Cobalt(II) 4-nitrobenzoates having pyridine as ancillary ligands and their catalytic role in the TBHP oxidation of alcohols

Article abstract of DOI:10.1016/j.poly.2010.03.014

By making use of the Co²⁺/pyridine (py)/O₂N-4-C₆H₄CO₂^- (4-nbz) system it has been possible to isolate two new complexes with markedly different stabilities: a purple form [Co(4-nbz)₂(py)₂] (1) and a pink form [Co(4-nbz)₂(py)₂(H₂O)₂] (2). These compounds have been characterized by single crystal X-ray diffraction. Compound 1 may be described as approximately tetrahedral, while compound 2 has Co(II) in a nearly octahedral environment. Both π-π stacking and weak C-H···O interactions appear to contribute to the moderate stability of the tetrahedral complex in the solid state. 2 displays an interesting supramolecular layered structure which is stabilized further by interlayer C-H···O interactions. Facile interconversion between 1 and 2 is found to occur in solid as well as solution phases. Apart from the synthesis, structure and spectral properties of these compounds, catalytic activity of 1 in the tert-butyl hydroperoxide (TBHP) oxidation of alcohols is also discussed in this paper.

Full text of DOI:10.1016/j.poly.2010.03.014

Polyhedron 29 (2010) 2006–2013
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cobalt(II) 4-nitrobenzoates having pyridine as ancillary ligands
and their catalytic role in the TBHP oxidation of alcohols
*
Stutee Chakravorty, Birinchi K. Das
Department of Chemistry, Gauhati University, Guwahati 781 014, Assam, India
a r t i c l e i n f o
a b s t r a c t
By making use of the Co2+/pyridine (py)/O2N-4-C6H4CO2 (4-nbz) system it has been possible to isolate
two new complexes with markedly different stabilities: a purple form [Co(4-nbz)2(py)2] (1) and a pink
form [Co(4-nbz)2(py)2(H2O)2] (2). These compounds have been characterized by single crystal X-ray dif-
fraction. Compound 1 may be described as approximately tetrahedral, while compound 2 has Co(II) in a
ꢀ
Article history:
Received 14 January 2010
Accepted 20 March 2010
Available online 30 March 2010
nearly octahedral environment. Both
p
ꢀ stacking and weak C–HꢁꢁꢁO interactions appear to contribute to
p
Keywords:
Crystal structures
Cobalt(II) complexes
Graph-set analysis
Tetrahedral–octahedral interconversion
Alcohol oxidation
the moderate stability of the tetrahedral complex in the solid state. 2 displays an interesting supramolec-
ular layered structure which is stabilized further by interlayer C–HꢁꢁꢁO interactions. Facile interconversion
between 1 and 2 is found to occur in solid as well as solution phases. Apart from the synthesis, structure
and spectral properties of these compounds, catalytic activity of 1 in the tert-butyl hydroperoxide (TBHP)
oxidation of alcohols is also discussed in this paper.
Ó 2010 Elsevier Ltd. All rights reserved.
TBHP oxidant
1. Introduction
solid products [9]. In fact, tetrahedral complexes among cobalt(II)
carboxylates having pyridyl ligands as ancillaries are quite uncom-
Catalytic as well as biological roles of mixed-ligand transition
metal complexes containing carboxylate anions and nitrogen base
ligands are widespread [1,2]. For example, metal salts of aromatic
carboxylic acids are known to display anti-bacterial and anti-fun-
gal properties [3,4]. Previous studies from this laboratory have
shown that complexes of cobalt and copper containing carboxyl-
ates and pyridines as ligands may act as oxidation catalysts in
the transformation of alkylaromatics, alcohols and alkenes under
environmentally friendly conditions [5–8]. In this context, we note
here that while preparing the cobalt(III) cubane compounds of type
[Co4O4(O2CR)4L4] for the above purpose a methanolic solution con-
mon. A search in the Cambridge Structural Database (CSD) [10]
showed only one example of such tetrahedral cobalt(II) complexes,
viz. bis(g
1-benzoato)bis(4-methylpyridine)cobalt(II) [11], for
which a crystal structure is known. Earlier attempts to isolate
four-coordinate cobalt(II) species led to the formation of only
six-coordinate complexes [12]. In view of favorable thermody-
namic effects Co(II) complexes of the octahedral and tetrahedral
geometries display facile structural interchange. Solid-state effects
including supramolecular interactions are thus likely to be impor-
tant in the stabilization of the four-coordinate species. Herein we
report the syntheses, crystal structures and physicochemical prop-
erties of two Co(II) complexes, viz. Co(4-nbz)2(py)2 (1), [Co(4-
nbz)2(py)2(H2O)2] (2) containing 4-nbz as monodentate ligands.
