) fumarate and
Ni(II) oxalate with r-donor diamines like ethylenediamine
(3), and a mono-dap adduct [Ni(dap)(ox)]Æ2H O (4) even
2
after reacting the polymeric metal carboxylates with excess
of the diamine under reflux condition. In this work we
report the crystal and molecular structures of 1 and 2 in
detail, as well as the spectral and thermal features of all
the four adducts 1–4.
2. Experimental
(
en) and 1,3-diaminopropane (dap) both having chelating
properties but with different Dq values and report
structural, spectroscopic and thermal properties of two
crystalline tris–en products [Ni(en) ](fum)Æ3H O (1) and
The reagents 1,2-diaminoethane (en), fumaric acid,
K (C O ), NiCl Æ6H O and NaOH used were E-Merck
2
2
4
2
2
(India) samples. 1,3-Diaminopropane (dap) was purchased
from Merck, Germany and used as received.
3
2
[
Ni(en) ](ox) (2) (fum = fumarate dianion, ox = oxalate
3
dianion) obtained as molecular products after fragmenta-
tion. We also present some related properties of the
two dap-incorporated products of these metal dicarboxyl-
ates, [Ni(dap) (fum)] (3) and [Ni(dap)(ox)]Æ2H O (4) (dap =
2.1. Fragmentation of polymeric nickel-fumarate and -oxalate
by diamines and synthesis of molecular products
2
2
1
,3-diaminopropane), which are seen to exist not as
We have made attempts to interact and fragment poly-
meric nickel-fumarate and -oxalate with diamines under
various conditions, which include refluxing a suspension
of the insoluble metal dicarboxylate with the diamines in
an appropriate solvent medium at various compositions
and also exposing the solid polymer to the vapors of the
diamines in a closed glass chamber for a prolonged time
followed by solubilization and crystallization from water
or methanol. It is seen that irrespective of the relative
MDC:diamine ratio taken the stable products formed are
tris–en derivatives, [Ni(en) ](fum)Æ3H O (1) and [Ni(e-
fragmented products but as the diamine incorporated
polymeric systems.
Metal tris–ethylenediamine complexes, in general, are of
special interest owing to their diverse configurational and
conformational features they can exhibit because of the rel-
ative geometric tilts of the –CH –CH – spacer moieties
2
2
possible in the three en ligands in such complexes. It is
known that the relative dispositions of the –CH –CH –
2
2
units of the three en moieties in them can give rise to var-
ious conformational forms of K or D configurations for the
resulting complexes. There are ample proofs to show that
the reaction conditions and the nature of counter anions
play major role in deciding the type of conformers in metal
tris–ethylenediamine products [22–24]. Except in the case
of metal tartrates [24d–g] the tris–en complexes containing
any dicarboxylate ion as the counter anion have not yet
been investigated. Since the nature of the associated anion
and H-bonding features of the amine protons with the
neighboring species can have significant role in deciding
3
2
n) ](ox) (2) in the case of en adducts, while the diamine
3
dap is never seen to give any metal-tris(dap) product but
only polymeric products like [Ni(dap) (fum)] (3) and
2
[Ni(dap)(ox)]Æ2H O (4). Systematic steps involved in the
2
synthesis of these compounds in high yield are given below.
