New quinoline α-diimine ligands as fluorescent probes for metal ions: Ultrasound-assisted and conventional synthetic methods

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<ul><li><p>resnt</p><p>loso, 32de Nrue</p><p>noqacte56 aZn</p><p>favorable geometry for coordinating two metal cations, fact that is conrmed by the synthesis of thedinuclear complex (4), with similar molecular geometry.</p><p> 2011 Elsevier B.V. All rights reserved.</p><p>quinoich alklogicauticals</p><p>titative chemical determinations of Zn(II) and other metal ions[1618]. These quinolines are non-uorescent, and the presenceof an intramolecular hydrogen bond between the heterocyclicnitrogen atom and the 8-substituted group (i.e. OH or NH2)</p><p>skeleton. Classical and ultrasound-assisted synthetic methods willalso be discussed.</p><p>2. Experimental</p><p>2.1. General</p><p>All reactions andmanipulations of solutionswere performed un-der an argon atmosphere using Schlenk techniques, except whennoted. Organic reagents and transition-metal salts were purchased</p><p> Corresponding authors. Address: REQUIMTE, Departamento de Qumica, Fac-uldade de Cincias e Tecnologia, Universidade Nova de Lisboa, Caparica 2829-516,Portugal. Tel.: +34 988368894; fax: +34 988387001 (B. Pedras), tel.: +351212948559; fax: +351 212948550 (T. Avils).</p><p>Inorganica Chimica Acta 381 (2012) 143149</p><p>Contents lists available at</p><p>Inorganica Ch</p><p>w.eE-mail addresses: b.pedras@dq.fct.unl.pt (B. Pedras), tap@dq.fct.unl.pt (T. Avils).aminoquinoline, and some of its derivatives are strong drugs usedto treat malaria and other parasitic diseases, such as leishmaniasis[29]) and they are also widely used in synthetic chemistry [10].</p><p>In 1971, Zatka et al. reported the synthesis of a tetradentate b-diketiminate ligand that contained quinoline pendant arms [11].UVVis spectroscopy studies showed that this ligand formed 1:1complexes with Zn(II) and Cd(II). Work done in the following yearsconrmed the afnity of quinoline ligands for Zn(II), producingcomplexes of high stability and high molar absorptivity values[1215].</p><p>8-Aminoquinoline and 8-hydroxyquinoline (and theirderivatives) have found application as uorogenic sensors for quan-</p><p>(which come from a fusion of pyridine with aniline to form a N,N0</p><p>chelating motive), 8-aminoquinoline and its derivatives have re-cently been given credit for their antiprotozal and other medicinalproperties [22]. They also have been used to prepare highlyconducting co-polymers [23]. Different functionalized moleculesof 8-aminoquinoline have been recently reported [2426].</p><p>As part of our ongoing research project in the design and syn-thesis of new versatile uorescent chemosensors [27] (uorescentprobes in solution and active-recognition matrices in gas-phase),we report here the synthesis, characterization and metal ion inter-action studies of a new family of uorescent compounds (1)(4),containing one or two emissive quinoline units in a Schiff-baseKeywords:Quinolinea-DiimineChemosensorsZn(II)Cu(II)</p><p>1. Introduction</p><p>Manynatural products include theing block in their architecture, of whNumerous quinolines exhibit physiothe door to applications in pharmace0020-1693/$ - see front matter 2011 Elsevier B.V. Adoi:10.1016/j.ica.2011.08.063linemolecule as a build-aloids [1] are examples.l activities that opened(e.g. primaquine, an 8-</p><p>[19,20] allow these compounds to remain unaffected by pH changes[21]. However, when these compounds chelate Zn(II) and otherme-tal ions, they present some intense yellow-greenish uorescence.Cations like Zn(II), Ca(II) and Al(III) can break hydrogen bondingin these compounds, and therefore enhance uorescence, whichpermits their selective detection. Besides its chelating propertiesSpecial Issue monitoring the changes in absorption and uorescence spectra, and complemented by calculation ofmetalligand stability