Cationic cobaltammines as anion receptors: Synthesis and characterization of [Co(NH3)6](4-np)3.4H2O, [Co(NH3)6](2,4-dnp)3 and [Co(NH 3)6](2,4,6-tnp)3

Article abstract of DOI:10.1016/j.molstruc.2005.06.007

In an effort to utilize [Co(NH₃)₆]³⁺ cation as a new host for mono-, di- and tri-substituted nitrophenolate anions, yellow solids of composition [Co(NH₃)₆](4-np) _3.4H₂O, [Co(NH₃)₆](2,4-dnp) ₃, and [Co(NH₃)₆](2,4,6-tnp)₃ were obtained in high yields by reacting sodium salts of 4-np, 2,4-dnp and 2,4,6-tnp (where np=nitrophenolate, dnp=dinitrophenolate and tnp=trinitrophenolate,) with [Co(NH₃)₆]Cl₃ respectively in 3:1 molar ratio in hot aqueous medium. These cobalt(III) complex salts have been characterized by elemental analyses and spectroscopic techniques (UV/Visible, IR and NMR). Single crystal X-ray structure determination of [Co(NH₃) ₆](4-np)_3.4H₂O revealed the presence of discrete ions, [Co(NH₃)₆]³⁺ and C ₆H₄NO₃^- in addition to four lattice water molecules in the solid state. A strong network of hydrogen bond interactions (N-H?O (phenolate), N-H?O (nitro), O-H?O (water) stabilize the crystal lattice. The formation of these salts and measurement of solubility products suggests that [Co(NH₃) ₆]³⁺ present in [Co(NH₃)₆]Cl ₃ may be used as anion receptor for the nitrophenolate ions.

Full text of DOI:10.1016/j.molstruc.2005.06.007

Journal of Molecular Structure 752 (2005) 170–177
www.elsevier.com/locate/molstruc
Cationic cobaltammines as anion receptors: Synthesis
and characterization of [Co(NH3)6](4-np)3.4H2O, [Co(NH3)6](2,4-dnp)3
and [Co(NH3)6](2,4,6-tnp)3 (where npZnitrophenolate,
dnpZdinitrophenolate, tnp, trinitrophenolate), single crystal
X-ray structure and packing of [Co(NH3)6](4-np)3.4H2O
*
Raj Pal Sharma , Ritu Bala, Rajni Sharma, Paloth Venugopalan
Department of Chemistry, Panjab University, Chandigarh-160014, India
Received 13 May 2005; revised 2 June 2005; accepted 2 June 2005
Available online 21 July 2005
Abstract
In an effort to utilize [Co(NH3)6]3C cation as a new host for mono-, di- and tri-substituted nitrophenolate anions, yellow solids of
composition [Co(NH3)6](4-np)3.4H2O, [Co(NH3)6](2,4-dnp)3, and [Co(NH3)6](2,4,6-tnp)3 were obtained in high yields by reacting sodium
salts of 4-np, 2,4-dnp and 2,4,6-tnp (where npZnitrophenolate, dnpZdinitrophenolate and tnpZtrinitrophenolate,) with [Co(NH3)6]Cl3
respectively in 3:1 molar ratio in hot aqueous medium. These cobalt(III) complex salts have been characterized by elemental analyses and
spectroscopic techniques (UV/Visible, IR and NMR). Single crystal X-ray structure determination of [Co(NH3)6](4-np)3.4H2O revealed the
presence of discrete ions, [Co(NH3)6]3C and C6H4NO3K in addition to four lattice water molecules in the solid state. A strong network of
hydrogen bond interactions (N–H/O (phenolate), N–H/O (nitro), O–H/O (water) stabilize the crystal lattice. The formation of these salts
and measurement of solubility products suggests that [Co(NH3)6]3C present in [Co(NH3)6]Cl3 may be used as anion receptor for the
nitrophenolate ions.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Anion receptor; Cobalt(III); Coordination chemistry; Inorganic synthesis; Nitrophenolates; X-ray crystallography
1. Introduction
higher complexity in design and synthesis of receptors and
(c) pH sensitivity. Anion receptors could be valuable if they
are capable of anion binding in wastes or recognizing their
presence. During last thirty years, many organic based anion
receptors have been reported e.g. cationic cyclic poly-
amines[10–12], tripodal amines [13], calixarenes [14],
sapphyrins [15], azacryptands [16] and azacages or open
chain counterparts [17] but sporadic reports are available
with cationic transition metal complexes based anion
receptors i.e. alkoxide-bridged dinuclear Zn(II) complex
[18], urea based Pt(II) receptor [19] and bipyridine Ru(II)
complexes [20]. Therefore, the development of selective
anion binding agents, which may find potential applications
[21] and especially in polar solvents such as water is of
particular interest [22–23].
