Iron(II) and nickel(II)-thiocyanato complexes of 1-alkyl-2-(arylazo) imidazole: Single crystal X-ray structure of [Fe(MeaaiEt)2(NCS) 2] (MeaaiEt = 1-ethyl-2(p-tolylazo)imidazole) and [Ni(MeaaiMe)(NCS)2(H2O)2]·2DMF (MeaaiMe = 1-methyl-2(p-tolylazo)imidazole)

Article abstract of DOI:10.1016/j.ica.2004.11.020

The reaction of green vitriol (FeSO₄,7H₂O) with 1-alkyl-2-(arylazo)imidazoles [RaaiR′ where R = H (a), Me (b); R′ = Me (1/3/5), Et (2/4/6)] and NH₄SCN in 1:2:2 mole proportion affords mononuclear bis-chelated [Fe(RaaiR′)₂(SCN)₂] (3, 4) complexes. Reaction of NiSO₄·6H₂O with 1-alkyl-2(arylazo)imidazole and ammonium thiocyanate (1:2:2 molar ratio) in methanol solution forms mononuclear monochelated thiocyanato complexes of the type [Ni(RaaiR′)(H₂O)₂(SCN)₂] ·2H₂O (5, 6). Slow diffusion of DMF solution of [Ni(MeaaiMe)(H₂O)₂(NCS)₂] in methanol has isolated the X-ray quality crystals. All the complexes are characterized by elemental, UV-Vis, IR, thermal and magnetic studies. In both cases, the distorted octahedral structure was confirmed by single crystal X-ray diffraction study of [Fe(MeaaiEt)₂(NCS)₂] (4b) and [Ni(MeaaiMe)(H₂O)₂(NCS)₂]·2DMF (7). Variable temperature susceptibility measurements of [Fe(MeaaiEt) ₂(NCS)₂] (4b) do not exhibit spin crossover property, which may be due to high structural distortion.

Full text of DOI:10.1016/j.ica.2004.11.020

Inorganica Chimica Acta 358 (2005) 1019–1026
www.elsevier.com/locate/ica
Iron(II) and nickel(II)-thiocyanato complexes of
1-alkyl-2-(arylazo)imidazole: single crystal X-ray structure
of [Fe(MeaaiEt)2(NCS)2] (MeaaiEt = 1-ethyl-2(p-tolylazo)imidazole)
and [Ni(MeaaiMe)(NCS)2(H2O)2] Æ 2DMF
(MeaaiMe = 1-methyl-2(p-tolylazo)imidazole)
a
Umasankar Ray , Debasis Banerjee , Jian-Cheng Liou , Chun-Nan Lin ,
a
b
b
b
Tian-Huey Lu , Chittaranjan Sinha
c,
*
a
Department of Chemistry, The University of Burdwan, Burdwan 713 104, India
b
Department of Physics, National Tsing-Hua University, Hsinchu 300, Taiwan, PR China
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India
c
Received 31 July 2004; accepted 10 November 2004
Available online 11 February 2005
Abstract
The reaction of green vitriol (FeSO4,7H2O) with 1-alkyl-2-(arylazo)imidazoles [RaaiR0 where R = H (a), Me (b); R0 = Me (1/3/5),
Et (2/4/6)] and NH4SCN in 1:2:2 mole proportion affords mononuclear bis-chelated [Fe(RaaiR0)2(SCN)2] (3, 4) complexes. Reaction
of NiSO4 Æ 6H2O with 1-alkyl-2(arylazo)imidazole and ammonium thiocyanate (1:2:2 molar ratio) in methanol solution forms
mononuclear monochelated thiocyanato complexes of the type [Ni(RaaiR0)(H2O)2(SCN)2] Æ 2H2O (5, 6). Slow diffusion of DMF
solution of [Ni(MeaaiMe)(H2O)2(NCS)2] in methanol has isolated the X-ray quality crystals. All the complexes are characterized
by elemental, UV–Vis, IR, thermal and magnetic studies. In both cases, the distorted octahedral structure was confirmed by single
crystal X-ray diffraction study of [Fe(MeaaiEt)2(NCS)2] (4b) and [Ni(MeaaiMe)(H2O)2(NCS)2] Æ 2DMF (7). Variable temperature
susceptibility measurements of [Fe(MeaaiEt)2(NCS)2] (4b) do not exhibit spin crossover property, which may be due to high struc-
tural distortion.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Arylazoimidazole; Iron(II), Nickel(II)-complexes; X-ray structure, electrochemistry
1. Introduction
production of polymetallic complexes of magneto-
chemical interests. We have been interested to design
functionalized imidazole ligands like arylazoimidazoles.
For the past several years, we [6–17] and others [18–20]
have been engaged in the exploration of the chemistry of
arylazoimidazoles. The molecule bears azoimine, –
N@N–C@N–, functional group. This is an efficient
p-acid system for the stabilization of low oxidation me-
tal ions. The platinum chemistry of this function is
known in more detail [10–20] than 3d transition metals
[6–9]. Present work stems from our continued interest
There has been a continuous interest in the chemistry
of metal complexes of imidazole and related heterocy-
cles [1–5]. Imidazole and its derivatives have been widely
used in chemistry and biology due to their appearance in
biomolecules, metalloproteins and their versatility in the
*
Corresponding author. Tel.: +91 909830632450; fax: +91 033
24146584.
