Selective picomolar level fluorometric sensing of the Cr(VI)-oxoanion in a water medium by a novel metal–organic complex

Abstract

A novel metal–organic complex (MOC) of Cu(II) with n-butylmalonate ligands and protonated 2-aminopyridimium rings has been synthesized and structurally characterized by single-crystal X-ray diffraction. An intriguing supramolecular interaction i.e. hydrogen-bonding pattern (like N–H⋯O and O–H⋯O) assisted lone-pair⋯π/π⋯π assembly is observed in the crystalline form of the MOC. The role of different non-covalent contacts in the formation of the MOC in the solid-state has also been scrutinized through Hirshfeld Surface Analysis. The luminescence properties of the MOC in the water solution were also experimentally investigated. The aqueous solution of the MOC acts as a selective picomolar level fluorescent sensor for Cr(VI)-oxoanion in water medium. Even the presence of several other cations like Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Hg²⁺, Zn²⁺, another chromium source like Cr³⁺ and versatile anions including F⁻, Cl⁻, Br⁻, SO₄²⁻, N₃⁻, NO₃⁻, BF₄⁻, ClO₄⁻, AsO₃³⁻, PO₄³⁻ do not interfere with the picomolar level Cr(VI)-oxoanion sensing ability of the MOC in water medium. The probable sensing mechanism of the Cr(VI)-oxoanion by the MOC has been investigated experimentally and theoretically.

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1. Introduction

Chromium is one of the most harmful environmental pollutants. Due to massive-scale industrial discharge,¹ increasing chromium contamination in ecosystems and human health demands our immediate attention. Among its diverse oxidation states ranging from −IV to +VI, Cr(VI) is especially toxic. Heavy industrial areas are most affected by Cr(VI) toxicity through effluent from the electrochemical, tanning and paper industries and petroleum refineries. Hazardous Cr(VI) enters the human body through inhalation. Prolonged exposure to Cr(VI) disturbs the human respiratory tract with septum damage, bronchitis, pulmonary malfunction, pneumonia etc.^2,3 Cr(VI) in high doses also affects the liver, kidneys, and the gastrointestinal and immune systems. It may even cause critical dermatitis.^4,5

Hence, it is expedient to detect the presence of the Cr(VI) ion in a water medium for our own safety. A number of indirect techniques including electrochemical,⁶ voltammetric,⁷ potentiometric,⁸ and colorimetric,^9,10 and optical fiber method,¹¹ atomic absorption spectroscopic method,^12–15 microcantilever sensing method,¹⁶ X-ray spectrometric technique,¹⁷ luminescent nanoparticle based sensing method,¹⁸ and chromatographic techniques,^19,20 and even carbon dot-based nanosensor via a fluorescent inner filter effect²¹ are reported as Cr(VI)-sensing approaches. But, reports of the picomolar range detection of toxic Cr(VI)-oxoanion in a water medium through direct fluorescence spectroscopy are rare in the literature.

Over the last few years we have been involved in exploring toxic metal ion-sensing applications via exploiting novel metal–organic complexes in a water medium and bio-systems.^22,23 As an extension of this idea, we have tried to develop a potent chromium sensor for the exclusive detection of the Cr(VI)-oxoanion as a source of toxic hexavalent inorganic chromium in a water medium by a simple water soluble MOC. The strong affinity of the Cr(VI)-oxoanion towards protonated pyridinium moieties, observed in pyridinium dichromate (PDC), motivates us to synthesize an intriguing MOC with protonated pyridinium derivatives for the development of a toxic Cr(VI)-oxoanion sensor in aqueous medium. To realize this idea we have tried to develop a novel metal–organic complex based on pyridine rings. There are several reports of pyridinium ring based metal–organic complexes where the pyridinium moieties exist as protonated 2-aminopyridinium rings in several metal–organic complexes.^24–26 Following these reports, here we have developed a novel metal–organic complex of Cu(II) with n-butylmalonate and protonated 2-aminopyridimium rings. The luminescent feature of the water solution of the MOC is exploited for the Cr(VI)-oxoanion sensing activity in a water medium. We have invented a selective sensor in the picomolar range for the Cr(VI)-oxoanion in a water medium by a straightforward, economic and reliable fluorescence spectroscopic method.

