New Bis-Pyrazole-Bis-Acetate Based Coordination Complexes: Inﬂuence of Counter-Anions and Metal Ions on the Supramolecular Structures

the Abstract: A new ﬂexible bis-pyrazol-bis-acetate ligand, diethyl 2,2’-(pyridine-2,6-diylbis (5-methyl-1H-pyrazole-3,1-diyl))diacetate ( L ), has been synthesised, and three coordination complexes, namely, [Zn( L ) 2 ](BF 4 ) 2 ( 1 ), [Mn L Cl 2 ] ( 2 ) and [Cd L Cl 2 ] ( 3 ) have been obtained. All ligands and complexes were characterised by IR, mass spectroscopy, thermogravimetric analysis and single-crystal X-ray diffraction. Single crystal X-ray diffraction experiment revealed that the primary supramolecular building block of 1 is a hexagonal chair shaped 0D hydrogen bonded synthon (stabilised by C–H ··· O hydrogen bonding and C=O ··· π interactions), which further built into a 2D corrugated sheet-like architecture having a 3-c net honeycomb topology, and ﬁnally extended to a 3D hydrogen bonded network structure having a ﬁve nodal 1,3,3,3,7-c net, through C–H ··· F interactions. On the other hand, the two crystallographically independent molecules of 2 exhibited two distinct supramolecular structures such as 2D hydrogen bonded sheet structure and 1D zigzag hydrogen bonded chain, sustained by C–H · O and C–H ··· Cl interactions, which are further self-assembled into a 3,4-c network structure, and 3 showed a 2D hydrogen bonded sheet structure. The supramolecular structural diversity in these complexes is due to the different conformations adopted by the ligands, which are mainly induced by different metal ions with coordination environments controlled by different anions. Hirshfeld surface analysis was explored for the qualitative and quantitative analysis of the supramolecular interactions.


Introduction
Designing coordination complexes by using supramolecular self-assembly is an important research area in materials chemistry [1]. The use of relatively simple organic ligands and metal ions through their kinetically labile and thermodynamically stable coordination bonds attracted many research groups due to their various potential applications [2][3][4][5][6][7]. Such self-assembly resulted in channels or void spaces, wherein host-guest chemistry played a role for the incorporation of small molecules or anions within such empty spaces [8]. In most of the coordination complexes so far reported, the ligands having only one hetero nitrogen as the donor atom such as pyridine [9], picoline [10], isoquinoline [11] etc. were used. On the other hand, ligands having two hetero nitrogen atoms such as imidazole [12], pyrazole [13,14] and pyrazine [15] are not much explored in the coordination chemistry of transition metals.
Our research group has recently started a research programme on coordination complexes built from pyrazole ligands. For example, we have reported the crystal structures of Co(II)/Cu(II) coordinated complexes of pyrazole-dicarboxylate acid ligand and established their supramolecular structures [16]. In another work, we have demonstrated the effect of the aliphatic backbone of the bis-pyrazole-bis-carboxylate ligand on the supramolecular structures of their Co(II)/Cu(II)/Cd(II) coordination complexes [17]. In a further account, we have studied the effect of anions and hydrogen bonding on the supramolecular structural diversities of Cu(II) and Mn(II) coordination complexes obtained from a novel bis-pyrazole ligand [18]. More recently, two new pyrazole-acetamide ligands and their solid-state structures of coordination complexes caracterised by their remarkable antioxidant activity have been reported too, in the context of the effect of hydrogen bonding on the self-assembly process [19]. Last but not least, we have reported the crystal structure-bioactivity correlation of three mononuclear coordination complexes of a pyrazolyl-benzimidazole ligand [20].
etc. were used. On the other hand, ligands having two hetero nitrogen atoms such as imidazole [12], pyrazole [13,14] and pyrazine [15] are not much explored in the coordination chemistry of transition metals.
