Polyhedron 61 (2013) 161–171

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Polyhedron

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Benzazolate complexes of pentacoordinate nickel(II). Synthesis,spectroscopic study and luminescent response towards metal cations

0277-5387/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.poly.2013.05.047

⇑ Corresponding authors. Tel.: +34 868887458; fax: +34 868884148 (M. DoloresSantana).

E-mail addresses: dsl@um.es (M. Dolores Santana), ilgarcia@um.es (I. López-García).

M. Dolores Santana a,⇑, Rocío García-Bueno a, Gabriel García a, María José Piernas a, José Pérez b,Luís García b, Ignacio López-García c,⇑a Departamento de Química Inorgánica, Universidad de Murcia, E-30071 Murcia, Spainb Departamento de Ingeniería Minera, Geológica y Cartográfica, Área de Química Inorgánica, Universidad Politécnica de Cartagena, E-30203 Cartagena, Spainc Department of Analytical Chemistry, Faculty of Chemistry, University of Murcia, E-30071 Murcia, Spain

a r t i c l e i n f o

Article history:Received 5 December 2012Accepted 23 May 2013Available online 1 June 2013

Keywords:Macrocyclic ligandsNickelLuminescenceNMR spectroscopyN,O ligands

a b s t r a c t

[Ni(mcN3)(hpbY)](PF6) (1a–3b) [mcN3 = Me3–mcN3 (2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene);Me4–mcN3 (its 9-methyl derivative); hpbY = hpbm (2-(20-hydroxyphenyl) benzimidazolate); hpbx(2-(20-hydroxyphenyl) benzoxazolate); hpbt, 2-(20-hydroxy phenyl)benzothiazolate] were synthesizedvia acid–base method and structurally characterized. The complexes exhibit moderate fluorescenceintensities. Hetero-atoms of benzazole ligands present in the Ni(II) complexes influence their lumines-cent properties. These complexes are used for the recognition of different cations of biological and envi-ronmental interest. The study of luminescence properties of these complexes reveals that they displaygood sensitivity towards transition metal ions ZnII, CuII and HgII. The complexes act as sensitive fluores-cent ON�OFF probe for CuII. Other metal ions such as HgII and ZnII gave an enhanced fluorescenceresponse.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The design and synthesis of fluorescent sensors for the recogni-tion of cations with biological and environmental interest arebecoming one of the most challenging realms, not only from theviewpoint of chemistry but also from their potential applications[1]. Thus, zinc is the second most abundant transition metal inmany biological systems behind iron, and it plays diverse rolessuch as regulation of enzymes, DNA binding or recognition,structural cofactors, neural signal transmission, and others [2].However, many important processes of zinc are still poorly under-stood [3] in addition, the sensing of zinc is spectroscopically ormagnetically silent because of its electronic configuration [4].Therefore, the detection of zinc is an interesting field of research.Furthermore, it is still a challenge to develop chemosensors thatcan discriminate zinc from cadmium [5], because both of themare in the same group and have similar properties, which usuallycause similar spectral changes after interacting with chemosen-sors. Copper is not only a significant metallic pollutant but alsoan essential element for living organisms [6]. Deficiency of copperor zinc can lead to growth or metabolism disorders [7]. Mercury is

one of the heavy metals more contaminant with chemical andphysical complex properties [8]. It is well-known that their ionstend to bioaccumulate in organisms causing dangerous intoxica-tions [9–11]. Due to their toxic effects and importance to organ-isms, it is important to develop new chemosensors to able tomonitor the presence of copper [12], zinc [13], cadmium [14]and mercury [15], quantitatively.

