NVP-2

Sunlight assisted SCSC dimerization of a 1D coordination polymer impacts the selectivity of Pd(II) sensing in water†

Mohammad Hedayetullah Mir, Sambhunath Bera, Samim Khan, Suvendu Maity, Chittaranjan Sinha and Basudeb Dutta
a Department of Chemistry, Aliah University, New Town, Kolkata 700 156, India.
b Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India

A one-dimensional coordination polymer (1D CP) [Cd(4-nvp)2(5-ssa)] middot;(4-nvp) (1) [4-nvp = 4-(1-naphthylvinyl)pyridine and 5-ssa = 5- sulfosalicylic acid] undergoes topochemical [2+2] cycloaddition by sunlight irradiation to generate a two-dimensional (2D) CP [Cd(rctt-4- pncb)0.5(4-nvp)(5-ssa)]·(4-nvp) (10) [rctt-4-pncb = 1,3-bis(40-pyridyl)-2, 4-bis(naphthyl)cyclobutane] in a single-crystal to single-crystal manner. Interestingly, 10 can be reverted back to 1 by heating and both the CPs selectively recognize Pd2+ in aqueous medium; however, the limit of detection is improved after photodimerization.
In modern trends of crystal engineering,1–4 solid-state reactions involving structural transformations5–7 have received considerable attention from researchers. This transformation is accredited to the involvement of a solvent free modest synthetic method and establishment of a product that may be otherwise difficult to obtain by conventional routes. Of the various solid-state reactions, photochemical [2+2] photodimerization is particularly interesting, by which stereo-selective cyclobutane rings are formed in photo- active organic as well as inorganic coordination compounds, and these have impacts in various applications.8–15 Moreover, such a transformation via a single-crystal to single-crystal (SCSC) process is further fascinating, in which the structure of the end product can be determined unambiguously.7 Recently, reversible [2+2] cycloaddition involving formation and cleavage of the cyclobutane ring has emerged as a new synthetic route for the fabrication of molecular switching, optical recording and storage devices, sensors, photochemical actuators and remotely controlled devices.16–20 In this regard, coordination polymers (CPs) have been proven to be good candidates for molecular dielectric switches.21,22 Furthermore, most photodimerization reactions in CPs occur via harmful UV radiation. Based on the idea of a green synthetic route, dimerization reactions using solar radiation are still uncommon.22
Recently, luminescent CPs have been found to be used as sensors to detect specific anions, cations and organic mole- cules. In this regard, fluorescence-based chemosensors for the detection of metal ions have attracted great attention because of their high sensitivity, portability, rapid response time and convenient visual detection.23–25 Among the elements present in the earth, palladium (Pd) is a metal with vast importance in the synthesis of new and unique organic and biologically significant molecules. Besides, Pd has a wide spectrum of applications in materials and electronics. Again, Pd(II)- complexes are used as catalysts in various organic coupling reactions.26,27 Although Pd(0) and Pd(IV) compounds are useful in several reactions, they have been erratically converted to Pd(II) by oxidation of Pd(0) or reduction of Pd(IV) in the reaction system.28 In whole purification processes, a little amount of Pd(II) could stay in the final product as an impurity and thus a substantive amount of Pd(II) is released into the environment in the long run.29 Therefore, there is a need to design a particular sensor that can specifically detect Pd(II) particularly in water medium.
Herein, we have synthesized a one-dimensional coordination polymer (1D CP) [Cd(4-nvp)2(5-ssa)]·(4-nvp) (1) [4-nvp = 4-(1-naphthylvinyl)pyridine and 5-ssa = 5-sulfosalicylic acid] that undergoes a topochemical [2 + 2] cycloaddition reaction simply in sunlight and generates a two-dimensional (2D) CP [Cd(rctt-4-pncb)0.5(4-nvp)(5-ssa)]·(4-nvp) (10) [rctt-4-pncb = 1, 3-bis(40-pyridyl)-2,4-bis(naphthyl)cyclobutane] via SCSC transformation. Compound 10 can also be obtained by keeping the reaction mixture in sunlight. Interestingly, the cyclobutane ring of 10 could be cleaved back to the original olefin by heating. Due to this structural change, there is a significant effect on the sensing property of the material. Compound 1 has a relatively better luminescence detection limit of Pd2+ at the ppm level in comparison to 10. Although a few luminescent CPs have been reported for sensing of metal ions by post-synthetic modifica- tion via [2+2] cycloaddition,30–32 to the best of our knowledge, these compounds are the first reported CPs for specific sensing of Pd(II) in aqueous medium.
