Syntheses, structural diversity and thermal behavior of first row transition metal complexes containing potential multidentate ligands based on 2,6-diacetylpyridine and benzyl carbazate

Article abstract of DOI:10.1016/j.poly.2017.11.009

The Schiff base ligand, N′-(1-{6-[1-(Benzyloxycarbonyl-hydrazono)-ethyl]-pyridin-2-yl}-ethylidene)-hydrazinecarboxylic acid benzyl ester (bc₂-dap), was synthesized by the reaction of 2,6-diacetylpyridine (dap) with benzyl carbazate (bc) in methanol. Also first row transition metal complexes of the general formula [M(NCS)₂(bc₂-dap)]·nH₂O; M = Mn (1), Fe (2), Co (3), Zn (4) or Co (5); n = 0 (1–4) and n = 1 (5), [Fe(bc₂-dap)(H₂O)₂]·2Cl·4H₂O (6), [Ni(NCS)₂(bc₂-dap)]·CH₃OH (7) and [Co(NCS)₂(bc-dap)] (8) have been synthesized using a template method and characterized by spectroscopic techniques and single crystal X-ray diffraction. Complexes 1–4 are isotypic, while compounds 1–6 comprises of discrete mononuclear units with pentagonal bipyramidal geometry. In contrast compounds 7 and 8 adopt octahedral and tetrahedral geometries, respectively. In all of the complexes 1–6, the Schiff base acts as a pentadentate ligand that binds via the pyridine nitrogen, azomethine nitrogen and carbonyl oxygen atoms. The Schiff base ligand occupies the equatorial plane and isothiocyanate anions for 1–5 and water molecules for 6 occupy the axial positions. Compound 7 has octahedral geometry in which the Schiff base ligand acts as a tetradentate ligand and in this case one of the carbonyl oxygen atoms is not involved in coordination. Furthermore, the N-bound isothiocyanate anions are coordinated as terminal ligands. In compound 8, the Schiff base behaves as a bidentate ligand through the azomethine nitrogen and pyridine nitrogen atoms leading to a tetrahedral geometry with the remaining two sites are occupied by isothiocyanate ions. Thermal reactivity of the ligand and complexes (1–8) was studied by TG-DTA and the complexes all undergo endo- followed by exothermic decomposition to give metal oxides, whereas the ligand decomposes completely to give gaseous products.

Full text of DOI:10.1016/j.poly.2017.11.009

Accepted Manuscript
Syntheses, structural diversity and thermal behavior of first row transition metal
complexes containing potential multidentate ligands based on 2,6-diacetylpyr-
idine and benzyl carbazate
Palanivelu Nithya, Jim Simpson, Subbiah Govindarajan
PII:
S0277-5387(17)30709-X
https://doi.org/10.1016/j.poly.2017.11.009
POLY 12911
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
10 July 2017
6 November 2017
Please cite this article as: P. Nithya, J. Simpson, S. Govindarajan, Syntheses, structural diversity and thermal
behavior of first row transition metal complexes containing potential multidentate ligands based on 2,6-
diacetylpyridine and benzyl carbazate, Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.11.009
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Syntheses, structural diversity and thermal behavior of first row transition metal
complexes containing potential multidentate ligands based on 2,6-diacetylpyridine and
benzyl carbazate
a
b
a*
Palanivelu Nithya , Jim Simpson and Subbiah Govindarajan
a
Department of Chemistry, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India
b
Department of Chemistry, University of Otago, Dunedin 9054, New Zealand
*
Corresponding author: E-mail: drsgovind@yahoo.co.in
Abstract
The Schiff base ligand, N’-(1-{6-[1-(Benzyloxycarbonyl-hydrazono)-ethyl]-pyridin-2-
yl}-ethylidene)-hydrazinecarboxylic acid benzyl ester (bc -dap), was synthesized by the reaction
of 2,6-diacetylpyridine (dap) with benzyl carbazate (bc) in methanol. Also first row transition
metal complexes of the general formula [M(NCS) (bc -dap)].nH O; M = Mn (1), Fe (2), Co (3),
Zn (4) or Co (5); n=0 (1-4) and n=1 (5), [Fe(bc -dap)(H O) ].2Cl.4H O (6), [Ni(NCS) (bc
dap)].CH OH (7) and [Co(NCS) (bc-dap)] (8) have been synthesized using a template method
and characterized by spectroscopic techniques and single crystal X-ray diffraction. Complexes
-4 are isotypic, while compounds 1-6 comprises of discrete mononuclear units with
2
2
2
2
2
2
2
2
2
2
-
3
2
1
pentagonalbipyramidal geometry. In contrast compounds 7 and 8 adopt octahedral and
tetrahedral geometries, respectively. In all of the complexes 1-6, the Schiff base acts as a
pentadentate ligand that binds via the pyridine nitrogen, azomethine nitrogen and carbonyl
oxygen atoms. The Schiff base ligand occupies the equatorial plane and isothiocyanate anions for
1
-5 and water molecules for 6 occupy the axial positions. Compound 7 has octahedral geometry
in which the Schiff base ligand acts as a tetradentate ligand and in this case one of the carbonyl
oxygen atoms is not involved in coordination. Furthermore, the N-bound isothiocyanate anions
1

are coordinated as terminal ligands. In compound 8, the Schiff base behaves as a bidentate ligand
through the azomethine nitrogen and pyridine nitrogen atoms leading to a tetrahedral geometry
with the remaining two sites are occupied by isothiocyanate ions. Thermal reactivity of the
ligand and complexes (1-8) was studied by TG-DTA and the complexes all undergo endo-
followed by exothermic decomposition to give metal oxides, whereas the ligand decomposes
completely to give gaseous products.
Keywords: Benzyl carbazate, 2,6-diacetylpyridine, Schiff base complexes, Crystal structures,
Thermal behavior
2