The use of metal complexes in the design of luminescentresponsive probes has been developed over the last years, sincethey potentially offer improved photophysical attributes over or-ganic fluorophores purely [16]. There have been a number of excel-lent reviews on transition metal complexes as ion and moleculesensors including several review articles on the metallo-supramo-lecular chemistry that describe their use in chemical sensing[16a,c]. The binding action of these complexes can be monitoredby the changes in the optical properties of the systems. On theother hand, it has been shown in previous reports [17,18] thatthe benzimidazole-based derivatives are a novel class of N,O-donorligands that could form sublimable luminescent complexes as pos-sible electroluminescent (EL) materials. Similarly, luminescentproperties of related ligands, containing different heteroatoms(NH, O, and S) in the aromatic rings can be expected [18b]. In thispaper, we report the syntheses, crystal structures, and luminescentproperties of pentacoordinate Ni(II) complexes of aromaticN,O-chelate ligands, namely benzazoles, in general, [Hhpbm,2-(20-hydroxyphenyl)benzimidazole, Hhpbx, 2-(20-hydroxyphenyl)

Chart 1. Ligands used in this work.

162 M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171

benzoxazole, and Hhpbt, 2-(20-hydroxyphenyl)benzothiazole](Chart 1). Extensive spectroscopic characterization and study ofphotophysical properties is also presented. The electronic transi-tions in the photoluminescent process, as well as the spectral vari-ations along with the effect of complexation and effect of differentheteroatoms were studied. The aim was to synthesize functionalluminescent complexes, which display variable response in pres-ence of a given cation of physiological and toxicological impor-tance such as ZnII, CdII, HgII or CuII. The versatility and syntheticaccessibility of these complexes makes especially attractive thesecation receptors.

2. Experimental

All chemicals were of reagent grade and were used without fur-ther purification. Solvents were dried and distilled by generalmethods before use. The complexes [Ni(mcN3)(l-OH)]2(PF6)2

(mcN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me3–mcN3) and its 9-methyl derivative (Me4–mcN3)) were preparedby the previously described procedure [19].

2.1. Physical techniques

C, H, N and S analyses were carried out with a microanalyzerCarlo Erba model EA 1108. IR spectra were recorded on a Perkin-El-mer 16F PC FT-IR spectrophotometer using Nujol mulls betweenpolyethylene sheets. Conductance measurements were performedwith a Crison 525 conductimeter (in acetone solution, c � 10�3 M).ESI-MS analyses were performed on an Agilent VL mass spectrome-ter. The ionization mechanism used was electrospray in positiveand negative ion full scan mode using acetonitrile as solvent andnitrogen gas for desolvation. The NMR spectra of (CD3)2CO or CD3-

CN solutions were recorded on a Bruker spectrometer (AC 200E orAC 300E). {1H–1H} COSY spectra were recorded on the Bruker200 MHz spectrometer at 20 �C in CD3CN solutions with 512 datapoints in the F1 dimension and 1024 data points in the F2 dimen-sion with a delay time of 500 ms. An unshifted sine-bell-squaredweighting function was applied prior to Fourier transformationfollowed by baseline correction in both dimensions and symmetri-zation. The UV–Vis spectra (in acetonitrile) were recorded on aUNICAM UV 500 spectrophotometer for 230–800 nm range. DRUVspectra were recorded with a Unicam UV-4 spectrophotometerequipped with a Spectralon RSA-UC-40 Labspere integratingsphere. The solid samples were mixed with dried silica gel to forman homogeneous powder. The mixtures were placed in a home-made cell equipped wtih a quartz window. The intensities were re-corded in Kubelka–Munk units: log[R/(1 � R)2], were R = reflec-tance. Excitation and emission spectra were recorded on a JobinYvon Fluorolog 3-22 spectrofluorometer with a 450 W xenon lampdouble-grating monochromators and a TBX-04 photomultiplier.The solution measurements were carried out in a right angle con-figuration using degassed acetonitrile solutions of the samples in10 mm quartz fluorescence.

2.2. X-ray data collection and structure determination

Diffraction data were collected in Bruker Smart Apex diffrac-tometer with graphite-monochromated Mo Ka radiation(k = 0.71073 Å). The diffraction frames were integrated using theSAINT package [20] and corrected for absorption with SADABS [21].Crystallographic data are shown in Table S1. The structures weresolved by direct methods and refined anisotropically on F2 [22].Hydrogen atoms were introduced in calculated positions.

