Spectral and structural studies of nickel(II) complexes of salicylaldehyde 3-azacyclothiosemicarbazones

Article abstract of DOI:10.1016/j.poly.2007.08.048

Mononuclear nickel(II) complexes with two ONS donor thiosemicarbazone ligands {salicylaldehyde 3-hexamethyleneiminyl thiosemicarbazone [H₂L¹] and salicylaldehyde 3-tetramethyleneiminyl thiosemicarbazone [H₂L²]} have been prepared and physico-chemically characterized. IR and electronic spectra of the complexes have been obtained. The thiosemicarbazones bind to the metal as dianionic ONS donor ligands in all the complexes except in [Ni(HL¹)₂] (1). In compound 1, the ligand is coordinated as a monoanionic (HL^-) one. The magnetic susceptibility measurements indicate that all the complexes are mononuclear and are diamagnetic. The complexes were given the formulae [Ni(HL¹)₂] (1), [NiL¹py] (2), [NiL¹α-pic] (3), [NiL¹γ-pic] · H₂O (4), [NiL²py] (5) and [NiL²γ-pic] (6). The structures of compounds 2 and 3 have been solved by single crystal X-ray crystallography and were found to be distorted square planar in geometry with coordination of azomethine nitrogen, thiolato sulfur, phenolato oxygen and pyridyl nitrogen atoms.

Full text of DOI:10.1016/j.poly.2007.08.048

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 35–43
www.elsevier.com/locate/poly
Spectral and structural studies of nickel(II) complexes
of salicylaldehyde 3-azacyclothiosemicarbazones
*
Leji Latheef, Maliyeckal R. Prathapachandra Kurup
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India
Received 12 June 2007; accepted 17 August 2007
Available online 24 October 2007
Abstract
Mononuclear nickel(II) complexes with two ONS donor thiosemicarbazone ligands {salicylaldehyde 3-hexamethyleneiminyl thio-
semicarbazone [H2L1] and salicylaldehyde 3-tetramethyleneiminyl thiosemicarbazone [H2L2]} have been prepared and physico-chem-
ically characterized. IR and electronic spectra of the complexes have been obtained. The thiosemicarbazones bind to the metal as
dianionic ONS donor ligands in all the complexes except in [Ni(HL1)2] (1). In compound 1, the ligand is coordinated as a mono-
anionic (HLꢀ) one. The magnetic susceptibility measurements indicate that all the complexes are mononuclear and are diamagnetic.
The complexes were given the formulae [Ni(HL1)2] (1), [NiL1py] (2), [NiL1a-pic] (3), [NiL1c-pic] Æ H2O (4), [NiL2py] (5) and [NiL2c-
pic] (6). The structures of compounds 2 and 3 have been solved by single crystal X-ray crystallography and were found to be
distorted square planar in geometry with coordination of azomethine nitrogen, thiolato sulfur, phenolato oxygen and pyridyl
nitrogen atoms.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Thiosemicarbazone; Nickel(II) complex; Salicylaldehyde; Crystal structure
1. Introduction
to the identification of a sulfur rich coordination environ-
ment in biological nickel centres, such as the active sites
of certain ureases, methyl-S-coenzyme-M-methyl reduc-
tase and hydrogenases [11].
The coordination chemistry of thiosemicarbazones
with transition metals [1–4], which began with Jensen’s
work [5], has been more intensively investigated as com-
pared to that of main group elements [6,7]. Thiosemicar-
bazones are an important group of multidentate ligands
with potential binding sites available for a wide variety
of metal ions [8–10]. These thiourea derivatives find sub-
stantial applications in different facets of contemporary
scientific research. Thiosemicarbazones and their metal
complexes are a broad class of biologically active com-
pounds [4]. Due to this biological activity, there is con-
siderable interest in metal complexes of heterocyclic
thiosemicarbazones [4]. Nickel(II) complexes containing
sulfur donors have received considerable attention due
Spectral and, in some complexes, structural investiga-
tions of a series of nickel(II) complexes of ONS donor
thiosemicarbazones [12–16] have been reported earlier.
The thiosemicarbazone ligand usually coordinates with
the metal through the imine nitrogen and sulfur atom.
The ligands feature more than two covalent sites, the
number of which depends on the aldehyde and on the
tautomeric equilibrium of the thiosemicarbazone,
although the most common way to coordinate is through
the thiolate form [17]. We have reported Cu(II) [18] and
Zn(II) [19] complexes of the present thiosemicarbazone
ligands recently. Here, we report six mononuclear nickel
(II) complexes of salicylaldehyde 3-hexamethyleneiminyl
and 3-tetramethyleneiminyl thiosemicarbazones. These
complexes were characterized by IR, electronic and
magnetic data.
*
Corresponding author. Tel.: +91 484 2575804; fax: +91 484 2577595.
