Calcium coordination compounds of dipicolinic and quinolinic acids: synthesis, characterization, crystal structures and DFT studies

Adedibu Clement Tella (  ac_tella@yahoo.co.uk ) University of Ilorin https://orcid.org/0000-0003-2090-4747 Adetola Christianah Oladipo Landmark University Victoria Tosin Olayemi Kwara State University Allen Gordon Nelson Mandela University Adeniyi Sunday Ogunlaja Nelson Mandela University Lukman Olawale Alimi Stellenbosch University Stephen Argent University of Nottingham Robert Mokaya University of Nottingham Guy Clarkson University of Warwick Richard Walton University of Warwick

the chlorine substituents were reported to lead to diversities in the crystal structures of the three calcium compounds obtained. Furthermore, a steric hindrance effect and the formation of O-H···Cl hydrogen-bond contributed to the enrichment of the structures [18]. A Ca(II) coordination compound of L-malic acid was synthesized and reported to have high thermal stability. In addition, the compound is capable of emitting its own characteristic sensitized luminescence [19]. A group of researchers synthesized and compared the structures of Ca(II), Mg(II) and Ba(II) Coordination Compounds of 1H-tetrazolate-5-acetic acid Ligand.
They concluded that the nature of metal ions is pivotal in determining their molecular framework [20].
Despite these existing reports, there are still fewer reports on coordination compounds of alkaline earth metals compared to those of transition metals, this is because their geometries are not predictable as their bonding mode is not governed by ligand eld stabilization [21].
Carboxylates of s-block metals have high a nity for oxygen donor e.g water [21]. Many researchers have reported their synthesis is aqueous media, e.g hydrothermal synthesis [22][23][24]. Herein, we report the synthesis of two calcium coordination compounds 1 and 2 in aqueous medium alongside suitable solvents, using solvothermal and room temperature synthetic reactions respectively. Furthermore, DFT study was employed to gain insight on the electronic properties of the compounds.

Materials and methods
All reagents and chemicals were of analytical grade and were used as received without further puri cation. 2,3-pyridinedicarboxylic acid and 2,6-pyridinedicarboxylic acid were purchased from Sigma Aldrich, Germany. Calcium nitrate tetrahydrate (Ca(NO 3

Instrumentation and characterization
The samples were characterized by elemental analysis using an Exeter Analytical CE-440 Elemental Analyser. The infrared spectra were recorded using a Bruker Alpha diamond module FT-IR spectrometer with attenuated total re ectance (ATR) attachment for solid samples. Powder X-ray diffraction (PXRD) patterns were measured on a PANalytical X'Pert PRO diffractometer with a Cu-Kα source (40 kV, 40 mA) with a step size of 0.02° and a 50 s time step. TGA was performed using an SDT-Q600 TA instrument.
The samples were heated in air with a heating rate of 10 ºC min -1 and the scan was recorded within the temperature range of 30-800 °C. Single crystal X-ray data for 1 were collected at 120 K on an Oxford Diffraction GV1000 diffractometer equipped with an Atlas S2 detector and mirror monochromated microfocus Cu source while 2 was collected at 150 K on Agilent Xcalibur Gemini diffractometer with a Ruby CCD area detector. Olex2 suite was used as a graphic user interface (GUI) and as imaging software [25]. The structures were solved with the ShelXT [26] structure solution program using intrinsic phasing and re ned with the ShelXL [27] re nement package using least squares minimisation. All nonhydrogen atoms were re ned with anisotropic displacement parameters and images were prepared via Mercury 4.1.0.