Our research group has recently started a research programme on coordination complexes built from pyrazole ligands. For example, we have reported the crystal structures of Co(II)/Cu(II) coordinated complexes of pyrazole-dicarboxylate acid ligand and established their supramolecular structures [16]. In another work, we have demonstrated the effect of the aliphatic backbone of the bis-pyrazole-bis-carboxylate ligand on the supramolecular structures of their Co(II)/Cu(II)/Cd(II) coordination complexes [17]. In a further account, we have studied the effect of anions and hydrogen bonding on the supramolecular structural diversities of Cu(II) and Mn(II) coordination complexes obtained from a novel bis-pyrazole ligand [18]. More recently, two new pyrazole-acetamide ligands and their solid-state structures of coordination complexes caracterised by their remarkable antioxidant activity have been reported too, in the context of the effect of hydrogen bonding on the self-assembly process [19]. Last but not least, we have reported the crystal structure-bioactivity correlation of three mononuclear coordination complexes of a pyrazolylbenzimidazole ligand [20].
In the present study, we aim to explore the effect of ligating topologies, counter anions and the metal ion nodes on the supramolecular structures of coordination complexes obtained from a conformationally flexible bis-pyrazol-bis-acetate ligand having a pyridine backbone (Scheme 1), namely diethyl 2,2'-(pyridine-2,6-diylbis(5-methyl-1H-pyrazole-3,1-diyl)) diacetate (L) because of the following reasons: (1) The ligand L is an N-heterocyclic tridentate pyrazolyl pyridine compound capable of forming various coordination modes, and ligating topology with transition metal ions [21]. (2) This type of pyridine ligand having pyrazolyl groups at the second and sixth position, possesses a wide range of interesting chemical and/or physical properties, such as catalytic [22,23], electrochemical [24], magnetic and photophysical properties [25].
Many coordination complexes containing both Lewis base donors and Lewis acid acceptors in the same ligand [26] such as 2,6-bis (pyrazolyl) pyridine have been reported [23][24][25][26][27] according to the Hard-Soft acid base theory [28]. (4) L is a new ligand, not yet reported. In this work, we shall investigate the coordination properties of L with Zn(II), Mn(II) and Cd(II), which are recognised non-biodegradable and toxic metal ions, toxic for both health and environment. For this purpose, we have reacted L with Zn(BF4)2•6H2O, In this work, we shall investigate the coordination properties of L with Zn(II), Mn(II) and Cd(II), which are recognised non-biodegradable and toxic metal ions, toxic for both health and environment. For this purpose, we have reacted L with Zn(BF 4 ) 2 ·6H 2 O, MnCl 2 ·4H 2 O and CdCl 2 ·2.5H 2 O in a 1:2 molar ratio, which led to single crystals which were systematically investigated by single crystals X-ray diffraction (Scheme 2).
The crystal structures of three coordination complexes were discussed in the context of their effect of conformation dependent ligating topology, counter anions and metal ion nodes on the supramolecular structural diversities. We have also present, for completeness, MnCl2•4H2O and CdCl2•2.5H2O in a 1:2 molar ratio, which led to single crystals which were systematically investigated by single crystals X-ray diffraction (Scheme 2).
The crystal structures of three coordination complexes were discussed in the context of their effect of conformation dependent ligating topology, counter anions and metal ion nodes on the supramolecular structural diversities. We have also present, for completeness, the crystal structure of the ligand L and of the intermediate compound 2-(5-methyl-1H-pyrazol-3-yl)-6-(3-methyl-1H-pyrazol-5-yl) pyridine B (See Scheme 1).

Scheme 2.
Schematic representation of the synthesis of coordination complexes 1, 2 and 3.

Materials and Methods
All solvents and chemicals, obtained from usual commercial sources, were of analytical grade and used without further purification. 1 H and 13 C NMR spectra were obtained on a Bruker AC 300 MHz spectrometer with the solvent proton peak as internal standard. High resolution mass spectrometry HRMS data were obtained with a Q Exactive Thermofisher Scientific ion trap spectrometer by using ESI ionisation. FT-IR spectra were recorded with KBr discs on a Perkin Elmer 1310 spectrometer. Thermogravimetric Analyses (TGA) were carried out on a Mettler Toledo TGA/SDTA 851e analyser by loading 3-4 mg of sample, and the mass loss was monitored under nitrogen on warming from room temperature to 900 °C at 10 °C/min. A suitable single crystal was selected and mounted onto a rubber loop using Fomblin oil. Single-crystal X-ray diffraction (SXRD) data of B, L, 1, 2, 3 were recorded on a Bruker Apex CCD diffractometer (λ (MoKα) = 0.71073 Å) at 150 K equipped with a graphite monochromator. Structure solution and refinement were carried out with SHELXS-97 [29] and SHELXL-97 [30] using the WinGX software package [31]. Data collection and reduction were performed using the Apex2 software package. Corrections for the incident and diffracted beam absorption effects were applied using empirical absorption corrections [32]. All the non-H atoms were refined anisotropically. The positions of hydrogen atoms were calculated based on stereochemical considerations using the riding model. Final unit cell data and refinement statistics for B, L, 1, 2, 3 are collected in Table 1.