The current interest [1–2] in developing new anion
receptors is due to the role played by anions in many
chemical, biological and environmental processes [3–7].
Several reasons why reliable sensing of anions is a
particularly challenging area of research are: (a) anions
are larger than isoelectronic cations and therefore have
lower charge-to-radius (surface) ratio, a feature that makes
the electrostatic binding of anions to the receptors less
effective [8–9]. (b) Anions have a wide range of geometries
and are often present in delocalized forms, which results in
*
Corresponding author. Tel.: C91 0172 25434433; fax: C91 0172
2545074.
E-mail address: rpsharmapu@yahoo.co.in (R.P. Sharma).
Hexaamminecobalt(III) cation possesses all the charac-
teristic features to act as a potential anion receptor (binding
agent): (i) hydrogen bond donor groups, (ii) a positively
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.06.007

R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
171
charged component for effective electrostatic interactions
and (iii) a suitable framework onto which these structural
components can be assembled. This cationic cobaltammine
is tri-positively charged, contains eighteen hydrogen bond
donor. N–H groups which can interact with properly
oriented atoms of the hydrogen bond accepters (oxygens
of phenolate and nitro groups as well as water molecule)
involving second sphere coordination and stable structural
framework. We have undertaken an extensive research
program to explore hexaamminecobalt(III) cation as anion
receptor because it is relatively easy to synthesize from
readily available materials and could be stored for months
without any noticeable decomposition.
1H-NMR and 13C-NMR spectra of title complex salts were
run in the solvent D2O at 25 8C by using JEOL AL 300 MHz
FT NMR spectrometer. The chemical shift values are
expressed as d value (ppm) down field from tetramethylsi-
lane as an internal standard.
2.2. Synthesis of [Co(NH3)6](4-np)3.4H2O
Hexaamminecobalt(III) chloride (0.50 g., 0.0018 mol)
was dissolved in 10 ml hot water taken in a beaker by
mechanical stirring. In another beaker 0.907 g
(0.0056 mol) of sodium 4-nitrophenolate was dissolved in
20 ml hot water. Both the solutions were mixed and
allowed to cool slowly, yellow solid precipitated immedi-
ately, which was washed with ice-cold water and air dried.
Orangish-yellow coloured shining crystals were obtained
from hot aqueous medium, which were filtered and dried in
air. The overall yield is nearly quantitative and salt
decomposes at 398 K. Solubility (25 8C) in water:
0.876 g/100 ml. Elemental Anal. Calc. for C18 H38 Co N9
O13, (F.W.Z647.50): C, 33.35; H, 6.86; N, 19.45; Co,
9.05. Found: C, 33.32; H, 5.84; N, 19.42, Co, 8.25%. IR.
nZ3410 (b), 3232(b), 1618 (m), 1573 (m), 1521 (m),
1347(s), 1322(s), 803 (m) cmK1. UV/Vis (solution):
lmaxZ346, 399. NMR (solution): 1HNMR (DMSO),
dZ7.6–7.7 (d,6H), 6.0–6.1(d,6H), 3.7 (s,18H), 13CNMR,
dZ179, 129, 127,119 ppm.
Mono-, di- and tri-substituted nitro-phenol and its salts
are biologically and commercially important class of
compounds. Nitrophenols are used to manufacture drugs
(e.g. acetaminophen), fungicides, insecticides, and dyes and
to darken leather [24]. Acute (short-term) inhalation or
ingestion of nitrophenols in humans causes headaches,
drowsiness, nausea, and cyanosis (blue color in lips, ears,
and fingernails) [25]. Contact with eyes causes irritation in
humans. Besides, picric acid (2,4,6-trinitrophenol) has been
used in ammunitions and explosives during World War I as
Lyddite or Melinite. Therefore, we report here the binding
of biologically and commercially important mono-, di- and
tri- substituted nitro-phenolate ions with the help of
hexaaminecobalt(III) cation i.e. synthesis and characteri-
zation of [Co(NH3)6](4-np)3.4H2O, [Co(NH3)6](2,4-dnp)3,
and [Co(NH3)6](2,4,6-tnp)3, (where np is nitrophenolate,
dnp is dinitrophenolate and tnp is trinitrophenolate).
And single crystal X-ray structure determination of one of
the newly synthesized salt, [Co(NH3)6](4-np)3.4H2O. We
have already reported the use of hexaamminecobalt(III) as
anion receptor for teraoxo-metallates [26–28], methanesul-
phatnate [29], carboxylates [30], fluoroanion [31] and
halates [32].