E-mail address: mrp@cusat.ac.in (M.R. Prathapachandra Kurup).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.08.048

36
L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
Table 1
2. Experimental
2.1. Materials
Stoichiometries, colors and partial elemental analysis of complexes
Compound
Anal. Found (Calc.) (%)
C
H
N
[Ni(HL1)2] (1)
[NiL1py] (2)
55.9(55.30)
55.20(55.23)
56.21(56.23)
54.11(53.90)
53.12(53.02)
54.02(54.16)
6.36(5.93)
5.35(5.37)
5.63(5.66)
6.02(5.89)
4.57(4.71)
5.70(5.05)
14.04(13.74)
13.55(13.56)
13.11(13.12)
12.59(12.58)
14.42(14.55)
14.16(14.04)
All the solvents used for the synthesis of the thiosemicar-
bazone ligands and their complexes were purified by distilla-
tion. Pyridine, a/b/c-picoline, Ni(OAc)2 Æ 4H2O (Central
drug house) were used as received. 4-Methyl-4-phenyl-3-thio-
semicarbazide was prepared as reported previously [20].
[NiL1a-pic] (3)
[NiL1c-pic] Æ H2O (4)
[NiL2py] (5)
[NiL2c-pic] (6)
2.2. Synthesis of the ligands
tibility measurements were carried out at IIT, Roorkee,
India, at 298 K in the polycrystalline state on a PAR model
155 Vibrating sample magnetometer at 5 kOe field strength
using Hg[Co(SCN)4] as a calibrant. The FT-IR spectra
were recorded on a Thermo Nicolet AVATAR 370 DTGS
FTIR spectrometer using KBr pellets in the range 400–
4000 cmꢀ1 at SAIF, Cochin University of Science and
Technology, Kochi 22, India, and far IR spectra were
recorded in the range 50–500 cmꢀ1 on a Nicolet Magna
550 FT-IR spectrophotometer using polyethylene pellets
at SAIF, IIT, Bombay, India. Electronic spectra were
recorded on a UVD-3500 UV–VIS-Double beam Spectro-
photometer from solutions in DMF. 1H NMR spectra were
recorded on a Bruker AMX 300 in CDCl3 with TMS as an
internal standard.
H2L1 was prepared using a reported procedure [21].
H2L2 was prepared as described by adapting the reported
procedure [20,21]. A solution of 4-methyl-4-phenyl thio-
semicarbazide (1 g, 5.52 mmol) in acetonitrile (5 ml) was
treated with salicylaldehyde (0.60 ml, 5.52 mmol) and pyr-
rolidine (0.50 ml, 5.52 mmol) and refluxed for 20 min. The
solution was allowed to cool and the fine colorless needles
of the compound separated out. It was filtered off, washed
with cold acetonitrile and dried in vacuo over P4O10. The
compound was recrystallized from methanol. The struc-
tures of the two ligands are given in Scheme 1.
2.3. Synthesis of complexes
Complex 1 was prepared by refluxing an ethanolic solu-
tion of H2L1 with a methanolic solution of Ni(OAc)2 Æ
4H2O in a 2:1 molar ratio for 4 h. The brown colored com-
plex that formed was filtered, washed with ethanol and
then with ether and dried over P4O10 in vacuo. The com-
plexes 2–6 were prepared by refluxing an ethanolic solution
of the corresponding ligands and heterocyclic bases with a
methanolic solution of Ni(OAc)2 Æ 4H2O in a 1:1:1 molar
ratio. The complexes that separated were filtered, washed
with ethanol and finally with ether and dried over P4O10
in vacuo. Crystals suitable for X-ray analysis for com-
pounds 2 and 3 were obtained by slow evaporation of solu-
tions of the compounds from an ethanol methanol mixture
(1:1 v/v) over 10 days.
2.5. X-ray crystallography
Reddish brown crystals of compounds 2 and 3, having
approximate dimensions of 0.26 · 0.21 · 0.17 and
0.35 · 0.30 · 0.20 mm3, respectively, were selected. The
crystal data for compounds 2 and 3 were collected by Cry-
sAlis CCD, Oxford Diffraction Ltd. with graphite-mono-
˚
chromated Mo Ka (k = 0.71073 A) radiation. In
compound 2, the atom C10 in the ring containing N3 is dis-
ordered as this carbon splits over two sets of positions.
Restraints were applied to assist the geometry of the disor-
dered atom. The ratio of major to minor disorder compo-
nents for the C10 atom is 54:46. Compound 3 similarly has
some disorder in the 7-membered ring. Even though refine-
ment with restraints was applied to the 7-membered ring,
the problems of the weak data could not be improved fur-
ther. In spite of several attempts, we were unable to isolate
single crystals of good quality.
The colors and analytical data of the complexes, elemen-
tal analysis and magnetic susceptibilities are presented in
Table 1.