Materials and Methods
All solvents and chemicals, obtained from usual commercial sources, were of analytical grade and used without further purification. 1 H and 13 C NMR spectra were obtained on a Bruker AC 300 MHz spectrometer with the solvent proton peak as internal standard. High resolution mass spectrometry HRMS data were obtained with a Q Exactive Thermofisher Scientific ion trap spectrometer by using ESI ionisation. FT-IR spectra were recorded with KBr discs on a Perkin Elmer 1310 spectrometer. Thermogravimetric Analyses (TGA) were carried out on a Mettler Toledo TGA/SDTA 851e analyser by loading 3-4 mg of sample, and the mass loss was monitored under nitrogen on warming from room temperature to 900 • C at 10 • C/min.
A suitable single crystal was selected and mounted onto a rubber loop using Fomblin oil. Single-crystal X-ray diffraction (SXRD) data of B, L, 1, 2, 3 were recorded on a Bruker Apex CCD diffractometer (λ (MoK α ) = 0.71073 Å) at 150 K equipped with a graphite monochromator. Structure solution and refinement were carried out with SHELXS-97 [29] and SHELXL-97 [30] using the WinGX software package [31]. Data collection and reduction were performed using the Apex2 software package. Corrections for the incident and diffracted beam absorption effects were applied using empirical absorption corrections [32]. All the non-H atoms were refined anisotropically. The positions of hydrogen atoms were calculated based on stereochemical considerations using the riding model. Final unit cell data and refinement statistics for B, L, 1, 2, 3 are collected in Table 1.

Synthesis, FT-IR and UV-Visible Spectroscopy
The ligand L was synthesized by following a three-step reaction, in which the dimethyl pyridine-2,6-dicarboxylate was converted to an intermediate compound bis-hydroxy-bisone (A) following a nucleophilic reaction with acetone and NaOMe, in the first step, and in the second step, the intermediate compound was treated by hydrazine hydrate resulting in 2- The resultant needle shaped crystals were characterised by using FT-IR, UV-visible spectroscopy, electrospray ionisation mass spectrometry (ESI-MS) and SXRD. More detail explanation of FT-IR and UV-visible spectroscopy are given in the SI.

Crystal Structures
The crystal data of B, L, 1, 2 and 3 are given in Table 1 Needle-type crystals were obtained by slow evaporation of dichloromethane and methanol in the case of L and a mixture of dichloromethane and ethanol in the case of the ligand B. Not surprisingly, the crystal structure of B contains an ethanol molecule, whereas no solvent was detected for L. Single crystal X-ray diffraction analysis of ligand B revealed that the ligand crystallises in the orthorhombic space group P2 1 2 1 2 1 , which is an achiral member of the Sohncke family defining chiral crystals ( Figure 1). The asymmetric unit is composed of one molecule of each B and lattice included ethanol. The solvent ethanol was found to be disordered over two positions. In the unit cell, four molecules of ligand and ethanol were present. The N-N bond distances are in the range of 1.343(4)-1.350(4) Å, which is characteristic for pyrazole [20]. From the crystal structure, it is found that the ligand exists as slightly non-planar, in which the planes of the pyrazole rings showed a difference in the angle of 8.  The chirality obtained from 2-fold rotational axes as a result of the molecular assemblies in the crystal lattice of an achiral component is an important topic in crystal engineering [33]. On the other hand, the ligand L crystallised in the centrosymmetric triclinic space group P-1. The asymmetric unit contains only one molecule of L, and there were two such molecules found in the unit cell, both related to each other by a centre of inversion symmetry. As expected, the ligand showed non-planar structure, which is revealed from the angle between the pyrazole rings (14.13°). Among various plausible conformations, the ligand L showed anti-anti-syn-anti-anti conformation in the crystal structure (Scheme S1). The N-N bond distance was in the range of 1.358(4)-1.350(4) Å, which is the characteristic N-N bond length for pyrazole [20]. The chirality obtained from 2-fold rotational axes as a result of the molecular assemblies in the crystal lattice of an achiral component is an important topic in crystal engineering [33]. On the other hand, the ligand L crystallised in the centrosymmetric triclinic space group P-1. The asymmetric unit contains only one molecule of L, and there were two such molecules found in the unit cell, both related to each other by a centre of inversion symmetry. As expected, the ligand showed non-planar structure, which is revealed from the angle between the pyrazole rings (14.13 • ). Among various plausible conformations, the ligand L showed anti-anti-syn-anti-anti conformation in the crystal structure (Scheme S1). The N-N bond distance was in the range of 1.358(4)-1.350(4) Å, which is the characteristic N-N bond length for pyrazole [20]. Moreover, the C=O, C-O and O-C bond lengths were in the range of 1.184(4)-1.206(8) Å, 1.318(5)-1.319(9) Å and 1.446(5)-1.458(8) Å, respectively [34], which confirms the presence of ethyl acetate functionality in L.
