Synthesis, structure and properties of mononuclear cobalt(II) and cobalt(III) pseudohalide complexes containing N-donor Schiff bases: Synthetic control of metal oxidation levels

Article abstract of DOI:10.1016/j.poly.2005.05.010

Two different series of cobalt(II) and cobalt(III) complexes of types [Co(L¹)(X)₂] (1a-1f) and [Co(L²)(X) ₃] (2a-2f), respectively [L¹ = N,N′-(bis(pyridin-2- yl)formylidene)propane-1,3-diamine (pfpd) or N,N′-(bis(pyridin-2-yl) benzylidene)propane-1,3-diamine (pbpd); L² = (N,N-diethyl,N′- (pyridin-2-yl)formylidene)ethane-1,2-diamine (pfed) or (N,N-diethyl,N′- (pyridin-2-yl)benzylidene)ethane-1,2-diamine (pbed); X=N₃ ^-, NCS^- or NCO^-] have been prepared and characterized using microanalytical, spectroscopic, magnetic, electrochemical and other physicochemical results. Structures of one member in each series, viz., [Co(pfpd)(NCS)₂] (1c) and [Co(pbed)(N₃)₃] (2b) are solved by X-ray diffraction measurements for the exact definition of the coordination sphere. Structural analysis reveals cobalt(II) centre in 1c adopts a distorted octahedral environment with a CoN₆ chromophore ligated by four N atoms of the tetradentate Schiff base and two N-coordinated NCS^- ions in trans orientation. In 2b, the three N atoms of the tridentate Schiff base and three N atoms of three terminal azides are found in meridional arrangement. The organic blockers are folded in ethylenic/propylenic part. Electrochemiocal electron transfer study in MeCN solutions shows cobalt(III)-cobalt(II) response as oxidative in 1 and reductive in 2. Available information shows that the structures 1a, 1b, 1d-1f are cognate with 1c, whereas 2a, 2c, 2d-2f are iso-structural with 2b. All the complexes display intraligand ¹(π-π*) fluorescence and intraligand ³(π-π*) phosphorescence in glassy solutions (MeOH at 77 K).

Full text of DOI:10.1016/j.poly.2005.05.010

Polyhedron 24 (2005) 1755–1763
www.elsevier.com/locate/poly
Synthesis, structure and properties of mononuclear cobalt(II)
and cobalt(III) pseudohalide complexes containing N-donor
Schiff bases: Synthetic control of metal oxidation levels
a
Sk Hafijur Rahaman , Habibar Chowdhury , Doyel Bose , Rajarshi Ghosh ,
a
a
a
b
Chen-Hsing Hung , Barindra Kumar Ghosh
a,
*
a
Department of Chemistry, The University of Burdwan, Golapbag, Rajbati, Burdwan 713 104, India
Department of Chemistry, National Changhua University of Education, Changhua, Taiwan, ROC
b
Received 15 March 2005; accepted 20 May 2005
Available online 18 July 2005
Abstract
Two different series of cobalt(II) and cobalt(III) complexes of types [Co(L1)(X)2] (1a–1f) and [Co(L2)(X)3] (2a–2f), respectively
[L1 = N,N0-(bis(pyridin-2-yl)formylidene)propane-1,3-diamine (pfpd) or N,N0-(bis(pyridin-2-yl)benzylidene)propane-1,3-diamine
(pbpd); L2 = (N,N-diethyl,N0-(pyridin-2-yl)formylidene)ethane-1,2-diamine (pfed) or (N,N-diethyl,N0-(pyridin-2-yl)benzylidene)-
ethane-1,2-diamine (pbed); X ¼ N3ꢀ, NCSꢀ or NCOꢀ] have been prepared and characterized using microanalytical, spectroscopic,
magnetic, electrochemical and other physicochemical results. Structures of one member in each series, viz., [Co(pfpd)(NCS)2] (1c)
and [Co(pbed)(N3)3] (2b) are solved by X-ray diffraction measurements for the exact definition of the coordination sphere. Structural
analysis reveals cobalt(II) centre in 1c adopts a distorted octahedral environment with a CoN6 chromophore ligated by four N
atoms of the tetradentate Schiff base and two N-coordinated NCSꢀ ions in trans orientation. In 2b, the three N atoms of the tri-
dentate Schiff base and three N atoms of three terminal azides are found in meridional arrangement. The organic blockers are folded
in ethylenic/propylenic part. Electrochemiocal electron transfer study in MeCN solutions shows cobalt(III)–cobalt(II) response as
oxidative in 1 and reductive in 2. Available information shows that the structures 1a, 1b, 1d–1f are cognate with 1c, whereas 2a, 2c,
2d–2f are iso-structural with 2b. All the complexes display intraligand 1(p–p*) fluorescence and intraligand 3(p–p*) phosphorescence
in glassy solutions (MeOH at 77 K).
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Cobalt(II)/cobalt(III) compounds; Schiff bases; X-ray structures; Electrochemistry; Luminescence
1. Introduction
be accessed using varied organic blockers and bridging
units. Self-assembly [8] is the most efficient approach to-
wards preparation of such materials. We are also active
[9–11] in this field particularly with pseudohalides and
Schiff bases. Pseudohalides [12] have long been known
for their versatile coordination modes to give mono-
meric, dimeric and polymeric complexes. Schiff bases
[9,13] are useful chelators because of their preparative
accessibilities, structural varieties and varied denticities.