E-mail address: c_r_sinha@yahoo.com (C. Sinha).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.11.020

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U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
in the development of transition metal chemistry of ary-
lazoimidazoles. Herein, we describe Fe(II) and Ni(II)–
thiocyanato complexes of 1-alkyl-2-(arylazo)imidazoles.
All other complexes were prepared by the same pro-
cedure. Crystalline products were obtained in the yield
of 75–80% and microanalytical data of the complexes
are as follows. [Fe(HaaiMe)2(NCS)2] (3a): Anal. Calc.
for C22H20N10S2Fe: C, 48.52; H, 3.67; N, 25.73. Found:
C, 48.46; H, 3.72 ; N, 25.68%. IR (KBr disk) (cmꢀ1):
m(SCN) 2066 (s); m(C@N) 1585 (m), m(N@N) 1428 (m);
l(BM), 5.04. [Fe(MeaaiMe)2(NCS)2] (3b): Anal. Calc.
for C24H24N10S2Fe: C, 50.34; H, 4.19; N, 24.47. Found:
C, 50.38; H, 4.24; N, 24.40%. IR (KBr disk) (cmꢀ1):
m(SCN) 2066 (s); m(C@N) 1588 (m), m(N@N) 1424 (m);
l(BM), 5.08. [Fe(HaaiEt)2(NCS)2](4a) Anal. Calc. for
C24H24N10S2Fe: C, 50.34; H, 4.19; N, 24.47. Found:
C, 50.28; H, 4.23; N, 24.41%. IR (KBr disk) (cmꢀ1):
m(SCN) 2065 (s); m(C@N) 1586 (m), m(N@N) 1422 (m);
l(BM), 5.06. [Fe(MeaaiEt)2(NCS)2] (4b) : Anal. Calc.
for C26H28N10 S2Fe: C, 52.0; H, 4.66; N, 23.33. Found:
C, 51.89; H, 4.72; N, 23.27%. IR (KBr disk) (cmꢀ1):
m(SCN) 2064 (s); m(C@N) 1586 (m), m(N@N) 1426 (m);
l(BM), 5.05.
2. Experimental
2.1. Materials
The compounds FeSO4 Æ 7H2O, NiSO4 Æ 6H2O and
NH4SCN were obtained from E. Merck, India and 1-Al-
kyl-2-(arylazo)imidazoles (RaaiR0) were prepared by
following the reported procedure [10]. All other chemi-
cals and solvents are of reagent grade. Solvents are used
after drying.
2.2. Physical measurements
Microanalyses (C, H, N) were performed using a Per-
kin–Elmer 2400 CHNO/S elemental analyzer. Spectro-
scopic measurements were carried out using the
following instruments: UV–Vis spectra, JASCO
UV–VIS/NIR model V-570; IR spectra (KBr disk,
4000–200 cmꢀ1), JASCO FT-IR model 420. Room
temperature magnetic moment was measured using
vibrating sample 155 magnetometer at 298 K. Molar con-
ductances (KM) were measured in a Systronics conduc-
tivity meter 304 model using ca. 10ꢀ3 M solutions in
MeOH. Electrochemical measurements were carried
out with the use of computer controlled EG&G PARC
VersaStat model 250 electrochemical instrument using
a Pt-disk working electrode. The solution was IR com-
pensated and the results were collected at 298 K. The re-
ported results are referenced to SCE in acetonitrile and
are uncorrected for junction potential. Voltammograms
were recorded in the potential range of +1.5 V to ꢀ1.5
V. Magnetic measurements were carried out on poly-
crystalline samples (20–30 mg) in the ꢀServei de Magnet-
2.4. Synthesis of [Ni(MeaaiMe)(H2O)2(NCS)2] Æ 2H2O
(5b)
A solution of NH4SCN (0.04 g, 0.53 mmol) in MeOH
(10 ml) was added to the stirred solution of NiSO4 Æ 6-
H2O (0.07 g, 0.27 mmol) in MeOH (10 ml) at 298 K.
1-Methyl-2-(p-tolylazo)imidazole (MeaaiMe) (1b) (0.1
g, 0.50 mmol) in the same solvent (10 ml MeOH) was
added dropwise to this solution. The resulting brown
solution was stirred for 15 min. The solution was filtered
and left undisturbed for few days. The dark brown crys-
talline products were separated, collected, washed with
aqueous methanol and finally with ether. It was dried
in vacuo. Yield was 0.12 g (54%).
All other complexes were prepared by the same pro-
cedure. Crystalline products were obtained in the yield
of 50–70% and microanalytical data of the complexes
are as follows. [Ni(HaaiMe) (H2O)2(NCS)2]. 2H2O (5a)
Anal. Calc. for C12H18N6S2O4Ni: C, 33.28; H, 4.16; N,
19.41. Found: C, 33.40; H, 4.24; N, 19.28%. IR (KBr
disk) (cmꢀ1): m(H2O) 3425 (b); m(SCN) 2092 (s);
m(C@N) 1595 (m), m(N@N) 1425 (m); l(BM), 2.82.
[Ni(MeaaiMe)(H2O)2(NCS)2]. 2H2O (5b) Anal. Calc.
for C13H20N6S2O4Ni: C, 34.92; H, 4.48; N, 18.80.