We are interested in the final supramolecular structure of this new ligand L, in which three distinct functionalities such as pyridine, pyrazole and ethyl acetate are present. The ] involving a -CH2spacer and the pyrazole N atom leading to the formation of a network structure having Schläfli symbol {4 8 .6 2 } and exhibiting a 5-c net unimodal topology [35]. In fact, such hydrogen bonding resulted in the formation of an eightmembered hydrogen bonded macrocycle of graph set R (8). Such pairs of 2D sheets are further self-assembled through weak van der Waals force (Figure 2).   In contrast to the anti-anti-syn-anti-anti conformation of the ligand (non-coordinated to the metal centre), the metal bound ligand L molecules showed two distinct conformations such as syn-syn-syn-syn-syn and syn-syn-syn-syn-anti in the coordination complex 1, with substantial molecular non planarity, which is evident from the corresponding dihedral angles of 9.00-15.77 • involving the terminal pyrazole rings. The crystallographically independent molecules of 1, showed weak C-H···O hydrogen bonding [C-H···O anions facilitated the self-assembly of a 2D corrugated sheet, into a three dimensional hydrogen bonded network structure having a five nodal 1,3,3,3,7-c net with Schläfli symbol {0}{3.5.6}{3 2 .5 2 .6 3 .7 3 .8 3 .9 2 }{4.5.7}2 (Figure 4).
The asymmetric units of the coordination complexes are shown in Figure 3. A colourless needle-shaped single crystal of 1 crystallised in the centrosymmetric monoclinic space group P21/c. The asymmetric unit contains two molecules of Zn(II) coordination complex, and four counter anions of tetrafluoroborate (BF4 -). The metal centre Zn(II) exhibited distorted octahedral geometry [<N-Zn-N = 74.27(9) -99.10(10)°] wherein all of the six coordination sites were occupied by the N atoms (both pyridine and pyrazole) of two molecules of the ligand L. In contrast to the anti-anti-syn-anti-anti conformation of the ligand (non-coordinated to the metal centre), the metal bound ligand L molecules showed two distinct conformations such as syn-syn-syn-syn-syn and syn-syn-syn-syn-anti in the coordination complex 1, with substantial molecular non planarity, which is evident from the corresponding dihedral angles of 9.00-15.   SXRD analysis revealed that single crystals of 2 belong to the centrosymmetric monoclinic space group P21/c. The asymmetric unit was comprised of two crystallographically independent molecules of coordination complex 2. The coordination complex 2 consists of Mn(II) ion, two chloride anions and one ligand L. In the unit cell, there were four such units of each crystallographically independent molecules of 2, which were symmetrically related by two-fold screw axis (21), glide plane and centre of inversion. The Mn(II) showed SXRD analysis revealed that single crystals of 2 belong to the centrosymmetric monoclinic space group P2 1 /c. The asymmetric unit was comprised of two crystallographically independent molecules of coordination complex 2. The coordination complex 2 consists of Mn(II) ion, two chloride anions and one ligand L. In the unit cell, there were four such units of each crystallographically independent molecules of 2, which were symmetrically related by two-fold screw axis (2 1   Coordination complex 3 crystallises in the centrosymmetric monoclinic space group P2/n. The asymmetric unit is comprised of one half of the molecule of 3, i.e., one half of Cd(II) metal ion, one half of the molecule of L and one chloride anion (both L and chloride anions were coordinated to Cd(II)). The two-fold axis is passing through the Cd(II) metal centre and N(1) and C(1) atoms of the ligand L. Due to the presence of this two-fold axis, the remaining half of Cd(II), ligand L and chloride anion are generated by symmetry. In Coordination complex 3 crystallises in the centrosymmetric monoclinic space group P2/n. The asymmetric unit is comprised of one half of the molecule of 3, i.e., one half of Cd(II) metal ion, one half of the molecule of L and one chloride anion (both L and chloride anions were coordinated to Cd(II)). The two-fold axis is passing through the Cd(II) metal centre and N(1) and C(1) atoms of the ligand L. Due to the presence of this two-fold axis, the remaining half of Cd(II), ligand L and chloride anion are generated by symmetry. In the crystal structure, the metal atom Cd(II) displays distorted trigonal bipyramidal geometry with angles ranging from 69.47(5)-104. 