Direct chemical and electrochemical syntheses of coordination compounds of benzazolyl azo ligands

Article abstract of DOI:10.1080/00958971003690421

The chelates of 2-hydroxy(aminophenylnaphthyl)-azo-1-alkylbenzimidazoles were obtained from electrochemical (anodic dissolution of zero-valent d-metals) and chemical (from acetates of the same metals) syntheses in methanol. The EXAFS and X-ray single crys

Full text of DOI:10.1080/00958971003690421

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Direct chemical and electrochemical
syntheses of coordination compounds
of benzazolyl azo ligands
Anatolii S. Burlov a , Alla S. Antsyshkina b , Georgii G. Sadikov
b , Yurii V. Koshchienko a , Ludmila N. Divaeva a , Sergei A.
Mashchenko a , Ali I. Uraev a , Dmitrii A. Garnovskii c , Igor
S. Vasilchenko a , Valery G. Vlasenko d , Yan V. Zubavichus e
,
Gennadii S. Borodkin a , Boris I. Kharisov f , Tomás C. Hernández
García f , Vladimir S. Sergienko b & Alexander D. Garnovskii a
a Research Institute of Physical and Organic Chemistry, South
Federal University, Rostov-on-Don, Russia
b Kurnakov Institute of General and Inorganic Chemistry, Moscow,
Russia
c South Research Center of Russian Academy of Sciences, Rostov-
on-Don, Russia
d Research Institute of Physics, South Federal University, Rostov-
on-Don, Russia
e Russian Research Centre “Kurchatov Institute”, Moscow, Russia
f Facultad de Ciencias Químicas, Universidad Autónoma de Nuevo
León, Monterrey, México
Published online: 07 Apr 2010.
To cite this article: Anatolii S. Burlov , Alla S. Antsyshkina , Georgii G. Sadikov , Yurii V.
Koshchienko , Ludmila N. Divaeva , Sergei A. Mashchenko , Ali I. Uraev , Dmitrii A. Garnovskii , Igor
S. Vasilchenko , Valery G. Vlasenko , Yan V. Zubavichus , Gennadii S. Borodkin , Boris I. Kharisov ,
Toms C. Hernndez Garca , Vladimir S. Sergienko & Alexander D. Garnovskii (2010) Direct chemical
and electrochemical syntheses of coordination compounds of benzazolyl azo ligands, Journal of
Coordination Chemistry, 63:6, 917-930, DOI: 10.1080/00958971003690421
To link to this article: http://dx.doi.org/10.1080/00958971003690421
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Journal of Coordination Chemistry
Vol. 63, No. 6, 20 March 2010, 917–930
Direct chemical and electrochemical syntheses of coordination
compounds of benzazolyl azo ligands**
ANATOLII S. BURLOVy, ALLA S. ANTSYSHKINAz, GEORGII G. SADIKOVz,
YURII V. KOSHCHIENKOy, LUDMILA N. DIVAEVAy, SERGEI
A. MASHCHENKOy, ALI I. URAEVy, DMITRII A. GARNOVSKIIx, IGOR
S. VASILCHENKOy, VALERY G. VLASENKO{, YAN V. ZUBAVICHUS?,
´
´
GENNADII S. BORODKINy, BORIS I. KHARISOV*k, TOMAS C. HERNANDEZ
´
GARCIAk, VLADIMIR S. SERGIENKOz and ALEXANDER D. GARNOVSKIIy
yResearch Institute of Physical and Organic Chemistry, South Federal University,
Rostov-on-Don, Russia
zKurnakov Institute of General and Inorganic Chemistry, Moscow, Russia
xSouth Research Center of Russian Academy of Sciences, Rostov-on-Don, Russia
{Research Institute of Physics, South Federal University, Rostov-on-Don, Russia
?Russian Research Centre ‘‘Kurchatov Institute’’, Moscow, Russia
kFacultad de Ciencias Quı
´
micas, Universidad Autonoma de Nuevo Leon,
´ ´
Monterrey, Mexico
´
(Received 2 June 2009; in final form 29 October 2009)
The chelates of 2-hydroxy(aminophenylnaphthyl)-azo-1-alkylbenzimidazoles were obtained
from electrochemical (anodic dissolution of zero-valent d-metals) and chemical (from acetates
of the same metals) syntheses in methanol. The EXAFS and X-ray single crystal diffraction
data revealed the existence of octahedral nickel, zinc, and cadmium complexes containing four
five-membered metallocycles with the N4N2 or N6 ligand environments. The formation of
five-membered coordination units, non-typical for azo ligands, is explained on the basis of
inner-chelate competitive metal binding of ambidentate ligand systems.
Keywords: Benzazolyl azo ligands; Direct electrochemical synthesis; EXAFS; X-ray diffraction;
Octahedral complexes
1. Introduction
Electrosynthesis, an important tool in coordination chemistry [1–15], is mostly carried
out by anodic dissolution of zero-valent metals (direct synthesis [7, 8, 10]), although
some metal salts could also be used [12, 13]. This method is part of the so-called ‘‘direct
synthesis’’ [7], i.e. preparation of metal complexes via cryosynthesis, electrosynthesis,
tribosynthesis, and metal dissolution in non-aqueous media. Progress in direct synthesis
*Corresponding author. Email: bkhariss@mail.ru
**Dedicated to Professor, Dr Sci. (Academician) Vladimir I. Minkin (Research Institute of Physical and
Organic Chemistry, South Federal University, Rostov-on-Don, Russia) on occasion of his 75th birthday.
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958971003690421

