Synthesis, structure and polymorphism in cobalt carboxylate complexes
2.2 Polymorphism in an aqua bridged dinuclear cobalt(II) carboxylate
Cobalt carboxylate complexes In last two steps loss of two carboxylic acids take place, one at each step. At the temperature range 390-500°C the complex loses one carboxylic acid which corresponds to the weight loss of 76.13% (theoretical loss 74.50%). In the final step, at the temperature range 500-580°C, the complex loses the remaining bridging carboxylate molecule and gets converted to cobalt(II) oxide. This corresponds to a weight loss of 86.21%. Theoretically loss of four pyridine, four carboxylic acids and a water molecule amounts to 88.21% weight loss.
The presence of aromatic guest such as benzoic acid and benzene in the lattice of 2.1 and 2.2 gives rise to a strong absorption at 1537 cm-1 (indicated by arrows in Figure 2.7). This absorption band is absent in the solid state FT-IR spectrum of complex 2.3 and 2.4 signifying the absence of any solvent molecule in the lattice (Figure 2.7).
Figure 2.7 The IR spectra of complexes 2.1-2.4
Cobalt carboxylate complexes
special interest in the process leading to a
olymorph is generally sensitive to the variation in the conditions of temperature, pressure g aspect to nderstand the structural, thermodynamic, and kinetic factors associated with the
pharmaceutical industries. The crystallisation p
and/or the manner in which the crystals are obtained275-276. It is a most challengin u
crystallization of molecular compounds. The various types of supramolecular interactions such as hydrogen bonding, C−H····π and π ····π interactions have become important implement in crystal engineering277. The delicate combination of hydrogen bonds and weak interactions, may efficiently contribute to the formation of different crystalline phases. The packing pattern and structural motifs of the resulting polymorphs depend remarkably on the different types of supramolecular interactions. The interconversion between polymorphs is possible through the mechanochemical processing of solid compounds278 or through the mixing of several components in the solid phase279. Consequently, aspects related to the synthesis, structure and polymorphism in metal coordination complexes are emerging as an active field of research280. In this subsection, we have described the synthetic and structural aspects of three polymorphs of an aqua–bridged dinuclear cobalt(II) complex.
The reaction of cobalt (II) chloride hexahydrate with 2-nitrobenzoic acid, sodium hydroxide and pyridine in methanol gives complex 2.5 (Polymorph I), which crystallises as pink rods in the triclinic space group PĪ (Scheme 2.5).
Ar Ar
X = H2O, Y = Pyridine 1) NaOH, Methanol
2) Pyridine + ArCO2H
1:2 stoichiometry
Y O
Co O O X
O
O Co Y
Y Ar
Y O
O O
Ar CoCl2.6H2O
Ar = 2-Nitro phenyl
2.5 (Polymorph I)
Scheme 2.5
In this complex each cobalt (II) centre is coordinated to a monodentate benzoate ligand and six-coordinate octahedral geometry is completed by two bridging
···A 167.0°] and O17−H···O2 [dO17····O2 2.508 Å,
<D−H····A 176.8°] interactions. The crystal structure of polymorph I is shown in Figure 2.8.
The Co1−O17 and Co2−O17 bond distances in the complex are 2.126(15) and 2.128(15)Å two pyridine molecules; the
2-nitrobenzoate groups and a bridging aqua ligand. The oxygen atom of monodentate 2- nitrobenzoate is intramolecularly hydrogen bonded to the bridging aqua group through O17−H···O14 [dO17····O14 2.578 Å, <D−H·
Cobalt carboxylate complexes and the Co1−O17−Co2 bond angle is 114.5(7)°. Moreover, the nitro groups of carboxylic acid are involved in weak intramolecular C−H····O interactions namely C6−H····O3 [dC6····O3 3.255 Å, <D−H····A 157.9°], C10−H····O12 [dC10····O12 3.281 Å, <D−H····A 150.2°] and C14−H···O12 [dC14····O12 3.429 Å, <D−H····A 158.4°]. The hydrogen bond parameters of the polymorph I are listed in Table 2.5. These interactions stabilize the out-out orientation of the nitro groups of 2-nitrobenzoate ligand relative to the Co1−O17−Co2 direction.
