Silver(I) complexes of the naphthyl-azoimine function: Single crystal X-ray structure of bis-[1-ethyl-2-(naphthyl-α-azo)imidazole]silver(I) perchlorate

Article abstract of DOI:10.1016/j.poly.2003.11.056

Reaction of AgNO₃ and 1-alkyl-2-(naphthyl-(α/β)-azo) imidazole (α-/β-NaiR) in MeOH followed by the addition of NaClO ₄ has isolated [Ag(α-/β-NaiR)₂](ClO ₄) (3/4). The complexes are characterized by microanalytical and spectral data. Single crystal X-ray diffraction study of bis-[1-ethyl-2- (naphthyl-α-azo)imidazole]silver(I) perchlorate confirms the structure of the complex. Argentophilicity of imidazole-N and steric crowding by naphthyl groups have forced a {2+2} distorted linear geometry. The structure shows intramolecular and intermolecular H-bonding in a 1D helical framework along with face-to-face π-π interactions.

Full text of DOI:10.1016/j.poly.2003.11.056

Polyhedron 23 (2004) 793–800
www.elsevier.com/locate/poly
Silver(I) complexes of the naphthyl-azoimine function:
single crystal X-ray structure of bis-[1-ethyl-2-
(naphthyl-a-azo)imidazole]silver(I) perchlorate
a
Joydev Dinda , Sk. Jasimuddin , G. Mostafa , C.-H. Hung , Chittaranjan Sinha
a
b
c
a,
*
a
Department of Chemistry, The University of Burdwan, Burdwan 713104, India
b
Department of Physics, National Tsing-Hua University, 101 Sec2, Kuang Fu Road, 300, Taiwan, ROC
c
Department of Chemistry, National Changhua University of Education, Changhua 50058, Taiwan, ROC
Received 24 July 2003; accepted 11 November 2003
Abstract
Reaction of AgNO3 and 1-alkyl-2-(naphthyl-(a=bÞ-azo)imidazole (a-/b-NaiR) in MeOH followed by the addition of NaClO4 has
isolated [Ag(a-/b-NaiR)2](ClO4) (3/4). The complexes are characterized by microanalytical and spectral data. Single crystal X-ray
diffraction study of bis-[1-ethyl-2-(naphthyl-a-azo)imidazole]silver(I) perchlorate confirms the structure of the complex. Argento-
philicity of imidazole-N and steric crowding by naphthyl groups have forced a {2+2} distorted linear geometry. The structure shows
intramolecular and intermolecular H-bonding in a 1D helical framework along with face-to-face p–p interactions.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Naphthylazoimidazole; Silver(I) complexes; Helical structure; EHMO
1. Introduction
dine/pyrimidine rings. The electronic property of the
molecule may be finely adjusted by changing substitu-
ents in the aryl group or complete replacement of the
aryl group by an a- and/or b-naphthyl group [9–12]. The
a- and b-naphthyl moieties also exhibit different ther-
modynamic stability [13] which may indirectly perturb
the electronic levels of the metal complexes.
We have recently reported the coordination chemistry
of platinum(II) [9], ruthenium(II) [10], osmium(II) [11]
and palladium(II) [12] naphthylazoimidazoles. In recent
years the coordination chemistry of coinage metals came
into the field for their (i) biological and medicinal im-
portance [14–16], (ii) metal–metal interaction between
closed shell (d10–d10) cations or the aurophilic interac-
tion [17–20] and (iii) supramolecular assemblies with
particular reference to helicity in silver(I) complexes [21–
25]. Several single and double stranded helical polymers
of silver(I) and their photophysical and spectrochemi-
cal properties are topics of developing interest. Because
of the flexibility in coordination number in Ag(I) [26,
27], covalent interaction may lead to the synthesis of
coordination polymers [28]. The assembly of coordina-
tion networks through H-bonding and p–p stacking
Imidazole is ubiquitous in chemistry and biology [1].
In this respect, the chemical modification of imidazole to
synthesize polydentate ligands is of current interest [2–
4]. We are also engaged to synthesize imidazole con-
taining ligands, arylazoimidazoles by anchoring an
arylazo (Ar–N@Nþ) group to the imidazole backbone.