2.3. Synthesis of [Co(NH3)6](2,4-dnp)3
Hexaamminecobalt(III) chloride (1 g, 0.003 mol) was
dissolved in 20 ml of hot water taken in a beaker by
mechanical stirring. In another beaker 2.33 g (0.011 mol) of
sodium 2,4-dinitrophenolate was dissolved in minimum
amount of hot water (20 ml). Both the solutions were mixed
with stirring and yellow solid precipitated immediately,
which was washed with ice-cold water and air-dried. The
yield was 79% and salt decomposes at 474 K. Solubility
(25 8C) in water: 0.056 g/100 ml. Elemental Anal. Calc. for
C18 H27 Co N12 O15, (F.W.Z781.9): C, 30.42; H, 3.80; N,
23.66; Co, 8.29 Found: C, 30.39; H, 3.75; N, 23.64, Co,
8.05%. IR. nZ3232(b), 1602 (m), 1554 (m), 1515 (m),
1331(s), 1324(s), 781 (m) cmK1. UV/Vis (solution):
2. Experimental
2.1. General remarks
Analytical grade reagents were used without any further
purification. [Co(NH3)6]Cl3 has been prepared by air
oxidation of Co(II) salt in ammonical solution in the
presence of activated charcoal as a catalyst according to
method described by Bjerrum and McReynold [33]. Cobalt
was determined by standard gravimetric method of
estimation [34] and C, H, N were estimated micro
analytically by automatic PERKIN ELMER 2400CHN
elemental analyzer. The non-crystalline samples were
dried in the desiccator before elemental analyses. UV/
Visible spectra were recorded using HITACHI 330
spectrometer in water as solvent. Infrared spectra of the
title complex salts were recorded using PERKIN ELMER
spectrum RX FT-IR system using Nujol mull in KBr plates.
1
lmaxZ356, 395. NMR (D2O): HNMR, dZ8.69 (s,3H,),
7.8–7.9 (d,6H), 6.4–6.5 (d, 6H), 3.45(18H), 13CNMR,
dZ170, 135, 128, 127, 126, 124 ppm.
2.4. Synthesis of [Co(NH3)6](2,4,6-tnp)3
Hexaamminecobalt(III) chloride (1 g, 0.003 mol) was
dissolved in 20 ml of hot water taken in a beaker by
mechanical stirring for 10–15 min. In another beaker 2.83 g
(0.011 mol) of sodium 2,4,5-trinitrophenolate was dissolved
in minimum amount of hot water (20 ml). Both the solutions
were mixed with stirring and yellow solid precipitated
immediately, which was washed with ice-cold water and

172
R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
Table 1
air-dried. The yield was 80% and the salt decomposes at
483 K. Solubility (25 8C) in water: 0.039 g/100 ml. Elemen-
tal Anal. Calc. for C18 H24 Co N15 O21, (F.W.Z844.9): C,
25.56; H, 2.84; N, 24.85, Co, 6.97) Found: C, 25.52; H,
2.84; N, 24.81; Co, 6.43%. IR. nZ3232(b), 1602 (m), 1554
(m), 1515 (m), 1331(s), 1324(s), 781 (m) cmK1. UV/Vis
(solution): lmaxZ355, 395. NMR (D2O): 1HNMR, dZ8.69
(s,3H), 7.8–7.9 (d,6H), 6.4–6.5 (d, 6H), 3.45(18H),
13CNMR, 170, 135, 128,127, 126, 124 ppm.
Crystal data and structure refinement parameters of
[Co(NH3)6](4-np)3.4H2O
Empirical formula
Formula weight
Temperature (K)
C18 H38 Co N9 O13
647.50
293(2)
˚
Radiation used, wavelength (A)
Crystal system, space group
Unit cell dimensions
Mo Ka 0.71073
Monoclinic, P21/c
˚
aZ16.250(1) A
˚
bZ23.932(2) A, bZ95.35(1)8
˚
cZ7.248(1) A
3
˚
Volume
Z, Calculated density (Mg/m3)
Absorption coefficient (mmK1
2806.4(5) A
2.5. Crystal structure determination
4, 1.532
)
0.690
Good single crystal of [Co(NH3)6](4-np)3.4H2O suitable
for X-ray diffraction studies were grown from aqueous
solution by slow evaporation method. A single crystal with
dimension 0.28!0.19!0.17 mm was mounted along the
largest dimension and used for data collection. The intensity
data were collected on Siemens P4 single crystal
diffractometer equipped with molybdenum sealed tube
F(000)
1360
Crystal size (mm)
0.28!0.19!0.17
0.880, 0.868
Max. and min. transmission
Theta range for data collection (8)
Scan type
2.12–24.00
2qKq
Scan speed
Variable, 2.0–60.08/min. in u
0.908 plus Ka separation
Stationary crystal and stationary
counter at the beginning and end of
scan, each for 25.0% of total scan
time
Scan range (u)
Background measurement
˚
(lZ0.71073 A) and highly oriented graphite monochroma-
tor. Three reflections were used to monitor the stability and
orientation of the crystal and were re-measured after every
97 reflections. The data were corrected for Lorentz and
polarization factors and an empirical absorption correction
based on psi scan method has also applied. All other
relevant information about the data collection is presented
in Table 1. The structure was solved by Direct Methods
using SHELX-97 [35] package and also refined using the same
one. All other information regarding the refinement is also
recorded in Table 1.