Table 2.5: Hydrogen bond (Å, °) of polymorph I
D−H···A d(D−H) d(H···A) d(D···A) <D−H···A geometry
O(17)−H(17A)····O(14) 0.85(3) 1.74(3) 2.578(2) 167.0(3) O(17)−H(17B)····O(2) 0.90(4) 1.62(4) 2.508(3) 176.8(4) C(2)−H(2)····O(7) [-1+x, y, z] 0.93 2.52 3.279(5) 139.4
C(6)−H(6)····O(3) 0.93 2.37 3.255(4) 157.9
C(10)−H(10)····O(12) 0.93 2.44 3.281(5)
C(14)−H(14)····O(12) [x, 1+y, z] 0.93 2.55 3.429(4) 158.4
C(29)−H(29)····O(13) 0.93 2.65 3.572 169.4
C(40)−H(40)····O(7) 0.93 2.64 3.504 155.1
C(42)−H(42)····O(11) 0.93 2.70 3.437 136.
150.2
5
Th plex 2.5 (Polymorph II) was prepared by the reaction of cobalt (II) acetate tetrahydrate with 2-nitrobenzoic acid and pyridine in methanol. This complex crystallizes from methanol as pink plates in the monoclinic C2/c space g
e com
roup. In polymorph II, each obalt (II) centre is coordinated to a monodentate 2-nitrobenzoate and two pyridine
molecules; two br omplete the six-
coordinate octahedral geometry (Fig Crys re o ph II the
C 616 Å an plex is stabilised by intramolec n
b onodentate enzo the qua
O −H····A 157.4°] interaction.
The corresponding ···O9 distance is 2.142(9) Co1' bon is
115. ymorph 10B
m ce through weak intermolecular dC .229 Å,
<D −H····O4 [dC 293 Å, <D H····A 127.0°], C14−H····O8 [dC14····O8
3.36 and C22−H····O4 [ 22····O4 3 −H····A 177.5°]
mportant hydrogen bond distances are listed in Table 2.6. In polymorph II, c
idged 2-nitrobenzoate groups and the bridging aqua ligand c
ure 2.8). tal structu f polymor shows that
o1···Co1' distance is 3. d the com ular hydroge
onding between the m 2-nitrob ate and bridging a group through 9−H···O2 [dO9 ···O2 2.581 Å, <D
Co1· Å and the Co1−O9− d angle
1 (7)°. The packing structure of pol II is shown in Figure 2. , wherein the olecules are held in the latti C3−H····O2 [ 3····O2 3
−H····A 155.9°], C4 4····O4 3. −
7 Å, <D−H····A 135.8°] dC .602 Å, <D
interactions. The i
Cobalt carboxylate complexes the nitro groups of each carboxylate ligand project inwards and they therefore may be described as having in-in orientations.
Table 2.6: Hydrogen bond geometry(Å, °) of polymorph II
D−H···A d(D−H) d(H···A) d(D···A) <D−H···A
O(9)−H(9o)····O(2) 0.84 1.78 2.581(17) 157.4
C(3)−H(3)····O(2) [1/2+x, 1/2+y, z] 0.93 2.36 3.229(3) 155.9
C(4)−H(4)····O(4) 0.93 2.65 3.293 127.0
C(14)−H(14)····O(8) 0.93 2.64 3.367 135.8
C(22)−H(22)····O(4) 0.93 2.67 3.602 177.5
The reaction of cobalt(II) chloride hexahydrate, 2-nitrobenzoic acid and sodium hydroxide in the solid-state followed by addition of pyridine gave the dinuclear cobalt(II) complex, 2.5 (polymorph III). This complex is soluble in chloroform, benzene or toluene and can subsequently be crystallised from toluene as pink plates in P21/n space group.In this complex, the cobalt centers are in distorted octahedral environment. Each cobalt center has coordinated through two bridging and one monodentate o-nitrobenzoate ligand as shown in Figure 2.8.
The remaining coordination sites are occupied by two pyridine and a water molecule. The
distance between molecule have
distances 2.136 (15) Å and 2.157 (16) o1 and Co2 respectively. The ua
l nded with the monodentate carboxylate ugh
O · 1.9°], H····O 2.576 Å, <D–H····A
1 Besides that the ps of carboxylic acids are involved in weak
C ith aromatic hy ns to ze the lar assem ome of
t bonds are list able this p ph III two nitro groups
re in out-out orientation and the other two are in in-in orientation, so the overall orientation the two cobalt center is 3.651 Å and the bridging water
Å from C bridging aq
igand is hydrogen bo group thro intramolecular
17−H····O6 [dO17····O6 2.574 Å, <D–H ···A 17 O17− 2 [dO17····O2
69.2°] interactions. nitro grou
−H···O interactions w droge stabili molecu bly. S he significant hydrogen ed in T 2.7. In olymor
a
of nitro groups is out-in or in-out.