918
A.S. Burlov et al.
has been reached due to activation of metals (in particular, use of pyrophoric
Rieke metals [9, 16, 17]), ultrasonic, or ultraviolet irradiation [9, 10].
The advantages of the direct electrochemical method in comparison with standard
techniques (those using metal salts of carbonyls [8]) are as follows: the possibility to
carry out the synthesis at a relatively low temperature (down to 0ꢀC), variation of
non-aqueous solvents, high yields, and isolation of coordination compounds mostly
without salt anions [7–10].
The present work is devoted to a comparison of direct electrochemical (es) and
standard chemical (cs) synthesis (reactions (1) and (2)) of metal chelates 3 and 4 on the
basis of 2-(2-aminophenylnaphthyl(hydroxy))azo-1-alkylbenzimidazoles 1 and 2 and
elucidation of the product structures by the EXAFS and X-ray single crystal diffraction
techniques.
M0
O
es
OH
N
R
5
R
N
M/2
(1)
N
N
5
M(OAc)2
cs
N
N
R1
N
R1
N
1: R = 5-Me, R1 = Me (a);
R = 5-OMe, R1 = Me (b);
R = 4-OMe, R1 = Me (c);
R = 5-Me, R1 = Et (d)
3: M = Co, Ni, Zn, Cd
M0
es
Ph
Ph
N
5
N
H
N
M/2
(2)
N
N
N
N
5
M(OAc)2
cs
N
N
Et
N
Et
2
4: M = Co, Ni, Zn, Cd
2. Experimental
2.1. Materials
All solvents, metal acetates, platinum wire, and metal sheets were purchased from Aesar
or Lancaster.
2.2. Synthesis of heterocyclic azo compounds 1 and 2
1-methyl- and 1-ethyl-2-aminobenzimidazole were prepared according to the literature
procedure [18]. A solution of 2.14 g (0.031 mol) of sodium nitrite in 2 mL of water was

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
919
added dropwise to a solution of 0.03 mol of 1-methyl-2-aminobenzimidazole or
1-ethyl-2-aminobenzimidazole in 90% H3PO4. The latter was prepared by dissolution of
22.5 g of P4O10 in 75 mL of phosphoric acid under intense stirring at 0–5ꢀC. After
adding all sodium nitrite, the mixture was kept cold for 30 min, and then the obtained
diazonium salt solution was slowly added to a solution of 0.03 mol of p-cresol, 3- or
4-methoxyphenol or 2-phenylaminonaphthalene. In 30 min, ice was added and the
resulting mixture was treated with saturated solution of Na2CO3 adjusted to pH 7–8.
The solids formed were filtered off, washed with water, and dried in air at room
temperature.
The obtained orange heterocyclic compounds were dissolved in toluene and purified
in a chromatographic column (Al2O3, eluent toluene). The first portions were thrown
away and the main substance was extracted with chloroform, which was then removed
in a rotary evaporator. Solids of azo compounds were recrystallized from ethanol.
2.3. Synthesis of 3 and 4
The synthesis was carried out by two techniques:
2.3.1. Direct es of the complexes. A solution of 0.001 mol of the corresponding azo
compound (1a–d, 2) and 0.01 g [Et4N]ClO4 (electrolyte) in 20 mL of methanol was
added in an electrochemical cell with a platinum cathode and the respective Co, Ni, Cu,
Zn, or Cd anode. The electrosynthesis was carried out at constant current (40 mA) and
initial voltage of 15 V for 1 h. The complexes were filtered, washed with methanol, and
recrystallized from a mixture chloroform : methanol (1 : 2).
2.3.2. Chemical synthesis. A solution of 0.0005 mol of the corresponding metal acetate
in 10 mL of methanol was added to a solution of 0.266 g (0.001 mol) of 1‘, 0.28 g
(0.001 mol) of 1b, 1c, 1d or 0.39 g (0.001 mol) of 2, respectively, in 20 mL of the same
solvent, and boiled for 1 h. After cooling, solids were filtered, washed with methanol,
and recrystallized from a mixture chloroform : methanol (1 : 2).
The reactions (1 and 2) afforded complexes 3 and 4 with the ML2 compositions
(LH ¼ 1a–d, 2) in yields 70%–93% (es) and 65%–85% (cs).
2.4. Studies of composition and structure of ligands and complexes
Yields, melting points, and elemental analyses of 1a–d, 2, 3, and 4 are shown in table 1.
IR , 1H-NMR, and magnetic susceptibility data are summarized in Supplementary
material.
2.4.1. IR spectra. The IR spectra of powder samples were recorded with a Varian
Fxcalibur-3100 FT-IR spectrophotometer.
2.4.2. 1H-NMR spectra. The 1H-NMR spectra were measured with Varian Unity-300
equipment (300 MHz) with the signal of residual 1H in CDCl3 as the internal standard.