2.4. Physical measurements
In both of these compounds restraints were applied to
assist the geometry of the disordered atoms. The trial struc-
ture was solved using SHELXS-97 [22] and refinement was
Elemental analyses were carried out using a Vario EL
III CHN analyzer at SAIF, Kochi, India. Magnetic suscep-
Scheme 1. The thiosemicarbazones H2L1 and H2L2.

L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
37
Table 2
The room temperature magnetic moments of the Ni(II)
complexes suggest that they are all essentially diamagnetic.
For the d8 electronic configuration, diamagnetism gener-
ally implies that the metal ion has a square-planar config-
uration. The electronic spectra of the complexes in DMF
solution are not well resolved owing to very intense charge
transfer bands extending into the visible portion of the
spectra.
Crystal data for compounds 2 and 3
(2)
(3)
Empirical formula
Formula weight
Temperature (K)
C19H22N4NiOS
826.35
150(2)
0.71073
orthorhombic
Pbca
C20H24N4NiOS
427.20
150(2)
0.71073
monoclinic
P21/a
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
3.1. Crystal structure of the compound [NiL1py] and
[NiL1a-pic]
˚
a (A)
13.7965(19)
21.954(4)
25.410(5)
90
90
90
7696(2)
16
1.426
1.132
10.567(3)
10.933(3)
17.439(6)
90
100.42(3)
90
1981.5(10)
4
1.432
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
The molecular structures of [NiL1py] (2) and [NiL1a-pic]
(3) along with the atom labelling schemes are shown in
Figs. 1 and 2, and selected bond lengths and bond angles
are summarized in Table 3. The ligand H2L1 gets doubly
deprotonated to behave as an ONS tridentate ligand, coor-
dinating via its phenolato oxygen, azomethine nitrogen and
thiolate sulfur atom in the deprotonated form after thiol
formation, giving a distorted square planar geometry
around the nickel atom. The fourth coordination site is
occupied by the nitrogen atom from the heterocyclic bases.
The E configuration of the thiosemicarbazone chain about
the C7–N1 bond is retained to facilitate the coordination of
the thiolate sulfur to Ni in the two complexes. This reveals
that no rotation has occurred about the azomethine bond
for coordination. The coordination results in one 5-
membered and one 6-membered chelate ring in the two
complexes.
3
˚
Volume (A )
Z
Dcalc (g/cm3)
Absorption coefficient
(mmꢀ1
F(000)
Crystal size (mm3)
h Range for data
collection (ꢁ)
1.102
)
3456
0.26 · 0.21 · 0.17
2.95–25
896
0.35 · 0.30 · 0.20
3.02–2
Index ranges
ꢀ16 6 h 6 16,
ꢀ26 6 k 6 25,
ꢀ30 6 l 6 19
37350
ꢀ12 6 h 6 12,
ꢀ13 6 k 6 12,
ꢀ20 6 l 6 20
17298
Reflections collected
Independent reflections
6769 (0.0865)
3476 (0.1093)
(Rint
)
Refinement method
Data/restraints/
parameters
Goodness-of-fit on F2
Final R indices
[I > 2r(I)]
full-matrix on F2
6769/0/479
full-matrix on F2
3476/52/232
0.761
0.787
Compound 2 crystallizes with two monomers per asym-
metric unit in the orthorhombic crystal system. The two
molecules in the asymmetric unit are almost identical. So
the discussion can be limited to one of the molecules.
The Ni1 coordination is almost square planar with a max-
imum mean deviation of the coordinating atoms (phenolic
oxygen O1, azomethine nitrogen N1, thiolate sulfur S1 and
R1 = 0.0362,
wR2 = 0.0636
R1 = 0.1028,
wR2 = 0.0732
R1 = 0.0600,
wR2 = 0.1418
R1 = 0.1798,
wR2 = 0.1691
R indices (all data)
carried out by full-matrix least squares on F2 (SHELXL) [22].
The molecular and crystal structures are illustrated by
DIAMOND version 3.1d [23] and MERCURY [24]. All
non-hydrogen atoms were refined anisotropically and
hydrogen atoms were geometrically fixed at calculated
positions. The crystallographic data and structure refine-
ment parameters for the complexes are given in Table 2.
3. Results and discussion
The elemental analyses data are consistent with the gen-
eral empirical formula [M(HL)2] for 1, MLB for 2, 3, 4, 5
and 6 where M is the nickel metal atom, L is the doubly
deprotonated thiosemicarbazone ligand and B is the het-
erocyclic bases viz. py and a/b/c pic. All the nickel com-
plexes are reddish brown in color, which is common for
complexes involving thiosemicarbazone coordination,
resulting from the sulfur to metal charge transfer bands
[25]. The complexes are insoluble in most common sol-
vents. They are however soluble in DMF and DMSO.
