A second crystal form of [Ni(2,2′-bipyridine)(H2O) 3(NO3)](NO3) featuring a different molecular orientation

Article abstract of DOI:10.1016/j.poly.2011.09.046

The molecular structure of a second form of [Ni(2,2′-bipyridine) (H₂O)₃(NO₃)](NO₃) is reported. The previous report is for a blue monoclinic polymorph. The second form is orthorhombic and crystallises as green blocks with unit cell parameters a = 9.1201(12) , b = 14.444(2) , c = 21.805(4) , V = 2872.4(8) ³, Z = 8. The complex was characterised by elemental analysis, infrared spectroscopy, UV-Vis spectroscopy, and thermogravimetry. The bipyridine acts as a bidentate ligand to Ni²⁺ and the octahedral coordination is completed by three water molecules and one monodentate nitrate ion. A second nitrate forms hydrogen bonds to the bound water molecules. The difference between the two forms in terms of the molecular geometry is described in relation to other similar compounds. The key difference between the two forms is the orientation of the two nitrate anions, and hence the hydrogen bonding present.

Full text of DOI:10.1016/j.poly.2011.09.046

Polyhedron 31 (2012) 345–351
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0
A second crystal form of [Ni(2,2 -bipyridine)(H O) (NO )](NO ) featuring a
2
3
3
3
different molecular orientation
a
a
b,
⇑
Apinpus Rujiwatra , Saranphong Yimklan , Timothy J. Prior
a
Department of Chemistry and Centre for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, UK
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
2 3 3 3
The molecular structure of a second form of [Ni(2,2 -bipyridine)(H O) (NO )](NO ) is reported. The pre-
Received 30 August 2011
Accepted 25 September 2011
Available online 12 October 2011
vious report is for a blue monoclinic polymorph. The second form is orthorhombic and crystallises as
green blocks with unit cell parameters a = 9.1201(12) Å, b = 14.444(2) Å, c = 21.805(4) Å,
3
V = 2872.4(8) Å , Z = 8. The complex was characterised by elemental analysis, infrared spectroscopy,
2+
UV–Vis spectroscopy, and thermogravimetry. The bipyridine acts as a bidentate ligand to Ni and the
octahedral coordination is completed by three water molecules and one monodentate nitrate ion. A sec-
ond nitrate forms hydrogen bonds to the bound water molecules. The difference between the two forms
in terms of the molecular geometry is described in relation to other similar compounds. The key differ-
ence between the two forms is the orientation of the two nitrate anions, and hence the hydrogen bonding
present.
Keywords:
Nickel
0
2
,2 -Bipyridine
Coordination complex
Crystal structure
Polymorph
Ó 2011 Elsevier Ltd. All rights reserved.
0
1
. Introduction
of a cation and 2,2 -bipyridine to synthesise similar frameworks.
Our aim was to control the initial pH and ratio of reactants to influ-
ence the product obtained from hydrothermal synthesis. Here we
0
2
,2 -Bipyridine is one of the most widely employed ligands in
the synthesis of metal complexes [1]. At present there are over
report the structure of a second crystal form of the compound
0
0
5
500 structures reported that feature 2,2 -bipyridine bound to a
[Ni(2,2 -bipyridine)(H
2
O) NO
3 3
](NO
3
) and demonstrate that the
transition metal [2]. The vast majority of these (>99.6%) feature
structure determined at low temperature persists above room
temperature. The relationship with earlier structure of the same
composition is described in detail.
0
2
,2 -bipyridine acting as a bidentate ligand. There are many further
0
examples that contain derivatives of 2,2 -bipyridine [1] and other
0
similar ligands [3]. Complexes containing 2,2 -bipyridine have
been widely used in studies of electron transfer [4] and chemilumi-
nescence [5], and as model systems for optical isomerism [6].