Index ranges
K18%h%18, K27%k%0,
0%l%8
4802
Reflections collected
Independent reflections
Refinement method
4410 [R(int)Z0.0190]
Full-matrix least-squares on F2
4410/0/371
Data/restraints/parameters
Goodness-of-fit on F2
Weighting scheme
1.013
1/[s2 (Fo2)C(0.0408P)2C1.00P],
PZðmaxðF2o; 0ÞC2 ꢀ F2cÞ=3
11.89:1
Data to parameter ratio
Final R indices, 3319 reflections
[IO2s(I)]
R1Z0.0345, wR2Z0.0806
R indices (all data)
R1Z0.0529, wR2Z0.0895
0.0025(3)
˚
0.198 and K0.256 e A
3. Results and discussion
Extinction coefficient
Largest diff. peak and hole
K3
3.1. Synthesis
3.2. Measurement of solubility products
Three new hexaamminecobalt(III) complex salts i.e.
[Co(NH3)6](4-np)3.4H2O, [Co(NH3)6] (2,4-dnp)3 and
[Co(NH3)6](2,4,6-tnp)3 have been synthesized by
reacting appropriate metal salt- Na(4-np), Na(2,4-dnp),
Na(2,4, 6-tnp), with [Co(NH3)6]Cl3 respectively in 3:1
molar ratio in hot aqueous medium. Single crystals of
[Co(NH3)6]-(4-dnp)3.4H2O were obtained from hot aqu-
eous medium by allowing it to evaporate at room
temperature. However, all efforts to obtain single crystals
of [Co(NH3)6](2,4-dnp)3 and [Co(NH3)6](2,4,6-tnp)3
suitable for X-ray structure determination were
unsuccessful. The chemical compositions of newly
synthesized complex salts were initially indicated by
elemental analyses. All the title complex salts are
highly soluble in DMSO but insoluble in other organic
solvents (e.g. chloroform, ethanol, acetone) except
[Co(NH3)6](2,4,6-tnp)3 which is partially soluble in
ethanol and acetone. These hexaamminecobalt(III) salts
are slightly soluble in water but more soluble in hot
water.
Solubility of ionic salts in water differs to a great
extent and on the basis of solubility criterion, the salts
are classified into three categories (a) solubility O0.1 M
(soluble), (b) solubility between 0.01 and 0.1 M (slightly
soluble) and (c) solubility !0.01 M (springly soluble).
The solubility measurements at room temperature
(Table 2) show that [Co(NH3)6](4-np)3$4H2O is slightly
soluble in water whereas [Co(NH3)6](2,4-dnp)3 and
[Co(NH3)6](2,4,6-tnp)3 are springly soluble. It is
Table 2
Solubility products (Ksp) of hexaamminecobalt(III) salts
Complex salts
Solubility (M)
Ksp
[Co(NH3)6]Cl3
0.26052
0.01352
0.00079
0.00046
0.123
[Co(NH3)6](4-np)3$4H2O
[Co(NH3)6](2,4-dnp)3
[Co(NH3)6](2,4,6-tnp)3
9.0!10K7
1.0!10K11
1.2!10K12

R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
173
observed that the binding (association) of 2,4,6-trinitro-
phenolate and 2,4-dinitrophenolate ions with
[Co(NH3)6]3C is much more as compared to
4-nitrophenolate ion i.e. the binding affinities are in the
order [Co(NH3)6](2,4,6-tnp)3O[Co(NH3)6](2,4-dnp)3[
[Co(NH3)6](4-np)3$4H2O. This may be ascribed to
increase in number of nitro groups on phenolate ion.