Fig. 1. Molecular structure of compound 2.

38
L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
Table 3
1
˚
Comparison of selected bond lengths (A) and bond angles (ꢁ) for H2L , 2
and 3
H2L1a
2
3
Bond lengths
Ni1–O1
Ni1–N1
Ni1–N4
Ni1–S1
Ni2–O2
Ni2–N5
Ni2–N8
Ni2–S2
S1–C8
1.837(2)
1.848(3)
1.907(3)
2.134(11)
1.848(2)
1.859(3)
1.925(3)
2.132(11)
1.753(4)
1.752(4)
1.305(4)
1.406(3)
1.304(4)
1.350(4)
1.326(4)
1.327(4)
1.302(4)
1.400(4)
1.293(4)
1.357(4)
1.340(4)
1.343(4)
1.859(4)
1.847(5)
1.906(4)
2.151(2)
1.684(3)
1.759(6)
S2–C27
N1–C7
N1–N2
N2–C8
1.269(3)
1.356(3)
1.362(3)
1.320(7)
1.393(6)
1.302(7)
1.362(7)
1.388
N3–C8
N4–C19
N4–C15
N5–C26
N5–N6
N6–C27
N7–C27
N8–C34
N8–C38
1.392
Fig. 2. Molecular structure of compound 3.
˚
pyridyl nitrogen N4) from their own plane of 0.0103 A for
N4. The thiosemicarbazonato moiety, C7–N1–N2–C8–S1–
N3, also is almost planar and has a maximum mean plane
Bond angles
O1–Ni1–N1
O1–Ni1–N4
N1–Ni1–N4
O1–Ni1–S1
N1–Ni1–S1
N4–Ni1–S1
O2–Ni2–N5
O2–Ni2–N8
N5–Ni2–N8
O2–Ni2–S2
N5–Ni2–S2
N8–Ni2–S2
C8–S1–Ni1
C27–S2–Ni2
C7–N1–N2
N2–N1–Ni1
C26–N5–N6
C26–N5–Ni2
N6–N5–Ni2
C26–N5–N6
96.56(12)
85.35(11)
96.5(2)
87.56(19)
174.4(2)
176.71(14)
86.77(19)
89.17(15)
˚
deviation of 0.037(4) A and an angle of 2.86(6)ꢁ to the
plane of the four donor atoms, while the phenyl ring has
an angle of 6.29(10)ꢁ to the donor plane of compound 2
indicating that the thiosemicarbazone moieties are close
to coplanar with the nickel’s coordination plane. Ring
puckering analysis and least-square plane calculations
show that the Cg(5) ring comprising of atoms N3, C9,
C10, C11, C12, C13 and C14, and the Cg(10) ring compris-
177.87(12)
176.35(8)
87.08(10)
91.01(8)
95.75(12)
86.96(11)
177.26(14)
176.83(8)
87.15(11)
90.16(9)
96.57(13)
96.11(16)
113.3(3)
122.8(2)
113.6(3)
124.0(3)
122.4(2)
113.6(3)
ing of atoms N7, C28, C29, C30, C31, C32 and C33 adopt
˚
a
chair conformation [QT = 0.696(6) A (Cg(5)) and
95.8(3)
˚
QT = 0.650(6) A (Cg(10))].
120.0(2)
112.4(5)
123.8(4)
The bond angles O1–Ni1–N1 [96.16(12)ꢁ], O1–Ni1–N4
[85.35(11)ꢁ] and N1–Ni1–S1 [87.08(10)ꢁ] also reveal the dis-
tortion of the square plane comprising of Ni1, O1, S1, N1
and N4. The dihedral angle formed by the least square
planes Cg(1) comprising of atoms Ni1, S1, N1, N2 and
C8, and Cg(2) comprising of atoms Ni1, O1, N1, C1, C6
and C7 is 3.47(12)ꢁ. The C7–N1 bond distance of
a
Ref. [17].
˚
1.305(4) A lengthens compared to the corresponding C7–
˚
N1 bond distance (1.269(3) A) in the ligand. The delocal-
uncomplexed thiosemicarbazone. The bond angles O1–
Ni1–N4 [85.35(11)ꢁ] and S1–Ni1–N1 [87.08(10)ꢁ] are also
analogous, as expected on the basis of the similar square
planar nature for all the complexes. Coordination of O1
ization of electron density from the coordinating nitrogen
onto the central metal atom gives rise to an elongated
˚
N1–N2 bond length (1.406(3) A) compared to the uncom-
˚
˚
plexed thiosemicarbazone (1.356(3) A). Also, the C7–N1–
results in O1–C1 decreasing by approximately 0.04–0.05 A.
