Mono- and binuclear cobalt(II)-azido complexes of arylazoimidazole: Synthesis, spectral characterization, electrochemistry and crystal structure

Article abstract of DOI:10.1016/S0277-5387(03)00347-4

Upon setting up proper reaction conditions 1-alkyl-2-(arylazo)imidazoles (RaaiR′ where R = H (a), Me (b); R′= Me (1/3/5), Et (2/4/6) may react with Co(OAc)₂·4H₂O and NaN₃ in methanolic solution to give two classes of azido complexes: mononuclear bischelated [Co(RaaiR′)₂(N₃)₂] (3,4) and binuclear μ-(1,1) azido bridged mono-chelated [Co(RaaiR′)(μ-(1)N₃)(μ-(1,1)N₃]₂ (5,6) complexes. They are characterized by UV-Vis, IR spectra, room temperature magnetism and they have been structurally confirmed by X-ray diffraction study of [Co(MeaaiMe)₂(N₃)₂] (3b) and [Co{(μ-1)N₃}(MeaaiMe)]₂ (5b). The mononuclear complex (3b) is distorted octahedral whereas the binuclear azide bridged complex (5b) has the distorted square pyramidal geometry around the cobalt(II) centre. Redox properties of the complexes are examined by cyclic voltammetry and show a high potential Co(III)/Co(II) couple along with ligand reductions. EHMO calculation and comparison between the two types of complexes explain the spectral and redox properties of the complexes.

Full text of DOI:10.1016/S0277-5387(03)00347-4

Polyhedron 22 (2003) 2587Á2594
/
www.elsevier.com/locate/poly
Mono- and binuclear cobalt(II)-azido complexes of arylazoimidazole:
synthesis, spectral characterization, electrochemistry and crystal
structure
Umasankar Ray a, Brojogopal Chand a, Golam Mostafa b, Jack Cheng b,
Tian-Huey Lu b, Chittaranjan Sinha a,*
a Department of Chemistry, The University of Burdwan, Burdwan 713104, India
b Department of Physics, National Tsing-Hua University, Hsinchu 30043, Taiwan, ROC
Received 7 February 2003; accepted 17 April 2003
Abstract
Upon setting up proper reaction conditions 1-alkyl-2-(arylazo)imidazoles (RaaiR? where Rꢀ
(2/4/6) may react with Co(OAc)2×4H2O and NaN3 in methanolic solution to give two classes of azido complexes: mononuclear bis-
chelated [Co(RaaiR?)2(N3)2] (3,4) and binuclear m-(1,1) azido bridged mono-chelated [Co(RaaiR?)(m-(1)N3)(m-(1,1)N3]2 (5,6)
complexes. They are characterized by UVÁVis, IR spectra, room temperature magnetism and they have been structurally confirmed
/
H (a), Me (b); R?ꢀMe (1/3/5), Et
/
/
/
by X-ray diffraction study of [Co(MeaaiMe)2(N3)2] (3b) and [Co{(m-1)N3}(MeaaiMe)]2 (5b). The mononuclear complex (3b) is
distorted octahedral whereas the binuclear azide bridged complex (5b) has the distorted square pyramidal geometry around the
cobalt(II) centre. Redox properties of the complexes are examined by cyclic voltammetry and show a high potential Co(III)/Co(II)
couple along with ligand reductions. EHMO calculation and comparison between the two types of complexes explain the spectral
and redox properties of the complexes.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Arylazoimidazoles; Cobalt(II) complexes; EO azido bridged complexes; X-ray structures
1. Introduction
heterocycle ring size [20]. In the scope of our continuous
research in this field we have extended our research to
design a new series of azoheterocycles in the imidazole
backbone. Imidazole and related compounds have been
widely used in chemistry and biology due to their
appearance as such in biomolecules, metalloproteins
Interest in the transition metal coordination chemistry
of the azoimine function (ꢀ
/
Nꢁ
/
Nꢀ
/
Cꢁ
/
Nꢀ
/
) is rapidly
expanding in several areas ranging from organometallic
chemistry, metal assisted organic transformation, radi-
cal chemistry, stabilization of low valent metal oxidation
[21Á23] and their versatility in the production of
/
state to DNA labelling and anticancer medicine [1Á
Incorporation of the arylazo group (Arꢀ N) into
/19].
polymetallic complexes of magneto-chemical interest
[24,25]. We have taken a strategy to incorporate an
/N
ꢁ ꢂ
/
Arꢀ/N
ꢁ ꢂ
/N imidazole backbone which leads to the
backbone of a N-heterocycle at the ortho position has
lead to the synthesis of arylazoheterocycles which have
an azoimine function. The activity of the group is
dependent on the nature of substituents, number of
heteroatoms, and ring size of the heterocycle. The most
significant change has been observed on changing the
synthesis of 2-(arylazo)imidazoles. Bridge forming abil-
ity of imidazole has been eliminated upon alkylation and
we have synthesized 1-alkyl-2-(arylazo)imidazoles (ab-
breviated RaaiR?) [26,27]. The present work stems from
our continuous interest to explore transition metal
chemistry of RaaiR? [26Á33]. Herein we describe
/
mono- and binuclear cobalt(II)-azide complexes of
RaaiR?. Synthesis, structures and electrochemistry are
described herewith.
