Indian Journal of ChemistryVol. 45B, May 2006, pp. 1269-1273

Synthesis, optical spectral studies of photoemmitive benzopyranone crown etherand its application for metal ion extraction

Sabir H Mashraqui*, Dhaval Vashi & S SubramanianDepartment of Chemistry, University of Mumbai, Vidyanagari, Santacruz (E), Mumbai 400 098, India

E-mail: sh_mashraqui@yahoo.com

Received 6 June 2005; accepted (revised) 29 December 2005

Synthesis of a new photoemmitive, benzopyranone 15-crown-5 (1) has been reported starting from 2-formyl benzo-15-crown-5 by using a straightforward protocol. Metal ion extraction profile revealed 1 to extract K+ more selectively over Na+and Li+ compared to the known benzo-15-crown-5. However, except for slight enhancements (8 to 27%) in the emissionintensities, no appreciable changes either in the absorption or emission spectra of 1 were noticeable in the presence of se-lected group I and II metal ions. The absence of significant optical perturbations suggests at best only weak interaction be-tween the metal ions and the benzopyranone chromophore.

Keywords: Fluoroionophore, metal ion extraction, UV-visible, tluorescence

IPC: Int.CI.s C 07 D

The detection and quantification of metal ions is ofgreat importance in biology, medical diagnostic, ma-terial and analytical fields 1,2. Not surprisingly, overpast two decades, an impressive range of chromo-ionophoric systems that function on the basis of a va-riety of optical effects has been developed. A basicdesign concept involves integrating an ion selectivereceptor with a suitable chromophoreltluorophoresuch that the electrostatic effect of ion complexationperturbs absorption/emission properties of fluoro-ionophores '. Typical fluoroionophores displaying ca-tion-induced optical changes possess anthracene,coumarin, naphthalene, pyrene and benzothiazole aschromophores and crown ethers as the metal bindingcomponeatsl

-4. In particular, ion-specific systems arebeing designed that deliver "off-on" or "on-off'switching effects.

Crown ethers encompassing xanthone and couma-rin moities'r" have been demonstrated to exhibitprominent carbonyl interaction with the metal ions, asevidenced from the metal-induced optical perturba-tions. In the present work, we have designed benzopy-ranone crown ether 1 in which a photoemmitive ben-zopyranone moiety'" has been linked at the ortho po-sition of benzo-Ic-crown-S (Bz-15-C-5) via a methyl-ene spacer. In analogy to well-known preference ofBz-15-C-5 for hard alkali and alkaline earth metalionsll

,12, we anticipated that the optical properties of 1

might be modified as a result of the interaction ofmetal ions with the carbonyl group of benzopyranonenucleus. Herein, we report synthesis of 1, its al-kali/alkaline earth metal ion extraction and the effectsof metal ions on its optical properties.

Synthesis and structure of benzopyranone crownether 1

The synthesis of 1 (Scheme I) starts with the basecatalyzed condensation (30% alcoholic KOH at O°CtoRT) of known 2-formyl benzo-15-crown-5 (2, ref.13)with 2-hydroxyacetophenone under N2 atmosphere.The reaction delivered aldol product 3, as a yellowcrystalline solid in 45% yield. Compound 3 on cata-lytic hydrogenation over 10% Pd/C in ethyl acetateafforded dihydro derivative 4 in 85% yield as a waxysolid, mp 74-76°C. Finally, heating 4 with an excessof N,N-dimethylformamide dimethyl acetal at 120-125°C for 2 hr led directly to the target molecule 1 insingle step involving formylation of the active meth-ylene group and subsequent ring closure via the phe-nolic -OH participation. The target molecule 1, mp65-67°C was isolated in 72% yield by Si02 columnchromatography.

