Lead Based Metal Organic Frameworks(Pb-MOFs): Anion Influence, Luminescence and Gas Adsorption Study

Two Lead based organic frameworks (1 & 2) have been synthesized using 4,6-dihydroxy-2mercaptopyrimidine (H2DMP) as organic linker. Lead nitrate (1) and Lead acetate (2) salts were used to investigate their influence on morphology of resulting MOFs, adsorption capacities and luminescence behaviour. Structure and morphology of these MOFs was established on the basis of UV/Vis, Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), 1H NMR, energy dispersive spectroscopy (EDS), thermogravimetric analysis (TGA) and X-ray diffraction technique. Brunauer-Emmett-Teller (BET) studies revealed better N2 adsorption capacity for 1 (surface area 814 m2/g) when compared with 2 (surface area 196 m2/g). Photoluminescence behaviour of (1 & 2) was investigated in solid state at room temperature and has been attributed to metal-centred s→p transition.

In past decade, hybrid inorganic organic frameworks have attracted much attention due to their appropriate properties and fascinating applications [1][2][3][4][5][6][7][8]. Several organic linkers particularly having carboxylate and hydroxyl moieties [9][10][11][12][13][14] have been widely employed to assemble frameworks of versatile structural design as well as excellent physical properties. Among metal ions, much attention have been paid on transition and lanthanide metal ions compared to main group elements. Among main group elements, Lead metal ion exhibits flexible coordination environment due to the lone-pair and large ionic radius. Moreover, it possesses large spin orbit coupling constant, an important characteristic for developing luminescent complexes [15]. Keeping in view versatile structural chemistry of lead ion and in continuation of our recent work [16], we present in this paper, synthesis of two MOFs originating from 4,6dihydroxy-2-mercaptopyrimidine (H 2-DMP) as organic linke. Comparative studies of anion influence, adsorption as well luminescence are also part of this manuscript.

Experimental part
Methods and Materials 4,6-dihydroxy-2-mercapto-pyrimidine, were procured from Sigma-Aldrich while lead nitrate and lead acetate trihydrate were purchased from Uni-chem. All chemicals were of reagent grade and used without further purification. Gallenhamp melting point apparatus was used for the melting point determination. The IR spectra were recorded using Cary 630 FTIR Agilent Technologies. 1 H NMR spectra were recorded on Avance AV-400 spectrometers. The morphology of the sample was done by NovaNano 450-SEM ûeld emission scanning electron microscope (FESEM) and model vega tescan LMU. Thermogravimetric Analyzer (TGA) was performed from 30 to 800 °C at a heating rate of 10 C/min under a nitrogen atmosphere using SDT Q600 instrument. The Powder x-ray diffraction patterns of MOFs (1 & 2) were obtained by diffractometer (Equinox 2000, Thermo scientific, USA) using CuKα 1 radiation. Also Brunauer-Emmett-Teller (BET) based N 2 gas adsorption studies were made by using Tri Star II 3020 version 2.00, a BET surface area measurement instrument.

Results and discussions
Lead based MOFs (1 & 2) were synthesized under hydrothermal conditions by reacting respective lead salts and (H 2 DMP) in 1:1 as outlined in Scheme 1. Both these MOFs (1 & 2) are air stable solids and insoluble in water and common organic solvents. These are only soluble in DMSO. Their reduced solubility might be attributed to their polymeric/framework nature. Moreover, both are colourless and have been obtained in good yield of (48% 1, 58% 2) has been obtained.
To establish the coordination mode of 4,6-dihydroxy-2mercaptopyrimidine (H 2 DMP) towards lead ion, FTIR spectra of this linker and both MOFs have been compared. The IR spectra of Pb MOFs (1 & 2) lacks band for -OH group thus indicating deprotonation and subsequent involvement with lead ion. Moreover, the stretching frequency for C=N moiety in H 2 DMP at 1643 cm -1 has been shifted at 1682 and 1618 cm -1 in MOF 1 and 2 respectively. This comparison revealed that each H 2 DMP linker behave in a monoanionic bidentate manner.   The crystallite size estimation 15.97 nm for 1 and 16.18 nm for 2 was made by Debye-Scherrer formula (23), thus presenting MOF 1 as better candidate for BET studies.

Thermal Stability
The thermal stability of MOFs (1 & 2) was determined by TGA in a nitrogen atmosphere from room temperature to 800 o ,C and TGA curves are presented in Figure 6. In MOF (1), first weight loss was observed between 30-155 o C that can be attributed to release of water molecules [24]. Finally, a gradual weight loss approximately 48% of their initial mass occurred from 200 to 790 o C thus leaving PbO as residue.
Compared to MOF 1, TGA curve of MOF (2) exhibits sharp weight loss in the range from 160 to 780 o C. This reveals that MOF (2) degraded thermally more easily relative to MOF (1).

Surface Area and Porosity Characterization
Figures 8-11 displayed N 2 adsorption isotherms for Pb-MOFs (1 & 24) measured at 77 K. These isotherms are described as Type II isotherm according to the IUPAC classification that corresponds to mono-and multilayer physical adsorption. The estimated BET surface area for MOF (1) is 814 m 2 /g and total pore volume was 0.12 cm 3 / g. While it is 196 m 2 /g with total pore volume 0.20 cm 3 /g for MOF (2). Because of more BET surface area, MOF (1) showed significant adsorption capacity for N 2 relative to MOF (2). Photoluminescence studies The luminescence behaviour of both MOFs (1 & 2) were investigated in solid state at room temperature and their spectra are shown in Figure 7 (a & b). Both these MOFs (1 & 2) display a similar emission band at 514 nm which can be assigned to a metal-centered s→p transition as proposed by Vogler (25). Similar emission bands have also been assigned in the literature [Pb-based MOFs like [Pb(H 2 O)(tip)] n , Pb 3 (µ 4 -O(tip) 2 ] n (tip = 5-tert-butylisophthalic acid) [26].

Conclusions
In this work, two lead-MOFs have been synthesized and structurally characterized.Both MOFs exhibited similar orthorhombic crystal form with different space groups. Both these MOFs display strong emission and could be considered as promising candidate for developing light emitting diode devices. BET studies of MOF 1 reveal it to be a better system for sequestring of gases and other pollutants from environment.