N2 bond angle decreases considerably compared to the
ligand H2L1. The loss of the proton bound to N2 in
H2L1 produces a negative charge which is delocalized on
the N1–N2–C8 system. This is indicated by the lengthening
Compound 3 crystallizes with only one monomer per
asymmetric unit in the monoclinic crystal system. The bond
angles N1–Ni1–O1 [96.5(2)ꢁ], O1–Ni1–N4 [87.56(19)ꢁ] and
N1–Ni1–S1 [86.77(19)ꢁ] reveal the distortion of the square
planar geometry. The dihedral angle formed by the least
square planes Cg(1) comprising of atoms Ni1, S1, N1,
N2 and C8, and Cg(2) comprising of atoms Ni1, O1, N1,
C1, C6 and C7 is 0.6(2)ꢁ. Similar to compound 2, the
˚
of the bond C8–S1, 1.753(4) A, compared to the value of
˚
1.684(3) A in the ligand. This thiolate formation is also
supported by the decrease in bond length of N2–C8,
˚
˚
1.304(4) A from the value of 1.362(3) A found in the

L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
39
C7–N1 bond length increases compared to the ligand. Thus
on complexation, the azomethine C–N bond lengthens
because of coordination of the azomethine nitrogen. Coor-
dination lengthens the thiosemicarbazone moiety’s C8–S1
bond length is shorter compared to other C–Npy bond
lengths in the complexes. This indicates that the bonding
is dominated by the thiosemicarbazone moiety.
In compound 2, adjacent molecules are arranged in an
offset manner within the unit cell when viewed along the
a axis, as seen in Fig. 3, as a result of diverse p–p stacking,
CH–p and hydrogen bonding interactions (Table 4).
The molecules of 3 are connected by various hydrogen
bonding interactions and are arranged parallel along the
a axis (Fig. 4). Though p–p interactions exist in the lattice
of compound 3, they are observed at distances greater than
˚
bond length to 1.759(6) A and shortens the N2–C8 bond
˚
length to 1.302(7) A. The maximum deviation from the
˚
least square planes around the Ni(II) ion is 0.0626(5) A
for N1. The thiosemicarbazonato moiety, C7–N1–N2–
C8–S1–N3, is almost planar with a maximum mean devia-
tion of 0.028(6) for N2 and is at an angle of 1.46(9)ꢁ to the
plane of the four donor atoms, while the dihedral angle
formed by the phenyl ring is 1.95(2)ꢁ to the donor plane.
The hexamethyleneiminyl ring adopts a chair conformation
˚
4.0 A, also there are no effective CH–p interactions (Table
5). adjacent molecules are arranged opposing directions
within the unit cell.
˚
[QT = 0.7269 A].
The dihedral angles formed by the least square planes
Cg(1) and Cg(2) for the compounds support the distorted
square planar conformation for these complexes, where
greater distortion is observed for compound 2. This small
deviation from coplanarity would certainly not hinder the
delocalization of electrons in the coordination sphere,
and the stability of the complex is sustained.
3.2. Infrared spectra
The characteristic IR bands (50–4000 cmꢀ1) for the four
free ligands differ from those of their complexes and pro-
Table 4
The Ni–Nazomethine bond lengths are shorter compared
to the Ni–Nbase bond lengths, indicating greater strength
of the former bonds compared to the latter in the two com-
plexes. The O1–Ni1–N1 bond angles are slightly greater
compared to the similar Ni(II) ONS donor complexes
[Ni(PTSC)(PPh3)] 95.32(9)ꢁ, [Ni(PTSC)(PPh3)].CHCl3,
95.4(1)ꢁ [12] and [Ni(C18H18N6O2S2)](ClO4)2 Æ 2CH4O,
83.64(12)ꢁ [26], and the N1–Ni1–S1 bond angles are com-
