Anion selectivity of Zn–salophen receptors: Influence of ligand substituents

Inorganica Chimica Acta 434 (2015) 1–6

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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Anion selectivity of Zn–salophen receptors: Influence of ligand substituents Ferran Sabaté a, Ilaria Giannicchi b, Laura Acón a, Antonella Dalla Cort b,⇑, Laura Rodríguez a,⇑ a b

Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain Dipartimento di Chimica and IMC-CNR Sezione Meccanismi di Reazione, Università La Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy

a r t i c l e

i n f o

Article history: Received 25 November 2014 Received in revised form 13 April 2015 Accepted 27 April 2015 Available online 14 May 2015 Keywords: Zn–salophen Anions Host–guest systems Substituent effects

a b s t r a c t Four Zn–salophen complexes that differ for the electrodonating/electrowithdrawing character of the substituents located in the positions para to the phenolic oxygens have been used for the molecular recognition of several anions. Spectrophotometric and spectrofluorimetric titrations carried out in ethanol show for three of them strong binding affinity and also marked selectivity in the recognition of ATP4, when methoxy electrodonating substituents are present. The finding that a Zn–salophen derivative with overall electrodonating (I < +R) substituents is a more selective receptor for this anion than those decorated with electron withdrawing groups, support the occurrence in this case of supramolecular p–p stacking interactions between the adenosine residue and the aromatic ligand surface that counterbalance the decrease in electrophilicity of the metal center. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction It is well known that anions play an important role in a wide range of chemical and biological processes [1,2]. They are ubiquitous in both the organic and mineral worlds, play different key roles in biology [3–6], and cause dramatic effects as environmental pollutants [7]. For these reasons, the supramolecular recognition of anions is a topic of constant interest that is witnessed by the rich literature focused on the design and preparation of new receptors addressing this target [8–13]. One of the most handy and simplest means of detection is the use of optical chemosensors that show changes in absorption or emission bands in the presence of target analytes. Chemosensors are indeed more convenient and inexpensive and they display high sensitivity and low detection limit [14]. Sal(oph)en-type derivatives represent a fundamental class of compounds in coordination chemistry that have been also extensively used in the molecular recognition of anions and in several interesting applications [15–17]. Such ligands, easily obtained through the condensation of o-phenylendiamine with two equivalents of salicylaldehyde, can indeed coordinate transition and main group metals to form stable complexes. The possibility of functionalizing the starting building blocks, i.e. the amine and the ⇑ Corresponding authors. Tel.: +39 0649913087; fax: +39 06490421 (A. Dalla Cort). Tel.: +34 934039130; fax: +34 934907725 (L. Rodríguez). E-mail addresses: [email protected] (A. Dalla Cort), [email protected] (L. Rodríguez). http://dx.doi.org/10.1016/j.ica.2015.04.032 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

salicylaldehydes, provides easy access to a variety of derivatives in which the role of the metal center and of its coordination geometry is fundamental to determine the properties of the complex. In the case of Zinc, the metal displays a square-planar arrangement formed by the N2O2-donor atoms, while the axial positions are available for coordinating solvent molecules or other guests endowed with donating groups [10]. Thus, the insertion of the metal ion in the organic frame of the salophen ligand provides an electron deficient Lewis acid coordinating site that can reversibly bind anions that donate a lone pair to the metal. In this work, we report the use of four differently substituted Zn–salophen complexes, see Chart 1, in the molecular recognition of several anions in ethanol. The effect of the electrodonating and electrowithdrawing character of the substituents has been examined together with the observed selectivity.

