Synthesis, structure and electrochemistry behavior of a cobalt(III) compound with azide and methyl 2-pyridyl ketone semicarbazone ligands

Article abstract of DOI:10.1016/j.molstruc.2013.03.049

An unprecedented cobalt(III) compound with methyl 2-pyridyl ketone semicarbazone (HL) and the auxiliary azide ligand, [Co(L)₂] [Co(L)(N₃)₃] (1) was synthesized and further characterized crystallographically and spectroscopically. Compound 1 crystallizes in the triclinic system and space group of P-1 and its structure consists of two mononuclear crystallographic units with metal chromophore comprising two cobalt(III) centers revealing distorted octahedral geometries and formed by distinct ligands in the inner coordination spheres. Interestingly, compound 1 represents the first complex formed by two distinct mononuclear units involving this ligand. As consequence of various donor and acceptor groups in both crystallographic units, there are several strong N-H?N and N-H?O hydrogen bonding interactions interconnecting adjacent moieties, ultimately leading to a three-dimensional supramolecular network. Furthermore, the electrochemical behavior of the HL and compound 1 were investigated.

Full text of DOI:10.1016/j.molstruc.2013.03.049

Accepted Manuscript
Synthesis, structure and electrochemistry behavior of a Cobalt (III) compound
with azide and methyl 2-pyridyl ketone semicarbazone ligands
Behrouz Shaabani, Ali Akbar Khandar, Farzaneh Mahmoudi, Salete S. Balula,
Luís Cunha-Silva
PII:
S0022-2860(13)00272-X
DOI:
http://dx.doi.org/10.1016/j.molstruc.2013.03.049
MOLSTR 19642
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
5 November 2012
23 March 2013
25 March 2013
Please cite this article as: B. Shaabani, A.A. Khandar, F. Mahmoudi, S.S. Balula, L. Cunha-Silva, Synthesis, structure
and electrochemistry behavior of a Cobalt (III) compound with azide and methyl 2-pyridyl ketone semicarbazone
ligands, Journal of Molecular Structure (2013), doi: http://dx.doi.org/10.1016/j.molstruc.2013.03.049
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1
Synthesis, structure and electrochemistry behavior of a Cobalt (III)
compound with azide and methyl 2-pyridyl ketone semicarbazone ligands
2
3
4
Behrouz Shaabani1*, Ali Akbar Khandar1, Farzaneh Mahmoudi1,
Salete S. Balula2 and Luís Cunha-Silva2*
5
6
7
1 Department of Inorganic Chemistry, Faculty of Science, Tabriz University 51666-14766, Iran
2 REQUIMTE & Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto,
4169-007 Porto, Portugal
8
9
* Corresponding authors:
Behrouz Shaabani, b.shaabani@yahoo.com, Tel: +98 411 3393144; Fax: +98 411 3340191
Luís Cunha-Silva, l.cunha.silva@fc.up.pt, Tel: +351 220402576; Fax: +351 220402659
10
11 Abstract
12 An unprecedented cobalt(III) compound with methyl 2-pyridyl ketone semicarbazone (HL) and the
13 auxiliary azide ligand, [Co(L)2] [Co(L)(N3)3] (1) was synthesized and further characterized
14 crystallographically and spectroscopically. Compound 1 crystallizes in the triclinic system and space
15 group of P-1 and its structure consists of two mononuclear crystallographic units with metal
16 chromophore comprising two cobalt(III) centers revealing distorted octahedral geometries and
17 formed by distinct ligands in the inner coordination spheres. Interestingly, compound 1 represents
18 the first complex formed by two distinct mononuclear units involving this ligand. As consequence of
19 various donor and acceptor groups in both crystallographic units, there are several strong N−H···N
20 and N−H···O hydrogen bonding interactions interconnecting adjacent moieties, ultimately leading to
21 a three-dimensional supramolecular network. Furthermore, the electrochemical behavior of the HL
22 and compound 1 were investigated.
23 Keywords: Semicarbazone ligand, Azide compound, Cobalt(III) center, Crystal structure, Cyclic voltammogram.
1