Cobalt is well suited [1] to this study as it permits a wide
range of symmetries and coordination numbers in its
Research on design and synthesis of mono- and poly-
nuclear coordination [1–3] compounds of cobalt in its
varied oxidation levels continues unabated for different
uses [4–7]. Exploiting the various coordination environ-
ments around the metal centre, different geometries may
*
Corresponding author. Tel.: +91 342 2558545; fax: +91 342
2530452.
E-mail address: barin_1@yahoo.co.uk (B.K. Ghosh).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.05.010

1756
S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
(CH2
)
3
(CH2
)
2
Caution! Perchlorate and azide compounds of metal
ions are potentially explosive especially in presence of
organic ligands. Only a small amount of the materials
should be prepared and handled with care.
N
N
N
C2H5
R
R
R
N
C
C
C
C2H5
N
N
N
2.2. Physical measurements
R = H, pfpd
R = Ph, pbpd
R = H, pfed
R = Ph, pbed
Elemental analyses (carbon, hydrogen and nitrogen)
were performed on a Perkin–Elmer 2400 CHNS/O ele-
mental analyzer. IR spectra (KBr discs, 4000–300
cmꢀ1) were recorded using a Jasco FT-IR Model 420
spectrometer. Molar conductances were measured using
a Systronics conductivity meter where the cell constant
was calibrated with 0.01 M KCl solution and dry MeCN
was used as solvent. Room-temperature magnetic
susceptibilities of powder samples were done on a
PAR 155 vibrating sample magnetometer, with Hg[Co-
(SCN)4] as the reference. Diamagnetic corrections were
made using Pascalꢀs constants. Ground state absorption
and steady-state fluorescence measurements were made
with a Jasco Model V-530 UV–Vis spectrophotometer
and Hitachi Model F-4010 spectrofluorimeter, respec-
tively. Time-resolved fluorescence measurements were
carried out using a time-correlated single photon count-
ing (TCSPC) spectrometer Edinburgh Instruments,
Model 199; hydrogen filled coaxial flash lamp with a
pulse width of 1.2 ns at FWHM and a Philips
XP-2020Q Photomultiplier tube were, respectively, used
as the excitation source and for the fluorescence detec-
tion. Electrochemical measurements (cyclic voltamme-
try, CV) were made with a computer controlled
EG&G PARC VersaStat Model 270 electrochemical
instrument using a platinum disk working electrode as
described elsewhere [15]. The solution was IR-compen-
sated and the results were collected at 298 K. The
following parameters and relations were used: scan rate
(m), 50 mV sꢀ1; formal potential Eꢁ = 0.5(Epa + Epc)
where Epa and Epc are anodic and cathodic peak poten-
tials, respectively; DEp is the peak-to-peak separation.
The potentials are referenced to a saturated calomel
electrode (SCE) and are uncorrected for junction
contributions.
Scheme 1.
different oxidation states. We have successfully isolated
six cobalt(II) and six cobalt(III) mononuclear complexes
of the types [Co(L1)(X)2] (1a–1f) and [Co(L2)(X)3]
(2a–2f) [L1 = N,N0-(bis(pyridin-2-yl)formylidene)propane-
1,3-diamine (pfpd) or N,N0-(bis(pyridin-2-yl)benzyli-
dene)propane-1,3-diamine (pbpd); L2 = (N,N-diethyl,
N0-(pyridin-2-yl)formylidene)ethane-1,2-diamine (pfed)
or (N,N-diethyl,N0-(pyridin-2-yl)benzylidene)ethane-1,2-
diamine (pbed), Scheme 1; X ¼ N3ꢀ, NCSꢀ or NCOꢀ]
using one-pot reactions of cobalt(II) salt, L and pseud-
ohalide in 1:1:2 and 1:1:3 molar ratios, respectively, at
room temperature in methanol–water solvent mixtures.
All the complexes are characterized using microanalyti-
cal, spectroscopic, magnetic, electrochemical and other
physicochemical results. In two representative cases,
[Co(pfpd)(NCS)2] (1c) and [Co(pbed)(N3)3] (2b), crystal
structures have been solved by X-ray diffraction
measurements to define the coordination spheres. This
paper has addressed the details of syntheses, structures
and properties of these new complexes and focused on
the stabilization of metal oxidation levels through judi-
cious choice of Schiff base-pseudohalide combinations.
2. Experimental
2.1. Materials
High purity 2-benzoylpyridine (Lancaster, UK), pyri-
dine-2-carboxaldehyde (Lancaster, UK), N,N-diethyle-
thane-1,2-diamine (SRL, India), propane-1,2-diamine
(SRL, India), cobalt nitrate hexahydrate (E. Merck,
India), sodium azide (E. Merck, India), ammonium thio-
cyanate (E. Merck, India), and sodium cyanate (Lancas-
ter, UK) were used as received. Cobalt perchlorate
hexahydrate was prepared on treatment of cobalt
carbonate (E. Merck, India) with perchloric acid
(E. Merck) followed by slow evaporation on a steam
bath, filtration through a fine glass-frit and preservation
in a desiccator containing concentrated sulphuric acid (E.
