FerroceneâTriazoleâPyrene Triads as Multichannel Heteroditopic

Article pubs.acs.org/Organometallics

Ferrocene−Triazole−Pyrene Triads as Multichannel Heteroditopic Recognition Receptors for Anions, Cations and Ion Pairs Marıá del Carmen González,† Francisco Otón,† Raúl A. Orenes,‡ Arturo Espinosa,† Alberto Tárraga,*,† and Pedro Molina*,† †

Departamento de Quı ́mica Orgánica, Facultad de Quı ́mica, and ‡Servicio de Apoyo a la Investigación (SAI), Campus de Espinardo, Universidad de Murcia, E-30100 Murcia, Spain S Supporting Information *

ABSTRACT: A number of functionalized ferrocene−triazole−pyrene triads, available from 1,1′-bis(azido)ferrocene by initial upper arm regioselective formation, through a coppercatalyzed click reaction, and subsequent nitrogen-containing bottom arm formation, through the remaining nitrogen functionality, have been designed, prepared, and structurally characterized as potential multichannel heteroditopic receptors for ions. The versatility of the bottom arm formation enables the decoration of the triad core with an additional nitrogenbased binding site, linked to substituents displaying different optical properties. As a consequence, the resulting disubstituted ferrocene derivatives have been shown to be excellent anion, cation, and ion pair multichannel recognition receptors. In order to gain insight into the coordination modes and into the nature of the anion contact ion pair formed, quantum chemical calculations were also carried out.

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INTRODUCTION The design of receptors that contain two quite different binding sites for the complexation of cationic and anionic guest species is an emerging and topical field of supramolecular chemistry.1 In this context, the redox-active organometallic scaffold ferrocene plays a key role as a remarkable redox-signaling unit. Given the richness of its chemistry, it has proved to be a robust building block for the preparation of functionalized ferrocene-containing ligands, including more complex ferrocene-based dendrimers,2 which have been considered as chemosensor molecules displaying interesting electrochemicalsensing properties. Because of this, the topic of the preparation and chemosensing behavior of ferrocene derivatives has been comprehensively reviewed.3 The advantage associated with the use of these functionalized ferrocene-containing ligands lies in the fact that, upon complexation with metal cations or hydrogen-bond formation with anions, they undergo significant perturbations of the ferrocene/ferrocenium redox couple and the values of the corresponding anodic or cathodic oxidation potential shifts are informative about the strength of the recognition event: the closer the binding site to the ferrocene unit, the higher the oxidation potential shift. The 1,2,3-triazole motif has proven to be a versatile ion recognition unit for both cations and anions. As a nitrogencontaining Lewis base, triazole-based ligands have been shown to coordinate transition-metal cations.4 In contrast, several triazole derivatives recognize anions through a cooperative triazole C−H···anion hydrogen bond.4p,q,5 Hence, the integration of the triazole motif into the design of heteroditopic © XXXX American Chemical Society

receptors for ion-pair recognition is an attractive proposition. Consequently, ferrocene−triazole derivatives are some of the systems that have potential applications in the field of electrochemical detection and sensing and host−guest chemistry.6 However, to the best of our knowledge, few examples of triazole-7 and ferrocene-containing8 ditopic receptors have been reported, whereas only three examples based on an unsymmetrically 1,1′-disubstituted ferrocene have been described.9 Pyrene has often been used as an effective fluorescence probe because of its high detection sensibility.10 The emission wavelength of pyrene has been proven to be extremely sensitive to the polarity of the local environment. Formation of the selfassembled complex results in a remarkable change in the fluorescence emission intensities of the pyrene excimer and monomer.11 Two informative parameters associated with the pyrene excimer are the intensity ratio of the excimer to the monomer emission (IE/IM) and the wavelength corresponding to the maximum of the excimer emission (λE). Although the IE/ IM parameter is sensitive to the structure of the pyrene-labeled systems, the corresponding pyrene λE is much less variable and is generally located at 475−485 nm. Unsymmetrically 1,1′-disubstituted ferrocene derivatives can lead to improved multifunctional systems for molecular recognition processes. Thus, in a monosubstituted ferrocene a different substituent at the other cyclopentadienyl ring can Received: April 4, 2014

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dx.doi.org/10.1021/om500358c | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Scheme 1. Synthesis of Receptors 4−7a

a Reagents and conditions: (a) CuSO4·5H2O; Na ascorbate; THF/H2O, room temperature; (b) PMe3, anhydrous THF; (c) H2O; (d) 1pyrenecarboxaldehyde, anhydrous THF, room temperature, 6 h; (e) acetic anhydride, anhydrous THF, room temperature, 4 h; (f) coumarine-3carboxylic acid chloride, anhydrous THF, room temperature, 3 h; (g) (E)-1-isocyanato-4-(4-nitrostyryl)benzene, anhydrous THF, room temperature, 2 h.

bis(azido)ferrocene15 and 1-ethynylpyrene in the presence of a copper(II) sulfate/sodium ascorbate mixture. Then, compound 1 underwent a Staudinger reaction with trimethylphosphine under anhydrous conditions to give the unisolable iminophosphorane derivative 2, which subsequently underwent an azaWittig reaction with 1-pyrenecarboxaldehyde in dry THF to yield the bis(pyrene) receptor 4 in 56% yield. The preparation of receptors 5−7 involves the previous formation of the intermediate aminoferrocene derivative 3 achieved by hydrolysis, under mild conditions, of the iminophosphorane 2.9c Conversion of compound 3 into the 1′-amido-functionalized derivatives 5 and 6 was achieved in 88% and 82% yields, respectively, by treatment with acetic anhydride or coumarin-3carboxylic acid chloride, respectively, while urea derivative 7 was obtained in 60% yield by treatment of 3 with (E)-1isocyanato-4-(4-nitrostyryl)benzene at room temperature. The structures of these compounds were elucidated using extensive spectral studies (1H NMR, 2D COSY, 13C NMR, and HMQC spectra, as well as electrospray mass spectra (ESI-MS)) and elemental analysis (see the Supporting Information). In general, the 1H NMR spectra of compounds 4−7 showed the presence of four pseudotriplets, integrating to two protons

induce an intermolecular interaction or a change in the electrochemical parameters.12 Importantly, known synthetic methodologies for the preparation of unsymmetrically 1,1′disubstituted ferrocene derivatives involve complex transformations.13 The methods of their preparation involve selective introduction of a second substituent at the 1′-position of a monosubstituted derivative or selective transformation of one substituent of symmetrically disubstituted compounds. In this paper we wish to report the synthesis, structural characterization, and sensing properties of a structural motif bearing a central ferrocene unit linked to a pyrene through a 1,2,3-triazole at the upper arm, whereas the bottom arm is decorated by a photoactive pyrene ring, a chromophoric azobenzene derivative, or a chromogenic coumarin ring linked through a nitrogen functionality displaying proven binding ability such as aldimine, amide, or urea.