2. Results and discussion 2.1. Synthesis and characterization The new compounds 1 and 2 were prepared according to the standard template procedure for salen and salophen metal complexes [18] using 5-cyanosalicylaldehyde and 1,2-diaminobenzene as starting materials, and by adding also the stoichiometric amount of zinc acetate salt to obtain compound 2. The reactants were stirred at r.t. in methanol for 24 h. The products precipitated as yellow solids in pure form. Characterization by different spectroscopic and

2

F. Sabaté et al. / Inorganica Chimica Acta 434 (2015) 1–6

N NC

CN

OH HO

NC

O

1

MeO

N

N O

Zn

N

N

N

Zn

CN

O

2

N

N OMe

O

Br

O

3

Zn

N

N Br

O

O2N

O

Zn

N NO2

O

5

4 Chart 1. Salophen derivatives studied in this work.

spectrometric techniques confirmed their structure. 1H NMR spectra show the expected signals of the salophen derivatives. Protons are clearly affected by metal coordination, e.g. while the signal of phenolic protons obviously disappear upon coordination, imine protons are ca. 0.2 ppm downfield shifted. HRMS-ESI spectra let us identify unequivocally the products since they display the corresponding protonated molecular peak in all cases.

Inorm (a.u.)

1.0

1 2

0.5

2.2. Photophysical characterization Absorption and emission spectra of all complexes were recorded in ethanol and the results are summarized in Table 1. In addition, spectroscopic characterization of 3–5, previously reported by us [19], was added for comparison purposes together with absorption spectra of the 3–5 uncomplexed ligands (3a–5a in SI, Fig. S1). Uncomplexed salophen ligand 1 shows a broad, unstructured absorption band in the 300–500 nm region (Fig. S2). The observed broadening can be assigned to a possible dimerization process [20]. Based on ZINDO/S semi-empirical electronic structure theoretical calculations and, as already reported for similar derivatives [16– 19], the observed transitions must be predominantly p–p⁄ (Fig. S3). Excitation at the lowest energy absorption band gives rise to a broad emission band centered at 430 nm attributed to an intraligand transition (Fig. 1). Coordination to Zn(II) metal, in complex 2, gives origin to an absorption pattern similar to those observed by us previously for the other complexes displaying substituents at the para position (see Fig. S1) [19]. Excitation at the lowest energy absorption band produces a broad emission band centered at 505 nm, 75 nm redshifted with respect to the unmetalated precursor 1 (Fig. 1) and slightly blue shifted with respect to the previously described derivatives 4–5 in accordance with the electrowithdrawing character of the substituents [19]. A demetalation process was previously

Table 1 Absorption and emission data of compounds 1–5.

a b c

Compound

Absorption (nm) (e  103, M1 cm1)

Emission (nm)a

QYc

1 2 3 4 5

280 383 300 290 295

470 505 551 507 476

– 0.008 0.13 0.016 0.009

(7.0), 337 (5.2) (2.5), 413 (17.74) (10.4), 432 (7.2) b (24.9), 400 (19.7) b (37.1), 380 (39.6) b

b b b

kexc = 420 nm. Data retrieved from Ref. [19]. Quantum yields referred to quinine sulfate in H2SO4 1 N as reference (/ = 0.54).

0.0 500

600

700

800

Wavelength (nm) Fig. 1. Normalized emission spectra of 1 (solid line) and 2 (dashed line).

observed for other Zn–salophen complexes in the presence of a guest molecules [21]. 2.3. Molecular recognition of anions UV–Vis spectroscopic titration experiments were performed by registering the spectra after the addition of increasing amounts of standard solution of the anions, 1  104 M or 1  103 M, to the solution of the complex, 5  107 M in ethanol. Sodium salts were chosen due to the innocent character of this counterion. Unfortunately the addition of all the anions to the solution of 2 leads to demetalation, as proven by 1H NMR and also by the resulting absorption and emission bands corresponding to the free ligand (Fig. S4). Kleij has indeed reported a similar demetalation process for some Zn–salophen derivatives in the presence of potential guest molecules [21]. For this reason we excluded this complex from our investigation and the experiments were limited to complexes 3–5 using seven different anions. As a matter of fact, very few anions produce significant changes on the absorption spectra of the zinc–salophen complexes.

Table 2 Association constant values (log Ka) for complexes between compounds 3 and 4 and different anionsa retrieved by spectrophotometric titrations. (Estimated error: 4%).

3 4 a

ATP4

ADP3

AMP2

P3O5 10

PO3 4

SO2 4

NO 2

7.6 7.0

– 7.7

– –

– 8.0

– 7.3

– –

– –

Sodium salts.