As a part of our continuing program of research in the
field of azohetereocyclic molecules we have designed
arylazoheterocycles of pyridine, pyrimidine and imid-
azole [5–7]. They differ in their p-acidity in the order of
imidazole < pyridine < pyrimidine [8]. Arylazoheterocy-
cles bear the functional group azoimine, –N@N–C@N–,
which is an efficient group to stabilize the low valent
metal redox state [9–12].
Imidazole, being a five-membered heterocycle, is less
p-acidic and a better p-donor than six-membered pyri-
* Corresponding author. Tel.: +91-342-57683; fax: +91-342-64452/
2530452.
E-mail addresses: buclib@satyam.net.in, c_r_sinha@yahoo.com
(C. Sinha).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2003.11.056

794
J. Dinda et al. / Polyhedron 23 (2004) 793–800
the help of an EG&G PARC 270 computer controlled
Versastat using a Pt-disk milli working electrode. All
results were collected at 298 K with the saturated calo-
mel electrode (SCE) as reference. Reported potentials
are uncorrected for junction effects.
Caution! Perchlorate salts of metal complexes are
explosive in nature and should be handled carefully.
2.3. Preparation of bis-[1-ethyl-2-(naphthyl-a-azo)imid-
azole]silver(I) perchlorate, [Ag(a-NaiEt)2](ClO4) (3b)
Scheme 1.
The ligand a-NaiEt (0.50 g, 2 mmol) in dry MeOH
(20 ml) was added to a methanolic solution of AgNO3
(0.17 g, 1 mmol). The red-brown solution was stirred for
4 h in a dark environment and covered with carbon
paper. The solution volume was reduced to half of its
original volume by bubbling nitrogen gas. The com-
pound was collected as the perchlorate salt by adding
NaClO4(1 g in 2 ml aqueous solution). The brown-red
mass was recrystalized from diffusion of a CH2Cl2 so-
lution of the complex into hexane. Yield was 0.60 gm
(90%). Anal. Calc. for C30H28N8ClO4Ag (3b): C, 50.78;
H, 3.87; N, 15.90; Ag, 15.03. Found: C, 50.89; H, 3.96;
N, 15.83; Ag, 15.24%. IR (KBr, cmꢀ1): 1384 (N@N),
1582 (C@N).
All other complexes were prepared following an
identical procedure and yields were 75–90%. Anal. Calc.
for C28H24N8ClO4 Ag (3a): C, 49.35; H, 3.91; N, 15.78;
Ag, 16.15. Found: C, 49.46; H, 3.59; N, 16.49; Ag,
15.87%. IR (KBr, cmꢀ1): 1386 (N@N), 1585 (C@N).
Anal. Calc. For C40H32N8ClO4Ag (3c): C, 57.60; H,
3.78; N, 13.39; Ag, 13.11. Found: C, 57.74; H, 3.85; N,
13.47; Ag, 13.25%. IR (KBr, cmꢀ1): 1381 (N@N), 1590
(C@N). Anal. Calc. For C28H24N8ClO4 Ag (4a): C,
49.39; H, 3.51; N, 16.43; Ag, 15.69. Found: C, 49.46; H,
3.59; N, 16.49; Ag, 15.87%. IR (KBr, cmꢀ1): 1382
(N@N), 1587 (C@N). Anal. Calc. for C30H28N8ClO4Ag
(4b): C, 50.80; H, 3.90; N, 15.73; Ag, 15.84. Found: C,
50.89; H, 3.96; N, 15.83; Ag, 15.24%. IR (KBr, cmꢀ1):
1384 (N@N), 1582 (C@N). Anal. Calc. for
C40H32N8ClO4Ag (4c): C, 57.69; H, 3.80; N, 13.43; Ag,
13.02. Found: C, 57.74; H, 3.85; N, 13.47; Ag, 13.25%.
IR (KBr, cmꢀ1): 1381 (N@N), 1585 (C@N).
interactions has been of great interest in recent years
[29,30]. Presence of suitable oxo counter ions (e.g.
NO3ꢀ, ClO4ꢀ, CF3SO3ꢀ, etc) in the complexes may re-
sult in secondary interactions, C–Hꢁ ꢁ ꢁO and act as a
determining factor for crystal packing and helicity [31].