* Corresponding author. Tel.: ꢁ
253-0452.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
/
91-342-255-7683; fax: ꢁ91-342-
/
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00347-4

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U. Ray et al. / Polyhedron 22 (2003) 2587Á2594
/
2. Experimental
4.01; N, 37.99; Co, 11.04. Calc. for C20H20N14Co: C,
46.60; H, 3.88; N, 38.06; Co, 11.44%.
[Co(HaaiEt)2(N3)2](4a) Anal. Found: C, 48.71; H,
2.1. Materials
4.20; N, 36.08; Co, 11.12. Calc. for C22H24N14Co: C,
10.85%.
Co(OAc)2×/4H2O and NaN3 were purchased from
48.62;
H,
4.42;
N,
36.10;
Co,
Loba chemicals, Bombay, India and 1-alkyl-2-aryla-
zoimdazoles (RaaiR?) were prepared following the
literature procedures [31]. All other chemicals, solvents
are reagent grade as received. Solvents are used after
drying.
[Co(MeaaiEt)2(N3)2] (4b): Anal. Found: C, 50.38; H,
2.56; N, 34.39; Co, 10.19. Calc. For C24H28N14Co: C,
50.44; H, 2.45; N, 34.32; Co, 10.32%.
2.4. Preparation of binuclear azido bridge complexes,
[Co(MeaaiMe){m-(1)N3}{m-(1,1)N3}]2 (5b)
2.2. Physical measurements
NaN3 (0.065 g, 1 mmol) in MeOH (10 ml) was added
4H2O (0.062
Microanalyses (C, H, N) were performed using a
dropwise to a stirred solution of Co(OAc)2×
/
PerkinÁ
troscopic measurements were carried out using the
following instruments: UVÁVis spectra, JASCO UVÁ
VisÁNIR model V-570; IR spectra (KBr disk, 4000Á200
/Elmer 2400 CHNO/S elemental analyzer. Spec-
g, 0.25 mmol) in MeOH (10 ml) at 298 K. The brown
solution was stirred for 15 min and MeaaiMe (1b) (0.05
g, 0.25 mmol) in MeOH (10 ml) was added. The color of
the solution became dark brown. The solution was
filtered and then left undisturbed for a week. The dark
brown crystal of compound (5b) was obtained. It was
then washed with water, methanol and ether. Finally, it
was dried in vacuo. Yield was 0.128 g, (75%). The
microanalytical data of the complex is as follows,
/
/
/
/
cmꢂ1), JASCO FT-IR model 420 spectrophotometers.
Room temperature magnetic moment was measured
using a Vibronic sample magnetometer. Molar conduc-
tances (LM) were measured in a Systronics conductivity
meter 304 model using ca. 10ꢂ3 M solutions in MeOH.
Electrochemical measurements were carried out with the
use of a computer controlled EG & G PARC VersaStat
model 250 electrochemical instrument using a Pt-disk
working electrode in acetonitrile at 298 K. The solution
was IR compensated and the results were collected at
298 K. The reported results are referenced to a Ag/AgCl,
Clꢂ electrode in the presence of [nBu4N][ClO4] as
supporting electyrolyte and are uncorrected for junction
potential. Estimation of cobalt was carried out by the
complexometric titration method [34].
[Co(MeaaiMe){m-(1)N3}{m-(1,1)N3}]2
(5b):
Anal.
Found: C, 38.37; H, 3.56; N, 40.89; Co, 17.07. Calc.
for C22H24N20Co2: C, 38.49; H, 3.49; N, 40.82; Co,
17.18%.
All other complexes in this series were prepared by the
same procedure. The yield was varied from 70Á75% and
/
microanalytical data of the complexes are as follows.
[Co(HaaiMe){m-(1)N3}{m-(1,1)N3}]2 (5a): Anal. Found:
C, 36.53; H, 3.10; N, 42.65; Co, 17.12. Calc. for
C20H20N20Co2: C, 36.48; H, 3.04; N, 42.56; Co,
17.12%. [Co(HaaiEt){m-(1)N3}{m-(1,1)N3}]2 (6a): Anal.
Found: C, 38.63; H, 3.54; N, 41.02; Co, 17.13. Calc. for
C22H24N20Co2: C, 38.49; H, 3.49; N, 40.82; Co, 17.18%.
[Co(MeaaiEt){m-(1)N3}{m-(1,1)N3}]2 (6b): Anal. Found:
C, 40.45; H, 4.01; N, 39.38; Co, 16.72. Calc. For
C24H28N20Co2: C, 40.34; H, 3.92; N, 39.22; Co, 16.51%.
2.3. Preparation of mononuclear azido complexes,
Co(MeaaiMe)2(N3)2 (3b)
1-Methyl-2-(p-tolylazo)imidazole (MeaaiMe) (0.1 g,
0.50 mmol) in MeOH (10 ml) was added dropwise to a
stirred solution of Co(OAc)2×
/
4H2O (0.062 g, 0.25 mmol)
in MeOH (10 ml) at 298 K. The brown solution was
stirred for 15 min followed by the addition of NaN3
(0.03 g, 0.50 mmol) in MeOH (10 ml). The brown color
of the solution became intensified. The solution was
filtered and then left undisturbed for a week. A dark
brown crystal was obtained. It was washed with water,
methanol and ether. Finally, it was dried in vacuo. Yield
was 0.108 g, (80%). The microanalytical data of the
complex is as follows, [Co(MeaaiMe)2(N3)2] (3b): Anal.