Syntheses, structural diversity and thermal behavior of first row transition metal
complexes containing potential multidentate ligands based on 2,6-diacetylpyridine and
benzyl carbazate
a
b
a*
Palanivelu Nithya , Jim Simpson and Subbiah Govindarajan
a
Department of Chemistry, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India
b
Department of Chemistry, University of Otago, Dunedin 9054, New Zealand
*
Corresponding author: E-mail: drsgovind@yahoo.co.in
1
. Introduction
Metal complexes containing esters of hydrazinecarboxylic acid, that is, carbazates have
been of considerable interest due to their interesting thermal and structural properties [1-8].
Carbazates are interesting as ligands in view of their variety of potential oxygen and nitrogen
donor atoms and these neutral molecules are expected to exhibit only one a common
coordination mode, namely bidentate N, O-chelation. In recent years, we have explored the
coordination chemistry of methyl and ethyl carbazates and have now turned our attention to
benzyl carbazate (NH
2
NHCOOCH
2
C
6
5
H , bc) that is structurally related to these alkyl
carbazates. Further, the coordination chemistry of benzyl carbazate has not been explored
extensively. Guerra et al. reported [9, 10] the syntheses, spectroscopic characterizations, DFT
studies and cytotoxic activity of [M(X)
-methoxybenzylcarbazate (4-MC), benzyl carbazate (BC), 4-fluorophenoxyacetic acid
hydrazide (4-FH), 3-methoxybenzoic acid hydrazide (3-MH) and tert-butyl carbazate (TC)},
M(EC) ] {M = Pd or Pt, EC = ethyl carbazate} and [Pt(4-HH) ].2.5H O {4-HH = (4-
2 2
Cl
]
{where
M
=
Pd or Pt, X =
4
[
2
I
2
2
I
2
2
hydroxy-phenyl)-acetic acid hydrazide (4-HH)}. Apart from its coordinating ability, benzyl
carbazate can also undergo condensation reactions; the hydrazinic part of the terminal amine
group reacts with the carbonyl groups of aldehydes or ketones to form Schiff bases. However,
3

Schiff bases and their metal complexes derived from benzyl carbazate have not been studied till
date except our own recent reports [11-13] of Schiff bases derived from benzyl carbazate with
alkyl and heteroaryl ketones and their metal complexes.
In the recent past, the coordination properties of 2,6-diacetylpyridine based Schiff base
ligands have been investigated in detail and are shown to be particularly versatile in the variety
of coordination modes that they adopt. Among such Schiff base ligand systems considerable
interest has been shown in the coordination chemistry of metal complexes based on 2,6-
diacetylpyridine with hydrazinic bases such as hydrazines [14-18], hydrazides [19-23],
semicarbazides [24, 25] and thiosemicarbazides [26-32]. In addition to this, dithiocarbazates are
among the most important hydrazinic bases because of their structural similarities to the
carbazates and a considerable number of studies involving Schiff base complexes with
dithiocarbazates and 2,6-diacetylpyridine have been reported [33-41].
Although Schiff bases and their metal complexes derived from dithiocarbazates with
diacetylpyridine have been reported, the corresponding oxygen analogues, namely the
carbazates in general and benzyl carbazate in particular, have not been previously reported. As
part of our work focuses on Schiff base metal complexes derived from benzyl carbazate, we
report here the synthesis of 2,6-diacetylpyridine bis(benzyl carbazate) and the template
synthesis and structural characterization of its complexes with Mn(II), Fe(II), Co(II), Ni(II) or
Zn(II). The thermal stability of each of these complexes is also discussed.
4

2
. Experimental
2
.1. Materials
Benzyl carbazate and 2,6-diacetylpyridine was purchased from Sigma-Aldrich. All other
chemicals and solvents were obtained from a commercial source and were used as received
without further purification.
2
.2. Synthesis of the ligand
.166 g (1 mmol) of benzyl carbazate and 0.082 g (0.5 mmol) of 2,6-diacetylpyridine
0
were combined in 20 mL of methanol. Slow evaporation of the solvent from the mixture at room
temperature resulted in a white crystalline precipitate which was isolated and washed with
doubly- distilled water.
Ligand bc
2
-dap: Colorless, 80 % yield with respect to the amount of base (benzyl carbazate)
taken. Anal. Calcd for C25
H
25
N
5
O
4
: C, 65.36; H, 3.96; N, 15.25%. Found: C, 65.15; H, 3.40;
-1
1
N, 14.95%. IR (cm ): 3246, 1694, 1573, 1038. H NMR (400 MHz, DMSO-d
6
): δ 10.55 (s, 1H),
1
2
2
2
0.44 (s, 1H), 7.82-8.22 (m, 3H), 7.33-7.46 (m, 10H), 5.24 (s, 2H), 5.23 (s, 2H), 2.39 (s, 3H),
.35 (s, 3H).
.3. Synthesis of the complexes
2 2 2
.3.1. [M(NCS) (bc -dap)].nH O; M = Mn (1), Fe (2), Co (3), Zn (4) or Co (5); n=0 (1- 4) and
n=1 (5)
Benzyl carbazate (0.166 g, 1 mmol) was dissolved in 10 mL of a propanol solution of
,6-diacetylpyridine (0.082 g, 0.5 mmol). To this solution the various metal nitrates
M(NO .6H O, (Mn=0.144 g, Co=0.146 g, Zn=0.149 g), 0.5 mmol; Fe(NO .9H O, 0.202 g,
.5 mmol) were added together with ammonium thiocyanate (0.076 g, 1 mmol) dissolved in 10
2
(
3
)
2
2
3
)
3
2
0
mL of doubly-distilled water. The resulting mixture was kept at room temperature for
5