Ag(I)-azoimidazole complexes have been reported by
a different researcher [32–36]. In this work, we have se-
lected closely related ligands, 2-(naphthyl-(a=bÞ-
azo)imidazole (a=b-NaiR) (Scheme 1) which provide
larger steric crowding and better electron donating
ability than a conventional aryl group. In this work we
have explored the coordination chemistry of Ag(I)-
naphthylazoimidazoles. Synthesis, spectral, electro-
chemical studies have been carried out and a single
crystal X-ray structure of Ag(I)-naphthylazoimidazoles
are described.
2. Experimental
2.1. Materials
1-Alkyl-2-(naphthyl-(a=bÞ-azo)imidazoles (a-NaiR
(1), b-NaiR (2)) were synthesized by a reported proce-
dure [10,12]. AgNO3 and a-/b-naphthylamines were
obtained from E. Merck, Germany and NaClO4 was
obtained from Fluka, USA. Purification of solvents was
performed as in the reported procedure [12]. All other
solvents and chemicals were of reagent grade and were
used without further purification.
2.2. Physical measurements
2.4. Crystallographic data collection and refinement
Microanalytical (C, H, N) data were collected using a
Perkin–Elmer 2400 elemental analyzer. UV–Vis spectra
were recorded by a JASCO V-570 UV/Vis/NIR spec-
trophotometer. IR data were collected from a JASCO-
420 FTIR (KBr disk, 4000–200 cmꢀ1). 1H NMR spectra
were collected in CDCl3 using a 500 MHz FT-NMR
spectrometer. Electrical conductivity was measured us-
ing a Systronics 305 digital conductivity meter with a
solution concentration of ꢂ10ꢀ3 M cmꢀ3. Silver was
analyzed gravimetrically [37]. Electrochemical measure-
ments were carried out under dinitrogen conditions with
A crystal suitable for X-ray diffraction study was
obtained after several trials by slow diffusion of a
CH2Cl2 solution of the complex into a hexane layer. A
suitable single crystal of complex [Ag(a-NaiEt)2](ClO4)
(3b) was mounted on a Siemens CCD diffractometer
equipped with graphite monochromated Mo Ka (k ¼
ꢀ
0:71073 A) radiation. The intensity data were corrected
for Lorentz and polarization effects. Details of data
collection and structure solution are given in Table 1.
The structure was solved by Patterson syntheses and

J. Dinda et al. / Polyhedron 23 (2004) 793–800
795
Table 1
Summarized crystallography data for [Ag(a-NaiEt)2][ClO4] (3b)
sized by the reported procedure [10–12]. The ligands are
an unsymmetric bidentate N,N0 chelating type.
Crystal parameters
These ligands in MeOH–CHCl3 mixture react with
AgNO3in MeOH and the complexes have been isolated
as their perchlorate salts. They were purified by crys-
tallization by diffusion of CH2Cl2 solution of the
complex into hexane. The complexes are highly photo-
sensitive and decomposed with separation of black
metallic silver from solution upon exposure to light. The
complexes are characterized by elemental analyses, IR,
UV–Vis (Table 2), 1H NMR (Table 3) data and support
the formulation [Ag(a-/b-NaiR)2][ClO4] (3, 4). The
complexes are diamagnetic and 1:1 electrolytes in ni-
tromethane. The structural configuration has been de-
termined by a single crystal X-ray diffraction study of
[Ag (a-NaiEt)2][ClO4] (3b).
Empirical formula
Formula weight
Crystal system
Space group
C30H28N8ClO4Ag
707.9
tetragonal
P-4n2
16.376(4)
16.376(4)
22.971(8)
90
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
90
90
3
ꢀ
V (A )
6160(3)
0.71073
1.52
ꢀ
k (A)
qcalc (Mg mꢀ3
)
Z
T (K)
8
293(2)
0.791
37,885
7091
l(Mo Ka) (mmꢀ1
Total data
)
3.2. Spectral properties
Unique data
Observed data (I > 2rðIÞ)
2938
397
The complexes [Ag(NaiR)2]þ exhibit characteristic IR
transmissions for m(N@N) and m(C@N) at around 1382
and 1485 cmꢀ1. The free ligand values are 1410 and 1600
cmꢀ1, respectively. The observation is interesting. The m
(C@N) is red-shifted by >100 cmꢀ1. This supports very
strong interaction of Agþ with imidazolic-N compared
to azo-N. This is also confirmed by bond length data
(Table 4). The perchlorate (ClO4ꢀ) exhibits bands at
1147, 1108 and 1090 cmꢀ1 along with a weak band at
620–625 cmꢀ1. The splitting of m(ClO4ꢀ) supports re-
duction in symmetry, which may be due to some hy-
drogen bonding interaction. This is observed, indeed
(vide infra).