In addition, we also describe our results on the use of complex 1
in the catalytic oxidation of both primary and secondary alcohols
using tert-butyl hydroperoxide (TBHP) as the oxidant.
taining Co2+, RCO2 and L (which is either pyridine or a substituted
ꢀ
pyridine) is generated before producing the cobalt(III) tetramers by
subsequent H2O2 oxidation [5,6]. However, with R = C6H4-4-NO2
and L = py (C5H5N) the above mixture gives a solid product prior
to H2O2 oxidation. In our efforts to identify this species, which is
now known to be a six-coordinate Co(II) complex, we have also
been able to isolate a four-coordinate mixed-ligand complex of
Co(II) containing two each of nitrobenzoato and pyridine ligands
constituting an approximate tetrahedral coordination geometry
around the cobalt center.
2. Experimental
Materials used in this work were obtained from commercial
sources and used as received. Co(NO3)2ꢁ6H2O was obtained from E.
Merck (India). The sodium salt of 4-nitrobenzoic acid (NaO2CC6H4-
4-NO2) was prepared by neutralizing the acid with NaOH.
While benzyl alcohol and 1-phenyl ethanol were obtained from
E. Merck (India), 4-nitrobenzyl alcohol, 1-phenyl-1-propanol and
TBHP (70% aqueous solution) were purchased from Aldrich (USA).
Benzhydrol and benzoin were purchased from Loba Chemie India).
Formation of both tetrahedral and octahedral complexes is a
common feature of the chemistry of cobalt(II). However, in pres-
ence of moisture tetrahedral complexes are not easy to obtain as
* Corresponding author. Tel.: +91 361 2570535; fax: +91 361 2700311.
E-mail addresses: Birinchi.Das@hotmail.com, das_bk@rediffmail.com (B.K. Das).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.03.014

S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
2007
1-(40-methyl)-phenyl ethanol was prepared by reduction of the
corresponding ketone, 4-methyl acetophenone (E. Merck, India) by
literature method [13].
1514(vs), 1487(w), 1449(s), 1403(w), 1347(w), 1347(w), 1319(w),
1217(w), 1103(s), 1076(w), 1039(w), 1013(m), 881(m), 797(s),
760(m), 724(s), 701(s), 630(w) and 526(w). [s, strong; m, medium;
w, weak; br, broad].
The mid-IR infrared spectra in the 4000–450 cmꢀ1 region were
recorded using a Perkin–Elmer RX1 FT-IR spectrophotometer as
KBr pellets. C–H–N analyses were done using a Perkin–Elmer
2400 Series II CHNS/O instrument. Cobalt analysis was performed
gravimetrically by precipitating out cobalt as Hg[Co(NCS)4] follow-
ing standard procedure. While the solution UV–Vis spectra were
recorded on a Perkin–Elmer Lambda 40 spectrophotometer, the
diffuse-reflectance UV–Vis–NIR (200–2600 nm) spectra were ob-
tained using a Hitachi U-4100 spectrophotometer equipped with
an integrating sphere. Magnetic susceptibilities were recorded at
room temperature using a benchtop Sherwood MK-1 balance by
Evans Method. The susceptibility data were corrected for diamag-
netism by making use of Pascal’s constants. Gas chromatography
was done by using a Varian 450-GC instrument equipped with a
FID detector and a CP-Sil 8CB capillary column. Retention times
of reaction products were confirmed by comparison with authentic
samples. Details of single crystal X-ray data collection and struc-
ture determination of complexes are described below in a separate
section.
2.3. X-ray crystallographic procedures
Crystals of suitable size were picked for intensity data collection
at room temperature using graphite-monochromatized Mo–K
a
radiation (k = 0.71073 Å) at 293 K on a Bruker SMART CCD diffrac-
tometer [14]. The crystals were stable during the measurements
at room temperature, no intensity decay was observed. The struc-
tures were solved by direct methods (SHELXS-97) and standard Fou-
rier techniques and refined on F2 using full matrix least squares
techniques (SHELXL-97) [15] using the
WinGX [16] platform available
for personal computers. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The aromatic hydrogen
atoms were placed at calculated positions and refined by making
use of a riding model. The hydrogen atoms of the water molecules
for compound 2 were located in difference Fourier maps and re-
fined with isotropic atomic displacement parameters. The crystal
data and structure refinement details are given in Table 1. The
crystal structures were analyzed using
Mercury [17] and PLUTON
[18], while the structural diagrams were drawn using Diamond 3
[19].