24 1. Introduction
25 For decades, Schiff bases have been played an extraordinary role as chelating ligands in main group
26 and transition metal coordination chemistry, due to their uncomplicated synthesis, wide availability
27 and structural versatility associated with their potential in diverse applications [1–10]. Numerous
28 Schiff bases and coordination complexes with this extensive family of ligands have revealed
29 remarkable antimicrobial activity, being potent antibacterial, antifungal, anticancer and antiviral
30 compounds[11–21]. In particular the semi and thiosemicarbazones are versatile ligands that have
31 attracted the scientific interest of researchers, not only due to their interesting coordination
32 proprieties since they can coordinate in both neutral and anionic forms [22–24], but also as
33 consequence of their potential in pharmacological applications [25–36]. Frequently, their metal
34 complexes revealed enhanced antimicrobial activities than the free Schiff base ligands.
35 Consequently, the design and preparation of novel coordination complexes with transition metals and
36 semicarbazones ligands to improve and tune the proprieties trough the discovery of new structures is
37 still a notable scientific challenge. In an earlier paper [36] we have reported the synthesis of ligand
38 namely pyridine-2-carbaldehyde semicarbazone, here we report the synthesis and characterization of
39 a novel ligand methyl 2-pyridyl ketone semicarbazone (HL) (scheme 1) and the cobalt(III)
40 compound of it. The methyl 2-pyridyl ketone semicarbazone ligand has been utilized in the
41 preparation of metal complexes, A survey in the Cambridge Structural Database showed
42 demonstrated five crystal structures involving this ligand, all of them featuring mononuclear
43 complexes. On the other hand, a basic possibility to improve the structural features of the
44 semicarbazones based coordination complexes is the use of auxiliary bridging ligands, such as
−
45 pseudohalides. These can form bridging complexes with transition metals where the groups N3 ,
46 NCS−, NCO− coordinate in the end-to-end and end-on bridging modes [37–46]. Recently mono and
2

47 dinuclear complexes of Schiff base ligands and pseudohalides like azide with Cobalt(III), have been
48 well established [44–50].
49
Following our interest in the design and development of unprecedented transition metal
50 coordination complexes with semicarbazone-type ligands and a second bridging ligand, we
51 synthesized the Co(III) compound [Co(L)2] [Co(L)(N3)3] (1), with the HL and azide ligand. This
52 compound was characterized by IR and UV spectroscopies and the solid-state structure was unveiled
53 by single-crystal X-ray diffraction revealing two distinct mononuclear Co(III) unites, [Co(L)2] and
54 [Co(L)(N3)3]. The supramolecular interations between the different unities were investigated, and the
55 electrochemical behavior of the free semicarbazone ligand and the compound 1 was studied.
56
57
58
Scheme 1. The Schiff base ligand methyl 2-pyridyl ketone semicarbazone (HL)
59 2. Experimental
60 2.1. Materials and general methods
61 All chemicals and solvents were of reagent grade and used as received from Merck or Fluka.
62 Microanalyses were carried out using a Heraeus CHN–O– Rapid analyzer. Melting points were
63 measured on an Electrothermal 9100 apparatus and are uncorrected. IR spectra were recorded on a
64 FT-IR Spectrometer Bruker Tensor 27 in the region 4000–400 cm-1 using KBr pellets. UV spectra
65 were recorded on a Shimadzu, UV-1650 PC spectrophotometer. The solid state electronic absorption
66 spectra were recorded by diffuse reflectance spectra (DRS) on a Scinco 4100 in the range of 200-
67 1100 nm using BaSO4 as reference. Cyclic voltammetric measurements were performed using an
3