2.3. Syntheses

In separate experiments, the corresponding benzazole(0.232 mmol) was added to a solution of [Ni(Me3–mcN3)(l-OH)]2(-PF6)2 (100 mg, 0.116 mmol) in acetone (20 mL). The reaction mix-ture was stirred at room temperature for 2 h. The solution wasconcentrated to approximately 5 mL and Et2O (25 mL) was added.The resulting precipitate was filtered and the green solid waswashed with several aliquots of diethyl ether and vacuum-driedto give the expected complexes 1a–3a. The powder isolated in eachcase was air stable.

The experimental procedure was similar to that describedabove using [Ni(Me4–mcN3)(l-OH)]2(PF6)2 (100 mg, 0.112 mmol)and the corresponding benzazole (0.224 mmol) in acetone(20 mL) to prepare complexes 1b–3b. The green powder isolatedin each case was air stable.

[Ni(Me3–mcN3)(hpbm)](PF6) (1a): Green solid, Yield: 120.7 mg,83%; elemental Anal. Calc. for C25F6H36N5NiO2P: C, 46.7; H, 5.6;N, 10.9. Found: C, 46.5; H, 5.5; N, 10.9%. KM: 115.8 S cm2 mol–1.MS-ESI: m/z (%): 478.2 (100) [M]+. IR (Nujol): m = 3379, 3262,3168, 1652, 1603, 1549, 841, 558 cm�1.

[Ni(Me3–mcN3)(hpbx)](PF6) (2a): Green solid, Yield: 112.6 mg,77%; elemental Anal. Calc. for C28F6H39N4NiO3P: C, 49.2; H, 5.7;N, 8.2. Found: C, 49.1; H, 5.6; N 8.5%. KM: 117.9 S cm2 mol–1. MS-ESI: m/z (%): 479.2 (100) [M]+. IR (Nujol): m = 3274, 3255, 1654,1614, 1552, 836, 558 cm�1.

[Ni(Me3–mcN3)(hpbt)](PF6) (3a): Green solid, Yield: 129.9 mg,87%; elemental Anal. Calc. for C25F6H33N4NiOPS: C, 46.8; H, 5.2;N, 8.7; S, 5.0. Found: C, 46.5; H, 5.5; N, 8.5; S, 4.8%. KM:134.6 S cm2 mol–1. MS-ESI: m/z (%): 495.1 (100) [M]+. IR (Nujol):m = 3260, 3246, 1655, 1600, 1540, 840, 558 cm�1.

[Ni(Me4–mcN3)(hpbm)](PF6) (1b): Green solid, Yield: 110.6 mg,77%; elemental Anal. Calc. for C26F6H36N5NiOP: C, 48.9; H, 5.7; N,10.9. Found: C, 49.2; H, 5.7; N, 10.5%. KM: 120.6 S cm2 mol–1. MS-ESI: m/z (%): 492.2 (100) [M]+. IR (Nujol): m = 3371, 3295, 1628,1603, 1549, 837, 558 cm�1.

[Ni(Me4–mcN3)(hpbx)](PF6) (2b): Green solid, Yield: 133.6 mg,93%; elemental Anal. Calc. for C26F6H35N4NiO2P: C, 48.9; H, 5.5;N, 8.8. Found: C, 48.9; H, 5.4; N, 8.9%. KM: 131.1 S cm2 mol–1.MS-ESI: m/z (%): 493.0 (100) [M]+. IR (Nujol): m = 3292, 1629,1615, 1554, 840, 557 cm�1.

[Ni(Me4–mcN3)(hpbt)](PF6) (3b): Green solid, Yield: 123.9 mg,84%; elemental Anal. Calc. for C26F6H35N4NiOPS: C, 47.7; H, 5.4;N, 8.5; S, 4.9. Found: C, 47.3; H, 5.5; N, 8.6; S, 4.8%. KM:128.6 S cm2 mol–1. MS-ESI: m/z (%): 509.1 (100) [M]+. IR (Nujol):m = 3249, 1646, 1599, 1541, 841, 558 cm�1.

3. Results and discussion

3.1. Synthesis and crystal structures

The complexes under investigation [Ni(mcN3)(hpbY)](PF6)(1a–3b) were synthesized by acid–basic reaction between[Ni(mcN3)(l-OH)]2(PF6)2 [19] and the corresponding benzazole(Chart 1) in acetone at room temperature, as depicted in Scheme 1.