920
Table 1. Elemental analysis data for azo compounds 1, 2 and their complexes 3, 4.
Element content (found/calculated) (%)
A.S. Burlov et al.
Compound Yield (%) m.p. (ꢀC) Brutto formula
C
H
N
M
1‘
20
239–240
C
15H14N4O
67.74/67.75 5.28/5.30
21.15/21.04
3‘-Zn
cs 83 4250
es 89
60
cs 75 4250
es 85
cs 65 4250
es 70
cs 81 4250
es 90
cs 80 4250
es 85
C30H26N8O2Zn cs 60.57/60.46 4.45/4.40
es 60.58/60.46 4.48/4.40
18.79/18.80 10.87/10.47
18.85/18.80 10.60/10.47
19.87/19.85
1b
3b-Ni
293–294 C15H14N4O2
63.94/63.82 5.12/5.00
C30H26N8O4Ni cs 58.11/58.00 4.32/4.22
es 58.17/58.00 4.25/4.22
C30H26N8O4Cu cs 57.47/57.55 4.21/4.19
es 57.60/57.55 4.25/4.19
C30H26N8O4Zn cs 57.48/57.38 4.07/4.17
es 57.42/57.38 4.19/4.17
C30H26N8O4Cd cs 53.47/53.38 3.90/3.88
es 53.45/53.38 3.92/3.88
18.17/18.04 9.56/9.45
18.14/18.04 9.38/9.45
17.86/17.90 10.38/10.15
17.92/17.90 10.28/10.15
17.78/17.85 10.62/10.41
17.89/17.85 10.52/10.41
6.65/6.60 16.51/16.45
6.70/6.60 16.54/16.45
19.77/19.85
3b-Cu
3b-Zn
3b-Cd
1c
3c-Ni
30
228–229 C15H14N4O2
199–200 C30H26N8O4Ni cs 58.11/58.00 4.32/4.22
es 58.15/58.00 4.28/4.22
63.98/63.82 5.09/5.00
cs 76
es 85
88
cs 75 4250
es 85
cs 70 4250
es 87
cs 85 4250
es 90
cs 65 4250
es 85
68
cs 65
es 70
cs 70
es 80
cs 75 4250
es 85
18.15/18.04 9.52/9.45
18.10/18.04 9.58/9.45
20.05/19.99
1d
3d-Ni
105–106
C16H16N4O
C32H30N8O2Ni cs 62.37/62.26 4.88/4.90
es 62.28/62.26 4.95/4.90
C32H30N8O2Cu cs 61.87/61.77 4.79/4.86
es 61.79/61.77 4.91/4.86
C32H30N8O2Zn cs 61.62/61.59 4.75/4.85
es 61.65/61.59 4.90/4.85
C32H30N8O2Cd cs 57.38/57.28 4.62/4.51
es 57.18/57.28 4.68/4.51
C25H21N5
68.64/68.55 5.79/5.75
18.26/18.15 9.64/9.51
18.17/18.15 9.68/9.51
18.15/18.01 10.32/10.21
18.20/18.01 10.11/10.21
17.78/17.96 10.64/10.48
17.87/17.96 10.54/10.48
16.84/16.70 16.68/16.75
16.65/16.70 16.82/16.75
17.72/17.89
3d-Cu
3d-Zn
3d-Cd
2
4-Co
218–219
76.71/76.70 5.51/5.41
222–223 C50H40N10Co cs 71.62/71.51 4.91/4.804.77/4.80 16.57/16.67 7.17/7.02
es 71.54/71.51
236–237 C50H40N10Ni cs 71.64/71.53 4.75/4.80
es 71.60/71.53 4.64/4.80
16.45/16.67 7.19/7.02
16.58/16.68 7.15/6.99
16.70/16.68 7.00/6.99
16.61/16.59 7.74/7.53
16.62/16.59 6.75/7.53
16.47/16.55 7.78/7.72
16.50/16.55 7.79/7.72
4-Ni
4-Cu
4-Zn
C50H40N10Cu cs 71.21/71.11 4.68/4.77
es 71.22/71.11 4.71/4.77
C50H40N10Zn cs 70.92/70.96 4.68/4.76
es 70.95/70.96 4.82/4.76
cs 80 4250
es 93
2.4.3. Magnetic susceptibility. The magnetic susceptibility of a solid phase was
determined by the Faraday method at room temperature [19].
2.4.4. EXAFS measurements. The NiK-, ZnK-, CdK- and CuK-edge EXAFS spectra
for the complexes were obtained at the Station J1.3b ‘‘Structural Materials Science’’ of
the Kurchatov Center for Synchrotron Radiation and Nanotechnology (KCSRNT,
Moscow, Russia) [20]. The storage ring with electron beam energy of 2.5 GeV and a
current of 70–90 mA was used as the source of radiation. All the spectra were recorded
in the transmission mode using a channel-cut Si(1 1 1) (NiK-, ZnK-, and CuK-edges) or
Si(2 2 0) (CdK-edge) monochromators and two N2- or Ar-filled ionization chambers
as detectors. The EXAFS data were analyzed using the IFEFFIT data analysis
package [21]. The EXAFS data reduction followed standard procedures for pre-edge
subtraction and spline background removal.
The radial pair distribution functions around Ni, Zn, Cd, and Cu were obtained
by Fourier transformation of the k3-weighted EXAFS functions over the range of

