A highly specific ‘turn-on’ fluorescent detection of Mg²⁺ through a xanthene based fluorescent molecular probe

Abstract

Two acid hydrazones incorporating xanthene as a metal chelating centre (B-XAN and N-XAN) have been synthesized, characterized and evaluated for the optical sensing of Mg²⁺ and Al³⁺. The replacement of the naphthyl moiety in N-XAN by the phenyl moiety in B-XAN leads to total specificity of the latter one towards Mg²⁺. The B-XAN exhibited a highly sensitive and selective ‘turn-on’ fluorescent response towards Mg²⁺ through intense blue fluorescence enhancement. This measurement was free from any interference by other cationic analytes including the Ca²⁺ and Al³⁺ also. The binding constant, stoichiometry as well as the mechanistic aspect of the above sensing phenomenon were also studied through various spectroscopic methods.

---

Introduction

In recent years, myriad fluorescent chemosensors for Mg²⁺ have been reported due to their biological,¹ analytical² as well as environmental³ importance. Magnesium is the second most common intracellular cation and the fourth most abundant cation in the human body.⁴ Mg²⁺ is indispensable for mediating many enzyme-catalysed reactions participating in many biological functions such as, a cofactor in the phosphorylation of glucose during carbohydrate metabolism,⁵ proper functioning of nerves and immune systems, and muscle and bone remodelling and skeletal development.^1b,6 Furthermore, it has also been found to be involved in modulation of signal transduction, various transporters, ion channels and stabilization of DNA conformation etc.⁷ Mg²⁺ also plays important roles in a wide variety of physiological and pathological processes, such as cerebral infarction, lung cancer, congestive heart failure and muscle dysfunction.⁸ A trace amount of Mg²⁺ causes hypomagnesemia; which is often associated with diabetes, hypertension, neuronal injury, metabolic syndrome etc. The hypermagnesemia is a less frequent condition and is associated with chronic renal failure.⁹ Moreover, the Mg²⁺ concentration varies between 0.1 and 6 mM, for e.g. cardiac cells (0.5–1.2 mM), hepatocytes (0.37 mM), 0.3 mM in synaptosomes, while 0.44–1.5 mM in normal serum¹⁰ in efflux and influx processes. We can apply these concentration limits to diagnose the diseased states of human body. Therefore, in order to protect the environment and human health, monitoring of Mg²⁺ concentration level is an important issue.

A number of probes for Mg²⁺ have been reported in literature are mostly fluorescent ones incorporating crown ether,¹¹ diketone,¹² porphyrin,^1b calix[4]arene,¹³ coumarins,¹⁴ Schiff bases.¹⁵ A thorough study of literature regarding the chemical probes for the detection of Mg²⁺ revealed that detection of same in the presence of Ca²⁺ is a common bottle neck for most of the probes reported hitherto.¹⁶ Beside this, comparatively lesser number of efficient fluorescent probe for the optical sensing of Mg²⁺ have been reported in the literature due to its spectroscopic silence.^1b,2,11 Many reported fluorescent probes for Mg²⁺ yet involve lengthy preparation time and high cost of synthesis.¹⁷ In this context, the probes being reported herein by us is a highly relevant, worthy one as well as addresses most of the challenges mentioned above. We have designed, synthesised and evaluated xanthene based fluorescent probe B-XAN. We also studied the mechanistic aspect of the above sensing process through fluorescence spectroscopy, FT-IR, ¹H NMR titrations, job's continuous variation, ESI-MS spectral studies as well as through DFT calculations.

Xanthene based host–guest chemistry of these architecture haven't been investigated much yet. In order to investigate the mechanistic aspect of the interaction of B-XAN with Mg²⁺ we also synthesised a control compound N-XAN where phenyl moiety was replaced by naphthyl moiety (Fig. 1). The N-XAN gave response towards Al³⁺ also besides Mg²⁺, hence specificity of N-XAN was poor than B-XAN. This designing principle can be understood in terms of electron density control by having phenyl moiety at the place of napthyl moiety. The resonance energy per benzene ring of naphthalene is lower than that of benzene hence the electron donating capability of naphthyl moiety is expected to be higher than that of phenol moiety of B-XAN. Thus the N-XAN is capable to interact with a metal ion like Al³⁺ having much higher demand of electron density as compared to the Mg²⁺.