Refined parameters
R
0.0394
0.0920
1.00
wR2
Goodnees-of-fit
P
P
P
P
2
2
2
2 2 1=2
R¼ jjFojꢀjFcjj= jFoj. wR2 ¼½ wðjFoj ꢀjFcj Þ = wðjFoj Þ ꢄ
,
2
w¼1=½r2ðFo2Þ þ ð0:0299PÞ ꢄ; P ¼ðFo2 þ 2Fc2Þ=3. Goodness-of-fit (GOF)
1=2
is defined as ½wðFo ꢀ FcÞ=ðn0 ꢀ nvÞꢄ
where n0 and nv denote the
number of data and variables, respectively.
followed by successive Fourier and difference Fourier
syntheses. Full-matrix least-squares refinements on F 2
were carried out using SHELXL-97 with anisotropic
displacement parameters for all non-hydrogen atoms.
Hydrogen atoms were constrained to ride on the re-
spective carbon atoms with an isotropic displacement
parameter equal to 1.2 times the equivalent isotropic
displacement parameter of their parent atom. Since the
complex was crystallized in a non-centrosymmetric
space group, the absolute configuration was determined
(Flack ꢃ parameter ¼ )0.00(3)) [38]. Complex neutral
atom scattering factors were used throughout this pro-
gram. All calculations were carried out using SHELXS 86
[39] SHELXL 97 [40], PLATON 99 and ORTEP-3 [41,42]
programs.
Solution electronic spectra of the complexes were
recorded in dichloromethane in the range of 600–200
nm. Bathochromic shift relative to the free ligand indi-
cates interaction between Agþ and the ligand (Table 2).
The silver(I) complexes exhibit the absorption at ca. 400
nm and are assigned to intraligand charge transitions.
1
The ligands and complexes show highly resolved H
NMR spectra in CDCl3 at 298 K. 1-Alkyl-2-(naphthy-
lazo)imidazoles (NaiR) have been characterized previ-
ously [10,12] and serve as guides to determine the
structure of the silver(I) complexes. The spectral data
are collected in Table 3. The 1-alkyl groups in
[Ag(NaiR)2]þ show significant shifting downfield by
0.1– 0.2 ppm from that of the free ligand values [10–12].
A significant difference is observed in the resonance
signal of imidazole protons 4- and 5-H from that of
naphthyl protons. Imidazole protons 4- and 5-H appear
upfield (6.9–7.2 ppm) and naphthyl protons (8-H–15-H)
resonate downfield, 7.5–8.0 ppm. In [Ag(NaiR)2]þ the
signal pattern shows that imidazole protons are shifted
to the downfield side by 0.4–0.5 ppm and naphthyl
protons are shifted to upfield side except 9-H in a-NaiR
and 8, 10-H in b-NaiR compared to the free ligand
values.
3. Results and discussion
3.1. Synthesis
The two classes of ligands used in the present study
belong to the azoimine (–N@N–C@N–) family. 1-Alkyl-
2-(naphthyl-a-azo)imidazoles are abbreviated as
a-NaiMe (1a), a-NaiEt (1b), a-NaiBz (1c); 1-alkyl-2-
(naphthyl-b-azo)imidazoles are abbreviated as b-NaiMe
(2a), b-NaiEt (2b) and b-NaiBz (2c). They are synthe-

796
J. Dinda et al. / Polyhedron 23 (2004) 793–800
Table 2
Electronic spectroscopica and cyclic voltammetricb;c data
c
Compound
UV–Vis spectral data
max, nm (10ꢀ3 e, Mꢀ1 cmꢀ1
Ligand reductions
azo/azoꢀ
Redox data
k
)
d
e
E0 (V)
Es (V)
3a
3b
3c
4a
4b
4c
480 (1555)f , 434 (2011), 418 (9305), 384 (10,043)
472 (1240)f , 436 (1665), 354 (7450)
)0.087
)0.093
)0.088
)0.081
)0.085
)0.091
0.503
0.495
0.505
0.462
0.449
0.476
)0.551
)0.569
)0.578
)0.679
)0.761
)0.881
484 (1465)f , 436 (1911), 354 (8940)
414 (1675)f , 400 (2430), 378 (9450)
416 (2248)f , 400 (3260), 384 (10,936)
414 (1765)f , 402 (2050), 384 (9467), 378 (9840)
a Solvent CH2Cl2.