2.1. Preparation of Co(O2CC6H4-4-NO2)2(C5H5N)2 (1)
Co(NO3)2ꢁ6H2O (0.5 mmol, 0.145 g) was dissolved in 5 mL of
methanol. NaO2CC6H4-4-NO2 (1 mmol, 0.189 g) and pyridine
(1.5 mmol, ꢂ0.15 mL) were added to this solution and the mixture
was stirred at room temperature for 1 h. The purple precipitate
was filtered and washed with diethyl ether under suction to obtain
a dry crystalline powder. Yield: 61% (based on cobalt). To obtain
suitable crystals for single crystal X-ray diffraction work, a 1 mM
methanolic solution of NaO2CC6H4-4-NO2 (3 mL) was slowly added
to 2 mL of a methanolic solution of Co(NO3)2ꢁ6H2O (0.5 mmol) con-
taining pyridine (1.5 mmol) and the mixture was left undisturbed
in a test tube. After 1–2 days a good crop of transparent purple
crystals was obtained. The product was stored in a desiccator over
fused CaCl2 for further analysis. Anal. Calc. for C24H18N4O8Co 1: C,
52.43; H, 3.28; N, 10.19. Found: C, 52.01; H, 3.62; N, 10.64%.
2.4. Oxidation of alcohols: typical procedure
To a stirred solution of benzyl alcohol (0.54 g, 5 mmol) in
CH3CN solvent (5 mL) taken in a two-necked round-bottomed flask
equipped with
a reflux condenser was suspended 1.5 mol%
(0.0412 g) of complex 1 (0.075 mmol). 2 mL (ꢂ15 mmol) of 70%
aqueous tert-butyl hydroperoxide (TBHP) was then added to the
above mixture, and the contents were stirred at 82 °C for 5 h. The
course of the reaction was monitored by TLC in all cases using
petroleum ether–ethyl acetate (9:1 v/v) as the solvent system. At
the completion of the reaction, the excess TBHP was destroyed
by adding sodium metabisulfite into the reaction mixture and then
the mixture of products was extracted with diethyl ether. After
l
eff(298 K) = 4.15 BM. IR spectral data (KBr pellet, m
max/cmꢀ1):
3427(m), 2924(w), 2852(w), 1624(s), 1588(vs), 1514(vs),
1485(w), 1447(s), 1410(w), 1371(s), 1341(vs), 1317(w), 1215 (w),
1103(w), 1066(w), 1046(w), 1012(w), 876(m), 833(s), 798(m),
762(m), 723(s), 697(s), 642(w) and 572(w). [s, strong; m, medium;
w, weak; br, broad].
Table 1
Crystallographic data for Co(4-nbz)2(py)2 (1) and Co(4-nbz)2(py)2(H2O)2 (2).
1
2
Formula
Formula weight
Crystal system
Space group
a (Å)
C24H18N4O8Co
549.35
monoclinic
C2/c
14.9862 (6)
6.3167 (2)
24.8559 (10)
91.964 (4)
2351.56 (15)
4
C24H22CoN4O10
585.39
monoclinic
P21/c
14.4946 (16)
11.6071 (13)
7.9534 (10)
105.636 (4)
1288.6 (3)
2
2.2. Preparation of [Co(O2CC6H4-4-NO2)2(C5H5N)2(H2O)2] (2)
b (Å)
c (Å)
2.2.1. Method 1
Co(NO3)2ꢁ6H2O (3 mmol, 0.873 g), NaO2CC6H4-4-NO2 (6 mmol,
1.134 g) and pyridine (6 mmol, ꢂ0.5 mL) were dissolved in water
(20 mL) and the mixture was stirred at room temperature for 1 h.
The resulting faint orange product was filtered, washed with water
and methanol, and dried. Yield: 78% (based on cobalt).
b (°)
V (Å3)
Z
Crystal size (mm)
0.43 ꢃ 0.23 ꢃ 0.12
0.48 ꢃ 0.28 ꢃ 0.18
Dcalc (g cmꢀ3
)
1.552
1.509
l
(Mo K
a
) (mmꢀ1
)
0.789
0.730
Tminimum/Tmaximum
Reflections collected
(unique), Rint
Parameters refined
R1/wR2 (all data)
R1/wR2 (Fo P 4r(Fo))
0.7279/9113
11652 (2746), 0.0238
0.7208/0.8798
15224 (3060), 0.0342
2.2.2. Method 2
The transparent crystals of 1 obtained as above were stored along
with the methanolic mother liquor at room temperature for about a
month to convert the purple crystals quantitatively to faint orange
crystals which are suitable for single crystal X-ray diffraction work.