2. Experimental section

2.1 Materials

Copper(II) chloride hexahydrate, n-butylmalonic acid, 2-aminopyridine, and chloride salts of Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Hg²⁺, Zn²⁺, Cr³⁺ and sodium salts of F⁻, Cl⁻, Br⁻, SO₄²⁻, N₃⁻, NO₃⁻, BF₄⁻, ClO₄⁻, AsO₃³⁻ and PO₄³⁻ were purchased from Sigma-Aldrich chemical company and used as received. Double distilled water was used throughout.

2.2 Characterization

Elemental analysis (C, H, N) was carried out using a Perkin–Elmer 2400C elemental analyzer. Solid-phase IR spectroscopy was measured using a Shimadzu FTIR-8400S spectrometer between 400 and 4000 cm⁻¹, using the KBr pellet method. Absorption and fluorescence spectra of the aqueous solution of the MOC were measured in a Shimadzu UVPC-3200 spectrophotometer and a Perkin–Elmer LS55 fluorimeter, respectively. Electrospray ionisation mass spectroscopy (ESI-MS) experiments were carried out using a Water’s QtoF Model YA 263 spectrometer in positive ion ESI mode. For lifetime measurements, the samples were excited at 295 nm using a picosecond diode laser (IBH-NanoLED source N-295) having a fwhm of IRF of ∼810 ps. The emission was collected at magic angle polarization using a Hamamatsu MCP Photomultiplier (Model R-3809U-50). The decays were analyzed with DAS-6 decay analysis software.

2.3 Crystallographic data collection and refinement

A suitable single crystal of the MOC was mounted on a Bruker SMART diffractometer equipped with a graphite monochromator and MoKα (λ = 0.71073 Å) radiation. The structure was resolved by the Patterson method using SHELXS97. Subsequent difference Fourier synthesis and least-squares refinement revealed the positions of the remaining non-hydrogen atoms. Non-hydrogen atoms were refined with independent anisotropic displacement parameters. Hydrogen atoms were placed in their idealized positions and their displacement parameters were fixed to be 1.2 times larger than those of the attached non-hydrogen atoms. All calculations were carried out using SHELXS97,²⁷ SHELXL97,²⁸ PLATON 99,²⁹ ORTEP-32³⁰ and WinGX system Ver-1.64.³¹ Data collection and structure refinement parameters and crystallographic data for the MOC are given in Table S1 in the ESI.† The selected bond lengths, and bond angles are given in Tables S2 and S3 in the ESI.†

2.4 Synthesis of complex 1 {[Cu(butylmalonate)₂(H₂O)](2-APH)₂·(H₂O)} where 2-APH = protonated 2-aminopyridine

Cu(II) acetate monohydrate (0.199 g, 1 mmol) and n-butylmalonic acid (0.320 g, 2 mmol) were dissolved in 25 mL of water to give a clear blue solution. To this solution, 2-aminopyridine (0.190 g, 2 mmol) dissolved in 10 mL of water was slowly added and the pH of the solution was adjusted to 4.7 using an aqueous solution of NaBF₄. The resulting deep blue coloured solution was heated to ∼70 °C and stirred continuously for 1 hour. Then the solution was allowed to cool. It was filtered off and kept for crystallization. Deep blue block shaped single crystals suitable for X-ray analysis were separated after a few days from the mother liquor by slow evaporation at room temperature. The crystals were filtered off, washed with cold water, and dried in air. The yield was 69% based on Cu. Anal. calcd for C₂₄H₃₈CuO₁₀N₄: C, 47.51; H, 6.27; N, 9.24. Found: C, 47.25; H, 6.19; N, 9.17.