Scheme 1. Reaction scheme.

Fig. 1. ORTEP plot of the cation of [Ni(Me3–mcN3)(hpbm)](PF6) 1a.

Fig. 2. ORTEP plot of the cation of [Ni(Me3–mcN3)(hpbx)](PF6) 2a.

Table 1Selected bonds (Å) and angles (�) for 1a and 2a.

1a 2a

Ni(1)–O(1) 2.006(2) 1.990(2)Ni(1)–N(1) 2.051(3) 2.037(3)Ni(1)–N(3) 2.051(3) 2.047(3)Ni(1)–N(4) 2.036(3) 2.060(3)Ni(1)–N(2) 2.058(3) 2.056(3)O(1)–Ni(1)–N(1) 175.6(2) 174.86(10)O(1)–Ni(1)–N(3) 92.73(11) 93.43(10)N(1)–Ni(1)–N(3) 89.14(12) 89.64(11)O(1)–Ni(1)–N(4) 85.12(11) 87.03(10)N(1)–Ni(1)–N(4) 98.56(12) 95.70(11)N(3)–Ni(1)–N(4) 103.04(12) 111.35(11)O(1)–Ni(1)–N(2) 84.93(11) 83.79(10)N(1)–Ni(1)–N(2) 90.61(12) 91.62(11)N(3)–Ni(1)–N(2) 99.75(12) 100.17(11)N(4)–Ni(1)–N(2) 155.49(11) 147.62(11)

M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171 163

The crystal structures of the cations of complexes 1a and 2a areshown in Figs. 1 and 2 and selected bond lengths and bond anglesin Table 1.

The coordination geometry around the nickel atom is distortedsquare pyramidal for 1a and 2a; this can be ascertained by theAddison’s s parameter [23] this parameter shows a value of 0.33and 0.45 for complexes 1a and 2a respectively. The three nitrogenatoms of the macrocyclic ligand constitute a trigonal face of thepyramid. The O1 and N4 atoms from the benzazolate and twonitrogens (N1 and N2) of the macrocycle form the basal plane.The nickel ion lies above this plane and is displaced 0.2168(15) Å(1a) and 0.3070(14) Å (2a) toward the N(3) nitrogen, which consti-tutes the axial donor. The Ni–N distances (2.060–2.036 Å) liewithin the range reported in complexes containing the {Ni(mcN3)}moiety [24]. The dihedral angles between the phenolate(O1,C13,C18,C19 and N4) and heterocycle (N4,C19,C20,C21,N5or O2) moieties of 1a and 2a are 19.69(19)� and 26.49(9)�, respec-tively, therefore away from planarity. In these complexes, there aresome intra and intermolecular hydrogen bonding in their struc-tures of 1a and 2a (Table S2 in Supplementary data).

3.2. 1H NMR Spectroscopy

Compounds 1a–3b were characterised by IR and 1H NMR spec-troscopies, absorption and fluorescence measurements. 1H NMR

Table 2Chemical Shift Data (ppm) for complexes 1a - 3b.

Complex Macroclicic Ligands Benzazolate Ligands

Ha Hb 4-Me

4-Me

2-Me 9-Me H1 H2 H3 H4 H5 H6 H7 H8

1a 214.8, 168.3, 121.9, 115.1, 83.9,75.1, 37.0, 33.4

0.8, �10.5, �10.8, �27.2,�31.2 (2H)

42.8 22.0 �14.1 – �13.0 8.2 �6.0 4.1 14.8 15.7 26.2 31.5

2a 227.7, 186.3, 125.2, 121.1, 91.7,49.9, 37.2 (2H)

�8.8, �10.8, �13.0, �28.2,�31.9 (2H)

