A Co-Crystallised Cobalt(II) Cluster of Pyridinedicarboxylic Acid (PDC) as a Luminescent Material for Selective Sensing of Methanol

A luminescent Cobalt(II) co-crystal [Co13(PDC)16(H2O)24.7H2O] 1 (where H2PDC = 2,6-pyridinedicarboxylic acid) have been prepared by oven-heating and slow evaporation of solvent. Single crystal X-ray diffraction (SCXRD) analysis revealed that 1 is a mixture of complexes that crystallizes in the triclinic space group P-1 and the geometry around the Co(II) ions is octahedral. The structure is extensively imbued with hydrogen bonding that helps in stabilizing the complex. Thermogravimetric analysis indicates that 1 is thermally stable up to 364 οC. The luminescence properties of 1 revealed a strong emission centered at 437 nm (λex = 345 nm) assigned to ligand to metal charge transfer (LMCT). The luminescence sensing of 1 towards volatile organic molecules were also examined. However, 1 displayed a turn off towards methanol compared to other molecules with high quenching efficiency and low limit of detection (3.5 × 10−4 vol%). The results show excellent selectively and high sensitivity. Powder X-ray diffraction studies revealed that the structural integrity of the complex was maintained after exposure to methanol vapour. Theoretical studies also revealed small binding energy (−413.2 au) and low energy gap (1.19) for 1-CH3OH adduct.


Introduction
Dipicolinic acid or 2,6-pyridinedicarboxylic acid has been widely used as a ligand binding to metal transition ions and may present various modes of coordination such as bidentate, tridentate, or chelate bridging via O, N, O atoms. Moreover, depending on the groups being deprotonated, dipicolinic acid can serve as either proton acceptor or proton donor [1]. Because of its low toxicity, the ligand has recently gained so much interest, making it useful in applications involving biological studies, chemical sensing and environmental remediation [2,3]. To the best of what we know, the crystal structures of dipicolinic acid with Cu(II), Zn(II), Ni(II) and Co(III) have been reported [4][5][6][7]. Also, some other N, O-donor complexes have been studied for their photoluminescent properties [8][9][10].
Methanol, an aliphatic alcohol is often used as a solvent in paint manufacture, chemical and pharmaceutical industries, and as anti-freeze agent [11]. It has been known to have adverse effect on the health of human beings when it is ingested or inhaled due to its high toxicity. Acute exposure of the body to methanol vapour typically affects the central nervous system (CNS), causes headache, vertigo, fatigue, nausea, vomiting, blurred vision, blindness and even death [12]. Yet despite its toxicity, methanol is commonly used in other areas as a cheaper and more readily available raw material. For instance, the manufacture of herbal medicines typically uses methanol as a solvent to extract natural ingredients. [13]. Thus, it is important to identify traces of methanol for the safety of consumers. Methanol is mainly determined using analytical techniques such as high performance liquid chromatography (HPLC), gas chromatography and mass spectrometry (GC-MS) [14]. While these techniques have been shown to be highly sensitive, the high cost of the equipment, immobility and the complexity of sample preparation restrict its large use in everyday life [15]. Coordination complexes have unique features such as high selectivity, quick response time, low-cost and luminescent properties resulting from weak interactions between the coordination cluster and the target analyte. Sensor technology is much simpler in instrumental implementation and sample preparation. There has been a growing interest towards synthesis of transition metal complexes as optical probes for the recognition and sensing of small organic molecules especially methanol vapour in ambient environment which will be beneficial to human health and safety. As part of our work on synthesis of transition metal complexes for sensing volatile organic molecules [16][17][18], a co-crystallised cluster synthesized from Cobalt(II) ion and dipicolinic acid is reported for recognition and sensing of methanol vapour.

Materials
All reagents and chemicals are of analytical grade and have been used as received without further purification. In a typical reaction, 2,6-Pyridinedicarboxylic acid (0.167 g, 1 mmol) and cobalt chloride dihydrate, CoCl 2 ·2H 2 O (0.237 g, 1 mmol) were dissolved in two separate 50 mL beakers with 10 mL ethanol and 10 mL distilled water respectively. The two solutions were gently mixed, and a clear pink solution was formed which was heated in the oven at 80 ο C for 4 h. The clear solution formed was kept standing on the bench undisturbed and a crystalline purple solid was observed after 24 h. The dark purple crystals were separated from the mother liquor by filtration and then washed in ethanol-water solution

Luminescent Sensing of Methanol
The method described by Mehlana [19] was modified and adopted for the vapour sensing studies. 3 mg of the compounds were first activated by heating under vacuum for 5 h at 120 ο C. The activated compounds were placed in small vials and then these small vials were placed in previously evacuated larger vials containing various dry solvents (ethanol, methanol, chloroform, acetone, dimethylformamide, dichloromethane). The large vials were sealed tightly and left to stand at room temperature for 24 h (Scheme 1). The sensitivity of the compounds towards the different vapours detected were examined by exposing the compounds to different concentrations of the analytes ranging from 0.5-4.0 vol%.