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RESULTS AND DISCUSSION Synthesis and Characterization. The target ferrocene− triazole derivatives 4−7 were prepared by the synthetic routes depicted in Scheme 1. The previously reported8c intermediate 1 was synthesized by using a click reaction14 between 1,1′B


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each, assigned to the eight protons within the two asymmetrically monosubstituted cyclopentadienyl (Cp) rings of the ferrocene unit, with one singlet corresponding to the H-5 proton of the triazole bridge together with the pattern of signals corresponding to the linked 1-pyrenyl fragment. Furthermore, each receptor also shows the characteristic set of signals associated with the units placed at the bottom arm of the ferrocene core. X-ray Structural Characterization. Single-crystal X-ray diffraction was also used to confirm the connectivity of the structure of the 1,1′-asymmetrically disubstituted ferrocene derivatives 4 and 6 (Tables S1 and S6, Supporting Information). Compound 4 crystallizes from dichloromethane as a red needle in the triclinic space group P1̅. The Cp rings are twisted from the eclipsed conformation (the average torsion angle C1− centroid(Cp)1−centroid(Cp)2−C6 is 24.03°) and have an almost parallel orientation with a tilt angle of 1.6°. The angle between the mean planes of the triazole ring and the C1−Cp ring is 19.5°. The pyrene moiety linked to the triazole is almost parallel to the C1−Cp ring (angle between mean planes 3.3°) and is rotated respect to the triazole by 22.5°. The second pyrene moiety attached to the imide carbon C40 is almost parallel to the C6−Cp ring, the angle between mean planes being 2.2°. The two pyrene moieties of the molecule are parallel (the angle between mean planes is 5.3°) and the distance between planes is approximately 3.5 Å, showing interactions via π−π stacking (Figure 1). However, the overlapping of the pyrenes is only partial, with a parallel displacement of ∼1.5 Å.

Figure 2. Formation of dimers via π−π stacking in the crystal structure of 4.

the triazole mean plane by 10.8°. The organic ligand linked to the amide carbon atom C41 is also rotated with respect to the C6−Cp ring by 10.8° and is almost parallel to the pyrene (the angle between mean planes is 2.1°). The distance between the organic ligand and pyrene planes is ∼3.5 Å, showing interactions via π−π stacking between the pyrene and the benzene ring of the ligand (Figure 3). The disposition of the organic ligand is fixed due to the intramolecular hydrogen bond N4−H04···O2 (d = 1.914 Å).

Figure 3. Partial overlapping of aromatic moieties in 6. Selected bond lengths (Å) and angles (deg): N4−C41, 1.357; O1−C41, 1.228; O2− C50, 1.229; C2−C1−N1−N2, 0.80; C11−C12−C21−C22, −7.40; C10−C6−N4−C41, −10.95; N4−C41−C42−C50, 1.03.

Figure 1. Partial overlapping of the pyrene moieties in 4. Selected bond lengths (Å) and angles (deg): N4−C40, 1.282; N4−C40−C41− C42, 8.39; C2−C1−N1−N2, −18.92; C11−C12−C21−C22, 20.44.

In the crystal structure, neighboring molecules with opposite orientation are interlinked by two intermolecular hydrogen bonds C5−H5···O1 to form dimers (d = 2.454 Å). The dimers are stacked due to π−π interactions between the pyrene ring of one molecule and the organic ligand of the other, forming ribbons along the a axis (Figure 4). Cation and Anion Sensing Studies. Taking into account the fact that the densely decorated bottom arm in the ferrocene−triazole−pyrene molecular framework could play a key role in the recognition events toward ions, we have investigated the sensing capabilities of receptors 4−7 toward cations (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Pb2+, as their perchlorate or triflate salts),16 and anions (F−, Cl−, Br−, AcO−, NO3−, HSO4−, H2PO4−, and HP2O73−, as tetrabutylammonium (TBA+) salts) by using electrochemical (linear sweep voltammetry (LSV), cyclic voltammetry (CV), and Osteryoung square-wave voltammetry (OSWV))17 techniques, as well as spectrophotometric (UV−vis and fluorescence spectroscopy) and 1H NMR experiments. The titration

In the crystal, molecules with opposite orientations stack to form dimers, the pyrene moiety of one molecule interacting via π−π stacking with the Cp ring of the other. The angle formed by the mean planes of the stacked pyrene and the Cp ring is 4.3°, and the distance is approximately 3.4 Å. As the asymmetric unit cell is formed by two molecules related by an angle of 72.6° between their pyrene rings, there are two orientations of stacked molecules: perpendicular to the a axis and perpendicular to the b axis (Figure 2). Compound 6 crystallizes from dichloromethane as a red needle in the triclinic space group P1̅. The Cp rings are arranged in an almost eclipsed conformation (average torsion angle C1−centroid(Cp)1−centroid(Cp)2−C6 of 2.88°) and have an almost parallel orientation (the tilt angle is 2.2°). The triazole ring attached to the C1 atom is parallel to the C1−Cp ring, the angle between mean planes being 1.0°. The pyrene moiety attached to the triazole ring is rotated with respect to C


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Moreover, the direct linkage of this redox-active unit to a 1,2,3triazole ring, containing two quite different binding sites such as sp2-hybridized N atoms, able to bind metal cations, and a CH, able to promote CH···anion interactions, together with the presence of additional binding sites such as imine, amide, and urea groups directly located in the bottom arm of the 1,1′disubstituted ferrocene moiety allows the evaluation of the chemosensing properties of these ligands by electrochemical techniques. The stepwise addition of substoichiometric amounts of the appropriate guest ionic species to the receptors revealed two different electrochemical behaviors: a “shifting behavior” or a “two wave behavior”.20 In the former, the changes observed in the voltammetric response involve ion-induced shifts of the oxidation potential corresponding to the receptor’s original redox couple. Thus, the oxidation potential gradually shifts as the ion concentration is increased, but resolved voltammetric waves for the free receptor and the complex are never observed. In the latter, the addition of the ionic guest species give rise to the appearance of a new redox couple while the original couple has diminished current levels. Thus, as the concentration of the complex formed increases, as a consequence of the addition of the corresponding ions, the new redox couple also increases at expense of the original couple, corresponding to the free receptor. Moreover, a possible way to reveal the formation of hydrogen-bonded complexes between the free receptors and basic anion guests, under electrochemical titration conditions, is to suppress the possible deprotonation process by adding a small amount of acetic acid. Additionally, another way to solve the deprotonation/coordination dualism which could take place upon titration of the free receptor with basic anions is based on a comparison of the electrochemical results obtained by titration of the receptors with a strong base, such as nBu4NOH, which definitely leads to deprotonation, as thus resulting from the titration with the anionic species.8a,21 Within this context, we found that addition of up to 20 equiv of acetic acid affected neither the CV nor OSWV of the electrochemical solutions of receptors 4−7. In contrast, in all cases, titration with the strong base n-Bu4NOH induced a remarkable cathodic shift of their oxidation peaks (Table 1) as a