Fig. 1 The molecular structure of B-XAN and N-XAN.

Results and discussion

The synthetic pathways of two new fluorogenic xanthene based derivatives B-XAN [(E)-N-(2-hydroxybenzylidine)-9H-xanthene-9-carbohydrazone] and N-XAN [(E)-N′-((2-hydroxynaphthalen-1-yl)methylene)-9H-xanthene-9-carbohydrazide] are shown in Scheme 1. The corresponding aldehydes such as 2-hydroxy benzaldehyde and 2-hydroxy naphthaldehyde were added separately to the 20 mL methanolic solution of 9H-xanthene-9-carbohydrazide at room temperature with constant stirring for ∼4–5 hours resulting into white and light yellow solids respectively which were filtered, washed and dried under vacuum. These probes were fully characterized (see the Experimental section).

Scheme 1 Synthesis of B-XAN and N-XAN.

X-ray crystallographic studies

The single crystals of B-XAN were obtained by slow evaporation of its saturated DMF solution. The structure of B-XAN was fully confirmed through its single crystal X-ray analysis. B-XAN crystallizes into a monoclinic system with space group P21/n. An ORTEP view of the asymmetric unit of B-XAN is shown in Fig. 2(a), while crystal data and structural refinement details are listed in (Table 1; ESI†).

Fig. 2 (a) Single crystal of B-XAN with 50% probability. (b) Supramolecular architecture showing intra and intermolecular hydrogen bondings in B-XAN.

The bond angles C16–C15–N2 and N2–N1–C14 are 119.84° and 117.99° respectively. The dihedral angle C16–C15–N2–N1 is 178.04° which reveals that these atoms are in the same plane. The dihedral angles C12–C13–C14–O2 and C1–C13–C14–O2 are −74.62° and 45.79° respectively which shows that xanthene moiety gets bent with respect to remaining plane of molecule shown in Fig. 2(a).

The molecule (B-XAN) adopts an (E)-configuration about the [double bond splayed left]C=N bond (C15–N2) having bond distances 1.277 Å appreciably close to that of a [double bond splayed left]C=N bond (1.300 Å). The N1H of xanthene moiety forms strong classical intermolecular hydrogen bonding with O2 of another xanthene moiety (N1–H⋯O2 = 1.976 Å). On the other hand O3H and N2 atoms undergo strong intramolecular hydrogen bonding (O3–H⋯N2 = 1.857 Å). Finally a nice supramolecular architecture knitted through intermolecular hydrogen bonding was seen in Fig. 2(b). The weak fluorescent nature of B-XAN seems as a consequence of partial quenching of PET due to intramolecular hydrogen bonding between O3⋯H and N2.

Photo physical studies of B-XAN

In order to study the effect of slight structural modification in B-XAN and N-XAN and their sensitivity towards Mg²⁺ we did photophysical studies. The UV-visible study of B-XAN and N-XAN were performed in their 10 μM acetonitrile solutions. As illustrated in (Fig. S13a and b and S14a and b; ESI†). Free B-XAN and N-XAN exhibit maximum absorption bands centered at 320 and 355 nm as their charge transfer (CT) bands respectively. The same solution of B-XAN and N-XAN were allowed to interact separately with 10 equivalents each of a variety of cations including viz. Li⁺, Na⁺, K⁺, Ba²⁺, Al³⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ as their chloride salts. None of the above was found to perturb either the UV-visible spectral pattern of B-XAN and N-XAN or their visible appearance to naked eye to a significant extent (Fig. S13a and b and S14a and b; ESI†).

Due to non-conclusive spectral and visible colour responses of B-XAN and N-XAN towards aforementioned cations including Mg²⁺, the metal chelating behavior of B-XAN and N-XAN were further checked by fluorescence spectroscopy. Surprisingly the N-XAN was not able to differentiate between Mg²⁺ and Al³⁺ in fluorescence emission upon excitation at 355 nm ((Fig. S15a and b); ESI†). Nevertheless in contrast, the 1 μM solution of B-XAN was excited at 320 nm and showed a weak emission band at 447 nm. Upon separate additions (10 equiv.) of the above mentioned metal ions to the 1 μM solution B-XAN, a broad significant peak at 458 nm was observed selectively with Mg²⁺ (Fig. 3).