b Solvent MeCN, working electrode Pt-disk, reference SCE, supporting electrolyte [nBu4N][ClO4] (0.1 M), solute concentration ꢂ10ꢀ3 M, scan rate
50 mV, potential is expressed in V, Epa stands for anodic peak potential.
c Epc cathodic peak potential in V and ipa/ipc lies between 0.5 and 0.6.
d Threshold potential to reduction of Ag(I) ! Ag(0) on the electrode surface.
e Anodic stripping potential for Ag(0) ! Ag(I).
f Shoulder.
Table 3
1H NMR data recorded in CDCl3 at 298K
Compound
d (ppm (J, Hz))
c
d
4-Ha
5-Ha
8-Ha
9-Hb
10-H
11-Hc
12-H
13-Hd
14-Hc
15-Hb
N-R
(3a)
(3b)
(3c)
(4a)
(4b)
7.58
7.61
7.65
7.55
7.58
7.52
7.59
7.63
7.45
7.50
8.44 (7.10) 7.77 (7.0)
7.46 (7.0)
7.44 (7.2)
7.46 (7.2)
7.67 (7.5)
7.65 (7.5)
7.08
7.04
7.12
7.50
7.49
7.08
7.04
7.12
7.42
7.41
7.19 (7.2)
7.15 (7.2)
7.23 (7.5)
7.42 (7.5)
7.41 (7.2)
7.04 (7.5)
6.91 (7.5)
7.06 (7.8)
7.62 (7.5)
7.60 (7.8)
4.13e
4.44f (12.0)
5.67g
4.11e
4.35f
(12.0)
8.44 (8.1)
8.45 (8.4)
7.80 (7.5)
7.82 (7.5)
7.78 (8.1)
7.75 (8.1)
8.03
8.04
g
(4c)
7.63
7.55
8.08
7.83 (8.4)
7.71 (8.1)
7.5
7.47
7.47 (8.1)
7.69 (7.8)
5.74
a Singlet broad.
b Doublet.
c Triplet.
d Multiplet.
e 1-CH3:
f Quartet for –CH2– and d(CH2)–CH3: 5b, 1.51(7.0)b; 6b, 1.47(7.0)b ppm.
g 1-CH2(Ph) and d(Ph): 7.4–7.5 ppm.
Table 4
linear with a further two weak interactions to azo-N of
the azoimine group. The N@N distances are equivalent
ꢀ
Selected bond distances (A) and angles (°) for [Ag(a-NaiEt)2](ClO4)
(3b) along with their esd in parentheses
ꢀ
in the two azoimine chelates (1.266(5), 1.265(5) A). Free
ꢀ
Bond distances (A)
Bond angles (°)
ligand bond parameters are available for 1-ethyl-2-
(naphthyl-azo)imidazolium hexafluorophosphate, [a-
NaiEtH]þ[PFꢀ6 ] [43] and the N–N distance is 1.267(3)
Ag–N(1)
Ag–N(5)
2.124(5)
N(1)–Ag–N(4)
N(5)–Ag–N(8)
N(1)–Ag–N(5)
N(4)–Ag–N(8)
N(3)–N(4)–Ag
N(7)–N(8)–Ag
C(3)–N(1)–Ag
C(18)–N(5)–Ag
65.45(2)
64.86(2)
174.54(2)
74.88(1)
106.6(3)
106.6(4)
122.8(4)
123.8(5)
2.128(5)
2.851(5)
2.859(5)
1.266(5)
1.265(5)
1.360(7)
1.333(7)
1.310(7)
1.369(7)
Ag–N(4)
Ag–N(8)
ꢀ
A. The N–N distance in 1-methyl-2-(phenylazo)imi-
ꢀ
dazolium perchlorate is 1.250(1) A [44]. The elongation
N(3)–N(4)
N(7)–N(8)
C (3)–N(3)
C (3)–N(1)
C (18)–N(5)
C (18)–N(7)
is likely to be a result of a high steric crowding provided
by the naphthyl group relative to the phenyl group. This
may be due to better r-donor character of the ligand
compared to the aryl group. The Ag–N distances are as
follows: Ag–N (imidazole) [Ag–N(1), 2.124(5); Ag–N(5),
ꢀ
ꢀ
2.128(5) A]. The distance is shortened by >0.7 A than
that of the proposed Ag–N(azo) distance [Ag–N(4),
3.3. Molecular structure
molecular view of the complex [Ag(a-Na-
iEt)2][ClO4] (3b) is shown in Fig. 1 and the bond pa-
rameters are given in Table 4. A review of the bond
lengths and angles shows that the complex is essentially
ꢀ