In contrast to purely organic systems, it is somewhat unusual to
observe polymorphism in inorganic compounds, particularly
where two or more polymorphs are stable at the same tempera-
ture. Often the difference in crystal packing is accompanied by a
change in colour, as is the case for some compounds containing
2
. Materials and methods
2.1. Microwave synthesis
Crystals of the title compound were grown from a solution of
0
.5826 g nickel(II) nitrate hexahydrate (Ni(NO
3
)ꢀ6H
2
O, Carlo Erba
0
0
0
99%) with 0.1562 g 2,2 -bipyridine (C10
H
N
8 2
, Fluka >99%) and
2
2
,2 -bipyridine. For example, Pt(2,2 -bipyridine)Cl exists as two
3
0
.1670 g terephthalic acid (C
6
H
4 2
(COOH) , BDH 97%) in 10.00 cm
different crystal forms under ambient conditions depending on
the solvent employed: a red form [7] that crystallises in space
0
deionized water (Ni:nitrate:2,2 -bipyridine:terephthalic:water mo-
lar ratio = 2:2:1:1:556). Reagents were loaded into a 23 cm Teflon
lined pressure vessel. The reaction was performed for 3 h, under an
autogenous pressure generated at 630 W (96 °C) using a domestic
microwave oven (Whirlpool XT – 25ES/S, 900 W, 2.45 GHz). The ini-
tial pH of the solution was 3, measured using universal pH strips
3
group Cmcm and a yellow form [8] that adopts space group Pbca.
0
2
,2 -Bipyridine has been utilised as an auxiliary (non-frame-
work) ligand in the synthesis of coordination polymers. For exam-
ple it has been used with metal ions and the framework formers
1
,3,5-benzenetricarboxylate [9], squarate [10], and succinate [11].
(
Merck, 1.09535.0001). It may be noted that the pH of the solution
We sought to employ 1,4-benzenedicarboxylic acid in the presence
did not change after the reaction.
The crystals obtained were present as aggregates of mid-green
plates, separated from supernatant by filtration, then washed with
deionized water and dried in air. The sample appeared visibly
⇑
Corresponding author. Tel.: +44 (0)1482466389; fax: +44 (0)1482466410.
E-mail address: t.prior@hull.ac.uk (T.J. Prior).
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.046

3
46
A. Rujiwatra et al. / Polyhedron 31 (2012) 345–351
0
2+
homogeneous under magnification. X-ray diffraction analysis of
several crystals at room temperature confirmed this. Satisfactory
chemical analysis data were obtained for 2.
with Z = 1. Each Ni is approximately octahedral in coordination
0
geometry, bound to a bidentate molecule of 2,2 -bipyridine, three
molecules of water, and a monodentate nitrate. The lengths of
the coordination bonds about nickel are in good agreement with
others of similar complexes. The CCDC [2] has 133 structures
containing nickel, bipyridine, and ligated water. For these, the
mean Ni–O (water) and Ni–N bond lengths are 2.08(4) and
.07(4) Å, respectively, while the mean N–Ni–N angle is
79.0(15)°. For form 2 at 120 K, the bond lengths are 2.054(4),
.078(4) and 2.083(5) Å (Ni–O), and 2.050(5) and 2.081(5) Å (Ni–
N), while the N–Ni–N angle is 79.4(2)°. For 63 structures with
monodentate nitrate and at least two pyridyl donors, the mean
Ni–O (nitrate) bond length is 2.10(7)Å and for 2 this bond length
is 2.082(4). A further uncoordinated nitrate anion forms hydrogen
bonds to the bound water. The distance between hydrogen atoms
of water and unbound nitrate are: 1.91(3) Å (O1–H1BꢀꢀꢀO10) and
1.94(3) Å (O2–H2BꢀꢀꢀO9). An ORTEP representation of 2 is shown
in Fig. 1. Basic crystal data are given in Table 1.
The bound nitrate forms a single hydrogen bond to water in an
adjacent complex. Every hydrogen atom of the three water mole-
cules is involved in hydrogen bonding to nitrate. A full list of these
is given in Table 2. There is a tendency to short, linear, hydrogen
bonds with few bifurcated interactions (Fig. 2b). The combination
of hydrogen bonds between the complexes and unbound nitrate
generates infinite puckered sheets of thickness 5.552 Å that extend
in the xy plane. These sheets are two molecules thick and the
2.2. X-ray diffraction analysis
Routine X-ray diffraction data collection and structure solution
2
procedures were adopted.