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
921
photoelectron wavenumbers 2.2–12.0 Aꢁ1. The structural parameters, including
interatomic distances (R), coordination numbers (CNs) and the distance mean-square
deviation Debye–Waller factors (ꢀ2), were found by non-linear fit of theoretical spectra
to experimental. Experimental data were simulated using the theoretical EXAFS
amplitude and phase functions, which were calculated using the FEFF7 program [22].
The amplitude reduction factor, S20, and threshold energy, E0, were calibrated by fitting
the EXAFS data for crystallographically characterized models. The amplitude
reduction factor, S20, was found equal to 0.9 in all cases.
˚
The quality of fits was estimated from discrepancy factors between the experimental
and simulated functions (Q-factor).
2.4.5. X-ray crystallography. Dark blue crystals of bis-[2-(hydroxy-5-methyl-
phenylazo)-1-ethylbenzimidazolato]zinc(II) C32H30N8O2Zn (3d-Zn) are triclinic:
ꢀ
ꢀ
˚
˚
˚
a ¼ 10.471(2) A, b ¼ 11.026(2) A, c ¼ 13.956(4) A, ꢁ ¼ 69.22(3) , ꢂ ¼80.64(2) , ꢃ ¼
80.48(2) , V ¼ 1476.1(2) A , M ¼ 624.01, F(0 0 0) ¼ 648, ꢄcalcd ¼1.404 g cmꢁ1, ꢅ ¼
3
ꢀ
˚
0.876 g cmꢁ1, Z ¼ 2, space group P-1.
Dark blue crystals of bis-[2-(phenylaminonaphthylazo)-1-ethylbenzimidazolato]-
˚
˚
nickel(II) C50H40N10Ni (4-Ni) are triclinic: a ¼ 11.305(2) A, b ¼ 12.337(2) A, c ¼
3
17.293(3) A, ꢁ ¼ 96.08(1)ꢀ, ꢂ ¼ 105.82(2)ꢀ, ꢃ ¼ 110.70(1)ꢀ, V ¼ 2115.1(3) A , M ¼
419.82, F(0 0 0) ¼ 876, ꢄcalcd ¼ 1.318 g cmꢁ1, ꢅ ¼ 0.507 g cmꢁ1, Z ¼ 2, space group P-1.
Experimental data for 3d-Zn and 4-Ni were obtained with an Enraf-Nonius CAD-4
difractometer (ꢆ Mo-Kꢁ, graphite monochromator, ꢇ/2ꢇ-scanning, ꢇmax ¼ 25ꢀ).
For 3d-Zn, intensities of 5126 reflections were measured; 7274 reflections were obtained
for 4-Ni.
˚
˚
The structures of both compounds were determined by direct methods (SHELXS-86)
[23], refined by full-matrix least-squares on F2 (SHELXL-97) [24] in the anisotropic
approximation for all non-hydrogen atoms. Hydrogens were placed in the calculated
positions and refined according to the riding model with fixed Debye factors of
2
˚
UH ¼ 0.08 A .
The final fitting parameters for 3d-Zn are: R1 ¼ 0.0478, wR2 ¼ 0.1373 on 2858
reflections with Fo ꢂ 2ꢀ(Fo), R1 ¼ 0.1252, wR2 ¼ 0.1821 on all reflections, 388 variable
parameters, index range: ꢁ11 ꢃ h ꢃ 11, 0 ꢃ k ꢃ 12, ꢁ15 ꢃ l ꢃ 16, goodness-of-fit ¼ 0.983,
ꢁ3
˚
residual electronic density Dꢄmin ¼ ꢁ0.453, Dꢄmax ¼ 0.525 e A
.
The fitting parameters for 4-Ni are: R1 ¼ 0.0386, wR2 ¼ 0.1162 on 3976 reflections
with Fo ꢂ 2ꢀ(Fo), R1 ¼ 0.1200, wR2 ¼ 0.1776 on all reflections, 549 variable parameters,
index range: ꢁ12 ꢃ h ꢃ 12, 0 ꢃ k ꢃ 13, ꢁ19 ꢃ l ꢃ19, goodness-of-fit ¼ 0.843, residual
ꢁ3
˚
electronic density Dꢄmin ¼ ꢁ0.266, Dꢄmax ¼ 0.254 e A
.
3. Results and discussion
3.1. Ligands structure
According to the IR and 1H-NMR data (Supplementary material), 1 and 2 exist mostly
in the hydroxy (amino) azo form. The IR spectra of 1a–d have a broad stretching
vibration at 2800–3400 cmꢁ1 typical of an OH involved in an intramolecular hydrogen