Found: C, 48.81; H, 4.57; N, 36.01; Co, 11.25. Calc. for
C22H24N14Co: C, 48.62; H, 4.42; N, 36.10; Co, 10.85%.
All other complexes were prepared by the same
procedure. In all cases, crystalline products were ob-
2.5. X-ray crystal structure analyses
Data were collected with a Siemens SMART CCD
diffractometer using graphite-monochromatized Mo Ka
˚
0.71073 A) at 293 K for 3b and 295 K for
radiation (lꢀ
/
5b. Unit cell parameters were determined from least-
squares refinement of setting angles of 2848 reflections
for 3b and 1956 reflections for 5b with 2u in the range
4.25
/
2u 5
/
558 and 4.45
/
2u 556.68, respectively. A
/
summary of the crystallographic data and structure
refinement parameters are given in Table 1. 15 955
reflections were collected in the range 4.25
with 5744 unique reflections for 3b and 8548 reflections
were collected 4.452u 556.68 with 3257 unique reflec-
/
2u 5558
/
tained. The yield varied from 75Á
/80% and microanaly-
/
/
tical data of the complexes are as follows.
[Co(HaaiMe)2(N3)2] (3a): Anal. Found: C, 46.75; H,
tions for 5b. Data were corrected for Lorentz polariza-
tion effects and for linear decay. Empirical absorption

U. Ray et al. / Polyhedron 22 (2003) 2587Á
/2594
2589
Table 1
Summarised crystallographic data for [Co(MeaaiMe)2(N3)2] (3b) and
[Co(MeaaiMe)(N3)2(1,1-m)]2] (5b)
[Co(MeaaiMe)2(N3)2]
(3b)
[Co(N3)2(1,1-
m)(MeaaiMe)]2 (5b)
Empirical for-
mula
C22H24N14Co
C22H24N20Co2
Formula weight 543.48
Temperature (K) 293
686.47
295
Crystal system
Space group
monoclinic
P21/c
monoclinic
P21/n
Crystal size (mm) 0.27ꢃ/0.20ꢃ/0.14
0.25ꢃ
/
0.20ꢃ0.13
/
Unit cell dimen-
sions
The versatility of azido ligand (N3ꢂ) is well estab-
lished. It can act as monodentate m-(1) [35] and
˚
a (A)
7.5873(5)
7.9633(9)
15.4677(17)
11.8266(13)
91.55(1)
1456.2(3)
2
˚
b (A)
16.9677(12)
19.9773(14)
100.499(2)
2528.8(3)
4
˚
c (A)
bidentate bridging m-(1,1)×
/
(EO), m-(1,3)×
/
(EE), m-(1,1,1),
b (8)
3
˚
V (A)
m-(1,1,3), alternating m-(1,1) and m-(1,3) type [36Á
/41].
Z
Rational control in the construction of mono-/bi-/poly-
nuclear networks is a great challenge in coordination
polymers, supramolecules and crystal engineering.
Upon manipulation of reaction condition (solvent
˚
l (A)
0.71073
0.720
0.71073
1.192
m (Mo Ka)
(mmꢂ1
)
Dcalc (Mg mꢂ3
Refine para-
meters
)
1.428
338
1.566
199
and temperature), metalÁ
/
ligandÁazido ratio and se-
/
quence of mixing of reactants and reagents these two
classes of azido compounds, mononuclear bis-chelated
Total reflections 15 955
2848
8548
1956
Unique reflec-
tions
Co(RaaiR?)2(N3)2 (3,4) and binuclear m-(1,1)×
bridged mono-chelated
[Co(RaaiR?)(m-(1)N3)(m-
(1,1)N3)]2 (5,6) have been isolated. Bis-chelated mono-
nuclear MX2(RaaiR?)2 (Xꢀhalogen), MꢀRu, Os)
/(EO) azido
a
R
[I ꢀ
/
2s (I)] 0.0370
0.0402
0.0989
0.907
b
wR2
0.0519
Goodness-of-fit 0.969
/
/
ꢂ3
˚
Dmax (e A
˚
Dmin (e A
)
0.589
0.384
0.300
pseudooctahedral complexes can, in principle, exist in
five geometrical isomeric forms. The isomers are named
ꢂ3
)
ꢂ
/
ꢂ/0.316
a
cis Á
/
trans Á
/
trans (ctt), trans Á
/
trans Á
/
trans (ttt), trans Á
/
Rꢀ
/
ajFojꢂ
wR2ꢀ
[aw(Fo2ꢂ
and wꢀ
1//[s2(Fo2)ꢁ(0.049P)2] for 5b where Pꢀ
/
jFcj/ajFoj.
b
/
/
Fc2)2/awFo4]1/2; wꢀ
/
1/[s2(Fo2)ꢁ
/
(0.0055P)2] for 3b
2Fc2)/3.
cis Á
/
cis (tcc), cis Á
/
cis Átrans (cct) and cis Á
/
/
cis Ácis (ccc)
/
/
/
/
(Fo2ꢁ
/
with reference to the sequence of coordination pairs
X,X; N,N; N?,N? respectively. We have established two
isomers tcc and ctc in the case of ruthenium(II) by single
crystal X-ray crystallographic study [29] and four
corrections were applied. The structure was solved by
heavy atom methods using SHELXS-97 and successive
difference Fourier syntheses. All non-hydrogen atoms
were refined anisotropically. The hydrogen atoms were
fixed geometrically and refined using the riding model.
