Synthesis, structural characterization and electrochemical studies of nickel(II), copper(II) and cobalt(III) complexes of some ONS donor ligands derived from thiosemicarbazide and S-alkyl/aryl dithiocarbazates

Article abstract of DOI:10.1007/s11243-012-9634-z

Four tridentate ONS ligands, namely 2-hydroxyacetophenonethiosemicarbazone (H2L1), the 2-hydroxyacetophenone Schiff base of S-methyldithiocarbazate (H2L2), the 2-hydroxy-5-nitrobenzaldehyde Schiff base of S-methyldithiocarbazate (H₂L₃), and the 2-hydroxy-5- nitrobenzaldehyde Schiff base of S-benzyldithiocarbazate (H2L4), and their complexes of general formula [Ni(HL¹)], [ML] (M = NiII or CuII; L = L¹, L², L³ and L4), [Co(HL)(L); L = L1, L², L³ and L⁴] and [ML(B)] (M = NiII or CuII; L = L² and L ⁴; B = py, PPh³) have been prepared and characterized by physico-chemical techniques. Spectroscopic evidence indicates that the Schiff bases behave as ONS tridentate chelating agents. X-ray crystallographic structure determination of [NiL²(PPh₃)] and [CuL ₄(py)] indicates that these complexes have an approximately square-planar structure with the Schiff bases acting as dinegatively charged ONS tridentate ligands coordinating via the phenoxide oxygen, azomethine nitrogen and thiolate sulfur atoms. The electrochemical properties of the complexes have been studied by cyclic voltammetry. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11243-012-9634-z

Transition Met Chem (2012) 37:651–659
DOI 10.1007/s11243-012-9634-z
Synthesis, structural characterization and electrochemical studies
of nickel(II), copper(II) and cobalt(III) complexes of some ONS
donor ligands derived from thiosemicarbazide and S-alkyl/aryl
dithiocarbazates
•
Mohammad Akbar Ali Ai Ling Tan
•
•
Aminul Huq Mirza Jose H. Santos
•
Aimi Hanisah Bte Hj Abdullah
Received: 10 May 2012 / Accepted: 11 July 2012 / Published online: 27 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Four tridentate ONS ligands, namely 2-hydro-
xyacetophenonethiosemicarbazone (H2L1), the 2-hydroxy-
acetophenone Schiff base of S-methyldithiocarbazate
(H2L2), the 2-hydroxy-5-nitrobenzaldehyde Schiff base of
S-methyldithiocarbazate (H2L3), and the 2-hydroxy-5-
nitrobenzaldehyde Schiff base of S-benzyldithiocarbazate
(H2L4), and their complexes of general formula [Ni(HL1)2],
[ML] (M = NiII or CuII; L = L1, L2, L3 and L4),
[Co(HL)(L); L = L1, L2, L3 and L4] and [ML(B)]
(M = NiII or CuII; L = L2 and L4; B = py, PPh3) have
been prepared and characterized by physico-chemical
techniques. Spectroscopic evidence indicates that the
Schiff bases behave as ONS tridentate chelating agents.
X-ray crystallographic structure determination of
[NiL2(PPh3)] and [CuL4(py)] indicates that these com-
plexes have an approximately square-planar structure with
the Schiff bases acting as dinegatively charged ONS tri-
dentate ligands coordinating via the phenoxide oxygen,
azomethine nitrogen and thiolate sulfur atoms. The elec-
trochemical properties of the complexes have been studied
by cyclic voltammetry.
Introduction
Metal complexes of Schiff bases have been extensively
investigated because of their industrial, antifungal, antibac-
terial, anticancer and herbicidal applications [1]. It is well
known that coordination of metals to Schiff base ligands may
enhance their biological activities [2, 3]. Many new Schiff
bases and their metal complexes reported in the literature
have been studied electrochemically, using various elec-
trodes and solvents, with a view to understand the mecha-
nism of their biological activities [4–6]. Tridentate Schiff
bases with N, O and S donor atoms are of special interest
because of their ability to form stable transition metal com-
plexes with interesting physico-chemical properties. For
example, the dimeric nickel(II) complex of the ONS tri-
dentate thiosemicarbazone, 2-hydroxy-5-methyl acetophe-
none N,N-dimethylthiosemicarbazone, is known to catalyze
reaction 1 below, which is a key reaction of the metalloen-
zyme, CO dehydrogenase [7]. The nickel(II) complex of 2-
hydroxyacetophenonethiosemicarbazone, another ONS
chelating agent, also catalyzes the silane alcoholysis reac-
tion 2, which is a related electrophilic activation of small
molecules [8].
Reaction 1: CO þ H2O þ CO2 þ 2Hþ þ 2eþ;
Reaction 2: R3SiH þ R0SiOR þ R3SiOR0 þ H2
In view of the interesting physico-chemical properties of
metal complexes of ONS donor ligands and as part of our
study of metal complexes of sulfur–nitrogen chelating agents,
we report here the synthesis, spectroscopic, electrochemical
and structural characterization of some nickel(II), copper(II)
and cobalt(III) complexes of four tridentate ONS ligands
derived from thiosemicarbazide and S-alkyl/aryl dith-
ioicarbazates (Fig. 1), together with the X-ray crystal and
molecular structures of [NiL2(PPh3)] and [CuL4(py)].