As reported in the experimental section, when the
appropriate amounts of the reactants were mixed in
minimum amount of water the precipitation occurred
immediately resulting in the formation of title complex
salts. This is because ionic products are greater than
solubility products, Ksp (Table 2).
complexes are observed around 470 and 340 nm respect-
ively producing the familiar orange yellow colour for
number of classical coordination compounds containing
cobalt(III). For these entire complex salts the lmax were not
observed at 470 and 340 nm (showing d–d transitions
typical for low spin Co(III) d6 octahedral complex) but
shifted to left i.e. 355 and 395 this is due to outer-sphere
charge transfer from nitro substituted phenolate ion to
central metal ion. These values of lmax in the newly
synthesized complex salts are also in agreement with those
of the earlier reported complex salts [39] but quite different
from our earlier reported hexaamminecobalt(III) salts
[26–32].
1
In the title complex salts, H and 13C NMR chemical
shift values are in good agreement with the literature values
reported for mono-, di- and tri- substituted nitrophenols
1
[40]. The H NMR spectrum of all complex salts shows a
3.3. Spectroscopy
The vibrational spectra of newly synthesized hexaammi-
necobalt(III) complex salts shows that the stretching
vibration of the coordinated NH3 molecule are lower than
those of the free NH3 molecules for two reasons, one is the
effect of coordination and the other is the effect of the counter
ions i.e. 4-np, 2,4-dnp and 2,4,6-tnp. This is attributed to the
weakening of the N–H bond due to the formation of N–H/O
type of hydrogen bond. It is seen that the antisymmetric
stretch and symmetric NH3 stretch, NH3 degenerate
deformation, NH3 symmetric deformation and NH3
vibrations appear in the regions of 3400–3000, 1650–1550,
1370–1200 and 800–900 cmK1 respectively for [Co(NH3)6]
Cl3 [36] which are comparable with newly synthesized
hexaamminecobalt(III) complex salts. For the anionic
groups, i.e. for mono, di and tri substituted nitrophenolates,
the hydrogen bonding weaken the NaO bond resulting in
absorption of lower frequency and the sharp peaks are
observed in the range 1521–1515 and 1331–1347 cmK1 for
NO2 (ns) and NO2 (nas) respectively. The peak assignments
have been made in consultation with literature [37] values.
As reported in the literature [38] the two transitions
broad peak at 3.45 ppm, which is a characteristic of
coordinated NH3 group [41]. In 13C NMR, chemical shift
values for carbon atom (C–OK) in 4-np, 2,4-dnp and
2,4,6-tnp salts are 179, 170 and 160 ppm respectively. The
lowering trend is due to presence of number of electron
withdrawing groups on phenolate rings.
3.4. X-ray crystallography
The X-ray crystal structure of the [Co(NH3)6]
(4-np)3.4H2O has been unambiguously determined by single
crystal X-ray crystallography. The salt crystallizes in
Monoclinic, space group P21/c, having cell dimensions,
˚
˚
˚
aZ16.250(1) A, bZ23.932(2) A, cZ7.248(1) A bZ
3
˚
95.35(1)8, VZ2806.4(5) A , ZZ4 and RZ0.0345. The
crystal structure not only conclusively establishes the
existence of single salt of [Co(NH3)6](C6H4NO3)3.4H2O
but also rules out the possibility of a mixture of salts or
double salt. Furthermore, this revealed for the first time that
it is a first ionic salt which contains discrete ions
[Co(NH3)6]3C and C6H4NOK3 in addition to four lattice
1
1A1g/T1 g and A1g/1T2 g for hexaamminecobalt(III)
Fig. 1. ORTEP Diagram of [Co(NH3)6](4-np)3.4H2O.

174
R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
water molecules in the solid state. The ORTEP diagram of
[Co(NH3)6](4-np)3.4H2O complex salts is shown in Fig. 1.