Found: C, 35.10; H, 4.36; N, 18.69%. IR (KBr disk)
(cmꢀ1): m(H2O) 3421 (b); m(SCN) 2091 (s); m(C@N)
1596 (m), m(N@N) 1428 (m); l(BM), 2.66. [Ni(Haa-
iEt)(H2O)2(NCS)2] Æ 2H2O (6a) Anal. Calc. for
C13H20N6S2O4Ni: C, 34.92; H, 4.48; N, 18.80. Found:
C, 34.78; H, 4.29; N, 19.02%. IR (KBr disk) (cmꢀ1):
m(H2O) 3429 (b); m(SCN) 2089 (s); m(C@N) 1597 (m),
m(N@N) 1432 (m); l(BM), 2.76. [Ni(Meaa-
iEt)(H2O)2(NCS)2] Æ 2H2O (6b) Anal. Calc. for
C14H22N6S2O4Ni: C, 36.47; H, 3.91; N, 18.23. Found:
C, 36.62; H, 3.74; N, 18.09%. IR (KBr disk) (cmꢀ1):
´
oquımica, Universitat de Barcelonaꢁ with a Quantum
Design MPMS SQUID susceptometer operating at a
magnetic field of 0.1 T within the temperature range
2–300 K. The diamagnetic corrections were evaluated
from Pascalꢁs constants. TIP was assumed to be
200 · 10ꢀ6 cm3 molꢀ1
.
2.3. Preparation of Fe(MeaaiEt)2(NCS)2 (4b)
2-Methoxy-ethanol solution (15 ml) of 1-ethyl-2-(p-
tolylazo)imidazole (MeaaiEt) (0.11 g, 0.50 mmol) was
added dropwise to a methanol solution (25 ml) contain-
ing a mixture of FeSO4, 7H2O (0.07 g, 0.25 mmol) and
NH4SCN (0.04 g, 0.53 mmol) at 298 K. The resulted
green solution was left undisturbed for two weeks. The
dark green crystals were collected by filtration, washed
with methanol/water (1:1, v/v), and dried in vacuo for
1 h. Yield was 0.12 g (84%).

U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
1021
Table 1
Summarized crystallographic data for [Fe(MeaaiEt)2(SCN)2] (4b) and
[Ni(MeaaiMe)(H2O)2(SCN)2] Æ 2DMF (7)
m(H2O) 3423 (b); m(SCN) 2093 (s); m(C@N) 1598 (m),
m(N@N) 1437 (m); l(BM), 2.82.
Empirical formula
C
26H28N10S2Fe
C13H16N6NiO2S2,
2(C3H7NO)
557.34
2.5. X-ray crystal structure analysis
Formula weight
Temperature (K)
Crystal system
600.55
295
[Fe(MeaaiEt)2(NCS)2] (4b) has been crystallized from
synthetic mixture in methanol solution. Crystals ob-
tained from methanol solution of NiSO4 Æ 6H2O, Mea-
aiMe and NH4CNS in the molar ratio of 1:2:2
disintegrate upon exposure to X-ray. Upon addition of
DMF to the reaction mixture, X-ray quality crystals
[Ni(MeaaiMe)(H2O)2(NCS)2] Æ 2DMF (7) have been
isolated.
294
monoclinic
P21/c
monoclinic
C2/c
Space group
Crystal size (mm)3
Unit cell dimensions
0.20 · 0.20 · 0.20
0. 20 · 0.30 · 0.40
˚
a (A)
19.3315(15)
14.4434(11)
13.0915(10)
125.99
2957.6(4)
4
14.511(14)
13.581(13)
13.977(13)
100.54(2)
2708(4)
4
˚
b (A)
˚
c (A)
b (ꢁ)
V (A)
3
˚
Data were collected with Siemens SMART CCD dif-
fractometer using graphite-monochromatized Mo Ka
Z
˚
k (A)
0.71073
0.685
1.349
0.71073
0.910
1.367
˚
l (Mo Ka) (mmꢀ1
)
radiation (k = 0.71073 A) at 295 K for 4b and 293K
Dcalc (mg mꢀ3
Refined parameters
)
for 7. Unit cell parameters were determined from least-
squares refinement of setting angles of 2170 reflections
for 4b and 6464 reflections for 7 with 2h in the range
3.8 6 2 6 56.6ꢁ and 4.2 6 2h 6 56.8ꢁ, respectively. A
summary of the crystallographic data and structure
refinement parameters is given in Table 1. Reflection
data were recorded using the x scan technique. Data
were corrected for Lorentz polarization effects and for
linear decay. Semi-empirical absorption corrections
based on w-scans 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.
In the final difference Fourier map, the residual minima
177
3541
427
6464
Total no. of reflection
No. of reflections obtained
2170
3661
a
R1 [I > 2r (I)]
wR2
Goodness-of-fit
0.0592
0.1386
0.86
0.0391
0.0943
0.87
b
a
R = RjFo ꢀ Fcj/RFo.
ꢁ
ꢀ
P
ꢂ
P
1=2
2
b
wR2 ¼
wF 2o ꢀ F 2c
wF 4o
w ¼ 1=½r2ðF 2oÞ þ ð0:0985PÞ ꢁ for
2
4b and w ¼ 1=½r2ðF o2Þ þ ð0:0419PÞ ꢁ for 7, where P ¼ ðF o2 þ 2F 2c Þ=3.