2.2. Synthesis of diamine incorporated metal-fumarates
and -oxalates
the overall configuration and conformations of the
2.2.1. Synthesis of [Ni(en) ](fum)Æ3H O (1)
3
2
n+
[
M(en)3] cations it was thought worthwhile to generate
A solution of fumaric acid (0.116 g, 1 mmol) dissolved
in aqueous NaOH (2 mmol, in 10 ml water) was added
drop-wise to a hot aqueous solution (10 ml) of NiCl Æ6H O
a few new dicarboxylate bearing metal tris–en systems
and look at their conformational preferences. Unlike the
2
2
n+
earlier reports on [M(en)3] containing halides, nitrates
(0.238 g, 1 mmol) with constant stirring. Since the solution
pH was seen to be slightly acidic a small amount of dilute
NaOH was added drop-wise and with stirring to bring the
pH slightly below 7.0 to get pale green precipitate of
[Ni(fum)(H O) ] . The solid product obtained was filtered,
and acetate anions [22a,b–25], we report here the structural
2
+
and spectral features of [Ni(en)3] bearing dicarboxylate
anions like fumarate and oxalate and look at how the nat-
ure of anions and associated H-bonding modulate their
structural, electronic and thermal properties. Knowing that
chelating r-donor ligands like en can fragment polymeric
nickel(II) fumarate and oxalate effectively to generate
molecular ionic products, we have also made an attempt
to generate closely related tris-(1,3-diaminopropane)
adducts of these metal dicarboxylates, as 1,3-diaminopro-
pane (dap) ligand is expected to exhibit interesting confor-
mational features like en. However, we find that dap does
not yield any tris–diamine type of adducts of nickel(II)
fumarate or oxalate like in the case of en, but produces
2
4 n
washed with distilled water till free from chloride ions. An
aqueous solution (10 ml) containing 0.24 ml (3 mmol) 1,2-
diaminoethane (en) was added drop-wise to an aqueous
suspension of the polymeric nickel fumarate (0.244 g,
1 mmol) prepared as above with constant stirring and heat-
ing. The clear violet solution obtained was filtered and kept
for crystallization in open air. Violet crystals of 1 suitable
for single crystal X-ray analysis were obtained after 3 days.
Anal. Calcd (%) for Complex (1) C H NiN O (407.13):
1
0
32
6
7
C, 29.47; H, 7.85; N, 20.63. Found C, 29.52; H, 7.87; N
20.66.
only a bis-(1,3-diaminopropane) product, [Ni(dap) (fum)]
2

38
M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
2
.2.2. Synthesis of [Ni(en) ](ox) (2)
3
Table 1
Crystallographic data and details of structure refinements for 1 and 2
An aqueous solution (10 ml) of 0.184 g dipotassium oxa-
late (1 mmol) was added drop-wise to a hot aqueous solu-
1
2
tion (10 ml) of 0.238 g NiCl Æ6H O (1 mmol) with constant
Formula
M
C10H32N NiO
407.13
293(2)
0.71073
C H N NiO
4
2
2
6
7
8
24
6
stirring. The green precipitate of nickel(II) oxalate formed
was washed with distilled water till free from chloride ions.
To an aqueous suspension of polymeric nickel(II) oxalate
r
327.04
293(2)
0.71073
Trigonal
PÀ31c
8.8770(13)
8.8770(13)
10.482(2)
90
90
120
715.3(2)
2
1.518
Temperature (K)
Wavelength (A)
Crystal system, space group Monoclinic
˚
(
0.182 g, 1 mmol) prepared as above a solution (10 ml) con-
Space group
C2/c
˚
taining 0.24 ml (3 mmol) 1,2-diaminoethane (en) was
added drop-wise with constant stirring and heating. The
nickel oxalate gets dissolved completely giving a deep pink
solution. The filtered solution was kept for crystallization
in open air. Pink needle shaped crystals of 2 suitable for
single crystal X-ray analysis were obtained after 3 days.
Anal. Calcd (%) for Complex (2) C H NiN O (327.04):
a (A)
17.899(4)
11.747(2)
10.748(2)
90
125.59(3)
90
1837.7(6)
4
1.471
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
˚
3
V (A )
Z
8
24
6
4
À3
d
calc (g cm
)
C, 29.35; H, 7.33; N, 25.68. Found C, 29.07; H, 7.55; N,
À1
l (mm
F(000)
)
1.099
872
1.377
348
2
5.87.
Crystal size (mm)
h range
Reflections collected
Unique reflections
Rint
Absorption correction
Max. and min. transmission 0.9997–0.8659
Data/restraints/parameters
Final R indices (I > 2r(I))
0.40 · 0.15 · 0.15
2.23–25.96
1895
1800
0.0311
0.40 · 0.15 · 0.15
2.65–25.98
1490
474
00281
w-scan
0.9998–0.8771
422/0/60
2
.2.3. Synthesis of [Ni(dap) (fum)] (3)
2
We tried to synthesize the tris-1,3-diaminopropane
derivative of nickel(II) fumarate the same way as 1 by
reacting 0.2 g nickel(II) fumarate (1 mmol) with an aque-
ous solution (10 ml) of dap containing 0.25 ml (3 mmol)
of diamine with constant stirring and heating. However,
we could isolate only the bis-1,3-diaminopropane deriva-
tive 3 even after carrying out the reaction with excess
dap. The blue solution formed after reacting the metal
fumarate with the diamine yielded blue crystalline com-
w-scan
1525/0/126
= 0.031,
wR = 0.077
= 0.044,
R
1
R
1
= 0.024,
wR = 0.060
= 0.030,
2
2
Final R indices (all data)
R
1
R
1
wR = 0.081
2
wR = 0.063
2
Largest diff. peak and hole
0.290, À0.237
0.181, À0.196
˚
À3
(
e A
)
pound 3 after 3 days which analyzed as [Ni(dap) (fum)].