Computational Details
Theoretical studies were performed for 1 and 2. The input les were taken from the CIF obtained from reported X-ray single crystal measurements. The geometries were optimized by minimizing energies with respect to all geometrical parameters without imposing any molecular symmetry constraints.
Computational studies were carried out using the Gaussian 09 software package [28]. The calculations were performed by using B3LYP method (standard hybrid density functional method) with a basis set of 6-311++G** (2p, 2d) level [29]. Compound 1 crystallized in the orthorhombic space group Pccn. The asymmetric unit comprises of one crystallographically independent Ca(II) ion, one 2,6-H 2 pdc anion and a coordinated water molecule. The Ca(II) center coordinates to two oxygen atoms (O8) from two 2,6-H 2 pdc ligands, two oxygen atoms (O11) from two half deprotonated carboxylate groups, two oxygen atoms (O13) from two coordinated water molecules and two nitrogen atoms (N1) from two 2,6-H 2 pdc ligands resulting in a distorted dodecahedral geometry ( Fig. 1a). Selected bond lengths and angles are given in  [30], possibly due to distortion by metal ion coordination.
Interestingly, the O11-H11 group of the half deprotonated carboxylic group forms a strong intermolecular hydrogen bond with adjacent carboxylate group (O11-H11•••O8 = 2.5732 Å). The coordinated water also forms a network of intermolecular hydrogen bonds and acts as the hydrogen bond donor (O13-H13A•••O9) and (O13-H13B•••O9) with D···A distances of 2.834 Å and 2.724 Å respectively (Fig. 1b). The selected hydrogen bonding parameters for 1 are shown in Table S2. The crystal packing shows that the molecules are connected by hydrogen bonds from coordinated water molecules which help in stabilizing the structure (Fig. 1b). The entire framework is further stabilized by π···π stacking between the centroid of one pyridyl ring (i1) and the centroid of the adjacent pyridyl ring (i2) within a layer ( Fig. 1c and Table S3).
Compound 2 belongs to the monoclinic system with space group Pc, with a dimeric formula [Ca 2 (2,3pdc) 2 (H 2 O) 6 ] n . It has two coordination centers [Ca1 and Ca2], inducing a complete deprotonation of H 2 pdc, which in turn affects the structural functionalities of the carboxylic residues. The asymmetric unit comprises of two Ca(II) ions, two 2,3-H 2 pdc anions and six coordinated water molecules (Fig. 2). The coordination number of Ca1 in 2 is seven, the polyhedron being pentagonal bipyramid formed by one N, O set from one 2,3-H 2 pdc molecule, two oxygen atoms from two 2,3-pdc molecules and three oxygen atoms of three water molecules, one of which bridges Ca1 and Ca2. The coordination number of Ca2 is also seven (one N, O set from one 2,3-H 2 pdc molecule, one oxygen atom from one 2,3-H 2 pdc molecule, and four oxygen atoms of four water molecules (including the bridging water molecule) and exhibits pentagonal bipyramid geometry (Fig. 3 Table S1. There are two modes of coordination of the 2,3-H 2 pdc residue. In the rst mode around Ca1, one of the carboxylate groups (O101) coordinates monodentately and bridges Ca1 atom, while the other carboxylate group coordinates bidentately, with one of the oxygen atoms (O110) bridging a Ca1 atom and the other (O109) chelating another Ca1 atom with the pyridine nitrogen (N107). In the second mode around Ca2, both carboxylate groups coordinate monodentately, with the coordinating oxygen of one group (O201) bridging a Ca2 atom and the coordinating oxygen of the other group (O210) chelating another Ca2 atom with the pyridine nitrogen (N207). These features were observed in heteronuclear compounds of H 2 pdc [9].
The distance between Ca atom and the chelating oxygen atom (O109) is slightly longer than that of the bridging oxygen atom (O110), indicating the stronger bond of the latter. The plane of the carboxylic group of 2,3-H 2 pdc ligand (C204-C203-C202-O201) is anti-coplanar with that of the aromatic ring, with a torsional angle of 103.08°. Apart from these interatomic distances between the centre metal and other heteroatoms, the crystal packing of compound 2 reveals there are O-H···O hydrogen bonding interactions between the coordinated water molecules and the carboxylate of the 2,3-H 2 pdc molecules with donoracceptor (D···A) distance in the range of 1.990-2.911 Å ( Fig. 4 and Table S4). In addition to the O-H···O interactions, there exists C-H···O intermolecular interactions when viewed along the crystallographic c axis. These interactions connect two adjacent 2,3-H 2 pdc molecules involving the hydrogen atom from one 2,3-H 2 pdc ring and oxygen atom of the carboxylate of the neighbouring 2,3-H 2 pdc molecule (Fig. 5).
Compound 2 possesses a 3D framework due to an extended system of hydrogen bonds involving carboxylates and coordinated water molecules as well as the perpendicular π···π interactions between the 2,3-pdc layers ( Fig. 6 and Table S5). The 3D framework is shown in Fig. 7.

FTIR Studies
The comparison of the FTIR spectra of the free ligands and compounds 1 and 2 are shown in Fig. S1 and S2. For 1, the characteristic ν(C=O) and ν(C-O) absorption band of the parent ligand were observed at The FT-IR spectrum of 2 shows signi cant changes in absorption frequencies that occur at coordination sites, notably is the -OH band which appears around 3100 cm −1 in the spectrum of 2,3-H 2 pdc but disappeared in that of compound 2 due to the coordination of the deprotonated -OH group to Ca (II) metal. The IR spectra shows useful information related to the carboxylate bands from the metal

PXRD analysis
The powder X-ray diffraction (PXRD) data of compounds 1 and 2 were collected to ascertain their phase purity. The PXRD patterns of the as-synthesized samples were plotted against the simulated patterns as shown in Fig. S3 and S4. There was excellent agreement between the simulated and the experimental patterns which indicates high bulk purity of the as-synthesized phases.