RaaiR0and NH4SCN in methanol in 1:2:2 mole propor-
tion, crystalline compounds have been isolated. How-
ever, the products are of different compositions
(Scheme 1). Colour of the compounds in crystalline
states is dark green for [Fe(RaaiR0)2(NCS)2] (3, 4) and
brown for [Ni(RaaiR0)(NCS)2(H2O)2] Æ 2H2O (5, 6).
Microanalytical data support the composition of the
complexes and in two cases the structures have been
established by single crystal X-ray diffraction studies.
The thermal decomposition profile of the compounds
also supports the structure and composition. They were
heated to 600 ꢁC in nitrogen. The TGA data of iron(II)
complexes (3b, 4b) suggest loss of two moles of ligands
(observed 67.7%, calculated, 69.9% for 3b; observed
71.1%, calculated 71.4% for 4b) between 250 and 350
ꢁC and decomposition of Fe(NCS)2 above 400 ꢁC. The
weight loss continued up to 500 ꢁC, and the final residue
was black and amorphous.
3
˚
and maxima were [ꢀ0.14, 0.22 and ꢀ0.27, 0.28 (e/A ) for
4b and 7, respectively] carried out using SHELXL-97.
3. Results and discussion
3.1. Synthesis and general characteristics
Four 1-alkyl-2-(arylazo)imidazole ligands [RaaiR0,
where R = H(a), Me(b); R0 = Me(1), Et(2)] are used in
this work. Ligands belong to unsymmetric N,N0-biden-
tate chelator, where N and N0 abbreviated to N(imidaz-
ole) and N(azo) donor centres, respectively. Upon
reaction of MSO4, [M = Fe(II) (3, 4), Ni(II), (5, 6)] and
N
NCS
R'
N
N'
N'
NCS
N'
N
OH2
OH2
N
R
N
N
N '
Fe
N
Ni
N
NCS
NCS
RaaiR/ (1, 2)
R = H (a), Me (b);
3, 4
5, 6
R/ = Me (1/3/5), Et (2/4/6)
Scheme 1.

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U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
The TGA data for [Ni(MeaaiMe)(H2O)2(NCS)2]
and their assignments are given below with aid of molec-
ular orbital calculation.
2H2O (5b) showed three steps weight loss. In the first re-
gion 80–110 ꢁC, it loses two water molecules (observed,
1.11%; calculated, 1.08%) followed by loss of another
two molecules of water (observed, 1.11%; calculated,
1.08%) at 200–250 ꢁC. On further heating, it loses one li-
gand (observed, 45.02%; calculated, 44.8% for 5b) and
decomposition of Ni(NCS)2 above 300 ꢁC. The weight
loss continued upto 500 ꢁC and the final residue was
black and amorphous. [Ni(MeaaiMe)(H2O)2(NCS)2]
2DMF exhibits loss of 2 moles of water at 160–200 ꢁC,
2 moles of DMF and one mole of MeaaiMe in over-
lapped steps in the temperature range of 250–350 ꢁC.
The EHMO calculation using crystallographic param-
eters of the complexes has been performed to find out the
approximate composition of frontier orbitals. It is amaz-
ing that the HOMO of [Fe(MeaaiEt)2(NCS)2] (4b) is dom-
inated by Fe-orbitals (72% which is shared by 24% and
26% dxy and dxz orbitals, respectively), while [Ni(Mea-
aiMe) (H2O)2(NCS)2] Æ 2DMF (7) shows exclusively li-
gand characteristic of which azoimidazole shares 81%.
The LUMO in both the cases have been participated by
metal and ligand orbitals: in 4b Fe, 52%; azoimidazole,
42% and in 7 Ni, 48%; azoimidazole, 38%. Thus, the
HOMO ! LUMO transition may be assigned as admix-
ture of MLCT and d–d in case of Fe(II) complexes, while
it is LMCT in nature in Ni(II) complexes. The composi-
tion of HOMO ꢀ 1 (4b: Fe, 9%; azoimidazole, 65%. 7:
Ni, 1%; azoimidazole, 70%) and LUMO + 1 (in 4b Fe,
9%; azoimidazole, 75% and in 7 Ni, 59%; azoimidazole,
13%) also suggests that HOMO ꢀ 1 ! LUMO/
LUMO + 1 involve LMCT and intraligand charge trans-
fer (n ! p*, p ! p*) transitions.
3.2. Spectral studies
The complexes are sparingly soluble in common or-
ganic solvents like alcohols, toluene, and hexane but
fairly soluble in CHCl3, CH2Cl2, CH3CN, DMF, and
DMSO. The complexes are non-conducting. The solu-
tion spectra were recorded in CH3CN and exhibit four
transitions in the wavelength range 300–800 nm. Data
are listed in Table 2. Transitions below 400 nm are
highly intense (e ꢂ 104) and have been assigned to intra-
molecular (n ! p* and p ! p*, assumed as HOMO -
1 ! LUMO/LUMO + 1, vide infra) charge transfer
Infrared spectra of the complexes (3, 4) exhibit a
strong band at 2065–2070 cmꢀ1 along with a weak band
at 2050–2055 cmꢀ1 corresponding to m(CN) of bounded
SCN. Medium to strong bands observed at 760–770 and
680–695 cmꢀ1 have been assigned to m(CS). The m(NCS)
appear at 450–470 and 400–420 cmꢀ1 [22]. The chelated
azoimine exhibits strong stretches at 1425–1430 and
1540–1550 cmꢀ1 corresponding to m(N@N) and
m(C@N), respectively [6]. IR spectra of 5 and 6 exhibit
a sharp, strong band at 2089–2093 cmꢀ1 corresponding
to m(CN) of bounded SCN. A broad band at 3423–3429
cmꢀ1 corresponds to m(H2O) is observed for the com-
plexes 5, 6.