922
A.S. Burlov et al.
bond (IMHB). The IMHB considerably decreases the absorption intensity for HNPh,
which becomes non-characteristic for 2. In the IR spectrum of the benzazolylazo 2,
vibrations of the secondary amino group occur from 2800 to 3400 cmꢁ1 as a broad
low-intensity band, observed for the majority of o-aminoarylazo compounds [25].
For the 3 and 4 IR bands from 1600 to 1618 cmꢁ1 attributable to the benzimidazole
fragment does not change compared to 1 and 2. Frequencies of vibrations for the azo of
heterocyclic azo compounds at 1330–1350 cmꢁ1 slightly increase upon complexation.
At 1227–1280 cmꢁ1, ꢈ(Ph–O) absorption bands occur for 1a–1d, which shift upon
complexation to shorter wavenumbers by 10–29 cmꢁ1 due to the metal chelate
formation [26].
Distinct signals related to OH-protons (3.53–4.9 ppm) 1 and HNPh (13.16 ppm) 2 are
1
observed in H-NMR spectra (Supplementary material).
3.2. Structure of the coordination compounds
IR and 1H-NMR spectra of 3 and 4 (Supplementary material) do not reveal absorptions
of proton-donor fragments. Proton signals of the methyl (methoxyl) groups from
aromatic fragments and ethyl substituents of benzimidazole of azo compounds in
1H-NMR spectra of zinc and cadmium complexes slightly shift to stronger field by
0.10–0.18 ppm on average. Aromatic proton signals upon complexation also shift to
stronger field by 0.27–0.82 ppm on average.
Magnetic properties of copper, nickel, and cobalt complexes were studied at room
temperature (Supplementary material). The values of ꢅeff ¼ 3.12–3.35 BM for nickel
and 4.32 BM for cobalt complexes are characteristic for chelates with tetrahedral or
octahedral structure. Probably, copper complexes (ꢅeff ¼ 1.72–2.09 BM) also possess an
analogous structure.
Fourier transforms (MFT) of the NiK-, ZnK-, and CuK-edge EXAFS data for 3
and 4 are shown in figures 1–3. The best-fit local structural parameters of the respective
metal ions in 3 and 4 are compiled in tables 2–4.
˚
There is an intense metal-nearest neighbor feature at R ¼ 1.5 A, together with two
˚
resolvable features at longer distances. For MFT of all 3 and 4, the first peak at 1.5 A is
associated with the M ꢄ ꢄ ꢄ O/N (M ꢄ ꢄ ꢄ N for 4) first-shell contribution, while the second
˚
and following peaks within a range 2.45–3.5 A are essentially due to the M ꢄ ꢄ ꢄ C/N,
M–C second- and third-shell contributions as well as M–C ꢄ ꢄ ꢄ N, M–N ꢄ ꢄ ꢄ N multiple
scattering signals. Multiple scattering refers to the event when the X-ray excited
photoelectron is scattered by two (or more) atoms of the heterocyclic ligands prior to
returning to the central atom. All the EXAFS spectra of the Ni, Zn, and Cu complexes
were fitted with the inclusion of a second low-Z shell at a longer distance.
Similarity of the coordination geometry around nickel in 3b, 3c, 3d, and 4 and around
zinc in 3a, 3b, and 4 has been verified by the quantitative EXAFS spectra analysis.
All four Ni spectra can be fitted with a single low-Z (oxygen/nitrogen) nearest neighbor
˚
shell at an average distance of 2.05 ꢅ 0.01 A with a CN of six. The resultant single-shell
fits are characterized by relatively large Debye–Waller factors, indicating disorder in the
metal–ligand bond lengths. For 3a, 3b, and 4, the nearest neighbor shell consists of
˚
6 N/O atoms at average distances Zn ꢄ ꢄ ꢄ O of R ¼ 2.07–2.10 A and Zn ꢄ ꢄ ꢄ N of
˚
R ¼ 2.11 A (3a, 3b, and 4), assuming octahedral coordination. The best fits for the
cadmium complexes 3b and 3d were also obtained within the model with