2.5 Methodology of theoretical calculations

The structures of the molecules were optimized at the Density Functional (DFT) level of theory, with TZVPP and SV basis sets^32,33 and the popular (B3LYP) functional.^34–36 The first set was used for Cu and Cr atoms, and the SV basis was used for the rest. Solvent effects were incorporated with the conductor-like screening model (COSMO) of Klamt^37,38 Time-dependent DFT (TDDFT) calculations were carried out on the optimized structures with the same basis sets and functional. The Orca program suite³⁹ was used in all calculations. For visualization of the MOs, MaSK software⁴⁰ and the Gaussian-09W program suite⁴¹ were used.

2.6 Methodology of Hirshfeld surface analysis

Molecular Hirshfeld surfaces^42–48 in the crystal structure of the MOC are determined based on the electron distribution calculated as the sum of the spherical atom electron densities.^49,50 For a specified crystal structure and set of spherical atomic electron densities, the Hirshfeld surface is unique.⁵¹ The Hirshfeld surfaces provide additional insight into the intermolecular interactions of molecular crystals. The Hirshfeld surface enclosing a molecule is identified by points where the impact to the electron density from the molecule of interest is equal to the contribution from all the other molecules. For every point on that isosurface two distances are defined as d_e, the distance from the point to the nearest nucleus external to the surface, and d_i, the distance to the nearest nucleus internal to the surface. The normalized contact distance (d_norm), depending on d_e, d_i, and the vdW radii of the atoms, shown in eqn (1), assists us in denoting the regions of specific significance to intermolecular interactions.⁴²

d_norm = (d_i − r^vdw_i)/r^vdw_i + (d_e − r^vdw_e)/r^vdw_e (1)

Due to the symmetry between d_e and d_i in the expression for d_norm, where two Hirshfeld surfaces associate, the value of d_norm becomes negative or positive when the intermolecular contacts are shorter or longer than the vdW separations, respectively. In this instance both will display a red spot identical in colour intensity along with size and shape. The 2D fingerprint plot is the combination of d_e and d_i, and provides a summary of the intermolecular contacts in the crystal.⁵² Molecular Hirshfeld surfaces^{41–48,53,54} are mapped with d_norm and associated 2D-fingerprint plots^53,54 were calculated and analysed using the program CrystalExplorer 3.1.⁵⁵ CrystalExplorer 3.1 automatically regulates all bond lengths to hydrogen to standard neutron values (C–H = 1.083 Å, N–H = 1.009 Å and O–H = 0.983 Å), when comparing one structure with another through this program for analysis. Molecular Hirshfeld surfaces mapped with d_norm used a red-white-blue colour scheme in the graphical plots, where red highlights shorter contacts, white is used for contacts around the vdW separation, and blue is for longer contacts. Moreover, two further coloured properties (shape index and curvedness) based on the local curvature of the surface can be specified. The Hirshfeld surfaces mapped with d_e use a fixed colour scale of 0.65 (red) to 2.2 Å (blue). The d_norm surfaces are mapped over a fixed colour scale of −0.75 (red) to 1.10 (blue). The fingerprint plots displayed each use the standard 0.4–2.6 Å view with the d_e and d_i distance scales displayed on the graph axes. The connectivity of d_e and d_i is expressed in the form of a 2D fingerprint plot.^53,54 The 2D fingerprint plot shows the different intermolecular contacts within the crystal system.⁴²