26(5) • . The axial coordination sites of Cd(II) were occupied by the two pyrazole nitrogen atoms of L whereas the equatorial sites are occupied by nitrogen atom of pyridine moiety of L and two chloride anions. Like in 2, the ligand L showed anti-syn-syn-syn-syn conformation with slight non-planarity in 3, which is revealed from the angle (6.33 • ) between the pyrazole rings.

Hirshfeld Surface Analyses
To investigate more about the supramolecular interactions in the crystal structures of B, L, 1, 2 and 3, Hirshfeld surfaces have been calculated for all the structures. From Hirshfeld surface [36] analysis, we can quantify various supramolecular interactions present in the crystal structure. We used CRYSTAL EXPLORER [37] to plot the Hirshfeld surfaces [38] and calculate their respective 2D fingerprint plots [39].
The 3D maps of Hirshfeld surface (HS) assist us to find out the main interactions between molecules, and the 2D fingerprint plot (FP) help us to understand the distances

Hirshfeld Surface Analyses
To investigate more about the supramolecular interactions in the crystal structures of B, L, 1, 2 and 3, Hirshfeld surfaces have been calculated for all the structures. From Hirshfeld surface [36] analysis, we can quantify various supramolecular interactions present in the crystal structure. We used CRYSTAL EXPLORER [37] to plot the Hirshfeld surfaces [38] and calculate their respective 2D fingerprint plots [39].
The 3D maps of Hirshfeld surface (HS) assist us to find out the main interactions between molecules, and the 2D fingerprint plot (FP) help us to understand the distances among atoms involved in those interactions. More precisely, 3D HS and 2D FP enable us to give insights into qualitative and quantitative analysis of supramolecular interactions, respectively, present in the molecule. The 3D HS plots of B, L, 1, 2 and 3 are presented in Figure 7, exhibiting the surface map over the normalised contact distance (d norm ), which can be determined from the d e (the distance between the Hirshfeld surface and adjacent nucleus outside the surface), d i (the distance between the Hirshfeld surface and nearest inside the nucleus) and the van der Waals radii of the atoms (r vdW i or r vdW ie ) from Equation (1): The corresponding shape index and curvedness of B, L, 1, 2 and 3 are shown in Figures S1-S4 (ESI). In the dnorm map, the red spots indicate the closeness of atoms to the HS from outside, meaning a strong hydrogen bonding exists between the HS and the nearest atoms outside. While the white areas on the 3D HS designate the contacts with distances equal to the sum of van der Waals radii, the blue colour regions indicate the longer distances than the van der Waals radii as shown in Figure 7a  The corresponding shape index and curvedness of B, L, 1, 2 and 3 are shown in Figures S1-S4 (ESI). In the d norm map, the red spots indicate the closeness of atoms to the HS from outside, meaning a strong hydrogen bonding exists between the HS and the nearest atoms outside. While the white areas on the 3D HS designate the contacts with distances equal to the sum of van der Waals radii, the blue colour regions indicate the longer distances than the van der Waals radii as shown in Figure 7a,b. The HS of B was generated by using a standard (high) surface resolution with 3D d norm surfaces mapped to a range −0.6557 to 1.3709 a.u. From the d norm mapping, it is revealed that strong hydrogen bonding interactions such as N-H···N (between the pyrazole moieties) and N-H···O (between pyrazole and solvated ethanol) were present in the crystal lattice of B, as observed from the bright red spots on the HS. On the other hand, 3D d norm surfaces mapping (ranges between -0.6823 to 1.4926 a.u.) of L, showed bright red spots near to C=O of ester, pyrazole and -CH 2 -spacer of neighbouring molecules of L, confirming C-H···O and C-H···N hydrogen bonding interactions.
