Solid-state fluorescence sensing of amine vapours by an anthracene-based Zn(II) complex, its phosphatase activity and theoretical calculations

Abstract

Here we have synthesized a novel mononuclear Zn(II) complex, [ZnLCl₂], of an anthracene-based ligand (N¹-(anthracen-9-ylmethyl)-N²,N²-dimethylethane-1,2-diamine). The characterization of the complex was done by UV-vis, IR, and NMR spectroscopy and ESI-MS. We have also performed X-ray crystallography of the complex to confirm its actual structure. X-ray crystal structure analysis shows that the molecular unit is mononuclear and the Zn(II) centre is tetra-coordinated with two N-donors of the ligand and two chlorine atoms, obtaining a distorted tetrahedral geometry. Phosphatase-like activity of the complex was studied spectrophotometrically using 4-nitrophenylphosphate (PNPP) as the substrate and the investigation shows that the complex efficiently catalyses phosphate ester hydrolysis. A kinetic study of PNPP hydrolysis was performed by the initial rate method at 25 °C, which yielded the k_cat value as 4.78 × 10² h⁻¹. Therefore, we can conclude that the complex is a good catalyst for phosphate ester hydrolysis. Also, the complex in the solid state can act as a potential fluorescent sensor for detecting volatile amines. When the solid complex was exposed to vapours from different amines it was found to quench the fluorescence of the complex, and the extent of quenching is dependent on the nature of the amines. From the photoluminescence study we have compared the quenching ability of different amines at a fixed temperature.

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Introduction

Zinc(II)-containing metalloenzymes constitute one of the largest studied categories, with significant advancements in crystallographic and spectroscopic techniques aiding the identification of zinc-dependent enzymes.¹ Zinc ions are integral to various enzyme classes, such as oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases, each characterized by a Zn(II) active site.^2–7 Among these, phosphatase activity stands out due to the diverse enzymatic functions of zinc ions.⁸ This has led to extensive research on synthetic Zn(II) and other transition metal complexes as models for phosphate ester hydrolysis,^9–12 due to their unique chemical properties, such as high Lewis acidity, redox stability, and relevance to biological systems.¹³

Amines have found extensive applications in food and dyeing industries, gas treatment plants, and in medical diagnostics.¹⁴ The development of highly sensitive and selective sensors for detecting volatile amines, which can contribute to serious air pollution, has garnered significant attention due to their increasing concentration in soil and wastewater.^15–17 Detection of volatile amines is also very important for monitoring food spoilage, particularly fish, meat and dairy products.^18–21 The most common volatile amines are ammonia, dimethylamine and triethylamine.^18,19 Although substantial progress has been made in the development of amine sensors for liquid-phase detection,²² the detection of amines in the vapor phase remains both critical and challenging due to the low vapour pressure of amines at room temperature. Among various analytical tools, fluorescent chemosensors stand out as a superior technique for amine detection, as it offers high signal output, simple detection methods, low cost, reliability, low background noise, and a broad operating range.^23–25

Herein, we have synthesized a zinc complex with the ligand (N¹-(anthracen-9-ylmethyl)-N²,N²-dimethylethane-1,2-diamine) (Scheme 1). This complex efficiently catalyses phosphate ester hydrolysis (Fig. 1). The complex is highly emissive both in solution and in the solid state. Strong π⋯π and CH⋯π interactions contribute to the fluorescence of the complex in the solid state. In the presence of various amine vapours, the solid-state fluorescence of the complex is quenched and we utilized this property to sense amine vapours.

Scheme 1 Schematic representation of the synthesis of the [ZnLCl₂] complex.

Fig. 1 Chemical reaction describing phosphatase activity.

Experimental

Synthesis of (N¹-(anthracen-9-ylmethyl)-N²,N²-dimethylethane-1,2-diamine) (2)

N,N-Dimethylethylenediamine (5 mmol, 0.44 g) and 9-anthracenealdehyde (5 mmol, 1.031 g) were added in methanol. The yellow solution was refluxed for 3 h and cooled to room temperature. The intermediate Schiff base was reduced with an excess of NaBH₄ (10 mmol) in methanol containing a few drops of NaOH solution, when the yellowish colour of the solution fades slowly and becomes colourless. After that the solution was acidified with HCl to pH 6. Then, DCM was added, the mixture was shaken well and kept for some time for the layers to separate. The DCM layer was taken out, and the solvent was removed on a rotary evaporator. The crude compound was purified by column chromatography over silica gel (60–120 mesh size) using a dichloromethane/methanol (95 : 5, v/v) mixture as the eluent for the separation and purification of the desired compound (Scheme 1). Finally, the compound was characterized by ESI-MS, IR, UV-Vis and ¹H NMR spectroscopic measurements.

