Role of second-sphere coordination in anion binding: Synthesis, characterization and X-ray structure of hexaamminecobalt(III) chloride hydrogen phthalate trihydrate and sodium hexaamminecobalt(III) benzoate monohydrate

Article abstract of DOI:10.1016/j.molstruc.2005.03.028

In an effort to utilize [Co(NH₃)₆]³⁺cation as a new host for carboxylate ions, orange coloured crystalline solids of composition [Co(NH₃)₆]Cl(C₈H₅O ₄)₂·3H₂O (1) and Na[Co(NH ₃)₆](C₇H₅O₂) ₄·H₂O (2) were obtained by reacting hot aqueous solutions of hexaamminecobalt(III) chloride with potassium hydrogen phthalate and sodium benzoate in 1:3 molar ratio, respectively. The title complex salts were characterized by elemental analyses and spectroscopic studies (IR, UV/Visible and NMR). Single crystal X-ray structure determinations revealed the formation of second-sphere coordination complexes based on hydrogen bond interactions. In complex salt 1 only two out of three ionisable chloride ions present in [Co(NH₃)₆]Cl₃ were replaced by two C8H5O4- ions whereas in complex salt 2 all the three ionisable chloride ions present in [Co(NH₃)₆]Cl₃ were replaced and the final product was an adduct with another mole of sodium benzoate in solid state. The crystal lattice is stabilized by electrostatic forces of attraction and predominantly N-H?O interactions.

Full text of DOI:10.1016/j.molstruc.2005.03.028

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Journal of Molecular Structure 748 (2005) 143–151
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Role of second-sphere coordination in anion binding: Synthesis,
characterization and X-ray structure of hexaamminecobalt(III) chloride
hydrogen phthalate trihydrate and sodium hexaamminecobalt(III)
benzoate monohydrate
Raj Pal Sharmaa, ,1, Ritu Balaa, Rajni Sharmaa, B.M. Kariukib, ,2
*
*
,
c
Urszula Rychlewskac, ,3, Beata Warzajtis
*
˙
aChemistry Department and Centre of Advanced studies in Chemistry, Panjab University, Chandigarh 160014, India
bSchool of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
´
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
c
Received 5 February 2005; revised 14 March 2005; accepted 15 March 2005
Abstract
In an effort to utilize [Co(NH3)6]3Ccation as a new host for carboxylate ions, orange coloured crystalline solids of composition
[Co(NH3)6]Cl(C8H5O4)2$3H2O (1) and Na[Co(NH3)6](C7H5O2)4$H2O (2) were obtained by reacting hot aqueous solutions of
hexaamminecobalt(III) chloride with potassium hydrogen phthalate and sodium benzoate in 1:3 molar ratio, respectively. The title
complex salts were characterized by elemental analyses and spectroscopic studies (IR, UV/Visible and NMR). Single crystal X-ray structure
determinations revealed the formation of second-sphere coordination complexes based on hydrogen bond interactions. In complex salt 1 only
two out of three ionisable chloride ions present in [Co(NH3)6]Cl3 were replaced by two C8H5OK4 ions whereas in complex salt 2 all the three
ionisable chloride ions present in [Co(NH3)6]Cl3 were replaced and the final product was an adduct with another mole of sodium benzoate in
solid state. The crystal lattice is stabilized by electrostatic forces of attraction and predominantly N–H/O interactions.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Anion binding agent; Cobalt(III); Coordination chemistry; Second-sphere coordination; Supramolecular chemistry; X-ray crystallography
1. Introduction
and synthesis of receptors for carboxylates has attracted
much interest for the last three decades. The different
geometry and higher basicity arising from greater charge
density on the carboxylate oxygen can be expected to affect
the anion-receptor interactions. Benzoate and hydrogen
phthalate ions are two such carboxylates with a number of
applications [9]. The former is used in the manufacturing of
benzoyl compounds, preservation of food, as a mordant in
calico painting and as a standard in volumetric and
calorimetric analysis. The hydrogen phthalate is used as a
primary standard for preparing volumetric alkali solutions,
as buffer in pH solutions as well as in pharmaceuticals (as
antifungal). Therefore, the search for new anion receptor
that may find potential applications [10] in the areas of
analytical chemistry, biology, catalysis and coordination
equilibria remain unabated.
Anions play important roles in chemistry, biology and
environmental science and therefore, the molecular recog-
nition of anions or anion binding is an area of current
interest [1–6]. Due to their varied shapes and sizes [7],
anions pose a greater challenge than cations. One important
class of mono or multiple charged oxoanions is the
biologically significant carboxylate ions [8]. The design
*
Corresponding authors.
E-mail addresses: rpsharma@pu.ac.in (R.P. Sharma), b.m.kariuki@
bham.ac.uk (B.M. Kariuki), urszular@anil.edu.pl (U. Rychlewska).
1 Tel.: C91 172 2534433.
2 Tel.: C44 121 C147481.
3 Tel.: C48 61 8271268.
The idea that ligands in the primary coordination sphere
of a transition metal complex can interact in an ordered
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.03.028