2.851(5); Ag–N(8), 2.859(5) A]. This accounts for the
A
argentophilicity [17,18] of imidazole-N compared to
azo-N donor centers. On considering the weak interac-
tion of Ag(I) with the azo-N leading to the chelated
structure, the proposed chelate angles are: N(1)–Ag–

J. Dinda et al. / Polyhedron 23 (2004) 793–800
797
Fig. 2. Dimensions of chelate rings of (a) Ag(a-NaiEt)2þ and (b)
Ag(papm)þ.
reduction in r-donation capacity of a-NaiEt with re-
spect to papm. The naphthyl group is a better electron
donor, and provides higher steric strain relative to the
phenyl ring and that plays a crucial role to monitor the
r-/p-acidity of the azo function [46]. The p-acidity of
naphthyl < phenyl as well as imidazole < pyrimidine [8].
This may support the shortening of the Ag–N(4) bond in
Ag-(papm) [28] compared to the present silver(I) com-
plex. The same argument cannot explain the shortening
Fig. 1. X-ray crystal structure of [Ag(a-NaiEt)2] (ClO4) (3b).
ꢀ
of bond length (by ꢂ0.15 A) of Ag–N(1) (Ag–N (het-
N(4), 65.45(2)° and N(5)–Ag–N(8), 64.86(2)°. This ex-
ample bears the lowest chelate bite angles so far re-
ported in the azoimine family [9–12]. This smallest
chelate angle may lead to distortion in the tetrahedral
geometry. The naphthyl ring is no longer planar with the
chelate plane. The plane-1 (Ag–N(1)–C(3)–N(3)–N(4))
inclines at an angle of 23.4(2)° with the a-naphthyl
group whereas plane-2 (Ag–N(5)–C(18)–N(7)–N(8))
makes an angle of 49.89(2)°. Thus, the two naphthyl
groups are skewed by 18.9(2)°. The bulky size of the
naphthyl groups may provide high steric interaction and
may be responsible for the unusual distortion in geom-
etry. The strain is manifested in stereochemical change
from tetrahedral to linear symmetry. This has led us to
conclude that the structure may be described as a {2+2}
distorted linear geometry tending to tetrahedral sym-
metry. Usually macrocycles are described by [m+n] no-
menclature. For brevity we use the {2+2} symbol to
designate bi-coordinated linear geometry with some
distortion towards tetrahedral symmetry.
erocycle)) in Ag(a-NaiEt)2þ relative to that of Ag-
(papm). The strain that is developed due to replacement
of pyrimidine by imidazole has been released by the
elongation of the Ag–N(azo) bond length in the Ag(a-
NaiEt)2þ system compared to that of the Ag-(papm)
complex. However, the –N@N– bond length is unusu-
ally shortened in Ag-papm, which may be due to the
helical network and triangular Ag(N)3 coordination in
the complex. The phenomena is more likely to be a re-
sult of bond alteration in Ag(papm) vs. a bond delo-
calization in Ag(a-NaiEt) in C3–N3–N4 bond lengths,
probably caused by electronic effects.
The packing view of the crystal structure shows
(Fig. 3) that Ag(a-NaiEt)2þ contains an intramolecular
and intermolecular H-bonded 1D helical framework.