parable. The N1–Ni1–S1 bond angles are greater compared
to [Ni(Et-fbt)2], 85.35(11)ꢁ, [Ni(fbt)2], 84.90(10)ꢁ [27] and
[Ni(NQTS)2] Æ 2DMSO 81.47(16)ꢁ [28]. The C–Nazomethine
Interaction parameters of compound 2
˚
Cg(I)-Res 1ꢁ ꢁ ꢁCg (J)
Cg–Cg (A)
a (ꢁ)
3.28
6.74
17.19
17.19
3.28
b (ꢁ)
p–p interactions
Cg(1) [1] ! Cg(9)a
Cg(2) [1] ! Cg(9)a
Cg(3) [1] ! Cg(8)b
Cg(8) [1] ! Cg(3)b
Cg(9) [1] ! Cg(1)c
Cg(9) [1] ! Cg(2)c
3.747(2)
3.624(2)
3.856(2)
3.856(2)
3.747(2)
3.624(2)
25.77
24.05
27.64
12.31
22.53
24.30
6.74
Equivalent position code: a = 1/2 ꢀ x, 1/2 + y,z; b = 1/2 + x, y, 1/2 ꢀ z;
c = 1/2 ꢀ x, ꢀ1/2 + y,z
a = Dihedral angle between planes I and J (ꢁ)
b = \Cg(I) ! Cg(J) or Cg(I) ! Me vector and normal to plane I (ꢁ)
Cg(1) = Ni1, S1, C8, N2, N1; Cg(2) = Ni1, O1, C1, C6, C7, N1;
Cg(3) = N4, C15, C16, C17, C18, C19
Cg(8) = N8, C34, C35, C36, C37, C38; Cg(9) = C20, C21, C22, C23, C24,
C25
˚
˚
XH(I)ꢁ ꢁ ꢁÆCg(J)
Hꢁ ꢁ ꢁCg (A)
X–Hꢁ ꢁ ꢁCg (ꢁ)
Xꢁ ꢁ ꢁCg (A)
CH–p interactions
C(17)–H(17) [1] ! Cg(6)a
C(18)–H(18) [1] ! Cg(1)a
C(35)–H(35) [1] ! Cg(9)a
C(37)–H(37) [1] ! Cg(4)a
2.69
2.69
2.67
2.90
144
139
153
141
3.4857
3.4420
3.5223
3.6669
Equivalent position code: a = 1/2 + x,y, 1/2 ꢀ z
Cg(1) = Ni1, S1, C8, N2, N1; Cg(4) = C1, C2, C3,C4, C5, C6;
Cg(6) = Ni2, S2, C27, N6, N5; Cg(9) = C20, C21, C22, C23, C24, C25
˚
˚
˚
D–Hꢁ ꢁ ꢁA
H-bonding
Dꢁ ꢁ ꢁH (A) Hꢁ ꢁ ꢁA (A) Dꢁ ꢁ ꢁA (A) D–Hꢁ ꢁ ꢁA (ꢁ)
C15–H15ꢁ ꢁ ꢁO2a 0.930
2.967(2)
2.827(2)
2.895(3)
2.755(3)
2.962(3)
2.859(3)
2.863(2)
2.699(3)
3.539(5)
3.460(5)
3.589(5)
3.530(6)
3.445(5)
3.424(5)
3.723(5)
3.358(5)
121.23(25)
126.33(26)
132.41(26)
141.42(29)
113.90(27)
120.36(28)
154.33(25)
128.52(24)
C16–H16ꢁ ꢁ ꢁO2a 0.930
C3–H3ꢁ ꢁ ꢁN7b
0.930
C17–H17ꢁ ꢁ ꢁN5c 0.930
C18–H18ꢁ ꢁ ꢁN1c 0.930
C18–H18ꢁ ꢁ ꢁN2c 0.930
C36–H36ꢁ ꢁ ꢁO1c 0.930
C19–H19ꢁ ꢁ ꢁN2c 0.930
D, donor; A, acceptor. Equivalent position code: a = x, y, z; b = x, 1/
Fig. 3. Unit cell packing diagram of compound 2.
2 ꢀ y, 1/2 + z; c = 1/2 + x, ꢀ z + 1/2 + 1.

40
L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
orated with the decrease in stretching frequency by 60–
80 cmꢀ1 for m(CO) as well as the appearance of a band in
the 410–430 cmꢀ1 region due to a m(Ni–O) stretch in the
spectra of the complexes [29]. It indicates coordination
via the phenolic oxygen. In complex 1, the phenolic group
is not coordinated to the metal. The non-involvement of
the phenolic group of the same thiosemicarbazone ligand
in a zinc complex [19] has been proven by us. However,
there is X-ray crystal structure evidence for the existence
of the phenol rather than the phenolate form in a closely
related salicylaldehyde thiosemicarbazone [30], with the
ligand acting as an ONS donor.
On coordination of the azomethine nitrogen,
m(C@Nazo) shifts to lower wavenumbers by 10–20 cmꢀ1
,
as the band shifts from 1613 cmꢀ1 in the uncomplexed
thiosemicarbazone spectrum to ꢂ1596 cmꢀ1 in the spec-
tra of the complexes. Coordination of the azomethine
nitrogen is confirmed by the presence of a new band
in the range 450–490 cmꢀ1, assignable to m(Ni–N) for
these complexes [31]. The m(N–N) band of the thiosem-
icarbazone is found in the 1030–1075 cmꢀ1 range. The
increase in the frequency of this band in the spectra
of the complexes, due to the increase in the bond
strength, again confirms coordination via the azomethine
nitrogen [19].
Fig. 4. Unit cell packing diagram of compound 3.