Thermal Analysis
The TGA curves of compounds 1 and 2 are presented in Fig. 8. TGA curve of 1 shows a mass loss between 213°C to 292°C (56.10% found (calc. 50.49%)) assigned to the loss of two water molecules and a molecule of 2,6-H 2 pdc ligand. The large difference between the experimental and calculated values may suggest the presence of excess surface water. A steady mass loss was further observed up to 474°C (22.90% found (calc. 19.55%)) after which the framework collapsed leaving behind a residue of CaO. Compound 2 shows three thermal mass loss stages. The six molecules of water are lost in the rst step, and this amounts to 21% (20.8%) of the total weight. The two 2,3-H 2 pdc residues of weight 54% were lost in two indistinct steps, rst between 400-530°C and the second between 530-725°C. CaO residue was left till 800°C. The melting points were found to range between 248-250 ο C and 186-188°C for 2,6-H 2 pdc and 2,3-H 2 pdc ligands respectively. The results of the thermal analysis show that the compounds are more thermally stable than some metal coordination compounds of pyridinedicarboxylate [9,12]. However, the thermal stability of beyond 400 ο C was reported for some calcium coordination compounds [19,34].

DFT Studies
Optimized Molecular Structure The full geometry of 1 and 2 in Pccn and Pn symmetry was optimized (Fig. S5a & S5b), and the optimized crystal structures were very similar to those obtained by X-ray diffraction. The calculated bond parameters are presented in Table S2

HOMO-LUMO energy gap
The HOMO−LUMO gap describes the stability of molecules, and it predicts reactivity between species by providing the electrical transport properties as well as electron carrier and mobility in molecules [35][36][37]. The calculated HOMO and LUMO energies of 1 and 2 are summarized in Table 3. As shown in Fig. 9, the charge density distribution of 1 illustrates that the HOMO density is mainly located around the 2,6pyridinedicarboxylic acid (2,6-H 2 pdc) ligand, especially their nitrogen atom/imine and caboxylic groups, there are some indication that calcium atom may partly contribute to HOMO (Fig. 9). The LUMO density is localized on around the 2,6-pyridinedicarboxylic acid (2,6-H 2 pdc) ligand. With 2, the HOMO density is mainly located around the 2,6-pyridinedicarboxylic acid (2,6-H 2 pdc) ligand and the LUMO density is centred on the 2,6-pyridinedicarboxylic acid (2,6-H 2 pdc) ligand (around nitrogen and oxygen atoms) and the calcium atom (Fig. 10). Compound 2 displayed the smallest energy gap (5.156 eV) which is indicative of the softest molecule with better polarizability. Similarly, the lowest LUMO energy (E LUMO = 1.874 eV), which means that it can be the best acceptor of electrons [38]. 1 is characterized with an energy gap (ΔE gap = 9.221 eV) and this may indicate a molecule with high excitation energy, good stability and a high chemical hardness ( Table 2). The HOMO−LUMO orbitals in 1 and 2 are presented in Fig. 9 and 10, respectively.
The ionization potential (I), electron a nity (A), chemical potential (µ), electronegativity (χ), global hardness (η), global softness (S) and global electrophilicity (ω) values were calculated using the HOMO and LUMO energy values [39]. The HOMO energy (E HOMO ) is related to ionization potential (I) by Koopman's theorem and LUMO energy (E LUMO ) is related to electron a nity (A) [40]  Absolute electronegativity (χ) is related to average value of HOMO and LUMO energies de ned by Mulliken [41].  6) where µ is the chemical potential takes the average value of ionization potential (I) and electron a nity (A) [44]. The lowest value of the potential ionization (I = 3.282 eV) for 2 con rm that it is the better electron donor [45]. Chemical hardness and softness value of 1 (η = 4.611 eV, S=0.217 eV) is greater (lesser) than that of 2, respectively. Thus, it shows a molecule which is less reactive nature. 1 possesses higher electronegativity value (χ = 2.603 eV) than 2, hence making it a better electron acceptor. The value of ω for 1 (ω = 0.734 eV) indicates that it is the stronger electrophiles.

Conclusions
Two calcium coordination compounds of dipicolinate and quinolinate ligands were successfully synthesized. Structural characterization revealed that the Ca(II) ion in 1 interacts with adjacent carboxylate groups via strong intermolecular hydrogen bonds. On the other hand, 2 revealed two calcium centers, Ca1 and Ca2. For the two compounds, the torsional angles between the carboxylate group and the aromatic ring showed that they exhibit anti-coplanar conformation. Optimized molecular structures by DFT simulations are comparable to the crystal structure geometry obtained by X-ray diffraction. Further, theoretical calculations of the HOMO-LUMO energies con rmed that compound 1 is a better electron acceptor with less reactivity while compound 2 with smaller energy gap is more reactive.

Funding
The authors appreciate the Royal Society of Chemistry for funding. The School of Chemistry, University of Nottingham and the University of Warwick's Research Technology Platforms, for the provision of facilities for the analysis of the compounds. We are grateful to the Center for High Performance Computing (CHPC), Capetown, South Africa for providing the platform to carry out molecular modelling studies using the Gaussian09 software.

Con ict of Interest
There are no con icts to declare.
Availability of data and material

Code availability
The datasets and codes for DFT modelling generated during the current study will be made available upon request.   The asymmetric unit of compound 2.

Figure 3
ORTEP diagram of 2 with thermal ellipsoids drawn at 50% probability level.