0
A
single crystal of dimensions
.21 ꢁ 0.11 ꢁ 0.06 mm was cut from a larger aggregate, coated
in perfluoropolyether oil and mounted at the end of a glass fibre.
Data were collected in series of -scans using a Stoe IPDS2 diffrac-
3
2
x
tometer operating with Mo radiation. The crystal temperature was
maintained using an Oxford Instruments nitrogen gas cryostream.
The structure was solved by direct methods [12]. Full matrix least
2
squares refinement against F implemented within SHELXL [12] was
employed for structure refinement.
0
Hydrogen atoms attached to 2,2 -bipyridine were positioned
and refined using a riding model. Those of water were located in
difference Fourier maps and their positions refined subject to the
restraint that all O–H bond lengths were the same with a standard
deviation 0.03 Å. Sensible restraints were also applied to the geom-
etry of the water molecules.
For the first data collection the crystal was held at 120 K and a
full set of data collected. Subsequently, the crystal was glued to the
fibre and further sets of data frames collected, sufficient to deter-
mine the unit cell. The unit cell was determined at a further nine
temperatures. A set of twenty data frames were collected at
0
+
[
2 3 3
Ni(2,2 -bipyridine)(H O) NO ] ions are arranged such that the
aromatic rings of the bipyridine are approximately perpendicular
to the layers and project above and below them. The sheets are
c in ABAB fashion at a separation of c/2
10.903(4) Å). This arrangement facilitates two types of intermo-
3
53 K and the data from these were used to refine the structure
at this temperature.
stacked along
Routine X-ray diffraction data collection and structure solution
(
procedures were adopted for form 1 using a crystal of dimensions
3
lecular interactions. The hydrogen bonding within the layer is
illustrated in Fig. 2. Interdigitation of the aromatic rings on adja-
0
.48 ꢁ 0.12 ꢁ 0.11 mm .
cent layers leads to relatively close approach of the
p-systems of
2
.3. Spectroscopic characterisation
neighbouring bipyridine ligands. The distance between these is of
i
the order of 3.43 Å; for example C7 (i = x ꢂ ½, y, 1 ꢂ z) lies
IR spectra were collected from ground crystals of 2 as a KBr disc,
ꢂ1
3.376(6) Å above the ring formed from N1 and C1–C5. This close
using a Bruker Tensor 27 FT-IR spectrometer (4000–400 cm , res-
olution 0.5 cm ).
ꢂ1
approach is suggestive of a p–p interaction. In addition the prox-
ii
imity of H3 to an adjacent bipyridine (H3ꢀꢀꢀC9 = 2.80 Å, ii = ꢂx,
UV–Vis reflectance spectra were collected from lightly ground
crystals loaded in a BaSO
Spectrophotometer.
4
matrix using a Cary 5E UV-vis-NIR
2.4. Thermogravimetry
Samples were placed into platinum pans, loaded into a Mettler-
Toledo TGA/DSC1 Thermogravimetric Analyzer and heated under a
flow of nitrogen from room temperature to 1000 °C at a ramp rate
ꢂ1
of 30 °C min
.
3
. Results and discussion
3.1. Crystal structure analysis
0
A report of the structure of [Ni(2,2 -bipyridine)(H
at room temperature has appeared previously [13]. This compound
hereafter 1) crystallises in the centrosymmetric monoclinic space
group P2 /c and is isostructural with the manganese analogue [14].
2 3 3 3
O) NO ](NO )
(
1
Here we describe a second crystal form (2) for this compound ob-
tained under microwave-assisted hydrothermal conditions that
displays the same connectivity as 1, but differs in the orientation
of the groups present. A description of the structure of 1 and its
relation to 2 appears later.