68 AMEL Instruments Model 2053 as potentiostate connected with a function generator (AMEL Model
69 568). In all electrochemical studies, a three-electrode system was used consisting of a glassy carbon
70 as the working electrode, a platinum wire auxiliary electrode, and an Ag/AgCl as the reference
71 electrode. All of the electrochemical experiments were carried out under nitrogen atmosphere at
72 room temperature using solution of compounds with concentration about 10-3 M in DMSO solvent
73 containing 0.1 M lithium perchlorate as the supporting electrolyte. Electrochemistry of HL and its
74 metal compound and azide, were studied by cyclic voltammetry in scan rate of 0.01 Vs-1. Ferrocene
75 (Fc) was used as the internal standard and all redox potentials were referenced to the Fc+/0 couple.
76
77 2.2. Synthesis of methyl 2-pyridyl ketone semicarbazone (HL)
78 The HL was prepared by condensation of methyl 2-pyridyl ketone with semicarbazide hydrochloride
79 [22, 23]. 0.8 g (7 mmol) of semicarbazide hydrochloride was added to a solution of 2-methyl Pyridyl
80 ketone (0.85 ml, 7 mmol) in 20 ml of methanol in a round bottom flask under continuous stirring.
81 The mixture was refluxed for 5 hours to produce a white suspension. The final reaction mixture was
82 filtered off and the filtrate was kept in room temperature for 10 hours. The resulting white
83 precipitation (m.p. 206 ºC) was filtered off, washed with methanol and dried in air. Yield: 1.30 g
84 (5mmol, 80%), Anal. Calc. for C8H10N4O: C, 44.72; H, 4.66; N, 26.08. Found: C, 44.65; H, 4.46;
85 N, 26.08. Characteristic IR absorbtions (cm-1): 3431, 3279 m, ν(NH2); 3192 m, ν(N-H); 1578 s,
86 ν(C=N); 1105 s, ν(N-N); 1685 s, ν(C=O).
87
88 2.3. Synthesis of [Co(L)2][Co(L)(N3)3] (1)
89 Crystals of 1 are obtained by slow diffusion in an H-shaped Tube already described in the literature
90 [37, 51, 52]. By using 0.2 mmol of HL, 0.8 mmol of Co(OAC)2.4H2O and 1.5 mmol of NaN3 in
4

91 ethanol, dark brown crystals of 1 were formed (decomposition at 300 ºC) after seven days, which
92 were isolated, filtered off, washed with acetone and diethyl ether and dried in air. Yield: 0.12 g, 30%.
93 Anal. Calc. for C24H27Co2N21O3: C, 37.13; H, 3.48 ; N, 43.32. Found: C, 37.03; H, 3.28 ; N, 43.20.
94 Characteristic IR absorbtions (cm-1): 3441, 3301m, ν(NH2); 1604 s, ν(C=N); 1182 m, ν(N-N); 591 m,
-
95 ν(Co-N azomethine), 2023 s, ν(N3 ).
96
97 2.4. X-ray crystallography
98 Crystalline material suitable for single-crystal X-ray diffraction analysis of the complex
99 [Co(L)2][Co(L)(N3)3] (HL is methyl 2-pyridyl ketone semicarbazone) was manually harvested and
100 mounted on a CryoLoop using appropriated oil [53]. Data were collected at 150(2) K on a Bruker X8
101 Kappa APEX II Charge-Coupled Device (CCD) area-detector diffractometer (Mo Kα graphite-
102 monochromated radiation was used, λ = 0.71073 Å) controlled by the APEX2 software package [54],
103 and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely by the
104 Cryopad software [55]. Images were processed with SAINT+ [56], and data were corrected for
105 absorption by the multi-scan method implemented in SADABS [57] . The structure was solved by
106 the direct methods of SHELXS-97 [58,59] .allowing the immediate location of the Co atoms, with
107 the remaining non-hydrogen atoms being identified in the difference Fourier maps calculated from
108 successive full-matrix least-squares refinement cycles on F2 using SHELXL-97 [59, 60]. All non-
109 hydrogen atoms were successfully refined with anisotropic displacement parameters, and hydrogen
110 atoms bounded to the carbon atoms wer placed in geometrica geometrical position [59–62].
111 Hydrogen atoms of the NH2 groups were markedly visible in difference Fourier maps and included
112 in the structure with the N–H and H···H distances restrained to 0.90(2) and 1.50(2) Å, respectively,
113 and with Uiso fixed at 1.5 × Ueq of the parent nitrogen atom. The highest peak (0.59 eÅ-3) was found
5

114 at 1.09 Å from Co2 and the deepest hole (-0.69 eÅ-3) at 0.71 Å from Co2. Refinement details are
115 summarized in the Table 1.
116
117
Table 1 - Crystal data and structure refinement parameters for [Co(L)2][Co(L)(N3)3] (1).
Chemical Formula
Formula weight
Crystal description
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
C24H27Co2N21O3
775.53
Plate, brown
0.17 × 0.09 × 0.03
150
Triclinic
P -1
8.6140(4)
13.2566(7)
13.6042(7)
86.445(4)
84.286(3)
82.026(3)
1529.00(13)
2
b (Å)
c (Å)
α (°)
β(°)
γ(°)
Volume (Å3)
Z
ρcalculated (g cm-3)
µ (mm-1)
1.684
1.154
θ range (°)
3.66 - 25.41
0.0432( 4292)
0.1115(5551)
0.574 and -0.692
Final R indices [I>2σ(I)]
Final R indices (all data)
Largest diff. peak and hole (eÅ–3)
118
119 3. Results and discussion
120 3.1. IR spectra
121 The IR spectra of HL and compound 1, and the shifts of IR bands of ligand due to its coordination to
122 Co(III) cation are given in Table 2. For more information the FT-IR spectra curves of HL and 1 are
6