crystallization. After two days, the manganese, 1, iron, 2, and zinc, 4, complexes were obtained
as crystalline solids. In the case of cobalt, both red, 3, and orange, 5, crystals were observed in
the solution after two days and were isolated by filtration. These crystals were separated
manually and used for further analysis.
Compound 1: Lemon yellow, 75 % yield with respect to the amount of metal salt taken. Anal.
Calcd for C27
H
25
N
7
O
4
2
S Mn: Mn, 8.71; C, 51.38; H, 3.96; N, 15.54; S, 10.15%. Found: Mn, 8.25;
-1
C, 50.10; H, 3.20; N, 14.95; S, 10.05%. IR (cm ): 3153, 2080, 2005, 1678, 1525, 1047.
Compound 2: Brown, 78 % yield with respect to the amount of metal salt taken. Anal. Calcd for
C
27
H
25
N
7
O
4
S Fe: Fe, 8.85; C, 51.36; H, 3.96; N, 15.53; S, 10.15%. Found: Fe, 8.30; C, 51.10; H,
2
-1
3
.40; N, 15.30; S, 9.95 %. IR (cm ): 3140, 2087, 2009, 1676, 1525, 1047.
Compound 3: Red, 20 % yield with respect to the amount of metal salt taken. Anal. Calcd for
Co: Co, 8.50; C, 51.06; H, 3.94; N, 15.44; S, 10.09%. Found: Co, 8.30; C, 51.05;
C
27
H
25
N
7
O S
4 2
-1
H, 3.50; N, 15.10; S, 10.25 %. IR (cm ): 3141, 2098, 2011, 1680, 1528, 1052.
Compound 4: Light yellow, 70 % yield with respect to the amount of metal salt taken. Anal.
Calcd for C54
H
50
N
14
O
8
S
4
Zn
2
: Zn, 10.20; C, 50.54; H, 3.90; N, 15.29; S, 9.98%. Found: Zn,
-1
1
0.10; C, 50.00; H, 3.30; N, 15.25; S, 9.10%. IR (cm ): 3183, 2097, 2009, 1695, 1521, 1045.
1
H NMR (400 MHz, DMSO-d
0H), 5.48 (s, 2H), 5.24 (s, 2H), 2.50 (s, 3H), 2.37 (s, 3H).
Compound 5: Orange, 65 % yield with respect to the amount of metal salt taken. Anal. Calcd for
Co: Co, 8.26; C, 49.65; H, 4.14; N, 15.02; S, 9.81%. Found: Co, 8.10; C, 49.30; H,
6
): δ 12.48 (s, 1H), 10.47 (s, 1H), 8.08-8.14 (m, 3H), 7.43-7.97 (m,
1
C
27
H
27
N
7
O S
5 2
-1
1
4
.05; N, 14.50; S, 9.55%. IR (cm ): 3418, 3115, 2103, 1686, 1546, 1052. H NMR (400 MHz,
DMSO-d ): δ 13.60 (s, 2H), 10.28 (s, 1H), 10.50 (s, 1H), 7.97-8.56 (m, 3H), 7.35-7.66 (m, 10H),
.25 (s, 2H), 5.20 (s, 2H), 2.48 (s, 3H), 2.36 (s, 3H).
6
5
6

2
2 2 2
.3.2. [Fe(bc-dap)(H O) ]. 2Cl. 4H O (6)
To a methanol solution (10 mL) of benzyl carbazate (0.166 g, 1 mmol) and
,6-diacetylpyridine (0.082 g, 0.5 mmol), an aqueous solution (10 mL) of iron(II) chloride
2
hydrate (0.072 g, 0.5 mmol) was added drop wise. The resulting solution was allowed to slowly
evaporate at room temperature. X-ray quality brown colored crystals were isolated from mother-
liquor, washed with doubly-distilled water and air dried.
Compound 6: Brown, 73 % yield with respect to the amount of metal salt taken. Anal. Calcd for
C
25
H
35
N
5
O
10Cl Fe: Fe, 8.85; C, 43.33; H, 5.06; N, 10.11 %. Found: Fe, 8.30; C, 43.05; H, 4.70;
2
-
1
1
N, 10.20 %. IR (cm ): 3596, 3141, 1668, 1538, 1061. H NMR (400 MHz, DMSO-d
s, 1H), 7.35-7.88 (m, 3H), 6.42-6.66 (m, 10H), 5.15 (s, 2H), 2.47 (s, 3H), 2.27 (s, 3H).
.3.3. [Ni(NCS) (bc -dap)]. CH OH (7)
A mixture of nickel(II) nitrate hexahydrate (0.146 g, 0.5 mmol) and ammonium
thiocyanate (0.076 g, 1 mmol) dissolved in doubly-distilled water (10 mL) was mixed with a
methanol solution (10 mL) of benzyl carbazate (0.166 g, mmol) and
,6-diacetylpyridine (0.163 g, 1 mmol). The solution was left to stand overnight and the resulting
6
): δ 10.42
(
2
2
2
3
1
2
light green crystalline compound was filtered off, washed with distilled water and air dried.
Compound 7: Green, 73 % yield with respect to the amount of metal salt taken. Anal. Calcd for
C
28
H
29
N
7
O
5
S Ni: Ni, 8.81; C, 50.42; H, 4.35; N, 14.71; S, 9.60%. Found: Ni, 8.25; C, 50.20; H,
2
-1
1
4
.00; N, 14.25; S, 8.95%. IR (cm ): 3581, 3169, 2106, 1708, 1618, 1520, 1043. H NMR (400
MHz, DMSO-d ): δ 10.41 (s, 1H), 7.79-7.91 (m, 3H), 7.29-7.40 (m, 10H), 6.78 (s, 1H), 5.16 (s,
H), 3.17 (s, 3H), 2.49 (s, 3H), 2.28 (s, 3H).
6
4
7

2
2
.3.4. [Co(NCS) (bc-dap)] (8)
To a 10 mL propanol solution of benzyl carbazate (0.083 g, 0.5 mmol) and
,6-diacetylpyridine (0.082 g, 0.5 mmol), a 10 mL aqueous solution containing cobalt(II) nitrate
2
hexahydrate(0.146 g, 0.5 mmol), zinc(II) nitrate heptahydrate (0.149 g, 0.5 mmol) and
ammonium thiocyanate (0.076 g, 1 mmol) was added slowly with constant stirring. The
resulting orange solution was kept at room temperature for slow evaporation of the solvent.
X-ray quality green crystals of 8 were obtained after two days and separated as before.
Compound 8: Dark green, 72 % yield with respect to the amount of metal salt taken. Anal. Calcd
for C17
H
14
N
5
O
2
2
S Co: Co, 13.30; C, 46.06; H, 3.16; N, 15.80; S, 14.45%. Found: Co, 13.10;
-1
1
C, 45.90; H, 3.05; N, 15.45; S, 14.10%. IR (cm ): 3068, 2091, 1734, 1694, 1518, 1053. H NMR
(
(
400 MHz, DMSO-d
s, 3H), 2.38 (s, 3H).
6
): δ 10.57 (s, 1H), 7.91-8.22 (m, 3H), 7.33-7.46 (m, 5H), 5.23 (s, 2H), 2.50
2
.4. Physical measurements
The metal content in all the complexes was determined volumetrically by EDTA titration
42]. Elemental analyses for C, H, N and S were performed on a Vario-ELIII elemental analyzer.
The FT-IR spectra were recorded on a JASCO-4100 spectrophotometer as KBr pellets in the
[
-1
1
1
range of 4,000-400 cm . H NMR spectra were recorded on a Bruker Ultrashield 400 MHz ( H)
spectrometer using tetramethylsilane (TMS) as an internal reference. Chemical shifts are
expressed in parts per million (ppm). Electronic absorption spectra were measured on a JASCO
V-630 UV-vis spectrophotometer. The simultaneous TG-DTA studies were undertaken on a
PerkinElmer SII Thermal Analyzer, and the thermograms were obtained in air using platinum
-1
cups as sample holders with 5-10 mg of each sample at a heating rate of 10 ˚C min .
8