3. Results and discussion

3.1 Supramolecular aspect of the MOC in the solid-state

Single crystal structural analysis designates that the MOC of Cu(II) crystallizes in the triclinic space group P1̄ (Fig. 1, Table S1 and Fig. S1 for the ORTEP diagram given in the ESI†). Each asymmetric unit of the MOC contains a primary unit of square-pyramidal geometry, where the four oxygen atoms (i.e. O1, O3, O5, and O7) of two of the butylmalonate units chelate to the Cu(II) centre and form the basal plane of the square pyramid with a water molecule (O1W) at an apical position, and a secondary unit constructed of two protonated 2-aminopyridine rings designated as Cg1 and Cg2 (where, Cg1 is formed by the N1, C15, C16, C17, C18, and C19 atoms and Cg2 is made by the N3, C20, C21, C22, C23, and C24 atoms) along with one free guest water molecule (O2W) (Fig. 1A). The elongated axial Cu–O1W bond distance (i.e. 2.310(7) Å) is due to the Jahn–Teller effect, typically observed for a d⁹ system of Cu(II). Distortion from perfect square pyramidal geometry is indicated by the ‘τ’ value of 0.29 for Cu in the MOC.⁵⁶ Most interestingly, novel hydrogen-bonding patterns including N–H⋯O (i.e. N1–H1⋯O2, N2–H2A⋯O1, N3–H3⋯O7 and N4–H4C⋯O8) and O–H⋯O (i.e. O1W–H2W1⋯O6) interaction assisted lone-pair⋯π/π⋯π supramolecular assembly of the Cu(II)-butylmalonate unit (i.e. [Cu(n-butylmalonate)₂(H₂O)]⁻²) and two different protonated 2-aminopyridine rings have been observed in the MOC in the solid-state (Fig. 1B).

Fig. 1 Crystallographic view of the MOC of Cu(II). Here, (A) the supramolecular recognition of protonated 2-aminopyridine rings with the [Cu(n-butylmalonate)₂(H₂O)]⁻² dianionic unit through the R₂²(8) and guest water (O2W) aided R₃³(10) hydrogen-bonded motifs are observed. (B) N–H⋯O and O–H⋯O hydrogen-bonding pattern assisted lone-pair⋯π/π⋯π supramolecular assembly of the [Cu(n-butylmalonate)₂(H₂O)]⁻² units and protonated 2-aminopyridine rings (labelled Cg1 and Cg2) where the lone pair electrons of the free oxygen atoms (O8) from the carbonyl group of the Cu(II)-coordinated n-butylmalonate ligand are involved. Here, the H, C, O, N and Cu atoms are designated as light pink, gray, bronze, deep violet and blue coloured balls, respectively. The tables for the different supramolecular interactions are given in the ESI as Tables S4–S6.†

3.2 Hirshfeld surface of the MOC

The Hirshfeld surface of the MOC is presented in Fig. 2 (Fig. S2 in the ESI†), showing surfaces that have been mapped over a d_norm range of −0.623 to 1.481 Å, a shape index range of −1.000 to 1.000 Å and a curvedness range of −4.000 to 0.400 Å. The surfaces are transparent for the understanding of the molecular architecture, in a similar orientation for all structures. The information present in the ESI as Tables S4–S6† is summarized effectively in the spots, with the large circular depressions (deep red) visible on the d_norm surfaces indicating hydrogen bonding contacts and other visible spots due to H⋯H contacts. The dominant interactions between O⋯H, N⋯H and carbonyl O atoms in the MOC can be seen in the Hirshfeld surface plots as the red areas marked as a, b, and c in Fig. S2 in the ESI.† The small extent of the area and light colour on the surface indicate weaker and longer contacts compared to the hydrogen bonds. The O⋯H/H⋯O interactions appear as distinct spikes in the 2D fingerprint plots. Complementary regions are visible in the fingerprint plots where one molecule acts as a donor (d_e > d_i) and the other as an acceptor (d_e < d_i). The fingerprint plots are deconstructed to highlight the particular atom-pair close contacts. The proportion of O⋯H/H⋯O interactions make up 32.7% of the Hirshfeld surfaces for each molecule of the MOC.

Fig. 2 Hirshfeld surface analysis of the MOC showing the d_norm, fingerprint plot and the comparative roles of diverse non-covalent interactions.

The O⋯H interactions are represented by a spike (d_i = 1.5838 Å, d_e = 1.5961 Å) in the fingerprint plot of the MOC, and the H⋯O interactions are represented by a spike (d_e = 1.5961 Å, d_i = 1.5838 Å) in the fingerprint plot (Fig. S3 in the ESI†). Other non-covalent intermolecular interactions like C⋯H/H⋯C, N⋯H/H⋯N, C⋯O/O⋯C and C⋯C also appear as distinct points in the 2D fingerprint plot (Fig. S3†). The proportion of C⋯H/H⋯C, N⋯H/H⋯N, C⋯O/O⋯C and C⋯C interactions are 7.4%, 2.5%, 1.2% and 0.9% of the Hirshfeld surfaces of the MOC. A notable difference between the molecular interactions in the MOC in terms of the H⋯H interactions is reflected in the distribution of scattered points in the fingerprint plots, which spread only up to d_i ≈ d_e ≈ 1.5 Å in the MOC (Fig. S2 for the Hirshfeld surface mapped with d_norm, d_i, d_e, shape index and curvedness for the MOC and Fig. S3 for the fingerprint plots of the MOC showing the percentages of contacts contributing to the total Hirshfeld surface area of the MOC are shown in the ESI†).