Yield: 61% (based on cobalt). Anal. Calc. for C24H22N4O10Co 2: C,
49.19; H, 3.76; N, 9.57; Co, 10.07. Found: C, 49.10; H, 3.88; N, 9.80;
168
187
0.0405/0.0824
0.0312/0.0773 (2292
reflections)
1.034
0.001
0.294, ꢀ0.185
0.0994/0.2673
0.0874/0.2589 (2455
reflections)
1.078
0.000
0.803, ꢀ0.606
Goodness of fit on F2
(shift/esd)max
D
qmaximum,minimum (e Åꢀ3
)
Co, 10.02% (gravimetric).
(KBr pellet,
max/cmꢀ1): 3519(m), 3306(w), 3071(vs), 2924(w),
2852(w), 1614(s), 1599(vs), 1577(vs), 1558(vs), 1624(s), 1588(vs),
leff(298 K) = 5.04 BM. IR spectral data
[w(Fo ꢀ Fc2)2]/
R
2
m
R and Rw values are for all unique data; R =
[w(Fo2)2]}1/2 where w = 1/[
R
||Fo| ꢀ |Fc||/
R
|Fo|; Rw = {
R
r
2(Fo2) + (aP)2 + bP ] with P = [2Fc2 + Max(Fo2,0)]/3.

2008
S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
removal of most of the diethyl ether solvent, the acid part of the
product was separated from the above solution via extraction with
a saturated NaHCO3 solution and then by adding conc. hydrochlo-
ric acid to the resultant aqueous solution the acid was obtained as
a pure solid. For other alcohols, after completion of the reaction,
similar work-up procedures were followed up to the diethyl ether
extraction of product(s). The extract was then dried with anhy-
drous Na2SO4 and after removal of the solvent, the crude product
was purified by flash chromatography using petroleum ether-ethyl
acetate (9:1 v/v) as the eluent.
nate complex, while substitutional lability of Co(II) leads to the
conversion of the purple tetrahedral species to the pink octahedral
species under moist conditions.
3.2. Crystal structures
X-ray crystallographic studies have shown that compound 1 is a
nearly tetrahedral 4-coordinate cobalt(II) complex (Fig. 1 and Ta-
ble 2) wherein the coordination sphere consists of two pyridyl N
atoms and two oxygen atoms from two monodentate carboxyl
groups. The cobalt atom lies on a twofold axis of symmetry and
thus the asymmetric unit in the crystal structure has one each of
the 4-nbz and py ligands. While the CoꢀO distance is 1.970(1) Å,
the CoꢀN distance stands at 2.048(1) Å. The CoꢀO bond for the
monodentate nitrobenzoato ligands in 1 is shorter than the corre-
sponding distances of 2.183(1) Å and 2.091(2) Å observed for the
same ligand in octahedral Co(II) complexes [20,21]. However, the
observed CoꢀO bond is somewhat longer than the corresponding
distance of 1.922 Å found in the structurally related complex
3. Results and discussion
3.1. Preparation of complexes
Syntheses of the Co(II) complexes 1 and 2 have been accom-
plished via simple mixing of a Co(II) salt with the sodium salt of
4-nitrobenzoic acid and pyridine at room temperature. The syn-
thetic procedures followed by us are illustrated in Scheme 1. For
the self-assembly of the purple colored species 1, Co(NO3)2ꢁ6H2O,
NaO2CC6H4-4-NO2 and pyridine were reacted in a 1:2:3 molar ratio
in methanol. It is important (i) to keep the Co2+:py ratio at 1:3, (ii)
to use only a small volume of methanol and (iii) to isolate the prod-
uct from the reaction mixture at the earliest. While storing the
reaction mixture along with the purple crystals of 1 for over three
weeks, complete conversion of the purple crystals to faint orange
crystals of 2 occurs. The crystalline species produced by this route
is same with samples of 2 prepared via an alternative route by
making use of the above reaction mixture in the 1:2:2 ratio in an
aqueous medium.