M. A. Ali (&) ꢀ A. L. Tan ꢀ A. H. Mirza ꢀ
J. H. Santos ꢀ A. H. B. H. Abdullah
Chemistry Programme, Faculty of Science, Universiti Brunei
Darussalam, Jln. Tungku Link, BE 1410 Negara Brunei
Darussalam, Brunei
e-mail: palonkuri@yahoo.com
123

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Transition Met Chem (2012) 37:651–659
OH
R1
OH
R2
R2
N
N
R
NH
S
R
N
SH
R1
(a)
(b)
Fig. 1 The thione (a) and thiol (b) tautomeric forms of the Schiff
bases: H2L1 (1) (R = H, R1 = CH3, R2 = NH2, the 2-hydroxyace-
tophenonethiosemicarbazone); H2L2 (2) (R = H, R1 = CH3,
R2 = SCH3,the 2-hydroxyacetophenone Schiff base of S-
methyldithiocarbazate); H2L3 (3) (R = NO2, R1 = H, R2 = SCH3,
the 2-hydroxy-5-nitrobenzaldehyde Schiff base of S-methyldithiocar-
bazate); H2L4 (4) = the 2-hydroxy-5-nitrobenzaldehyde Schiff base
of S-benzyldithiocarbazate (R = NO2, R1 = H, R2 = SCH2C6H5)
Experimental
157.42 (C–OH), 153.21 (CN), 130.82, 120.96, 120.12,
117.13, 110.84 (ArC), 17.50 (CH3). ESI–MS: m/z = 210.0
([M ? H]?).
Materials and methods
S-Methyldithiocarbazate [9] and S-benzyldithiocarbazate
[10] were prepared according to published procedures. All
other chemicals were purchased from Sigma-Aldrich and
used without further purification. Electronic spectra were
recorded on a Shimadzu UV-300 spectrophotometer. IR
spectra were recorded as KBr discs on a Philips SP3100
spectrophotometer. 1H NMR spectra were run in CDCl3 or
DMF-d6 on a Bruker DPX300 FT NMR spectrometer using
TMS as internal standard. Microanalyses for C, H and N
were performed at the Micronalytical Laboratory of the
National University of Singapore. ESI–MS spectra in either
dichloromethane or methanol were recorded on a Finnigan
LCQ mass spectrometer with a heated capillary tempera-
ture of 200 °C, spray voltage 4 kV and capillary voltage
34 V. FAB mass spectra were measured using 3-nitro-
benzyl alcohol as a matrix by means of a Finnigan MAT95
mass spectrometer.
Preparation of H2L2 (2) and H2L3 (3)
H2L2 was prepared by the method of Akbar Ali et al. [11].
Yield (0.98 g, 47 %) m.p. 169–171 °C, lit. 171–172 °C
[11]; Anal. Calc. for C10H12N2OS2 : C, 50.0; H, 5.0; N,
1
11.7. Found C, 50.1; H, 4.7; N, 11.7 %. H NMR d 11.40
(1H, s, OH), 10.0 (1H, s, NH), 6.95 (m, ArH), 7.34 (m,
ArH), 2.41 (3H, s, CH3), 2.67 (3H, s,SCH3), 7.50 (d, ArH),
7.00 (d, ArH); 13C NMR: d 13.09 (CH3), 17.99 (SCH3),
199.31 (CS), 153.00 (C=N), 158.48 (C–OH), 132.26,
128.34, 118.34 (ArC); ESI–MS: m/z : 240.9 [M ? H]?.
H2L3 was prepared by the method of Akbar Ali et al.
[12]. Yield (2.50 g, 77 %); M.p. 181–183 °C, lit. 185 °C
[12]; Anal. Calc. for C9H9N3O3S2 : C, 39.8; H, 3.3; N,
1
15.5. Found C, 40.2; H, 3.2; N, 15.5 %. H NMR d 13.43
(1H, s, OH), 12.04 (1H, s, NH), 8.74 (s, CH=N), 8.68 (d,
ArH), 8.20 (dd, ArH), 7.20 (1H, d, ArH), 2.64 (3H, s,
SCH3); 13C NMR: d 17.39 (SCH3), 199.78 (CS), 142.66,
127.70, 126.63, 121.02, 117.83 (ArC), 117.51 (ArC); ESI–
MS: m/z = 270.1, [M ? H]?.
All the Schiff bases were washed with ethanol or
methanol after filtration and dried in a vacuum desiccator
over anhydrous silica gel.