octahedral geometry. The Co–N bond distances are in the
˚
range 1.958(2)–1.970(19) A, while cis N–Co–N bond
In [Co(NH3)6]3C
,
the cobalt(III) metal ion is
angles are in the range 88.02(8)–92.33(8)8 and trans
N–Co–N bond angles are in the range 180.00(9)–
180.00(12)8. This study showed that octahedron is
surrounded by six nitrogen atoms originating from six
coordinating ammonia molecules resulting in a nearly
Table 3
˚
Selected bond lengths [A] and angles [8] of [Co(NH3)6](4-np)3.4H2O
Co(1A)–N(2A)
1.958(2)
1.958(2)
1.961(2)
1.961(2)
1.9690(19)
1.9690(19)
1.311(3)
1.237(4)
1.235(4)
1.434(4)
1.402(4)
1.409(4)
1.371(4)
1.383(4)
1.382(4)
1.371(4)
1.302(3)
1.226(3)
1.242(3)
1.427(3)
1.408(4)
Co(1B)–N(2B)#2
1.9645(19)
1.9645(19)
1.9665(19)
1.9665(19)
1.9708(19)
1.9708(19)
1.302(3)
1.240(3)
1.242(3)
1.422(3)
1.414(4)
1.418(4)
1.366(4)
1.390(4)
1.382(4)
1.365(4)
1.409(4)
1.369(4)
1.380(4)
1.383(4)
1.368(4)
180.00(11)
89.42(8)
90.58(8)
90.58(8)
89.42(8)
180.0
Co(1A)–N(2A)#1
Co(1B)–N(2B)
Co(1A)–N(3A)
Co(1B)–N(1B)
Co(1A)–N(3A)#1
Co(1B)–N(1B)#2
Co(1A)–N(1A)
Co(1B)–N(3B)
Co(1A)–N(1A)#1
Co(1B)–N(3B)#2
O(1A)–C(1A)
O(1B)–C(1B)
O(2A)–N(4A)
O(2B)–N(4B)
O(3A)–N(4A)
O(3B)–N(4B)
N(4A)–C(4A)
N(4B)–C(4B)
C(1A)–C(6A)
C(1B)–C(6B)
C(1A)–C(2A)
C(1B)–C(2B)
C(2A)–C(3A)
C(2B)–C(3B)
C(3A)–C(4A)
C(3B)–C(4B)
C(4A)–C(5A)
C(4B)–C(5B)
C(5A)–C(6A)
C(5B)–C(6B)
O(1C)–C(1C)
C(1C)–C(2C)
O(2C)–N(4C)
C(2C)–C(3C)
O(3C)–N(4C)
C(3C)–C(4C)
N(4C)–C(4C)
C(4C)–C(5C)
C(1C)–C(6C)
C(6C)–C(5C)
N(2A)–Co(1A)–N(2A)#1
N(2A)–Co(1A)–N(3A)
N(2A)#1–Co(1A)–N(3A)
N(2A)–Co(1A)–N(3A)#1
N(2A)#1–Co(1A)–N(3A)#1
N(3A)–Co(1A)–N(3A)#1
N(2A)–Co(1A)–N(1A)
N(2A)#1–Co(1A)–N(1A)
N(3A)–Co(1A)–N(1A)
N(3A)#1–Co(1A)–N(1A)
N(2A)–Co(1A)–N(1A)#1
N(2A)#1–Co(1A)–N(1A)#1
N(3A)–Co(1A)–N(1A)#1
N(3A)#1–Co(1A)–N(1A)#1
N(1A)–Co(1A)–N(1A)#1
O(3A)–N(4A)–O(2A)
O(3A)–N(4A)–C(4A)
O(2A)–N(4A)–C(4A)
O(1A)–C(1A)–C(6A)
O(1A)–C(1A)–C(2A)
C(6A)–C(1A)–C(2A)
C(3A)–C(2A)–C(1A)
C(2A)–C(3A)–C(4A)
C(5A)–C(4A)–C(3A)
C(5A)–C(4A)–N(4A)
C(3A)–C(4A)–N(4A)
C(6A)–C(5A)–C(4A)
C(5A)–C(6A)–C(1A)
O(2B)–N(4B)–O(3B)
O(2B)–N(4B)–C(4B)
O(3B)–N(4B)–C(4B)
O(1B)–C(1B)–C(6B)
O(1B)–C(1B)–C(2B)
C(6B)–C(1B)–C(2B)
C(6C)–C(5C)–C(4C)
180.00(9)
N(2B)#2–Co(1B)–N(2B)
N(2B)#2–Co(1B)–N(1B)
N(2B)–Co(1B)–N(1B)
N(2B)#2–Co(1B)–N(1B)#2
N(2B)–Co(1B)–N(1B)#2
N(1B)–Co(1B)–N(1B)#2
N(2B)#2–Co(1B)–N(3B)
N(2B)–Co(1B)–N(3B)
N(1B)–Co(1B)–N(3B)
N(1B)#2–Co(1B)–N(3B)
N(2B)#2–Co(1B)–N(3B)#2
N(2B)–Co(1B)–N(3B)#2
N(1B)–Co(1B)–N(3B)#2
N(1B)#2–Co(1B)–N(3B)#2
N(3B)–Co(1B)–N(3B)#2
O(2C)–N(4C)–O(3C)
O(2C)–N(4C)–C(4C)
O(3C)–N(4C)–C(4C)
O(1C)–C(1C)–C(6C)
O(1C)–C(1C)–C(2C)
C(6C)–C(1C)–C(2C)
C(3C)–C(2C)–C(1C)
C(2C)–C(3C)–C(4C)
C(3C)–C(4C)–C(5C)
C(3C)–C(4C)–N(4C)
C(5C)–C(4C)–N(4C)
C(5C)–C(6C)–C(1C)
C(3B)–C(2B)–C(1B)
C(2B)–C(3B)–C(4B)
C(5B)–C(4B)–C(3B)
C(5B)–C(4B)–N(4B)
C(3B)–C(4B)–N(4B)
C(6B)–C(5B)–C(4B)
C(5B)–C(6B)–C(1B)
90.74(9)
89.26(9)
89.26(9)
90.74(9)
180.00(10)
88.55(9)
91.45(8)
89.24(9)
90.76(9)
91.45(8)
88.55(9)
90.76(9)
89.24(9)
180.0
91.98(8)
88.02(8)
87.67(8)
92.33(8)
88.02(8)
91.98(8)
92.33(8)
87.67(8)
180.00(12)
121.2(2)
120.5(2)
118.3(3)
122.0(2)
121.9(2)
116.1(2)
122.4(2)
119.5(2)
120.3(2)
120.0(2)
119.7(2)
121.8(3)
121.6(3)
119.3(2)
120.9(2)
119.2(3)
119.9(3)
119.8(3)
121.4(2)
121.8(3)
119.4(3)
118.8(3)
121.2(2)
121.9(2)
116.9(2)