123 presented in Figures 1a and 1b. A medium band in 3192 cm-1 in the free ligand HL, due to a ν(2N–H)
124 vibration, disappears in the spectra of 1, providing strong evidence for ligand coordination to the
125 Co(III) ion in the deprotonated enolate form [20,27,32]. The strong bands at 3431 and 3279 cm-1 in
126 the spectra of HL, assigned to ν(4N–H), shifts to higher energies in 1. Bands in 1578 and 1485 cm-1
127 suffer significant shifts in the spectra of 1, which can be attributed to ν(C=N) and ν(C=C) vibration
128 modes, and their mixing patterns are different from that in the spectra of HL. The positive shift of
129 bands corresponding to ν(C=N) in the range 1578 cm-1 in the HL to 1604 cm-1 in 1, is consistent with
130 the coordination of the azomethine nitrogen to the Co(III) ions [21–24,35]. Medium bands at 591
131 cm-1 corresponding to ν(Co–N) further support azomethine nitrogen coordination [35,38,39]. The
132 enolization of HL is supported by the increase in ν (N–N) by 77 cm-1. The absence of the ν (C=O)
133 bands, present in HL at1685 cm-1, indicates the enolization of the amide functionality upon
134 coordination to the Co(III) center. Instead strong bands at 1645 cm-1 are observed which are
135 characteristic of the coordination of HL in enolate forms [24-27]. Presence of a band at 1552 for 1,
136 is assigned to the newly formed ν (N=C) after enolization [24]. The low energy pyridine ring in-
137 plane and out-of-plane vibrations observed in the spectra of HL at 625 and 432 cm-1 are shifted to
138 higher frequencies at 638 and 445 cm-1 but for 1, which is a good indication of the coordination of
139 nitrogen of pyridine heterocycle to the metal center [24,27]. In 1, the well resolved peak at 2023
140 cm−1 were obtained, which are assigned to coordinated azido group [40–50].
141
Table 2. IR spectral assignment (cm-1) for HL and 1
2
4
Compound
ν
ν
ν
ν
ν(Co-N azomethine)
ν
ν
(N-N)
(C=O)
( N-H)
( N-H)
(N3)
(C=N)
HL
1
1578
1604
1105
1182
1685
-
3192
-
3431,3279
3441, 3301
-
-
591
2023
142
143
7

144
145
146
147
148
149
150
151
(a)
152
153
154
155
(a)
156
157
158
159
160
161
162
163
164
165
166
167
(b)
Figure 1. The FT-IR spectral curves for HL(a) and compound 1(b).
.
8

168 3.2. Electronic spectra
169 Electronic spectral data of HL and 1 are presented in Table 3. The UV absorption bands of HL in
170 DMSO solvent exhibits charge transfer transitions at 260 and 320 nm attributed to intra-ligand
171 π → π* transitions of the pyridyl ring and imine function of the semicarbazone moiety and a band at
172 365 nm attributed to n→ π* transition of imine function. Metallation of HL, causes a red shift of
173 these electronic transitions for 1 (see Table 3). Also the absorption band at 420nm is attributed to
174 LMCT from azido ligand to cobalt(III) ions. For 1, containing distorted octahedral cobalt(III)
175 centers, the absorption band at 600 nm appeared due to d−d transition as reported in the literature
176 [44,45,47] and the higher energy band was overlapped by ligand to metal charge transfer bands. As
177 seen in Table 3 and comparing the results of electronic spectra of HL and 1 in DMSO, DMF and
178 solid state, the results demonstrate that with dissolving, intense charge-transfer bands of HL and 1
179 appear at low wavelengths. Such a large blue shift may be attributed to the strong interaction
180 between the solvent molecules with 1. Also by comparing the electronic spectra of HL and 1 in
181 DMSO and DMF, it is concluded that by using a polar solvent the absorption bands are shifted to
182 short wavelength which is known as hypsochromic shift.
183
Table 3. Electronic absorption data of HL and 1
π → π*pyridine
)
(M-1Cm-
Compound
π → π*imine
n → π*imine
LMCT
d-d
1
HL(DMSO)
260(sh)
261(sh)
278(sh)
263(sh)
265
8000
320
323
351
336
340
361
25000
365(sh)
2000
2000
-
-
HL(DMF)
7000
24000
362(sh)
-
-
HL(Solid state)
-
-
-
-
-
-
Compound 1(DMSO)
Compound 1(DMF)
Compound 1 (Solid state)
10000
8000
16000
15000
420
423
462
14000
13000
600
604
643
250
100
280
184
185
9