The structural assignment for 1 follows from itselemental composition (C24H2607,M+ at mlz 450) andspectral data. Its IR spectrum showed a prominent

1270 INDIAN J. CHEM., SEC. B, MAY 2006

("'O~

~

o JI 0

~ 0 JCHO LJO

~IlAOH•

KOHI Ethanol

o

2

7'

1

Pd/C•

o

3 4

Scheme I-Synthesis of benzopyranone-crownether 1

band at 1645 em" due to the benzopyranone carbonylfunction and the IH NMR (CDCI3, 400 MHz) re-vealed a sharp singlet at 0 3.83 (2H) for the benzylic -CH2 group, whereas the oxyethylene protons appearas overlapping multiplets (16H) in the region 0 3.67to 4.22. A high field doublet at 0 6.7 (1H, J3.4 = 8 Hz)is due to the C3 proton that is coupled to the C4 pro-ton appearing in the form of a triplet (J= 8 Hz) at 06.98. A doublet located at 0 6.95 (h5= 8 Hz) can beassigned to the C5 proton. The olefinic proton of thebenzopyranone ring is seen as a singlet at 0 7.62whereas a low field doublet at 08.23 (l H, J5',6'= 7Hz)could be attributed to the C5' proton. An overlappingmultiplet essentially made up of a doublet and a triplet(2H) at 0 7.37-7.42 belongs to the C6' and C8' pro-tons. Finally, a triplet centered at 0 7.65 (h',s' = 7 Hz)is assigned to the remaining C7' proton.

Optical spectral studies and metal ion extraction

The UV -visible spectral data of 1recorded in sol-vents ranging from low polar cyclohexane to high

polar methanol are complied in Table I. A single,major absorption band attributable to n-n" transition isobserved in a narrow range of 306 to 309 nm withonly marginal differences in the E rnax values in thesesolvents. Absence of solvatochromism implies thatground or excited state structures are not appreciablyperturbed by solvent polarity. The fluorescent spectraobtained upon excitation at 305 nm revealed Aem inthe range of 356 to 367 nm with Stokes shifts varyingfrom a low of 50 nm in cyclohexane to a high of 58nm in chloroform. In line with the absorption spectra,the fluorescence behaviour also remains essentiallyunperturbed by solvent polarity. However, the highestemission intensity was recorded in the non-polar cy-clohexane, whereas increase in solvent polarity re-sulted in progressive quenching in the order: cylohex-ane < chloroform < acetonitrile < methanol. This re-sult implies that the excited state in polar solvents aresignificantly deactivated via non-radiative proc-esses". Furthermore, in methanol solvent an addi-tional non-radiative pathway involving H-bonding isavailable to diminish the fluorescence intensity.

MASHRAQUI et al.: SYNTHESIS OF BENZOPYRANONE CROWN ETHER 1271

Table 1- UV-visible and emission spectral data of 1 in different solvents

Solvent Cyclohexane Chloroform Acetonitrile Methanol

A.b (nm) 306 309 307 306

Emax (X 104) 1.54 1.47 1.31 1.35

A..m(nm) 356 367 359 360Intensity 134 98 40 IO

Stokes shift (Anrn) 50 58 52 54

Two phase extraction profile of selected alkali andalkaline metal ion picrates alongwith the spectral be-haviour of 1in the presence of these metal ions is re-ported in Table II. The order of alkali metal ion ex-traction, K > Na > Li parallels the trend reported forthe parent Bz-15-C-5 systemll

. However, compared toBz-15-C-5, crown 1 showed relatively inferior ex-traction of alkali metal ions. Thus, while Bz-15-C-5 isreported'<'" to extract 43.1, 24.4 and 4.3% of K, Naand Li ions, respectively, for the case of crown 1 thecorresponding values determined are 20.8, 1.76 and0.45%. However, the relative extraction of K ionsover Na and Li ions by 1 (K+/Na+ '" 12 and K+/Li+ '"46) is much superior vis a vis Bz-15-C-5 (K+/Na+ '" 2and K+/Li+ '" 20). Thus, the system 1 displays moreselective extraction of K ions over Na and Li ionscompared to Bz-15-C-5. Consistent with the poor ex-tractability of alkaline earth metal ions by Bz-15-C-5,crown 1 also displayed poor extraction of Ca, Ba andMg ions. Relatively poor extraction of cations by 1compared to that of Bz-15-C-5 may be a consequenceof reduced accessibility of sterically congested crown-ether motif to metal ions.