Fig. 1. ORTEP plot of the asymmetric unit of 2. Atoms are shown as 30% thermal
ellipsoids. Hydrogen bonds are illustrated by dashed lines. Selected bond lengths:
Ni1–N1 2.081(5) Å, Ni1–N2 2.050(5) Å, Ni1–O1 2.054(4) Å, Ni1–O2 2.078(4) Å, Ni1–
O3 2.083(5) Å, Ni1–O4 2.082(4) Å.
The structure of 2 was initially determined at 120 K. At this
0
temperature [Ni(2,2 -bipyridine)(H
O)
2 3
3 3
NO ](NO ) was found to
crystallise in the centrosymmetric space group Pbca (number 61)

A. Rujiwatra et al. / Polyhedron 31 (2012) 345–351
347
assigned to the transitions A2g ? 3T2g and A2g ? 3T1g (F), respec-
3
3
Table 1
Basic crystal structure and refinement data for form 1 and form 2.
ꢂ1
tively [15]. The third band expected at around 27000 cm due to
the transition A2g ? T1g (P) is obscured by broad charger transfer
3
3
0
Identification code
[Ni(2,2 -bipyridine)(H
2
O)
3 3
NO ](NO
3
)
absorption that extends into the visible region from the UV. The re-
Form 1
Form 2
0
gion of the spectrum due to charge transfer transitions to the 2,2 -
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
C
10
H
14
N
4
NiO
9
10 14 4 9
C H N NiO
ꢂ1
bipyridine (above ca. 23000 cm ) is rather different in the two
392.96
120(2)
0.71073
392.96
120(2)
0.71073
Orthorhombic
Pbca
a = 9.1201(12) Å
b = 14.444(2) Å
c = 21.805(4) Å
compounds. This is in line with the observed colours of the two
compounds; the green form (2) absorbs more light in the blue re-
Monoclinic
P2 /c
gion of the spectrum than does form 1. The inset within Fig. 4 high-
Space group
Unit cell dimensions
1
ꢂ1
lights the difference in the features around 32000 cm
.
a = 11.2206(13) Å;
b = 9.3513(7) Å;
c = 14.8649(19) Å;
The IR spectrum of 2 displays bands characteristic of the species
present: bipyridine, nitrate, and water. The presence of hydrogen
bonds between nitrate ions and water molecules is in good agree-
ment with the broadening effect observed in the IR spectrum (sup-
plied in the Supplementary data). In addition to the characteristic
a
= 90°
b = 109.793(9)°
= 90°
c
3
2872.4(8) Å3
8
1.817
Volume
Z
Density (calculated)
1467.6(3) Å
4
1.778
0
absorptions of the 2,2 -bipyridine ligands, an intense and broad
ꢂ1
absorption band in the range 3020–3670 cm
corresponds to
ꢂ3
(
mg m
)
coordinated water molecules [16]. A single absorption at 735 and
Absorption coefficient
1.380
808
1.411
ꢂ1
doublet absorptions at 825 and 815 cm , and at 1382 and
ꢂ1
(
mm
)
ꢂ1
1
414 cm should correspond to the fundamental
m
4
,
m
2
and
m
3
F(000)
1616
Crystal size (mm)
Theta range for data
collection (°)
0.48 ꢁ 0.12 ꢁ 0.11
0.21 ꢁ 0.11 ꢁ 0.06
modes of the nitrate, respectively [17,18]. The lowered symmetry
2.62–27.51
2.80–26.00
of nitrate in 2 results in the presence of
m
at 1025 and
1
ꢂ1
1
017 cm in the IR spectrum, which otherwise be absent. Due
Index ranges
ꢂ14 ꢃ h ꢃ 14,
ꢂ11 ꢃ k ꢃ 12,
ꢂ19 ꢃ l ꢃ 19
11700
3371 [Rint = 0.0856]
99.9
ꢂ11 ꢃ h ꢃ 10,
ꢂ15 ꢃ k ꢃ 17,
ꢂ17 ꢃ l ꢃ 26
8454
2763 [Rint = 0.1057]
98.1
to the presence of both monodentate and unbound nitrates, three
absorption bands should be expected due to a combination of
ꢂ1
Reflections collected
Independent reflections
Completeness to
[m
+
m
] in a region 1700–1800 cm , two of which from the mono-
3
4
ꢂ1
dentate nitrate should separate by ca. 20–25 cm [19]. The spec-
trum of 2 however shows only two absorptions (1790 and
theta = 26.00° (%)
ꢂ1
1
762 cm ) in this region, which is similar to earlier reports. The
Absorption correction
Maximum and minimum
transmission
analytical
0.8701 and 0.6508
analytical
0.9201 and 0.7560
suppression of the third band on account of hydrogen bonding
has been noted previously [13]. The difference in relative intensi-
ties of these two bands between 1 and 2 may be noted. While they
Refinement method
full-matrix least-
full-matrix least-
squares on F2
2763/19/237
0.906
2
squares on F
are nearly equivalent in intensity in 1 [13], the absorption at
Data/restraints/parameters
Goodness-of-fit on F
3371/51/257
0.951
ꢂ1
ꢂ1
2
1790 cm
shows significantly lower intensity than that at
Final R indices [I > 2
R indices (all data)
r
(I)]
R
1
= 0.0358,
R
1
= 0.0581,
1762 cm in the case of 2.