44.2 21.4 �14.2 – �4.7 8.8 �3.8 4.2 13.7 17.5 25.1 32.6

3a 348.8, 328.3, 236.1, 235.5, 171.4,121.5, 105.2, 82.1

�11.2 (2H), �13.2, �26.8,�29.4, �30.5

43.7 21.1 �14.2 – �5.6 5.3 1.2 3.4 9.1 11.6 23.6 28.4

1b 270.5, 242.8, 217.4, 165.2, 117.8,91.3, 63.6, 46.1

�4.3, �5.6, �19.9, �24.0,�28.7, �29.4

29.6 19.8 �13.6 107.7 �9.7 15.1 4.0 9.7 16.3 24.4 27.9 34.8

2b 281.0, 252.1, 218.8, 180.8, 127.5,95.6, 65.5, 51.6

�2.7, �4.5, �5.5, �20.7,�24.6, �28.5

30.1 19.8 �13.4 111.0 �7.1 14.8 4.3 9.7 16.1 23.7 27.5 35.3

3b 275.0, 251.9, 174.2, 127.5, 107.3,90.0, 61.1, 54.4

�4.6, �10.0, �11.0, �26.9,�28.8, �33.2

29.7 17.1 �14.4 115.1 �5.7 13.7 5.2 11.0 14.6 21.5 24.3 31.6

164 M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171

spectroscopy has been shown to be a powerful tool for the charac-terisation of five-coordinate nickel(II) complexes [24]. The 11HNMR spectra of complexes 1a–3b exhibit sharp isotropicallyshifted signals from 350 ppm (down field) to �35 ppm (up field),in acetone or acetonitrile solution (Table 2). The spectra showthe resonance line pattern observed for the mcN3 ligands thatwas assigned on the basis of previous studies of nickel macrocycliccomplexes [24]. A representative proton NMR spectrum for com-plex 1a is shown in Fig. 3.

The eight resonance signals due to a-methylene protons shiftdownfield whereas the six resonance signals for b-methylene pro-tons shift upfield with regard to the diamagnetic position becauseof a dominant spin polarization mechanism [25]. Some resonancescan occur overlapped [24–26]. The isotropically shifted 1H NMRsignals observed for the methyl groups (2-Me, 4-Me(a,b) and9-Me–N) can be initially assigned by inspection of their peak areas.

Fig. 3. 1H RMN spectrum of 1a (in [D3]

The complete assignment of these isotropically shifted signalsrequired an application of two-dimensional 1H NMR techniques(Figs. 4–6).

A magnitude {1H–1H}COSY spectrum of 3a recorded at 20 �C(Figs. 5 and 6), clearly shows cross signals between the resonancesat �5.6 and 5.3 ppm and also between resonances at 1.2, 3.4 and5.3 ppm. These signals can be assigned to H1 (�5.6), H2 (5.3), H3

(1.2) and H4 (3.4) protons of the benzazolate ligand, respectively(Fig. 4).

{1H–1H}COSY spectrum of 3a also shows cross signals betweenresonances at 9.1 and 11.6 ppm which are assigned to H5 and H6

protons, respectively. Furthermore, cross signals between reso-nances at 11.6, 23.6 and 28.4 ppm are observed; these signals areassigned to H6, H7 and H8 protons, respectively (Fig. 6).

All assignments are consistent with {1H–1H} COSY spectra ofcomplexes 1a–3b, see Table 2. All the resonances of the heterocyclic

acetonitrile at room temperature).

Fig. 4. Portion of 1H RMN spectrum of 1a (in [D3]acetonitrile at room temperature), only the region relevant to assign benzazolate resonances is shown.

Fig. 5. Portion of the {1H–1H} COSY spectrum of complex 3a (in [D3]acetonitrilesolution at 20 �C, 300 MHz). Only the relevant region to assign benzazolateresonances (H1–H4) is shown in the top trace.

Fig. 6. Portion of the {1H–1H} COSY spectrum of complex 3a (in [D3]acetonitrilesolution at 20 �C, 300 MHz). Only the relevant region to assign benzazolateresonances (H5–H8) is shown in the top trace.

Table 3Bands in the absorption (acetonitrile solutions) and DRUV spectra of complexes 1a–3b at room temperature.