Material Characterization Techniques
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 reflectance (ATR) attachment for solid samples. Powder X-ray diffraction (PXRD) patterns were measured on a PANalytical X'Pert PRO diffractometer with a Cu-Kα light source (40 kV, 40 mA) with a step size of 0.02°a nd a 50 s time step. UV-Vis absorption spectra were measured in the solid-state in the range 200 to 800 nm using an Agilent Cary Spectrophotometer with a slit width of 2 nm. The fluorescence emission spectra were recorded on an Agilent Cary Eclipse Spectrophotometer set at 2 nm Scheme 1 Experimental set-up for solvent vapour sensing Fig. 1 a ORTEP view of 1 drawn at 50% ellipsoid. All hydrogen atoms are omitted for clarity. b Packing mode in 1 along b-axis, showing centroid to centroid distance. c Hydrogen bonds in 1 slit width for both the excitation and emission. TGA was performed using an SDT-Q600 TA instrument. The samples were heated in air with a heating rate of 10°C min −1 and the scan was recorded within the temperature range of 30-600°C. SEM images were obtained on a Philips (FEI) XL30 SEM. The image scans reveal the morphology, shape and topology of each sample.

X-Ray Crystallography
Single crystal X-ray data for 1 was collected at 120 K on An Oxford Diffraction GV1000 diffractometer equipped with an Atlas S2 detector and a mirror-monochromated Cu-Kα radiation source. The Olex2 suite was used as a graphic user interface (GUI) and as imaging software [20]. The structures were solved with the ShelXT [21] structure solution program using Intrinsic Phasing and refined with the ShelXL [22] refinement package using Least Squares minimisation. All non-hydrogen atoms were refined with anisotropic displacement parameters and images were prepared via Mercury 4.1.0.

Refinement Experimental Details
Disorder is observed for in pyridine dicarboxylate ligand moiety H and for the entire metal-ligand complex of the residue Co3 with ligands E and F. The relative occupancies of all disorder components are refined and constrained to sum to unity. The geometries of all pyridine dicarboxylate ligand moieties are restrained to be similar (SAME). The minor disorder components are refined with isotropic displacement parameters.
Rigid bond and similarity restraints have been applied to the isotropic and anisotropic displacement parameters of all non-hydrogen atoms in the structure (RIGU, SIMU). The anisotropic displacement parameter of water residue O13W is restrained to have more isotropic character (ISOR).
The aryl hydrogen atoms were geometrically placed and refined with a riding model. Many water hydrogen atoms were observed in the electron difference map before being refined under the influence of geometric restraints on their bond distances (DFIX) and angles (DANG). The isotropic displacement parameters of all refined hydrogen atoms are fixed at 1.5 times the Ueq of their parent oxygen atom.
Where electron density peaks indicated three possible hydrogen atom positions that made plausible hydrogen bonds with nearby acceptors, hydrogen atoms were refined at all three positions with a fixed occupancy of 0.66666 each. Charge neutrality dictates that there will be an additional two protons associated with the [Co(II) 3 (pyridinedicarboxylate) 4 (H 2 O) 4 ] complex residue. The location of these extra two hydrogen atoms was not clear from the electron density map; no attempt has been made to assign their whereabout on the basis of this data. Only one hydrogen atom was observed and modelled for water ligand O11W (Fig. 1).

Results and Discussion
Description of Crystal Structure 1 crystallizes in the triclinic space group P-1, isomorphous with the corresponding Fe(II) complex reported previously [ 2 3 ] . a = 1 5 . 5 8 8 0 Å , b = 1 5 . 6 3 8 9 Å , c = 16.2808 Å ( Table 1). The asymmetric unit comprises of four cobalt complexes and seven lattice water. One of the complexes is trinuclear while two complexes are dinuclear and one is mononuclear. The ORTEP diagram with atom numbering is represented in Fig. 1a. The Co-O bond lengths range between 1.976(16) Å to 2.347(15) Å while the Co-N distances fall in the range 2.024(2) Å to 2.051(3) Å and are in the range usually reported in literature for cobalt complexes [24]. Also, the bond angles around the Co 2+ ions in the complexes are in the range 72.1(5) to 180.00(11) which is in good agreement with literature [25]. Selected interatomic bond lengths and angles are presented in Table 2. The Co(II) ions in the   is surrounded by four aqua ligand and two carboxylate oxygen atoms in the same plane. Three of the complexes are fully deprotonated while two are partially deprotonated. The structures revealed some π−π interactions arising from the pyridine rings of the mononuclear and trinuclear complexes with a centroid-centroid distance of 3.655 Å (Fig. 1b). The Co⋯Co intercluster separations are in the range 5.992 Å-6.828 Å shorter than reported ones [26]. The interstitial water molecules form a hydrogen bond network that bridges between the four complex cations in the asymmetric unit. Hydrogen bonds are also observed directly between the coordinated water ligands and donor and acceptor atoms of adjacent complexes. A network of intermolecular hydrogen bonding was also observed (Fig. 1c). The selected hydrogen bonding parameters for 1 are shown in Table 3.