wR ¼ 0:0825
F2
wR ¼ 0:1237
F2
Thermogravimetric data for 2 are contained the Supplementary
data. The compound loses mass in two relatively well-defined
steps. An initial weight loss in the region 30–200 °C corresponds
R
1
= 0.0526,
wRF
¼ 0:0871
0.950 and ꢂ0.795
R
1
= 0.1115,
wRF
¼ 0:1383
0.698 and ꢂ1.237
2
2
Largest difference peak and
ꢂ3
to loss of the ligated water and some decomposition of nitrate to
hole (e Å
)
0
give a composition Ni(2,2 -bipyridine)O0.33(NO
3
)
1.67. At approxi-
mately 360 °C the phenanthroline ligands are lost to generate a
composition with approximate RMM 118, which we tentatively as-
½
+ y, ½ ꢂ z) may suggest a C–Hꢀꢀꢀ
p
interaction. Therefore the
2
sign as NiO0.5(NO ). This further decomposes to give a mixture of
NiO and Ni above 600 °C.
structure is separated into two parts: hydrogen-bonded layers
and regions where there is relatively close approach of the bipyri-
dine aromatic rings; hydrophilic and hydrophobic parts, as shown
in Fig. 3.
It was surprising that a different structure for this compound
had been reported previously at room temperature and therefore
we explored the crystal chemistry of 2 as a function of tempera-
ture. It was proposed that 2 might be a low temperature form of
this compound that would change to 1 upon heating. The crystal-
lographic unit cell was measured as a function of temperature up
to 353 K. At this temperature the structure was determined again.
The unit cell volume expanded smoothly as a function of temper-
ature and there was no evidence for a change in the crystal
symmetry. Although the crystal began to decay upon heating
above 323 K, it was possible to refine the crystal structure using
3.2. Further characterisation
The diffuse reflectance UV–Vis spectra for crystals of 1 and 2 are
shown in Fig. 4. The low energy regions of the two spectra (5000–
ꢂ1
2
0000 cm ) are remarkably similar. In each form the coordination
2
+
about the Ni ion is approximately octahedral, hence the broad
absorptions centred around 10000 and 16000 cm
ꢂ1
can be
Table 2
Hydrogen bonds within form 2.