3.3 Fluorescence spectroscopic sensing of the Cr(VI)-oxoanion in a water medium

UV-vis absorption and fluorescence-emission spectral data of the water solution of the MOC are collected (Fig. 3 and 4). The aqueous solution of the MOC displays an intense broad absorption band centered at 300 nm and a fluorescence emission maximum centred at 365 nm (λ_ex = 300 nm) at 298 K and atmospheric pressure (Fig. 4).

Fig. 3 UV-vis absorption pattern of the MOC in a water medium at 298 K and atmospheric pressure.

Fig. 4 Fluorescence spectroscopic pattern of the MOC in the presence of different aqueous solutions of the Cr(IV)-oxoanion at different concentrations (i.e. 49.75, 99, 243.9, 476.2, 804.6, 1111.1, 1489.4, 1836.7, 2156.9, 2452.8, 2727.3, 3220.3, 3846.2, 4366.2, 4805.2, 5180.7, 5698.9, 6116.5, and 6460.2 pM of the Cr(IV)-oxoanion). The fluorescence pattern of the aqueous solution of the pure MOC is denoted by the top black coloured line. Each reading has been taken after the addition of an aqueous solution of the Cr(IV)-oxoanion into the aqueous solution of the MOC. Here, [MOC] = 1 × 10⁻⁶ (M).

The theoretical study specifies that mixed metal-ligand type transitions are primarily responsible for the fluorescent feature of the MOC in a water solution. Due to the presence of protonated 2-aminopyridine rings in the water medium of the MOC we have measured the Cr(VI)-oxoanion sensing activity of the MOC in a water medium. Aqueous solutions of the Cr(VI)-oxoanion with different concentrations ranging from the picomolar (pM) level (i.e. 49.75 pM) were added to the fluorescent water solution of the MOC and the resultant spectral output shows that the fluorescence intensities of the subsequent aqueous solutions of the probe and the Cr(VI)-oxoanion were decreased monotonically (Fig. 4).

The fluorescence lifetimes (τ) of the aqueous solution of the pure MOC probe in the absence and presence of the Cr(VI)-oxoanion are 7.73 ns and 6.33 ns, respectively (Fig. 5, and see the ESI for Table S7†). The difference in lifetime (τ) supports the possibility of interactions between the fluorescent monomeric unit of the MOC and the Cr(VI)-oxoanion in a water medium.

Fig. 5 Measurement of the fluorescence lifetimes (τ) of the aqueous solution of the pure MOC probe in the absence and presence of the aqueous solution of the Cr(VI)-oxoanion. Here, [MOC] = 1 × 10⁻⁶ (M) and [Cr(VI)-oxoanion] = 74.4 pM.

3.3.1 Effect of different cations and anions on the fluorometric sensing of the Cr(VI)-oxoanion in a water medium. The infinitesimal effect of diverse cations found in the output of the fluorescence spectral investigations (i.e. I/I₀ vs. concentration plot, Fig. 6) indicates that the MOC in a water medium is highly specific towards the Cr(VI)-oxoanion at its concentration range starting from the picomolar level. Similarly, the fluorescence responses of the MOC in the presence of some selective anions including F⁻, Cl⁻, Br⁻, SO₄²⁻, N₃⁻, NO₃⁻, BF₄⁻, ClO₄⁻, AsO₃³⁻ and PO₄³⁻ were also tested and the fluorescence spectral results (i.e. I/I₀ vs. concentration plot, Fig. 6) clearly indicate that these anions cannot affect the picomolar range fluorescent sensitivity of the MOC towards the Cr(VI)-oxoanion in a water medium.