Fig. 3 Relative fluorescence changes of B-XAN (1.0 μM) after treatment with 10 equiv. of chloride salt of various metal ions in acetonitrile at λ_ex = ∼458 nm along with inset fluorescence spectral graph.

On the other hand, it shows interference with the certain metal ions like Cr³⁺, Fe³⁺, Ni²⁺, Cu²⁺ (Fig. S16; ESI†). B-XAN displayed a unique fluorogenic (non-fluorescent to intense blue fluorescent) response. Hence the same could easily be detected by the naked-eye under UV light (Fig. 4).

Fig. 4 Fluorescence response of B-XAN (50 μM) in the presence of different metal ions under UV light (365 nm).

In the spectrofluorometric titration experiment, upon incremental addition of Mg²⁺ (0–6.0 equiv.) to the 1 μM solution of B-XAN in acetonitrile, the intensity of peak centered at 458 nm underwent enhancement by ∼16 fold to that of the probe itself (Fig. 5).

Fig. 5 Titration profile of 1 μM acetonitrile solution of B-XAN upon concomitant additions of Mg²⁺ (0-6.0 equiv.).

The detection limit (LOD) was worked out to be 9.08 × 10⁻¹⁰ M (R² = 0.991) with a linearity range of 2.0 × 10⁻⁷ to 1.2 × 10⁻⁶ M (3σ/slope)¹⁸ (Fig. S17; ESI†).

This result indicates that the B-XAN shows quite high sensitivity toward trace amount of Mg²⁺ as compared to most of previously reported optical probes^1a,2,19 for Mg²⁺ under similar condition.

Furthermore, by means of fluorescence intensity measurement we also studied the time dependence response of B-XAN towards Mg²⁺. The results revealed that the reaction of 1 μM solution of B-XAN with 10 equiv. of Mg²⁺ was completed within 30 seconds (ESI; Fig. S19; ESI†).

Reversibility check for B-XAN with EDTA

The reversibility is an important criterion for any sensing device hence the response of B-XAN towards Mg²⁺ was also checked. The addition of EDTA disodium salt (40 equiv.) to the mixture of B-XAN and Mg²⁺ (20 equiv.) solution resulted in a reduction of the emission intensity at 458 nm and a regeneration of free B-XAN. This reversibility is beneficial to the fabrication of reusable optical devices for sensing of Mg²⁺ (Fig. S20; ESI†).

To further explore the practical applicability of the B-XAN, we performed sensing of Mg²⁺ in different water samples such as pond, tap, and ground water. B-XAN (100 μM) shows an excellent selectivity towards Mg²⁺ over other common metal ions present in different samples such as ground, pond and tap water (Fig. 6).

Fig. 6 Fluorescence response of B-XAN (100 μM) in ACN : H₂O (1 : 1) solution after adding 10 eq. Mg²⁺ ion in different water samples, (G = ground water, T = tap water, P = pond water).

Binding behavior of Mg²⁺ towards B-XAN

Stoichiometry is an important parameter for having an insight of binding interaction during sensing event. Job's plot analysis shows 2 : 1 binding stoichiometry between B-XAN and Mg²⁺ (Fig. S18; ESI†).

Moreover, mass spectral studies were also conducted to further elucidate the binding mode of B-XAN with Mg²⁺. The ESI mass spectrum of B-XAN in acetonitrile medium, showed a prominent molecular ion peak m/z at 345.1 corresponding to [B-XAN + H]⁺, while adding 4 equiv. of Mg²⁺ (chloride as the counter anion), the formation of 2 : 1 complex between B-XAN and Mg²⁺ was further confirmed by the appearance of the molecular ion peak in positive ion mode at m/z = 711.21 (Fig. S12; ESI†). The corresponding binding constant of B-XAN with Mg²⁺ has been determined by the non-linear fitting of fluorescence titration data using the 2 : 1 binding equation.²³ The value of the binding constant (c) was found to be 7227.67 M^−1/2 with a satisfactory correlation coefficient value (R² = 0.9796) (ESI; Fig. S21†). In the given equation²³ ‘n’ is the number of Mg²⁺ ions bound to each B-XAN (here n = 0.7). The value of ‘n’ confirmed the 2 : 1 stoichiometry for the (B-XAN)₂Mg complex.