2
.5. X-ray crystallography
Crystallographic data for 1-5 are detailed in Table 1 with those for 6-8 in Table 2. All
X-ray measurements were performed on an Agilent SuperNova, Dual, Cu at zero, Atlas
diffractometer using a mirror monochromator and Cu-Kα radiation (λ = 1.5418 Å) for the
structures of 1-4, 7 and 8 with Mo-Kα radiation (λ = 0.71073 Å) providing better data for the
structures of 5 and 6. All data were collected at 100(2) K. The data collection, cell refinement,
data reduction and absorption corrections were applied using CrysAlisPro [43].
The structures of 1, 2, 6 and 8 were solved with SHELXT [44] and those of 3, 4, 5, and 7
2
with SHELXS97 [45] and refined by full-matrix least-squares on F using SHELXL-2014/7 [46]
and TITAN2000 [47]. All non-hydrogen atoms were assigned anisotropic displacement
parameters. The H atoms on N3 and N3' in all of the complexes were located in a difference
Fourier and their coordinates were refined with Ueq = 1.2Ueq (N). All H-atoms bound to carbon
were refined using a riding model with d(C-H) = 0.95 Å, Uiso =1.2Ueq (C) for aromatic, 0.99 Å,
U
iso =1.2Ueq (C) for methylene and 0.98 Å, Uiso =1.5Ueq (C) for the CH H atoms. When all of
3
the non-hydrogen atoms had been found in the structures of complexes 5 and 6 additional high
peaks were found in the difference Fourier maps that could be refined as the O atoms of solvent
water molecules; one for 5 and four for 6. For 6, one of these O atoms lies on a mirror plane. The
H atoms of these solvent water molecules were all found in subsequent Fourier maps and their
coordinates were refined with Ueq = 1.5Ueq(O). For 7, two additional high peaks appeared that
were found to be from a methanol solvate molecule. The H atoms for this solvent were treated as
riding for the methyl group while the coordinates of the OH hydrogen atom were refined.
Crystals of 2 were of particularly poor quality. Despite collecting data on several different
samples and with both Cu and Mo radiation the results presented here detail the best that could
9

be obtained. However, despite the poor final residuals, the structure solved and refined
satisfactorily and the poor data is reflected in the uncertainties of all the parameters reported for
this structure. All molecular plots and packing diagrams were drawn using Mercury [48]. Other
calculations were performed using PLATON [49] and tabular material was produced using
WINGX [50].
3
. Results and Discussion
The ligand (bc -dap) can be readily prepared in good yield by the reaction of
,6-diacetylpyridine (dap) with benzyl carbazate (bc) in methanol (Scheme 1). However, the
2
2
reaction of an aqueous methanolic solution of a metal salt in presence of ammonium thiocyanate
with bc -dap in 1:2:1 molar proportions did not yield any one of the desired complexes in stable
2
and crystalline forms. Therefore, the synthesis of complexes of this ligand was achieved by a
template method.
One-step synthesis of Schiff base complexes through a template condensation between
diacetylpyridine and benzyl carbazate in the presence of various metal salts and ammonium
thiocyanate at room temperature yielded a number of products, Scheme 2. Reaction of the ligand
generated in-situ in the presence of the metal salts is very interesting as, depending on the metal
and its associated counter anion, the reaction conditions and the solvents used in the preparation,
three different types of metal complexes were obtained as homogeneous crystalline solids
(
Scheme 2).
In the first type, I(a), bc
2
-dap acts as a pentadentate ligand and forms isotypic crystalline
complexes with Mn(II), Fe(II), Co(II) or Zn(II) labelled as compounds 1-4 respectively. In each
the metals are seven coordinate binding to the multidentate ligand through the carbonyl oxygens,
azomethine nitrogen atoms and the pyridine nitrogen. The remaining two sites are occupied by
10

nitrogen atoms of anionic thiocyanato ligands that ensure the neutrality of the complexes.
Closely related to these products is the Co(II) complex 5, with comparable coordination
geometry but which crystallises with a solvate water molecule. Type I(b) is represented by the
Fe(II) complex 6, the only cationic complex reported here, all the others being neutral. The
coordination geometry is similar to that of 1-5 except that neutral water molecule replaces the
thiocyanato ligands with two uncoordinated chloride anions for neutrality. This complex
co-crystallizes with four solvent water molecules. A stable product of this type could only be
obtained with iron and no comparable cationic complexes could not be isolated with other metal
ions.
In the type II nickel compound, the bc -dap ligand formed in-situ is tetratendate
2
coordinating to Ni(II) through the pyridyl nitrogen, both azomethine nitrogen atoms but only one
of the carbonyl oxygen atoms. In attempts to prepare 7, if the nickel nitrate, ammonium
thiocyanate, benzyl carbazate and 2,6-diacetylpyridine mole ratio was 1:2:2:1, as was used for
the preparation of 1-6, the resulting product was polycrystalline in nature. Although analysis of
this polycrystalline solid was consistent with the formula C28
H
29
N
7
NiO
5
2
S , X-ray quality crystals
could not be obtained. However, the reaction was carried out in the mole ratio 1:2:2:2, a
crystalline form of compound 7 formed that was suitable for single crystal X-ray diffraction.
The third type of complex 8 is formed for Co(II) from an alternative ligand formed in-
situ. Here only one benzyl carbazate has condensed with the diacetyl pyridine to produce the
mono-Schiff base (bc-dap). A notable feature of this type III complex is that the Co(II) cation is
tetrahedrally coordinated, binding to the pyridine and azomethine N atoms of the bidentate
bc-dap ligand. Two N-bound thiocyanato ligands complete the tetrahedral coordination sphere
of the neutral complex. This compound was obtained as an unintentional product of an attempt to
11