Fig. 6 The fluorescence patterns i.e. the plot of I/I₀ vs. ions with different concentrations. Here, I₀ and I are the fluorescence intensities of the water solution of the MOC in the absence and presence of different ions including the Cr(VI)-oxoanion.

Moreover, the crucial specific sensing ability of the water solution of the MOC towards the picomolar range Cr(VI)-oxoanion has also been scrutinized in the presence of the cations and anions mentioned above at a micromolar concentration level in a water medium (Fig. 7 and 8). Accordingly after getting the fluorescence spectral outcomes of the aqueous solutions of the MOC in the presence of the above mentioned cations and anions separately, a water solution of the picomolar level Cr(VI)-oxoanion was added to each of these solutions containing the respective cations or anions and the corresponding fluorescence spectral results were collected (Fig. 7 for cations and Fig. 8 for anions). These fluorescence spectroscopic data (Fig. 7 and 8) illustrate that the aqueous solution of the MOC is extremely selective for the Cr(VI)-oxoanion in the presence of different cations and anions in a water medium.

Fig. 7 Comparison of the fluorescence intensity ratios of the water solution of the MOC in the presence of the Cr(VI)-oxoanion and other cations. Each reading has been taken after the addition of aqueous solutions of different cations with individual concentrations of 50 μM into the aqueous solution of the MOC. Here, [Cr(VI)-oxoanion] = 49.75 pM. Here, I₀ and I are the fluorescence intensities of the water solution of the MOC in the absence and presence of different ions including the Cr(VI)-oxoanion.

Fig. 8 Comparison of the fluorescence intensity ratios of the water solution of the MOC in the presence of the Cr(VI)-oxoanion and other anions. Each reading has been taken after the addition of aqueous solutions of different anions with individual concentrations of 50 μM into the aqueous solution of the MOC. Here, [Cr(VI)-oxoanion] = 49.75 pM. Here, I₀ and I are the fluorescence intensities of the water solution of the MOC in the absence and presence of different ions including the Cr(VI)-oxoanion.

3.3.2 Probable sensing mechanism of the Cr(VI)-oxoanion by the MOC. The possible mechanistic pathway for the Cr(VI)-oxoanion sensing ability of the MOC in a water medium has been explored through infrared spectroscopic measurements (Fig. 9, and Fig. S4 and S5 in the ESI†). IR results (Fig. 9) especially show that in the presence of the Cr(VI)-oxoanion there is a change in the C–H, O–H and N–H stretching frequencies with respect to the pure probe (i.e. the MOC) and this might be due to the possible non-covalent type interactions between the Cr(VI)-oxoanion and protonated 2-aminopyridine rings in the MOC in a water medium. Thus, the non-covalent type hydrogen-bonding interactions between the Cr(VI)-oxoanions and protonated 2-aminopyridine rings of the MOC in a water medium may be accountable for the Cr(VI)-oxoanion detection by the MOC in a water solution.

Fig. 9 IR spectra of the solid MOC and the dried (using a CaCl₂ based closed desiccator) sample of the water solution of the MOC and Cr(VI)-oxoanion. Here, IR signals are omitted for clarity. These two IR spectra with signals are separately given in Fig. S4 and S5 in the ESI.†

Moreover, the ESI-MS result of the aqueous solution containing the MOC and Cr(VI)-oxoanion also clearly displays the Cr(VI)-oxoanion sensing aptitude of the monomeric unit of the MOC in a water medium (see the ESI for Fig. S6†). The ESI-MS spectral output also clarifies that the association between the protonated 2-aminopyridine rings of the MOC and the Cr(VI)-oxoanion in a water medium is quite stable (Fig. S6†).