186 3.3. Crystal structure of [Co(L)2][Co(L)(N3)3](1)
187 The structure of 1 is depicted in Figure 2, and the crystallographic data and structure analysis for it,
188 is summarized in Table 1. The structure of 1 consists of two distinct mononuclear crystallographic
189 units, which will be denoted as parts A and B. In part A with formula [Co(L)2], the cobalt(III) center
190 is uniquely coordinated by two deprotonated HL. The structural study reveals that the Co1 center
191 exhibit a distorted octahedral environment and the equatorial sites of Co1 center are occupied by two
192 nitrogen and one enolate oxygen atoms of the HL (N5, N6 and O2) and one nitrogen atom of the
193 second HL (N2), and the axial sites are occupied by one nitrogen and one enolate oxygen atoms of
194 the second HL (N1, O1). The coordination polyhedron around Co1 may be described as an axially
195 elongated octahedral (for more information about the bond length see Table 4). The Co(III)–N
196 distances are shorter for the azomethine nitrogen atoms, indicating their stronger coordination than
197 pyridyl nitrogens. The angles around the Co1 center deviate significantly from 90º, the N2–Co1–O2
198 and N5–Co1–N2 angles have opened up to 95.16º and 98.67º, respectively and the N6–Co1–O2 and
199 N5–Co1–N6 angles have reduced to 82.68º and, 83.18º indicating distortion from a regular
200 octahedron [44,46]. The chelate bite angles in the two five-membered rings are close [O1–Co1–N2,
201 92.09º ; N2–Co1–N1, 93.12º and O2–Co1–N6, 82.68º; N6–Co1–N5, 83.51º]. In part B with formula
202 [Co(L)(N3)3], the Co2 center is uniquely hexa coordinated by a tridentate Schiff base HL and three
203 terminal azides. The coordination polyhedron around Co2 is best described as distorted octahedral.
204 Distortion from the ideal octahedral geometry is mainly due to the asymmetric nature of the bond
205 tridentate Schiff base ligand. Three atoms (N9, N10 and O3) of the tridentate HL in combination
206 with three nitrogen atoms (N13, N16 and N19) of three different azides in a meridional alignment
207 complete octahedral coordination of the Co2 ion. The equatorial positions are occupied by the two
208 nitrogen and one enolate oxygen atoms of the HL (N9, N10 and O3) and one nitrogen atom of the
10

209 terminal azide (N16), while other nitrogen atoms of the remaining azides (N13 and N19) are placed
210 at the axial positions with the angle of N13–Co2–N19; 178.05(12) º. The chelate bite angles in the
211 two five-membered rings are very close [O3–Co2–N10 is 82.00º, N10–Co2–N9 is 83.15º]. The
212 coordination polyhedron around Co2 may be described as an axially elongated octahedral. As seen,
213 the Co(III)–N distance is shorter for the azomethine nitrogen atom indicating its stronger
214 coordination than pyridyl and azide nitrogens. The terminal azide ligands show asymmetric N–N
215 distances of 1.198(4) /1.160(4) Å, 1.207(4) /1.167(4) and 1.208(4) /1.144(4) Å with the longer N–N
216 bonds involving the N atoms coordinated to the metal centre. The bond length for Co–N(azido) with
217 Co–N13; 1.971(3) Å, Co-N16; 1.949(3) Å , Co-N19; 1.956(5) Å are similar with that reported in the
218 literature [44-47]. The azides are quasi-linear with N–N–N angles: [N13–N14–N15; 176.7(4) º, N16–
219 N17–N18; 177.9(4) º and N19–N20–N21; 176.6(4) º], these N–N–N angles are similar with that
220 reported in the literature [44, 47].
221
222
223
224
225
Figure 2. The two neutral mononuclear crystallographic units that compose the structure of 1,
showing the labeling scheme for the Co(III) coordination centers. For clarity reasons only the
hydrogen atoms of the amine groups are shown.
11