transitions. Intense band ((e ꢂ 104 dm3 molꢀ1 cmꢀ1
)
appearing in the range 420–440 nm for iron (II) com-
plexes [Fe(RaaiR0)2(NCS)2] (3, 4) may be assigned to
d(Fe) ! p ligand) charge transfer transitions. Two
broad bands at longer wavelengths (710–720 and 630–
635 nm) are also fairly intense and the molar extinction
coefficient data warrant to consider them not only as d–
d bands but rather an admixture of d–d and MLCT
transitions [21]. The azoimine (–N@N–C@N–) function
is p-acidic enough to insist such MLCT transitions.
Nickel (II) complexes [Ni(RaaiR0)(H2O)2(NCS)2] Æ
2H2O (5, 6) also exhibit three transitions in the wave-
length range 450–500 nm. Those are sufficiently intense
3.3. Molecular structure
Complexes [Fe(MeaaiEt)2(NCS)2] (4b) and [Ni(Mea-
aiMe)(NCS)2(H2O)2].2DMF (7) were characterized by
Table 2
UV–Vis spectraa, cyclic voltammetric datab and magnetic moment (l) data
Compound
UV–Vis spectral data
(kmax/nm) (10ꢀ3e Mꢀ1 cmꢀ1
Fe(III)/Fe(II) Ligand reductions
EL, V
l(BM)
)
E1/2, V,
(DEp, mV)
(DEp, mV)
[Fe(HaaiMe)2(SCN)2] (3a)
[Fe(HaaiEt)2(SCN)2] (4a)
716(1.25), 628(1.08), 442(4.98)
716(2.10), 632(1.74), 440(5.15)
718(1.41), 626(1.18), 434(3.02)
712(1.59), 628(1.57), 448(7.29)
496(0.73), 464(2.23), 456(2.41)
497(0.85), 466(3.10), 458(3.57)
0.82(120)
0.77(125)
0.76(100)
0.71(96)
ꢀ0.39 (130) ꢀ0.70 (170)
ꢀ0.42 (140) ꢀ0.73 (170)
ꢀ0.42 (120) ꢀ0.81 (160)
ꢀ0.44 (120) ꢀ0.76 (140)
5.04
5.06
5.08
5.05
[Fe(MeaaiMe)2(SCN)2] (3b)
[Fe(MeaaiEt)2(SCN)2] (4b)
[Ni(HaaiMe)(H2O)2(SCN)2] Æ 2H2O (5a)
[Ni(HaaiEt)(H2O)2(SCN)2] Æ 2H2O (6a)
[Ni(MeaaiMe)(H2O)2(SCN)2] Æ 2H2O (5b) 496(0.86), 464(3.21), 456(3.61)
ꢀ0.28 (130) ꢀ0.62 (160) ꢀ1.24c 2.82
ꢀ0.31 (140) ꢀ0.68 (150) ꢀ1.25c 2.76
ꢀ0.32 (120) ꢀ0.66 (140) ꢀ1.22c 2.66
ꢀ0.35 (130) ꢀ0.70 (150) ꢀ1.25c 2.82
[Ni(MeaaiEt)(H2O)2(SCN)2] Æ 2H2O (6b) 498(0.80), 464(2.38), 454(2.45), 424(2.38)
a
Solvent: Acetonitrile (CH3CN).
b
Solvent: Acetonitrile (CH3CN), Pt-disk working electrode, reference SCE, supporting electrolyte [nBu4N][ClO4], E1/2 = 0.5(Epa + Epc), V;
DEp = jEpa ꢀ Epcj, mV; Epa = anodic peak potential, Epc = cathodic peak potential.
c
Epc (cathodic peak potential).