Intramolecular H-bonding is playing between the azo-N
and a-H of the naphthyl group [N(4)ꢁ ꢁ ꢁH(12), 2.477(8);
ꢀ
N(8)ꢁ ꢁ ꢁH(27), 2.531(3) A]. This enhances the acidity of
the C(12)–H(12)/C(27)–H(27) group leading to activa-
2ꢀ
tion of this bond. This is observed by adding PdCl4 to
1-Ethyl-2-(naphthyl-a-azo)imidazoles (a-NaiEt) and
2-(phenylazo)pyrimidines (papm) [45] have an azoimine,
–N@N–C@N–, chelating function with an inbuilt imine
(–C@N–) group in a heterocyclic back bone. Imidazole
is a five-membered meta related two-N heterocycle while
pyrimidine is a six-membered two-N heterocycle system.
The replacement of a six-membered heterocycle by a five
membered one reduces both the r-donor and p-acceptor
character of the system [8] and also introduces angular
strain in the chelate ring [2]. This is reflected in the
comparison of chelate ring structures of Ag(I)-naph-
thylazoimidazole and Ag(I)-2-(phenylazo)pyrimidine
(Ag(papm) system [28]) molecules (Fig. 2). The elonga-
the solution of this complex and the cyclopalladated
complex has been synthesized by a transmetallation re-
action followed by the C–H activation process [12]. In-
termolecular H-bonds operate between the two
ꢀ
tion of the Ag–N (azo) [Ag–N(4)] distance by ꢂ0.3 A in
Fig. 3. Helical 1D polymer showing intramolecular and intermolecular
H-bonding.
Ag(-NaiEt) compared to Ag(papm) is a support of the

798
J. Dinda et al. / Polyhedron 23 (2004) 793–800
Table 5
ꢀ
Hydrogen bonding and weak interaction geometry (A, °), H-bonding interaction
ꢀ
D–H (A)
ꢀ
ꢀ
D–Hꢁ ꢁ ꢁA
Hꢁ ꢁ ꢁA (A)
Dꢁ ꢁ ꢁA (A)
\D-Hꢁ ꢁ ꢁA (°)
C12–H12ꢁ ꢁ ꢁN4
C27–H27ꢁ ꢁ ꢁN8
C15–H15ꢁ ꢁ ꢁO1
C1–H1ꢁ ꢁ ꢁO3
0.93
0.93
0.93
0.93
0.93
2.478
2.521
2.554
2.440
2.568
2.812(9)
2.835(8)
3.461(14)
3.199(13)
3.314(12)
101.3
100.1
165.5
138.7
137.8
C16–H16ꢁ ꢁ ꢁO4
C–Hꢁ ꢁ ꢁp interaction
X–Hꢁ ꢁ ꢁCg(J)
Hꢁ ꢁ ꢁCg
H-Perp
3.332
\X–Hꢁ ꢁ ꢁCg
Xꢁ ꢁ ꢁCg
C14–H14ꢁ ꢁ ꢁCg(1)
3.365
81.97
3.363(13)
p–p Interaction
Cg(I)–Cg(J)
Cg(1)–Cg(4)
Cg(2)–Cg(6)
Cg(3)–Cg(5)
Cg–Cg
Dihedral (I,J ring)
Perp. From CgI to J
3.8404(52)
3.7900(40)
3.9209(44)
0.71
2.40
19.38
3.307
3.261
3.842
Cg(I) ! centroid of ring I. Ring(1) ! N1–C1–C2–N2–C3; Ring(2) ! N5–C16–C17–N6–C18; Ring(3) ! C6–C7–C8–C9–C10–C11; Ring(4)
!C10–C11–C12–C13–C14–C15; Ring(5) ! C21–C22–C23–C24–C25–C26; Ring(6) ! C25–C26–C27–C28–C29–C30.
fragments around the azo group. The C–H group of the
imidazole moiety ortho to metal coordinated-N (imid-
azole) [C(1)–H(1)/C(16)–H(16)] develops an electrostatic
interaction with perchlorate-O(1)/O(4) while naphthyl
C(15)–H(15) gives a H-bonding interaction with the
second perchlorate ion. In addition to these, the com-
plex experiences C–H–O as well as face-to-face p–p in-
teractions (Table 5).