Yield: 1.08 g (78%). Anal calc for C₁₉H₂₂N₂: C, 81.97; H, 7.97; N, 10.06. Found: C, 81.54; H, 7.37; N, 9.98%. ESI-MS(+ve ion mode): m/z: 279.33 (100%) [M+H⁺] (Fig. S1), ¹H NMR (DMSO-d₆): δ_H (ppm); 9.69 (s,1H), 8.82 (s,1H), 8.62 (d,2H), 8.20 (d,2H), 7.67 (t,2H),7.62 (t,2H), 5.29 (s, 2H), 3.87 (d,2H), 3.55 (d,2H), 2.86 (s, 6H) (Fig. S2). Electronic spectrum in methanol λ_max/nm (ε_max/M⁻¹ cm⁻¹): 314 (1024), 331 (2280), 346 (4263), 364 (6361), 384 (5766) (Fig. 3), FT-IR (cm⁻¹): 3217 (ν_N–H) (Fig. S3).

Synthesis of [ZnLCl₂] (3)

Ligand (2) (0.278 g, 1 mmol) and ZnCl₂ (0.136 g, 1 mmol) were added to 5 mL of methanol. The reaction mixture was stirred for three hours at room temperature when a light-yellow precipitate was formed. The mixture was filtered and the residue was washed with diethyl ether. Then, the filtrate was allowed to evaporate slowly at room temperature. After 2 days light-yellow, square shaped, shiny crystals of the complex suitable for X-ray diffraction studies were obtained (Scheme 1).

Yield: 0.290 g (70%). Anal calc for C₁₉H₂₂Cl₂N₂Zn: C, 55.03; H, 5.35; N, 6.76. Found: C, 55.25; H, 5.28; N, 6.70%. ESI-MS (+ve ion mode): m/z: 437.56 (100%) [M+Na⁺] (Fig. S4), ¹H NMR (DMSO-d₆): δ_H (ppm); 8.65 (s,1H), 8.44 (d,2H), 8.14 (d,2H), 7.63 (t,2H), 7.58 (t,2H), 4.87 (s,2H), 2.76 (d,2H), 2.59 (d,2H), 2.38 (s, 6H), (Fig. S5). Electronic spectrum (in MeOH) λ_max/nm (ε_max/M⁻¹ cm⁻¹): 314 (1122), 332 (3040), 347 (5965), 365 (8642), 385 (7287) (Fig. 3). FT-IR (cm⁻¹): 3236 (ν_N–H) (Fig. S6).

Kinetic measurements of the hydrolysis of PNPP

Here, the substrate was disodium (4-nitrophenyl) phosphate hexahydrate (PNPP) and aqueous DMF was chosen as the solvent.²⁶ The solutions of PNPP (2 × 10⁻² M) and the complex (1 × 10⁻⁴ M) were freshly prepared in a DMF : H₂O (2 : 3 v/v) mixture and the total reaction volume was maintained at 2.5 mL. An electronic spectral study was performed for 1 h to obtain the kinetic data. The rate of PNPP hydrolysis in the presence of the complex was treated by the initial rate method, taking the absorption increase at 421 nm due to the production of the 4-nitrophenolate ion in aqueous DMF (ε = 18 500 M⁻¹ cm⁻¹) at 25 °C. The study included 6 sets having a catalyst concentration of 0.1 mmol and 20–100 equivalents of substrate relative to the catalyst. The reaction was started by adding the metal complex to PNPP solutions of concentrations 2 mmol (20 equiv.), 4 mmol (40 equiv.), 6 mmol (60 equiv.), 8 mmol (80 equiv.) and 10 mmol (100 equiv.) and the spectra were recorded after fully mixing at 25 °C. The visible absorption increase was recorded for a total period of 1 h at regular intervals of 3 min. All measurements were performed three times, and the average value was taken for further measurement. Michaelis–Menten parameters (K_M, V_max, and k_cat) were determined using the Michaelis–Menten equation.²⁷

The reactions were corrected for the degree of ionisation of the 4-nitrophenol at 25 °C using its molar extinction coefficient at 421 nm. The final A_α value for each set was found after 2 days (at 25 °C).