U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
1023
single crystal X-ray crystallography. Perspective views
of the molecules are shown in Figs. 1 and 2, respec-
tively. Selected bond lengths and angles are given in Ta-
bles 3 and 4 for 4b and 7, respectively. In discrete
molecule, iron(II) is surrounded by two ligands and
two pseudohalides. Ligand serves as unsymmetric
N,N0-chelating agent (N refers to imidazole-N and
N0refers to azo-N donor centres) and two ligands che-
late in a manner to give trans-N,N and cis-N0,N0
arrangement. Thiocyanato (NCS) coordinates in a cis-
fashion around Fe(II). Thus, coordination sequence of
NCS, imidazole-N(N) and azo-N(N0) describes cis–
trans–cis stereochemical orientation. In the FeN6 coor-
dination environment, the equatorial plane is formed
by the two azo nitrogens (each from individual ligand)
and two NCS donor centres. Two axial positions are
occupied by two imidazole-N donor centres of two
MeaaiEt. The Fe–N bond distance data account the ax-
ial compression; since they follow the ordering Fe–
N(imi) < Fe–N(NCS) < Fe–N(azo). The coordination
environment around nickel in the complex is also dis-
torted octahedral, NiN4O2 type. The ligand MeaaiMe
acts as N,N0-chelator (N refers to imidazole-N and
N0refers to azo-N donor centres). There are two coordi-
nated water present in the cis-fashion, while two thiocy-
anato present as trans-N-donor groups in the axial
positions. Two molecules of DMF have been associated
via hydrogen bonds with coordinated H2O. Thus, they
are disposed in cis-fashion (Fig. 2). In 4b, the N–Fe–N
bond angle subtended by the chelating ligands N(1)–
Fe–N(3)/ N(1*)–Fe–N(3*) is 71.21(8)ꢁ. Similarly, in
[Ni(MeaaiMe)2(NCS)2(H2O)2]2DMF (7) the \N(1)–
Ni(1)–N(4) is 75.64(7)ꢁ and \O(1)–Ni(1)–O(2) is
90.44(8)ꢁ. Thiocyanates are trans oriented, SCN(5)–
Ni(1)–N(6)CS is 169.52(8)ꢁ. The deviation of the angle
from ideal 90ꢁ for a regular octahedron is a result of
steric requirements of the bidentate ligands. Other an-
Fig. 2. ORTEP view of [Ni(MeaaiMe)(NCS)2(H2O)2] Æ 2DMF (7).
gles in the chelated motif are also deviated in either
direction from the regular five membered ring. Presence
of two chelating ligands, MeaaiMe, in Fe(II) complex
may cause larger steric interaction than [Ni(Mea-
aiMe)(NCS)2(H2O)2] Æ 2DMF, where one chelator is
present. This may be the reason for larger distortion
in the former, which is manifested by very low chelate
angle (av. 69ꢁ). Arylazoimidazole moiety is almost pla-
˚
nar with deviation from the least square plane <0.04 A.
The chelate rings constitute good planes (mean
˚
deviation ꢂ 0.04 A). The pendant aryl ring makes dihe-
dral of 33ꢁ with the chelate plane. The proposed molec-
ular planes in an octahedral geometry are found to be
heavily distorted in these structures.
There are three types of N-donor centres: N(imidaz-
ole), N(azo) and N(thiocyanato) in Fe(MeaaiEt)2-
(NCS)2. Out of them, N(azo) requires considerably
more room than, other two types N-donors. There are
at least two steric reasons for this. First, the N(azo) be-
longs to exocyclic N@N along with a pendant aryl
group; thus, the van der Waals repulsion will be greater
than N(imidazole). The lowering in overall symmetry is
manifested by the elongation of axial bonds and two
N(azo) donor centres occupy two axial positions. The
˚
Fe–N(azo) distances lie in the range of 2.6–2.7 A. The
Table 3
Bond distances and bond angles of [Fe(MeaaiEt)2(SCN)2] (4b)
˚
(A)
Bond distance
Bond angle
(ꢁ)
Fe–N(1)
Fe–N(3)
Fe–N(5)
2.371(2)
2.103(2)
2.061(2)
1.624(3)
1.271(3)
1.329(3)
1.145(4)
N(1)–Fe–N(3)
N(1)–Fe–N(5)
N(1)–Fe–N(1*)
N(1)–Fe–N(3*)
N(1)–Fe–N(5*)
N(3)–Fe–N(5)
N(3)–Fe–N(3*)
N(3)–Fe–N(5*)
N(5)–Fe–N(5*)
71.21(8)
96.66(9)
76.01(8)
88.64(8)
162.92(8)
103.77(10)
154.65(8)
93.47(9)
94.29(9)
S(1)–C(1)
N(1)–N(2)
N(3)–C(7)
N(5)–C(10)
Fig. 1. ORTEP view of [Fe(MeaaiEt)2(NCS)2] (4b).

1024
U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
Table 4
Bond distances and bond angles of [Ni(MeaaiMe)(H2O)2(SCN)2] Æ 2DMF (7)
˚
(A)
Bond distance
Bond angle
(ꢁ)
Ni(1)–N(1)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–N(6)
Ni(1)–O(1)
Ni(1)–O(2)
S(1)–C(12)
S(2)–C(13)
N(1)–N(2)
N(2)–C(9)
N(4)–C(9)
N(5)–C(12)
N(6)–N(13)
2.252(3)
2.052(3)
2.027(3)
2.031(3)
2.097(3)
2.070(3)
1.623(3)
1.630(3)
1.274(3)
1.380(3)
1.318(3)
1.144(3)
1.151(3)
O(1)–Ni(1)–O(2)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(4)
O(1)–Ni(1)–N(5)
O(1)–Ni(1)–N(6)
O(2)–Ni(1)–N(1)
O(2)–Ni(1)–N(4)
O(2)–Ni(1)–N(5)
N(1)–Ni(1)–N(4)
N(1)–Ni(1)–N(5)
N(1)–Ni(1)–N(6)
N(4)–Ni(1)–N(5)
N(4)–Ni(1)–N(6)
N(5)–Ni(1)–N(6)
90.44(8)
104.48(7)
177.66(8)
85.23(8)
87.42(8)
165.02(8)
89.51(8)
95.25(9)
75.64(7)
87.46(7)
87.17(7)
92.45(8)
94.92(8)
169.52(8)
˚
d(D–H) (A)
˚
˚
D–Hꢃ ꢃ ꢃA
Hydrogen bonds
d(Hꢃ ꢃ ꢃA) (A)
d(Dꢃ ꢃ ꢃA) (A)
\DHA (ꢁ)
O(1)–H(13)ꢃ ꢃ ꢃO(3)
O(1)–H(14)ꢃ ꢃ ꢃO(4)
O(2)–H(15)ꢃ ꢃ ꢃO(3)
O(2)–H(16)ꢃ ꢃ ꢃO(4)
C(4)–H(5)ꢃ ꢃ ꢃO(1)
O(2)–H(1)–O(4)#1
O(1)–H(2)–O(4)
0.81(3)
0.75(3)
0.82(3)
0.75(3)
0.88(3)
0.92(8)
0.80(8)
0.74(8)
0.72(7)
1.92(3)
2.05(3)
1.95(3)
2.04(3)
2.45(2)
1.84(6)
1.92(7)
2.05(3)
2.05(7)
2.708(4)
2.784(4)
2.746(4)
2.761(4)
3.276(4)
2.74(2)
2.70(7)
2.78(6)
2.75(9)
163.0(3)
166.0(3)
165.1(3)
164.2(3)
157.6(19)
161.5(8)
161.8(1)
166.2(8)
162.7(4)
O(1)–H(3)–O(3)
O(2)–H(4)–O(3)#1
Symmetry: 1, ꢀx, ꢀy + 2, ꢀz.