Merck, India) for subsequent use. Purification of MeCN
and preparation of supporting electrolyte ([Et4N][ClO4])
for electrochemical work were executed following
reported method [14]. All other chemicals were of analyt-
ical grade and were used without further purification.
2.3. Preparation of Schiff bases
The Schiff bases pfpd, pbpd, pfed and pbed were pre-
pared following a reported [11a] procedure with a little
modification.
2.3.1. N,N0-(bis(pyridin-2-yl)formylidene)-propane-1,
3-diamine (pfpd) and N,N0-(bis(pyridin-2-yl)-
benzylidene)propane-1,3-diamine (pbpd)
Propane-1,3-diamine (0.074 g, 1 mmol) and pyridine-
2-carboxaldehyde (0.214 g, 2 mmol) were mixed together
and refluxed in dehydrated alcohol (25 cm3) for 10 h.
The yellow solution that resulted was concentrated on

S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
1757
a steam-bath and dried in vacuo over CaCl2 to a yellow
solid. Yield: 0.176 g (70%). pbpd was prepared (yield,
74%) using similar procedure except that 2-bezoylpyri-
dine instead of pyridine-2-caboxaldehyde was used.
Anal. Calc. for C15H16N4 (pfpd): C, 71.4; H, 6.4; N,
22.2. Found: C, 72.0; H, 6.7; N, 22.5%. IR (KBr disc,
cmꢀ1): m(C@N) 1594. UV–Vis (MeCN): kmax, 260 and
392 nm. Found: C, 80.4; H, 5.8; N, 14.2%. Anal. Calc.
C27H24N4 (pbpd): C, 80.2; H, 6.0; N, 13.9%. IR (KBr
disc, cmꢀ1): m(C@N) 1595. UV–Vis (MeCN): kmax, 260
and 390 nm.
72%) similarly except that pbpd instead of pfpd was
used. Anal. Calc. for C15H16N10Co (1a): C, 45.58; H,
4.08; N, 35.43. Found: C, 45.72; H, 4.02; N, 35.65%.
Anal. Calc. for C27H24N10Co (1b): C, 59.23; H, 4.42;
N, 25.58. Found: C, 59.42; H, 4.45; N, 25.82%.
2.4.2. [Co(pfpd)(NCS)2] (1c) and [Co(pbpd)(NCS)2]
(1d)
These were prepared (yield, 70–75%) using the same
reaction condition and reaction stoichiometry as
described in 1a/1b except that NH4NCS (2 mmol) in
water (5 cm3) instead of NaN3 was added. The reaction
solution was processed as above to result in crystalline
1c/1d. Anal. Calc. for C17H16N6S2Co (1c): C, 47.77; H,
3.77; N, 19.66. Found: C, 47.68; H, 4.72; N, 19.18%.
Anal. Calc. for C29H24N6S2Co (1d): C, 60.09; H, 4.17;
N, 14.49. Found: C, 60.35; H, 4.22; N, 14.28.%.
2.3.2. (N,N-diethyl,N0-(pyridin-2-yl)formylidene)-
ethane-1,2-diamine (pfed) and (N,N-diethyl,N0-(pyridin-
2-yl)benzylidene)-ethane-1,2-diamine (pbed)
The procedure is similar to the synthesis of pfpd/pbpd
except that the reaction time is longer. N,N-diethyle-
thane-1,2-diamine (0.116 g, 1 mmol) and pyridine-2-car-
boxaldehyde (0.107 g, 1 mmol) were refluxed in
dehydrated alcohol (30 cm3) for 12 h. The yellow solu-
tion was processed as above in case of pfpd to yield a
semisolid, which was stored in vacuo over CaCl2 for
subsequent use. Yield: 0.153 g (75%). Pbed was prepared
(yield, 78%) similarly using 2-benzoylpyridine instead of
pyridine-2-carboxaldehyde. Anal. Calc. for C12H19N3
(pfed): C, 70.2; H, 9.3; N, 20.4. Found: C, 70.4; H,
9.0; N, 20.7%. IR (KBr disc, cmꢀ1): m(C@N) 1592.
UV–Vis (MeCN): kmax, 242 and 370 nm. Found: C,
76.3; H, 8.4; N, 14.6%. Anal. Calc. C18H23N3 (pbed):
C, 76.8; H, 8.3; N, 14.9%. IR (KBr disc, cmꢀ1):
m(C@N) 1590. UV–Vis (MeCN): kmax, 245 and 372 nm.
2.4.3. [Co(pfpd)(NCO)2] (1e) and
[Co(pbpd)(NCO)2] (1f)
These were also prepared (yield, 70–75%) using the
same procedure as described in 1a/1b except that NaN-
CO (2 mmol) in water (5 cm3) instead of NaN3 was
added. Anal. Calc. for C17H16N6O2Co (1e): C, 51.65;
H, 4.08; N, 21.26. Found: C, 51.68; H, 4.10; N,
21.23%. Anal. Calc. for C29H24N6O2Co (1f): C, 63.62;
H, 4.42; N, 15.35. Found: C, 63.65; H, 4.40; N, 15.38%.