Figure 4. Formation of dimers by hydrogen bonds in the crystal structure of 6.

experiments were further analyzed using the computer program SPECFIT.18 Electrochemical Study. The reversibility and relative oxidation potential of the ferrocenium/ferrocene (Fc+/Fc) redox couple in receptors 4−6 were determined by cyclic voltammetry (CV) and Osteryoung square-wave voltammetry (OSWV) in solutions of CH3CN/CH2Cl2 (4/1) containing 0.1 M [(n-Bu)4N]PF6 (TBAHP) as supporting electrolyte. However, the limited solubility of 7 in this solvent forced us to carry out such studies in DMSO solutions. As expected, the CV of receptors 4−6 show a reversible oneelectron oxidation wave at E1/2 = 195, 120, and 190 mV, respectively, versus the ferrocenium/ferrocene (Fc+/Fc) redox couple. However, the electrochemical response of 7 showed a nonreversible one-electron oxidation peak at Ep = 65 mV versus Fc+/Fc (Table 1). In all cases, the potential values are identical with those obtained from the corresponding OSWV peaks. Chemical receptors bearing redox-active ferrocene moieties as sensing units have been broadly studied to recognize and sense not only metal cations2 but also anionic species.19

Table 1. Characteristic Electrochemical Data of the Free Receptors 4−7 and Their Metal and Anion Complexes compound 4 [4·Pb2+] [4·Zn2+] [4·HP2O73−] [4·F−] [4·H2PO4−] [4·AcO−] [4·BzO−] [4·OH−] 5 [5·HP2O73−] [5·F−] [5·H2PO4−] [5·AcO−] [5·BzO−] a

E1/2 (ΔE1/2)a

compound −

195 395 (200) 240 (45) 70 (−125);b 83 (−112)c 100 (−95)b; 112 (−83)c 105 (−90);b 103 (−92)c 110 (−85);b 117 (−78)c 115 (−70);b 120 (−75)c −192 (−387) 120 −250 (−370);b −90 (−210);b −85 (−205)c −245 (−365);b 55 (−65);b 48 (−72)c −75 (−195);b −55 (−175)c 5 (−115);b 18 (−102)c 20 (−100);b 5 (−115)c

[5·OH ] 6 [6·HP2O73−] [6·F−] [6·H2PO4−] [6·AcO−] [6·OH−] 7 [7·HP2O73−] [7·F−] [7·H2PO4−] [7·AcO−] [7·BzO−] [7·OH−] [7·Hg2+]

E1/2 (ΔE1/2)a −250 (−370) 190 −140 (−330);b 35 (−155);b 55 (−135)c −135 (−325);b 105 (−85);b 125 (−65)c 60 (−130)b; 85 (−105)c 60 (−130);b 100 (−90)c −145 (−335) 65 −435 (−500);b −105 (−170);b −95 (−160)c −435 (−500);b −65 (−130);b −55 (−120)c −90 (−155);b −70 (−135)c −110 (−175);b −80 (−145)c −75 (−140);b −55 (−120)c −435 (−500) 95 (30)

ΔE1/2 = E1/2(complex) − E1/2(free receptor), in mV vs Fc+/Fc. bIn the absence of acetic acid. cIn the presence of 20 equiv of acetic acid. D


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perturbation of the oxidation peak of the free receptors, the addition of F−, HP2O73−, H2PO4−, AcO−, and BzO− anions to 5 or F−, HP2O73−, H2PO4−, and AcO− anions to 6 induced characteristic changes in their oxidation peaks, although they were dependent on the anion studied. Thus, the oxidation peak associated with the free receptor underwent a cathodic shift upon addition of H2PO4−, AcO−, and BzO− anions to 5 (Figure 6 and Figures S12−S15 (Supporting Information)) or of H2PO4− and AcO− anions to 6 (Figures S16−S18 (Supporting Information)).

consequence of the formation of the corresponding deprotonated receptors. The anion binding properties of 4 (c = 3 × 10−4 M in CH3CN/CH2Cl2 4/1) revealed that only addition of F−, H2PO4−, HP2O73−, AcO−, and BzO−, anions promotes remarkable responses. In all cases a “shifting behavior” was observed, in which the redox peak is negatively shifted, in comparison to the free receptor (Table 1, Figure 5, and Figures

Figure 5. Evolution of the OSWV of 4 (c = 3 × 10−4 M in CH3CN/ CH2Cl2 4/1, TBAHP as supporting electrolyte), versus the Fc+/Fc redox couple, scanned at 0.1 V s−1 in the presence of increasing amounts of H2PO4− anion: (magenta) 0 equiv; (orange) 0.5 equiv; (pale green) 1 equiv; (green) 5 equiv; (blue) 10 equiv.

Figure 6. Evolution of the OSWV of 5 (c = 5 × 10−4 M in CH3CN/ CH2Cl2 4/1, TBAHP as supporting electrolyte), versus the Fc+/Fc redox couple, scanned at 0.1 V s−1 in the presence of increasing amounts of BzO− anion: (magenta) 0 equiv; (orange) 0.5 equiv; (pale green) 1 equiv; (green) 2 equiv; (blue) 3 equiv; (dark blue) 4 equiv.