Complex kmax/nm (e/dm3 mol�1 cm�1) DRUV/nm

1a 287 (40707), 370 (38049) 3652a 288 (37599), 385 (30800) 293(sh), 3803a 289 (37820), 407 (24654) 324, 400(sh)1b 291 (34172), 369 (31109) 287(sh), 362

M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171 165

moiety are downfield to TMS in accordance with a dominantr-delocalization pattern of spin density consistent with the groundstate of NiII. However, the shift direction alternation of the pheno-late protons is characteristic if the p-contact shift is dominant.The net spin density in the dp orbitals could be polarized by the un-paired electrons in the dx2�y2 and dz2 orbitals via spin-orbit coupling.A similar behavior has been observed elsewhere [24e,f].

2b 289 (40805), 385 (34168) 288(sh), 3803b 288 (45014), 333 (32741), 410 (16831) 322, 401(sh)

sh = shoulder.

3.3. Absorption and diffuse-reflectance spectra

Table 3 summarizes the absorption properties of the complexes1a–3b.

As shown in Table 3, the highest energy absorption bands ofpowdered samples of 1a–3b appear between 320 and 380 nm,

such transitions can be assigned to p–p⁄ transitions of the benzaz-olate ligands [18b,27], while the corresponding ligands, Hhpbm,Hhpbx, and Hhpbt, have absorption bands at 342, 356, and

Fig. 7. Luminescent photographs of 1a, 2a and 3a in CH3CN solutions r.t. underbench UV lamp (irradiation kex = 365 nm).

Table 4Luminescent spectral data for compounds 1a,b–3a,b at 298 and 77 K, in acetonitrile solut

Complex Acetonitrile solution

298 K 77 K

kmax(exc)/nm kmax(em)/nm U/%a s(ls) kmax(exc)/nm

1a 310 455 0.29 15.43(29) 3052a 330 470 0.13 7.87(18) 3203a 390 455 0.46 9.12(27) 3101b 315 455 0.36 8.14(21) 3052b 370 430 0.15 9.39(13) 3153b 380 455 0.24 5.90(16) 300

sh = shoulder.a Quantum yields were determined relative to [Ru(bpy)3](PF6)2.

Fig. 8. Emission spectra (CH3CN,

166 M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171

364 nm, respectively [18b]. Absorption spectra in solution (MeCN)display transition associated with the coordinate ligands. The highenergy bands (<400 nm) for all complexes are assigned to intra-li-gand transitions from benzazolates associated with p–p⁄ transi-tions in these ligands [18b]. However, MC and CT absorptionsmay thus be obscured by IL benzazolate bands [28,29].

3.4. Emission spectra

All the complexes studied are moderately blue-luminescent atroom temperature and at 77 K in the solid state as well as insolution (Fig. 7).

Emission spectra in acetonitrile solution at room temperature of1a–3b display a broad structureless band in the range of430–470 nm (Table 4, Fig. 8).

These emissions could be attributed predominantly to a ligand-centered (LC) excited state [18b], in fact, emission energies of thefrozen solutions (even in the solids) are red-shifted, so it is not ob-served rigidochromism. Although this phenomenon can occureven, if there is a contribution of metal-to-ligand charge transfer(MLCT) to the excited state [30]. It has been reported that thedeprotonation and coordination of organic ligands to metal ionscan reduce the energy gaps between the HOMOs and LUMOs[18a,31]. According to that, the luminescence of 1a–3b should bered-shifted compared with their corresponding ligands [32]. The

ion and solid state.

Solid state

298 K 77 K

kmax(em)/nm kmax(exc)/nm kmax(em)/nm kmax(exc)/nm kmax(em)/nm

455 320 490 320 490475, 495(sh) 310 480 340 490485 340 535 340 525455 320 500 335 490540 335 510 335 510510 350 530 350 510

298 K) of compounds 1a–3b.

Fig. 9. Fluorescence response of sensor 1a (10�5 M) in CH3CN/EtOH (10:1) solution upon addition of 1 equiv. metal ions.

M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171 167

spectra of the benzazole ligands have been included as SupportingInformation (Fig. S1). It is worth mentioning that the luminescenceof transition metal complexes are greatly affected by the accessi-bility of the metal-centered excited state through population ofempty d orbitals, which opens efficient canals of non-radiativedecay.