F. Sabaté et al. / Inorganica Chimica Acta 434 (2015) 1–6

However the nearly insensitiveness to the presence of axial ligands shown by these derivatives (see Fig. S5 and Table 2) is not unprecedented [16,22,23]. The absorption titration data here reported allow some considerations. Compounds 3 and 4 show evident binding affinities toward selected anions, Table 2, while the UV–Vis spectrum of 5 does not reveal any variation upon addition of anions. The association constants are quite high considering the competiveness of the medium [20] that contends for the binding site of the receptor. Complex 3 shows a remarkable selectivity toward ATP4, over the other investigated anions, (log K = 7.6), Fig. 2. Moreover the recognition process induces changes on the color of the solution that can be easily observed by naked eye (Fig. 3). ESI-MS(+) mass spectrometry evidenced the formation of the host:guest complex. Peaks at 1097.2 (3 + ATP + 2 EtOH + 3 H2O), 901.2 (3 + ATP-NH2-2OH), 881.2 (3 + P3O10 + CH2C5H4(OH)2O + EtOH + H2O), 757.3 (3 + P3O10 + 3 H2O), 668.7 (3 + P3O8) clearly confirm it (Fig. S6). Also complex 4 binds this anion with a good, slightly lower, association constant (log K = 7.0), but in this case the same selectivity is not observed since it binds also ADP3 (log K = 7.7) and, like 3, does not show any affinity toward AMP2. This might be related to the lower charge of the anion. In both cases, Job’s Plot analysis of data suggests the formation of a 1:1 host:guest complex. Mass spectrometry confirmed the formation of the 1:1 adduct (Figs. 4 and S7). As an example, typical peaks for the 4:ATP4 adduct are listed here: 1169.0 (4 + ATP + 2 EtOH + 2 H2O), 1148.0 (4 + ATP2 + 2 EtOH + H2O + H+), 1105.0 (4 + ATP + EtOH + H2O), 1080.0 (4 + ATP + 2 H2O), 1062.1 (4 + ATP + H2O), 1045.0 (4 + ATP), 1028.7 (4 + ATP  NH2). A reliable description of the interactions occurring between these metal–salophen complexes and ATP4 can be provided on the basis of previous experiments [16,19]. The binding of ATP4 causes a progressive decrease of the lowest energy absorption band centered at ca. 420 nm, while the absorbance of the band at 295 shows a 20% decrease in intensity. A new band also appears at ca. 345 nm in both cases. Clear isosbestic points are detected at ca. 290, 330, and 395 nm. These changes and the comparison with former studies suggest that ATP4 coordinates to the metal atom of 3 and 4 via phosphate (establishment of Zn–O(phosphate) bond) and, at the same time, p–p stacking interactions take place between the aromatic part of the host skeleton and the adenosine unit (Fig. S8, left) [16]. Interaction between ADP3 and derivative 4 produces a general decrease in molar absorptivity (hypochromism) and a small bathochromic effect (2–5 nm). Such behavior, reported for DNA intercalation processes, can be associated to a charge transfer process (Fig. S8, right) [19]. Among the inorganic anions, 3 only the highly charged, P3O5 10 and PO4 ones, induce changes on the absorption spectra of 4 that indeed shows a strong affinity for triphosphate anion. In this case, the variations of the electronic absorption and comparison with the data obtained from titrations with ATP4 rule out the occurrence of p–p stacking interactions only in this case. The similarity on the recorded changes displayed in the presence of ADP3 and the inorganic phosphates indicate similar trends, i.e. direct formation of a Zn–O(phosphate) bond and no p–p stacking in the case of ADP3. It should be pointed out that the additional variations recorded in the case of 4 with respect to 3, could be due to the lowest electronic density of this complex that favors the interaction with electron rich anions. For this reason, higher negative charged anions induce the formation of host:guest complexes, being the driving force of these processes. Titration experiments were also carried out by recording at each point the emission spectra of the salophen complexes. The results are summarized in Table 3 and Fig. 5.