S5−S7 (Supporting Information)). Addition of these anions to an electrochemical solution of receptor 4, in the presence of 20 equiv of acetic acid, induced a cathodic shift of the corresponding oxidation peak similar to that observed in the absence of acid, thus indicating that a recognition process has taken place. On the other hand, the results obtained on the stepwise addition of substoichiometric amounts of the aforementioned set of metal cations show that only the addition of Zn2+ and Pb2+ induced the appearance of a new oxidation peak, in the OSWV, at a remarkably more positive potential (Table 1). Particularly, while addition of Pb2+ induces an anodic shift of ΔE1/2 = 200 mV in which a “two wave behavior” is observed, the addition of Zn2+ only causes a moderate anodic shift (ΔE1/2 = 45 mV) of the redox wave of the receptor. It is worth noting that while addition of Zn2+ and Pb2+ metal cations to 4 promotes the formation of the corresponding complexes, addition of Cu2+ and Hg2+ induces the oxidation of the ferrocene moiety present in the free receptor. Thus, LSV (linear sweep voltametry) studies carried out upon addition of Cu2+ and Hg2+ to the electrochemical solutions of receptor 4 in CH3CN/CH2Cl2 showed a significant shift of the sigmoidal voltammetric wave toward cathodic currents, indicating that these metal cations promote the oxidation of the free receptor. In contrast, the same experiments carried out upon addition of Zn2+ and Pb2+ metal cations revealed a shift of the linear sweep voltammogram toward more positive potentials, which is in agreement with the complexation process previously observed by OSWV (Figure S11 (Supporting Information)). The electrochemical recognition abilities of receptors 5 and 6, bearing an amide unit connected to the bottom arm of the ferrocene moiety, were also investigated in the presence of anions and cations. The results obtained demonstrate that, while addition of any of the cations tested did not induce

In contrast, the changes promoted by the addition of HP2O73− and F− to 5 and 6 consist of the progressive appearance of two new cathodically shifted oxidation peaks (Table 1). To rule out possible deprotonation processes of the free receptors, we have also carried out additional electrochemical experiments either by adding Bu4NOH to the electrochemical solution of the free receptors or by titrating with these anions in the presence of acetic acid, in which case the deprotonation process is prevented. First, titration of 5 and 6 with the strong base Bu4NOH gave rise to the appearance of a new oxidation peak, cathodically shifted. This magnitude is quite similar to that observed for the cathodic shift of one of the oxidation peaks resulting upon addition of HP2O73− and F− (Table 1) but different from those found after addition of H2PO4−, AcO−, and BzO− anions to 5 or of H2PO4− and AcO− anions to 6. Second, titrations of these receptors in the presence of 20 equiv of AcOH almost did not affect the results obtained in the electrochemical titrations carried out by using H2PO4−, AcO−, and BzO− anions alone, whereas addition of HP2O73− and F− to an electrochemical solution of 5 and 6, under the same acidic conditions, gave only rise to one oxidation peak, cathodically shifted in a magnitude which is equivalent to that observed for the other oxidation peak appearing in the absence of acid (Table 1 and Figures S13, S14, and S17−S19 (Supporting Information)). These results suggest that the perturbations observed upon addition of H2PO4−, AcO−, and BzO− anions should be associated with a recognition event, involving the formation of a hydrogenbonded complex between the receptors 5 or 6 and the corresponding anionic species. However, the results obtained upon titration with HP2O73− and F− anions showed that in the absence of an acidic medium both a deprotonation and a E


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Table 2. Characteristic UV−Vis Data for the Receptors 4−7 and Representative Metal and Anion Complexes UV−vis λmax (10−3ε)a

compound d

5d [5·HP2O73−] [5·F−] [5·H2PO4−]

237 (65.71), 243 (bs, 62.12), 282 (42.83), 357 (42.75), 411 (bs, 14.21) 236 (bs, 84.18), 244 (93.60), 271 (bs, 48.23), 279 (60.47), 318 (bs, 20.59), 332 (bs, 35.90), 347 (48.06), 397 (bs, 6.27). 237 (bs, 69.03), 243 (76.10), 269 (bs, 38.28), 279 (49.58), 317 (bs, 19.84), 332 (bs, 33.22), 347 (46.85), 395 (bs, 9.17) 271 (sh, 18.43), 280 (23.35), 349 (19.37) 273 (sh, 18.92), 282 (21.06), 353 (16.03), 398 (bs, 4.60), 448 (1.82) 273 (sh, 17.54), 282 (20.18), 354 (16.27), 398 (bs, 4.25), 448 (1.60) 271 (sh, 18.03), 281 (21.97), 351 (18.35), 398 (bs, 4.06)

284, 322, 359 285, 320, 362 266, 285, 319, 359

[5·AcO−]

271 (sh, 19.17), 281 (22.08), 351 (17.79), 398 (bs, 4.06)

265, 285 316 357

[5·BzO−]

350 (18.14), 397 (4.29)

318, 360

[5·OH−] 6d [6·HP2O73−] [6·F−] [6·H2PO4−]

273 271 271 272 271

[6·AcO−]

271 (sh, 27.55), 280 (34.89), 344 (26.25), 398 (bs, 3.38)

287, 363

[6·OH−] 7e [7·Hg2+]

271 (sh, 30.25), 281 (35.41), 354 (25.14), 393 (bs, 3.33), 440 (2.90) 273 (sh, 35.33), 284 (40.15), 359 (33.74), 415 (24.07) 360 (40.4), 413 (bs, 21.16)

284, 362 406

[7·HP2O73−] [7·F−] [7·H2PO4−]

275 (sh, 30.36), 284 (33.30), 365 (26.79), 408 (19.66), 610 (4.4) 276 (sh, 32.52), 284 (34.83), 364 (29.24), 408 (22.53), 613 (4.1) 274 (sh, 32.72), 284 (36.69), 359 (31.93), 432 (22.88)

287, 320, 376, 460 287, 375, 460 288, 344, 424

[7·AcO−]

275 (sh, 30.35), 284 (34.43), 361 (31.02), 430 (22.81)

288, 319, 435

[7·BzO−]

360 (31.75), 425 (23.05)

318, 436

[7·OH−]

275 (sh, 32.20), 284 (34.57), 364 (28.40), 415 (22.27), 615 (4.33)

288, 309, 375, 477

4 [4·Pb2+] [4·Zn2+]

a

IPb

(sh, (sh, (sh, (sh, (sh,

21.81), 27.55), 28.05), 25.78), 26.28),

281 282 280 282 280

(24.47), (35.34), (33.59), (31,51), (33.58),

353 343 348 345 344

(18.49), (27.93) (26.05), (24.87), (25.98),

230, 252, 284, 315, 355 234, 249, 265, 283, 311, 354

398 (bs, 5.97), 448 (2.48)

319, 361

394 (bs, 5.27), 434 (2.58) 394 (2.60), 438 (1.89) 399 (bs, 2.80)

285, 366 284, 363 287, 365

Kas (error)