The nature of the binding mode of B-XAN with Mg²⁺ was further analyzed through ¹H NMR experiment carried out in CD₃CN. The amidic proton ([double bond splayed left]NH) of the B-XAN appeared at 11.15 δ ppm and become disappeared upon gradual addition of Mg²⁺ (0.0 to 3.5 equiv.), indicating the deprotonation of amidic ([double bond splayed left]NH) protons. On the other hand, the hydroxyl proton of the B-XAN experienced an obvious down field shift from 10.15 to 10.29 δ ppm along with decrease in the intensity of the aldimine proton (–CH=N–), indicating the involvement of oxygen of –OH group and nitrogen of imine (–CH=N–) group in the binding with Mg²⁺ ion. There was no significant change in the position of aromatic protons. Finally the change in chemical shifts of protons during titration with Mg²⁺ clearly indicated that –OH, –NH and –CH=N– have been involved in binding interactions with Mg²⁺ (Fig. 7).

Fig. 7 Partial ¹H NMR spectra of B-XAN and successive addition of Mg²⁺ in a CD₃CN solution.

This was further confirmed through FT-IR analysis of B-XAN which showed two bands for –CH=N– and [double bond splayed left]C=O at 1611 and 1657 cm⁻¹ respectively, in the absence of Mg²⁺ (Fig. S1; ESI†). On addition of Mg²⁺ to B-XAN, the band at 1657 cm⁻¹ ([double bond splayed left]C=O) got shifted to 1633 cm⁻¹ because of the Mg²⁺ binding to the carbonyl group indicating its involvement in coordination. Moreover the co-ordination of imine (–HC=N–) atom in binding was also confirmed by its lowering in terms of wavenumber (from 1611 to 1575 cm⁻¹) upon interaction of B-XAN with Mg²⁺, suggesting the complexation between the B-XAN and Mg²⁺.

Finally, on the basis of above job's plot, ESI-MS, FT-IR and ¹H NMR titration study, we proposed a rational coordinated mode in which Mg²⁺ is hexa-coordinated with two tridentate probe having O–N–O type donor sites involving phenolic –OH of 2-hydroxybenzaldehyde, imine nitrogen (–HC=N–) and carbonyl oxygen atom of hydrazide (Scheme 2) leading to rigidify of the molecular assembly by restricting the C=N isomerization. The same finally resulted in significant fluorescent enhancement through the process of chelation-enhanced fluorescence (CHEF) in consonance with previous literature report.²²

Scheme 2 Proposed binding mode of interaction of Mg²⁺ with B-XAN.

Computational studies

In order to fortify the experimental results regarding binding of B-XAN producing (B-XAN)₂Mg complex the theoretical studies at DFT level were also performed through B3LYP method using 6-31g** as a basis set.²⁰ The optimised structures and their HOMO–LUMO have been shown in (Fig. 8 and 9). It has also been observed that the HOMO–LUMO gap of B-XAN decreased up to 0.4773 eV upon its complexation with Mg²⁺ (Fig. 9). The bond length between donor atoms of B-XAN and Mg²⁺ are well within the acceptable range and favors the strong binding of Mg²⁺ with B-XAN²¹ (Fig. 8). Thus coordination of Mg²⁺ with B-XAN was proved beyond doubt.

Fig. 8 Optimized geometry of complex [(B-XAN)₂Mg] showing important bond lengths.

Fig. 9 Frontier molecular orbitals of B-XAN and (B-XAN)₂Mg together with their energy levels (in eV) obtained from DFT calculations using Gaussian 03 program.