Brown colour block shaped single crystals were developed in a narrow necked (60% yield) tube by slow diffusion of 4-nvp and 5-ssa (deprotonated by Et3N) in ethanol into a solution of aqueous Cd(NO3)2·4H2O. Single crystal X-ray diffraction (SCXRD) reveals that compound 1 crystallizes in the triclinic crystal system with space group P1% and Z = 2. The asymmetric unit contains a Cd(II) centre, two 4-nvp and a 5-ssa anion. The asymmetric unit further contains a free 4-nvp ligand which undergoes a p·· ·p stacking interaction with the ligated 4-nvp ligand. Each Cd(II) centre has a distorted octahedral geometry, wherein the equatorial plane is described by four oxygen atoms from three different 5-ssa ligands in monodentate fashion along with an aqua ligand and the axial position is occupied by two 4-nvp appended in trans-fashion (Fig. S1, ESI†). How- ever, two such Cd(II) centres are connected by two linking 5-ssa ligands to form a dimeric unit which spreads along the a-axis to generate a 1D double chain ladder polymer (Fig. 1). The axial 4-nvp ligands are appended on two sides of the dimeric units of the ladder, resulting in the formation of empty spaces between 4-nvp ligands of the chain. This enables the 1D ladders to be interdigitated to form a 2D layer structure (Fig. S3, ESI†). Interestingly, the 4-nvp ligand of one ladder is closely parallel in a head-to-tail fashion to the 4-nvp ligand of an adjacent ladder (Fig. 1). The bond distance between the centre of adjacent CQC bonds is 3.79 Å, which directs (according to Smith’s topochemical principle) the prospect of a photochemi- cal [2+2] cycloaddition reaction. In this structural arrangement of the dimeric unit, only one 4-nvp undergoes alignment and thus 50% of the ligated 4-nvp ligands have the prospect of undergoing [2+2] cycloaddition. The free 4-nvp ligands present in the crystal lattice also do not align.
Interestingly, when a single crystal of 1 is exposed to open sunlight, the shape and transparency of the crystal remain intact after irradiation, which persuades us to explore SCSC transformation of the crystal of 1. SCXRD indicates the for- mation of a 2D sheet like structure of dimerized product 1′ with 50% conversion of coordinated 4-nvp to rctt-4-pncb (Fig. 1). In the solid-state structure, both the CPs contain a non- coordinated free 4-nvp ligand, which undergoes p·· ·p interac- tions with the dimeric unit (Fig. S3 and S4, ESI†). The 1H NMR study of compound 1′ in d6-DMSO shows the appearance of a sharp peak at d = 5.17 of the cyclobutane proton of rctt-4-pncb, which is evidence of photo-cycloaddition (Fig. S5, ESI†). Besides, the phase purity of the CPs is verified with powder X-ray diffraction (PXRD) and the patterns show that almost all the peaks of the as-synthesized compounds match well with the corresponding simulated arrangements (Fig. S6 and S7, ESI†). Thermogravimetric analyses (TGA) indicate that both the com- pounds are fairly stable above 200 1C (Fig. S8 and S9, ESI†). These results prompt us to check the thermal reversibility of 1′ to 1. Interestingly, 1′ returns back to 1 when heating the crystals at 200 1C for 5 h as confirmed by the disappearance of the cyclobutane protons in the 1H NMR spectrum (Fig. S10, ESI†). The PXRD data also supports this observation (Fig. S11, ESI†). Keeping in mind the fluorescence activity of 4-nvp moities, the photo-physical properties of 1 and 1′ were studied to understand the molecular characteristics of the compounds. Consequently, the photoluminescence spectra of 1 and 1′ in aqueous medium at ambient temperature (Fig. S12 and S13, ESI†) show that the maximum emission wavelengths are located at about 445 nm and 448 nm at an excitation wave- length of 340 nm. Finely crushed powders of 1 and 1′ (2 mg) in H2O (2 mL) were inspected for their ability in selective detection using various analytes such as Na+, Al3+, Cu2+, Hg2+, Cd2+, Co2+, Mn2+, Fe3+, Pb2+, K+, Ba2+, Ni2+, Zn2+, Pd2+, Ca2+ and Cr3+ metal ions with their corresponding chloride, acetate and nitrate salts at a concentration of 1 mmol L—1. The addition of 1 mmol L—1 ions into the suspensions of 1 and 1′ in H2O shows that the fluorescence intensity is slightly changed or most of the cases remain unchanged (Fig. 2). However, in the case of Pd2+ (1 mmol L—1), the original emissions of both the compounds were decreased extremely and the extents of quenching were excellent. To evaluate the sensitivity of the compounds toward Pd2+, the fluorescence intensity of 1 and 1′ (1 mg) dispersed in aqueous medium (2 mL) was