3

Emission data confirm the selective recognition of ATP4 by the methoxy substituted derivative 3. The anion causes indeed strong variations on the emissive properties of the complex with a quenching effect of 80%, Fig. 5. The particularly good electron donating nature of the OCH3 groups increases the electronic density on the metal center and therefore decreases its electrophilicity, in accord to what would expected. Nevertheless with ATP4 the p– p stacking interactions that take place, favor the molecular recognition process, as observed by the changes in absorption spectra, leading to the marked specificity that we found. For compounds 4 and 5 with bromide and nitro substituents, respectively, an increase of the quenching effect is observed during titrations, Fig. 5. While the increase of this effect for compound 4 seems to reflect the increase of anion charge, this is not observed for 5 and the variation is almost constant for all the tested anions (plus 20–35%), Table 2. Both substituents have an electron withdrawing character, much stronger in the case of the nitro group that has, as previously reported, the lowest LUMO energy orbital [19]. Substituents are located in a position in which resonance effects come into play and become quite important. The decrease of the electronic density on the metal center, should increase its electrophilicity and this is more marked in the case of nitro substituents for which resonance and inductive contributions produce an overall electron withdrawing effect (R, I). So the almost constant trend of the quenching effect in anion binding to complex 5 (Figs. S9, S10) might reflect the formation of host–guest complexes in which the host has become a stronger Lewis acid that binds a series of Lewis bases, of different strengths, without showing any marked selectivity.

3. Conclusions The molecular recognition of several anions has been investigated by considering four Zn–salophen receptors with different substituents on the para positions to the phenolic oxygens of the ligand. Three of these substituents are electron withdrawing groups (–CN, –Br, –NO2) while the fourth, –OCH3, although electron-withdrawing via induction (I), is overall electronreleasing due to resonance effects (+R). First of all, it was observed that the cyano derivative 2 demetallates in the presence of anions. All the other three complexes are able to preferentially recognize ATP4, as observed by measuring the emission quenching. The data indicate that substituents indeed can influence the properties of the zinc–metal salophen complexes. In our case, all the substituents are located in the position para to the phenolic oxygens of the ligand allowing resonance effects to become quite important. The finding that complex 3 with the methoxy substituents (+R > I) selectively binds ATP4 can be related to the occurrence of supramolecular interactions of the adenosine residue with the aromatic part of the salophen ligand that counterbalance the weaker Lewis acid character of the metal. The two complexes, 4–5, substituted with overall electron withdrawing groups, bromine and nitro, show peculiarities that are difficult to rationalize within the series, but that in general can be ascribed to the increased electrophilicity of the metal center. Complex 5, which is the strongest Lewis acid, binds the investigated anions without showing any marked selectivity, while compound 4 (–Br) shows a slight dependency on the anion charge. We believe that further studies are needed to shed light completely on these issues, nevertheless our data show the possibility of tuning the selectivity of recognition processes involving Zn–salophen complexes through the appropriate choice of substituents. This can be quite important for the future development of new, efficient anion receptors based on such easily synthetically accessible derivatives.

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0.2

0.08

(a)

(b)

0.04

0.02

0.02

0.1

0.0

-5

-5

1.0x10

2.0x10 4-

[ATP ]

xhost· ΔA

A420nm

A

0.03

0.06

0.01

0.00

0.0 300

400

500

0.2

600

0.4

0.6

0.8

1.0

xhost

Wavelength (nm) 7

Fig. 2. (a) Absorption spectra of a 5  10 M ethanol solution of compound 3 upon titration with Na4ATP at room temperature. Inset: Plot of the absorption variation at 420 nm vs. concentration of anion. (b) Job plot representation for 3 and ATP4. Here x = [3]/([3] + [ATP4]).

Table 3 Association constant values (log K) for complexes between compounds 3–5 and different anions obtained by spectrofluorometric titrations (Estimated error: 4%).

3 4 5

ATP4

ADP3

AMP2

P3O5 10

PO3 4

SO2 4

NO 2

7.7 7.0 6.1

– 7.7 6.5

– 6.1 6.2

– 8.0 6.0

– 7.4 6.0

– 5.8 6.3

– 5.7 6.4

synthesized according to previously reported methodology (see SI for characterization data) [19]. 4.2. Physical measurements Fig. 3. 5  107 M ethanol solution of complex 3 (left) and complex 3 in the presence of 1 equivalent of ATP4 (right).