Experimental

Synthesis of B-XAN

The syntheses of B-XAN and N-XAN have been shown in Scheme 1. The 2.0 mM absolute methanolic solution of 2-hydroxybenzaldehyde and 2-hydroxynapthaldehyde was added to an equimolar absolute methanolic solution of 9H-xanthene-9-carbohydrazide respectively at room temperature with constant stirring for ∼4–5 hours. As a result of this, white and light yellow solids were precipitated respectively which were finally dried under vacuum over anhydrous CaCl₂. B-XAN and N-XAN were characterized through various spectroscopic techniques like IR, ¹H & ¹³C NMR spectral studies along with ESI-mass determination (Fig. S1–S8; ESI†) and single crystal XRD analysis (Table 1; ESI†).

Spectroscopic characterization data for B-XAN

Yield: 78%; IR/cm⁻¹: 3433, 3187, 3173, 3040, 2890, 1657, 1622, 1611, 1556, 1481, 1354, 1224, 1095, 991, 847, 703; ¹H NMR (300 MHz, CD₃CN, TMS): δ ppm = 11.152 (s, 1H, –NH), 10.153 (s, 1H, –OH), 8.312 (s, 1H, –HC=N–), 7.383–7.271 (m, 6H, Ar), 7.168–7.104 (m, 4H, Ar), 6.939–6.872 (M, 2H, Ar), 5.025 (s, 1H, CH); ¹³C NMR (75 MHz, DMSO-d₆) δ ppm = 167.46, 157.26, 151.20, 150.92, 148.20, 131.53, 129.03, 128.90, 128.66, 128.57, 123.41, 120.16, 119.37, 118.57, 116.48, 116.32, 43.98; ESI-MS: m/z for C₂₁H₁₆N₂O₃, calculated [M] = 344.3, found [M + H]⁺ = 345.1.

Spectroscopic characterization data for N-XAN

Yield: 70%; IR/cm⁻¹ of N-XAN 3417, 3194, 3042, 1649, 1624, 1576, 1551, 1481, 1455, 1418, 1391, 1326, 1257, 1221, 1185, 1098, 1033, 985, 821, 745, 696, 533, 474, 432; ¹H NMR (300 MHz, DMSO-d₆, TMS): δ ppm = 12.203 (s, 1H, –NH), 12.185 (s, –OH), 9.243 (s, –HC=N–), 8.283–8.254 (d, –2H, Ar), 7.855–7.793 (t, –2H, Ar), 7.530–7.481 (t, 2H, Ar), 7.365–7.341 (d, 2H, Ar), 7.303–7.257 (t, 2H, Ar), 7.214–7.194 (d, 2H, Ar), 7.142–7.052 (q, 2H, Ar), 5.035 (s, 1H, CH); ¹³C NMR (75 MHz, DMSO-d₆) δ ppm, 167.36, 157.94, 150.95, 147.04, 132.92, 131.47, 128.98, 128.78, 127.82, 123.49, 121.20, 119.32, 118.71, 116.54, 108.50, 43.98 ESI-MS: m/z for C₂₅H₁₈N₂O₃, calculated [M] = 394.4, found [M + H]⁺ = 395.2.

Synthesis of complex (B-XAN)₂Mg

A 10 mL acetonitrile solution of MgCl₂ (508.25 mg, 2.5 mM) was added slowly to a magnetically stirred 10 mL acetonitrile solution of B-XAN (344.36 mg, 1 mM). The reaction mixture was stirred at room temperature for ∼3 h whereby a fluorescent bluish colour solution was formed which was dried under vacuum over anhydrous CaCl₂. The complex was characterized by IR, ¹H &¹³C NMR along with mass spectral studies (Fig. S9–S12; ESI†).

Spectroscopic characterization data for complex (B-XAN)₂Mg

Yield: 60%; IR/cm⁻¹: 3573, 3232, 2233, 2024, 1844, 1633, 1555, 1482, 1454, 1389, 1258, 1149, 940, 865, 749, 637, 475; ¹H NMR (300 MHz, CD₃CN, TMS): δ ppm = 10.226 (s, –OH), 8.310 (s, –HC=N–), 7.381–7.280 (m, 6H, Ar), 7.168–7.107 (m, 4H, Ar), 6.947–6.878 (m, 2H, Ar), 5.035 (s, 1H, CH); ¹³C NMR of (B-XAN)₂Mg (125 MHz, DMSO-d₆) δ ppm = 167.55, 157.29, 150.97, 148.43, 141.25, 131.53, 129.18, 128.89, 128.64, 128.57, 128.45, 123.40, 123.32, 119.53, 119.40, 119.36, 118.54, 116.47, 116.33, 116.16, 43.95. ESI-MS: m/z calculated for [C₄₂H₃₀N₄O₆Mg] = 710.20, found = 711.21.