Dlimc

5.4 × 105 (±1.109)f 7.4 × 103 (±1.117)f

4.2 × 10−6

2.1 × 1013 (±1.303)g 2.7 × 1013 (±1.670)g 2.1 × 1011 (±1.216)g

1.3 × 10−5

5.8 × 1012 (±1.889)g 2.3 × 1012 (±1.608)g

1.7 × 10−5

2 × 104 (±1.106)f

4.9 × 10−6

8.43 × 1012 (±1.097)g 4.44 × 1013 (±1.070)g 8.24 × 1012 (±1.107)g

1.8 × 10−5

4 × 10−6

6.9 × 10−6 2.4 × 10−5

5.7 × 10−6

6.6 × 10−6 5.5 × 10−6

ε in dm3 mol−1 cm−1. bIsosbestic points in nm. cDetection limits in M. dCH3CN/CH2Cl2 4/1 solution. eDMSO solution. fIn M−1. gIn M−2.

which completely disappeared after addition of 4 equiv of anion (Figure S22 (Supporting Information)). Absorption and Emission Study. Ion recognition properties of these receptors toward the aforemetioned set of metal cations and anions have also been studied by using absorption and emission techniques (Table 2). The UV−visible spectra of these receptors show an absorption band with a maximum between 343 and 360 nm which can safely be ascribed to a high-energy ligand-centered π−π* electronic transition (L−π*) (HE band) consistent with the UV−vis data of most ferrocenyl chromophores.22 In addition to this band, in receptors 4 and 7 another weaker absorption band appears at 411 and 415 nm, respectively, which is assigned to another localized excitation band with a lower energy produced by two nearly degenerate transitions, by a Fe(II) d−d transition,23 or by a metal−ligand charge transfer (MLCT) process (dπ−π*) (LE band). This assignment is in accordance with the theoretical treatment (model III) reported by Barlow et al.24 Such spectral characteristics confer a orange color to the solution of these receptors. Titration experiments of receptor 4 (c = 1.25 × 10−5 M in CH3CN/CH2Cl2 4/1) toward the ions under study revealed that, while the anions tested did not give rise to any significant changes in the absorption spectrum of 4, the results obtained upon addition of the metal cations confirm the electrochemical results previously shown in the sense that only Pb2+ and Zn2+

recognition process should simultaneously take place, while in the presence of a small amount of acetic acid only the corresponding hydrogen-bonded complexes are formed (Table 1). Electrochemical titration studies carried out by using receptor 7 (c = 5 × 10 −4 M in DMSO) with the aforementioned set of metal cations demonstrated that only the addition of Hg2+ caused a variation in the redox response of the receptor. Therefore, upon addition of 0−2 equiv of Hg2+, a clear evolution of the oxidation peak, from Ep = 65 mV to Ep = 95 mV (ΔE1/2 = 30 mV) was observed. On the other hand, in the presence of anions, it was found that only F−, H2PO4−, HP2O73− AcO−, and BzO− caused changes in the electrochemical oxidation peak. However, the evolution of the OSWV was dependent on both the type of anion and the number of equivalents of the anion added. Thus, the addition of H2PO4− induced a cathodic shift of the oxidation peak corresponding to the free receptor, which reaches a maximum (ΔEp = −155 mV) upon addition of 6 equiv of anion. However, in the cases of AcO− and BzO− anions, a typical “two wave behavior” was observed for the evolution of the peak at Ep = 65 mV, which consists of the progressive appearance of a second oxidation peak at more negative potentials (ΔEp = −175 mV for AcO− and ΔEp = −140 mV for BzO−), due to the anion complexed species, together with that corresponding to the free receptor, F


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The changes in the fluorescence spectrum of 4 upon addition of metal cations and anions were also examined. The free receptor 4 (c =1.25 × 10−5 M, CH3CN/CH2Cl2, 4/1) displays, on excitation at λexc 345 nm, typical emission bands at λ 388 and 405 nm (Φ = 2.5 × 10−3), which are attributed to the pyrene monomeric emission. However, while no emission changes were observed upon addition of the metal cations, receptor 4 modified its fluorescence emission in the presence of F−, H2PO4−, HP2O73− AcO−, and BzO−. In general, the addition of such anions is accompanied by a remarkable decrease in the intensity of the monomer emission band with a concomitant increase of a structureless band at λ 450 nm, associated with the pyrene excimer emission, the ratio of the fluorescence intensity (λ358/λ450) monomer/excimer being lower than that in the free ligand (λ388/λ450 = 4.80) (see Table 3 and the Supporting Information). From the fluorescence titration experiments it was observed that the complexation process ends with 6 equiv for F− (Φ = 2.2 × 10−3) and HP2O73− (Φ = 2.5 × 10−3) while 40 equiv of H2PO4− (Φ = 2.7 × 10−3), AcO− (Φ = 2.5 × 10−3), and BzO− (Φ = 2.3 × 10−3) is needed to complete the recognition process (see the Supporting Information). Empirical 1/1 binding models for the complexes formed were determined by using the fluorescence titration data, with association constant values of Ka = 104 M−1 (±1.361) for F−, Ka = 4.8 × 104 M−1 (±1.238) for HP2O73−, Ka = 2.7 × 103 M−1 (±1.201) for H2PO4−, Ka = 4.9 × 103 M−1 (±1.170) for AcO−, and Ka = 5 × 103 M−1 (±1.211) for BzO− anions. The calculated detection limits for the different anions are in the range shown in Table 3. Dilution experiments demonstrate that upon anion complexation an intramolecular excimer is formed because the ratio between the intensities of the monomer and excimer emission bands of the complexes formed is independent of the dilution (Figure S28 (Supporting Information)). The binding abilities of receptors 5 and 6 were also studied by UV/vis and emission spectra changes in their mixed organic solutions produced by addition of the set of anionic and cationic species tested. Thus, addition of H2PO4−, AcO−, and

induced variations in the UV−vis spectrum of the receptor, as a consequence of its coordination to these metal cations. The changes observed during such coordination processes are similar for both metal cations and basically consist of a hypsochromic shift of the band at λ 357 nm (Δλ = 10 nm) with a simultaneous decrease in the intensity of the band that appears as a shoulder at λ 411 nm (Figure 7). It was found that

Figure 7. UV−vis titration of receptor 4 (c = 1.25 × 10−5 M, CH3CN/ CH2Cl2 4/1) with Pb2+ (from 0 to 4 equiv). The inset shows color changes of the solution of 4 upon addition of Pb2+ and a Job plot of the titration with the metal cation measured at λ 400 nm.

the addition of proper amounts of these metal cations results in a fast change in the color of the solution from orange to yellow, while the presence of other metal ions showed no observable color change in the receptor solution. Binding assays using the method of continuous variations (Job plot) (Figure 7) suggests a 1/1 binding model (metal:ligand) with Ka = 5.4 × 105 M−1 (±1.109) and Ka = 7.4 × 103 M−1 (±1.117) for Pb2+ and Zn2+, respectively. Moreover, the calculated detection limits25 were 4.2 × 10−6 M for Pb2+ and 4.0 × 10−6 M for Zn2+.