Preparation of H2L1 (1)
Preparation of H2L4 (4)
Thiosemicarbazide (1.22 g) was dissolved in a 1:1 mixture
of boiling absolute ethanol (10 mL) and dicholoromethane
(10 mL). The resulting colorless solution turned yellow
when a solution of 2-hydroxyacetophenone (1.2 mL) in hot
methanol (24 mL) was added. After the addition of a few
drops of 10 % HCl, the reaction mixture was heated under
reflux for 3 h to obtain a yellow solution which was left to
cool to room temperature. The resulting white precipitate
was filtered off. Yield (1.78 g, 64 %) m.p. 173–175 °C;
Anal. Calc. for C9H11N3OS.2H2O : C, 51.7; H, 5.3; N,
To a boiling solution of 2-hydroxy-5-nitrobenzaldehyde
(0.33 g, 1 mmol) in absolute ethanol (25 mL), a hot solu-
tion of S-benzyldithiocarbazate (0.41 g, 1 mmol) in the
same solvent (25 mL) was added, giving a pale yellow
solution. After the addition of a few drops of glacial acetic
acid to the mixture, it was heated under reflux for 3 h. The
volume of the reaction mixture was then reduced to half of
its original by heating on a water bath, when yellow
crystals were obtained. Yield (0.14 g, 41 %); m.p.
196–198 °C; Anal. Calc. for C15H11N3O3S2 : C, 51.9; H,
3.8; N, 12.6. Found: C, 51.5; H, 3.5; N, 12.1 %. 1H NMR d
13.49 (s, OH), 12.08 (s, NH), 8.76 (s, ArH), 8.65 (d, ArH),
8.20 (dd, ArH), 8.02 (1H, s, CH=N), 7.50 (d, ArH),
7.27–7.39 (m, ArH), 7.20 (d, ArH), 4.70 (s, SCH2–). 13C
1
20.1. Found: C, 51.6; H, 5.0; N, 20.5 %. H NMR d 2.20
(3H, s, –CCH3), 6.80–7.00 (br, 2H, NH2), 8.50 (s, ArH),
8.30 (s, ArH),7.80 (s, ArH),7.20 (s, ArH), 8.70 (s,1H, NH),
10.65 (1H, s, OH); 13C NMR (DMSO-d6), d 181.22 (CS),
123

Transition Met Chem (2012) 37:651–659
653
resulting precipitates was filtered off, washed with cold
ethanol and dried in vacuo.
Table 1 Crystallographic data for the compounds
Compound
[NiL2(PPh3)].0.5CH2Cl2 [(Cu L4(py)]
[Ni(HL1)2] (5); Yield (0.1 g, 19 %); 1H NMR d 2.39 (s,
CH3), 4.68(s, 2H, NH2), 6.91–7.61 (m, ArH), 11.06 (s,
OH); Anal. Calc. for C18H18N6O2S2Ni; C, 45.7; H, 3.4; N,
16.8. Found: C, 45.6; H, 4.6; N, 16.5 %. FAB–MS: 475.1
(52 %) [M]?.
Empirical formula
Formula weight
Crystal system
Space group
C28.5H26ClN2NiOPS2
601.76
C20H16CuN4O3S2
488.03
Triclinic
Triclinic
P-1
P-1
[NiL2] (6); Yield (0.3717 g, 48 %); Anal. Calc. for
C10H10N2OS2Ni: C, 40.4; H, 3.3; N, 9.4. Found: C, 40.3; H, 4.0;
N, 9.5 %. ESI–MS: m/z = 534.8, [(NiL2)2 - Ni ? 3H]?.
˚
a (A)
8.8073(4)
8.5974(16)
9.9938(19)
11.962(2)
101.621(3)
99.527(3)
97.147(3)
980.6(3), 2
1.653
˚
b (A)
12.5239(6)
13.8526(7)
78.5440 (10)
71.4910 (10)
70.125(10)
1355.56(11), 2
˚
c (A)
1
[NiL3] (7); Yield (0.46 g, 86 %); H NMR d 2.37 (s,
a (°)
b (°)
c (°)
SCH3), 6.32 (CH=N), 8.03–8.65 (m, Ar–H); Anal. Calc. for
C9H7N3O3S2Ni: C, 33.0; H, 2.2; N, 12.8. Found: C, 32.1; H,
2.4; N, 12.5 %. FAB–MS: m/z: 597.2(44 %) [(NiL3)2–Ni]?.
[NiL4].H2O (8); Yield (0.59 g, 86 %); 1H NMR: d
8.57–9.15 (m, 3H, O2N–Ar–H), 7.06–7.58 (6H, Ar–H,
N=C–H), 4.35–4.78 (d, 2H, CH2); Anal. Calc for
C15H11N3O3S2NiꢀH2O: C, 43.4; H, 2.9; N, 10.1. Found: C,
42.7; H, 3.1; N, 10.0 %. FAB–MS: m/z = 402.9(100 %),
[NiL4]?; 806.0(7 %) [(NiL4)2]?.
CuL1ꢀH2O (9); Yield (0.055 g, 10 %); Anal. Calc. for
C9H9N3OSCuꢀH2O:C, 37.2; H, 3.8; N, 14.6. Found: C,
37.2; H, 3.4; N, 14.3 %; leff (BM) = 0.54; ESI–MS:
m/z = 544.7(25 %) [(Cu L1)2 ? H]?.