Anal. Calcd (%) for 3 C H NiN O (321): C, 37.38; H,
2
2
GOF on F
1.064
1.122
1
0
22
4
4
6
.85; N, 17.44. Found C, 37.35; H, 6.93; N, 17.25.
were carried out on a Shimadzu DTG-60 simultaneous
DTA-TG apparatus.
2
.2.4. Synthesis of [Ni(dap)(ox)]Æ2H O (4)
2
As in the case of 2 we tried to synthesize the tris–dap
adduct of nickel(II) oxalate by reacting 0.182 g nickel(II)
oxalate (1 mmol) with an aqueous solution (10 ml) of dap
containing 0.25 ml (3 mmol) of diamine with constant stir-
ring and heating. Contrary to our expectation the blue
solution obtained after the reaction yielded only the
mono-dap adduct 4 as a light blue crystalline powder after
2
.4. X-ray crystallographic studies
Diffraction data were collected at 293 ± 2 K on a Enraf-
Nonius CAD4 Diffractometer. Intensity data and cell
dimensions were measured using graphite monochromated
MoKa radiation with k = 0.71073 A. Raw intensities were
´
˚
3
days. This was filtered, washed with water and acetone
corrected for Lorentz and polarization effects, and for
and dried. Anal. Calcd (%) for 4 C H NiN O (256.69):
C, 23.37; H, 5.45; N, 10.9. Found C, 23.08; H, 5.96; N,
5
14
2
6
absorption by empirical method based on w-scan data
[
26]. The structures were solved by direct methods and
1
0.86.
subsequent difference Fourier map synthesis. The final
2
full-matrix least-square refinement was based on F .
Computations were performed using the SHELX-97
program package [27]. Details of crystal data, collection
and refinement are listed for 1 and 2 in Table 1.
2
.3. Physical measurements
Elemental analyses (C, H and N) were performed using
an Elementar Vario EL III elemental analyzer. The IR
spectrum was taken using Shimadzu FTIR-8400S spectro-
3. Results and discussion
À1
photometer as KBr pellets in the range 4000–400 cm
.
UV–Vis absorption spectra were recorded between 400
and 1100 nm on a Shimadzu UV-2400 PC series spectro-
photometer at room temperature. Magnetic susceptibility
measurements were done using MSB mk 1 model magnetic
balance of Sherwood Scientific Ltd, England. TG analyses
3.1. Synthesis
The fragmentation of polymeric [Ni(fum)(H O) ] and
2
4 n
[Ni(ox)] was seen to be very facile when they are interacted
n
with en in aqueous solution. Chemical analysis shows that

M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
39
both 1 and 2 are tris–(ethylenediamine) nickel(II) species
containing the dicarboxylate moieties in ionic form with
compositions [Ni(en) ](fum)Æ3H O and [Ni(en) ](ox),
respectively. Compared to this observation the depolymer-
ization of the parent nickel(II) fumarate and oxalate is seen
to be not easy with 1,3-diaminopropane (dap). We have
carried out the interaction with large excess of dap under
reflex condition for 3 days also but only a bis-dap product
for the nickel(II) fumarate with composition [Ni(dap)2
late moiety. In contrast, in the oxalate derivative 4 the
À1
m (OCO) is seen at 1604 cm and m (OCO) at 1350 and
as
s
À1
À1
1315 cm with its d(OCO) mode at 798 cm clearly sug-
gestive of g -bis-chelating form of the oxalate [25e]. In
3
2
3
4
comparison with the values of the parent nickel(II) oxalate,
À1
which has m (OCO) at 1624 cm and the two symmetric
as
À1
m (OCO) vibrations at 1361 and 1316 cm and d(OCO)
mode at 825 cm , all the corresponding vibrations are
s
À1
red-shifted in its dap adduct 4. The reason for such an
(
fum)] (3) could be obtained in all the cases. Anticipating
observation is the enhanced back-donation possible from
the filled d orbital of the metal to the empty p orbital(s)
*
the possibility of formation of similar bis-dap adduct of
nickel(II) oxalate we tried reacting it with large excess of
dap under reflux conditions for 4 days but only a mono-
dap derivative of composition [Ni(dap)(ox)]Æ2H O (4)
could be isolated in this case. No indication of the forma-
tion of any bis-dap or tris–dap adduct could be seen with
of the coordinated oxalate moiety because of the significant
electron density accumulated on Ni from the r-donating
dap. In all the four adducts the set of bands observed in
2
À1
the region 3350–3250 cm indicate the coordinated form
of both en and dap [28].