226
Table 4 - Selected bond lengths (Å) and angles (deg) for 1.
Co1−N2
Co1−N6
Co1−O2
Co1−N5
Co1−N1
Co1−O1
1.860(3)
1.861(3)
1.896(2)
1.909(3)
1.916(3)
1.922(2)
Co2−N10
Co2−N9
Co2−O3
Co2−N16
Co2−N19
Co2−N13
1.862(3)
1.917(3)
1.944(3)
1.949(3)
1.956(3)
1.971(3)
N2−Co1−N6
N2−Co1−O2
N6−Co1−O2
N2−Co1−N5
N6−Co1−N5
O2−Co1−N5
N2−Co1−N1
N6−Co1−N1
O2−Co1−N1
N5−Co1−N1
N2−Co1−O1
N6−Co1−O1
O2−Co1−O1
N5−Co1−O1
N1−Co1−O1
177.83(12)
95.16(10)
82.68(10)
98.67(12)
83.50(12)
166.15(10)
83.15(11)
97.04(11)
89.80(11)
90.98(11)
82.09(10)
97.74(10)
92.39(10)
90.38(11)
165.21(10)
N10−Co2−N9
N10−Co2−O3
N9−Co2−O3
83.13(13)
82.03(11)
165.15(11)
177.61(13)
98.81(13)
96.02(12)
87.44(12)
88.59(12)
90.36(12)
91.21(13)
90.65(12)
90.92(12)
89.64(11)
90.72(12)
178.06(12)
N10−Co2−N16
N9−Co2−N16
O3−Co2−N16
N10−Co2−N19
N9−Co2−N19
O3−Co2−N19
N16−Co2−N19
N10−Co2−N13
N9−Co2−N13
O3−Co2−N13
N16−Co2−N13
N19−Co2−N13
227
228
As consequence of various donor and acceptor groups in both the parts, there are several strong
229 N−H···N and N−H···O hydrogen bonds interconnecting adjacent moieties. The details about the
230 hydrogen bonds interconnecting are summarized in Table 5. While N4−H4B···O1ii and
231 N8−H8B···N3iii interconnect only crystallographic equivalent parts of type A, the remaining
232 hydrogen bonds (N4−H4A···N11i, N8−H8A···N13 and N12−H12B···N7iv) are interaction involving
233 distinct parts, i.e. between parts A and B. These strong hydrogen bonding network lead to the
12

234 formation of a three-dimensional supramolecular network. These hydrogen interactions and a three-
235 dimensional supramolecular network for the compound are shown in the Figures. 3a and 3b.
236
237
238
239
240
Figure 3. (a) N−H···N and N−H···O hydrogen bonds interconnecting adjacent parts A and B. (b)
Extended crystalline packing showing the 3D hydrogen bonding network. Hydrogen bonds are
represented as dashed lines (golden color) and for clarity only the H-atoms involved in the
hydrogen bonds are shown.
241
242
Table 5. Selected hydrogen bonding parameters for 1.
D−H···A
d(D−H)(Å)
0.886(17)
0.874(17)
0.869(17)
0.868(17)
0.880(18)
d(H···A)(Å)
2.09(2)
d(D···A)(Å) (D−H···A)(°)
N4−H4A···N11i
N4−H4B···O1ii
N8−H8A···N13
N8−H8B···N3iii
N12−H12B···N7iv
2.943(4)
2.898(3)
2.969(4)
2.963(4)
3.322(4)
161(3)
174(3)
164(3)
165(3)
147(4)
2.028(19)
2.12(2)
2.12(2)
2.55(3)
Symmetry transformations used to generate equivalent atoms: (i) x-1,y,z ;
(ii) -x,-y+1,-z+1 ; (iii) x+1,y,z; (iv) -x+1,-y+1,-z+1
243
244
245
246
.
13