3
˚
Volume (A , Z)
Density (calculated) 1.474
(Mg/m3)
Absorption
coefficient
(mm-1
1.053
1.358
)
F(000)
622
498
Crystal size (mm3) 0.60 9 0.46 9 0.38
0.46 9 0.20 9 0.12
1.77–27.49
Theta range for data 1.56–27.500
collection (°)
Reflections
collected
17,7480
12,593
CuL2 (10); Yield (0.45 g, 92 %); Anal. Calc. for
C10H10N2OS2Cu: C, 39.8; H, 3.3; N, 9.3 .
Independent
reflections
6222
[R(int) = 0.0247]
4479
[R(int) = 0.0272]
Found: C, 40.0; H, 3.3; N, 9.2 %.; leff (BM) = 0.59.
CuL3 (11); Yield (0.05 g, 27 %); Anal. Calc. for
C9H7N3O3S2Cu: C, 32.5; H, 2.1; N, 12.6.; Found: C, 32.4;
H, 2.2; N, 12.3 %; leff (BM) = 0.56; FAB–MS:
m/z = 331.8 [CuL3]?, m/z = 663.8 [(CuL3)2]?.
CuL4 (12);Yield (0.08 g, 20 %); Anal. Calc. for C 15H11
N3 O3S2Cu: C, 44.1; H, 2.7; N, 10.3. Found: C, 44.4; H,
3.1; N, 10.0 %.; leff (BM) = 0.57; FAB–MS: m/z = 407.8
(CuL4) (100 %); m/z = 817.8, [(CuL4)2]? (18 %).
Maximum and
minimum
transmission
0.7456 and 0.6519
0.8540 and 0.5738
Data/restraints/
parameters
6222/15/339
1.042
4479/0/271
1.148
Goodness-of-fit
on F2
Final R indices
[I [ 2sigma(I)],
R1, wR2
0.0421, 0.1145
0.0416, 0.1018
R indices (all data); 0.0446, 0.1164
R1,wR2
0.0447, 0.1037
Preparation of Co(L)(HL)
Largest diff. peak
-3
1.456 and -1.233
0.640 and -0.309
˚
and hole (e A
The complexes [Co(Lx)(HLx)] (x = 1–4) were prepared by
the following general method. Co(NO3)2ꢀ6H2O (1 mmol)
was dissolved in a 1:1 mixture of methanol (6 mL) and 2,2-
dimethoxypropane (6 mL) and mixed with a solution of the
appropriate ligand (2 mmol) in hot acetonitrile (20 mL).
The mixture was heated on a water bath for ca. 5 min then
allowed to cool to room temperature. The resulting pre-
cipitate was filtered off, washed with methanol and dried in
a desiccator over anhydrous silica gel.
)
NMR: d 38.82 (SCH2), 117.51 (ArC), 198.01 (CS),
120.98–142.96 (m, ArC). ES–MS: m/z = 346.2
[M ? H]?.
Preparation of NiL and CuL
The complexes MLx (M = Ni(II), Cu(II); x = 1–4) were
prepared by mixing a boiling solution of the appropriate
Schiff base (1 mmol) in a 1:1 mixture of absolute ethanol
(25 mL) and dichloromethane (25 mL) with a hot solution
of the metal(II) acetate (1 mmol) in methanol (ca. 50 mL)
and heating the mixture on a water bath for ca. 5 min. The
1
[Co(HL1)(L1)] (13); Yield (0.18 g, 86 %); H NMR d
1.56 (s, CH3), 7.26 (m, ArH), 12.3 (s, NH). Anal. Calc. for
C18H20N6S2O2Co.5H2O: C, 37.9; H, 4.1; N, 14.8. Found:
C, 38.2; H, 3.5; N, 14.9 %. FAB–MS: m/z = 474.9 [M]?.
[Co(HL2)(L2)] (14); Yield (0.4902 g, 62 %); 1H NMR d
7.84 (1H, d, ArH), 7.16 (1H, tr, ArH), 6.97 (2H, m, ArH),
123

654
Transition Met Chem (2012) 37:651–659
Table 2 Selected IR and UV–Vis absorption bands
Compound
IR assignments (cm-1
)
UV–Vis bands (nm)
mO–H/m(N–H)
m(C=N)
m(N–N)
m(CSS)
Solvent
kmax (log e/mol-1dm3cm-1
)
1
3391br
1618s
1106m
1055m
1062m
1053m
1102m
1003m
1102s
1097s
1155s
1104s
1104m
1103m
1041s
1009s
1029m
1010s
1003s
1010s
906m
954m
948s
DMSO
DMF
331(4.21), 302 (4.48)
399(4.18), 294(3.30)
469(4.17), 389 (3.5)
471(4.26), 398 (3.69)
379, 268
2
3429br
1596s
3
3428br
1606s
DMF
4
3511br
1602s
943m
923m
945m
942m
949m
937m
947m
945m
930m
962m
945m
945m
951m
960m
950m
DMF
5
–
1595m
1602m
1603m
1601m
1598m
1607m
1608s
Nujol
Nujol
Nujol
DMF
6
–
370, 269
7
–
301, 384
8
–
407(4.10), ca.380 sh
9
–
DMF
580(3.53), 386(3.53), 354 (3.51)
610(3.20), 421(3.21), 413(4.21)
408, ca.354sh
10
11
12
13
14
15
16
17
18
–
DMSO
Nujol
DMSO
Nujol
DMF
–
–
1603m
1595m
1597m
1558m
1596s
632(3.14),466(4.25),401(3.46)
408, 350
3290br
3406br
3490br
3465br
–
416(4.17), 351(3.88)
DMF
415 (4.00), 289 (3.39)
414(4.10), 361(3.85)
DMF
1561m
1570s
DMSO
DMSO
363(4.265), 277 (4.37)
632(3.26), 394(4.24), 302(4.22)
–
Fig. 2 Possible phenolic
oxygen- and thiolate sulfur
bridging in the 1:1 ML
complexes
N
S
O
N
O
S
M
M
M
M
O
S
N
S
O
N
2.43 (3H, s, N=C–CH3), 2.67 (3H, s, SCH3). Anal. Calc. for
C20H21N4O2S4Co: C, 44.8; H, 3.9; N, 10.4. Found: C, 44.9;
H, 3.8; N, 10.4 %. ESI–MS m/z: 536.9 (20 %), [M ? H]?.