The luminescence of 1a–3b complexes is originated from LC ex-cited state and its quantum yield would be affected by the pres-ence of a d–d metal-centered state but, maybe, not enough toquench it completely. Luminescence quantum yields and lifetimesin degassed acetonitrile solutions at room temperature are set outin Table 4. Complex 2a has the lowest quantum yield and relativelyshort lifetime but 3b has the shortest lifetime and high quantum

Fig. 10. Fluorescence response of sensor 2a (10�5 M) in CH3CN/

yield. Similar trend has also been observed for p–p⁄ and ILCT tran-sitions on nickel(II) complexes [33,34].

3.5. Luminescent properties towards cations

The presence of different heteroatoms (NH, O or S) in benzazolerings may lead to different electronic effects that can affect theluminescent properties of complexes 1a–3b, to highlight theseeffects and use them to recognize cations, complexes 1a–3a weretreated with different metal cations of biological interest, includingalkali and alkaline earth, copper and zinc, and other cations ofenvironmental importance such as cadmium, mercury and silver.The changes of the fluorescence properties of 1a–3a were

EtOH (10:1) solution upon addition of 1 equiv. metal ions.

Fig. 11. Fluorescence response of sensor 3a (10�5 M) in CH3CN/EtOH (10:1) solution upon addition of 1 equiv. metal ions.

Fig. 12. Fluorescence titration spectra upon addition of ZnII ion (0.0–2.0 equiv.) to a solution of 1a (10�5 M) in CH3CN/EtOH (10:1).

Fig. 13. Fluorescence titration spectra upon addition of ZnII ion (0.0–4.0 equiv.) to a solution of 2a (10�5 M) in CH3CN/EtOH (10:1).

168 M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171

Fig. 14. Fluorescence titration spectra upon addition of ZnII ion (0.0–1.0 equiv.) to a solution of 3a (10�5 M) in CH3CN/EtOH (10:1).

Scheme 2. Interaction of metal cations with the heterocycle of the benzazole.

M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171 169

investigated after addition of a range of 1 equiv. cations includingNaI, KI, MgII, BaII, ZnII, CdII, HgII, CuII and AgI in CH3CN/EtOH(10:1) solution at 20 �C. The metal ions that were investigatedcan be divided into three groups. Group 1 consisted of NaI, KI, MgII,BaII, CdII and AgI in which a slight decrease in the fluorescence re-sponse was observed; groups 2 and 3 formed by ZnII, HgII and CuII

which display quenching or enhanced fluorescence responsedepending of the complex used because of the different heteroat-oms in the same site of the aromatic ring. The fluorescence re-sponse of selected metal ions with complexes 1a, 2a and 3a areshown in Figs. 9–11.

The chemosensory behavior of 1a is depicted in Fig. 9: the CH3-

CN/EtOH (10:1) solution of 1a displayed a remarkable fluorescentemission quenching (at 455 nm) when ZnII or CuII was added; bycontrast when HgII or ZnII was added into the solution of 2a (or3a) was observed a emission increase and upon addition of CuII

into the solution of 2a (or 3a) a significant fluorescent emissionquenching was observed (Figs. 10 and 11).

By contrast, relatively insignificant spectral changes were ob-served upon addition of most of the other metal ions, so it can beconcluded that 1a, 2a and 3a have a high selectivity for recognitionof CuII, ZnII and HgII, even the compounds 1a, 2a and 3a have a dif-ferent response to ZnII or CdII, therefore compounds 1a, 2a and 3adistinguish between both cations. The sensitivity of fluorescenceemission response of 1a, 2a and 3a toward ZnII, HgII and CuII wassubsequently examined under the same condition with variousmetal ion concentrations (Figs. 12–15). To have better understand-ing of the affinity of 1a for ZnII ions, fluorescence titration studieswere performed by addition of Zn(ClO4)2�6H2O (0.0–2 equiv in CH3-

CN/EtOH, 10:1) to a solution of 1a, Figs. 12.Titration with ZnII for 1a was consistent with complex forma-

tion, a single isosbestic point was observed in emission spectrum,suggesting equilibrium was reached during complexation. Whilethe emission intensity at 455 nm decreases, the emission at408 nm is increased, though much less, so the intensity ratio be-tween 408 and 455 nm decreases linearly with the concentrationof ZnII up to a mole ratio (1a/Zn2+) of 1:1. These results suggest that1a in solution could form 1:1complex with ZnII. The fluorescenceemission spectra of complexes 2a and 3a with ZnII displayed en-hanced fluorescence response, accompanied by blue-shift in wave-length for 2a (Figs. 13 and 14).