4. Experimental 4.1. General The best commercially available chemicals were used without further purification unless otherwise stated. Compounds 3–5 were

Infrared spectra were recorded on a FT-IR 520 Nicolet Spectrophotometer. 1H and 13C NMR (d(TMS) = 0.0 ppm) spectra were obtained on a Varian Unity 400. Elemental analyses of C, H, N and S were carried out at the Serveis Científico-Tècnics in Barcelona. ES(+) mass spectra were recorded on a Fisons VG Quatro spectrometer. Absorption spectra were recorded on a Varian Cary 100 Bio UV-spectrophotometer and emission spectra on a Horiba-Jobin-Yvon SPEX Nanolog spectrofluorimeters. Total luminescence quantum yields were measured at 298 K relatively to quinine sulfate in 1 N H2SO4 (U = 0.54) [24].

Fig. 4. ESI-MS(+) spectrum of 4ATP4.

F. Sabaté et al. / Inorganica Chimica Acta 434 (2015) 1–6

5

Fig. 5. Plots of % quenching vs. anions for compounds 3–5.

Titrations were carried out at 25 °C in air-equilibrated ethanol by addition of aliquots of 1  104 M or 1  103 M solution of the corresponding anion to a 5  107 M solution of 2–5. The association constants of the complexes with the anions were obtained from the fit of the spectrophotometric or fluorimetric titrations data with the general equation derived by Lehn coworkers [25]. Molecular modeling and semi-empirical calculations. Electronic structure and electronic transitions were calculated with ZINDO/S semi-empirical method with configuration interaction (99 singly excited configurations) in structures previously optimized with MM + molecular mechanics method, both methods included in the software package HYPERCHEM 7.0 (Hypercube (2005) [26]. 4.3. Synthesis and characterization 4.3.1. Synthesis of N,N0 -(o-phenylene)bis(5-cyanosalicylideneimine), 1 A methanol (8 mL) solution of 2-hydroxy-5-cyanobenzaldehyde (0.417 g, 2.8 mmol) and 1,2-diaminobenzene (0.318 g, 2.9 mmol) was stirred for 24 h. The mixture was then filtered and the final product was obtained as orange solid in 75% yield. 1H NMR (300 MHz, DMSO-d6) d 12.26 (bs, 2H, OH), 8.87 (s, 2H, N@C–H), 7.86 (d, J1 = 2.4 Hz, 2H, CH), 7.53–7.39 (m, 6H, CH), 6.91 (d, J1 = 8.7 Hz 2H, CH) ppm. 13C NMR (75 MHz, DMSO-d6) d 163.27, 154.88, 152.26, 142.91, 128.19, 121.14, 119.96, 119.89, 118.00, 115.10, 55.97 ppm. HRMS-ESI-TOF calc.: 367.37; found: 367.47 (M+H+). IR (KBr, cm1): 2216 s, m(C„ N). Elemental analysis: Calc. for C22H14N4O2: C, 72.12; H, 3.85; N, 15.29. Found: C, 72.14; H, 3.86; N, 15.27 (%). 4.3.2. Synthesis of N,N0 -(o-phenylene)bis(5cyanosalicylideneiminato)zinc(II), 2 A methanol (14 mL) solution of 2-hydroxy-5-cyanobenzaldehyde (0.501 g, 3.4 mmol), 1,2-diaminobenzene (0.371 g, 3.4 mmol) and Zn(OAc)22H2O (0.170 g, 3.7 mmol) was stirred for 24 h. The mixture was then filtered and the final product was obtained as yellow solid in 80% yield. 1H NMR (300 MHz, DMSO-d6) d 9.04 (s, 2H, N@C–H), 7.85 (d, J = 3.6 Hz, 2H, CH), 7.44 (dd, J1 = 6.0 Hz, J2 = 3.6 Hz, 2H, CH), 6.94–6.90 (m, 4H, CH), 6.64–6.61 (m, 2H, CH) ppm. 13C NMR (75 MHz, DMSO-d6) d 168.12, 162.30, 147.56, 139.53, 127.25, 124.81, 124.20, 117.78, 116.52, 116.06, 55.67 ppm. HRMS-ESI-TOF calc.: 430.75; found: 430.75 (MH+). IR (KBr, cm1): 2224 s, m(C„N). Elemental analysis: Calc. for