All calculations were carried out using SHELXL-97,
ORTEP-3 and PLATON-99.
isomers (tcc, ctc, cct and ccc) by solution spectroscopic
1
NIR and H NMR) data [30]. In this case,
(UVÁ
/
VisÁ
/
Co(N3)2(MeaaiMe)2, we find for the first time the ccc-
isomeric disposition around the metal ion (vide infra).
Co(OAc)2 ꢁ2RaaiR?ꢁ2NaN3 M0eOH
stir: 298
K
Co(N3)2(RaaiR?)2ꢁ2NaOAc
(1)
(2)
3=4
Co(OAc)2 ꢁRaaiR?ꢁnNaN3 M0eOH
3. Result and discussion
298
K
(nꢀ4)
[Co(mꢂ(1)N3)(mꢂ(1; 1)N3)(RaaiR?)]
5=6
3.1. The ligands and complexes
Bi-azido-bis-chelated Co(N3)2(RaaiR?)2 (3,4) have
been isolated from a reaction where a composition
Co(OAc)2:RaaiR?:NaN3 of 1:2:2 (Eq. (1)) is maintained.
The complexes 5,6 have been prepared mixing
Two
classes
of
1-alkyl-2-(arylazo)imidazoles
(RaaiR?), total of four have been used as chelating
ligands in the present work. They are unsymmetric,
N,N?-bidentate chelator where N and N? refer to
N(imidazole) and N(azo) donor centers.
Co(OAc)2, RaaiR? and NaN3 in 1:1:excess (ꢀ4 mole)
/
mole proportion in MeOH (Eq. (2)). Dimers (5,6) are
also isolated from the reaction in which the sequence of

2590
U. Ray et al. / Polyhedron 22 (2003) 2587Á2594
/
addition is Co(OAc)2ꢁ
/
NaN3 (ꢀ
/
4 mole) followed by 1
Table 2
˚
Selected bond distance (A) and angles (8) for [Co(MeaaiMe)2(N3)2]
(3b)
mole of RaaiR?. In both cases, the complexes are
isolated from the reaction mixture on slow evaporation
under ambient conditions. The complexes are non-
electrolytes in MeCN. The composition of the com-
plexes was supported by C, H, N data. Metal analyses
were carried out by complexometric titration [34].
˚
Bond distance (A)
Bond angle (8)
Coꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
N1ꢀ
N2ꢀ
N4ꢀ
N5ꢀ
N12ꢀ
N11ꢀ
N22ꢀ
C102ꢀ
N21ꢀC101
C101ꢀN31
/
N11
N31
N12
N32
N1
2.4045(17)
2.0461(18)
2.5199(18)
2.0768(18)
2.026(2)
2.0188(17)
1.169(3)
1.161(3)
1.182(3)
1.157(3)
1.266(2)
1.276(2)
1.385(3)
1.325(3)
1.390(3)
1.316(3)
N11ꢀ
N12ꢀ
N4ꢀCoꢀ
N4ꢀCoꢀ
N4ꢀCoꢀ
N4ꢀCoꢀ
N4ꢀCoꢀ
N1ꢀCoꢀ
N1ꢀCoꢀ
N1ꢀCoꢀ
N1ꢀCoꢀ
N11ꢀCoꢀ
N11ꢀCoꢀ
N31ꢀCoꢀ
N31ꢀCoꢀ
CoꢀN4ꢀ
N4ꢀN5ꢀ
CoꢀN1ꢀ
N1ꢀN2ꢀ
/
Coꢀ
/
N31
71.40(6)
69.88(7)
100.33(9)
98.71(7)
81.90(7)
145.19(8)
90.23(7)
97.97(8)
167.84(7)
105.99(8)
82.49(7)
170.79(6)
109.53(6)
77.16(6)
99.74(7)
135.96(17)
177.2(2)
134.86(18)
177.5(3)
/
/
Coꢀ
/
N32
/
/
/
N1
/
/
/
N32
N12
N31
N11
N32
N12
N31
N11
3.2. Structure description
/
/
/
/N4
/
/
/
N2
N3
N5
N6
/
/
3.2.1. [Co(MeaaiMe)2(N3)2] (3b)
/
/
/
The molecular structure of [Co(MeaaiMe)2(N3)2] (3b)
is shown in Fig. 1. Selected bond distances and angles
are given in Table 2. The molecule shows distorted
octahedral arrangement about Co(II) with cis-(N3),(N3)
cis-N(imidazole), N(imidazole) and cis-N(azo), N(azo)
/
/
/
/
/
/
/
N22
N21
C102
/
/
/
/
/
N32
N12
N12
N32
/
/
/
/N32
/
/
donor centers and has been named thereafter cis Á
cis or ccc-[Co(MeaaiMe)2(N3)2]. The NꢀCoꢀN bond
angles subtended by the chelating MeaaiMe ligands are
N(11)ꢀCoꢀN(31), 71.3(9)8, N(12)ꢀCoꢀN(32), 69.8(8)8.
/
cis Á
/
/
/
/
/
/
/
/
/
N5
/
/N6
/
/
N2
/
/
/
/
/
/N3
The deviation from the ideal 908 for a regular octahedral
geometry is a result of steric requirements of the
bidentate ligands. Other angles in the chelated motif
(
) are also deviated in either direction from
constitute three planes. The planes are mutually ortho-
gonal extending dihedral angles in the range 85Á978.