X-ray crystallography

Data was collected on a GV1000 Atlas four circle diffractometer using ω-scan and Cu-Kα radiation (λ = 1.54184 Å) at 150 (2) K. Data was compiled and reduced using CrysAlisPro.²⁸ Empirical absorption correction was applied using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm. The direct method was used to solve the structure using SHELXS and the structure was refined using the SHELXL program interfaced on an Olex2 platform.^29,30 Hydrogen atoms were treated using a mixed model. H-atoms belonging to all C(H) groups, C(H,H) groups and N(H) groups were assigned a fixed U_iso of 1.2 times that of their parent atoms, whereas all C(H,H,H) groups were assigned a U_iso of 1.5 times that of their parent atoms. Hydrogen atoms belonging to aromatic moieties, CH₂ groups, and the N–H hydrogen were refined with riding coordinates. The methyl groups were refined as rotating groups. A summary of the crystallographic data is given in Table S1. Crystal data have been deposited at the CCDC with deposition number 2530388.

Theoretical method (DFT calculations)

Geometry optimizations were carried out, in MeOH as the solvent, by the density functional theory (DFT) method, using the B3LYP hybrid exchange correlation functional^31,32 and IEFPCM model³³ of solvation. The 6-31+G(2d,p) basis set was used for C, H, and N atoms. The LanL2DZ basis set, with an effective core potential, was employed for the Zn and Cl atoms.^34–36 Vibrational frequency calculations were performed to ensure that the optimized geometries were local minima on the potential energy surface and only positive eigenvalues were obtained. All calculations were carried out using the Gaussian09 (G09) program³⁷ with the aid of the GaussView, Version 5, visualization program.³⁸

Results and discussion

The Zn(II) complex (3) was successfully obtained by reaction of ZnCl₂ with the ligand (2) in a 1 : 1 ratio in methanol at room temperature. In the complex, the Zn(II) centre is tetrahedrally coordinated by two nitrogen atoms of the chelating ligand and two chlorine atoms. From IR spectral data, it is observed that the ligand shows a sharp peak at 3217 cm⁻¹ (N–H stretching), which in the complex is shifted to 3236 cm⁻¹.

Description of the X-ray crystal structure

The complex crystallizes in the triclinic space group P121/c1. The structural representation of the complex in the form of ORTEP diagram is shown in Fig. S7, and important bond lengths and bond angles are listed in Table 1. Single crystal X-ray diffraction analysis reveals that the metallic centre is four coordinated by two nitrogen atoms of the ligand and two chloride ions in a distorted tetrahedral arrangement, with the τ₄ value for the Zn centre being 0.84 (τ₄ = [360 − (α + β)]/141; where α and β are the two largest angles around the metal centre, and τ₄ = 0 and 1 for ideal square planar and tetrahedral geometry, respectively). The average of the Zn-containing angle is 108.7° and this is very close to the ideal tetrahedron angle. The Zn–N bond lengths are 2.0897(13) and 2.1044(13) Å. Due to the small bite angle of the ligand, the N1–Zn–N2 angle shows a value of only 86.53(5)°. The Zn–Cl distances lies within the expected range of 2.22–2.24 Å.^39,40

Table 1 Bond distances (Å) and angles (°) for the complex

Bond lengths/Å			Bond angles/°
	XRD	DFT		XRD	DFT
Zn1–N1	2.0897(13)	2.1877	Cl1–Zn1–Cl2	115.724(17)	110.49
Zn1–N2	2.1044(13)	2.1926	N1–Zn1–Cl2	101.48(4)	106.704
Zn1–Cl1	2.2160(5)	2.3977	N1–Zn1–Cl1	126.15(4)	119.7656
Zn1–Cl2	2.2363(4)	2.4137	N1–Zn1–N2	86.53(5)	84.4495
			N2–Zn1–Cl2	109.37(4)	118.037
			N2–Zn1–Cl1	113.49(4)	115.2769

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As mentioned earlier, there is also π–π stacking interaction between the two anthracene units, related by the symmetry 2 − x, 1 − y, −z, with a Cg–Cg distance of 3.7423 (8) Å (Fig. 2a). The Cl1 atom enters into H-bonding interaction with the N–H hydrogen of another unit at 1 − x, 1 − y, −z (Cl1⋯H1 = 2.66(2) Å). The other Cl atom (Cl2) enters into several H-bonding interactions with adjacent molecules in the lattice: Cl2⋯H6 (2 − x, −½ + y, ½ − z) = 2.89(2) Å; Cl2⋯H15A (x, ½ − y, ½ + z) = 2.73(2) Å and Cl2⋯H18B (x, ½ − y, −½ + z) = 3.10 Å (Fig. 2b and Table 2).