constants. The results indicate that compound (3) is the one that presents the mostNew quinoline a-diimine ligands as uoUltrasound-assisted and conventional sy</p><p>Bruno Pedras a,b,, Vitor Rosa b, Richard Welter c, Cara Physical-Chemistry Department, Faculty of Science, Ourense Campus, University of VigbREQUIMTE, Departamento de Qumica, Faculdade de Cincias e Tecnologia, Universidac Laboratoire DECOMET, Institut de Chimie, CNRS UMR 7177, Universit de Strasbourg, 4</p><p>a r t i c l e i n f o</p><p>Article history:Available online 17 September 2011</p><p>Fluorescence Spectroscopy: from SingleChemosensors to Nanoparticles Science </p><p>a b s t r a c t</p><p>Three new emissive 8-amisynthesized and fully charcence emission between 4fraction. The effect upon</p><p>journal homepage: wwll rights reserved.cent probes for metal ions:hetic methods</p><p>Lodeiro a,b, Teresa Avils b,004 Ourense, Spainova de Lisboa, Caparica 2829-516, PortugalBlaise Pascal, F 67070 Strasbourg, France</p><p>uinoline derived probes (1)(3) and one dinuclear Zn(II) complex (4) wererized. Their absorption spectra show maxima at 310336 nm, and uores-nd 498 nm. Compound (1) was characterized by single crystal X-ray dif-(II) and Cu(II) coordination to compounds (1)(3) was studied by</p><p>SciVerse ScienceDirect</p><p>imica Acta</p><p>l sevier .com/locate / ica</p></li><li><p>from Merck and Aldrich and used as received. Elemental analyseswere performed at the Analytical Services of the Laboratory ofREQUIMTE, Departamento de Qumica, Universidade Nova deLisboa, on a Thermo Finnigan CE Flash-EA 1112-CHNS instrument.IR spectra were recorded as Nujol mulls on NaCl plates using aMattson Satellite FTIR spectrometer. NMR spectra were recordedusing a Bruker AVANCE II at 400 MHz and processed with theTOPSPIN 2.0 software (Bruker). Mass spectrometry analyses wereperformed at the Analytical Services of the Laboratory of REQUI-MTE-Departamento de Qumica, Universidade Nova de Lisboa,using a MALDI-TOF-MS model voyager DE-PRO BiospectrometryWorkstation equipped with a nitrogen laser radiating at 337 nmfrom Applied Biosystems (Foster City, United States). UVVisabsorption spectra of the compounds were measured with a JAS-CO-650 UVVis spectrophotometer. Fluorescence emission spectrawere carried out with a HORIBA-JOBIN IVON Spectramax 4spectrouorometer.</p><p>All spectrophotometric and spectrouorimetric titrations forcompounds (1), (2) and (3) were performed as follows: stock solu-tions of the compound (ca. 103 M) were prepared by dissolving anappropriate amount of the compound in a 20 mL volumetric askand diluting to the mark with CH2Cl2 UVA-sol. All measurementswere performed at 298 K. The titration solutions ([C] = 1.0 105 M) were prepared by appropriate dilution of the stock solu-tions. Titrations of the compounds were carried out by addition</p><p>of microliter amounts of standard solutions of zinc(II) in the formof Zn(BF4)2 salt and copper(II) as Cu(BF4)2 salt in acetonitrile. Forcompound (4), given its poor solubility in most solvents, the stocksolution (ca. 104 M) was prepared in DMSO. Luminescence quan-tum yields of all the compounds in different solvents were mea-sured using a solution of quinine sulfate in H2SO4 (0.1 M) as astandard (UF = 0.546) [28].</p><p>2.2. Synthesis</p><p>2.2.1. Synthesis of (1)Amixture of ferrocenaldehyde (0.1 g, 0.47 mmol), 8-aminoquin-</p><p>oline (0.13 g, 0.93 mmol) and 0.5 g of silica was added to a Schlenktube, degassed for 15 min and kept under argon atmosphere. Abso-lute ethanol (5 mL) was degassed and added to the mixture, whichwas then sonicated in an ultrasonic bath at room temperature for aperiod of 1 h. After this period, the solution was ltered, and thesilica was washed with CHCl3 until a complete extraction of thereaction solution was achieved. The red solution obtained was thenevaporated to dryness, to render dark-red oil. This oily residue