longest Fe–N distance in the series is the Fe–N(azo) dis-
tance. However, the Fe–N(azo) distances are less than
the sum of the van der Waals radii of Fe(II) and
N(azo) [23]. This implies the existence of covalent inter-
action of N(azo) and Fe(II). The Fe(II) has some prefer-
ential binding to N(imidazole) compared to N(azo).
However, Ru(II) [10–12] and Os(II) [15] show higher
affinity to N(azo) than N(imidazole) centre. The Pd(II)
[13] and Pt(II) [16] prefer imidazole-N centre. The struc-
tural studies of Co(II) [8], Zn(II) [24], Cd(II) [22] and
Hg(II) [25] complexes of arylazoimidazoles show better
affinity to imidazole-N than N(azo). The sum of the
equatorial bond angles (plane is constituted by two
azo–N and two pseudohalide-N donor centres) at Fe is
363.62ꢁ in [Fe(MeaaiEt)2(NCS)2]. It shows that the me-
tal ion is essentially in the mean plane. A significant
deviation from trans angle is observed for two axial N-
donors than that of equatorial N-donors in the chelated
fragment. In the equatorial plane, the trans angle
\N(1)–Fe–N(5*)/\N(1*)–Fe–N(5) is 162.92(8)ꢁ in
[Fe(MeaaiEt)2(NCS)2]. The axial \N(3)–Fe–N(3*) is
154.65(8)ꢁ in [Fe(MeaaiEt)2(NCS)2]. The large deviation
(ꢂ25ꢁ) from the trans angle (180ꢁ) is undoubtedly from
the combined contribution of short chelate angle and
steric requirement of azo-N centres. The azo, N@N, dis-
may be due to the manifestation of structural strain aris-
ing out of small chelate bite angle and steric requirement
of the chelated azoimine fragment as well as weak
d(Fe) ! p azo back donation.
The
molecular
structure
of
[Ni(MeaaiMe)
(NCS)2(H2O)2] Æ 2DMF shows that two molecules of
DMF are associated in a secondary coordination sphere
around the complex at the aquated zone. Each DMF
serves as ternary bridging agent through hydrogen
bonding, Ni–O(H)–Hꢃ ꢃ ꢃO(@CHNMe2)ꢃ ꢃ ꢃH–(H)O–Ni
(Fig. 3). Two DMF molecules from two sites of cis-
Ni(OH2)2 bridge two molecular units; altogether, there
are four hydrogen bridging bonds. The bonding
arrangement provides an opportunity to form eight
membered supracycle, [ꢃ ꢃ ꢃH–O–Hꢃ ꢃ ꢃO(DMF)–]2 and
twelve membered metallo-pseudo-macrocycle. The pat-
tern explains the thermal stability of Ni(OH2)2 unit (vide
supra). In the dimer, two MeaaiMe chelating ligands ar-
range in a head-tail manner; driving force to such
arrangement may be coming from better p–p interaction
between imidazole and phenyl rings than that of phenyl–
phenyl/imidazoleꢃ ꢃ ꢃimidazole rings. The existence of
weak pꢃ ꢃ ꢃp interaction amongst dimer units through
hetero-ring systems [(phenyl–imidazole): intra-dimer
˚
distance, Ph(1)ꢃ ꢃ ꢃImz(1), 4.153(2) A; inter-dimer dis-
˚
˚
tances in both the complexes lie 1.26–1.27 A, which is
slightly elongated compared to the free ligand value
tance, Ph(1)ꢃ ꢃ ꢃImz(2), 4.004(5) A, where 1 stands for
rings from a dimeric unit and 2 stands for respective
ring(s) from neighbouring dimer unit where symmetry
˚
˚
(1.250(1) A) [26]. A small elongation (0.01–0.02 A)

U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
1025
Fig. 3. Hydrogen bonding dimer of [Ni(MeaaiMe)(NCS)2(H2O)2] 2DMF (7).
operators are for 1: 1ꢀX, ꢀY, ꢀZ; and for 2: X, 1/
2ꢀY, ꢀ1/2 + Z] makes a supramolecular system.