2.4.4. [Co(pfed)(N3)3] (2a) and [Co(pbed)(N3)3]
(2b)
To a methanolic solution (5 cm3) of Co(ClO4)2 Æ
6H2O (0.183 g, 1 mmol) were added pfed (0.103 g, 1
mmol) in the same solvent (5 cm3) and NaN3 (0.098 g,
3 mmol) in water (5 cm3). The deep wine red solution
was filtered and was kept for slow evaporation. After
7 days 2a was separated, washed with methanol and
dried in vacuo over CaCl2. Yield: 0.146 g (75%).
Compound 2b was prepared (yield, 72%) using a similar
procedure as described above except that pbed instead
of pfed was used. Anal. Calc. for C12H19N12Co (2a):
C, 36.93; H, 4.91; N, 43.06. Found: C, 36.76; H, 3.84;
N, 43.24%. Anal. Calc. for C18H23N12Co (2b): C,
46.35; H, 4.97; N, 36.04. Found: C, 46.27; H, 4.89; N,
36.22%.
2.4. Preparation of the complexes
Mononuclear type 1 complexes were prepared from
both nitrate and perchlorate salts of cobalt(II) using
1:1:2 mole ratio of the metal salts, tetradentate block-
ers (pfpd and pbpd) and pseudohalides (azide, thiocy-
anate and cyanate). Type 2 compounds were isolated
using a 1:1:3 ratio of the cobalt(II) salts, tridentate
Schiff bases (pfed and pbed) and the pseudohalides.
The typical syntheses with perchlorate salt are des-
cribed below.
2.4.1. [Co(pfpd)(N3)2] (1a) and [Co(pbpd)(N3)2]
(1b)
2.4.5. [Co(pfed)(NCS)3] (2c) and [Co(pbed)(NCS)3]
(2d)
A methanolic solution (5 cm3) of faint yellow pfpd
(0.126 g, 1 mmol) was added dropwise to a pink Co-
(ClO4)2 Æ 6H2O (0.183 g, 1 mmol) solution (5 cm3) in
the same solvent. To this NaN3 (0.065 g, 2 mmol) in
water (5 cm3) was added slowly. The final red solution
was filtered and left for slow evaporation in air at room
temperature. After 3–4 days fine red microcrystalline
product of 1a separated out, which was filtered, washed
with methanol and dried in vacuo over CaCl2. Yield:
0.138 g (70%). Compound 1b was prepared (yield,
These were prepared (yield, 70–72%) using the same
reaction condition and reaction stoichiometry as
described in 2a/2b except that NH4NCS (3 mmol) in
water (5 cm3) instead of NaN3 was added. The reaction
solution was processed as above to result in crystalline
2c/2d. Anal. Calc. for C15H19N12S3Co (2c): C, 34.48;
H, 3.66; N, 32.17. Found: C, 34.60; H, 3.58; N,
32.10%. Anal. Calc. for C21H23N12S3Co (2d): C, 42.13;
H, 3.87; N, 28.08. Found: C, 42.25; H, 3.76; N, 28.25%.

1758
S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
2.4.6. [Co(pfed)(NCO)3] (2e) and [Co(pbed)(NCO)3]
(2f)
on w scans was employed [16]. The structures were
solved by the heavy atom method using SHELXS-97 [17]
and successive difference Fourier syntheses. All non-
hydrogen atoms were refined anisotropically. The
hydrogen atoms were fixed geometrically and refined
using a riding model. In the final difference Fourier
map the residual maxima and minima for 1c and 2b were
These were also prepared (yield, 70–74%) using the
same procedure as described in 2a/2b except that Na-
NCO (3 mmol) in water (5 cm3) instead of NaN3 was
added. Anal. Calc. for C15H19N12O3Co (2e): C, 37.98;
H, 4.04; N, 35.44. Found: C, 37.66; H, 4.07; N,
35.53%. Anal. Calc. for C21H23N12O3Co (2f): C, 45.82;
H, 4.21; N, 30.54. Found: C, 45.85; H, 4.01; N, 30.71%.
0.268 and ꢀ0.241 and 0.501 and ꢀ0.346 e Aꢀ3, respec-
˚
tively. All calculations were carried out using SHELXL-
97 [18].
2.5. X-ray crystallographic analysis
Red single crystals of 1c and dark brown crystals of
2b suitable for X-ray analyses were obtained by slow
evaporation of MeOH–H2O (2:1) solutions of the reac-
tion mixtures at room temperature (298 K). Diffraction
data were measured at 293(2) K on a Siemens SMART
CCD diffractometer equipped with a graphite mono-
3. Results and discussion
3.1. Synthesis and formulation
Thehexacoordinatedmononuclearcomplex[Co(pfpd)-
(NCS)2] (1c) was initially formed in aqueous methanolic
solution containing a 1:1:1 mixture of Co(ClO4)2 Æ
6H2O, pfpd and NH4NCS – a reactant ratio expected
to yield a dithiocyanato bridged dinuclear species of
the composition [(pfpd)CoII(NCS)2CoII(pfpd)](ClO4)2.
However, microanalyses showed a 1:1:2 ratio of metal,
blocking ligand and pseudohalide, and in the IR spec-
trum, no presence of perchlorate bands was noticed.