þ
Fig. 4. HOMO, LUMO, HOMO-1 and LUMO+1 of Ag(a-NaiEt)2
.

J. Dinda et al. / Polyhedron 23 (2004) 793–800
799
Table 6
Relative percent atomic contribution to MOs of the complex Ag[a-NaiEt]2þ
MO
Energy (eV)
Ag
N(imidazole)
Azo(N@N)
Naphthyl ring
Percentage of contribution
92 (HOMO-1)
91 (HOMO)
)11.575
)11.491
)10.806
)10.783
5.5
2
6.6
3.3
78.9
77
6.6
7.7
90 (LUMO)
89 (LUMO+1)
0
21.4
24.5
52.5
55.31
25.5
20.2
0
3.4. Redox studies
contribution, it is mainly ligand orbital, 99.4% [52.5%
azo (N@N), 25.5% naphthyl group and 21.4% N(imid-
azole)]. Other low-lying orbitals (HOMO-1, HOMO-2)
show a greater metal contribution but other higher en-
ergy orbitals (UMOs) show no metal contribution. It can
be concluded that there should be no M ! Ligand charge
transfer and this happens indeed. The complexes show
only the interligand (between imidazole and naphthyl
group) charge transition in the lower wavelength region.
Because of the weak M–L interaction, Agþ is quickly
separated from the ligands claw under the electrode field
potential and has been deposited on the electrode sur-
face. However, Cu(I)-complexes show somewhat differ-
ent results because of the higher contribution of
copper(I) to the HOMO and other occupied MOs [48].
The electrochemical properties of the complexes were
examined by cyclic voltammetry in MeCN using
[nBu4N][ClO4] as supporting electrolyte at the scan rate
50 mV sꢀ1. The complexes of silver exhibit an irrevers-
ible oxidation–reduction response with the Pt-disk
working electrode and saturated calomel electrode
(SCE) as reference electrode. The complexes undergo
reduction processes, which, given the redox non-inno-
cence of the azoimine group [10], are assigned to ligand-
centred-processes. The quasireversibility of the reductive
process is observed from the peak separation (DE > 130
mV) and the reduction is referred to the accommodation
of an electron to the azo function dominated orbital of
the bound ligand. Such processes, coupled to chemical
complications for the reduction of Ag(I) on the electrode
surface, result in that the current ratio ipa/ipc is always
less than unity (ipa/ipc lies between 0.5 and 0.6). In this
connection, the presence of anodic stripping peaks in the
backscan (Epaꢂ+0.5 V, Table 2) looks likely to testify
for reoxidation of electro-deposed silver metal. This
means that the reduction processes more than likely
trigger release of silver ion with its consequent reduction
to silver metal [47]. The reoxidation of Ag(0) ! Ag(I) is
associated with very high anodic current at positive to
SCE (stripping potential ꢂ+0.5 V) which on repeat scan
progressively increases the current. Besides, the ap-
proach to more negative potential followed by backscan
increases the peak current height. The increase in po-
tential gradient enhances the reaction rate which may
cause deposition of larger amounts of Ag on the surface.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 172833 for bis-[1-ethyl-2-
(naphthyl-a-azo)imidazole]silver(I) perchlorate. Copies
of this information may be obtained free of charge from
the Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336033; E-mail: deposit
@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
Financial support from the Department of Science
and Technology is gratefully acknowledged. One of us
(GM) is thankful to the National Science Council,
ROC for support under grants NSC 89-2811-M-007-
0086 and NSC 89-2112-M-007-083. We are also
thankful to Prof. T. -H. Lu, Department of Physics,
National Tsing Hua University, Taiwan, ROC for his
help.
3.5. EHMO calculation and correlation with spectral and
electrochemical properties
In order to highlight the electronic properties of sil-
ver(I)-naphthylazoimidazoles, an MO calculation has
been performed in the framework of extended Huckel
formalism. Crystallographic data of 3b are used for the
EHMO calculation. The MO pictures are shown in Fig. 4
and the percentage contribution of participating frag-
ment/atomic functions to the MOs are given in Table 6.
The HOMO(91) is contributed from 2% silver and 88%
[77% azo (N@N), 7.7% naphthyl group and 3.3%
N(imidazole)] ligand orbital. The LUMO has no metal
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