(lav = 2.8 BM), which corresponds to two unpaired elec-
trons ðd8,t26g e2gÞ. This information also determines octa-
hedral geometry of the complexes.
The vMT vs T plot for Fe(RaaiEt)2(NCS)2 (4b) (vM is
the molar magnetic susceptibility for one Fe(II) ion) is
shown in Fig. 4. vMT value at 300 K is 3.3 cm3 molꢀ1 K.
The vMT values continuously decrease to 1.49
cm3 molꢀ1 K at 2 K. These magnetic data indicate no
spin-crossover in this complex. Likely, the great distor-
3.4. Cyclic voltammetry
Voltammetrc data are set out in Table 2. Fe(R-
aaiR0)2(NCS)2 show an oxidative couple at positive
to SCE in the potential range 0.7–0.8 V. Quasirevers-
ibility of the couple is inferred from peak potential sep-
aration, DEp > 90 mV. This couple has been assigned
to the Fe(III)/Fe(II) couple (Eq. 1) [6]. This interpreta-
tion is also supported by EHMO calculation (vide su-
pra). The HOMO is composed of 72% Fe(II)
character and oxidation is electron extraction from
HOMO.
˚
tion created by the long distances Fe–N(azo) (2.371 A)
(Table 3) avoids this possibility.
When Fe(II) is octahedrally coordinated, the ground
5
state is a T2g. This implies the presence of the strong
first-order spin–orbit coupling. In this case, it would be
possible to fit the experimental data through the formula
½FeðRaaiR0Þ ðNCSÞ ꢁþ þ e ꢀ ½FeðRaaiR0Þ ðNCSÞ ꢁ ð1Þ
2
2
2
2
Redox couples negative to SCE are reductive in nat-
ure and have been assigned to reductions of chelated
RaaiR0. Arylazoheterocycles accommodate two elec-
trons at LUMO, which is mostly azo in character. So,
these two consecutive reductions are referred to azo/
azoꢀ redox reactions of two chelated ligands. Nickel(II)
complexes also exhibit two quasireversible reductions at
negative to SCE along with an irreversible peak at
<ꢀ1.2 V.
3.5
3.0
2.5
T
2.0
M
1.5
3.5. Magnetic data
0
50 100 150 200 250 300
T / K
These complexes are paramagnetic at room tempera-
ture (298 K). Fe(RaaiR0)2(NCS)2 (3, 4) show magnetic
moment about 5.0 BM at 298 K, which is in support
of high spin d6 ðt24g eg2Þ electronic configuration. Nickel
(II) complexes 5, 6 exhibit magnetic susceptibility
Fig. 4. Plot of the vMT vs T from 300 to 2 K, with increasing and
decreasing temperature. Solid line represents the best fit using the
formula for a S = 2 state with zero-field-splitting parameter (see text
for the results).

1026
U. Ray et al. / Inorganica Chimica Acta 358 (2005) 1019–1026
given in the book of Mabbs and Machin [27] for this kind
of T systems. However, when there is great asymmetry,
the complex cannot be considered as octahedral, the
degeneracy of the T term is lost and the system can be con-
sidered as derived from a very distorted T term (very dif-
ficult to treat). In this case, it is better to consider the
system as such a non degenerate ground state with four
unpaired electrons S = 2, which implies the typical zero-
field-splitting (D parameter) [28].
Trying to fit the experimental results with the formula
that considers the spin–orbit coupling [27], the fit was
not possible. However, trying to fit with the formula gi-
ven by Boca in his recent review on zero-field-splitting
[28], the fit is rather good, giving D = 5.0 cmꢀ1 and
g = 2.07, typical values for Fe(II) ions in strongly dis-
torted octahedral geometry.
the National Science Council, Taiwan, for financial sup-
port under Grant NSC 92-2112-M-007-046.
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Crystallographic data for the structures have been
deposited with the Cambridge Crystallographic data cen-
ter, CCDC Nos. 246253 for [Fe(MeaaiEt)2(SCN)2] (4b)
and 246254 for [Ni(MeaaiMe)(H2O)2(SCN)2] Æ 2DMF
(7). Copies of this information may be obtained free of
charge from the Director, CCDC, 12 Union Road, Cam-
bridge CB2, 1EZ, UK (e-mail: deposit@ccdc.cam.ac.uk
or www:http://www.ccdc.cam.ac.uk).
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Acknowledgements
Financial support from the Council of Scientific and
Industrial Research, New Delhi is gratefully
acknowledged. One of us (U.S.R.) thanks UGC, for fel-
lowship. Our sincere thanks are due to Prof. J. Ribas,
Universitat de Barcelona, Spain for his helpful discus-
sion on magnetism of the complexes. We also thank
[25] B.G. Chand, U.S. Ray, P.K. Santra, G. Mostafa, T.-H. Lu, C.
Sinha, Polyhedron 22 (2003) 1205.
[26] D. Das, Ph.D. Thesis, 1998, Burdwan University, Burdwan, 713
104, India.
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Complexes, Chapman and Hall, London, 1973.
[28] R. Boca, Coord. Chem. Rev. 248 (2004) 757.