The data show that all receptors 1a–3a have excellent sensitiv-ity for ZnII. The different heteroatoms (NH, O, S) of ligands (hpbm,hpbx, hpbt) present in complexes (1a, 2a, 3a) can lead to differentinteractions with the cations ZnII increasing the electron transferand hence energy and/or intensity of the emission, Scheme 2.

In addition, as shown in Fig. 9 a strong decrease of the emissionintensity was observed upon addition of CuII. The fluorescence re-sponse of CuII with 1a is shown in Fig. 15, complexes 2a and 3aexhibited similar responses; when CuII was added into solutionsof them a significant fluorescent emission quenching was observed.

After addition of 0.6 equiv of CuII the fluorescence intensity at455 nm of 1a was reduced to 65% of the initial one and after addi-tion 2 equiv the fluorescence intensity at 455 nm of 1a was re-duced to 1.5%. After addition of 2 equiv of CuII the fluorescenceintensity of 2a at 470 was reduced to 0.8% of the initial one. Hence,1a, 2a and 3a can be used as turn-off molecular probes toward CuII.As shown in Table 4 all complexes 1a, 2a and 3a have moderatefluorescence intensities, but after addition of 1 equiv HgII the fluo-rescence intensity of 2a and 3a is increased by about fourfold. The

Fig. 15. Fluorescence titration spectra upon addition of CuII ion (0.0–2.0 equiv.) to a solution of 1a (10�5 M) in CH3CN/EtOH (10:1).

170 M. Dolores Santana et al. / Polyhedron 61 (2013) 161–171

fluorescence emission spectra of complexes 2a and 3a with HgII

(Figs. S2–S4) displayed increased of the fluorescence responses,accompanied by a blue-shift in wavelength for HgII with 3a(Figs. S2 and S4). The interaction of the metal cations could takeplace in the atom, NH, O or S, of the corresponding heterocycle ofthe benzazole ligand (Scheme 2).

Electronic transfer from the heterocyclic atom (NH, O or S) tothe metal cation decreases the internal transfer charge in thebenzazole. This produces a blue shift of the emission with anenlargement of intensity for ZnII and HgII and complexes 2aand 3a. Most strikingly, the coordination of ZnII to 1a inducesblue-shifted emission with significantly reduced intensity at455 nm but an isosbestic point was also observed. By contrast,a significant luminescence emission quenching was observedwhen CuII was added to solutions of sensors 1a–3a. This can beattributed to an energy/electron transfer quenching of the p⁄

emissive state through low-lying metal-centred unfilled d-orbi-tals for CuII.

4. Conclusions

In summary, a new class of complexes containing benzazole li-gands have been synthesized and characterized. The complexes ex-hibit moderate fluorescence intensities. The study of luminescenceproperties of these complexes reveals that they display sensitiveresponse of luminescence towards ZnII, HgII and CuII ions. The dif-ferent heterocycles of the benzazole ligands and response of lumi-nescence of their complexes are features that enabled thedescribed compounds to be a very promising new type of sensingmaterials. The presented approach appears as an emerging strategyfor the design and construction of new luminescent materialsbased on benzazole derivatives.

Acknowledgments

The authors are grateful to the Spanish Comunidad Autónomade la Región de Murcia (CARM, Fundación Seneca Projects:08670/PI/08 and 11796/PI/09) for financial support.

Appendix A. Supplementary data

CCDC 877821 and 877822 contain the supplementary crystallo-graphic data for compounds 1a and 2a, respectively. These data canbe obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Cen-tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-ciated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.poly.2013.05.047.

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