Two chelating arrangements of atoms Co, N31, C101,
the regular five-membered ring. The angles around
Co(II) in the CoN6 coordination geometry suggest axial
binding of one N3 group and the other axial position is
occupied by imidazole-N (N(31)) from one of the two
MeaaiMe ligands. The steric requirement of the N(azo)
centers may force them to take position in the equatorial
site leading to the ccc-configuration. The atomic ar-
/
˚
N21, N11 (deviation B
C102, N32 (deviation B
/
0.03 A) and Co, N12, N22,
˚
0.04 A) constitute excellent
/
planes and make a dihedral angle of 718. The pendant p-
tolyl rings are nearly planer with their respective chelate
planes and their average dihedral angle is 88.
rangements Co, N1, N11, N12, N32 (deviation B
/
0.07
˚
˚
There are six unequal Coꢀ
/
N bonds in the CoN6
sphere to support distorted octahedron geometry. The
A) and Co, N4, N11, N31, N32 (deviation B
/
0.08 A)
˚
0.02 A)
and Co, N1, N4, N12, N3 (deviation B
/
Fig. 1. ORTEP plot of [Co(MeaaiMe)2(N3)2] (3b).

U. Ray et al. / Polyhedron 22 (2003) 2587Á
/2594
2591
˚
Coꢀ
bonds are shortest in this series and are comparable with
literature values [42]. The CoꢀN(azo) [CoꢀN(11),
N(12), 2.519(9) A] bonds are the longest
N family which are reasonably less than the
/
N(azido) [Coꢀ
/
N(1), 2.026(4), Coꢀ
/
N(4), 2.019(1) A]
Table 3
˚
Selected bond distance (A) and angles (8) for [Co(MeaaiMe)(N3)2(1,1-
m)]2 (5b)
/
/
˚
2.404(5), Coꢀ
in the Coꢀ
/
˚
Bond distance (A)
Bond angle (8)
N2
/
Coꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
N1ꢀ
N5ꢀ
N6ꢀ
N8ꢀ
N9ꢀ
N3ꢀ
N2ꢀ
Co
/
N1
N2
N5
N8
N8*
2.328(2)
2.010(2)
1.948(2)
1.994(2)
2.161(2)
1.278(3)
1.167(4)
1.148(5)
1.204(4)
1.137(4)
1.368(4)
1.331(4)
3.151
N1ꢀ
N1ꢀ
N1ꢀ
N1ꢀ
N2ꢀ
N2ꢀ
N2ꢀ
N1ꢀ
N3ꢀ
Coꢀ
Coꢀ
N8ꢀ
Coꢀ
N5ꢀ
N5ꢀ
Coꢀ
N5ꢀ
Coꢀ
Co*ꢀ
N8ꢀN9ꢀ
/
Coꢀ
N3ꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
Coꢀ
C8ꢀ
N2ꢀ
N1ꢀ
Coꢀ
N8ꢀ
Coꢀ
Coꢀ
N5ꢀ
N6ꢀ
N8ꢀ
/
74.33(9)
112.0(2)
96.84(9)
sum of the van der Waal’s radii of participating atoms
[43] and accounts for covalent interaction. The steric
requirement of the bidentate ligands and short chelate
rings may be manifested by elongation of the Coꢀ
N(azo) bond [44]. However, Co(II) shows some prefer-
ential binding to N(imidazole) centers [CoꢀN(31),
N(32), 2.076(9) A] compared to its
/
/
/
C8
/
/
/
N8
/
/
/
N8*
N8*
N8
167.54(9)
95.28(9)
/
/
/
/
/
N3
N6
N7
N9
N10
C8
/
/
116.49(10)
123.96(11)
97.37(10)
126.0(3)
/
/
/
N5
/
/
/
/
N5
˚
2.046(1); Coꢀ
/
/
/
/N2
N(azo) cousin. The structures of transition metal com-
plexes of RaaiR? reveal that ruthenium(II) [29] and
osmium(II) [31] prefer azo-N coordination while the
imidazole-N center is preferred by palladium(II) [26,27]
and platinum(II) [28]. Back donation ability of the metal
center may distinguish the binding preferences. High
spin cobalt(II) (t52geg2) is not a good p-donor [44] to form
/
/
/
C8
N3
114.67(19)
112.89(17)
81.45(9)
/
/
/
/
C8
/
/
N8*
Co*
N8
/
ꢀ ꢀ ꢀ
/Co*
/
/
98.55(10)
119.53(11)
94.17(11)
135.9(3)
/
/
/
/
N8*
N6
/
/
/
/
N7
176.0(4)
/
/
N9
133.8(2)
a strong Coꢀ
bond length is recommended. Two axial Coꢀ
tances [CoꢀN(31) and CoꢀN(4)] are shorter than their
respective twins [CoꢀN(32) and CoꢀN(11)]. This sup-
ports axial compression. The azido NꢀNꢀN bond
lengths and angles are as usual [35]. The NꢁN(azo)
bond lengths [N(11)ꢀN(21), 1.275(6); N(12)ꢀN(22),
1.266(4) A] are longer than the free ligand value
/
N(azo) bond and hence elongation of this
/
N8ꢀ
/
N9
N10
125.11(19)
178.1(3)
/
N dis-
/
/
/
/
/
/
/
/
/
/
/
the angles subtended at the metal ion (Table 3).