The contributions of the interatomic contacts (C···H, N···H, and O···H) present in B and L are revealed from the 2D FP ( Table 2). The C···H interatomic contacts present in B and L are due to the C-H···π involving C-H of pyrazole and pyrazole ring, and C-H of spacer and pyridine ring, respectively. Weak π···π stacking (C···C = 0.9%), lone pair···π (C···O = 0.9%), and stacking of the aromatic rings (C···N = 1.8%) were also present in the crystal structure of L.   Table 2). The C···H interatomic contacts were also present in the crystal structures of 1, 2 and 3, due to the C-H···π interactions (C-H of ester and pyrazole/pyridine ring in 1, C-H of ester and pyrazole ring in 2, and C-H of the methyl group of pyrazole/pyridine ring in 3). Supramolecular interactions such as π···π stacking (C···C = 2.7% in 2 and 3.1% in 3), lone pair···π (C···O = 2.0% in 1) were also present in the crystal structures of the coordination complexes (Table 2). Weak Van der Waals interactions were also found in 2 and 3 (Cl···O = 0.5% in 2 and 0.6% in 3). Moreover, the H···H contacts in B, L, 1, 2 and 3 comprise of the major contributors to the contact list of 2D FP, such as 49.3%, 57.3%, 51.9%, 46.6% and 43.9%, respectively, within the HS. This is due to the high share of hydrogen atoms present in their crystal structures. Interestingly, the presence of sharp spikes was found in the 2D FPs of 1, 2, and 3

Influence of Counter-Anion and Metal Ion on the Conformation of the Ligand L and the Supramolecular Structures of the Coordination Complexes
The coordination complexes discussed herein showed supramolecular structural diversity in their crystal structures. The fundamental reason behind such diversity is due to the influence of various metal ions and counter anions, during the crystallisation process, which induced the conformational changes of the ligand L in the coordination complexes [40][41][42]. As shown in Scheme S1, there are several possible conformations of L, which can contribute to the coordination with metal ions. Indeed, due to the small energy barrier between various conformations of the flexible ligand L, it can display a particular conformation required for the coordination driven self-assembly of a metal ion. However, predicting such specific conformation is generally challenging, because of various hurdles such as the diversity in the possible orientations of the ligands in the crystals, the less precision in estimating the energies of ligand for its coordination with metal ion, and difficulty to predict the thermodynamic and kinetic contributions for the crystal growth. Hence, it is very important to recognise the supramolecular synthon present in the crystal structure, which is the sub-structural motif in the crystal.
The ligand L showed anti-anti-syn-anti-anti conformation in the crystal structure. Once it undergoes coordination with Zn(II) and form coordination complex 1, the ligand L displayed two distinct conformations (two molecules of L are present in 1 such as syn-synsyn-syn-syn and syn-syn-syn-syn-anti. The coordination geometry of Zn(II) and the BF 4 anions present in the crystal lattice induce such conformations of L, in 1. From the overlay structure of L with the 1 (Figure 9a,b), we can easily understand that the pyrazole rings of L rotate around 180 • . Additionally, the self-assembly of L having these conformations, with the distorted octahedral Zn(II) via C-H···O hydrogen bonding resulted in a hexagonal chairshaped 0D hydrogen bonding synthon, the main sub-structural motif of 1, which further extended into a 2D corrugated sheet structure through weak C=O···π. In fact, the BF 4anions present in the crystal lattice of 1, further assisted the self-assembly process via C-H···F hydrogen bonding leading to the formation of a 3D hydrogen bonded network. On the other hand, L showed anti-syn-syn-syn-syn conformation in both 2 and 3, where the counter anion is common, viz. chloride. In both coordination complexes 2 and 3, chloride anions are coordinated to the metal ions. The difference between them is the metal ions, Mn(II) in 2 and Cd(II) in 3, present in the coordination complex. Another clear difference is the presence of two crystallographically independent molecules of coordination complex in 2, wherein in the case of 3, only one molecule was present in the asymmetric unit. The difference in the ionic radius of Mn(II) (0.75 Å) and Cd(II) (0.87 Å) is one of the crucial factors for such variances. As a result, the primary supramolecular synthons of 2 and 3 were also different; while the crystallographically independent molecules of 2 showed hexagonal shaped supramolecular synthon through C-H···O, which further extended to a 2D hydrogen bonded sheet structure, the C-H···Cl interaction assisted the formation of a 1D zigzag hydrogen bonded chain (such chains are further packed top and bottom of the sheets). The C-H···O hydrogen bonding in 3 gives a 1D hydrogen bonded chain as the primary supramolecular structure, which further extended to a 2D hydrogen bonded sheet structure with the support of C-H···Cl interactions. From the overlay of the structure of 2 and 3 over the ligand L, a difference in the conformations is observed (Figure 9c,d). Although the conformation of L is identical in 2 and 3, the supramolecular packing is different due to the packing of the molecules induced by anion and metal ion as a result of symmetry difference.