nickel(II) oxalate. This could be attributed to the fact that
The electronic spectra of all the complexes were
recorded in aqueous condition except for 4, which was
found to be insoluble in all solvents. All the adducts gave
characteristic three band spectra suggestive of an overall
octahedral geometry for the complexes. The energies of
these bands and their assignments are given in Table 2. It
is interesting to note the progressive energy ordering
(1 ꢀ 2 > 3 > 4) seen in the compounds for each of these
4
in the polymeric [Ni(ox)] the oxalate is coordinated in g -
n
1
bis-chelating form compared to g -bis-monodentate mode
in [Ni(fum)(H O) ] . In the latter case it is easy to break
2
4 n
1
the g -bonded fumarate moiety from the coordinated
form by the dap moiety to get the product in soluble form.
4
In the former case the g -bis-chelating oxalate has much
stronger chelating ability (and higher D value) because
q
of its unique coordination mode compared to weakly che-
lating dap and therefore the diamine does not have the
ability to delink the oxalate moiety to make it ionic. Con-
bands. Such a trend is seen in their D values also. The
q
solid state spectrum of 4 shows the presence of two addi-
À1
tional weak bands at 21,740 and 13,260 cm which could
4
sequently, the oxalate moiety is retained in the same g -
be assigned to the two spin forbidden transitions
1
3
1
3
bis-chelating mode (as evident from the IR data) in the
product and the single dap moiety coordinates to the
metal to make it as an octahedrally coordinated species.
The weaker chelating ability of dap compared to that of
en could be attributed to the less favorable bite-angle pos-
sible for the chelating dap compared to that of en. Besides
the chemical analysis, both IR and TG data indicate the
E ‹ A and T ‹ A , respectively [29]. We could
g
2g
2g
2g
evaluate the spectral parameters like D , Racah parameter
q
(B) and nephelauxetic parameter (b) for the adducts 1–3
employing relevant equations [29]. All these parameters
along with the assignment of spectral transitions are pre-
sented in Table 2. The observed range for m /m values
2
1
found for all the complexes is suggestive of their octahedral
presence and extent of H O in 1 and 4. Though 1 and 2
geometry. As expected, 3 has lower D value compared to 1
2
q
gave good quality single crystals for X-ray diffraction
studies, the adducts 3 and 4 yielded only either crystalline
or powdery samples.
and 2 owing to poor donor ability of dap. The magnetic
moments (l ) for the adducts are in the range 2.79–2.92
eff
BM (Table 2) which are well within the spin-only value
expected for Ni(II) in an octahedral ligand field.
3
.2. Spectroscopic studies
3.3. Description of crystal structures
We have made use of the nature of asymmetric
(
m (OCO)) and symmetric (m (OCO)) stretching frequencies
as s
3.3.1. Structural details of [Ni(en) ](fum)Æ3H O (1)
3
2
of the carboxylate moieties in all the four compounds 1–4
to decipher the nature of coordination of –COO in them.
The molecular form of 1 showing tris–ethylenediamine
moiety, [Ni(en)3] , and non-coordinated fumarate anion
À
2+
These two strong vibrations observed at 1583 and
along with the atom labeling scheme for the complex are
shown in the ORTEP plot (Fig. 1). The geometry around
À1
À1
1
395 cm , respectively, in 1 with Dm of 188 cm is clearly
suggestive of ionic form of the fumarate moiety in the com-
the Ni(II) is seen to be octahedral with NiN chromophore
6
pound [28]. In the dap adduct 3, however, these bands are
and has slightly varying Ni–N bond lengths [Ni–N(1)
À1
˚
˚
seen at 1590 and 1369 cm , respectively, with Dm of
2.117(2) A; Ni–N(2) 2.125(2) A]. The fumarate moiety
stays as a non-coordinating and dianionic unit in the
adduct.