bis(
g
1-benzoato)bis(4-methylpyridine)cobalt(II) [11]. The ob-
served CoꢀN bond distance in 1 also compares well with the CoꢀN
In open air the purple crystals of 1 slowly transform into opaque
blocks of pink colour. But, samples of 1 are indefinitely stable if
stored in tightly closed vials or in a dry atmosphere. Compound
2 is stable in air in the solid state; however, it dissolves readily
in dichloromethane and acetonitrile to form purple solutions con-
taining 1. From these solutions, it is possible to precipitate out 1 by
adding petroleum ether or diethyl ether. It is believed that favor-
able thermodynamic effects allow Co(II) complexes to undergo fac-
ile structural interchange between the octahedral and tetrahedral
coordination geometries. On one hand, solid-state effects including
supramolecular interactions are likely to be important in the sta-
bilization of the four-coordinate species relative to the six-coordi-
Fig. 1. Molecular structure of Co(O2CC6H4-4-NO2)2(C5H5N)2 (1) with non-hydrogen
atoms drawn as ellipsoids at 25% probability level.
Table 2
Selected interatomic distances (Å) and angles (°) for Co(O2CC6H4-4-NO2)2(C5H5N)2
(1), Co(O2CC6H4-4-NO2)2(C5H5N)2(H2O)2 (2).
Compound 1
Co(1)–O(1)
O(1)–C(6)
O(3)–N(2)
1.9704(12)
1.281(2)
1.210(2)
Co(1)–N(1)#1
O(2)–C(6)
O(4)–N(2)
2.0482(14)
1.229(2)
1.211(2)
O(1)–Co(1)–O(1)#1
O(1)–Co(1–N(1)#1
C(6)–O(1)–Co(1)
C(5)–N(1)–Co(1)
O(3)–N(2)–O(4)
98.71(7)
O(1)-Co(1)-N(1)
N(1)#1–Co(1)–N(1)
C(1)–N(1)–Co(1)
O(1)–C(6)–O(2)
106.12(5)
105.25(8)
121.48(11)
123.69(15)
120.88(6)
109.36(10)
120.37(12)
#1 ꢀx + 1,y, ꢀz + 3/2
Compound 2
Co(1)–O(1)
Co(1)–N(1)
O(1)–C(6)
2.082(3)
2.164(4)
1.247(6)
1.154(13)
Co(1)–O(5)
O(2)–C(6)
O(4)–N(2)
2.107(4)
1.259(6)
1.204(14)
O(3)–N(2)
O(1)–Co(1)–O(5)
O(1)–Co(1)–N(1)#1
O(5)#1–Co(1)–N(1)#1
O(1)–C(6)–O(2)
89.09(15)
91.54(16)
87.07(16)
126.3(5)
O(1)#1–Co(1)–O(5)
O(5)–Co(1)–N(1)#1
O(1)–Co(1)–N(1)
O(3)–N(2)–O(4)
90.91(15)
92.93(16)
88.46(16)
122.0(9)
#1 ꢀx, ꢀy, ꢀz
Scheme 1.

S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
2009
Fig. 2. (a) Crystal packing diagram showing the C–Hꢁꢁ O and pꢀp interactions in the structure of compound 1. (b) centroid–centroid proximity of 4-nbz ring planes.
distances reported for various tetrahedral Co(II) complexes with
gestive of
p p interactions [24]. On the other hand, the long cen-
ꢀ
pyridine ligands: e.g., [Co(
g
1-O2CC6H5)2(4-Mepy)4], 2.034 Å [11];
troid to centroid separation of 4.805 Å for the pyridine rings are
quite long for such an interaction. But, CꢀHꢁꢁꢁO interactions be-
tween the CꢀH group at the para position of the pyridine ring
[Co(py)4]+, 2.00(2) Å [22] and [Co(py)C13]ꢀ, 2.059(9) Å [23]. The
shorter O(2)–C(6) bond compared with O(1)–C(6) in 1 (Table 2)
agrees well with its expected partial double bond character in
the monodentate mode of coordination of the carboxyl group,
which also is responsible for the appearance of
m(COO)asym at a rel-
atively higher energy of 1624 cmꢀ1
.
The OꢀCoꢀO, NꢀCoꢀN and OꢀCoꢀN bond angles of 98.71(7)°,
105.25(8)° and 106.12(5)°/120.88(6)°, respectively in 1 deviate
considerably from the ideal tetrahedral angle. The only other
structurally characterized tetrahedral Co(II) complex having the
[Co(carboxylate)2(pyridine)2] type composition, [Co(g
1-O2CC6H5)2-
(4-Mepy)4], [11] also shows substantial distortion from ideal
tetrahedral geometry. For this complex, the O–Co–O, N–Co–N
and O–Co–N bond angles are reported to be 135.00°, 103.05° and
95.15°/112.73°. It appears that the elongation of the CoꢀO and
CoꢀN bonds in 1 compared to the corresponding distances in
[Co(g
1-O2CC6H5)2(4-Mepy)4] may be due the inductive effects of
the substituents at the 4-positions of the carboxylato and pyridyl
ligands in these two complexes. On the other hand, the differences
in the bond angles in the coordination spheres of the cobalt centers
are believed to be related to crystal packing effects.