Table 3. Emission Data for the Receptors 4−7 and Representative Metal and Anion Complexes λmax

compound d

4 [4·HP2O73−]d [4·F−]d [4·H2PO4−]d [4·AcO−]d [4·BzO−]d 5d [5·H2PO4−]d [5·AcO−]d [5·BzO−]d 6d [6·H2PO4−]d [6·AcO−]d 7e [7·Hg2+]e [7·H2PO4−]e [7·AcO−]e [7·BzO−]e a

388, 389, 389, 390, 388, 389, 388, 389, 389, 388, 388, 388, 392, 388, 388, 388, 388, 388,

405 451 450 408, 433 406, 435 407, 428 (bs) 407 410, 440 408, 441 408, 435 405 409 (bs), 446 410 (bs), 443 405, 430 (bs) 405, 430 (bs) 406, 453 406, 452 406, 452

Φ

IPa

2.5 × 10−3 2.5 × 10−3 2.2 × 10−3 2.7 × 10−3 2.5 × 10−3 2.3 × 10−3 3.3 × 10−3 3.6 × 10−3 5.6 × 10−3 4.2 × 10−3 8 × 10−3 1 × 10−2 9 × 10−3 1.06 × 10−2 3.5 × 10−3 1.03 × 10−2 1.25 × 10−2 9.3 × 10−3

Imon/Iexc

I/I0b

Dlimc

f

429 429 421 425 423 422 416 416 421 420

425 426 424

4.80 0.19f 0.23f 1.22f 0.78f 1.48f 3.03g 0.72g 0.41g 0.60g 4.44h 0.29h 0.36h 8.58f 5.47f 0.48f 0.77f 1.14f

× × × × ×

10−6 10−6 10−5 10−6 10−6

0.14 0.15 0.67 0.44 0.73

3.6 6.9 1.5 3.1 5.0

0.44 0.36 0.38

1.0 × 10−5 1.5 × 10−5 9.7 × 10−6

0.21 0.46

9.1 × 10−6 4.6 × 10−6

0.34 0.27 0.46 0.46

3.9 1.9 1.9 8.3

× × × ×

10−6 10−5 10−6 10−7

Isoemissive points in nm. bMeasured at 388 nm. cDetection limits in M. dCH3CN/CH2Cl2 (4/1) solution. eDMSO solution. fI388/I450. gI388/I440. I388/I445.

h

G


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BzO− to a solution of 5 led to a progressive decrease in intensity of the band at λ 349 nm and the appearance of a new shoulder at around λ 400 nm. Compound 6 also showed similar UV−vis spectral patterns when titrated with H2PO4− or AcO−. Clear isosbestic points were found during the titrations, indicating that a neat interconversion between the uncomplexed and complexed species occurs. In both cases, a Job plot analysis exhibited a maximum at 0.5 mol fraction of the anion (H2PO4−, AcO−, and BzO− in the case of 5, and H2PO4− and AcO− in the case of 6), suggesting the formation of the corresponding complexes with a 1/1 stoichiometry (see Table 3 and Figures S30−S33 (Supporting Information)). On the other hand, on excitation at λexc 345 nm, receptors 5 and 6 (c = 1.5 × 10−5 M, CH3CN/CH2Cl2 (4/1)) display two well-resolved emission bands at 388 and 405 mm, with a rather low quantum yield (Φ = 3.3 × 10−3 for 5 and Φ = 3.2 × 10−3 for 6), which in principle were ascribed to the monomeric emission band of the pyrene moiety present in both receptors. At this point, it must be taken into account that chemosensor 6 also contains a coumarin group, which has been extensively used as a fluorescent signal unit.26 Consequently, it might be thought that the aforementioned emission bands could also be associated with the coumarin moiety. However, such a possibility was ruled out because the photophysical properties exhibited by 6 are totally identical with those showed by 5, on excitation at the same λexc (345 nm) (Figures S34−S36 (Supporting Information)). The fluorescence titration experiments demonstrate that, while the emissive properties of 5 and 6 did not change upon addition of the monovalent and divalent metal cations tested, the addition of H2PO4−, AcO−, and BzO− to 5 or of H2PO4− and AcO− to 6 induced a clear ratiometric fluorescence changes, where a red-shifted structureless excimer emission band at around 450 nm increases and its monomer emission decreases (see in Table 3 the ratios I388/I440 for receptor 5 and I388/I446 for receptor 6 together with the magnitudes of these ratios for their corresponding anionic complexes), with concomitant appearance of an isoemissive point (Figure 8). Although from the UV−vis titration experiments an empirical 1/1 stoichiometry was established for the anionic complexes formed by 5 and 6, the observed pyrene fluorescence excimer

emission points out that the real stoichiometry for those complex species might be 2/2. To prove that this hypothesis is rational, dilution experiments were carried out once the anionic complexes were formed. The results obtained demonstrated that the emission intensity ratio of the monomer band to the excimer band of the complexes formed increases with increasing dilution. This result is consistent with the formation of an intermolecular excimer as a consequence of the interaction of two pyrene units belonging to two different molecules of the corresponding anion complex formed. Consequently, these results allow us to confirm our hypothesis about the real stoichiometry in the formation of the anionic complexes and also to rule out the possibility of formation of any mixed excimer27 between the two fluorophores present in 6. From these titration data, the detection limits were also calculated (see Table 3). Receptor 7 also shows notable changes in the absorption spectrum only upon addition of H2PO4−, HP2O73−, F−, AcO−, and BzO− anions. However, the changes observed are strongly dependent on the type of anion tested. Thus, the addition of H2PO4−, AcO−, and BzO− anions to a DMSO solution of 7 (c = 2.5 × 10−5 M, DMSO) results in a decrease in the intensity of the band appearing at λ 415 nm in the free receptor and the simultaneous appearance of a new red-shifted band (Δλ = 17 nm for H2PO4−, Δλ = 15 nm for AcO−, and Δλ = 10 nm for BzO−). In contrast, UV−vis titrations of 7 with F− and HP2O73− behave differently and a broad and intense band appears near λ 600 nm, similar to that found when the titrations were carried out with tetrabuthylammonium hydroxide. On the basis of these results we could conclude that both anions are interacting with 5 via deprotonation of the urea moiety. Because of such changes are accompanied by a color change from yellow to deep blue, clearly visible to the naked eye (Figure S39), receptor 5 could be considered as a chemodosimeter for the detection of F− and HP2O73−. With regard to the behavior of 7 toward metal cations it is worth mentioning that only the addition of Hg2+ induced slight changes in its UV−vis absorption spectrum. The best results were observed in the presence of 10 equiv of the metal cation, the most significant changes being a general hyperchromic effect for the band at λ 360 nm accompanied by a small hypsochromic shift (Δλ = 2 nm) and a hypochromic effect for the band at λ 415 nm. The binding profile and Job plot from the titration data suggest a 1/1 stoichiometry for the Hg2+ metal complex formed (Figure S40 (Supporting Information)), with a binding constant of Ka = 2 × 104 M−1, the calculated detection limit being 4.9 × 10−6 M. The chemosensor properties of 7 toward the same set of cations and anions were also evaluated by using fluorescence spectroscopy. Receptor 7 (c = 1.5 × 10−5 M) exhibits the monomer emission of its pyrene unit at λ 388, 405 nm (Φ = 1.06 × 10−2) on excitation at λexc 345 nm. Addition of AcO−, H2PO4−, and BzO− anions promotes a decrease in the intensity of the monomer emission bands and the concomitant appearance of a new band at λemi 452 nm, attributable to the excimer emission of the pyrene unit (Figure S41 (Supporting Information)). In contrast, the important changes caused in the fluorescence spectrum of 7 upon addition of Hg2+ consist of the progressive quenching of the emission bands until the metal complex was completely formed (10 equiv, Φ = 3.5 × 10−3) (Figure 9). Further additions of this metal cation (up to 40 equiv) did not give rise to any other changes in the fluorescence emission, confirming the cation recognition process and