121.4(3)
119.9(3)
120.3(3)
119.2(3)
120.6(3)
119.8(3)
121.8(3)
121.4(2)
119.2(3)
119.4(3)
121.4(2)
121.8(2)
116.8(2)
120.0(3)
Symmetry transformations used to generate equivalent atoms: #1 KxC1,KyC1,KzC2 #2 Kx,KyC1,KzC1.

R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
175
slightly distorted. In the hexaamminecobalt(III) chloride
˚
[42], the average Co–N distance is 1.963 A and the
structural features. There are four molecules of water as
solvent of crystallization in the lattice. They are all
clustered together with [Co(NH3)6]3C cations forming
linear columns perpendicular to the bc plane (Fig. 2). Out
of these four water molecules, there are two hydrogen
bonded water dimers formed between O4, O6 [O6/H4AZ
maximum deviation of cis N–Co–N bond angles from
their ideal values is 2.18 with a standard deviation of
0.78. These values of the [Co(NH3)6](4-np)3.4H2O salt
are also in the agreement with the earlier reported
hexaamminecobalt(III) complex salts [26–32]. The
selected bond angles and bond lengths are listed in
Table 3.
˚
1.828 A, O4–H4A/O6Z165.708] and O5, O7 [O7/
˚
H5BZ1.955 A, O5–H5B/O7Z168.228]. These water
dimers form an intricate network of hydrogen bonds with
[Co(NH3)6]3C cations and stabilize the linear columns (see
a typical one in the middle of Fig. 3). Six of such columns
present at the inversion center of the unit cell forms a tubular
cavity in which the 4-nitrophenolate anions reside, forming
hydrogen bonding linkages to adjacent columns from both
ends (using both –OK and –NO2 groups) i.e. head to tail
bonding of 4-np ion which we have observed for the first
time. It is interesting that all the four water molecules of the
columns are involved in holding the anions at the middle of
the tubular cavity through strong hydrogen bonds with the
negatively charged phenolate oxygen atoms. Indeed these
oxygen atoms are involved in multiple hydrogen bonds to
impart maximum stability (see the last six entries in
Table 4). The other side of the anion (nitro part) is also
anchored to the linear columns by hydrogen bonds
simulating a molecular ladder. It is noteworthy that
The conformation of the 4-np ion is similar to that found in
other 4-np salts/complexes [43–48]. The comparison of the
C–O bond length of the 4-np ion in the various 4-np
containing compounds indicate that there is no dependence
of the C–O bond length of the para-nitrophenolate ion on the
metal ion present (ionic and coordinated). The average C–O
˚
˚
bond length is 1.305(3) A and 1.311(2) A in Na(C6H4
NO3)$2H2O [45] and Mg(C6H4NO3)$8H2O [46], respect-
ively, whereas in the [Co(NH3)6] (C6H4NO3)3.4H2O salt is
˚
1.305(3) A. This indicate that the complex under study
in ionic.
In the crystal lattice, there are two independent
[Co(NH3)6]3C cations and both are placed on center of
inversion. As a result, they have near octahedral coordi-
nation geometry around the central metal ion (Table 3). The
packing of title complex salt shows some interesting
Fig. 2. Packing of [Co(NH3)6](4-np)3$4H2O viewed down a-axis. Note the formation of columns in which [Co(NH3)6]3C cation and water molecules
participate through H-bonding. These columns are linked by 4-np ions on both sides. (hydrogen bonds are shown with dotted lines).