C22H12N4O2Zn: C, 61.49; H, 2.81; N, 13.04. Found: C, 61.50; H, 2.82; N, 13.03 (%). Acknowledgements The support and sponsorship provided by COST Action CM1005 is acknowledged. Authors are also grateful to the Ministerio de Ciencia e Innovación of Spain ‘‘Project Project CTQ2012-31335’’. A.D.C., and I.G. acknowledge ‘‘Ricerca scientifica di Ateneo 2013’’ and MIUR ‘‘PRIN 2010CX2TLM’’. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.04.032. References [1] C. Männel-Croisé, C. Meister, F. Zelder, Inorg. Chem. 49 (2010) 10220. [2] J.M. Llinares, D. Powell, K. Bowman-James, Coord. Chem. Rev. 240 (2003) 57. [3] P.A. Gale, N. Busschaert, C.J.E. Haynes, L.E. Karagiannidis, I.L. Kirby, Chem. Soc. Rev. 43 (2014) 205. [4] P.A. Gale, T. Gunnlaugsson, Chem. Soc. Rev. 39 (2010) 3595. [5] L. Rodríguez, J.C. Lima, A.J. Parola, F. Pina, R. Meitz, R. Aucejo, E. García-España, J.M. Llinares, C. Soriano, J. Alarcón, Inorg. Chem. 47 (2008) 6173. [6] V. Amendola, M. Bonizzoni, D. Esteban-Gómez, L. Fabbrizzi, M. Licchelli, F. Sancenón, A. Taglietti, Coord. Chem. Rev. 250 (2006) 1451. [7] M.T. Albelda, J.C. Frías, E. García-España, H.J. Schneider, Chem. Soc. Rev. 41 (2012) 3859. [8] P. Ballester, Chem. Soc. Rev. 39 (2010) 3810. [9] A. Frontera, Coord. Chem. Rev. 257 (2013) 1716. [10] A. Dalla Cort, P. De Bernardin, G. Forte, F. Yafteh Mihan, Chem. Soc. Rev. 39 (2010) 3863. [11] J. Mosquera, S. Zarra, J.R. Nitschke, Angew. Chem., Int. Ed. 53 (2014) 1556. [12] M. Inclán, M.T. Albelda, E. Carbonell, S. Blasco, A. Bauzá, A. Frontera, E. GarcíaEspaña, Chem. Eur. J. 20 (2014) 3730. [13] A. Shokri, S.H.M. Deng, X.-B. Wang, S.R. Kass, Org. Chem. Front. 1 (2014) 54. [14] Y. Wang, S.-H. Kim, Dyes Pigm. 102 (2014) 228. [15] (a) P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717; (b) A.W. Kleij, Dalton Trans. (2009) 4635; (c) E. Bedini, G. Forte, C. De Castro, M. Parrilli, A. Dalla Cort, J. Org. Chem. 78 (2013) 7962; (d) F. Yafteh Mihan, S. Bartocci, M. Bruschini, P. De Bernardin, G. Forte, I. Giannicchi, A. Dalla Cort, Aust. J. Chem. 65 (2012) 1638. [16] M. Cano, L. Rodríguez, J.C. Lima, F. Pina, A. Dalla Cort, C. Pasquini, L. Schiaffino, Inorg. Chem. 48 (2009) 6229. [17] A. Dalla Cort, G. Forte, L. Schiaffino, J. Org. Chem. 76 (2011) 7569. [18] A.L. Singer, D.A. Atwood, Inorg. Chim. Acta 277 (1998) 157. [19] I. Giannicchi, R. Brissos, D. Ramos, J. de Lapuente, J.C. Lima, A. Dalla Cort, L. Rodríguez, Inorg. Chem. 52 (2013) 9245.

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