MeaaiMe binds in a usual chelating fashion and the
chelate angle is 74.33(9)8. The molecule shows a center
of inversion and the bridging parallelogram is con-
structed by Co2N2 (N(azido)) atom participation. The
˚
˚
˚
(1.250(1) A) [45,46]. This small elongation (Â
may be due to steric requirement of the chelated
azoimine group as well as weak d(Co)0p*(azo) back
bonding charge transference.
/
0.02 A)
/
bridging angles are Coꢀ
N(8)ꢀCoꢀN(8)*, 81.45(9)8. The azido (N3) group
bridges the two cobalt centers in a m-(1,1) fashion. The
five CoꢀN bond lengths in 5b are comparatively less
than the respective CoꢀN bond lengths from the
/
N(8)ꢀ
/
Co*, 98.55(10)8 and
/
/
3.2.2. [Co(MeaaiMe){(m-1)N3}{m-(1,1)N3}]2 (5b)
The single crystal structure of the dimer 5b is shown in
Fig. 2. Selected bond distances and angles are given in
Table 3. The CoN5 coordination sphere around each
Co(II) is distorted square pyramidal as can be seen from
/
/
mononuclear compound Co(MeaaiMe)2(N3)2 (3b). It
may be due to a steric effect provided by two chelate
˚
N(1), 2.328(2) A] bond
rings in 3b. The Coꢀ
/
N(azo) [Coꢀ
/
length is longer than Coꢀ
/
N(imidazole) distances [Coꢀ
N(2), 2.010(2) A]. The axial CoꢀN(azido) [CoꢀN(5),
1.948(2)] length is the shortest bond length in the Coꢀ
series. The bridged CoꢀN(azido) distances are unequal
[CoꢀN(8)*, 1.994(2); Coꢀ
/
˚
/
/
/
N
/
˚
N(8)*, 2.161(2) A] showing a
/
/
parallelogram pattern. Distortion in the square plane
constituted by Co, N1, N2, N8, N(8)* is observed and
the dihedral angle between the chelate plane (Co, N1,
N3, C8, N2) and the bridged parallelogram (Co, N8,
N(8)*, Co)* is 638. Axial Coꢀ
longer perpendicular to the square plane and is leaning
towards the imaginary edge of N(1) N(8)* of the
ꢀ ꢀ ꢀ
square plane. The bond angle data support this proposi-
tion: N1ꢀCoꢀN5, 97.37(10)8; N2ꢀCoꢀN5, 123.96(11)8;
N8*ꢀCoꢀN5, 94.17(11)8; N8*ꢀCoꢀN5, 119.53(11)8.
The bridged N3 and axial ꢀN3 bond lengths and angles
are as usual [36Á41].
/
N3 (N5ꢀ
/
N6ꢀ
/
N7) is no
/
/
/
/
/
/
/
/
/
/
/
Fig. 2. ORTEP plot of [Co(MeaaiMe){m-(1)N3}{m-(1,1)N3}]2 (5b).
/

2592
U. Ray et al. / Polyhedron 22 (2003) 2587Á2594
/
3.3. Spectral studies
[Co(RaaiR?)2(N3)2] exhibit m in the range 4.4Á4.7 BM,
/
slightly less than the spin only value. Binuclear com-
plexes, [Co(RaaiR?){(m-1)N3}{m(1,1)N3}]2 (5/6) show
The IR spectra of the complexes were recorded on
200 cmꢂ1. The bands were
assigned on comparing with free ligand data. The most
KBr discs in the range 4000Á
/
magnetic moment 3.6Á3.8 BM. Reduced m in binuclear
/
derivatives (5/6) may be due to anti-ferromagnetic spin-
coupling via m-(1,1)-N3 bridging. This is in support of
end-on bridged azido complexes [36]. We have not been
able to carry out variable temperature susceptibility
measurements due to the lack of instrumental facilities.
significant observation is the appearance of a very
2045 cmꢂ1 for 3/4
strong sharp single band at 2030Á
/
and a doublet at 2090Á2095 and 2050Á
/
/
2060 cmꢂ1 for 5/
6. These correspond to nasym(N3). This supports mono-
nuclear mono-dentate N3ꢂ bonding in 3/4 [35] and
bidentate bridging end-on (m-(1,1)) type in 5/6 [36]. The
chelated ligand RaaiR? shows characteristic transmis-
3.5. Redox properties
sion at 1600Á
at 1585Á1600 and 1420Á
and n(NꢁN), respectively [26Á
Solution electronic spectra of the complexes were
recorded in acetonitrile solution in the UVÁVis region
(250Á900 nm). The data are listed in Table 4. In the
visible region the complexes exhibit an intense band 5
400 nm. These transitions are presumably due to intra-
ligand pÁp*/ and nÁp* transitions [26,27]. There are
three transitions in the range 470Á640 nm. The intensity
pattern of the bands are different; the transition 470Á
485 nm is of high intensity (o Â
104) and is sharp while
the other two transitions 520Á575 and 620Á640 nm are
weak (o Â
102Á103) and broad. Based on the low
intensities, low energy nature of these bands they are
considered to be possible dÁd transitions. The high
intense transition at 470Á485 nm may be assigned to
MLCT transitions. The splitting pattern of dÁd transi-
/
370 cmꢂ1. Moderately intense stretching
The complexes exhibit both metal and ligand centred
/
/
1430 cmꢂ1 are due to n(Cꢁ
28].