FTIR Spectra
The FTIR spectrum of the ligand revealed the characteristic absorption bands of ν(C=O) and ν(C-O) at 1583 cm −1 and 1305 cm −1 . These absorption bands were shifted to 1608 cm −1 and 1284 cm −1 respectively in the spectrum of 1 suggesting coordination of the Co(II) ion to the ligand (Fig. S1). The sharp peaks at 1572 cm −1 and 1370 cm −1 are assigned to the   Additionally, a sharp absorption band due to aromatic ν(C-N) stretching vibration was observed at 1187 cm −1 , this band was seen at higher frequency in the spectrum of the free ligand. This indicates pyridine N-group chelation to the Co(II) center [27]. The weak peaks at 532 cm −1 and 428 cm −1 are assigned to ν(Co-N) and ν(Co-O) respectively.

Electronic Spectra
The electronic data of 1 and its ligands were recorded in the solid state at room temperature in the range 200 nm to 800 nm These absorption bands are assigned to 4 T 1g (F) → 4 T 2g , 4 T 1g (F) → 4 T 1g (P), and 4 T 1g (F) → 4 A 2g of the spin allowed transitions of the octahedral Co 2+ ions, respectively [28].

Thermogravimetric Analysis
The TGA curve of 1 revealed a three-step decomposition fashion (Fig. 2a). The first step showed a loss of seven molecules of lattice water and six molecules of coordinated water in the temperature range between 111 ο C to 149 ο C, 10.95% found (calc. 9.70%)). The second decomposition step corresponds to the loss of Co 3 (PDC) 4 Fig. 2b. SEM provides high resolution and clear images of the particles of the examined compounds, which helps in understanding the morphological changes upon metal complexation. The SEM micrograph shows prism-like homogenous aggregate morphology of the complex having tiny uniform crystals at the surface.

Photoluminescent Property
The photoluminescence (PL) spectra of 1 and the parent ligand were recorded in solid-state at room temperature (Fig.  2c). The ligand displayed a distinct emission band with high intensity centred at 437 nm (λex = 345 nm). The emission band is assigned to n -π* / π-π*. Compound 1, showed a similar emission at 437 nm (λex = 345 nm) but with much lower intensity which may be due to the partially filled d orbital of the Co 2+ (3d 7 ) ion leading to Ligand to Metal charge transfer (LMCT) [29,30]. The similarity in the emission wavelength of the free ligand and the title compound suggests that the luminescence of 1 is attributed to ligand-based emissions [31].

Sensing of Volatile Organic Molecules
The PL spectra of 1 upon exposure to different organic molecules is presented in Fig. 3a. From the structural and fluorescent properties of the Co(II) complex, it is expected that the free water molecules could be exchanged with other type of small organic molecules. The luminescence intensities are highly dependent on the solvents and their selectivity is in the sequence DMF > CHCl 3 > DCM > Acetone > EtOH > MeOH with a quenching efficiency of 79% (Fig. S3). To investigate the sensitivity of 1 towards methanol, it was exposed to the solvent in varying concentrations. Gradual decrease in photoluminescence emission intensities were observed upon increasing the concentration of methanol leading to about 97% quenching at 4.0 vol% acetone content (Fig. 3b). However, the phenomenon of energy transfer can be used to explain the quenching mechanism. The photoluminescent emission of 1 is highly dependent on Ligand to metal charge transfer, therefore, it is logical to suggest that interaction of methanol molecules with the ligand may disrupt energy transfer from ligand to Co 2+ thereby, drastically quenching the luminescence of the complex (Fig. 3c). This mechanism is similar to those proposed in previous literature [32]. Furthermore, thermogravimetric analysis revealed that the activated complex was stable up to 246 ο C indicating the lattice is free of solvent molecules prior to interaction with methanol molecules. The TGA curve of 1@methanol reveals the loss of methanol at 68 ο C. (Fig. 3d). As shown in Fig. S4, the spectrum of 1@activated revealed a partial disappearance of the broadband observed in the region 3101-3466 cm −1 (ν(OH)) in the spectrum of the assynthesised complex. This is ascribed to the removal of lattice water upon activation. Moreover, the FTIR spectrum of 1@MeOH displayed new peaks found between 2825 and 3131 cm −1 corresponding to ν(O-H) stretching of alcohol this indicates the presence of MeOH. PXRD pattern of 1 before and after interaction with methanol remains identical depicting the structural rigidity of the complex (Fig. S5).
In addition, the Stern Volmer plot also displayed excellent linearity (R 2 = 0.9789) in the methanol concentration range from 0.5 to 4.0 vol% and the quenching constant (KS v ) was  . 4 Optimized structures of (a) 1 and complexes between (b) 1-CH 3 OH and (c) 1-C 2 H 5 OH 9.03 × 10 3 /vol% while the limit of detection (LOD) was found to be 0.00035 vol% which is below the permissible occupational exposure limit for methanol (0.025 vol%) [33]. Noteworthy colour changes were also observed with the naked eye before and after solvent vapour interaction as the displayed in the photographs in Fig. S6.