Figure 8. Evolution of the emission spectrum of 6 (c = 1.5 × 10−5 M) in CH3CN/CH2Cl2 (4/1) (λexc 345 nm), upon addition of increasing amounts (0−35 equiv) of H2PO4−. The inset gives naked-eye fluorescence changes observed: (left) free receptor 6; (right) anionic complex. H


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Figure 9. Variations in the fluorescence of 7 (DMSO, c = 1.5 × 10−5 M) upon addition of increasing amounts (from 0 to 10 equiv) of Hg2+.

excluding the heavy metal effect.28 Moreover, none of the other cations or anions tested induced changes in the fluorescence emission or interfered with the observed complexation processes. Although from the spectrophotometric titration data a 1/1 binding model is deduced for the complexes formed between 7 and AcO−, H2PO4−, and BzO− anions, the appearance of a clear excimer emission band during the fluorescence titrations suggests that those anions induced the formation of anionic complexes with a real 2/2 stoichiometry. Then, the apparent association constants were calculated to be β = 4.44 × 1013 M−2 for AcO−, β = 8.24 × 1012 M−2 for BzO−, and β = 8.43 × 1012 M−2 for H2PO4−. Additionally, the detection limits24 were found to be 6.6 × 10−6 M for AcO−, 5,5 × 10−6 M for BzO−, and 1.8 × 10−5 M for H2PO4−. 1 H NMR Study. To support the results obtained from electrochemical, UV−vis, and fluorescence experiments, and in order to obtain additional information about the nature of the binding process of receptor 4 with ions, we also performed a 1H NMR spectroscopic analysis. The most significant changes observed during the titration experiments with Pb2+ and Zn2+ are the clear upfield shift promoted in the iminic proton (Δδ = −0.28 ppm for Pb2+ and Δδ = −0.24 ppm for Zn2) while the αand β-protons within the 1,1′-disubstituted ferrocene ring as well as the triazole proton experienced an important downfield shift due to the coordination of the metal ion: ΔδHα = 0.49 ppm for Pb2+ and 0.47 ppm for Zn2+, ΔδHβ = 0.50 ppm for Pb2+ and 0.45 ppm for Zn2+, and Δδtriazole = 0.29 ppm for Pb2+ and 0.28 ppm for Zn2+ (Figure 10 and Figure S42 (Supporting Information)). On the other hand, Figures S43−S47 (Supporting Information) illustrate the slight variations of the 1 H NMR spectra obtained upon addition of increasing amounts of F−, HP2O73−, H2PO4−, AcO−, and BzO− anions, the most significant change being the slight deshielding promoted by these anions on the H-5 triazole proton. In agreement with the electrochemical and UV−vis responses, receptors 5 and 6 do not experience any modification of their 1H NMR spectrum upon addition of the set of metal cations tested, indicating no cation binding. However, the anion binding properties of 5 demonstrate that, upon addition of H2PO4−, AcO−, and BzO−, very significant downfield shifts were observed for the NH amide proton, for the H triazole, and for the H22 and H33 protons within the pyrene unit (see Table S13 (Supporting Information)). Taken together, these results gave support to the participation of the

Figure 10. 1H NMR spectra of the titration of 4 with Pb2+ (c = 3 × 10−3 M) in CD2Cl2 upon addition of 0 (bottom), 0.3, 0.6, 1, 1.5, and 2.0 equiv (top) of the metal cation.

amide NH group in hydrogen bonding with those anions along with the CH triazole protons (Figure 11).

Figure 11. 1H NMR spectra of the titration of 5 with AcO− (c = 3 × 10−3 M) in CD2Cl2 upon addition of 0 (bottom), 0.3, 0.6, 1, 1.5, and 2.0 (top) equiv of the anion.

In contrast, upon addition of the H2PO4− and AcO− anions to receptor 6 the only important variation observed in the spectra was a small downfield shift of the triazole proton (Δδ = 0.04 ppm for both anions), indicating that the interaction with the anion should mostly take place through this proton (Figures S50 and S51 (Supporting Information)). However, the signal corresponding to the amide NH proton remains unaltered during the titration, which is probably due to the formation of an intramolecular hydrogen bond with the carbonyl group within the coumarin ring. With a view to shedding light on the coordination modes of 7 and on the nature of the complexes formed by this receptor with the H2PO4−, AcO−, and BzO− anions and with Hg2+ I