formed were filtered off, washed with cold ethanol and
dried in a desiccator over anhydrous silica gel.
[NiL2 (PPh3)] was obtained by dissolving [NiL2] (0.07 g;
0.25 mmol) in hot acetonitrile (100 mL) and adding tri-
phenylphosphine (0.13 g; 0.5 mmol) to the solution. The
complex crystallized out of the solution as diffraction quality
crystals which were filtered off, washed with cold ethanol
and dried in a desiccator over anhydrous silica gel.
1
[Co(HL3)(L3)] (15); Yield (0.055 g, 10 %); H NMR d
2.62 (s, SCH3), 6.56–6.59 (HC=N), 7.84–8.96 (m, Ar–H).
Anal. Calc. for C18H15N6O6S4Co: C, 36.1; H, 2.5; N, 14.0.
Found: C, 36.0; H, 2.5; N, 9.5. ESI–MS: m/z =
598.8(20 %), [M ? H]?.
[Co(HL4)(L)] (16); Yield (0.17 g, 14 %); 1H NMR d 8.1
(s, Ar–H), 8.4 (m, ArH), 8.6 (s, ArH), 7.9 (m, 5H, ArH), 7.2
(s, N=C–H), 5.16 (s, CH2S); Anal. Calc for
C30H24N6O6S4Co : C, 47.9; H, 3.2; N, 11.2; Found: C, 48.3;
H, 3.3; N, 11.3 %. ESI–MS: 750.9 (20 %), [M ? H]?.
[NiL2(PPh3)]0.0.5CH2Cl2 (17); Yield: 0.06 g; 40 %;
Anal. Found: C, 58.3; H, 4.4; N, 4.9. Calc. for
C28.5H26ClN2NiOPS2: C, 58.8; H, 4.5; N, 4.8 %.
Diamagnetic.
[CuL4 (py)] (18); Yield, 0.29 g; 50 %. Anal. Found: C,
49.2; H, 3.2; N, 11.8; S, 12.5; Calc. for C20H16CuN4O3S2:
C, 50.0; H, 3.3; N, 11.5; S, 13.1 %. l(BM) = 1.73.
Preparation of [ML(py)] and (NiL2(PPh3)]
[NiL2 (py)] and [CuL4 (py)] were obtained by dissolving
[MLx] (M = Ni or Cu) (0.05 g; 0.12 mmol) in hot pyridine
(10 mL) and letting the solution evaporate slowly at room
temperature over a period of 3 days. The crystals that
X-Ray crystallography
The diffraction data were obtained with a Bruker Smart
CCD Diffractometer using graphite monochromated Mo Ka
123

Transition Met Chem (2012) 37:651–659
655
were produced with ORTEP3 [16] and PLATON [17]. A
summary of the crystal data, structure solution and
refinement parameters is given in Table 1. Crystallographic
data in CIF format have been deposited at the Cambridge
Crystallographic Data Centre. CCDC numbers 871667 and
871666 contain the supplementary crystallographic data for
[NiL2(PPh3)] and [CuL4(py)], respectively. These data are
available free of charge from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: ?44-1223-336033; e-mail:depo-
sit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Results and discussion
Compounds containing the thioamide functional group
may exhibit thione–thiol tautomerism, and therefore, the
present Schiff bases (Fig. 1) can exist either as the thione
(Fig. 1a) or as the thiol (Fig. 1b) tautomeric forms, or as a
mixture of both. The absence of a m(SH) band at ca.
2,570 cm-1 in their IR spectra indicates that the thiol form
Fig. 3 ORTEP plot of [Ni L2(PPh3)]. Thermal ellipsoids are drawn at
30 % probability level
1
is not present in the solid state. Their H NMR spectra in
˚
Table 3 Selected bond lengths (A) and bond angles (°) of
DMSO-d6 also do not exhibit any signal due to the SH
proton at ca. 4.00 ppm, again suggesting that, even in a
polar solvent like DMSO, the thiol tautomer is not present.