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
923
35
Ni-N/O
Ni-C/N
30
3d-Ni
25
20
15
10
5
3c-Ni
3b-Ni
4-Ni
7
0
0
1
2
3
4
5
6
8
R, Å
Figure 1. MFT of the NiK-edge EXAFS spectra for the Ni complexes studied: experiment (solid lines) and
best-fits (open circles).
octahedral environment. The Cd ꢄ ꢄ ꢄ O and Cd ꢄ ꢄ ꢄ N distances obtained are considerably
longer than those in other studied complexes, according to the larger ionic radii of the
cadmium ion. However, for two copper complexes 3a and 4, best-fit simulations afford
the CN of the nearest neighbor shell less than 6, which demonstrates the possible
tetrahedral coordination.
In both cases of crystallographically characterized complexes 3d-Zn and 4-Ni
(see below), the agreement between the best-fit simulations and the crystallographic
data is very good, pointing to the correctness and reliability of the EXAFS data
analysis.

924
A.S. Burlov et al.
Zn-N/O
30
25
20
15
10
5
Zn-C/N
4-Zn
3b-Zn
3a-Zn
3d-Zn
7
0
0
1
2
3
4
5
6
8
R, Å
Figure 2. MFT of the ZnK-edge EXAFS spectra for the Zn complexes studied: experiment (solid lines) and
best-fits (open circles).
Cu-N/O
12
Cd-N/O
Cu-C/N
10
8
Cd-C/N
9
6
3
0
4-Cu
6
3b-Cd
4
2
3d-Cd
7
3a-Cu
7
0
0
1
2
3
4
5
6
8
0
1
2
3
4
5
6
8
R, Å
R, Å
Figure 3. MFT of the CuK-edge (left panel) and CdK-edge (right panel) EXAFS spectra for the Cu and Cd
complexes studied: experiment (solid lines) and best-fits (open circles).