A closer look reveals that intermolecular interactions play an
important role in the crystal structure of complex 1. As shown in
Fig. 2, the interplanar (centroid to centroid) distance of 3.998 Å
in case of the phenyl rings in the O2CC6H4-4-NO2 moieties is sug-
Fig. 3. Molecular structure of Co(O2CC6H4-4-NO2)2(C5H5N)2(H2O)2 (2) with non-
hydrogen atoms drawn as ellipsoids at 25% probability level.

2010
S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
Fig. 4. Unit cell packing diagram of 2 showing CꢀHꢁꢁ O interactions between 2-D sheets. The Cꢁꢁ O separations and CꢀHꢁꢁ O angles for the two distinct interactions in the
crystal are 3.363/3.502 Å and 127.84/151.06°, respectively.
and the uncoordinated oxygen atom of the carboxylato ligands
from an adjacent molecule appear to provide a significant supra-
molecular force in determining the crystal structure of the tetrahe-
dral complex. The weak CꢀHꢁꢁꢁO hydrogen bridges (2.652 Å) are
believed to be responsible for the deviation of the nitrobenzoato li-
gands from overall molecular planarity. It is seen that the planar
nitro and carboxyl groups of the nitrobenzoato ligand are twisted
away from the phenyl ring plane by 4.5(3)° and 11.1(2)°, respec-
tively. Such deviations from overall planarity of a coordinated li-
gand have been correlated earlier with higher lattice enthalpy
effects and enhanced solid-state stability in metal terephthalate
complexes [25].
Both identity and composition of compound 2 have been ascer-
tained by X-ray crystallography. The asymmetric unit of the crystal
structure consists of a half of the formula unit, with the Co atom
lying at the crystallographic center of symmetry (0 0 0) and all oth-
ers at general positions (x, y, z). The six-coordinate Co(II) complex
of Ma2b2c2 type has all three ligand pairs in mutually trans posi-
tions. (Fig. 3). The carboxylate anions coordinate in a monodentate
fashion as in related Co(II) complexes containing CoO2N2O2 coordi-
nation spheres [21,22,26–28]. Unlike in the case of 1, the carboxyl
and nitro groups are nearly coplanar with the phenyl ring plane in
the nitrobenzoato ligand present in 2. As expected, the observed
Co–O(carboxylate) and Co–N(pyridine) bond lengths in compound
2 are longer than the corresponding distances for 4-coordinate
Co(II) in 1 (Table 2). The Co(II)–O and Co(II)–N distances observed
in 2 are comparable with values in other related complexes
[21,22]. The deviations of the cis(O–Co–N) and cis(O–Co–O) bond
angles in the coordination sphere from the ideal value of 90° are
small. However, on the basis of disparate M–L distances, which
may also be subject to Jahn–Teller effect for the d7 ion, it is useful
to point out that the coordination geometry around the cobalt(II)
center is far from idealized octahedral.
Table 3
OꢀHꢁꢁ O hydrogen bonds for present in the crystal structure of 2.
D–HꢁꢁꢁA
O(5)–H(5A) ꢁꢁ O(2)#1
O(5)–H(5B) ꢁꢁ O(2)#2
d(D–H)
d(HꢁꢁꢁA)
2.14(8)
1.84(6)
d(DꢁꢁꢁA)
2.698(6)
2.742(6)
\(DHA)
0.67(8)
0.94(6)
142(8)
159(5)
Symmetry transformations used to generate equivalent atoms: #1 ꢀx + 1, ꢀy + 1,
ꢀz. #2 ꢀx + 1, y + 1/2, ꢀz + 1/2.
(Table 3) between the coordinated water molecules and the unco-
ordinated carboxyl oxygen atoms are responsible for the formation
of the supramolecular layers as illustrated in Fig. 5. The intricate
patterns of hydrogen bonds in the solid-state structure of 2 can
be understood and analyzed by graph-set theory [29]. It is possible
to identify an intramolecular hydrogen-bonded ring denoted by
S(6) which is formed by the ‘CoꢀO(carboxylate)ꢀCꢀO(carboxyl-
ate)ꢀ(OH2)’ moiety. The coordinated water molecule is further
linked with the uncoordinated oxygen atom of the carboxylate li-
gand of an adjacent molecule via intermolecular hydrogen bonding
to form a supramolecular chain. This chain is describable as C12ð4Þ.