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R.P. Sharma et al. / Journal of Molecular Structure 748 (2005) 143–151
manner with neutral or charged species to give a second
sphere or outer sphere complex was first advanced by
Werner in 1912 [11]. This was further supported by
spectroscopic and crystallographic techniques later on.
Although several examples of second-sphere complexes
with aqua and ammine coordination compounds have
appeared in the literature [12], the molecular recognition
and binding of anions by anion binding agents via second-
sphere coordination has not been exploited much.[13] The
pioneering work of Shimizu et al. [14] on the second sphere
aqua complexes of organo-sulphonate ligands and hexaam-
minecobalt(III) chloride needs special mention. The
second sphere-sphere coordination complexes of
[M(DABP)3]2Chas been reported to give supramolecular
assemblies {[M(DABP)3].(18-C-6)]}Cl2.4CH3OH and
{[Zn(DABP)3].(18-C-6)}(NO3)2.3/8 H2O with crown ethers
[15] and [Fe(DABP)3][p-nitrophenolate].2H2O with p-
nitrophenolate [16] where DABPZ5,50-diamino-2,20-
bipyridine. The organic framework solids (layered solids)
can be designed and assembled based on second-sphere
coordination to generate cavities or channels of various
sizes and shapes by appropriate choice of the building
blocks. These are of current interest because of a number of
potential applications [17]. We envisaged that cationic
metal complex, hexaamminecobalt(III) can serve as a
building block for larger Supramolecualar assemblies
because the free amino groups are good hydrogen bond
donors and may form second-sphere coordination com-
plexes with oxo-anions (hydrogen bond acceptors). This
paper reports synthesis, characterization and single crystal
X-ray structure determination of [Co(NH3)6]Cl(C8H5-
O4)2$3H2O (1) and Na[Co(NH3)6](C7H5O2)4$H2O (2) in
continuation of our interest [18] in cobalt(III) salts/
complexes. We have already reported the potential use of
hexaamminecobalt(III) cation for the binding of organic
oxo-anions [18a] and inorganic oxo-anions [18b–d] and
layered solids based on second-sphere coordination inter-
actions in copper(II) napthalenesulphonate[19].
clear supernatant solution gave second crop of crystals.
The overall yield is nearly quantitative and the salt
decomposes at 465 K. The elemental analyses is consistent
with the composition [Co(NH3)6]Cl(C8H5O4)2 3H2O
(Found (%): C, 33.02; H, 5.81; N, 14.44, Cl, 6.05; Co,
10.09, calculated: C, 33.08; H, 5.85; N, 14.47; Cl, 6.11; Co,
10.14).
2.2. Synthesis of Na[Co(NH3)6](C7H5O2)4$H2O(2)
Hexaamminecobalt(III) chloride (1.0 g, 0.003 mol) was
dissolved in 20 ml of hot water in which 25 ml hot solution
of sodium benzoate (1.625 g, 0.011 mol) was added. The
mixture of solutions was allowed to cool slowly, orange
shining crystals appeared after six days, which were filtered
and air dried. The reddish-orange coloured clear supernatant
solution gave second crop of crystals. The overall yield is
nearly quantitative and the salt decomposes at 493 K. The
elemental analyses is consistent with the composition
Na[Co(NH3)6](C7H5O2)4$H2O (Found (%): C, 48.93; H,
5.82; N, 12.24; Co, 8.57), calculated: C, 48.90; H, 5.79; N,
12.20; Co, 8.42).
Cobalt and chloride content were determined by standard
methods of estimation [21] and C, H, N were estimated
microanalytically by automatic PERKIN ELMER 2400CHN
elemental analyzer. UV/Visible spectra were recorded using
a HITACHI 330 spectrometer in water as solvent. Infrared
spectra were recorded using a PERKIN ELMER spectrum
RX FT-IR system using Nujol mull in KBr plates. 1H-NMR
and 13C-NMR spectra were recorded in D2O solvent at 25 8C
a using JEOL AL 300 MHz FT NMR spectrometer. The
chemical shift values are expressed as d value (ppm)
downfield from tetramethylsilane as an internal standard.
2.3. Crystal structure determination
Single crystals of 1 and 2 suitable for X-ray diffraction
studies were grown from aqueous solution by slow
evaporation (solution obtained by mixing the aqueous
solutions of the reactants in 1:3 molar ratio). The data
collection was performed at ambient temperature. The
intensity data for complex salt 1 were collected on Rigaku
R-axis IIc single crystal diffractometer equipped with an
image plate detector system and a molybdenum rotating
2. Experimental
Analytical grade reagents were used without any further
purification. [Co(NH3)6]Cl3 was prepared by air oxidation
of Co(II) salt in ammonical solution in the presence of
activated charcoal catalyst, according to the method
described by Bjerrum and McReynold [20].
˚
anode (lZ0.71069 A) and a graphite monochromator. The
intensity data for complex salt 2 were collected on KUMA
Diffraction KM-4 diffractometer equipped with CCD
detector. The absorption correction for the data set 2 was
performed with the procedure contained within the CrysAlis
software package [22] (Tmin/TmaxZ0.8219/0.9068). Both
structures were solved by direct methods, in the SHELXS-97
[23] program package followed by full-matrix least-squares
refinements on F2. The SHELXL-97 [23] system of programs
was used throughout. Non-hydrogen atoms were treated
anisotropically. The hydrogen atoms were placed at
calculated positions except for the water hydrogens which
2.1. Synthesis of [Co(NH3)6]Cl(C8H5O4)2$3H2O (1)
Hexaamminecobalt(III) chloride (1.0 g, 0.003 mol) was
dissolved in 20 ml hot water in which 25 ml hot solution of
potassium hydrogen phthalate (2.30 g, 0.011 mol) was
added. The mixture of solutions was allowed to cool slowly,
shining orange crystals appeared after few hours, which
were filtered and air dried. The reddish-orange coloured