In order to assess the potential of 1 as metal ionsensor, we recorded its UV-visible and fluorescencespectra in the presence of alkali and alkaline earthmetal perchlorates in CH3CN. The results are col-lected in Table II. However, no changes were ob-served in the positions of either t...b or lOmax up to a100-fold concentration of metal ions over that of 1.On the other hand, the fluorescence spectra revealedlittle or no change in the Aem, but slight to moderateenhancements in the fluorescence intensities (8 to27%) were observed in the concentration range of 1-2.5 x 104M, depending upon the metal ions used.Further increase in metal ion concentration led tovarying degree of quenching. The observed fluores-cence enhancements could be attributed to reducedphotoinduced electron transfer from metal-boundbenzo-crown to the excited benzopyranone chromo-

••

phore'". Since, the photophysical perturbations arerather weak, we discount the possibility of a stronginteraction between the metal ions and the benzopyra-none chromophore. The presence of a conformation-ally flexible spacer could at least in part be responsi-ble for the lack of effective electronic communicationbetween the crown ether and benzopyranone compo-nents.

In summary, we have synthesized a new fluoro-ionophore 1 by a straightforward route. Although,crown 1 exhibited lower extraction of alkali metalions relative to Bz-15-C-5, it offered superior selec-tivity for K ion extraction over Na and Li ions com-pared to that reported for Bz-15-C-5. However, thelack of substantial photophysical perturbations in thepresence of metal ions was disappointing. Work tomodify the structure of the fluoroionophore for betteroptical response is currently under progress.

Experimental SectionMetal perchlorates were prepared as described in

the literature" and dried under vacuum prior to use.Two phase metal ion extraction and optical spectralstudies were performed as described in our recentwork". The chemicals and spectral grade solventswere purchased from SO Fine Chemicals (India) andused as received. Melting points (uncorrected) weredetermined on a Gallenkamp apparatus. IR spectrawere recorded on a Shimadzu FTIR-420 spectropho-tometer. IH NMR spectra were recorded on a Bruker-AMX-500 spectrometer with TMS as an internalstandard. UV-visible spectra were taken on ShimadzuUV-visible spectrophotometer UV-2100 and Fluores-cence spectra were recorded on a Shimadzu spectro-fluoremeter RF-530 I Pc.

Hydroxychalcone 3: The known formyl-crown 2(0.7 gm, 2.5 mmoles) and 2-hydroxyacetophenone(0.34 gm, 2.5 mmoles) were added to a solution of30% KOH in ethyl alcohol (20 mL) at O°C and thereaction stirred at this temperature for 2 hr. The reac-tion mixture was then left overnight at room tern-

1272 INDIAN J. CHEM., SEe. 8, MAY 2006

System% extraction

Table 11- Metal ion extraction'" and optical spectral properties in the presence of metal ions

Metal perchlorates" Aem (nm) Intensity(% enhancements)

354 360d(-)

2.0 x JO.4M 354 456 (27)1.0 x JO.4M 356 404 (14)1.5 x 104M 355 434 (20)2.5 x JO.4M 354 416 (16)2.0 x 104M 353 390 (8)2.5 x JO.4M 354 412 (15)

11+ Li+1 + Na+1+ K+1+ Ba2+

1 +Mg2+

1+ Ca2+

0.451.76

20.81.3

0.49

0.51

Two phase extraction was performed using metal pictrate salts in CH2CI2.