The superscripts and subscripts in the symbol are respectively the
number of hydrogen bond acceptors and hydrogen bond donors. A
20-membered ring formed by periodic repetition of this chain gen-
erates a cyclic R48ð20Þ motif. This ring constitutes a characteristic
pattern repeating within 2-D supramolecular sheets of 2.
In addition to the moderately strong OꢀHꢁꢁꢁO hydrogen bonds,
weaker CꢀHꢁꢁꢁO interactions also characterize the crystal structure
under discussion. These intermolecular hydrogen bridges are
found to involve the O atoms of the nitro groups and aromatic
CꢀH groups from adjacent layers (Fig. 4). It appears to us that
although the nitro groups of the 4-nitrobenzoate ligands in 1 are
not involved in any significant intermolecular interaction, the C–
HꢁꢁꢁO interactions involving the nitro groups are playing a crucial
structure directing role in 2. This suggests that the NO2 groups in
the latter compound are more basic due to the somewhat weaker
cobalt–oxygen bonds for the carboxylato ligands.
Fig. 4 shows that the crystal structure of 2 is two-dimensional
(2-D) in which hydrogen-bonded layers run along the crystallo-
graphic bc plane. Both intra- and intermolecular OꢀHꢁꢁꢁO bonds

S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
2011
Fig. 5. Hydrogen bond patterns with graph-set descriptors in the crystal structure of 2. For clarity, pyridine ligands have been denoted by only the coordinated N atoms.
3.3. Electronic spectral properties
tion between tetrahedral and octahedral geometry of cobalt(II)
complexes can be made on the basis of their electronic spectra
[30,31]. We have studied the electronic spectra of complexes 1
and 2 in both solid as well as solution phases.
The visible band in tetrahedral Co(II) complexes generally ap-
pears as a multiple absorption envelope in the 500–700 nm range,
whereas the absorption band of octahedral Co(II) complexes in the
visible region appears near 500 nm and hence a clear cut distinc-
The solid-state UV–Vis–NIR spectra recorded for 1 and 2 are
shown in Fig. 6. The spectra in both cases are composed of two
broad absorptions occurring in the visible and near infrared re-
gions respectively. The electronic spectra for the complexes have
also been measured in the solution phase (Table 4). Comparing
the visible band for 1 with the corresponding band observed for
its CH2Cl2 solution we note the difference in shape between the
absorption bands displaying kmax values of 582 nm and 558 nm
respectively in the solid and solution phases. This observation sug-
gests a structural reorganization occurring upon dissolution. How-
ever, the molar absorptivity indicates that the species retains its
tetrahedral coordination sphere in the solution phase as well.
The observed kmax value of 517 nm for an MeOH solution of 2 is
also quite different from the corresponding band in the solid-state
spectrum with kmax = 476 nm to suggest a minor structural change
in the complex species present in solution. The broad near IR
absorptions in both cases are attributable to the
m
2[4A2 ? 4T1(F)]
and
m
1[4T1g(F) ? 4T2g] transitions respectively for 1 and 2. More-
over, bands at 291 nm and 288 nm (not shown in Fig. 6) are due
*
p p transitions of the organic groups present in complexes
to the
1 and 2, respectively.
ꢀ
Rather interestingly, both complexes show nearly identical
spectra in CH2Cl2. On the basis of absorption intensities, and band
shapes observed it is easy to conclude that the CH2Cl2 solutions of
both complexes contain the same species. The CH3CN solution also
gives nearly identical UV–Vis spectrum which is characteristic of
tetrahedral cobalt(II) complexes. On the other hand, complex 2 in
Table 4
Solution-phase electronic spectral data on 1 and 2 in the 400–800 nm range.
kmax, nm (e )
, L molꢀ1 cmꢀ1
1
2
CH2Cl2
CH3CN
MeOH
593 (sh)
558 (125)
522 (sh)
593 (sh)
560 (126)
521(sh)
591(sh)
566(85)
528(sh)
517 (4.8)
474 (sh)
Fig. 6. Solid-state UV–Vis–NIR spectra of 1 (a) and 2 (b).

2012
S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
Scheme 2.
methanol gives kmax = 517 nm with
e
ꢄ4.8 L molꢀ1 cmꢀ1 which is
Table 5
Optimization of conditions in the TBHP oxidation of benzyl alcohol catalyzed by
characteristic of cobalt(II) in an octahedral environment [32].