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R.P. Sharma et al. / Journal of Molecular Structure 748 (2005) 143–151
145
Table 1
[Co(NH3)6](C7H5O2)3$nH2O, the unexpected complex
salt 2, Na[Co(NH3)6](C7H5O2)4$H2O was obtained.
The composition 2 was initially indicated by elemental
analysis. The salt is soluble in water and DMSO but not in
organic solvents like acetone, ethanol and chloroform.
Repeated attempts to obtain [Co(NH3)6](C7H5O2)3 have
failed so far.
Crystal data and structure refinement for 1 and 2
1
2
M
580.87
686.58
T/K
˚
l/A
296(2)
293(2)
0.71069
Triclinic
P1
0.71073
Monoclinic
C2/c
Crystal system
Space group
˚
a/A
˚
b/A
UV/Visible spectra were recorded in water. Two
7.4140(13)
10.368(2)
16.181(3)
93.344(4)
90.565(5)
98.718(4)
1227.1(4)
2
16.3160(7)
32.4817(12)
6.6248(3)
90.0
1
1
transitions, A1g/1T1g and A1g/1T2g, at around 470
and 340 nm, respectively have been reported [24] for
complexes containing [Co(NH3)6]3C, producing the fam-
iliar orange-yellow colour observed for a number of classical
coordination compounds containing cobalt(III). The two
strong absorption maxima were observed at 474 and 337 nm,
for 1, and at 474 and 340 for 2, showing d–d transitions
typical for low spin Co(III) d6 octahedral complex salts.
The vibrational spectra show that the stretching vibrations
of the coordinated NH3 molecule are lower than those of the
free NH3 molecules. Two possible reasons are (a) the effect
of coordination and (b) the effect of the counter ion, i.e. ClK,
C7H5OK2 or C8H5OK4 . The counterion causes weakening of
the N–H bond due to the formation of N–H/ClK and N–
H/O types of hydrogen bonds. The antisymmetric 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 [25] which
are comparable with those for 1 and 2 (see Table 2). For the
hydrogen phthalate and benzoate anions, the hydrogen
bonding and resonance weaken the C]O bond, resulting
in absorption at lower frequency and the sharp peak at 1585
and 1594 cmK1 respectively, for C]O (nas). The peaks have
been assigned by comparison with the reported [26] values
given in Table 2.
˚
c/A
a/8
b/8
g/8
113.105(3)
90.0
3
˚
Volume/A
3229.3(2)
4
Z
scalc/Mg/m3
m/mmK1
1.572
1.412
0.875
0.605
F (000)
608
1440
Crystal size/mm
Reflections collected
Independent reflections
Rint
0.30!0.05!0.03
7176
0.40!0.30!0.20
14159
4067
3559
0.076
0.018
Parameters
336
209
R1, wR2 [IO2s(I)]
R1, wR2 (all data)
0.069, 0.143
0.081, 0.152
0.032, 0.087
0.041, 0.092
was located in a difference Fourier map. All H-atoms were
refined using the riding model, with their Uiso multiplied by
a factor of 1.2 compared to Ueq of atoms to which they were
bonded. Crystal and refinement data are shown in Table 1.
3. Results and discussion
Hexaamminecobalt(III) chloride may be expected to react
with potassium hydrogen phthalate or sodium benzoate in a
number of ways, depending upon the stoichiometric ratios:
H2O
½CoðNH3Þ6ꢀCl3 C3AC6H4CO2X ꢁꢁꢁ/ꢁ
(i)
1
In 1, the H NMR chemical shift value is observed at
7.35–7.55 (m, 8 aromatic protons, i.e. 3,11CH–,4,12CH–,
5,13CH–, 6,14CH–) ppm for phthalate ion. The values for 13
C
½CoðNH3Þ6ꢀðC6H4CO2XÞ3 C3ACl
NMR are 174.0 (4C, 7,15C–, 8,16C–), 133.5 (4C, 1,9C–, 2,10C–),
129.04 (4C, 6,14C–, 3,11C–) and 126.5 (4C, 4,12C–, 5,13C–) ppm
for the phthalate ion, which are comparable with literature
H2O
½CoðNH3Þ6ꢀCl3 C2AC6H4CO2X ꢁꢁꢁ/ꢁ ½CoðNH3Þ6ꢀ
(ii)
ClðC6H4CO2XÞ2 C2ACl
Table 2
Infrared spectral data (cmK1) and peak assignments for 1 and 2
H2O
½CoðNH3Þ6ꢀCl3 CAC6H4CO2X ꢁꢁꢁ/ꢁ ½CoðNH3Þ6ꢀ
(iii)
Complex salts
[Co(NH3)3]Cl3
Assignments
Cl2ðC6H4CO2XÞ CACl
1
2
with XZH, AZNaCor XZCO2H, AZKC.
–
3460
3278
3159
1585, 1548
–
3410
3272
3123
1594
1400
2
(O–H)n
Orange coloured crystals of 1 obtained by mixing 1:3
stoichiometric quantities of hexaamminecobalt(III) chloride
and potassium hydrogen phthalate gave elemental
analyses results, corresponding to the formula [Co(NH3)6]
Cl(C8H5O4)2$3H2O. This clearly indicated that only two out
of the three ionisable chloride ions present in [Co(NH3)6]Cl3
were replaced by two (C8H5O4)K ions (Eq. (ii)). The
product is soluble in water but not in acetone and ethanol.
When hexaamminecobalt(III) chloride and sodium ben-
zoate were mixed in a 1:3 molar ratio in the hope of getting
3240
3160
–
(N–H)nas
(N–H)ns
(COOK)nas
(COOK)ns
(N–H)dd
–
1619
–
a
1376
1337
813
–
(O–H)d
1326
831
–
1332
830
–
(N–H)ds
(N–H)rr
760,718,550
(1,2-substituted
phenyl ring)n
a, absorption hidden behind the carbonyl stretch.