Cobalt carboxylate complexes
Figure 2.8 Crystal structures of three polymorphs
Table 2.7: Hydrogen bond geometry(Å, °) of polymorph III
D−H···A d(D−H) d(H···A) d(D···A) <D−H···A
O(17)−H(1A)····O(6) 0.87(3) 1.71(3) 2.574(3) 171.9(3)
O(17)−H(1B)····O(2) 0.83(3) 1.76(3) 2.576(3) 169.2(3) C(2)−H(2)····O(1) [-x, 1-y, -z] 0.93 2.60 3.516(3) 168.7 C(8)−H(8)····O(2) [1/2-x, 1/2+y, 1/2-z] 0.93 2.52 3.313(4) 142.9 C(33)−H(33)····O(8) [3/2-x, 1/2+y,1/2-z] 0.93 2.42 3.074(7) 127.5
C(38)−H(38)····O(3) 4) 170.7
C(46)−H(46)····O(14) [1-x, 1-y, -z] 0.93 2.48 3.388(4) 164.9 [-x, -y, -z] 0.93 2.53 3.454(
A close inspection of the bond distances (li pp t olym s
that the coordination environment around each cobalt(II) centre is similar and the
c nt around the cobalt center is no d orie the
n ntres are c ated monodentate benzoate and two
p ce are bridged by two 2-nitrobenzoate and an
sted in A endix) in he three p orphs show
oordination environme t affecte due to the ntation of itro groups. The cobalt(II) ce oordin to a
yridine ligands and the two cobalt (II) ntres
Cobalt carboxylate complexes
a bonded to the carboxyl group of the
m s important that in p orph the roups
a er han morp an be by the 2-
itrobenzoate groups adopting in-in orientation, whereas the overall orientation of polymorph qua group, which is intramolecularly hydrogen
I each of
onodentate 2-nitrobenzoate. It i olym nitro g
h II c
dopts out-out orientation. On the oth d, poly identified two of n
III is out-in or in-out relative to the Co−O−Co unit. The orientations of the nitro benzoate groups in the three polymorphs are pictorially presented in Figure 2.9. The 2-nitrobenzoate groups are stabilized in these orientations by the weak intermolecular interactions. The packing structures of three polymorphs are shown in Figure 2.10.
Figure 2.9 Pictorial presentations of the three polymorphs
The torsional angles (Table 2.8) are different for the different orientations of the nitrobenzoate groups in the three polymorphs. Some of the selected bond distances and angles of three polymorps are listed in Table 5 (Appendix).
Table 2.8 : Torsional angles in polymorph I-III
2.5 (Polymorph I) out-out 2.6 (Polymorph II) in-in 2.7 (Polymorph III) in-out C18-C19-C20-N5 1.6(4) C6-C7-C8-N2 -8.5(3) C21-C22-C23-N8 -14.6(4) Monodentate nitro
benzoate groups C32-C33-C34-N8 11.2(3) C6'-C7'-C8'-N2' -8.5(3) C28-C29-C30-N 7 -9.6(5) C11-C12-C13-N6 -1.1(4) C18-C19-C20-N4 -7.7(3) C35-C36-C37-N6 7.2(4) Bridging nitro
benzoate groups C25-C26-C27-N7 -4.6(3) C18'-C19'-C20'-N4' -7.7(3) C42-C43-C44-N5 13.0(4)
The polymorphism originates due to the different orientations of the nitro groups in the three polymorphs. From these observations it may be concluded that the formation of the
olymorphs are controlled by three factors: (a) the synthetic route (solid or solution phase),
(b) the solvent of crysta y ligand.