Table 5
Interaction parameters of the compound 3
˚
˚
˚
D–Hꢁ ꢁ ꢁA
H-bonding
Dꢁ ꢁ ꢁH (A) Hꢁ ꢁ ꢁA (A) Dꢁ ꢁ ꢁA (A) D–Hꢁ ꢁ ꢁA (ꢁ)
The bands in the ranges 1320–1340 and 835–865 cmꢀ1
due to thioamide {which is partly m(C@S)} stretching and
bending vibrations, respectively, of the free ligands are
shifted to lower values, indicating coordination of the thio-
late sulfur to the Ni(II) ion. This negative shift of m(CS) in
the complexes has already been indicated by Campbell [1].
Coordination via the thiolato sulfur is confirmed with the
presence of a new band in the range 320–350 cmꢀ1, assign-
able to m(Ni–S) for these complexes [32]. In all the Ni(II)
complexes, another strong band is found in the range
1530–1540 cmꢀ1, which may be due to the newly formed
m(C@2N) bond formed as a result of enolization. The IR
spectra of the complexes, except 1, display bands character-
istic of coordinated heterocyclic bases [33].
C14–H14Bꢁ ꢁ ꢁS1a
0.970
2.517(2)
2.867(6)
2.858(5)
2.732(5)
3.021(8)
3.540(11)
3.771(7)
3.596(9)
112.25(47)
127.27(48)
167.25(27)
154.94(47)
C14–H14Aꢁ ꢁ ꢁN2b 0.970
C(17)–H17ꢁ ꢁ ꢁN2c 0.930
C18–H18ꢁ ꢁ ꢁO1d
0.930
D, donor, A, acceptor, Equivalent position code: a = x, y, z; b = x ꢀ 1/2,
ꢀy + 1/2, + z; c = x, y ꢀ 1, z; d = x ꢀ 1/2, ꢀ y ꢀ 1/2, + z.
vide significant indications regarding the bonding sites of
the ligands. IR spectral assignments of the ligands and
the complexes are listed in Table 6. A medium band in
the range 3070–3160 cmꢀ1 in the free ligands, due to the
m(2NH) vibration, disappears in the spectra of the com-
plexes, providing strong evidence for ligand coordination
around the Ni(II) ion in its deprotonated form.
3.3. Electronic spectra
The IR spectra of the ligands H2L1 and H2L2 exhibit
intermolecular hydrogen bonded m(OH) vibrations at
3315 and 3232 cmꢀ1, respectively, which disappear in the
spectra of the complexes except for 1. It is further corrob-
The electronic spectral assignments of the ligands and
their complexes are given in Table 7. The thiosemicarbaz-
ones show a ring p ! p* band at ꢂ36000 cmꢀ1 and an
Table 6
IR spectroscopic assignments for the ligands and their nickel complexes
Compound
m(C@ Nazo
)
m(C@2N) m(N–N) m/d[(C@S)/(C–S)] m(C–O) m(Ni–N) m(Ni–O) m(Ni–S) Bands due to heterocyclic base
H2L1
1612
1581
1597
1601
1037
1071
1070
1063
1068
1034
1061
1055
1324, 861
1267, 753
1288, 757
1278, 752
1270, 753
1338, 839
1290, 758
1275, 757
1271
1270
1207
1209
1208
1288
1210
1205
[Ni(HL1)2] (1)
[NiL1py] (2)
[NiL1a-pic] (3)
1530
1536
1531
1537
466
463
494
460
350
345
347
332
430
446
420
1447, 691
1448, 614
1449, 714
[NiL1c-pic]] Æ H2O (4) 1597
H2L2
1622
1600
1600
[NiL4py] (5)
[NiL4c-pic] (6)
1538
1538
490
476
422
425
334
325
1478, 693
1480, 607

L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
41
the 25000–28000 and 23000–24500 cmꢀ1 ranges. Their
positions are dependent on the steric requirements of
the N(4) substituents. That is, thiosemicarbazones with
bulkier N(4)-substituents have these bands at somewhat
higher energies. In most of the dianionic complexes,
LMCT maxima of the phenolate complexes show line
broadening with a tail running into the visible part of
the spectrum. This may result from a phenolate to Ni(II)
LMCT band being superimposed on the low energy side
of the S ! Ni(II) LMCT. Each complex has a broad d–d
combination band that appears as a shoulder on the
intraligand and charge-transfer bands. These nickel com-
plexes show a trend of increasing size of the N(4)-substi-
tuent and lower energy of the d–d band maximum,
presumably due to weakening of the coordinate bonding
with increased bulkiness of the ligands. The absence of
bands below 10000 cmꢀ1 confirms the square planar nat-
ure of the complexes [36,37].