Hydrogen bond donor (D–H) D–H bond length (Å) H. . .Acceptor distance (Å) D–H. . .A angle (°) D...A distance (Å) Acceptor atom (A) [symmetry operatory]
O1–H1A
O1–H1B
O2–H2A
O2–H2B
O3–H3A
O3–H3B
0.84(3)
0.83(3)
0.85(3)
0.83(2)
0.84(3)
0.83(3)
1.92(3)
1.91(3)
1.94(3)
1.94(3)
2.23(3)
2.00(3)
173(7)
170(7)
169(7)
168(7)
155(5)
167(7)
2.754(7)
2.726(5)
2.781(6)
2.761(6)
3.015(6)
2.816(7)
O11 [x ꢂ ½, ꢂy + ½, ꢂz]
O10
O5 [ꢂx + ½, y + ½, z]
O9
O10 [x ꢂ ½, ꢂy + ½, ꢂz]
O9 [ꢂx + 1, ꢂy + 1, ꢂz]

3
48
A. Rujiwatra et al. / Polyhedron 31 (2012) 345–351
0
Fig. 2. (a) View of a single hydrogen-bonding layer in 2. Bipyridine molecules have been omitted for clarity. (b) Two [Ni(2,2 -bipyridine)(H
2 3 3 3
O) NO ](NO ) units forming a
centrosymmetric embrace within the hydrogen bonding layer. Symmetry equivalent atoms are generated by the operator i = 1 ꢂ x, 1 ꢂ y, ꢂz.
Fig. 3. View down a of the crystal packing in 2 to illustrate the arrangement of hydrogen-bonding layers.

A. Rujiwatra et al. / Polyhedron 31 (2012) 345–351
349
Fig. 4. UV–Vis spectra for form 1 (blue) and form 2 (green). Data for form 1 are shown in black and for form 2 in grey. The inset shows an expanded region of the spectrum
ꢂ1
between 28000 and 38000 cm . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
data collected at 353 K. The principal structural features are
preserved at this temperature. This suggests that it is not possible
to convert between 1 and 2 by changing the temperature of the
crystal. Full results of the variable temperature diffraction experi-
ments are contained in the Supplementary information.
3.3. Recrystallisation and crystal structure of form 1
In the original report of form 1 [13] crystals were obtained from
aqueous solution. Recrystallisation of form 2 from water returned
green crystals of the same form, as demonstrated by X-ray diffrac-
tion. These crystals were dissolved in hot propan-2-ol to yield a
pale blue solution which was allowed evaporate over a period of
days and yielded light blue block-shape crystals, shown to be form
1
by X-ray diffraction. The structure of form 1 was redetermined at
1
20 K and found to be in good agreement with that published pre-
viously. Basic crystal data for form 1 are contained in Table 1 and
full details of the crystal structure are given in the Supplementary
information.
3.4. Comparison between 1 and 2
Fig. 5. Overlay of the asymmetric units of 1 and 2 viewed from approximately the
same orientation. Compound 1 is shown in black.
Compound 2 crystallises in the space group Pbca and although 1
crystallises in the space group P2 /c, a direct subgroup of Pbca,
1
there is no crystallographic relationship between the two cells.
There are two obvious physical differences between these materi-
als. Compound 1 is reported to crystallise from aqueous solution as
blue plate-like crystals, while 2 is obtained from microwave-as-
sisted hydrothermal synthesis as aggregates of green plates. The
density of the two forms is somewhat different: at 120 K the calcu-
ꢂ3
lated density of 1 is 1.778 g cm but for 2 the density is calculated
ꢂ3
to be 1.817 g cm
.
The connectivity in the two forms is the same, as shown in
Fig. 5, but the hydrogen bonding and relative arrangement of the
nitrate ligands is different. The rms deviation of equivalent bonds
between the two structures is 0.0217 Å (120 K), but there are
slightly more pronounced differences for bound and unbound ni-
trate. For 1 Ni–N bond lengths are 2.0477(18) and 2.054(2) Å, but
for 2, these are 2.050(5) and 2.081(5) Å. The Ni–O bond lengths
are similar: for 1 these are 2.0395(17), 2.0719(18), 2.0524(19),
Fig. 6. Arrangement of adjacent
p systems in 1 (A) and 2 (B).
and 2.1014(17) Å, but for 2 they are 2.083(5), 2.054(4), 2.078(4)
and 2.082(4) Å. For the six equivalent nitrate bonds the rms devia-
tion is 0.0258 Å, although the means of N–O for bound and

3
50
A. Rujiwatra et al. / Polyhedron 31 (2012) 345–351
Table 3
Comparison of the orientation of bound nitrate in 1 and 2 and the structures AFAYEL and NABPCU.