wasthen dissolved in a small volume of CHCl3 and by addition of petro-leum ether a precipitate was formed. It was then ltered off,washed with petroleum ether and dried under vacuum. The com-pound was isolated as a brownred solid (0.1 g, 44%). Anal. Calc.for C29H24FeN4: C, 71.90; H, 8.00; N, 11.57. Found: C, 71.60; H,</p><p>144 B. Pedras et al. / Inorganica Chimica Acta 381 (2012) 143149Scheme 1. Synthesis of quinoline-derived compounds. (i) Ethanol ultrasound; (ii) methroom temperature (vi) 2 equiv. ZnCl2, acetic acid reux.anol ultrasound; (iii) toluene reux; (iv) methanol room temperature (v) pentane</p></li><li><p>vigorous stirring at room temperature during 24 h. The solid wasrecovered by ltration of the solution and washed with pentaneand then dried under vacuum. Yield 0.14 g, 36%.</p><p>Anal. Calc. for C22H18N4: C, 78.08; H, 5.36; N, 16.56. Found: C,78.01; H, 5.47; N, 16.36%. 1H NMR (CDCl3): d = 8.76 (d, 2H, Ar);8.08 (d, 2H, Ar); 7.397.32 (m, 4H, Ar); 7.15 (d, 2H, Ar); 6.94 (d,2H, Ar) 2.33 (s, 6H, CH3) ppm. IR (Nujol mull/NaCl plates), cm1:1644 [(C@N)imine]. MALDI-TOF MS: m/z 338.19 ([3H+]).</p><p>2.2.4. Template synthesis of (4)A suspension of acenaphthenequinone (0.3 g, 1.65 mmol) and</p><p>ZnCl2 (0.45 g, 3.29 mmol) in glacial acetic acid (5 mL) was allowedto stir for 10 min, at room temperature. 8-Aminoquinoline (0.48 g,3.29 mmol) was then added, and the mixture was reuxed for 1 h,after which it was allowed to cool to room temperature. The solidwas recovered by ltration of the solution, washed with glacial ace-tic acid (3 5 mL), dichloromethane (3 5 mL), diethyl ether</p><p>himica Acta 381 (2012) 143149 1455.11; N, 11.31%. 1H NMR (CD2Cl2): d = 8.93 (d, 1H, Ar); 8.75 (d, 1H,Ar); 8.21 (d, 1H, Ar); 8.11 (d, 1H, Ar); 7.66 (d, 1H, Ar); 7.57 (d, 1H,Ar); 7.467.32 (m, 3H); 7.25 (d, 1H, Ar); 7.17 (d, 1H, Ar); 6.94 (d,1H, Ar); 5.03 (br s, 2H, NH); 4.88 (s, 2H, CpH); 4.55 (d, 2H, CpH);4.39 (s, 5H, CpH) ppm. IR (Nujol mull/ NaCl plates), cm1: 3382[m(NAH)secondary amine]. MALDI-TOF MS: m/z 341.1 [1-C9H7N2]+</p><p>(where C9H7N2 = a quinoline pendant arm).</p><p>2.2.2. Synthesis of (2)2.2.2.1. Method A. A mixture of acenaphthenequinone (0.1 g,0.55 mmol), 8-aminoquinoline (0.08 g, 0.55 mmol) and 0.5 g of sil-ica was added to a Schlenk tube, degassed for 15 min and kept un-der argon atmosphere. Methanol (5 mL) was degassed and addedto the mixture, followed by a few drops (ca. 0.5 mL) of formic acid.The suspension was then sonicated in an ultrasonic bath at roomtemperature for a period of 1 h, after which the solution was l-tered off, and the residue washed with MeOH, leaving a brown so-lid mixed with the silica. It was then extracted with CHCl3 and thesolvent removed by reduced pressure, rendering a greenbrownsolid (0.043 g, 26%).</p><p>2.2.2.2. Method B. A mixture of acenaphthenequinone (0.1 g,0.55 mmol), 8-aminoquinoline (0.08 g, 0.55 mmol), and a fewdrops of formic acid, in toluene (15 mL) were reuxed during24 h. The solution was ltered, and then the solvent was removed</p><p>Table 1Absorption and emission maxima and molar absorption coefcients of compounds(1)(4).</p><p>Compound kabsmax/nm emax/M1 cm1 kemmax/nm Stokes shift/nm</p><p>1 336 11428 456 1202 322 9151 498 1763 313 6344 461 1484 310 14223 488 178</p><p>Table 2Fluorescence quantum yields of compounds (1)(4), in different solvents. Parent:quinine sulfate in 0.5 M H2SO4.</p><p>1 2 3 4</p><p>DMSO 0.078 0.022 0.053 0.075THF 0.005 0.032 0.008 Acetonitrile 0.002 0.026 0.016 Ethyl acetate 0.004 0.024 0.016 Methanol 0.001 0.009 0.006 Dichloromethane 0.012 0.026 0.011 </p><p>B. Pedras et al. / Inorganica Cunder reduced pressure. The resultant greenbrown solid waswashed with pentane and dried under vacuum. Yield 0.035 g, 21%.</p><p>Anal. Calc. for C21H12N2OMeOH: C, 77.63; H, 4.74; N, 8.23.Found: C, 77.63; H, 4.33; N, 7.89%. 