Computational Details for the Proposed Interaction Study
Density functional theory (DFT) was employed to understand the mode of interactions between a unit of 1 and solvents (C 2 H 5 OH and CH 3 OH). Geometry optimisations and vibrational analyses of 1-C 2 H 5 OH (CH 3 OH) complexes were performed using the Gaussian16 software [34]. B3LYP [35] functional was employed with a 6-311 g++(d,p) basis set was employed for atoms C, H, O, N and Cl, while LANL2DZ for Na. The binding energy of formation was calculated by using Eq. 1 below.
The NBO and MEP for the compounds were also calculated. Optimized molecules were obtained with Chemcraft visualization program.

Theoretical Studies: Optimization Studies and Binding Energies
Interaction studies between 1 and alcohols (CH 3 OH and C 2 H 5 OH) was carried out because these molecules provide electron donating atom (O) for coordination/interaction with  1. The geometry of 1 was optimized using a unit of the crystal and it was observed that the structure were very similar to those of the X-ray diffraction unit. Furthermore, optimized structures presented close bond distance parameters compared to the crystal structure unit as shown in Fig. 4a. The optimization of 1-C 2 H 5 OH and 1-CH 3 OH (Fig. 4b and c) presented octahedral compounds coordinating via oxygen atom of alcohols to metal center (Co), with 1-C 2 H 5 OH presenting more favourable octahedral structure due to the observed low distortion.
The binding energies of 1 with C 2 H 5 OH and CH 3 OH relate to the feasibility of interaction between molecules. 1-CH 3 OH (−413.2 au) presented the smallest binding energies compared to 1-C 2 H 5 OH (−154.1 au) ( Table 4). The more negative binding energies confirmed the feasibility of interaction between 1 and CH 3 OH.

HOMO-LUMO Analysis
The difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) (HOMO−LUMO gap) describes the polarizability and reactivity of molecules, and it predicts reactivity between species by providing the electrical transport properties as well as electron carrier and mobility in molecules [36,37]. The images of HOMO and LUMO orbitals for 1, 1-C 2 H 5 OH and 1-CH 3 OH are shown in Fig. 5a-f. Generally, LUMO positions are distributed around ligand atoms (nitrogen (N), oxygen (O) and carbon (C)) and metal center (Co). HOMO is mostly centered round non-metals including nitrogen (N), oxygen (O) and carbon (C)). The change in HOMO and LUMO energies before and after adduct formation indicated an electron density transfer within the complexes/adducts. The HOMO, LUMO and band gap energies of the various complexes/adducts are provided in Table 5. Adduct, 1-CH 3 OH presented the least HOMO−LUMO energy gap which further confirms possible interactions. Adducts formed between 1 and CH 3 OH clearly indicated that the interaction is mainly driven by electron donation from HOMO to LUMO.

Conclusions
In summary, we have successfully synthesized a cocrystallised cobalt cluster with dipicolinate ligand comprising of hexacoordinated cobalt centers. The complex possesses excellent luminescence properties by virtue of extended πconjugation of the ligand. It exhibited selective sensing of methanol over other small organic molecules in a luminescence turn-off fashion. Moreover, negligible effect from other organic solvents such as ethanol, acetone, chloroform, dichloromethane and dimethylformamide was observed. The quenching mechanism could be described as resulting from energy transfer from the electron rich complex to methanol. Furthermore, PXRD, TGA and FTIR has confirmed the selective recognition of methanol vapour. The small binding energy and low energy gap obtained by theoretical analysis confirms possible interactions between complex 1 and methanol. These results authenticate the prospect of designing cheap transition metal complexes with suitable ligands for probing volatile organic molecules.