Conclusions

Thus we have demonstrated the judicious and tactful change in basic framework of N-XAN leads to inculcation of total selectivity of B-XAN towards fluorescent sensing of Mg²⁺. Moreover the detection limit of B-XAN is of the order of sub-nano molar level. To the best of our knowledge, the results reported in this work constitute the first example of a xanthene based probe which binds Mg²⁺ very selectively. Moreover we also demonstrated the practical applicability of B-XAN in detecting Mg²⁺ in different water samples such as pond, tap, and ground water etc.

Acknowledgements

KKU gratefully acknowledges the financial support [award no. 41-330/2012(SR)] from UGC, New Delhi. AP acknowledges the financial support from UGC [R./Dev./Sch.(UGC-JRF-410/S-01)] in the form junior research fellowship. SV acknowledges CSIR [award no. 09/013(0539)/2014-EMR-I] New Delhi, for junior research fellowship.

References

1. (a) Y. Zhao, B. Zheng, J. Du, D. Xiao and L. Yang, Talanta, 2011, 85, 2194; (b) G. Farruggia, S. Iotti, L. Prodi, M. Montalti, N. Zaccheroni, P. B. Savage, V. Trapani, P. Sale and F. I. Wolf, J. Am. Chem. Soc., 2006, 128, 344.
2. R. Alam, T. Mistri, A. Katarkar, K. Chaudhuri, S. K. Mandal, A. R. Khuda-Bukhsh, K. K. Das and M. Ali, Analyst, 2014, 139, 4022.
3. P. S. Hariharan and S. P. Anthony, RSC Adv., 2014, 4, 41565.
4. C. P. Hans, R. Sialy and D. D. Bansal, Curr. Sci., 2002, 83, 1456.
5. (a) B. O'Rourke, P. H. Backx and E. Marban, Science, 1992, 257, 245; (b) C. Schmitz, A. Perraud, C. O. Johnson, K. Inabe, M. K. Smith, R. Penner, T. Kurosaki, A. Fleig and A. M. Scharenberg, Cell, 2003, 113, 191.
6. (a) H. J. Kim and J. S. Kim, Tetrahedron Lett., 2006, 47, 7051; (b) A. Ajayaghosh, E. Arunkumar and J. Daub, Angew. Chem., Int. Ed., 2002, 41, 1766; (c) F. H. Nielsen and H. C. Lukaski, Magnesium Res., 2006, 19, 180.
7. (a) H. C. Politi and R. R. Preston, NeuroReport, 2003, 14, 659; (b) C. Schmitz, A. Perraud, C. O. Johnson, K. Inabe, M. K. Smith, R. Penner, T. Kurosaki and A. Fleig, Cell, 2003, 113, 191.
8. (a) S. Mahabir, Q. Y. Wei, S. L. Barrera, Y. Q. Dong, C. J. Etzel, M. R. Spitz and M. R. Forman, Carcinogenesis, 2008, 29, 949; (b) O. B. Stepura and A. I. Martynow, Int. J. Cardiol., 2009, 134, 145; (c) S. C. Larsson, M. J. Virtanen, M. Mars, S. Männistö, P. Pietinen, D. Albanes and J. Virtamo, Arch. Intern. Med., 2008, 168, 459.
9. (a) J. F. Navarro-González, C. Mora-Fernández and J. García-Pérez, Semin. Dial., 2009, 22, 37; (b) C. G. Musso, Int. Urol. Nephrol., 2009, 41, 357; (c) A. Cittadini and F. I. Wolf, Magnesium: from biochemistry and cell physiology to clinical implications, Mol. Aspects Med., 2003, vol. 24(1/3).