Reactant ratio corresponding to the product stoichiom-
etry afforded better yield of 1c. An effort to get dinuclear
˚
chromated Mo Ka (k = 0.71073 A) radiation. The unit
cell parameters were determined by least-squares meth-
od using all observed reflections. A summary of the crys-
tallographic data and structure refinement parameters is
given in Table 1. Of 12050 and 13562 collected reflec-
tions, 4378 and 4956 unique reflections were recorded
using x-scan technique for 1c and 2b, respectively. The
intensity data were corrected for Lorentzian polariza-
tion effects and an empirical absorption correction based
Table 1
Crystallographic data of [Co(pfpd)(NCS)2] (1c) and [Co(pbed)(N3)3] (2b)
Complex
1c
2b
Empirical formula
Formula weight
Colour
C
17H16N6S2Co
C18H23N12Co
466.41
427.41
orange
293(2)
dark brown
293(2)
T (K)
˚
k (A)
Crystal system
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
Space group
˚
a (A)
˚
8.3095(5)
9.5110(6)
24.6886(16)
98.2490(10)
1931.0(2)
4
7.9753(6)
13.4972(11)
20.3435(16)
99.789(2)
2158.0(3)
4
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
Dcalc (Mg mꢀ3
)
1.470
1.118
1.436
0.828
l (mmꢀ1
F(000)
)
876
0.38 · 0.27 · 0.22
2.30–27.51
968
0.40 · 0.15 · 0.10
1.82–27.54
Crystal size (mm3)
h ranges (ꢁ)
h/k/l
ꢀ10,10/ꢀ12,12/ꢀ19,31
12050
4378 (0.0235)
ꢀ9,10/ꢀ17,14/ꢀ26,16
13562
Reflections collected
Independent reflections (Rint
Completeness to h (ꢁ)
Tmax and Tmin
)
4956 (0.0310)
99.3
98.6
0.9486 and 0.7756
4378/0/253
0.788
0.9486 and 0.6709
4956/0/282
0.745
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
R1 = 0.0308, wR2 = 0.0578
R1 = 0.0647, wR2 = 0.0634
0.268 and ꢀ0.241
R1 = 0.0347, wR2 = 0.0619
R1 = 0.0750, wR2 = 0.0684
0.501 and ꢀ0.346
ꢀ3
˚
Largest peak and hole (e A
)

S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
1759
species, in which the starting material was changed to
Co(NO3)2 Æ 6H2O, and a 1:1:1 molar ratio of the nitrate
salt, ligand and pseudohalide followed by addition of
extraneous KPF6 (2 equivalent) produced 1c with lower
yield. Use of a 1:1:2 molar ratio of nitrate salt, Schiff
base and azide results in 1c in good yield. The different
syntheses are reproducible. Change of ligands from pfpd
The compounds are soluble in a range of common
organic solvents like methanol, acetonitrile, dimethyl-
formamide and dimethylsulphoxide but are insoluble
in water. In MeCN they behave as non-electrolytes as
are indicated [19] by their very low KM (ꢂ5 Xꢀ1 cm2
molꢀ1) values. Room-temperature solid-phase magnetic
susceptibility measurements show that type 1 complexes
behave as three-electron paramagnets with leff value
ꢂ3.9 BM whereas type 2 compounds are diamagnetic
(singlet ground state with t62g configuration) (Table 2).
In the IR spectra, the most striking observation in type
1 complexes is a sharp, single, intense band of the pseud-
ohalides. Compounds 1a and 1b show the asymmetric
to pbpd and the pseudohalides from NCSꢀ to N3 and
ꢀ
NCOꢀ showed similar behaviour and compounds
[Co(pfpd)(N3)2] (1a), [Co(pbpd)(N3)2] (1b), [Co(pbpd)-
(NCS)2] (1d), [Co(pfpd)(NCO)2] (1e) and [Co(pbpd)-
(NCO)2] (1f) were isolated. Changing the tetradentate
Schiff bases (L1 = pfpd or pbpd) to tridentate Schiff
bases (L2 = pfed or pbed) and use of metal perchlorate,
organic ligand and pseudohalide in 1:1:1.5 molar ratio –
a reactant ratio expected to yield a triazido bridged
dinuclear species of the composition [(L2)CoIII(N3)3-
CoIII(L2)]ClO4, resulted mononuclear complexes of the
stretch as
a .
strong band at about 2030 cmꢀ1
Compounds 1c and 1d exhibit mas(CN) stretching vibra-
tion at about 2100 cmꢀ1. The position of the signal
strongly suggests N-coordination of bound isothiocya-
nate [10]. Compounds 1e and 1f display asymmetric
cyanate stretching vibration at about 2050 cmꢀ1 which
is substantially higher than the free ion value [20] and
is consistent [21] with N-bonding rather O-bonding.
The single band in each case is inꢀline withꢀthe presence
of trans-CoX2 (X ¼ N3ꢀ, NCS , NCO ) frame in
[Co(L1)(X)2]. Three distinct splittings (vide Table 2) of
asymmetric stretching vibrations of the terminal pseud-
ohalides in type 2 compounds are strongly suggestive of
the less symmetric meridional structure over the more
symmetric facial arrangement. The m(C@N) stretching
vibrations of the metal bound Schiff bases are seen at
around 1590 cmꢀ1. Several bands in the range 2800–
3000 cmꢀ1 are assigned to the aliphatic m(CH) stretching
vibration of ligand. All other characteristic L vibrations
are seen in the 1600–600 cmꢀ1 range. X-ray structures of
1c and 2b show definite proof of this hypothesis.
type [Co(L2)(X)3] (L = pfed, X ¼ N ꢀ; 2ꢀa; NCSꢀ, 2ꢀb;
3
NCOꢀ, 2c; L = pbed, X ¼ N3ꢀ, 2d; NCS , 2e; NCO ,
2f). Microanalyses showed a 1:1:3 ratio of metal, block-
ing ligand and pseudohalide, and once again in the IR
spectrum, no presence of perchlorate bands was noticed.