À1
2
21 cm which is significantly higher than that in 1 and
1
characteristic of mono-coordinated (g -OCO) carboxylate
group [28]. In the complex 2 these vibrations are found
at 1610 and 1310 cm respectively, which are close to
The conjugation in the OCO units of the moiety is evi-
dent from the observed closeness in the carbon–oxygen
À1
˚
those observed for dipotassium oxalate (1597 and
bond lengths which are 1.252(3) A for C(4)–O(1) and
À1
˚
1
311 cm ), and are indicative of the ionic form of the oxa-
1.246(3) A for C(4)–O(2). The fumarate units are seen

40
M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
Table 2
Electronic spectral data of diamine adducts 1–4
*
Diamine adduct
Absorption maxima
Assignment
Molar absorptivity e
D
q
Racah
parameter B
Nephelauxetic
parameter b
m
2
/m
1
l
eff.
(BM)
À1
À1
À1
À1
(cm
)
(L M cm
)
(cm )
[
[
[
[
Ni(en)
3
]fumÆ3H
2
O (1)
29,325
18,215
m
3
m
2
m
1
m
3
m
2
m
1
m
3
m
2
m
1
m
3
m
2
147
60
70
135
88
80
166
100
50
1139
1126
1087
–
891
900
853
–
0.856
0.865
0.819
–
1.60
1.60
1.58
–
2.88
(aq. solution)
11,390
Ni(en)
3
]ox (2)
29,240
18,050
2.92
2.90
2.79
(aq. solution)
11,260
Ni(dap)
2
fum] (3)
28,170
17,240
(aq. solution)
10,870
Ni(dap)ox]. 2H
2
O (4)
27,020
16,550
–
(solid state)
2
1
1,740
3,260
3
#
#
3
*
3
3
3
3
1
3
1
3
m
1
= T2g(F) ‹ A2g; m
2
= T1g(F) ‹ A2g; m
3
= T1g(P) ‹ A2g;# = forbidden transitions ( E
g
‹ A2g and T2g ‹ A2g).
the mixed conformations like K(ddk), K(dkk) and also
K(kkk) reported in a few systems [22a,b] whose stability
is seen to be dictated by the nature of the counter anions
and their interactions with the chelating diamines. The rel-
evant bond lengths and bond angles observed in the com-
pound 1 are presented in Table 3. We have also made an
attempt to look at the nature of H-bonding involved in
the compound as it is known that H-bonding involving
both the anion and lattice water with the coordinated en
moieties contribute to the overall conformational prefer-
n+
ences of [M(en)3] moieties in their compounds. Presented
in Table 4 are the relevant H-bonding parameters in the
compound.
3.3.2. Structural details of [Ni(en) ](ox) (2)
3
As in the case of the fumarate derivative 1, the oxalate
Fig. 1. An ORTEP view of complex 1 with the atom labeling scheme (30%
thermal ellipsoids).
complex [Ni(en) ](ox) (2) also has the nickel ion existing
3
2+
as [Ni(en)3] unit with an overall octahedral geometry
having NiN as the chromophore. Given in Fig. 3 is the
6
ORTEP plot of the compound. All the Ni–N bond lengths
2
+
˚
sandwiched between [Ni(en)3] units but take part in H-
bonding with coordinated en moieties and also the lattice
water present in the compound. It is interesting to note
are seen to be similar in this case (Ni–N(1) 2.118(18) A, Ni–
˚
N(2) 2.118(18) A). The oxalate is non-coordinated and in
dianionic form. The C–O bond lengths of the –COO units
˚
the extensive H-bonding involved between NH groups of
the three coordinated en moieties and carboxylate O atoms
of the fumarate dianions along with the water molecules
Fig. 2) in the compound. This leads to a three-dimensional
network for the compound. Such a correlated interaction
involving dianionic and conjugated fumarate moiety, coor-
of the oxalate moiety are in the range 1.179(7) A for C(2)–
2
˚
O(1) and 1.249(7) A for C(2)–O(2) which indicate that the
carboxylate moieties are not completely conjugated. The
dissimilarity in the C–O bond lengths can also be due to
the preferential involvement of O(2) oxygen in the H-bond-
ing with the coordinated en moieties. The relevant bond
lengths and bond angles of 2 are presented in Table 5. Even
though there are significant H-bondings between the coor-
dinated en moieties and the oxalate ion in the compound,
there is no water molecules to participate in the H-bonding
in this molecular system, unlike in 1. The disorder observed
in the oxalate precludes the clear understanding of the moi-
ety and also the nature of its H-bonding with the diamine
(
dinated en units and also H O molecules makes the
Ni(en)3] moieties take configurations with preferred con-
2
2
+
[
formations. We could find that the crystal system of 1 is
made up of equal number of molecular units of both K
and D configurations. This is also evident from the centro-
symmetric requirement of the observed space group of C2/
c of the system [22a]. The observed conformations for these
two configurations in the racemic form of 1 are K(ddd) and
D(kkk) which are known to be highly preferred ones than
the mixed conformations. This is in contrast to some of
2
+
units of the [Ni(en)3] but the crystal system has an overall
3D structure because of the hydrogen bonding. The details
of the H-bonding interactions are given in Table 6. We

M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
41
Fig. 2. H-bonding interaction involving fumarate anion, coordinated en and lattice water in 1.