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
925
Table 2. Structural parameters of the nearest environment of the nickel ions in the complexes from the
EXAFS data analysis.
2
ꢀ2 (A )
Q (%)
3.5
˚
R (A)
˚
Compound
Atom
CN
4-Ni
N
6
6
2
14
6
6
2
6
6
2
2.05
2.92
3.32
4.22
2.04
2.93
3.30
2.05
2.88
3.28
2.04
2.92
3.24
0.0040
0.0065
0.0065
0.0065
0.0040
0.0066
0.0065
0.0047
0.0050
0.0050
0.0045
0.0063
0.0060
C/N
C/N
C
3b-Ni
3c-Ni
3d-Ni
N/O
C/N
C/N
N/O
C/N
C/N
N/O
C/N
C/N
4.6
4.8
4.9
6
6
2
R: the average distances; CN: coordination number; ꢀ2: Debye–Waller factors.
Table 3. Structural parameters of the nearest environment of the zinc ions in the complexes from the
EXAFS data analysis.
2
ꢀ2 (A )
Q (%)
˚
R (A)
˚
Compound
Atom
CN
3a-Zn
O
N
C/N
O
N
C/N
O
N
N
C/N
2
4
8
2
4
12
2
2
6
12
2.07
2.11
3.21
2.10
2.11
3.15
1.98
2.05
2.11
3.15
0.0045
0.0060
0.0046
0.0030
0.0045
0.0050
0.0045
0.0050
0.0030
0.0050
4.7
3b-Zn
5.7
3.9
6.1
3d-Zn
4-Zn
R: the average distances; CN: coordination number; ꢀ2: Debye–Waller factors.
Table 4. Structural parameters of the nearest environment of the copper and cadmium ions in the
complexes from the EXAFS data analysis.
2
ꢀ2 (A )
Q (%)
˚
R (A)
˚
Compound
Atom
CN
4-Cu
N
N
C/N
O
N
C/N
O
N
C/N
O
N
C/N
2
2
4
2
2
6
2
4
4
2
4
6
1.95
2.05
2.90
1.93
2.04
2.85
2.31
2.38
3.05
2.29
2.39
3.08
0.0037
0.0035
0.0050
0.0030
0.0035
0.0050
0.0030
0.0030
0.0040
0.0030
0.0030
0.0040
5.5
6.0
3a-Cu
3b-Cd
3d-Cd
3.5
3.3
R: the average distances; CN: coordination number; ꢀ2: Debye–Waller factors.

926
A.S. Burlov et al.
ꢀ
˚
Table 5. Selected bond lengths (A) and angles ( ) for 3d-Zn from crystallographic data.
Zn(1)–O(1)
Zn(1)–N(1)
Zn(1)–N(5)
2.107(4)
2.138(4)
2.124(4)
95.1(2)
149.6(2)
93.2(2)
95.0(2)
150.0(2)
166.8(2)
99.2(2)
Zn(1)–O(2)
Zn(1)–N(4)
Zn(1)–N(8)
2.094(4)
2.162(4)
2.188(4)
76.3(2)
110.9(2)
114.3(2)
76.7(2)
73.4(2)
95.7(2)
92.1(2)
113.2(4)
O(1)Zn(1)O(2)
O(1)Zn(1)N(4)
O(1)Zn(1)N(8)
O(2)Zn(1)N(4)
O(2)Zn(1)N(8)
N(1)Zn(1)N(5)
N(4)Zn(1)N(5)
N(5)Zn(1)N(8)
O(1)Zn(1)N(1)
O(1)Zn(1)N(5)
O(2)Zn(1)N(1)
O(2)Zn(1)N(5)
N(1)Zn(1)N(4)
N(1)Zn(1)N(8)
N(4)Zn(1)N(8)
Zn(1)O(1)C(1)
73.4(2)
ꢀ
˚
Table 6. Selected bond lengths (A) and angles ( ) for 4-Ni from crystallographic data.
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(7)
2.073(4)
2.073(4)
2.040(4)
77.6(2)
98.5(1)
89.9(2)
97.8(1)
108.7(2)
110.1(2)
77.3(2)
Ni(1)–N(2)
Ni(1)–N(6)
Ni(1)–N(8)
2.046(3)
2.093(4)
2.075(4)
153.7(2)
96.0(2)
N(1)Ni(1)N(2)
N(1)Ni(1)N(6)
N(1)Ni(1)N(8)
N(2)Ni(1)N(6)
N(2)Ni(1)N(8)
N(3)Ni(1)N(7)
N(6)Ni(1)N(7)
N(7)Ni(1)N(8)
N(1)Ni(1)N(3)
N(1)Ni(1)N(7)
N(2)Ni(1)N(3)
N(2)Ni(1)N(7)
N(3)Ni(1)N(6)
N(3)Ni(1)N(8)
N(6)Ni(1)N(8)
Ni(1)N(1)C(1)
76.9(1)
171.3(2)
91.0(1)
92.6(2)
153.4(2)
114.3(3)
76.7(2)
The most precise information on the structure was obtained on the basis of X-ray
single crystal diffraction data for 3d-Zn and 4-Ni as examples. The crystal structures are
built of octahedral complex molecules ML2 with coordination units ZnN4O2 and NiN6
(figure 4a and b). Bond lengths and angles in the corresponding coordination polyhedra
are summarized in tables 5 and 6.
The chelate molecules contain two ligands with similar structures whose planes are
perpendicular to each other. Each ligand is coordinated in a tridentate-chelate manner
forming an essentially planar system of five- (3d) and six-conjugated (4) rings, forming
the trans-meridional octahedral environment for the central metal atom. The ethyl
groups (present in both of the above ligands) and phenyl substituents in 4-Ni are also
nearly perpendicular to the corresponding main planes (the dihedral angle at the phenyl
ring C(45)–C(50) 76.8ꢀ is slightly smaller).
Distances to the metal (Zn or Ni) demonstrate that both complexes possess
approximate C2 symmetry, which is preserved up to most remote C atoms (tables 5
and 6). The experimental dispersion in formally equivalent distances (D) is comparable
to the nominal accuracy in the determination of atomic coordinates. Specific mutual
dispositions of the complex molecules in the crystal structures are worth noting. In
particular, in 3d-Zn centrosymmetric dimers exist with a distance between the Zn atoms
˚
and contacting planes of 6.47 and 3.21 A, respectively (figure 5). Dimers, in their turn,
are joined by ꢉ–ꢉ stacking interactions into supramolecular assemblies along the c-axis
˚
of the crystal (figure 6). The distance between contacting planes is 3.43 A (the distance