Fig. 2 X-ray crystal structure of 3.

Table 2 Hydrogen bonds (Å and °)

D–H⋯A	d (D–H)	d (H⋯A)	d (D⋯A)	<(DHA)
Symmetry transformations: a = 1 − x,1 − y, −z; b = 2 − x, ½ + y, ½ − z; c = x, ½ − y, −½ + z.
N1–H1⋯Cl1^a	0.84	2.66	3.434(14)	152.9
C6–H6⋯Cl2^b	0.95	2.89	3.747(2)	151
C15–H15a⋯Cl2^c	0.99	2.73	3.6996(16)	167

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Spectral study

The electronic spectra in methanol solution for both the ligand and the complex show a very strong band at 250 nm and a series of transitions between 300–400 nm, whose intensity is considerably lower than the 250 nm transition. Both these transitions are assigned to π → π* transitions of the anthracene moiety with the lower energy transition showing a vibrational structure (Fig. 3).

Fig. 3 UV-vis spectra of 2 (10⁻³ M) and 3 (10⁻³ M) in MeOH at 298 K.

The electronic spectrum in the solid-state shows that the lower energy transition is slightly red-shifted and the vibrational structure is diminished (Fig. 4a). The photoluminescence spectrum of the complex in methanol solution, obtained upon excitation at 345 nm, shows typical structured anthracene emission in the 390–500 nm region. However, when the fluorescence spectrum was recorded in the solid state, an unstructured broad emission, with a peak at 528 nm, was found upon excitation at 420 nm (Fig. 4b). This emission is assigned to the excimer of anthracene formed in the solid state.^41–43

Fig. 4 (a) Normalized absorption spectra and (b) normalized emission spectra of 3 in MeOH and the solid state.

It may be noted that the crystal structure of the complex (vide supra) clearly shows π–π stacking interaction with a Cg–Cg distance of 3.7423 (8) Å, which facilitates the formation of an excimer.

From the TCSPC study, the average lifetime of the complex in MeOH solution was evaluated as 3.08 ns (Fig. 5). In the solid state, the molecules are close together and less mobile. This allows excited molecules to pair and form excimers, which slows down the decay process. As a result, the fluorescence lifetime becomes longer.

Fig. 5 TCSPC spectra of 3 in solution and the solid state.

Computational studies of the complex

DFT calculations were carried out to understand the nature of the frontier orbitals of the complex (Table 2). It was found that HOMO, HOMO−1 and HOMO−3 are anthracene π-orbitals, whereas HOMO−4 and HOMO−5 are predominantly π-orbitals of chloride ions. Similarly, LUMO, LUMO+1 and LUMO+2 are π*-orbitals of anthracene, while LUMO+3 has major contributions from Zn, the amine N-atom and the chloride ions (Fig. S8).

Phosphatase activity

We monitored the changes in the spectral features of PNPP upon the addition of 3. The spectral changes were recorded over a period of 1 h, with measurements taken at 3 min intervals.

To assess the hydrolytic cleavage efficiency of phosphoester bonds in aqueous DMF, we tracked the time evolution of 4-nitrophenolate (λ_max = 421 nm) by performing a wavelength scan from 200 to 900 nm at 25 °C. The experiment was conducted with 20–100 equivalents of substrate relative to the catalyst (Fig. 6 and Fig. S9).

Fig. 6 Increase of the 4-nitrophenolate band at 421 nm after addition of 0.1 mmol of 3 up to 100 equivalents of PNPP solution in DMF/Water medium.

The kinetic study was performed using the initial rate method, monitoring the rate of increase in absorption at 421 nm, which corresponds to the rise in 4-nitrophenolate concentration. The initial first order rate constant for the cleavage of PNPP was obtained from the plot of log[A_α/(A_α − A_t)] versus time (Fig. 7a), which was linear with R² = 0.99108.