Table 3
Selected bond lengths (A) and angles (ꢀ) for complex 1
equal number of their mirror images with D(kkk) confor-
mations, forming a racemic mixture as required by the
space group of the two crystal systems.
˚
Ni–N(1)
Ni–N(1)#1
Ni–N(3)
Ni–N(3)#1
Ni–N(2)#1
Ni–N(2)
O(1)–C(4)
O(2)–C(4)
N(1)–Ni–N(1)#1
N(1)–Ni–N(3)
N(1)#1–Ni–N(3)
N(1)–Ni–N(3)#1
N(1)#1–Ni–N(3)#1
N(3)–Ni–N(3)#1
N(1)–Ni–N(2)#1
N(1)#1–Ni–N(2)#1
N(3)–Ni–N(2)#1
N(3)#1–Ni–N(2)#1
N(1)–Ni–N(2)
N(1)#1–Ni–N(2)
N(3)–Ni–N(2)
2.117(2)
2.1174(19)
2.121(2)
2.121(2)
2.125(2)
2.125(2)
1.252(3)
1.246(3)
81.94(11)
172.24(7)
93.07(8)
93.07(8)
172.23(7)
92.48(11)
95.21(8)
92.47(8)
90.94(8)
82.01(8)
92.47(8)
95.21(8)
82.01(8)
N(1)–C(1)
N(2)–C(2)
N(3)–C(3)
C(1)–C(1)#1
C(2)–C(3)
C(4)–C(5)
C(5)–C(5)#2
1.471(3)
1.471(3)
1.474(3)
1.518(5)
1.503(4)
1.507(3)
1.312(5)
3.4. Thermal properties
Since the thermal stability of metal-dicarboxylates and
their adducts are of relevance for any of their possible
applications we thought it worthwhile to study their ther-
mal dissociation features in some detail. Also we expected
N(3)#1–Ni–N(2)
N(2)#1–Ni–N(2)
C(1)–N(1)–Ni
C(2)–N(2)–Ni
C(3)–N(3)–Ni
N(1)–C(1)–C(1)#1
N(2)–C(2)–C(3)
N(3)–C(3)–C(2)
O(2)–C(4)–O(1)
O(2)–C(4)–C(5)
O(1)–C(4)–C(5)
C(5)#2–C(5)–C(4)
90.94(8)
169.83(11)
108.02(14)
107.35(14)
107.86(14)
108.23(15)
108.62(19)
109.01(18)
124.7(2)
the possibility of a step-wise loss of en from the two
2+
[
Ni(en)3] moieties to yield their bis-en products by ther-
mal reaction and also anticipated that the nature of the
anionic moiety may have some effect on the dissociation
features of 1 and 2. It was also our intention to assess
how the nature of chelating abilities of the two diamines,
en and dap, influence the dissociation features of their
adducts. Presented in Fig. 4 are the TG curves for both 1
and 2 and discussed below are a few interesting observa-
tions and some of the salient features of the thermal behav-
iour of 1–4. Relevant thermodynamic and kinetic
parameters of various thermal steps also could be evalu-
ated using Coats–Redfern equation [30].