10. E. Murphy, C. C. Freudenrich and M. Lieberman, Annu. Rev. Physiol., 1991, 53, 273.
11. (a) M. Tian and H. Ihmels, Eur. J. Org. Chem., 2011, 4145; (b) J. Brandel, M. Sairenji, K. Ichikawa and T. Nabeshima, Chem. Commun., 2010, 46, 3958.
12. X. Zhu, C. He, D. Dong, Y. Liu and C. Duan, Dalton Trans., 2010, 39, 10051.
13. A. Hamdi, S. H. Kim, R. Abidi, P. Thuery, J. S. Kim and J. Vicens, Tetrahedron, 2009, 65, 2818.
14. (a) Y. Dong, X. Mao, X. Jiang, J. Hou, Y. Cheng and C. Zhu, Chem. Commun., 2011, 47, 9450; (b) D. Ray and P. K. Bharadwaj, Inorg. Chem., 2008, 47, 2252; (c) S. B. Maity and P. K. Bharadwaj, J. Lumin., 2014, 155, 21.
15. S. Devaraj, Y. K. Tsui, C. Y. Chiang and Y. P. Yen, Spectrochim. Acta, Part A, 2012, 96, 594.
16. (a) P. Bühlmann, E. Pretsch and E. Bakker, Chem. Rev., 1998, 98, 159; (b) A. D. Dubonosov, V. I. Minkin, V. A. Bren, E. N. Shepelenko, A. V. Tsukanov, A. G. Starikov and G. S. Borodkin, Tetrahedron, 2008, 64, 3160; (c) J. Kim, T. Morozumi, N. Kurumatani and H. Nakamura, Tetrahedron Lett., 2008, 49, 1984.
17. (a) A. Sargenti, G. Farruggia, E. Malucelli, C. Cappadone, L. Merolle, C. Marraccini, G. Andreani, L. Prodi, N. Zaccheroni, M. Sgarzi, C. Trombini, M. Lombardo and S. Iotti, Analyst, 2014, 139, 1201; (b) X. Yu, P. Zhang, Q. Liu, Y. Li, X. Zhen, Y. Zhang and Z. Ma, Mater. Sci. Eng., C, 2014, 39, 73.
18. (a) IUPAC, Spectrochim. Acta, Part B, 1978, 33, 242; (b) USEPA, Appendix B to Part 136-Definition and Procedure for the Determination of the Method Detection Limit-Revision 1.11, Federal Register 49(209), 43430, October 26, 1984. Also referred to as “40 CFR Part 136”.
19. Y. Li, J. Wu, X. Jin, J. Wang, S. Han, W. Wu, J. Xu, W. Liu, X. Yao and Y. Tang, Dalton Trans., 2014, 43, 1881.
20. M. J. Frisch, et al., Gaussian 03, Revision D.01, Gaussian, Inc., Wallingford, CT, 2004.
21. (a) S. Song, H. Ma and Y. Yang, Dalton Trans., 2013, 42, 14200; (b) G. J. Moxey, F. Ortu, L. G. Sidley, H. N. Strandberg, A. J. Blake, W. Lewis and D. L. Kays, Dalton Trans., 2014, 43, 4838; (c) A.-X. Chen, G. Liu, H. Li, J.-D. Wang and F. Yue, J. Chem. Crystallogr., 2011, 41, 143.
22. (a) K. K. Upadhyay and A. Kumar, Org. Biomol. Chem., 2010, 8, 4892; (b) A. Kumar, V. Kumar and K. K. Upadhyay, Analyst, 2013, 138, 1891; (c) Neeraj, A. Kumar, V. Kumar, R. Prajapati, S. K. Asthana, K. K. Upadhyay and J. Zhao, Dalton Trans., 2014, 43, 5831.
23. C. R. Lohani, J.-M. Kim, S.-Y. Chung, J. Yoon and K.-H. Lee, Analyst, 2010, 135, 2079.

---

Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR, IR, mass spectrum of B-XAN and (B-XAN)₂Mg and crystal refinement data of B-XAN are also provided. CCDC 1059288. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra26531b

---