prepare mixed metal complexes of cobalt and zinc; green crystals of the Co(II) compound
appeared floating in the mother liquor in very low yield. Reaction of metal salts (cobalt and zinc
nitrates), ammonium thiocyanate, benzyl carbazate and 2,6-diacetylpyridine in the mole ratio of
½
:½:2:2:1 resulted in the 1:2:1:1 product with low yield.
Analytical results confirmed the proposed composition (1:2:1:1). Once the structure of
this complex was determined crystallographically, the reaction was repeated with the mole ratio
:½:2:1:1 (Co:Zn:NH SCN:bc:dap) that gave the green product in greater yield. It is interesting
½
4
to note here that, attempts to synthesize complex 8 without zinc nitrate present in the reaction
mixture were entirely unsuccessful. However, the same reaction carried out with other metal ions
instead of zinc did not yield any tractable products. The role of the zinc ion, in this reaction is not
clear at present.
However, the present Schiff base, irrespective nature of the metal salt used, yielded
complexes containing only the protonated form of the ligand. The result of these reactions
depends on the nature of the ligands, as well as on the coordination preferences of the metal ions.
Efforts to prepare the analogous chromium and copper complexes were unsuccessful, probably
due to the reducing nature of these metal ions. All the complexes that were obtained are stable in
air and insensitive to light. They are insoluble in water, methanol and ethanol, and soluble in
DMSO.
3
.1. Infrared spectra
The infrared spectra of the ligand (bc
confirm their structures and the IR spectra of ligand (bc
shown in Fig. S1-S6 as representative examples. The main IR bands of the ligand are found at
2
-dap) and complexes (1-8) were recorded to
-dap) and complexes (3 & 5-8) are
2
-1
1
694 and 1573 cm and may be assigned to C=O and C=N stretching vibrations, respectively.
12

-1
-1
These bands undergo shifts of 8-26 cm and 27-53 cm , respectively, to lower wavenumbers for
the complexes (1-6) upon binding the ligand to the central metal ion through the oxygen atoms of
both carbonyl groups and both azomethine nitrogen atoms. Shifts to lower wavenumber on
binding the azomethine N atom of a Schiff base to a metal atom has been observed previously
[
39]. Also a shift of the C=O stretching vibration to lower frequency on coordination to a metal
is a common observation [51]. Unlike the other compounds, complex 7 displayed a sharp peaks
-1
at 1708 and 1618 cm corresponding to the C=O stretching frequency of an uncoordinated and
coordinated carbonyl groups respectively. Complex 8 showed two discrete C=O stretching
-1
modes at 1736 cm due to the C=O stretching of carbonyl group of the diacetylpyridine and at
-1
1
695 cm from the amide C=O of benzyl carbazate. Further, evidence of coordination of
bc -dap to the metal ion through the azomethine nitrogen atom comes from the shift of the N-N
stretching of the free ligand (1028 cm ) to higher wave numbers in the spectra of the complexes
2
-
1
-1
(
to ~1050 cm in most cases) [52]. Coordination via the pyridine nitrogen atom is also indicated
-1
-1
by the out-of-plane ring deformation at 600 cm shifting to higher energies by 20 cm [53]. The
presence of both coordinated and solvent water molecules in 6 is indicated by the appearance of
-1
-1
O-H bands at 3141 and 3596 cm . Bands at 3418 and 3580 cm in complexes 5 and 7 have been
assigned to the O-H stretching frequencies of the uncoordinated solvent water and methanol
molecules. The IR spectra of the compounds (1-4) display a noticeable difference in the νCN band
-
-1
of SCN , showing two bands around 2080 and 2010 cm , whereas, there is only one band at
-1
2
100 cm in compounds 5, 7 and 8. Despite the superficial similarity between the isotypic
structures 1-4 and the similarly constructed hydrated complex 5, the M-N4-C16 angles in 1-4
differ by close to 50˚ whereas the difference for 5 is only approximately 12˚. Hence the variation
in the number of C≡N stretching modes between 1-4 and 5 is not surprising [54]. In 7 and 8, the
13

environments of the thiocyanate ligands are also reasonably similar suggesting a rationale for the
observation of a single stretching mode in each case.
1
.2. H NMR spectra
3
1
The H NMR spectra of the free ligand (bc
2
-dap) and complexes 4-8 were recorded in
1
DMSO-d
6
as typical examples. The assignments of the main signals in the H NMR spectra are
reported in the experimental section and the spectra are shown in figures in the supplementary
material (Fig. S7-S12). The spectrum of the ligand (bc -dap) exhibits signals at 2.35 and 5.23
ppm corresponding to the CH and OCH protons, respectively. Signals due to the aromatic
2
3
2
hydrogens of the phenyl rings appear within the range of 7.33-7.46 ppm while the heteroaromatic
protons of pyridine ring appeared as multiplets in the range 7.82-8.22 ppm. The singlets at 10.44
and 10.55 ppm are ascribed to the hydrazinic NH proton of the carbazate moieties. The signals
corresponding to these protons, in the spectra of complexes (4-8), are observed in the range
2
.28-2.67, 5.15-5.48, 6.42-7.66, 7.35-8.55 and 10.41-12.48 ppm, respectively. All proton
resonances found for the free ligand were also seen, albeit with some chemical shift differences,
in the spectra of the complexes suggesting that no deprotonation of the ligand occurred during
complex formation. Apart from these signals, the singlet at 1.24 ppm corresponds to the water
1
molecule in complex 5. Further, the H NMR spectrum of complex 5 shows a broad peak at
chemical shift δ 13.60 for the enolic OH protons, indicating that in solution, the ligand exists in
keto-enol tautomeric forms. The protons of water molecules in 6 appear as a broad signal at 1.27
ppm corresponding to both lattice and coordinated water molecules. A sharp peak at 3.17 ppm
and a broad signal at 6.78 ppm for 7 are those OCH and OH groups of the methanol solvent
3
molecule. Thus, NMR studies support the conclusions drawn from the infrared spectra about the
bonding mode of the Schiff bases.
14