116.3(2)
119.0(2)
123.9(3)
Symmetry transformations used to generate equivalent atoms: #1,
Àx,y,Àz + 3/2; #2, Àx + 1/2,Ày + 1/2,Àz + 1.
have also made an attempt to identify the conformational
forms of the [Ni(en)3] moieties in 1 and 2 which show
that both the systems are made up of K(ddd) units and
Despite the fact that both 1 and 2 have the same
[Ni(en)3] moieties, the fumarate derivative 1 shows three
step-wise loss of all the three diamine units while the
2
+
2+
Table 4
˚
Hydrogen bonds involved in 1 (A and ꢀ)
D–H
d(D–H)
d(H. . .A)
\DHA
d(D. . .A)
A
O3–H14
O4–H15
O4–H16
N1–H1
N1–H2
N2–H3
N2–H4
N3–H5
N3–H6
0.811
0.728
0.780
0.900
0.900
0.900
0.900
0.900
0.900
2.037
2.203
1.937
2.062
2.236
2.356
2.291
2.632
2.176
167.87
160.84
167.13
167.48
165.43
162.61
153.59
138.58
155.71
2.835
2.901
2.703
2.947
3.115
3.226
3.123
3.360
3.019
O1 [Àx + 1/2,yÀ1/2,Àz + 3/2]
O1 [Àx,y,Àz + 3/2]
O2 [xÀ1/2,y + 1/2,z]
O2 [Àx + 1/2,Ày + 1/2,Àz + 2]
O4 [Àx,y,Àz + 3/2]
O1 [Àx + 1/2,Ày + 1/2,Àz + 2]
O4
O4 [ÀxÀ1/2,Ày + 1/2,Àz + 1]
O3

42
M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
Fig. 3. An ORTEP view of complex 2 with atom labeling scheme (30% thermal ellipsoid) and its unit cell features.
Table 5
˚
Selected bond lengths (A) and angles (ꢀ) for complex 2
Ni–N#1
Ni–N
Ni–N#2
Ni–N#3
Ni–N#4
Ni–N#5
N#1–Ni–N
N#1–Ni–N#2
N–Ni–N#2
N#1–Ni–N#3
N–Ni–N#3
N#2–Ni–N#3
N#1–Ni–N#4
N–Ni–N#4
N#2–Ni–N#4
N#3–Ni–N#4
2.1188(18)
2.1188(18)
2.1188(18)
2.1188(18)
2.1188(18)
2.1188(18)
171.42(11)
92.33(9)
81.81(11)
92.33(9)
94.16(13)
92.32(9)
O(1)–C(2)
O(2)–C(2)
N–C(1)
C(1)–C(1)#2
C(2)–C(2)#8
1.179(7)
1.249(7)
1.470(3)
1.494(5)
1.490(7)
N#1–Ni–N#5
N–Ni–N#5
N#2–Ni–N#5
N#3–Ni–N#5
N#4–Ni–N#5
C(1)–N–Ni
N–C(1)–C(1)#2
O(1)#6–C(2)–O(2)#6
O(1)–C(2)–O(2)
O(1)#7–C(2)–O(2)#7
94.16(13)
92.32(9)
171.41(11)
81.81(11)
92.32(9)
107.71(14)
108.78(17)
125.2(5)
81.81(11)
92.32(9)
94.16(13)
171.42(11)
125.2(5)
125.2(5)
Symmetry transformations used to generate equivalent atoms: #1,
Àx + y,y,Àz + 1/2; #2, Ày + 1,Àx + 1,Àz + 1/2; #3, x,x À y + 1,
Àz + 1/2; #4, Àx + y,Àx + 1,z; #5, Ày + 1,x À y + 1,z; #6, Ày,
x À y + 1,z; #7, Àx + y À 1,Àx,z; #8, Ày,Àx,Àz + 1/2.