/N)
electroactivity in the potential range 91.7 V versus Ag/
/
AgCl, Clꢂ electrode. A representative voltammogram is
shown in Fig. 3 and data are given in Table 5. The
complexes display one quasireversible oxidation process
/
/
/
/
as evident from the peak-to-peak separation, DEp]
mV. Mononuclear [Co(RaaiR?)2(N3)] (3/4) complexes
show higher positive redox potential, 1.0Á1.2 V than
that of dinuclear derivatives, [Co(RaaiR?){(m-
1.0 V. The cathodic and
/
100
/
/
/
/
/
1)N3}{m(1,1)N3}]2 (5/6), 0.8Á
/
/
anodic peak heights (Ipc and Ipa) are equal. The one-
electron nature of the couple is confirmed on comparing
/
/
/
/
/
/
/
/
tions support six coordinate distorted octahedral sym-
metry around the cobalt center [44]. The coordinating
solvent MeCN may result in the required symmetry
around cobalt(II) in bridged dinuclear molecules.
3.4. Magnetism
Bulk magnetic moment (m) measurements of the
complexes at room temperature (300 K) suggest high
spin mononuclear and binuclear complexes (Table 4).
Fig. 3. Cyclic voltammogram of [Co(HaaiMe)2(N3)2] (3a) (*) and
[Co(HaaiMe){(m-1)N3}{(m-1,1)N3}]2 (5a) (----) in acetonitrile using a
Pt-disk electrode.
/
Table 4
UVÁ
Vis spectra a, IR b and magnetic moment (m) data
/
Compound
UVÁ
/
Vis spectral data (lmax (nm)) (10ꢂ3 o Mꢂ1 cmꢂ1
)
IR data
m (BM)
n(N3ꢂ
)
n(Cꢁ
/
N) n(NꢁN)
/
[Co(HaaiMe)2(N3)2] (3a)
[Co(HaaiEt)2(N3)2] (4a)
626(0.56), 575(0.61), 482(0.97), 406(13.08)
624(0.42), 576(0.52), 485(1.223), 400(12.73)
626(0.34), 570(0.47), 484(1.01), 400(11.51)
624(0.26), 572(0.31), 480(1.55), 402(9.75)
628(0.08), 570(0.79), 485(1.22), 404(7.83)
624(0.54), 574(0.62), 484(1.56), 400(12.91)
630(0.63), 566(0.84), 480(2.12), 400(14.51), 380(2.36)
628(0.51), 560(0.53), 474(2.39), 408(12.13)
2035
2030
2044
2038
1587
1596
1598
1595
1426
1428
1425
1428
1420
1430
1427
1430
4.34
4.53
4.74
4.62
3.71
3.77
3.76
3.72
[Co(MeaaiMe)2(N3)2] (3b)
[Co(MeaaiEt)2(N3)2] (4b)
[Co(N3){m-(1,1)N3}(HaaiMe)2] (5a)
[Co(N3){m-(1,1)N3}(HaaiEt)2] (6a)
[Co(N3){m-(1,1)N3}(MeaaiMe)2] (5b)
[Co(N3){m-(1,1)N3}(HaaiMe)2] (6b)
2090, 2056 1595
2080, 2050 1591
2085, 2053 1596
2084, 2052 1595
a
Solvent: MeCN.
In KBr disk.
b

U. Ray et al. / Polyhedron 22 (2003) 2587Á
/2594
2593
Table 5
Voltammetric data
MOs. The energy of the HOMO and LUMO and their
composition are set out in Table 6.
In general, the energy of the MOs of mononuclear
complexes are higher than those of dinuclear bridged
species. The HOMOs and LUMOs are composed of a
function from cobalt, MeaaiMe and azido group. In
Co(N3)2(MeaaiMe)2 the ligand functions dominate to
construct both the HOMO and LUMO: MeaaiMe
contributes 60% in the HOMO and 81% in the LUMO
(Table 6). Thus the high intense electronic spectral band
Compound
CV data E0 (V) (DEp, mV)
Co(II)/Co(III) Ligand reduction
[Co(HaaiMe)2(N3)2]
[Co(MeaaiMe)2(N3)2]
[Co(HaaiEt)2(N3)2]
[Co(MeaaiEt)2(N3)2]
1.17(110)
1.12(100)
1.13(120)
1.06(100)
ꢂ
/
0.22(110), ꢂ
/
0.50(140),
0.53(140),
0.50(130),
0.61(140),
ꢂ
/
1.10(200)
ꢂ
/
0.25(130), ꢂ
/
ꢂ
/
1.13(220)
ꢂ
/
0.24(110), ꢂ
/
ꢂ
/
1.15(190)
ꢂ
/
0.32(120), ꢂ
/
which is HOMO0
ligand-charge transfer transition. Additionally, the high
energy weak band in the spectra are due to dÁ
/
LUMO is considered as an intra-
ꢂ
/
1.37(240)
[Co(HaaiMe)(N3)2]2
[Co(MeaaiMe)(N3)2]2
[Co(HaaiEt)(N3)2]2
[Co(MeaaiEt)(N3)2]2
0.89(100)
0.80(110)
0.81(100)
0.75(120)
ꢂ
/
0.71(140), ꢂ
0.65(120), ꢂ
0.64(110), ꢂ
0.74(120), ꢂ
/
1.13(200)
1.19(220)
1.10(200)
1.20(220)
/d
ꢂ
/
/
ꢂ
/
/
transitions since both HOMO and LUMO contain 28
and 9% Co-atomic functions, respectively. Similar
results are observed in the dinuclear bridged complex.