Use of required molar ratios of the building units corre-
sponding to formulation of the product afforded better
yields of 2. With a view to getting tribridged dinuclear
species, the starting material was changed to Co(NO3)2 Æ
6H2O, and a 1:1:1.5 molar ratio of the nitrate salt,
ligand and pseudohalide followed by addition of extra-
neous KPF6 (1 equivalent). This also produced 2 but
in lower yield. Use of a 1:1:3 molar ratio of nitrate salt,
Schiff base and azide results in 2 in good yield. Once
again all these syntheses are reproducible.
The complexes 1 and 2 are sufficiently stable in air
and in the presence of moisture. Analytical data for all
the complexes are consistent with the calculated values.
The complexes exhibit several absorption bands in
MeCN solutions in the 200–900 nm range. The high
Table 2
Some characterization data of the complexes
Compound IRa (cmꢀ1) mas(X)
UV–Visb
k
max, nm (10ꢀ3 e Mꢀ1 cmꢀ1
)
KM (Xꢀ1 cm2 molꢀ1
)
leff (BM)
CoIII–CoII E2ꢁ98; V (DEp, mV)e
c
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
a
2025
2028
552 (0.55), 412 (0.73), 384 (12.76)
554 (0.54), 415 (0.70), 382 (12.98)
555 (0.52), 412 (0.72), 384 (12.78)
551 (0.55), 410 (0.75), 382 (13.01)
550 (0.56), 413 (0.71), 380 (13.12)
556 (0.52), 415 (0.69), 385 (12.68)
508 (0.68), 404 (0.74), 332 (16.82)
510 (0.66), 406 (0.71), 330 (16.71)
512 (0.64), 402 (0.76), 334 (16.23)
511 (0.63), 402 (0.75), 328 (17.08)
510 (0.65), 404 (0.73), 332 (16.79)
512 (0.63), 406 (0.70), 333 (16.74)
3
4
3
5
4
5
3
4
5
4
5
4
3.88
3.87
3.86
3.85
3.86
0.32 (70)
0.33 (70)
0.33 (70)
0.34 (70)
0.31 (70)
0.32 (70)
0.36 (70)
0.37 (70)
0.34 (70)
0.36 (70)
0.34 (70)
0.35 (70)
2095
2098
2250
2248
3.87
d
2002, 2010, 2025
2004, 2008, 2026
2061, 2077, 2095
2060, 2078, 2098
2255, 2235, 2218
2252, 2233, 2215
d
d
d
d
d
KBr disc.
b
In MeCN at 298 K.
In MeCN at 298 K.
Diamagnetic.
c
d
e
Working electrode is platinum.

1760
S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
Table 3
intensity band below 400 nm is of ligand origin assign-
able [11b] to intraligand n–p*/p–p* transition. Above
this wavelength the transitions are LMCT for cobalt(III)
complexes and MLCT type for cobalt(II) members as
are evident from their intensity values (Table 2). The
bands show systematic blue shift in going from type 1
to type 2 complexes. The spectral pattern supports [22]
six-coordinate distorted octahedral symmetry around
cobalt centre.
˚
Selected bond lengths (A) and bond angles (ꢁ) for [Co(pfpd)(NCS)2]
(1c)
˚
Bond distances (A)
Co(1)–N(2)
Co(1)–N(1)
Co(1)–N(4)
Co(1)–N(5)
Co(1)–N(6)
Co(1)–N(3)
S(1)–C(1)
2.0813(18)
2.0844(19)
2.0982(17)
2.1061(18)
2.1591(17)
2.1695(18)
1.621(2)
S(2)–C(2)
N(1)–C(1)
N(2)–C(2)
1.618(2)
1.160(1)
1.152(2)
3.2. X-ray crystal structures
3.2.1. Crystal structure of [Co(pfpd)(NCS)2] (1c)
An ORTEP view with atom numbering scheme of the
mononuclear dithiocyanato complex, 1c, is shown in
Fig. 1. Selected bond lengths and angles relevant to
the coordination sphere are listed in Table 3. The cobal-
t(II) centre is best described as a distorted octahedron
with a CoN6 chromophore. Distortion from the ideal
octahedral geometry is due to the asymmetric nature
of the bound tetradentate Schiff base and the deviations
of the refine angles (90ꢁ/180ꢁ) formed at the metal centre
(Table 3). The coordination includes two pyridine nitro-
gens [N(3), N(6)], two imine nitrogens [N(4), N(5)] of
the Schiff base and two nitrogens [N(1), N(2)] of the
terminal thiocyanates. The equatorial positions are
occupied with the four nitrogen atoms [N(3), N(4),
N(5) and N(6)] of the tetradentate ligand while the
nitrogens [N(1), N(2)] of the isothiocyanato groups are
placed at the axial positions. The degrees of distortion
of the coordination sphere are reflected in the cisoid
[77.11(7)–113.07(7)ꢁ] and the transoid angles [169.47(7)–
174.59(7)ꢁ]. The equatorial Co–N distances are in the
Bond angles (ꢁ)
N(2)–Co(1)–N(1)