Fig. 7 (a) Initial rate determination using the plot of log[A_α/(A_α − A_t)] versus time, R² = 0.99108. (b) Michaelis–Menten plot for hydrolysis of PNPP by the complex, R² = 0.99654.

The kinetic parameters (V_max, K_M, and k_cat) for the catalyzed reactions were determined by applying the Michaelis–Menten eq. for enzymatic kinetics and found as follows: V_max = 2.383 × 10⁻⁴ M min⁻¹ (±3.315 × 10⁻⁵) and K_M = 6.53 × 10⁻³ M (±5.86 × 10⁻⁴) (Fig. 7b). The results are listed in Table 3. The turnover frequency for this hydrolytic catalysis was determined as k_cat = 4.78 × 10² h⁻¹. A comparison of our data with some selected data from the literature on phosphatase activity of metal complexes is given in Table S2.^27,44–51 It may be seen that among the mononuclear Zn(II) complexes, only the complexes reported by Sanyal et al. have higher k_cat values than ours, suggesting considerably good catalytic performance of our complex.

Table 3 Kinetic parameters of the complex-catalyzed reactions

Complex	Wavelength (nm)	V max (M min^−1)	K M (M)	R ^2	k cat (h^−1)
3	421	2.38 × 10^−4	6.53 × 10^−3	0.996	478

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Mechanistic pathway

We have carefully examined the relevant literature regarding the mechanism of PNPP hydrolysis by metal complexes and propose a cleavage mechanism of PNPP by our complex.^44,52,53 Studies suggest that metal catalysts play a significant role in the catalytic cleavage of PNPP through Lewis acid activation leading to metal-substrate intermediates followed by nucleophilic attack on the substrate by in situ generated hydroxide formed by deprotonation of the metal-bound aqua ligand, and leaving group activation resulting in remarkably fast rates of phosphate hydrolysis. A plausible catalytic cycle is proposed in Fig. 8. A key element of this catalysis is the generation of the nucleophile from coordinated water molecules at the metal centres in the reaction medium. This step is fundamental to the reaction mechanism, followed by the attack of the metal-bound nucleophile on the phosphorus atom, as illustrated in the proposed mechanism.

Fig. 8 Plausible mechanism for PNPP hydrolysis by 3.

In aqueous DMF solution, the chloride ion coordinated to a Zn(II) complex dissociates, with the metal-coordinated water molecule playing a key role in facilitating the hydrolytic cleavage of PNPP. When PNPP is added to the complex, it binds in a monodentate fashion, likely through O⋯H interactions between the anionic oxide ion and the hydrogen atom of the coordinated water molecule. This interaction causes the water molecule to deprotonate, forming a hydroxide ion, which acts as a nucleophile. The resultant species undergoes a nucleophilic substitution, expelling a p-nitrophenolate ion and forming coordinated phosphate. Eventually, the system breaks down into the original complex or its aquated form (ZnL(H₂O)₂²⁺) and phosphoric acid.

Control experiments

Control experiments have been done to check if there is any role of the ligand used for complex formation and the metal-salt in the hydrolysis of PNPP and the results show that the free ligand and the metal-salt have no catalytic effect in the hydrolytic reaction (S10).^44,50 We therefore conclude that the metal-complex is actually responsible for the phosphatase activity.

Amine vapor sensing study

We also found that our reported complex can act as an amine vapor sensor in the solid state. To check the detection ability of our sensor, various types of volatile amines were taken. During the photoluminescence spectral study, upon excitation with 420 nm at 35 °C, fluorescence spectral quenching happens when we expose amine vapor to the complex. It was observed that maximum fluorescence quenching occurs in the case of methylamine, n-butylamine and dimethylamine, while that for benzylamine, di-isopropylamine, and di-isobutylamine was insignificant (Fig. 9a and Fig. S11). The complete order of the quenching ability of the investigated amines is MA > Nba > DIME > NH₃ > NNDA > AN » TEA > DTA> Py > DBUA > DIPA > BA (BA = benzylamine, DIPA = diisopropylamine, DBUA = dibutylamine, Py = pyridine, DTA= diethylenetriamine, TEA = triethylamine, AN = aniline, NNDA = N,N-dimethylethylenediamine, DIME = dimethylamine, Nba = n-butylamine, MA = methyl amine). Thus, our compound can act as a very good solid state fluorescence sensor for volatile amines like methyl amine, ammonia, n-butylamine, aniline, and triethylamine, which can be considered as reference amines in the laboratory for biogenic amines, which are markers for food spoilage.^18,19 The above order of quenching abilities of different volatile amines, in the gas phase, towards our complex is slightly different from that reported by Chen et al.⁵⁴

Fig. 9 (a) Bar plot of fluorescence response towards different amine vapours. (b) Emission spectra of 3 in the presence of aniline vapour at different temperatures.