1H NMR (CDCl3): d = 8.75 (m,1H, Ar); 8.27 (d, 2H, Ar); 8.118.04 (m, 3H, Ar); 7.84 (t, 2H, Ar);7.377.30 (m, 2H, Ar); 7.14 (d, 1H, Ar); 6.92 (d, 1H, Ar) ppm. IR (Nu-jol mull/NaCl plates), cm1: 1667 [(C@N)imine]. MALDI-TOF MS:m/z308.1 ([2H+]); 617.3 ([2(2H+)]).</p><p>2.2.3. Synthesis of (3)2.2.3.1. Method A. To a solution of 8-aminoquinoline (0.33 g,2.3 mmol) in methanol (10 mL) was added 2,3-butadione (0.1 mL,1.15 mmol) and the mixture was left under stirring for 12 h atroom temperature. The solvent was then evaporated under re-duced pressure. The residue was dissolved in a small volume ofdichloromethane, and slowly precipitated by addition of pentane.The solid was recovered by ltration of the solution, washed withpentane and dried under vacuum. The compound was isolated as adark-purple solid (0.16 g, 42%).</p><p>2.2.3.2. Method B. A mixture of 8-aminoquinoline (0.33 g,2.3 mmol) and 2,3-butadione (0.1 mL, 1.15 mmol) was left under(3 5 mL) and dried under vacuum. An orange solid was obtained(1.11 g, 95%). Solubility: only partially soluble in DMSO and DMF.Anal. Calc. for C30H18N4Zn2Cl4: C, 50.96; H, 2.57; N, 7.92. Found: C,50.84; H, 2.74; N, 7.73%. 1H NMR (DMF-d7): d = 8.76 (m, 2H, Ar);8.50 (d, 1H, Ar); 8.23 (d, 2H, Ar); 8.14 (d, 1H, Ar); 8.03 (m, 4H,Ar); 7.48 (m, 2H, Ar); 7.33 (t, 2H, Ar); 7.13 (d, 2H, Ar); 6.99 (d, 2H,Ar) ppm. IR (Nujol mull/NaCl plates), cm1: 1623 [(C@N)imine].</p><p>2.3. Crystal structure determination</p><p>A single crystal of compound (1), was mounted on a NoniusKappa-CCD area detector diffractometer (MoKa k = 0.71073 ).The complete conditions of data collection (Denzo software) andstructure renements are given below. The cell parameters weredetermined from reections taken from one set of 10 frames(1.0 steps in phi angle), each at 20 s exposure. The structure wassolved using direct methods (SHELXS97) and rened against F2 usingthe SHELXS97 software [29]. The absorption was not corrected. Allnon-hydrogen atoms were rened anisotropically. Hydrogenatoms were generated according to stereo-chemistry and renedusing a riding model in SHELXS97.</p><p>3. Results and discussion</p><p>3.1. Synthesis and characterization of compounds (1)(4)</p><p>The initial focus of the synthetic part of this work was the devel-opment of an optimized set of reaction conditions, by means of</p><p>Fig. 1. ORTEP view (ATOMS [32]) of compound (1) with partial labeling scheme.</p><p>Only one molecule of the asymmetric unit is presented, for clarity. The ellipsoidsenclose 50% of the electronic density. Dashed lines indicate intramolecularhydrogen bonds.</p></li><li><p>1-aminonaphthalene ones [30], we can observed that we do notachieve the same yields, with or without assisted ultrasound</p><p>Fig. 2. Packing diagram of compound (1) in projection in the (a and c) sheet. Only iron atoms are represented as spheres.</p><p>Table 3Analysis of hydrogen bonds in compound 1 with PLATON [33] (equivalent position</p><p>146 B. Pedras et al. / Inorganica Chimica Acta 381 (2012) 143149code: $1 = 1 x, 1 y, z; $2 = x, 2 y, z).</p><p>Donor H acceptor DH H A D A DH Aintramolecular</p><p>N(3) H(3N) N(4) 0.8802 2.3439 2.688(8) 103.40N(6) H(6N) N(1) 0.8795 2.2921 2.676(9) 106.34N(7) H(7N) N(2) 0.8793 2.3108 2.681(8) 105.38N(8) H(8) N(5) 0.8799 2.3588 2.681(10) 101.78</p><p>intermolecularC(22) H(22) N4_$1 0.9501 2.5787 3.441(11) 151.12C(52) H(52) N5_$2 0.9501 2.6078 3.503(9) 157.19reduction of reaction times without yield decrease, and wheneverpossible by improving the latter. For such a task, the synthesis ofcompounds (1) and (2) was ultrasound-assisted, which allowedthe reactions to be completed in 1-h periods, and the yields weresomehow satisfactory. For compound (3) the ultrasound assistedsynthesis gave inconclusive results, giving a mixture of productsthat we were not able to separate and characterize. For (4) a tem-plate synthesis was required, with reuxing glacial acetic acid, andit did not work in the ultrasonic bath. However, i...</p></li></ul>