247 3.3. Electrochemistry
248 3.3.1. Electrochemistry of the HL and azide ligands
249 It is evident from the cyclic voltammogram of HL displayed in Figure 4a, that HL is electroactive
250 over a range from 1.5 to −2.0 V in dimethylsulfoxide solvent. Two ligand-centered redox waves
251 observed at -1.15 and 1.18 V are attributed to the pyridine rings. The azo group (−N=N−) is known
252 as a potential electron transfer center [44] and the cyclic voltammogram of azide ligand displays only
253 one irreversible oxidation wave at 0.85 V (see Figure 4b).
254 3.3.2. Electrochemistry of [Co(L)2][Co(L)(N3)3](1)
255 As seen in Figure 5, the compound 1 exhibits both metal and ligand centred electroactivity in the
256 potential range −2 to 1V. The cyclic voltammogram of 1 in DMSO, displays two cathodic potentials
257 at about −0.54V (Ic1) and −1.68V(Ic2). The cathodic peaks of 1 at −0.54 (Ic1) and −1.68 (Ic2)
258 (versus Ag/AgCl) can be attributed for the reduction of two cobalt(III) ions in different environment
259 to cobalt(II). During the reverse scan oxidation waves (Ia1, Ia2 and Ia3) appear at −1.68 , −0.54 and
260 0.94 V. The oxidation waves (Ia1, Ia2) which are associated with the cathodic peaks Ic2 and Ic1,
261 respectively. These processes are completely reversible, characterized by a peak to peak separation
262 (ΔEp = EPa – Epc) of 0.18 and 0.16 V, respectively, which remains unchanged upon changing the scan
263 rate. There is also an equality in the anodic peak current and the cathodic peak current of 1 (ia1/ic2
264 =0.35, ia2/ic1= −.080 ), as expected for an reversible electron transfer process. The oxidation waves
265 (Ia3) displays at 0.94 V is attributed to the oxidation of azide ligand [29,33].
266
267
268
269
14

270
271
272
(a)
(b)
273
274
275
Figure. 4. Cyclic voltammogram of 10−3 mol/l HL(a) and azide(b) ligands in DMSO and lithium
perchlorate (0.1 mol/l); scan rate 10 mV s−1
276
277
278
279
Figure. 5. Cyclic voltammogram of 10-3 mol/l [Co(L)2][Co(L)(N3)3](1) in DMSO and lithium perchlorate (0.1 mol/l);
scan rate 10 mV s-1
280 4. Conclusion
281 A cobalt(III) compound [Co(L)2] [Co(L)(N3)3] (1), involving Schiff base methyl 2-pyridyl ketone
282 semicarbazone (HL) and the auxiliary azide ligand, has been synthesized and characterized.
283 Comparison of the IR spectra of HL and 1, indicates that HL acts as a tridentate Schiff base and is
284 chelated to the cobalt(III) centers in enolate form, which is confirmed by the solid-state structure
15

285 determinate by single crystal X-ray diffraction. In fact, compound 1 consists of two Co(III)
286 mononuclear crystallographic units with different coordination spheres, but both the Co(III) centers
287 are six coordinated with distorted octahedral geometries. This compound is the first compound
288 formed by two mononuclear units based in the methyl 2-pyridyl ketone semicarbazone ligand. As
289 consequence of various donor and acceptor groups in the two distinct units, there are several strong
290 N−H···N and N−H···O hydrogen bonds interconnecting adjacent moieties, ultimately leading to a
291 three-dimensional supramolecular network. Cyclic voltammetric experiments of 1 in DMSO reveals
292 oxidation of azide at 0.94 V and two oxidation potentials at −1.68, −0.54 V for two different
293 cobalt(III) centers. Currently, we are investigating other bridging auxiliary ligands to be used with
294 semicarbazone based molecules in the preparation of new metal coordination compounds, in order to
295 increase their structural complexity and improve their properties.
296
297 Acknowledgements
298 We are grateful to Tabriz University of Research council for the financial support of this research. We are grateful to the
299 Fundação para a Ciência e a Tecnologia, (FCT, MEC, Portugal) for their general financial support under the strategic
300 project PEst-C/EQB/LA0006/2011.
301
302 Appendix A. Supplementary data
303 Crystallographic data (excluding structure factors) for all the structures reported in this paper have been deposited with
304 the Cambridge Crystallographic Data Centre as supplementary publication no.CCDC- 894063. Copies of these data can
305 be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 2EZ, UK; FAX: (+44) 1223
306 336033, or online via www.ccdc.cam.ac.uk/data_request/cif or by emailing data_request@ccdc.cam.ac.uk.
307
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390 Dear Reviewer
391 We acknowledge the reviewer helpful and relevant comments. We revised the manuscript according
392 to your comments. we submit the revised manuscript that are highlighted in green for response to
393 your comments.
394 We will be grateful to publishing of this manuscript in the Journal of Molecular Structure.
395 Yours Sincerely
396 B. Shaabani
397
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