However, in the presence of metal(II) acetate salts, they
convert to the thiol tautomeric form, deprotonate and
coordinate as doubly negatively charged tridentate ligands
yielding 1:1 metal–ligand copper(II) and nickel(II) com-
plexes and 1:2 metal–ligand cobalt(III) complexes. The IR
spectral data together with assignments are shown in
Table 2. The IR spectra of the free ligands exhibits a broad
band in the range of 3,400–3,200 cm-1 whose envelope
includes both the m(O–H) and m(N–H) bands. These bands
are not present in the IR spectra of the nickel(II) and
copper(II) complexes indicating that the ligands are coor-
dinated to these metals in their doubly deprotonated forms.
However, in the IR spectra of the cobalt(III) complexes a
broad band in the range 3,200–3,500 cm-1 is observed,
which may be attributed to the m(O–H) of the monode-
protonated ligand.
[NiL2(PPh3)] and [CuL4(py)]
Bond
[NiL2 (PPh3)]
[CuL4 (py)]
M(1)–O(1)
1.8247(17)
2.1451(7)
1.9019(19)
2.2177(2)
1.314(3)
1.413(3)
1.278(3)
1.734(3)
1.749(3)
1.9186(18)
2.2802(8)
1.963(2)
2.041(2)
1.296(3)
1.401(3)
1.294(3)
1.782(3)
1.749(3)
M(1)–S(1)
M(1)–N(1)
M(1)–N(4/3)/P1
N(1)–C(7)
N(1)–N(2)
C(8/9)–N(2)
C(8/9)–S(1)
C(8/9)–S(2)
Bond angles (°)
O(1)–M(1)–N(1)
N(1)–M(1)–S(1)
O(1)-Ni(1)-N(3/4)/P1
N(1)–M(1)–N(3/4)/P1
N(3/4)/P1–M(1)–S(1)
O(1)–M(1)–S(1)
94.39(8)
88.03(6)
89.28(6)
172.01(6)
88.62(7)
176.57(6)
91.62(8)
84.97(6)
88.11(8)
172.92(8)
95.10(7)
176.28(6)
The IR spectra of the complexes also indicate that
bonding of the ligands to the metal occurs via the azome-
thine nitrogen atom since the m(CN) and m(NN) bands of
the free ligands is shifted in the spectra of the complexes
(Table 2). The presence of a m(M–O) band at ca. 740 cm-1
in the spectra of the complexes supports coordination of the
ligands via the phenolic oxygen [7].
˚
radiation (0.71073 A) and operating in the x - 2h mode
within the range 2° \ 2h \ 55°. An empirical absorption
correction (w scans) and data reduction [13] were per-
formed within the WINGX [14] suite programs. The
structures were solved by direct methods with SHELXS86
and refined by full-matrix least-squares analysis with
SHELXL97 [15]. Non-hydrogen atoms were refined with
anisotropic thermal parameters, whereas H-atoms were
included at estimated positions. Drawings of all molecules
The reactions of cobalt(II) nitrate in methanol with these
Schiff bases do not give cobalt(II) complexes, but rather
cobalt(III) complexes of general formula, [Co(HL)(L)]
(HL = monodeprotonated form of the ligand and
L = doubly deprotonated form of the ligand). These
123

656
Transition Met Chem (2012) 37:651–659
Table 4 Comparison of selected bond distances in square-planar copper(II) and nickel(II) complexes
Complexa
M–S
M–Nazomethine
M–Npy/P
M–O
Ref.
[NiL2(py)]
[NiL2(PPh3)]
2.157
2.1451
2.1260
2.129
2.157
2.166
2.2802
2.239
2.251
2.2254
2.227
1.872
1.9019
1.878
1.855
1.872
1.844
1.963
1.9584
1.944
1.939
1.952
1.930
2.2177
2.1923
–
1.825
2.041
1.857
1.851
1.825
1.858
1.9186
1.9316
1.95
[11]
This work
[29]
[Ni(hapNMe tsc)(PPh3)]
[Ni(atmtssA)]2
[Ni(acacsme)(py)]
[Ni(sal4Ph)(NH3)]
[CuL4(py]
[8]
1.930
1.921
2.041
2.239
2.029
1.95
[30]
[31]
This work
[32]
[Cu(ttacsme)(py)]
[Cu(SMPD)(Im)]
[Cu(PNSB)(NO3)]2
[Cu(mpsme)Cl]
a
[33]
1.949
2.060
[33]
–
[34]
hapNMetsc = doubly deprotonated form of the 2-hydroxyacetophenone-4N-methylthiosemicarbazone; atmttsA = doubly deprotonated form
of the 2-hydroxy-5-methylacetophenone thiosemicarbazone; acacsme = dianionic form of the 2,4-pentanedione Schiff base of S-meth-
yldithiocarbazate; sal4Ph = doubly deprotonated form of the salicylaldehyde 4N-phenylthiosemicarbazone; ttacsme = doubly deprotonated form
of the thenoyltrifluoroacetone Schiff base of S-benzyldithiocarbazate; SMPD = deprotonated form of 2-pyridinecarboxaldehyde-N-oxide Schiff
base of S-methyldithiocarbazate; ImH = imidazole); PNSB = deprotonated form of the 2-pyridinecarboxaldehyde Schiff base of S-ben-
zyldithiocarbazate); mpsme = monodeprotonated form of the methylpyruvate Schiff base of S-methyldithiocarbazate
complexes in DMF display a d–d band as a shoulder at ca.