3
.3. Electronic spectra
The electronic spectra of the complexes were recorded in DMSO and the spectra are
shown in Fig. S13-S18. The manganese 1 and zinc 4 complexes show no prominent absorption
5
in the visible region because Mn(II) with a high-spin d electronic configuration has no spin-
1
0
allowed d-d transition and Zn(II) with a d configuration also lacks d-d transitions [55]. The
intense band at 536 and 548 nm for iron complexes 2 and 6, respectively are attributed to ligand
charge transfer transitions for Fe(II) [56]. The d-d band cannot be observed as it is obscured by a
strong charge transfer transition in each case [53]. The absorption bands at 624 and 626 nm for
the cobalt complexes 3 and 5, respectively can be attributed to d-d transition [21] and support the
seven-coordinated pentagonalbipyramidal geometry around cobalt(II) ions with the CoN
5
O
2
chromophoric group. The electronic spectrum of nickel compound 7 shows three bands at 400,
3
3
3
3
3
6
59 and 745 nm corresponding to the A2g (F)
T
2g (F), A2g (F)
T
2g (F) and A2g (F)
3
T
1g (P) transitions respectively, confirming an octahedral geometry around the metal ion
4
4
[
57]. A broad band at 606 nm may be assigned to A
2
(F)
1
T (P) transition for Co(II) ion in a
tetrahedral coordination environment found in 8 [24].
3
3
.4. Crystal structures
.4.1. Molecular structures of 1-5
The molecular structures of 1-5 and their atom numbering schemes are shown in Fig.1.
Crystallographic data and structure refinement parameters are summarized in Table 1 and
selected bond distances and angles are given in Tables S1 and S2. Compounds 1- 4 are isotypic;
all four complexes have similar coordination spheres with the metal centres coordinated by the
pyridine nitrogen and two azomethine nitrogen atoms together with the two carbonyl oxygen
atoms of the pentadentate ligands. These complexes display the same coordination geometry and
15

all of them crystallize in the monoclinic space group P2 /n. Complex 5 is closely similar to
1
compounds 1-4 but, interestingly, 5 crystallizes as a monohydrate with a completely different
unit cell and crystal packing. All five complexes are monomeric and the metal ions are in seven
coordinate pentagonal bipyramidal environments. The two nitrogen atoms from the thiocyanato
2
anions are found in apical positions and the planar pentadentate ONNNO coordinated bc -dap
ligands each bind through the carbonyl oxygen atoms, the two azomethine nitrogen atoms and
the pyridine nitrogen atom to form the equatorial plane. The coordinated Schiff base ligand
presents an almost ideal pentagonal array of donor atoms (Fig. 2). Four of the angles subtended
at the metal atom by adjacent equatorial atoms vary only slightly than the value of 72˚ expected
for an ideal pentagonal bipyramidal arrangement, ranging from 68-71˚. In contrast the O8-M-O8'
angles are consistently wider across the series and their range of values is also much greater
ranging from 85-76˚, Table S1. An interesting structural feature of 1-5 is that the M-O bond
lengths are consistently longer than the M-N bond distances (Table S1).
A striking feature of the coordination of the two trans-axial thiocyanato ligands is the
'
'
sharp contrast between the values of the C16-N4-M and C16-N4-M angles in all four structures,
'
'
Table S2. The C16-N4-M angles do not differ severely from the ideal 180˚, (range 173.97(13)-
'
-
'
'
1
69.1(8)) such that the S1 C16-N4-M is reasonably linear, the corresponding range of C16-N4-
M angles is 125.3(7)-122.82(14) such that there is a significant bend at the N4 atom in each case,
-
widening the angle between the SCN and the trans axial line by up to 40˚. Similar significant
variations from linearity in the placement of thiocyanato ligands have been observed in one
polymorph of the complex [Co(SCN)
4
(ppz-H) ] (ppz = piperazine) [54]. As mentioned
2
previously, this is likely to be the reason for the occurrence of two νCN strethching modes in the
'
infrared spectra. In an interesting contrast, the bending at N4 and N4 for the hydrated analogue 5
16

is less severe and almost equivalent for each of the thiocyanato ligands with C16-N4-Co1 =
1
50.22(15) and C16'-N4'-Co1 = 161.97(16).
As expected for isostructural systems of 1- 4, the packing features of these complexes are
almost identical therefore the packing of compound 4 is used as a representative example. The
pyridine and C10/C15 phenyl rings form an offset π…π contact with a centroid to centroid
separation of 3.7831(9) Å. In addition, a C-H…π (ring) contact C71'-H71F…Cg3 also forms
linking each complex molecule to others (Fig. S19a). An eclectic mix of classical N-H…N and
N-H…S hydrogen bonds, Table S3, together with C-H…O and C-H…S contacts complete the
three dimensional network with complexes stacked along the b axis direction (Fig. S19b).
Packing diagrams for 1-3 appear in Figs. S20-S22 respectively.
Unsurprisingly, the crystal structure of the solvated complex 5 is somewhat different
from those of 1-4 complexes despite the similarities in the structures of the complexes
themselves. In particular the solvent water molecule is involved in O-H…S, N-H…O and
C-H…O hydrogen bonds, Table S4. Molecules of 5 are linked into chains through the N-H…S,
C-H…S and O-H…S interactions. The chains are arranged into sheets in the bc plane through
O-H…S and C-H…N hydrogen bonds. In addition to these interactions, C-H…π combine to
generate a 3-dimensional supramolecular network, Fig. S23.
3
.4.2. Molecular structure of 6
Compound 6 crystallized in the monoclinic space group P2 /m with two molecules in the
1
unit cell as the iron atom, Fe1, lies on mirror plane, Fig. 3. The O1, O2 and O2W atoms of water
molecules and their associated H atoms also lie on this plane as does the O3W atom. O1W is in a
general position so that four solvent water molecules in total co-crystallise with the complex
17

molecule. The charge on the Fe(II) complex is neutralized by two non-coordinating chloride
anions. The coordination geometry about the metal centre is pentagonal bipyramidal similar to
that adopted by complexes 1-5 with the neutral bc -dap ligand coordinating through its carbonyl
2
oxygen azomethine nitrogen and pyridine nitrogen atoms and forming the basal (pentagonal)
plane. Two coordinated water molecules occupy axial positions and complete the coordination
sphere. Selected bond distances and angles are detailed in Table S5. Unlike the situation for 1-5
where the M-Npy and M-Nazo distances were reasonably similar for a given complex, here
Fe-N1py at 2.205(5) Å is shorter than Fe-N2azo, 2.229(3) Å, with Fe-O8 = 2.236(3) Å. The Schiff
base ligand again adopts almost ideal pentagonal geometry with the N-Fe-N and O-Fe-N angles
close to the ideal 72˚. As was found in complexes 1-5, the O-Fe-O angle is much wider,
Fig. S24.
Packing in this structure is achieved through an extensive range of intermolecular
interactions, Table S6. Interestingly, there are no direct contacts between the complex cations;
the lattice chloride anions and water molecules act as donors and acceptors to bridge neighboring
cationic units. The cations are linked into a one-dimensional chain along the c axis through
C4-H4…O3W interactions. An extensive series of O-H…O, C-H…O, O-H…Cl and C-H...Cl
hydrogen bonds join adjacent chains forming two-dimensional sheets of cations, anions and
water molecules in the bc plane (Fig. S25a). These sheets are further interconnected by O-H…O
and C-H…O hydrogen bonds involving the solvent water molecules to generate a three-
dimensional network with the cations stacked along the c axis direction (Fig. S25b).
18