Table 7
Electronic spectral assignments of the ligands and their Ni(II) complexes
Compound
p–p*
n–p*
LMCT
d–d
H2L1
36110 29500
35340 30210
34840 30400
[Ni(HL1)2] (1)
[NiL1py] (2)
[NiL1a-pic] (3)
26190
26450
17240
16980
35420 30920, 33000 23640, 26700 17060
[NiL1c-pic] Æ H2O (4) 35580 30960, 33330 23980, 26740 17070
H2L2
[NiL2py] (5)
[NiL2c-pic] (6)
36490 30210
36020 32680
36230 30210, 33780 23980, 26380 18520
23790, 26040 18480
OH
S
3
9
2
10
4
8
N
N
11
12
NH
7
5
1
14
6
13
1
3.4. H NMR spectra
Fig. 5. The structure of the ligand H2L1 along with atom labelling scheme.
1
The numbering scheme used in the H NMR spectrum
of the ligand H2L1 is given in Fig. 5 and the spectra of
the complexes 1 and 4 are given in Figs. 6 and 7. The
1H NMR spectrum of the ligand H2L1 has been discussed
n ! p* band (involving transitions within the thiosemicar-
bazone moiety, mainly C(7)@N(1) and C(8)–S groups [16])
at ꢂ30000 cmꢀ1. These bands are slightly shifted upon
complexation. The shift of the p ! p* bands to a longer
wavelength region in the complexes is the result of the C–
S bond being weakened and the conjugation system being
enhanced on complexation [34,35]. The n ! p* bands in
the complexes show a blue shift due to donation of the lone
pair of electrons to the metal and hence the coordination of
the azomethine with a reduction of intensity.
1
earlier [19]. H NMR spectral assignments of the ligand
and its Ni(II) complexes 1 and 4 are given in Table 8.
2
The signals for NH are absent from the spectra of these
complexes, as expected, and the phenolic OH is absent in
the spectrum of complex 4 because of its loss on complex
formation, which is an evidence for the coordination of
the ligand as a doubly deprotonated anion. The spectra
of complexes 1 and 4 show a sharp singlet which
integrates as one hydrogen at d 7.44 and 7.83 ppm,
In the case of complexes having ONS donor ligands,
two ligand to metal charge transfer bands are found in
Fig. 6. 1H NMR spectrum of compound 1.

42
L. Latheef, M.R. Prathapachandra Kurup / Polyhedron 27 (2008) 35–43
Table 8
1H NMR (CDCl3) assignments of the ligand H2L1 and its Ni(II) complexes (d in ppm)
Compound
OH
Aromatic protons
8CH@
aCH2
bCH2
cCH2
H6Ph
H4Ph
H3Ph
H5Ph
H2L1
11.3
12.51
7.09
7.45
7.25
6.97
7.31
7.18
6.80
7.13
7.14
6.67
6.82
6.80
7.99
7.45
7.83
3.63
3.67
3.61
1.66
1.79
1.74
1.42
1.56
1.55
[Ni(HL1)2] (1)
[NiL1c-pic] Æ H2O (4)
Fig. 7. 1H NMR spectrum of compound 4.
7
respectively, corresponding to CH@ present in the thio-
semicarbazone moiety. Complex 1 shows a peak at d
12.51 ppm due to the presence of the uncoordinated
hydroxyl group. The more downfield shift of the OH pro-
tons compared to the uncomplexed thiosemicarbazone
may be due to the overall loss of electron density and
hydrogen bonding interactions. The 1H NMR spectrum
of 1 shows a multiplet which integrates as two hydrogens
at d ca. 6.82 ppm and is assigned to the protons attached
Acknowledgements
L. Latheef thanks the Council of Scientific and Indus-
trial Research, New Delhi, India, for financial support in
the form of a fellowship. M.R.P. Kurup is thankful to
the CSIR, New Delhi, India, for financial support. The
authors also thank the National Single Crystal X-ray
diffraction Facility, IIT, Bombay, for X-ray data
collection.
3
5
4
6
to C and C of the phenyl ring. CH and CH protons
of the phenyl ring appear at d 7.13 and 7.31 ppm, respec-
tively. In complex 4, the protons of the aromatic ring
Appendix A. Supplementary material
5
3CH and CH appear at d 6.58 and 6.80 ppm. The spec-
CCDC 603882 and 604196 contain the supplementary
crystallographic data for 2 and 3. The data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data
Centre (CCDC), 12 Union Road, Cambridge, CB2, IEZ,
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.poly.2007.
08.048.
trum also shows a multiplet which integrates as four
hydrogens at d ca. 7.15 ppm, corresponding to 4CH,
0
00
6CH and the base protons H2 and H2 . The upfield shift
of these base protons may be due to the presence of the
electron releasing methyl group present in the pyridine
0
00
ring. The heterocyclic base protons (H1 and H1 Þ are
present as a doublet, which integrates as two hydrogens
at d 8.66 ppm. The downfield shift of these protons
may be the result of the withdrawal of electron density
from the thiosemicarbazone moiety and the heterocyclic
base, due to coordination with the metal atom [38].
Methyl protons appear as a singlet at d 2.38 ppm (see
Fig. 7).
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