Clearly, the species present in the pink methanolic solution and
the purple dichloromethane solution of 2 are different. This leaves
no doubt that the absorbing complex species present in a solution
of [Co(4-nbz)2(py)2(H2O)2] (2) in CH2Cl2 is actually [Co(4-
nbz)2(py)2] (1). The nonpolar solvent medium induces the mole-
cules of 2 to shed the coordinated water molecules since in terms
of CFSE the 6- and 4-coordinate complexes are not too different.
This information coupled with the observed conversion of crystals
of the tetrahedral complex, 1, in hydrous methanol to the octahe-
dral species 2, demonstrate clearly that the four- and six-coordi-
nate complexes are interconvertible both in the solid as well as
solution states.
complex 1.
Entry Oxidant Catalyst Temperature Time Product yields (%)a
amount amount (°C)
(h)
(mL)
(mol%)
Benzaldehyde Benzoic
acid
1
2
3
4
5
1
2
2
2
2
1.5
0.5
1
1.5
1.5
82
82
82
82
27
5
5
5
5
33
7
48
64
77
88
4
7
5
18
49
Reaction conditions: benzyl alcohol = 5 mmol (0. mL); solvent (CH3CN) =5 mL.
a
GC yields.
Table 6
TBHP oxidation of alcohols catalyzed by [Co(4-nbz)2(py)2] (1).
Substrate
Time (h)
Products
Isolated yields (%)
4
Melting point (°C)
(l)
5
83
15
121
104
5
63
237
2
2
75
65
(l)
(l)
4
3
4
85
85
93
(l)
49
95
Reaction conditions: substrate, 5 mmol; CH3CN, 5 mL; catalyst amount, 1.5 mol%; aq. TBHP (70%), 2 mL; reaction temperature, 82 °C. l, liquid product.

S. Chakravorty, B.K. Das / Polyhedron 29 (2010) 2006–2013
2013
3.4. Catalytic studies
charge from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
In view of the solubility of complex 1 in acetonitrile we found it
instructive to examine if the complex is active as a homogeneous
catalyst in the oxidation of alcohols. In presence of a catalytic
amount (1.5 mol%) of complex 1, aqueous TBHP (70%) has been
found to effectively oxidize both primary and secondary alcohols
in acetonitrile medium. No other additives are required for the oxi-
dation of alcohols. Scheme 2 illustrates the reaction involving ben-
zyl alcohol.
Acknowledgments
Financial support from DST, Government of India (Grant No. SR/
S5/GC-18/2007) is gratefully acknowledged. SC is thankful to UGC
(India) for a research fellowship under its scheme for meritorious
students in science. We thank IIT Guwahati for kindly providing
us with X-ray diffraction data.
In Table 5 we present the results of our detailed studies on ben-
zyl alcohol oxidation mediated by complex 1. As may be seen from
entry 5 in the table, the rate of the reaction at room temperature is
slow and even after 18 h of reaction time, conversion of benzalde-
hyde is only 53%. However, unlike in the reactions carried out at
82 °C, benzaldehyde is the major product forming at 92% selectiv-
ity. In order to study the effect of the amount of catalyst with re-
spect to substrate, the benzyl alcohol oxidation was studied at
three different catalyst concentrations. On the basis of these stud-
ies, the highest conversion of benzyl alcohol is observed with
5 mmol of substrate, 15 mmol (2 mL) of TBHP and 1.5 mol% of
the catalyst dissolved in 5 mL of acetonitrile at 82 °C. The substrate
conversion also increases from 81% to 93% when the amount of
TBHP is increased from 1 to 2 mL. The reaction carried out in the
absence of the catalyst leads to only 2% conversion of the substrate.
As shown by the data presented in Table 6, carboxylic acids are
found to form as major products as a result of the oxidation of pri-
mary alcohols. In case of secondary alcohols, the corresponding
carbonyl compounds are obtained exclusively. These results thus
indicate that complex 1 is an effective catalyst in the oxidation of
both primary as well as secondary alcohols. In earlier studies
involving cobalt(II) complexes as catalysts for alcohol oxidation,
other authors also reported the formation of both aldehyde as well
as carboxylic acid as products from primary alcohols [33,34].
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4. Conclusions
Synthesis and crystal structure of two cobalt(II) 4-nitrobenzo-
ates having py as ancillary ligands are described. Spectral evi-
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themselves. Complex 1 acts as a catalyst in the TBHP oxidation of
primary and secondary alcohols, to form both aldehydes and car-
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5. Supplementary data
CCDC 760120 and 760121 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of