580 nm and intra-ligand bands in the regions 303–354 and
253–260 nm. A very strong band at ca. 420 nm in the
electronic spectra of the copper(II) complexes is attributed
to a S ? CuII LMCT transition. Similar LMCT bands have
also been observed in copper(II) complexes of the related
Schiff bases derived from dithiocarbazates [21, 22] and
thiosemicarbazones [23].
The diamagnetism of Ni(HL1)2, NiL2 and NiL4 is con-
sistent with a four-coordinate square-planar structure. In
contrast, NiL3 exhibits a room-temperature magnetic
moment of 2.54 B.M. indicating that it does not have a
four-coordinate geometry. It probably has a trimeric or a
polymeric structure involving oxygen and/or thiolate sulfur
bridges between the nickel(II) centres. Unfortunately, in
spite of repeated attempts, crystals of this complex suitable
Fig. 4 ORTEP diagram of the molecular structure of [CuL4(py)].
for X-ray diffraction analysis could not be obtained.
Thermal ellipsoids are drawn at 30 % probability level
However, its electrospray mass spectrum, although it does
not show any ion attributable to an intact trimer or tetra-
complexes are diamagnetic, indicating that they are not
cobalt(II) but low-spin cobalt(III). It seems that, while
reacting with the ligands, the cobalt(II) is readily oxidized.
Similar rapid oxidation of cobalt(II) in the presence of
dithiocarbazate ligands to yield cobalt(III) complexes has
been previously observed [18, 19].
mer, exhibits a peak at m/z = 948, with a relative abun-
dance of 100 % which may be assigned to the
[Ni2L33 ? Na ? 2H]? ion, suggesting the presence of a
tetramer.
The room-temperature magnetic moments of the CuL
complexes are less than that expected for a 3d9 metal ion in
a magnetically dilute environment. Since these complexes
contain dinegatively charged ONS tridentate ligands (as
unambiguously proven by X-ray crystallography in the
cases of [NiL2(PPh3)] and [CuL4(py)] (see below) and the
metal-to-ligand ratio in these complexes is 1:1, they must
dimerize so that each metal(II) centre adopts at least a four-
The electronic spectra of the nickel(II) complexes
(Table 2) exhibit a d–d band at around 696 nm. A band at
ca. 400 nm is attributed to the S ? Ni ligand-to-metal
charge transfer (LMCT) transition. Similar LMCT bands
have also been observed in the electronic spectra of other
square-planar nickel(II) complexes of related sulfur–nitro-
gen ligands [20]. The electronic spectra of the copper(II)
coordinate configuration.
A phenolic oxygen-bridged
123

Transition Met Chem (2012) 37:651–659
657
than that required for a 3d9 configuration in a magnetically
dilute environment. The m(NH) band of the free ligand at
3,195 cm-1 is absent in the IR spectra of the complexes
suggesting deprotonation of this ligand followed by coor-
dination to the copper(II). The m(C=N) band at 1,593 cm-1
is very little affected on coordination.
Table 5 Cyclic voltammetric data# for oxidation of the metal com-
plexes in DMSO solution using 3-mm glassy carbon electrode and
scan rate of 100 mV s-1
Complex Epa
Epc
DEp
Ipa
Ipc
Ipc/ Remarks
(mV) (mV) (mV) (lA) (lA) Ipa
6
7
8
956
747
174
A
–
17.1
4.6
A
A
–
–
Irreversible
Irreversible
A
–
Crystal structures of [NiL2(PPh3)] and [CuL4(py)]
150
24
0.8 0.5
0.62 Quasi-
reversible
0.90 Quasi-
reversible
Irreversible
0.52 Quasi-
reversible
Crystals of [NiL2(py)] and [CuL4(py)], suitable for X-ray
analysis, were obtained by dissolving the ML complexes in
a minimum amount of anhydrous pyridine at room tem-
perature and letting the solution evaporate slowly over a
period of 48 h. Crystals of [NiL2(PPh3)] were obtained
during the preparation of the complex.
9
499
200
200
1.0 0.9
10
11
852
496
A
–
8.7
A
–
263
233
2.5 1.3
13
16
813
251
A
A
–
–
27.1
2.0
A
A
–
–
Irreversible
Irreversible
The X-ray crystal and molecular structure of
[NiL2(PPh3)] is shown in Fig. 3, and bond lengths and
angles are compiled in Table 3. The nickel(II) centre in this
complex is four-coordinate, with the ligand coordinated in
a dinegatively charged tridentate mode via the thiolate
sulfur, the azomethane nitrogen atom and the phenolate
oxygen atom. The fourth coordination position is occupied
by the phosphorus atom of the triphenylphosphine ligand.