3.4 Theoretical study

The structures of the monomeric unit of the MOC and the molecule complexed with a dichromate anion (i.e. the MOC–chromate complex) in a water medium were optimized at the DFT level of theory (see Tables S8–S25 and Fig. S7–S19 in the ESI for details†), both in the gas-phase and in aqueous medium. A detailed description of the orbitals in both cases can be found in the ESI.† It is important to note that the frontier orbitals of the monomeric unit of the MOC are mixed metal–ligand type MOs (see the ESI for Fig. S8†). In fact, the molecular orbitals of the MOC and of the MOC-chromate complex near the frontier orbitals are mostly π type, with the SOMO and LUMO both being mixed metal–ligand orbitals. Hence, the significant electronic transition of the MOC is given by ΔE = 3.207455 eV (λ = 386.55 nm), which is composed of HOMO-1 → LUMO + 3(72.22%) and HOMO-1 → LUMO + 2(11.31%) transitions. This is an M–L to Ligand π → π* transition. Theoretical UV-visible spectra of the molecule (i.e. the MOC) and the molecule–chromate complex (i.e. the MOC–chromate complex), given in the ESI (Fig. S13 and S14†), show λ_max at 204.07 and 313.28 nm and λ_max at 217 and 320 nm respectively.

Theoretical calculated bond distances, including hydrogen bonds, bond angles and dihedral angles for the MOC, and the MOC–chromate complex, in the gas phase and in a water medium are given in the ESI as Tables S8–S24.† They correspond reasonably well with the experimental data. The table for the charges on the various centres (i.e. the Mulliken and Lowdin charges for the receptor and the MOC in a water medium) is also given in the ESI as Table S25.†

4. Conclusion

In brief, we have explored a facile synthetic process for generating a water soluble fluorescent MOC of Cu(II) which shows a remarkable N–H⋯O and O–H⋯O hydrogen-bonding pattern assisted lone-pair⋯π/π⋯π supramolecular assembly of the [Cu(n-butylmalonate)₂(H₂O)]⁻² units and protonated 2-aminopyridine rings in its crystalline form. The role of the diverse non-covalent bonds towards the formation of the MOC crystal is also investigated through Hirshfeld surface analysis. The fluorescent feature of the water solution of the MOC has also been exploited for chemo-sensing applications through fluorescence spectroscopy. The presence of the protonated 2-aminopyridine rings in the water solution of the MOC is actually responsible for the Cr(VI)-oxoanion sensing activity by the MOC. This has been demonstrated in various ways in the presented work. The aqueous solution of the MOC is extremely selective for the picomolar level of sensing the Cr(VI)-oxoanion in a water medium through a fluorescence spectroscopic method. Not only different metal ions like Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cd²⁺, Hg²⁺, Zn²⁺, and another chromium source such as Cr³⁺ but also several anions like F⁻, Cl⁻, Br⁻, SO₄²⁻, N₃⁻, NO₃⁻, BF₄⁻, ClO₄⁻, AsO₃³⁻ and PO₄³⁻ in a water medium do not interfere with the critical level fluorometric sensing activity of the aqueous solution of the MOC towards the picomolar level Cr(VI)-oxoanion. IR spectroscopic studies justify the possibility of non-covalent type hydrogen-bonding interactions between the Cr(VI)-oxoanions with available protonated 2-aminopyridine rings of the luminescent probe and this might be accountable for the critical level sensing activity of the aqueous solution of the MOC towards the Cr(VI)-oxoanion in a water medium. An ESI-MS investigation also supports the assertion that each monomeric unit of the MOC is a key factor for the selective Cr(VI)-oxoanion sensing activity in a water medium. Thus we have developed a direct, facile, economically doable, critically selective fluorometric probe for the picomolar level sensing of the Cr(VI)-oxoanion species as a source of toxic hexavalent inorganic chromium in a water medium.

Acknowledgements

B. D. is thankful to DST (New Delhi, India) for financial support for the research project (Project No.: SR/FT/CS-77/2011).

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Footnote

† Electronic supplementary information (ESI) available: The crystallographic data of the MOC, fluorescence life-time measurement, IR spectral results, ESI-MS spectral data and theoretical details of studies are given here as supporting information. Tables S1 to S25 and Fig. S1 to S19 are included as supporting information. CCDC 874430. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra12819j

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