We have investigated the thermal stability of L, and its coordination complexes, by thermo-gravimetric analysis (TGA) over the 25-900 • C under nitrogen atmosphere at a heating rate of 10 • C/min. As expected, the coordination complexes showed much higher thermal stability than the ligand L, with 230 • C, 310 • C and 270 • C, for 1, 2 and 3 respectively. Thus, their thermal stability can be ordered as follows: L < 1 < 3 < 2. While 1 showed a continuous one step thermal decomposition, 2 and 3 exhibited a three-step thermal degradation, with sharp profiles at steps one and two. The higher thermal stability of 2 is due to the presence of a higher quantity of C-H···Cl (18.2% in 2 compared to 17.8% in 3) and other weak interactions (46.6% in 2 compared to 43.9% in 3), as revealed from the 2D FPs and 3D HS. The lower stability of 1, compared to 2 and 3, is also revealed from the 2D FPs and 3D HS data; although 21.1% of strong C-H···F is present in 1, the quantity of C-H···O (13.8% in 1, 14.5% in 2, 15.8% in 3) and N-H···O (1.8% in 1, 4.9% in 2 and 5.8% in 3) in 1 is less, in contrast to 2 and 3. Moreover, the contributions from π···π stacking and anion···π interactions which were present in 2 and 3 (Table 2), were absent in 1 ( Figure 10). 1 Figure 10. The thermo-gravimetric analysis (TGA) comparison plot of L, 1, 2 and 3.

Conclusions
A new flexible bis-pyrazol-bis-acetate ligand L, and its Zn(II), Mn(II) and Cd(II) coordination complexes have been synthesised and structurally characterised by single crystal X-ray diffraction. The ligand L showed diverse conformations once reacted with transition metals to produce the coordination complexes 1, 2 and 3. In addition, the intermediate compound B, which was found to be an achiral molecule, showed supramolecular chirality obtained from 2-fold rotational axes. While L showed a pair of 2D hydrogen bonded sheet structure having a 5-c net uninodal topology, 1 exhibited a 2D corrugated sheet like architecture having a honeycomb topology which further extended into a 3D hydrogen bonded network structure. Interenstingly, two distinct topologies were observed in 2, due to the presence of crystallographically independent molecules of 2 in the unit cell, namely a 2D hydrogen bonded sheet structure along the 'bc' plane and 1D zigzag hydrogen bonded chains, which are packed on the top and bottom of the 2D sheet. Finally, 3 showed a 2D hydrogen bonded sheet structure. Thus, the influence of the counter anions in shaping up the coordination modes of the metal ions and the conformation of ligand, resulting in various supramolecular synthons which control the self-assembly of coordination complexes, was demonstrated. Remarkably, 1, 2 and 3 showed unusual thermal stability as revealed from thermogravimetric analyses, which can be justified by the presence of strong supramolecular interactions, as revealed by the crystal structure and Hirshfield surface analyses. Its unique thermal stability could provide stable hybrid materials upon grafting L to silica for metallic decontamination purposes, particularly towards Zn(II), Mn(II) and Cd(II) which are recognised as toxic metal ions. This technology is currently under investigation in our laboratory and already applied to real water samples (e.g., from natural rivers) [43][44][45][46][47][48][49][50][51][52].