The composition of the MOs suggests that the
ꢂ/
/
Solvent: MeCN, Pt-disk working electrode, Ag/AgCl, Clꢂ reference
electrode, Pt-wire auxiliary electrode, supporting electrolyte,
[n Bu4N][ClO4] (0.01 M); solute concentration, 10ꢂ3 M; scan rate,
0.05 V sꢂ1; DEpꢀ
cathodic peak potential, V; E1/2
HOMO0/LUMO charge transfer transition is asso-
/
jEpa
ꢂ
/
Epcj, mV; Epa
ꢀ
0.5(Epa
/
anodic peak potential, Epc
ꢀ
/
ciated with ILCT, MLCT and ligand-to-azido charge
transferences. The HOMO, HOMO-1 and other lower
energy orbitals are dominated by the metal atomic
function. The oxidation process in the electrochemical
experiment is referred to as electron extraction from the
HOMOs. This proposition may suggest ligand oxida-
tion. However, the observed results and the literature
reports [47] account on Co(III)/Co(II) redox response at
positive to the reference electrode system. Since the
HOMO includes both metal and ligand functions in the
ratio of about 1:2 we may assume that once the electron
leaves from the ligand based functional portion, the
metal electrons are piping out into it and ultimately we
may observe metal centred oxidation. Besides, Co(III) is
d6, diamagnetic (t26g) and hence the thermodynamic
stability prefers to consider cobalt(II) oxidation. Thus
ꢀ
/
ꢁEpc), V.
/
with the current height of the Fe(CN)63ꢂ/Fe(CN)64ꢂ
system. The complexes possess Co(II) and N3ꢂ as
oxidisable centers. On comparing with the electroche-
mical results of the cobalt(II)-2-(arylazo)pyridine system
[47], the oxidative couple is assigned to Co(III)/Co(II).
Presence of two p-acidic chelating RaaiR? in mono-
nuclear complexes 3/4 increases the Co(III)/Co(II)
potential compared to one RaaiR? ligand in dinuclear
complexes (5/6). The more positive the potential, the
stronger is the binding to the lower oxidation state.
Thus, the mononuclear complexes show better stabiliza-
tion to Co(II) than that of dinuclear species. The redox
responses negative to reference electrode are referred to
ligand reductions. The azo (ꢀ
/
Nꢁ
/
Nꢀ/) group in the
the oxidation is referred to Co(II)0/Co(III). The
ligand RaaiR? is known as a potential electron transfer
center. Each ligand can accept two electrons in the
electrochemically accessible LUMO which is predomi-
nantly azo in character [29,33]. So mononuclear com-
plexes (3/4) may exhibit four one-electron ligand based
reductions and dinuclear complexes 5/6 may show two
reductions. We observed two one-electron quasireversi-
LUMOs are dominated by ligand orbitals and reduction
is the accommodation of electrons in the LUMOs.
Reductive responses are thus considered in general
ligand reduction and in particular azo reductions.
ble reductions (DEp]
0.2 to ꢂ0.3 and ꢂ0.4 to ꢂ
(3/4) along with a two electron reduction (as evident
from current height) at ꢂ1.0 to ꢂ1.3 V. Dinuclear
complexes, 5/6 exhibit two quasireversible reductions at
0.6 to ꢂ0.7 V and ꢂ1.0 to ꢂ1.2 V.
/
100 mV) in the potential range ꢂ
/
/
/
/
0.6 V in [Co(N3)(RaaiR?)2]
4. Supplementary material
/
/
Crystallographic data for the structures have been
deposited with the Cambridge Crystallographic data
center, CCDC No. 203078 for [Co(MeaaiMe)2(N3)2]
(3b) and CCDC No. 203079 for [Co(MeaaiMe){m-
(1)N3}{m-(1,1)N3}]2 (5b). Copies of this information
may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2, 1EZ, UK
ꢂ
/
/
/
/
3.6. EHMO calculation
X-ray crystallographic parameters of the complexes
3b and 5b have been used in the EHMO calculations of
(fax: ꢁ
/
44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk).

2594
U. Ray et al. / Polyhedron 22 (2003) 2587Á2594
/
Table 6
Compound
HOMO
EHOMO (eV)
LUMO
ELUMO (eV)
% Co
% azide
% azo
% Co
% azide
25
% azo
[Co(MeaaiMe)2(N3)2]
[Co(MeaaiMe)2(N3){m-(1,1)N3}]
ꢂ
/
10.876
28
16
5
4
60(48)
59(10)
ꢂ
/
10.783
9
53
81(51)
10
ꢂ11.889
/
ꢂ11.796
/
[19] D. Graham, R. Brown, W.E. Smith, Chem. Commun. (2001)
1002.
Acknowledgements
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Proc. Natl. Acad. Sci. USA 97 (2002) 12.
Financial support from the Council of Scientific and
Industrial Research, and University Grants Commis-
sion, New Delhi are gratefully acknowledged. One of
them (U.S.R.) thanks the UGC, for fellowship and C.S.
also acknowledges UGC for providing him Visiting
Associateship to I.I.Sc., Bangalore. We thank Prof.
A.R. Chakraborty for his assistance and help.
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