3
.4.3. Molecular structure of 7
The asymmetric unit of the structure of 7 comprises a neutral Ni(II) complex that lies on
a general position in the orthorhombic space group P2
1
2
1
1
2 , and a methanol solvent molecule
Fig. 4. Selected bond distances and angles are given in Table S7. The structure of compound 7 is
type II, with highly distorted octahedral coordination around the Ni(II) cation. Unlike the
previous set of complexes 1-6, here the Schiff base provides only four donor atoms (N3O) from
the essentially planar ligand environment that forms the equatorial plane. The fifth and sixth
coordination sites of the octahedron are occupied by two nitrogen atoms of thiocyanate anions in
the axial positions. Fig. 4 clearly shows that the bc -dap ligand came close to achieving the
2
pentadentate coordination displayed in complexes 1-6 but the Ni-O8ʹ bond distance, 2.767(3) Å,
is too great to be considered a bonding interaction. As a consequence, the octahedral
coordination geometry of the molecule (Fig. 5) is severely distorted with N1-Ni1-N2, N1-Ni1-
N2ʹ and N2ʹ-Ni1-O8ʹ all very much less than the ideal 90˚ while in compensation, the O8ʹ-Ni1-
N2 angle is exceptionally large at 138.26(13)˚. The best fit plane through the four ligand donor
atoms has an rms deviation of 0.0126 Å. With the exception of the phenyl rings, the remaining
parts of the tetradentate ligands are also reasonably co-planar with an rms deviation 0.0976 Å for
7
from the best fit plane through the remaining 22 non-hydrogen atoms. The phenyl rings of the
benzyl units subtend angles of 72.62(9) and 68.58(4)˚ to this plane. The bonding parameters of
the terminal NCS ligands in 7 fall well within the range generally observed in the literature
[
58,59]. The terminal M-NCS linkages are again bent with C16-N4-Ni1 at 159.9(4)˚ more
markedly so in comparison to C16'-N4'-Ni1 at 177.0(3)˚. These angles fall in the range generally
2
+
-
found for Ni complexes with N bound NCS ligands [60].
19

In the crystal packing of 7, pairs of adjacent complex molecules are linked by C71-
7
ii
H71C…Cg5 hydrogen bonds, Table S8, and π…π stacking interactions (Cg4…Cg6 3.609(3)
Å, 7ii = x-1/2, -y+1/2, -z+1) to form chains of complex molecules parallel to (103), Fig. S26a.
(
Cg2, Cg4 and Cg6 are the centroids of the N1/C2-C6, C10-C15 and C10ʹ-C15ʹ rings
respectively). Classical N-H…S and non-classical C-H…S hydrogen bonds also link the
complex molecules. In addition, the methanol solvate molecule forms hydrogen bonds to the
complex molecules that involve all three hydrogen atoms of the methyl group, but no sensible
contact from the OH group is found. This variety of contacts combine to generate and extensive
three dimensional network with the complex molecules stacked along the a axis direction,
Fig. S26b.
3
.4.4. Molecular structure of 8
The molecular structure of the complex 8 is shown in Fig. 6 and the crystallographic
information with the selected bond parameters are provided in Tables 2 and S9 respectively. In
this instance the ligand that formed in-situ was a mono-Schiff base as the diacetyl substrate
condensed with only one benzyl carbazate molecule resulting in the bidentate bc-dap ligand.
The neutral complex 8 has a four-coordinate mononuclear cobalt(II) centre bound to the neutral
-
bidentate bc-dap ligand and two NCS anions. It is interesting to note that unlike other
complexes (1-7), compound 8 adopts tetrahedral coordination geometry. In the former, the ligand
has two Schiff base centres from the bc -dap ligands that can provide five donor atoms (N3O2)
2
in a basal plane. Here however, the smaller bc-dap ligand, uses only two donor atoms, the
pyridine nitrogen and an azomethine nitrogen atoms. The two remaining coordination sites are
occupied by the nitrogen atoms of two thiocyanate anions.
20

Tetrahedral coordination is confirmed for the complex by calculating the τ
= [360˚ - (α + β)]/141˚
α and β are the two largest angles subtended by the ligand donor atoms in the four-coordinate
complex. τ = 0 for perfect square planar and 1 for ideal tetrahedral coordination geometry [61].
The τ value 0.88 for 8 clearly confirms some distortion from tetrahedral coordination geometry
in this case as evidenced in particular by the angles N1-Co-N2 (76.27(15)˚) and N4-Co-N5
121.74 (19)˚) [62]. The Co-Nimine (2.135(4)Å) and Co-Npyridine (2.091(4)Å) bonds are slightly
longer than those to the nitrogen atoms of the thiocyanato anions (1.923(5) and 1.939(4)Å,
respectively); these are comparable to those found in (N(Me [Co(NCS) ] [63]. The two SCN-
4
index.
τ
4
4
4
(
4
)
2
4
ions are both almost linear (N4-C16-S1 = 176.7(5) and N5-C17-S2 = 178.9(5)˚), but a slight
bending occurs for both Co-N-C(-S) linkages with Co-N-C angles (162.6(5) and 173.1(5)˚)
respectively. These bond angles are comparable to those found in previously reported tetrahedral
Co(II) complexes with thiocyanato ligands [64].
In the crystal structure of 8, N3-H3N…N4 hydrogen bonds form inversion dimers and
2
8i
enclose R
2
(10) rings. These contacts are bolstered by π…π stacking interactions (Cg2…Cg3 =
3
.687 Å, Cg2 and Cg3 and are the centroids of the N1/C2…C6 and C10…C15 rings
respectively) Fig. S27a. C14-H14…S1 hydrogen bonds link adjacent dimers into double chains
of complex molecules along the ac diagonal. C4-H4…O8 hydrogen bonds combine with C71-
2
H71B…O21 hydrogen bonds to enclose R
2
(14) rings and link the molecules into zig-zag chains
along c. Parallel chains are linked in an obverse fashion by C-H…S hydrogen bonds with S2
acting as a bifurcated acceptor forming C3-H3…S2 and C15-H15…S2 hydrogen bonds. The
2
C3-H3…S2 hydrogen bonds form inversion dimers and generate R
2
(16) ring motifs, Fig. S27b.
21