The Ni–S, Ni–Nazomethine and Ni–O bond distances
(Table 3) of the complex are comparable and agree well
with those observed in other related four-coordinate,
approximately square-planar nickel(II) complexes of ONS
A indicates that no cathodic peak was observed in the reverse scan
Table 6 Cyclic voltammetric data# for reduction of the metal com-
plexes in DMSO solution using 3-mm glassy carbon electrode and
scan rate of 100 mV s-1
Complex Epc
Epa
DEp
Ipc
Ipa
Ipa/ Remarks
(mV) (mV) (mV) (lA) (lA) Ipc
5
6
-546
B
–
6.9
B
–
Irreversible
0.62 Quasi-
reversible
Irreversible
0.27 Quasi-
reversible
0.35 Quasi-
reversible
-608 -686 79
10.4 6.4
˚
ligands [8, 29–31]. The Ni–P bond length [2.2177(2)A] in
7
8
-689
B
–
11.6
B
–
[NiL2(PPh3)] also agrees well with that of the related
-601 -706 105
14.3 3.9
[Ni(hapNMetsc)PPh3] (hapNMetsc = doubly deprotonated
4
form of the 2-hydroxyacetophenone N-methylthiosemic-
¨
arbazone), whose structure has been reported by Ulku¨seven
12
-631 -740 109
12.4 4.3
13
15
-409
-748
B
B
–
–
5.7
26
B
B
–
–
Irreversible
Irreversible
et al. [29]. For the purpose of comparison, bond lengths of
some four-coordinate, approximately square-planar nick-
el(II) complexes of tridentate ONS ligands, for which
X-ray structures are known, are compiled in Table 4. The
data show that the Ni-donor atom distances in all these
complexes are similar.
The crystal structure of [CuL4(py)] is depicted in Fig. 4,
and selected bond angles and lengths are given in Table 3.
The structure, like the [NiL2(py)] complex [11], is also a
four-coordinate monomeric complex in which the Schiff
base is coordinated to the copper(II) as a dinegatively
charged tridentate ligand via the phenoxyl oxygen, the
azomethine nitrogen and the thiolate sulfur atoms, similar
to that observed in the nickel(II) complex described above.
The fourth coordination position is occupied by the pyri-
dine nitrogen atom. The Cu–S, Cu–Nazomethine and Cu–Npy
B indicates that no anodic peak was observed in the reverse scan
dimeric structure (Fig. 2) is very likely since iminothiolate
sulfur atoms tend not to form bridges with another metal if
a phenolic oxygen atom is also available for bridging [8,
24–26].
Bridging of the two nickel(II) and copper(II) centres in
the proposed dimers by the phenolate oxygen is further
supported by the presence of a medium intensity IR band at
720 cm-1 in the spectrum of the ML complexes, attribut-
able to the M–O–M bridging vibration [27, 28]. This band
is absent in the IR spectra of the free ligands as well as
those of [ML(B)] (M = Ni(II) or Cu(II); L = L2 or L4;
B = py or PPh3), for which X-ray crystallographic struc-
ture determination has unequivocally established a mono-
meric, approximately square-planar structure.
˚
bond lengths, 2.2802(8), 1.963(2) and 2.041(2)A, respec-
tively, compare well with those reported for other four-
coordinate, approximately square-planar copper(II) com-
plexes whose structures have been determined by X-ray
diffraction (see Table 4). Based on the data in Table 4, the
The room-temperature magnetic moments of the com-
plexes [CuL(py)] are in the range 0.54–0.59 B.M., lower
123

658
Transition Met Chem (2012) 37:651–659
present copper complex has the strongest Cu–O bond and
weakest Cu–S bond. Similar to that observed in the nick-
el(II) complexes described above, the Cu–Npyridine bond
˚
distance (2.041(2)A] in this complex is also longer than the
˚
Cu–Nazomethine bond [(1.963(2)A].
related 2-hydroxyacetophenonethiosemicarbazone, how-
ever, forms a bis-ligand nickel(II) complex but a mono-
ligand copper(II) complex. In methanolic solution, the
cobalt(II) centre is rapidly oxidized in the presence of these
Schiff bases to give spin-paired bis-ligand six-coordinate
cobalt(III) complexes. All the complexes studied here
undergo one-electron oxidation–reduction reactions which
are either irreversible or quasi-reversible.
The stereochemistry around the copper atom may be
considered as distorted square-planar, the distortion being
more pronounced in this compound than in the corre-
sponding nickel(II) complex. Thus, the angles N(1)–M(1)–
S(1) [84.97(6)°] and N(4)–M(1)–S(1) [95.10(7)°] deviate
by ca. 3° and 5°, respectively, from these required for an
ideal square-planar geometry. The five- and six-membered
chelate rings (S1–Cu1–N1–N2–C8) and (Cu1–O1–C1–C6–
C7–N1), respectively, are approximately planar. The angle
between the pyridine ring and the plane containing the
chelate rings is 172.92(8)°.
Acknowledgments We gratefully acknowledge the support pro-
vided for this work by the University of Brunei Darussalam, Bandar
Seri Begawan, Negara Brunei Darussalam.
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