p
llisation and (c) the stoichiometry of the auxiliar
Cobalt carboxylate complexes Cobalt carboxylate complexes
Figure 2.10 Packing structures of three polymorphs Polymorph III
The polymorph I and III, can be prepared by salt of 2-nitrobenzoic acid followed by the addition respectively. These two polymorphs are easily conver solvent like toluene and methanol. The intercove number of possible products of a reaction have si
nergy. However,
rated system. This may then transforms to the lowest energy product over
A B C
A) Polymorph I, B) Polymorph II, C)
the reaction of cobalt(II) chloride with sodium of pyridine in solution and solid state ted to polymorph II upon dissolution in rsion between the polymorphs occurs when a milar free energies. From an enthalpy point of view, the product which is most likely to form should have the lowest e
of view, the product which is most likely to form should have the lowest e
depending upon the reaction conditions, the product that quickly crystallises has high entropy in the supersatu
depending upon the reaction conditions, the product that quickly crystallises has high entropy in the supersatu
time, thereby facilitating the interconversion between the polymorphs.
O Co
O X O
O
O Co
O O
A B C
Figure 2.10 Packing structures of three polymorphs A) Polymorph I, B) Polymorph II, C) Polymorph III
The polymorph I and III, can be prepared by the reaction of cobalt(II) chloride with sodium salt of 2-nitrobenzoic acid followed by the addition of pyridine in solution and solid state respectively. These two polymorphs are easily converted to polymorph II upon dissolution in solvent like toluene and methanol. The intercoversion between the polymorphs occurs when a number of possible products of a reaction have similar free energies. From an enthalpy point nergy. However,
rated system. This may then transforms to the lowest energy product over time, thereby facilitating the interconversion between the polymorphs.
O Co
O X O
O
O Co
O O Py Py
Py Py
O O2N
NO2
O2N
NO2
O Co
O
O O
O Co
O O Py Py
Py Py
O
O2N O2N
NO2 NO2
X O
Co O X
O O
O Co O
O
Py Py
Py Py
O NO2 O2N
NO2 O2N
CoCl2. 6H2O ArCO2H, NaOH, Py
(in solid state, Toluene)
ArCO2H, NaOH, Py (in solution, Methanol)
Polymorph III Polymorph I
Methanol Toluene
Co(OAc)2. 4H2O
ArCO2H, Py (in solution, Methanol) Polymorph II
Ar =2-Nitrophenyl Py = Pyridine
X = H2O
Scheme 2.6
The powder XRD patterns of the polymorphs substantiate our observation of the distinct crystalline phases (Figure 2.11). The powder XRD patterns of all the three polymorphs are
Cobalt carboxylate complexes different, which supports the existence of the polymorphs. Subsequently, we performed powder XRD analysis of the samples after their transformation which confirm the conversion of both polymorphs I, and III into II as illustrated in Scheme 2.6. The PXRD patterns of three polymorphs are well in agreement with the simulated results from the single-crystal X-ray diffraction data, indicative of three distinct polymorphic forms.
Each of the polymorphs (I–III) shows broad signal around 3433 cm–1 (O–H stretching) in their solid-state FT-IR spectra. This corresponds to the presence of intramolecular hydrogen- bonding interactions involving the 2-nitrobenzoate group and the bridging aqua ligand. Solid- state FT-IR spectra of polymorphs I, II and III exhibit similar absorptions for monodentate carboxylate carbonyl C=O at around 1635 cm–1 and for the C–O group it appears at 1394 cm–
1. The asymmetric and symmetric band of bridging carboxylate groups appear at aroun 02 and 1487 cm–1 respectively. The asymm or nitro N–O stretching appears at the 531 cm–1 and for symmetric stretching it appears at 1364 cm–1. FT-IR spectra of polymorph
d 16 etric band f
1
III is shown in Figure 2.22. The magnetic moments of all the three polymorphs at room temperature are found to be around 6.56 BM.
Polymorph 1 Polymorph 2
Polymorph 3
Figure 2.11 Overlay PXRD patterns of the three polymorphs (black) with their simulated ones (blue).
The UV–Vis spectrum of the three polymorphs show a weak absorption band at 521 nm (ε = 39.18-39.23 M-1 dm3 cm-1) in methanol which is assigned to be the 4T1(F) → 4A2(F) transition for distorted octahedral cobalt(II) complexes. At room temperature, the ESR spectrum of the polymorphs show two signals one is around 3381.0G (g = 1.995) and another strong signal appears at 8688.3G (g = 0.776) which is an identification for the distorted octahedral geometry around the Co(II) center (Figure 2.12A).