measured upon gradual addition of Pd2+. The emission intensity of 1 in H2O was decreased by 96.25% at a Pd2+ concentration of 0.08 ppm. The fluorescent quenching efficiency was also calculated through the Stern—Volmer equation: I/I0 = 1 + KSV[Pd2+] (0–0.00001 mol L—1), where I0 and I are the fluorescence intensities before and after addition of Pd2+ and KSV represents the Stern–Volmer quenching coefficient. The Stern–Volmer plots were linear (R2 = 0.9449 and R2 = 0.9907, respectively) for 1 and 1′ over the concentration range 0–0.00001 mol L—1. The corresponding quenching rate constants (KSV) were calculated as 2.55 × 105 L mol—1 and 2.53 × 105 L mol—1 (Fig. S14 and S15, ESI†). Besides, the limit of detection (LOD) value for Pd2+ detection was calculated as 0.0089 ppm and 0.0072 ppm for 1 and 1′, respectively (Fig. S16 and S17, ESI†), which signifies the improvement of the LOD after dimerization.33,34 However, the LOD of both the CPs was lower than the level of Pd2+ in drinking water (5–10 ppm) recommended by the United States Environ- mental Protection Agency.
Selectivity is also very significant for sensors, and thus the selective sensing ability of the compounds for Pd2+ has been confirmed by performing competitive fluorescence experi- ments. As shown in Fig. 2, no outstanding changes in the emissions of the compounds are observed for various metal ions (Na+, Al3+, Cu2+, Hg2+, Cd2+, Co2+, Mn2+, Fe3+, Pb2+, K+, Ba2+, Ni2+, Zn2+, Ca2+ and Cr3+) with a concentration of 0.1 ppm in the absence of Pd2+ ions. The addition of competitive metal ions into the aqueous solution of CPs caused only a slight change of the fluorescence intensity. However, the fluorescence intensities were practically quenched upon subsequent addi- tion of Pd2+ ions (0.1 ppm), verifying that the Pd2+ sensing abilities of the CPs were independent of the occurrence of other competitive metal ions in H2O (Fig. S18 and S19, ESI†). The present CPs exhibit the sensitive and selective detection of trivial amounts of Pd2+ even in the presence of other contend- ing analogues and work as efficient sensors.
The probable mechanism behind the Pd2+ selectivity was analysed with respect to emission and absorption spectra. In the emission spectrum of 1 with Pd2+, the intensity quenching upon addition of an appropriate quantity of Pd2+ solution suggests the presence of significant interactions between the molecular framework and Pd2+. From the selective chromo- genic behaviour towards Pd2+ and the significant variation in the fluorescence spectrum, we anticipate that the observed selective sensing behaviour of the compounds for Pd2+ over other metal ions is due to the favourable interactions between the aromatic rings of the NVP-2 ligand and the p-philic Pd2+ ion.35 We have also checked the Pd2+ sensing ability of the component ligands, which indicates that they do not quench significantly as compared to the corresponding CPs (Fig. S20, ESI†). To get additional insight into the interactions between Pd2+ and the compounds, 1H NMR is recorded by excess addition of Pd2+ solution into the solution of the compounds. Some characteristic NMR peaks have been broadened and shifted approximately 0.1–0.2 ppm downfield from the corres- ponding proton chemical shift (Fig. S21 and S22, ESI†), which confirms the possibility of interactions. However, after dimer- ization, the extent of interactions has been improved as can be observed from the B0.05 ppm greater downfield shift of a few characterized protons with respect to their corresponding positions before dimerization (Fig. S21 and S22, ESI†). This could be the reason for the better LOD value of 1′. The lifetime experiments showed increasing lifetime values (Fig. S23 and S24, ESI†) upon addition of Pd2+, which also verifies the interactions of metal ions and the sensing process.
In summary, we have successfully synthesized a new lumi- nescent CP that undergoes topochemical [2+2] cycloaddition under sunlight irradiation. The reaction is thermally reversible and both the CPs show highly selective and sensitive detection of Pd2+ even in the concurrent presence of other competing metal ions, although the detection limit is improved after photodimerization. The Stern–Volmer plots of both the com- pounds give the highest quenching constant values among the CPs reported for detection of Pd2+ below the permissible limits set by the WHO. The present work demonstrates the potential use of fluorescent CPs for practical detection of hazardous metal ions for environmental and security concerns.