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
927
(a)
(b)
Figure 4. (a) Complex 3d-Zn. (b) Complex 4-Ni. Pseudo-axes C2 are perpendicular to the plane of the figure.
Figure 5. Molecular dimer in 3d-Zn.
˚
between the Zn atoms in the supramolecular chain is 8.82 A). In 4-Ni, specific
non-bonding intermolecular interactions are less pronounced. Nevertheless, pairs of
molecules related to each other by an inversion center forming a T-type intermolecular
contact can be recognized [27] (figure 7). The shortest distance between Ni atoms
˚
is 8.58 A.
Five-membered metallocycles are formed in 3 and 4; these structures are non-typical
for amino and hydroxyphenylazo compounds [28–30]. Low stability of four-membered

928
A.S. Burlov et al.
Figure 6. Supramolecular assembly in 3d-Zn. Planes of contacting phenyl rings are parallel to the plane of
figure.
Figure 7. Molecular dimer (T-type) in 4-Ni. The shortest distances between ligands are marked by dotted
lines.

Direct chemical and electrochemical syntheses of benzazolyl azo ligands
929
chelate fragments in comparison with five-membered fragments probably prevents
other possible combinations of polyhedra, for example two four- and two six-membered
units in 3 and 4.
The existence of the above structures with four five-membered metallocycles is caused
by high donor activity of the nitrogen of the benzimidazole fragment, which favors
formation of five-membered metallocycles. Similar results were reported earlier for
coordination units with the N2S2 ligand environment [8, 31].
4. Conclusions
The results obtained in the present work demonstrate the potential of the
electrochemical technique in the synthesis of chelates of amino and hydroxyazo
compounds [8, 28–30, 32, 33], of interest to address the problem of the inner-chelate
isomery of metal complexes [8, 34] on the basis of amino, hydroxy, and mercapthoazo
compounds. This problem is characteristic for the chelates, including azo fragments
which allow creation of metallocycles of distinct linkage (five; six; five and six).
This considerably expands the available toolset within the molecular design of
tautomeric ambidentate systems [35, 36], including azomethine [37–40], ꢂ-diketone [41]
ligands and their azo analogues [28, 41–43]. Competitive coordination of azo fragments
is observed not only upon the formation of metallocycles in chelates but also upon the
formation of molecular complexes with the protonated hydroxyazo on the basis of
1-(pyridilazo)-2-naphthol [33].
Supplementary material
The crystallographic information on the reported complexes (in the form of CIF files) is
deposited with Cambridge Crystallographic Data Centre: CCDC No. 720925 for 4-Ni
and No. 720926 for 3d-Zn.
Acknowledgments
This work was made possible due to the financial support of the grant of President of
Russian Federation (grant NSH 363.2008.3), Russian Foundation for Fundamental
Investigation (RFFI, grant nos. 08-03-00154, 07-03-00256, 07-03-00710), grant of
Russian Ministry of Education and Science within the project ‘‘Development of
Scientific Potential’’ (RNP 2.2.1.1/2348), program ‘‘Development of National
Universities’’ (internal SFU grants, 2009), and PAICyT-UANL (Monterrey, Mexico,
grant 2009).
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