Form 2
Form 2
Form 1
Form 1 (KEBFAY)
AFAYEL
NABPCU
Metal
Ni
Ni
Ni
Ni
Mn
Cu
Temperature (K)
N–M–N (°)
M–O(nitrate) (Å)
h (°)
120
353
120
294
293
293
79.4(2)
2.082(4)
229.7(5)
127.8(3)
2.2(8)
79.1(6)
2.08(2)
227.9(13)
126.5(17)
7(2)
79.71(8)
2.1014(17)
11.5(2)
131.03(14)
2.3(3)
79.5(1)
2.103(1)
8.0
131.6
ꢂ8.8
72.8
2.23
14.81
126.8
ꢂ10.2
81.6
2.64
9.46
131.8
ꢂ26.1
u
x
(°)
(°)
4
. Conclusion
0
The molecular structures of the two forms of [Ni(2,2 -bipyri-
2 3 3 3
dine)(H O) NO ](NO ) principally differ in the orientation of the
bound nitrate. This in turn has a profound effect on the crystal
packing and hydrogen bonding within the solid state. Although
the interconversion between the two forms involves only bond
rotations and not covalent bond breaking, it is not possible to con-
vert form 2 to form 1 thermally. However, this can be accom-
plished by recrystallisation from propan-2-ol. As is the case for
other coordination compounds [7,8], the crystallisation conditions,
particularly the solvent employed, have a direct influence on the
crystal form obtained.
Fig. 7. Principle structural unit in 1 and 2 and other similar structures.
unbound nitrate in the two structures are the same within error.
N–O bonds lengths in the bound nitrate are 1.260(3), 1.222(3),
Acknowledgements
1
.273(3) Å for 1, but 1.244(6), 1.253(6), 1.245(6) Å for 2; N–O bond
lengths in the unbound nitrate are: 1.230(3), 1.288(3), 1.235(3) Å
for 1, but 1.248(7), 1.255(6), 1.259(7) Å for 2.
This work is financially supported by the National Research
University Project under Thailand’s office of the Higher Education
Commission and the Thailand Research Fund. The authors are very
grateful to Mr. Ian Dobson for the collection of thermogravimetry
data and to Dr. Nigel Young for collection of UV–Vis spectra and
helpful discussions.
Differences in the positions of the bound and unbound nitrate
are obvious from Fig. 5. There is hydrogen bonding present
between water and nitrate and this assembles the complexes into
layers which are two molecules thick and extend in the yz plane.
The arrangement is similar to that in 2 but dominated by longer
bifurcated hydrogen bonds to the unbound nitrate. Projecting from
the layer and approximately perpendicular to it are the bipyridine
ligands. The distance between bipyridine molecules from adjacent
Appendix A. Supplementary data
CCDC 838752 and 837641 contain the supplementary crystallo-
graphic data for 1 and 2, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk.
The other supplementary data comprise thermogravimetry and
IR data of form 2, crystal structure data for 2 at 353 K and details of
variable temperature X-ray diffraction measurements, and full de-
tails of the structure of 1 at 120 K. Mol files for 1 and 2 are
included.
layers is 3.4869(14) Å which is suggestive of p–p interactions. The
overlap of adjacent bipyridine molecules is more pronounced than
in 2 as shown in Fig. 6, in line with other similar structures [20].
This suggests this is a more important stabilising factor in 1 than
in 2.
Aside from 2, there are three other unique structures in the
0
CCDC [2] that have the composition [M(2,2 -bipyridine)(H
2
O) NO
3 3
]
(
3
NO ). These are KEBFAY (1) (M = Ni), AFAYEL (M = Mn), and the
family based on NABPCU01 (M = Cu). The last two compounds have
very similar structures to 1. The orientation of the bound nitrate in
each of these structures can be quantified in terms of three angles
and a bond length. At 120 and 353 K the structures of 2 are similar.
The other three structures are distinct from these, but form a sim-
ilar group. Data for the nitrate orientation are given in Table 3 and
Fig. 7.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.09.046.
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(
1