For Bz-15-C-5, the reported extraction for Li+, Na+ and K+ are 4.4, 24.4 and 43.1 %, respectively.Concentration of metal perchlorates at which highest emission intensity is observed.d) Crown concentration used is 1.0 x lO·5 M.

perature. The reaction mixture was diluted with water,acidified to pH 3 and extracted with CHCI3. The or-ganic extract was washed with water, dried over an-hydrous Na2S04 and solvent distilled-off. The crudewas purified by Si02 column chromatography (50:50pet. ether-ethyl acetate) to afford 3 as a yellow solidin 45% yield (0.47 gm), m.p. 96-98°C. IR (KBr):3500, 2800, 1680, 1590, 1540, 1510, 1480, 1390,1340,1300,1240,1140,1060,960 em". IH NMR (60MHz, CDCI3): () 3.6 (s, 2H, -OCH2), 3.8-4.2 (m, 8H, -OCH2), 6.5-7.9 (9H, Ar), 10.8 (s, 1H, OH). Found: C,66.36; H, 6.60. Calc. for C23H2607; C, 66.66; H,6.28%.

Dihydro chalcone 4: Compound 3 (300 mg, 0.7mmole) was dissolved .in 35 mL of dry ethyl acetatecontaining 100 mg of 10% Pdlcharcoal. The reactionmixture was placed under the atmospheric pressure ofhydrogen and the reaction was allowed to proceed for2hr. The reaction mixture was then filtered through aWhatman filter paper and the filtrate concentrated.Purification by a short Si02 column chromatography(60:40 pet. ether-ethyl acetate) afforded 0.26 gm(85%) of the product 4 as a waxy solid, m.p. 74-76°C.IR (KBr): 3400, 2900, 2800, 1660, 1620, 1480, 1390,1280, 1100, 980 em". 'H NMR (60 MHz, CDCh): 82.6-2.8 (t, 2H, Ar-CH2), 3.3-3.2 (t, 2H, -CO-CH2), 3.6(s, 8H, -OCH2), 3.8-4.2 (m, 8H, -OCH2), 6.5-7.7 (9H,Ar), 9.7 (s, lH, -OH); Found: C, 66.50; H, 7.09. Calc.for C23H2807;C, 66.35; H, 6.73%.

Benzopyranone crown ether 1: Dihydro deriva-tive 4 (150 mg, 0.35 mmole) and N,N-dimethyl-form amide dimethyl acetal (2 mL) were heated to-gether at 120-125°C for 2 hr under N2 atmosphere.The reaction mixture was poured into 50 mL of 10%

HCI and extracted with CHCh. The organic extractwas washed thoroughly with water and dried overanhydrous Na2S04. The solvent was stripped off andthe crude purified by Si02 column chromatography

. (60:40 pet. ether-ethyl acetate) to give crown ether 1in 72% yield (0.11 gm) as a pale yellow solid, m.p.65-67°C. IR (KBr): 2900, 2800, 1650, 1630, 1600,1490, 1400, 1370, 1280, !220, 1140 em". 'H NMR(400 MHz, CDCh): 83.77-3.67 (m, 8H, -OCH2), 3.83(s, 2H, -CH2), 3.90-3.96 (m, 4H, -OCH2), 4.1-4.22 (m,4H, -OCH2), 6.78 (d, IH), 6.88 (d, IH), 6.97 (d, IH),7.35-7.44 (m, 2H), 7.63-7.65 (m, 2H), 8.23 (d, IH).Found: C, 67.38; H, 6.29. Calc. for C24H2607; C,67.61; H, 6.10%; ElMS: 450(M+).

AcknowledgementAuthors thank the B.R.N.S., Government of India

for a generous research grant and Prof. R. M. Kellogg,University of Groningen, the Netherlands for provid-ing the NMR spectra and his keen interest in ourwork.

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MASHRAQUI et al.: SYNTHESIS OF BENZOPYRANONE CROWN ETHER

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