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Fig. 1. (a) Anortep drawingof the assymetric unitof 1 and(b) thecrystalstructureof 1 vieweddownthe a axis showinghydrogenbondingcontactsasbrokenlines.
values.[27] ThenumberingschemeisasshowninFig.1(a).For
1
2, H NMR chemical shift values are observed at 7.72–7.73
4.1. Co(NH3)6]Cl(C8H5O4)2$3H2O (1)
(double doublet for 8 aromatic protons, i.e. 2,6CH–) and 7.31–
7.44 (multiplet for 12 aromatic protons, i.e. 3,4,5CH–) for
benzoate ion. The values for 13C NMR are 176.0 (4C, –
For 1, structure determination revealed the presence of
the following crystallographically independent components
in the solid state: [Co(NH3)6]3C, ClK, two (C8H5O4)2K and
three water molecules (Fig. 1(a)). This conclusively
established the existence of [Co(NH3)6]Cl(C8H5O4)2$3-
3H2O. The cobalt(III) metal ion is surrounded by six
nitrogen atoms originating from six coordinating ammonia
molecules resulting in a slightly distorted octahedral
geometry (Table 3). The Co–N bond distances are in the
1
COOK), 136.88 (4C, C–), 131.88 (8C, 2,6C–),129.40 (8C,
3,5C–)and128.93(4C,4C–)ppmforthebenzoateion,whichare
1
also comparable with literature values. [27] The H NMR
spectra for both complex salts show broad peaks at 3.52 ppm,
characteristic of coordinated NH3 group.[28]
˚
range 1.954(5)–1.974(5) A, the cis N–Co–N bond angles are
4. X-ray crystallography
in the range 88.9(2)–91.2(2)8 and trans N–Co–N bond
angles are in the range 179.1(2)–179.8(2)8. In the hex-
aamminecobalt(III) chloride [29], the average Co–N
˚
distance is 1.963 A and the maximum deviation of cis
N–Co–N bond angles from their ideal values is 2.1(7)8.
The crystal structures of both title complex salts have
been unambiguously established by single crystal X-ray
crystallography.