N(2)–Co(1)–N(4)
N(1)–Co(1)–N(4)
N(2)–Co(1)–N(5)
N(1)–Co(1)–N(5)
N(4)–Co(1)–N(5)
N(2)–Co(1)–N(6)
N(1)–Co(1)–N(6)
N(4)–Co(1)–N(6)
N(5)–Co(1)–N(6)
N(2)–Co(1)–N(3)
N(1)–Co(1)–N(3)
N(4)–Co(1)–N(3)
N(5)–Co(1)–N(3)
N(6)–Co(1)–N(3)
C(1)–N(1)–Co(1)
C(2)–N(2)–Co(1)
N(1)–C(1)–S(1)
174.59(7)
93.73(7)
91.64(7)
89.91(7)
90.54(7)
92.45(8)
87.27(7)
87.56(7)
169.80(8)
77.40(7)
89.49(7)
91.05(7)
77.11(7)
169.47(7)
113.07(7)
156.96(19)
170.25(19)
178.7(2)
178.7(2)
N(2)–C(2)–S(2)
˚
Co–N distances [2.0813(18)–2.0844(19) A] indicate
stronger coordination of the anionic isothiocyanate over
neutral Schiff base. The trans N1–Co1–N2 angle has a
value [174.59(7)ꢁ] close to the ideal 180.00ꢁ. The angles
N(6)–Co(1)–N(5), N(5)–Co(1)–N(4) and N(4)–Co(1)–
N(3) in the equatorial plane have values 77.40(7)ꢁ,
92.45(8)ꢁ and 77.11(7)ꢁ, respectively, which are much
less than the N(3)–Co(1)–N(6) angle [113.07(7)ꢁ] where
N(3) and N(6) belong to the two ends of the tetradentate
Schiff base. Both isothiocyanates are almost linear with
the same N–C–S angle 178.7(2)ꢁ. N–C [1.152(2)–
˚
range [2.0982(17)–2.1695(18) A], comparable to the
corresponding values in similar systems [1–3]. Axial
˚
˚
1.160(2) A] and C–S [1.618(2)–1.621(2) A] bond lengths
are as expected for N-coordination [12]. The bond
lengths and angles pertaining the ligand network are
close to expected values [9–11]. The propylenic part
[N(5)–C(11)–C(10)–C(9)–N(4)] of the Schiff base is to
some extent puckered. The sequestering of cobalt(II)
by four hard N donor centres of the tetradentate chela-
tor (pfpd) imparts hardness to the metal centre even in
low oxidation state; this influences preferential N-coor-
dination of both the terminal NCSꢀ ions through
electronegative hard N donor centres (Fig. 1) ignoring
the other possibilities, viz., both S-coordination and
N- plus S-coordinations.
Fig. 1. ORTEP view of [Co(pfpd)(NCS)2] (1c) with atom numbering
scheme and 40% probability ellipsoids for all non-hydrogen atoms.

S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
1761
3.2.2. Crystal structure of [Co(pbed)(N3)3] (2b)
atom [N(4)] of the terminal azide while other two nitro-
gen atoms [N(1), N(7)] of the two azides are placed at
the axial positions. Distortion from the ideal octahedral
geometry is due to the asymmetric nature of the bound
tridentate Schiff base and the deviations of the refine an-
gles (90ꢁ/180ꢁ) formed at the metal centre (Table 4). The
degrees of distortion from an ideal octahedral (90ꢁ)
geometry are reflected in the cisoid [87.64(8)–96.69(8)ꢁ]
and the transoid [167.90(7)–177.26(8)ꢁ] angles. Azide
An ORTEP diagram of the mononuclear triazido
compound 2b, is displayed in Fig. 2. Selected bond
lengths and angles pertaining to the coordination sphere
are set in Table 4. The cobalt(III) centre adopts a
distorted octahedral coordination sphere with a CoN6
chromophore. Three nitrogen atoms [N(10), N(11),
N(12)] of the tridentate Schiff base in combination with
three nitrogen atoms [N(1), N(4), N(7)] of three different
azides in a meridional alignment complete octahedral
coordination of the metal ion. The equatorial positions
are occupied by the three nitrogen atoms [N(10),
N(11), N(12)] of the tridentate blocker and one nitrogen
˚
Co–N distances [1.9451(19)–1.9801(18) A] indicate
stronger coordination of the anionic terminals over
˚
those [1.8858(17)–2.0544(18) A] of neutral Schiff base
chelator. The azide terminals are almost linear with
N–N–N angles 175.6(2)–177.7(2)ꢁ. N(1)–N(2) [1.185(2)
A], N(4)–N(5) [1.198(2) A] and N(7)–N(8) [1.198(2) A]
˚
˚
˚
˚
distances are longer than the N(2)–N(3) [1.158(2) A],
˚
˚
N(5)–N(6) [1.150(3) A] and N(8)–N(9) [1.160(2) A] in
usual manner [9–11] reflecting N(1), N(4) and N(7)
connection of the pseudohalides. The ethylenic part
[N(11)–C(13)–C(14)–N(10)] of the Schiff base is also to
some extent puckered (Fig. 2) which has significant
effect on variation of equatorial angles.