176
R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
Fig. 3. Packing of [Co(NH3)6](4-np)3$4H2O viewed down ‘ab’ plane showing the p stacking of aromatic planes. See all the four water molecules bonded to a
column of [Co(NH3)6]3C (hydrogen bonds are shown with dotted lines).
the hydrophilic parts of the structure are placed in the close
proximity and are linked through hydrogen bonds imparting
stability to the lattice. Due to such an arrangement, the
hydrophobic aromatic part of the anions is stacked one over
other generating additional stabilizing p/p stacking
interactions (the interplanar distance between the benzene
4. Conclusion
The selective complexation of hexaamminecobalt(III)
cation with nitrophenolates results in the formation of
yellow coloured complex salts which have been character-
ized on the basis of elemental analyses and spectroscopic
studies. The measurements of solubility products show that
relative affinities of nitrophenolate ions for hexaammine-
cobalt(III) is in the order [Co(NH3)6](2,4,6-tnp)3
O[Co(NH3)6](2,4-dnp)3[[Co(NH3)6](4-np)3$4H2O. In
the case of [Co(NH3)6](4-np)3$4H2O, the single crystal
˚
rings of the nearest neighbour is 3.54 A). Thus a view from
the one edge of a unit cell (Fig. 2) shows repeating array of
column-p/p stack, column-p/p stack., arrangement
that are stabilized through hydrogen bonds and p/p
interactions.
Table 4
˚
Hydrogen bonding parameters *(A) and (8) of [Co(NH3)6](4-np)3.4H2O
D-H/A
D-H
H/A
!DHA
D/A
Symmetry operations
N1A-H1AA/O2B
N1A-H1AC/O1B
N2A-H2AA/O4
N2A-H2AC/O1B
N3A-H3AC/O1B
N1B-H1BA/O3C
N1B-H1BB/O1A
N2B-H2BA/O1C
N2B-H2BB/O5
N2B-H2BC/O1A
N3B-H3BB/O3C
N3B-H3BC/O1C
O4-H4A/O6
O5-H5B/O7
O4-H4B/O1A
O5-H5A/O1C
O6-H6A/O1B
O6-H6B/O1C
O7-H7A/O2A
O7-H7B/O1A
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.890
0.988
0.872
0.891
0.970
0.932
0.961
0.958
0.904
2.271
2.113
2.095
2.272
2.022
2.104
2.169
2.183
2.205
2.077
2.460
2.082
1.828
1.955
1.916
1.767
1.918
1.931
2.027
1.996
148.16
165.95
163.45
145.33
164.01
157.63
161.74
166.71
156.24
158.43
125.88
174.82
165.70
168.22
164.01
162.58
153.92
165.04
169.63
172.17
3.063
2.985
2.959
3.047
2.888
2.946
3.027
3.056
3.041
2.923
3.068
2.969
2.796
2.814
2.783
2.709
2.786
2.870
2.975
2.894
[x, KyC3/2, zK1/2]
[KxC1, yK1/2, KzC3/2]
[KxC1, yK1/2, KzC5/2]
[Kx, KyC1, KzC1]
[Kx, yK1/2, KzC3/2]
[x, KyC3/2, zK1/2]
[x, KyC3/2, zC1/2]
[x, KyC3/2, zK1/2]
[x, KyC3/2, zK1/2]
[x, KyC1/2, zC1/2]
˚
Relatively strong hydrogen bonds are only depicted in this Table (distance less than 2.28 A between d(H/A)). There are short contacts in the range (2.28–
˚
2.6 A d(H/A)) present in the hydrophilic region of the structure providing additional stability to the lattice.

R.P. Sharma et al. / Journal of Molecular Structure 752 (2005) 170–177
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X-ray structure determination revealed the presence of
N–H/O, O–H/O and N–O/H charge assisted hydrogen
bond interactions apart from coulombic attractions in the
solid state which may be present in the other two title
complex salts also. The present study shows that
[Co(NH3)6]3C present in [Co(NH3)6]Cl3 may find
application as anion receptor for toxic and commercially
important mono, di and tri-subsituted nitrophenolate ions in
aqueous medium in contrast to other synthetic receptors
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5. Supplementary material
Crystallographic data for the structural analysis of
[Co(NH3)6](C6H4NO3)3.4H2O compound has been depos-
ited at the Cambridge Crystallographic Data Center,12
Union Road,Cambridge, CB2 1EZ, UK, and are available
free of charge from the Director on request quoting the
deposition number CCDC 250897 (Fax: 44 1223 336033,
email: deposit@ccdc.cam.ac.uk)
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Ritu Bala thanks the CSIR, New Delhi, India for the
financial support (Grant No.01 (1768)/02/EMR-II).
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