To investigate the temperature dependence in the amine vapor sensing, we have performed a temperature-dependent fluorescence spectral study using aniline and triethylamine at various temperatures. Here, we have observed that with increasing temperature, the fluorescence quenching becomes more (Fig. 9b and Fig. S12), which is expected considering that with an increase in temperature, the vapour pressure of the amines will increase.

As to the mechanism of the amine sensing, we suggest that the amine vapour enters into the crystal lattice and is trapped inside it through non-covalent interactions. This disrupts the π⋯π and CH⋯π interactions between adjacent anthracene moieties, thus preventing the formation of an excimer, resulting in quenching of the fluorescence at 528 nm. Additionally, the amines may also participate in a photoinduced electron transfer process (PET) with the excited state.

An examination of the quenching ability of the different amines, as observed above, suggests that the combination of several factors determines the quenching ability of a given amine. The primary factor seems to be the “sieve effect” where the solid sensor acts as a sieve or molecular tweezer, and the amine vapour that best fits in the sieve exerts higher quenching ability. As a result, sterically bulky amines (amines containing more substituents on the N-atom and more branching of the substituents) like triethylamine, diisopropylamine, and dibutylamine, despite being excellent donors (hence with greater ability to participate in the PET process), show poor quenching ability. Similarly, aromatic amines are found to be poorer quenchers than their aliphatic analogues, probably again due to their steric factor. Though ammonia is more volatile and has the least steric effect compared to methyl amine, n-butyl amine and dimethyl amine, it is found to have lower quenching ability than the last three amines, which suggests that the electronic factor also plays some role as methyl amine, n-butyl amine and dimethyl amine are better electron donors than ammonia. Methyl amine seems to have the most optimum balance between steric bulk, volatility, basicity and ability to enter into non-covalent interactions (CH⋯π interactions), to bind strongly to the crystal lattice and effect quenching. Although larger than dimethylamine, n-butyl amine exhibits higher quenching ability; this indicates that branching at the nitrogen (secondary/tertiary) hurts quenching more than a longer linear tail does.

A similar molecular tweezer effect was observed for a dinuclear Zn(II) Schiff-base complex, which can selectively detect biogenic di- or polyamines in preference to common primary, secondary or tertiary aliphatic amines in chloroform solution.⁵⁵ A paper-based sensor obtained from a mononuclear Zn(II) Schiff-base complex having a nearly identical ligand framework to that of the monomeric unit of the dinuclear complex mentioned above was reported by the same research group, which was found to preferentially detect n-butylamine among various volatile amines.⁵⁶

Conclusions

This study represents the synthesis and structural characterization of a Zn(II) complex bearing a fluorescent anthracene moiety and two labile chloride ligands. The fluorescence of the anthracene moiety makes it possible to use it as a fluorescent sensor for analytes like volatile amines. The lability of the chloride ligands conjures up the possibility of using the complex as a catalyst for hydrolytic reactions. In fact, the complex was tested as a model for phosphatase enzyme activity, using (4-nitrophenyl) phosphate (PNPP) as a substrate in an aqueous DMF medium. The complex demonstrated good efficiency in hydrolyzing phosphate ester bonds, with a catalytic rate constant (k_cat) of 4.78 × 10² h⁻¹. Also, the complex can be used as a solid-state fluorescence sensor for detecting amine vapours.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data for this work are reported as supplementary information (SI), which has been uploaded along with the submission. Supplementary information: details on materials and methods, ¹H NMR, mass spectrometry, and spectrophotometric analysis. See DOI: https://doi.org/10.1039/d6nj00629a.

CCDC 2530388 contains the supplementary crystallographic data for this paper.⁵⁷

Acknowledgements

A. M. thanks IIEST, Shibpur, for her PhD scholarship. All authors thank the Sophisticated Analytical Instruments Facility (SAIF), IIEST, Shibpur, for NMR and HRMS facilities. We gratefully acknowledge the DST-FIST for funding the purchase of the TCSPC instrument.

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