3 2 3
Fig. 4. Thermograms of [Ni(en) ](fum)Æ3H O 1 and [Ni(en) ](ox) 2.
oxalate derivative 2 is seen to exhibit much higher stability
and shows only two thermal steps. Among the four thermal
stages seen for 1 the first stage (endothermic, at Ti of 90 ꢀC)
corresponds to the loss of 3 moles of lattice water. The sec-
We have carried out isothermal heating of 1 separately
at temperatures 125, 200 and 250 ꢀC and established the
identity of the thermal products obtained in each case as
suggested as above, by chemical analysis and IR spectral
measurements. The elemental analysis of the product
obtained from the isothermal heating of 1 at 250 ꢀC for
6 h shows that it is a polymeric compound [Ni(en)(fum)]n
having Ni, en and fumarate in 1:1:1 ratio, the preparation
of which is not possible otherwise. Unlike in the parent
compound 1 the fumarate ion in this new product is seen
À1
À1
ond (Ti 160 ꢀC, E = 61 kJ M
,
DS = À240 J K
a
À1
À1
M
) and third (Ti 245 ꢀC, E = 190 kJ M , DS = À139
a
À1
À1
J K
M ) steps are also endothermic and the mass losses
suggest the dissociation of one molecule of en in each step.
In the fourth step (exothermic, Ti 300 ꢀC, E = 418
a
À1
À1
À1
kJ M , DS = 4 J K
M ) the combined decomposition
À1
of the third en and the fumarate is seen to occur.
to be coordinating in nature (IR bands at 1558 cm and
Table 6
˚
Hydrogen bonds for complex 2 (A and ꢀ)
D–H
d(D–H)
d(H. . .A)
\DHA
d(D. . .A)
A
N–H1
N–H1
N–H1
N–H2
N–H2
0.782
0.782
0.782
0.861
0.861
2.150
2.321
2.428
2.014
2.484
173.05
148.39
158.05
160.50
167.12
2.928
3.015
3.167
2.839
3.328
O2 [y,Àx + y,Àz]
O1 [Àx,Ày + 1,Àz]
O1 [x À y + 1,x + 1,Àz]
O1
O2 [Ày,x À y + 1,z]

M. Padmanabhan et al. / Journal of Molecular Structure 885 (2008) 36–44
43
À1
À1
1
385 cm for m (OCO) and m (OCO), respectively, with Dm
and also the incapability of the 1,3-diamine to form a
tris–dap product with both Ni(fum) and Ni(ox). Interesting
thermal dissociation features are seen in all the four prod-
ucts from which relevant thermodynamic and kinetic
parameters could be evaluated.
as
s
of 173 cm ) which has become necessary owing to the dis-
sociation of two en moieties from 1. We could verify such a
coordination mode of the fumarate moiety in this thermal
product by checking the IR spectra of both [Ni(fum)Æ4-
À1
H O] (m (OCO) at 1558 cm
and ms(OCO)s at
2
n
as
À1
À1
1
392 cm , Dm = 166 cm ) and its anhydrous polymeric
Acknowledgement
product [Ni(fum)]n obtained after heating at 250 ꢀC
À1
À1
(
m (OCO) at 1558 cm
and m (OCO) at 1385 cm
,
Partial support from the National Science Foundation
(Grant DMR-0422932) is gratefully acknowledged.
as
s
À1
Dm = 173 cm ). In contrast to this, the tris–en adduct of
the nickel oxalate, [Ni(en) ](ox), 2 is seen to be thermally
3
stable up to 250 ꢀC. Unlike 1, the adduct 2 loses 2 mole-
Appendix A. Supplementary data
cules of en in the first thermal step (endothermic, Ti
À1
À1
À1
2
50 ꢀC, E = 168 kJ M , DS = À161 J K
M
) itself
Crystallographic details and complete listings of com-
pounds 1 and 2 have been deposited at the Cambridge
Crystallographic Data Center (CCDC) as supplementary
data: CCDC-654087 (1) and CCDC-654088 (2). These data
can be obtained free of charge from the Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44
1223 336 033; E-mail: deposit@ccdc.cam.ac.uk).
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.10.012.
a
and then decomposes in a combined step (exothermic, Ti
À1
À1
À1
3
25 ꢀC, E = 220 kJ M , DS = À144 J K
M ) to yield
a
NiO as the final product.
The bis-dap adduct, [Ni(dap) fum] (3) is remarkably sta-
2
ble and shows the first dap dissociation only at 270 ꢀC
À1
À1
À1
(
endothermic, E = 481 kJ M , DS = 82 J K
M ) and
a
the loss of second dap at 310 ꢀC (exothermic, E = 242
a
À1
À1
À1
kJ M , DS = À111 J K
M ), followed by the decom-
position of Ni(fum) to NiCO and then finally to NiO.
3
The dap derivative of the oxalate compound [Ni(dap)(ox)]-
Æ2H O, 4 is also seen to have high thermal stability after the
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À1
2
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