3.3. Electrochemical properties
Fig. 2. ORTEP plot of [Co(pbed)(N3)3] (2b) with atom numbering
scheme and 40% probability ellipsoids for all non-hydrogen atoms.
The electroactivity of the complexes was examined in
MeCN solutions using cyclic voltammetry (CV) at a
platinum-working electrode. A representative voltam-
mogram is shown in Fig. 3. The complexes show
(Table 2) nearly reversible (DEp = 70–90 mV) oxidative
response (for type 1) and reductive response (for type
2) presumably due to CoIII–CoII couple as is shown in
the following reactions:
Table 4
˚
Selected bond distances (A) and bond angles (ꢁ) for [Co(pbed)(N3)3]
(2b)
˚
Bond distances (A)
Co(1)–N(1)
Co(1)–N(4)
Co(1)–N(7)
Co(1)–N(10)
Co(1)–N(11)
Co(1)–N(12)
N(1)–N(2)
N(2)–N(3)
N(4)–N(5)
N(5)–N(6)
N(7)–N(8)
N(8)–N(9)
1.9801(18)
1.9451(19)
1.9637(19)
2.0544(18)
1.8858(17)
1.9290(17)
1.185(2)
1þ
½CoIIIðL1ÞðXÞ ꢃ þ eꢀ ꢀ ½CoIIðL1ÞðXÞ ꢃ
ð1Þ
ð2Þ
2
2
1ꢀ
½CoIIIðL2ÞðXÞ ꢃ þ eꢀ ꢀ ½CoIIðL2ÞðXÞ ꢃ
3
3
1.158(2)
1.198(2)
1.150(3)
1.198(2)
1.160(2)
Bond angles (ꢁ)
N(2)–N(1)–Co(1)
N(5)–N(4)–Co(1)
N(8)–N(7)–Co(1)
N(1)–Co(1)–N(10)
N(4)–Co(1)–N(1)
N(7)–Co(1)–N(1)
N(12)–Co(1)–N(1)
N(11)–Co(1)–N(1)
N(4)–Co(1)–N(7)
N(12)–Co(1)–N(10)
N(4)–Co(1)–N(10)
N(7)–Co(1)–N(10)
N(3)–N(2)–N(1)
N(6)–N(5)–N(4)
N(9)–N(8)–N(7)
119.14(15)
122.72(16)
120.11(16)
94.12(8)
87.64(8)
177.26(8)
88.29(8)
90.20(8)
90.67(9)
167.90(7)
96.69(8)
88.22(8)
177.6(2)
175.6(3)
177.7(2)
Fig. 3. Cyclic voltammogram of 2b in MeCN at 298 K.

1762
S.H. Rahaman et al. / Polyhedron 24 (2005) 1755–1763
Table 5
Photophysical data for 1c and 2b
[Co(pbed)(N3)3] (2b), are solved by X-ray study.
Spectroscopic, electrochemical and other physicochemi-
cal properties strongly suggest that the structures 1a, 1b
and 1d–1f are cognate with 1c, whereas 2a and 2c–2f
with 2b. The neutral tetradentate Schiff bases in combi-
nation with two monodentate pseudohalide ions satisfy
hexacoordination of cobalt(II) as well as its charge; such
a combination simultaneously satisfying both coordina-
tion number and charge of central ion results in stabil-
ization of +2 oxidation state of the central. Similarly,
judicious choice of one tridentate neutral Schiff base
ligand and three pseudohalide ions satisfies hexacoordi-
nation of the central ion and its +3 oxidation level. This
work shows that control of oxidation state of metal ion
can be done through judicious choice of ligand matrices
so as to satisfy coordination number and charge of
metal ions simultaneously. The new complexes are good
examples of luminous materials.
Compound
Emission (k/nm)
Lifetime (ns)
Fluorescencea
Phosphorescenceb
1c
2b
a
478
476
550
548
2.33
2.42
In MeOH at room temperature (298 K).
In MeOH at 77 K.
b
5. Supplementary data
Crystallographic data for the structure analysis have
been deposited with the Cambridge Crystallographic
Data Centre Nos. 210735 for [Co(pfpd)(NCS)2] (1c)
and 240924 for [Co(pbed)(N3)3] (2b). Copies of this
information can be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.
cam.ac.uk or www:http://www.ccdc.cam.ac.uk).
Fig. 4. Fluorescence (- - - - - -) in MeOH solution at 298 K, phospho-
rescence (ꢄ–ꢄ–ꢄ–ꢄ) in MeOH glass at 77 K of 1c.
The responses are reproducible with no trace of decom-
position after a number of cycles. The formal potentials
lie close to ꢂ0.3 V versus SCE. One-electron nature of
the couple was verified with cobalt(II)azido complexes
of arylazoimidazole ligands [23].
Acknowledgements
The authors are grateful to the Department of Sci-
ence and Technology (DST) and Council of Scientific
and Industrial Research (CSIR) New Delhi, India for
financial support.
3.4. Luminescence properties
The spectroscopic data of selected complexes in
methanol solutions and glasses are listed in Table 5.
Compound 1c shows emission at 478 nm while com-
pound 2b has a band at 476 nm at 298 K. These are
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