These various contacts (Table S10) combine to stack the complex molecules along the a axis
direction, Fig. S27c.
3
.5. Thermal behavior
To examine the thermal stability of the various complexes, simultaneous TG-DTA of the
ligand (bc -dap) and complexes (1-8) have been recorded and thermograms of compounds 2, 4,
, 7 and 8 are shown in Fig. S28-S33 as representative examples. Thermal decomposition modes
of all complexes are detailed in Table S11.
The Schiff base ligand (bc -dap) undergoes three decomposition steps in the TG, in
2
6
2
accordance with the DTA results. The first peak at around 140 ˚C corresponds to melting. In the
TG curve, the first mass loss (observed: 47.28 %; calculated: 47.06 %) corresponds to the loss of
two benzyl alcohol molecules, which are seen as endotherms at 190 and 230 ˚C, in DTA. In the
second step, the observed mass loss of 65.46 % (calc. 64.92 %) in the temperature range 250-300
˚
C corresponds to the formation of 2,6-diacetylpyridine as an intermediate. This intermediate
further decomposes exothermically giving gaseous products, at 570 ˚C. Our efforts to separate
and further investigate these intermediates were unsuccessful due to the continuous
decomposition of the products, as evident from the TG curve. Hence, we have assigned the
nature of the likely intermediate from the mass loss observed in the TG analysis (Table S11),
which is consistent with the calculated mass loss.
From the thermograms (Fig. S29 & S30), it is clear that complexes 1-4 show two distinct
steps of decomposition in the TG. Compounds (1-4) are anhydrous and start to decompose above
1
30 ˚C, the weight loss in the first step is due to the loss of two molecules each of benzene and
carbon monoxide resulting in formic acid (1-{6-[1-(formyl-hydrazono)-ethyl]-pyridin-2-yl}-
ethylidene)-hydrazidebis(N-thiocyanato)metal(II), [M(X)(NCS) ], intermediates (see the
2
22

footnote to Table S11 for the structure of X). Further, these intermediates decompose to yield the
respective metal oxides as the final products. DTA shows exothermic peaks between 375 and
5
75 ˚C for this decomposition. Compounds 5 and 7 show three steps of decomposition in the TG.
The first step corresponds to the loss of water for 5 and methanol molecules for 7, respectively.
Corresponding to this, an endotherm is observed at 105 and 90 ˚C for 5 and 7, respectively, in
the DTA. These low temperature decomposition processes are consistent with the observation
that the solvent molecules are non-coordinated as found in the X-ray determinations. The
remaining steps are similar to those for complexes 1-4. After the removal of the solvent
molecules, the resulting desolvated compounds decompose at a lower temperature than the
previous set of complexes, as expected, due to pre heating.
Compound 6 undergoes three stages of mass loss upon heating. The first stage, which
occurs in the range 10-55 ˚C, is attributed to the loss of the four solvate water molecules. In the
DTA, this loss of water is observed as an endotherm around 40 ˚C and such a low temperature of
dehydration again supports the presence of lattice water molecules. The anhydrous compound,
then, decomposes in an endo- followed by an exothermic fashion to give a (1-{6-[1-(formyl-
hydrazono)-ethyl]-pyridin-2-yl}-ethylidene)-hydrazide)iron(II)
chloride,
[Fe(X)].2Cl
as the
intermediate. In the final step, this intermediate decomposes exothermically to give Fe
3
O
4
final residue.
In compound 8, the endotherm at 200 ˚C of the DTA is attributed to the loss of both
benzene and carbon monoxide molecules, which is supported by the weight loss in TG, to form
formic
Co(Y)(NCS)
intermediate further decomposes exothermically into cobalt oxide, in the range 235-625 ˚C. Our
acid
[1-(6-acetyl-pyridin-2-yl)-ethylidene]-hydrazidebis(N-thiocyanato)cobalt(II),
[
2
] as an intermediate (see the footnote to Table S11 for the structure of Y). This
23

effort to isolate the intermediates was again unsuccessful due to its continuous decomposition.
Once again we have assigned the nature of the likely intermediate from the mass losses observed
in the TG analysis (Table S11), which are consistent with the calculated values.
4
. Conclusion
Our present work is focused on the design and development of new molecular
coordination complexes. We have successfully synthesized three types of Schiff base complexes
derived from 2,6-diacetylpyridine and benzyl carbazate by a template method under different
reaction conditions. The products were characterized by various spectroscopic techniques and the
solid state structures of the complexes have been determined by X-ray crystallography. From
these studies, the variety of Schiff base ligands formed in-situ from
2
,6-diacetylpyridine and benzyl carbazate, the complexes showed interesting and perhaps
surprising structural diversity. Metal complexes 1-6 adopt pentagonal bipyramidal geometry with
the same polydentate ligand environment but different central metal ions, and ancillary ligands
and degrees of solvation. In 7 the Schiff base binds to the nickel ion in a tetradentate fashion
with one of the carbonyl oxygens no longer coordinated. This ligand occupies the equatorial
plane with thiocyanato anions in axial positions, such that the geometry around nickel is
octahedral. Finally complex 8 has a simpler ligand system with a single Schiff base unit. This
ligand binds to the cobalt centre as a neutral bidentate neutral chelator with the remaining sites of
the tetrahedral complex occupied by thiocyanate ions. Unlike the situation found with
dithiocarbazate ligands, the ligands in these complexes are invariably neutral. The
pentagonalbipyramidal, octahedral and tetrahedral complexes 1-8 undergo endo-/exothermic
decomposition to give complex intermediate, [M(X)(NCS)
2
]/[Fe(X)].2Cl/[Co(Y)(NCS) ] (see
2
24

Table S11). These intermediates decompose further in an exothermic fashion to form the
respective metal oxides as the final products.
Appendix A. Supplementary data
CCDC 1559691-1559698 contains the Supplementary crystallographic data for 1, 2, 6, 5,
7
,
3,
8
and 4. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
P. Nithya acknowledges the University Grant Commission (UGC-SAP), New Delhi,
India, for the award of a BSR - Senior Research Fellowship. S. Govindarajan would like to thank
UGC, New Delhi, for sponsoring a UGC-Emeritus fellowship. We thank the University of Otago
for the purchase of the diffractometer, and the Chemistry Department, University of Otago for
the support of the work of JS.
25

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