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cations, 1H NMR titrations experiments were also performed. The most significant features of the free ligand 7 are the following: (i) four broad singlets resonating at δ 4.13, 4.40, 4.76, and 5.23 ppm with relative intensities 2:2:2:2 assigned to Hα, Hα′, Hβ, and Hβ′ within the two differently monosubstituted Cp rings of the ferrocene moiety, (ii) the presence of two sets of doublets of doublets attributed to the two AB systems corresponding to the aromatic p-nitrophenyl moiety (δ 7.79 and 8.45 ppm) and to the p-disubstituted phenyl ring (δ 6.89 and 6.99 ppm) connecting the urea and azo units, (iii) a singlet at δ 9.04 ppm assigned to the triazole proton, and (iv) two broad signals at δ 8.56 and 9.54 ppm assigned to the chemically nonequivalent NH protons within the urea moiety. Figures S52−S54 (Supporting Information) show the 1H NMR spectrum of 7 before and after addition of different amounts of the appropriate anion. In general, it can be noticed that the 1 H NMR titration experiments carried out by using 7 and these anions resulted in significant downfield shifts for the urea and triazole protons, as well as the protons within phenyl ring directly linked to the urea moiety. In contrast, the ferrocenyl protons and those within the pyrene ring remaining essentially unaffected (see Table S15 (Supporting Information)). Taking into account these anion-induced chemical shift changes, it is plausible to suggest that the urea NH protons, placed in the vicinity of such most affected protons, should be involved in the interaction of these anions with the free receptor. In contrast, no measurable shifts were observed in the 1H NMR spectrum of 7 when the titration experiments were carried out with Hg2+ cations and, consequently, we were unable to accurately analyze the binding mode taking place. Ion Pair Recognition Study. The design and application of new heteroditopic receptor systems capable of the simultaneous coordination of both anionic and cationic guest species is an emerging field in supramolecular chemistry.29 Among the ion pair receptors reported, crown ether moieties, multidentate N or O ligands, and electron-rich aromatic rings are usually utilized as the cation binding sites,30 while the anion is coordinated using Lewis acidic, electrostatic, or hydrogenbonding interactions.31 Such systems are interesting not only as switches32 but also in molecular sensing.33 Despite these applications, the number of well-characterized ion pair receptors remains limited and only a few examples of the multichannel detection of ion pairs have been reported.8b,c,14,34 In this context, it is interesting to note that the studies discussed earlier clearly demonstrated that addition of metal cations did not induce any change either in the electrochemical response or in the absorption and emission spectra of 6. In contrast, positive responses were detected when some divalent metal cations were added to a solution of 6 containing 1 equiv of a coordinating anion (H2PO4− or AcO−).35 Interestingly, this behavior was not observed when receptors 4, 5, and 7 were used. These findings clearly show that small variations in the structure of ostensibly similar molecular receptors have a significant impact on their ability to recognize targeted substrates such as ion pairs. Thus, the oxidation peak of the H2PO4− and AcO− anion complexes formed by 6 undergoes a cathodic shift (see Tables S16 and S17 (Supporting Information)) upon addition of Mg2+, Ca2+, Cd2+, and Zn2+ metal cations (Figure 12 and Figures S67 and S68 (Supporting Information)). Such an observation confirmed that those divalent metal cations are only recognized when anions are previously bound to the anion binding site of 6.

Figure 12. OSWV in CH3CN/CH2Cl2 4/1 (c = 5 × 10−4 M, TBAHP as supporting electrolyte) of the free receptor 6 (black), the complex formed with H2PO4− ([6·H2PO4]−, red) and the ion pair complex with Zn2+ ([6·H2PO4·Zn]+, green).

Similarly, addition of Mg2+, Ca2+, Cd2+, and Zn2+ metal cations to the preformed receptor−anion complexes also induced slight perturbations of the absorption spectrum of 6, which are responsible for the change in color from yellow to pink. Binding assays using the method of continuous variations (Job plot) suggest a 1/1 binding model for ion pair complexes formed between the anionic complex and the metal cation, with Kas = 3.1 × 104, 4.5 × 104, 5.6 × 104, and 2.8 × 104 M−1 for the cases of Zn2+, Cd2+, Mg2+, and Ca2+, respectively. Moreover, this behavior was also supported by using emission spectroscopy. A ratiometric perturbation of the emission spectrum is observed when divalent metal cations are added to the anion complexes formed. Thus, addition of 8 equiv of Zn2+ to a solution containing the preformed complex [6·H2PO4]22− results in a dramatic enhancement of the monomeric emission band at 388 and 405 nm and the quantum yield (Φ = 7 × 10−2), resulting in a 9-fold increase (Figure S74 (Supporting Information)). Similar behavior was observed upon addition of Mg2+, Ca2+, and Cd2+ cations to this complex. When the addition of the corresponding anion and cation was done simultaneously as a mixture, identical results were obtained. In contrast, when the experiments were carried out with the [6· AcO]− complex, the disappearance of the excimer band and the strong decrease of the monomeric emission band were simultaneously observed (Figure S75). 1 H NMR and DFT calculations were used for further investigating the interactions between the anion-complexes and those divalent metal cations. Thus, upon addition of such metal cations to the previously formed [6·A−] complexes the following spectral changes were evidenced: the triazole proton (green) is downfield shifted (Δδ ∼ + 0.35 ppm); the coumarine protons (blue) are also significantly downfield shifted although showing the same pattern of signals as in [6· A−]; the pyrene H-33 proton (red) is upfield shifted (Δδ ≈ −0.60 ppm) (see Figure S1 (Supporting Information), Figure 13, and Table S20 (Supporting Information)). Quantum Chemical Calculations. QC calculations at the DFT level (see Computational Details) have allowed unveiling the structural and stoichiometric features of complexes derived from ligand 6. First of all, the structure of the ligand itself was computed, starting from the geometry obtained by X-ray diffraction studies (Figure 3). In the computed most stable geometry (Figure 14a), within the 3-coumarinylcarboxamido side arm the amide NH and the coumarin carbonyl group form J


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from formamide optimized at the same level, the magnitude of the H bonding was found to be 1.82 kcal/mol, from which the π-stacking is estimated to amount to 10.18 kcal/mol. Next, the behavior of 6 toward certain anions, as exemplified by H2PO4−, was explored theoretically. The starting geometry was chosen so as to fit with the experimentally found 1/1 ligand/anion stoichiometry, as well as the observed pyrene fluorescence excimer emission, therefore requiring intermolecular pyrene−pyrene parallel pairing by means of π stacking. Dihydrogen phosphate anions should be located in an antiparallel fashion, bridging both triazole units by acting as an H donor toward N3 of one triazole ring and as an H acceptor toward H5 in the other heterocyclic unit (Figure 15).

Figure 13. Evolution of the 1H NMR spectrum (aromatic region) of the free ligand (bottom), of the previously formed [6·H2PO4−] complex (second spectrum from the bottom) and upon addition of increasing amounts of Zn2+ (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4 equiv on top).

Figure 15. Sketched proposal for the 2/2 complex of 6 with H2PO4−. Anions are color coded in blue or gray to denote relative proximity to the viewer.

In such an arrangement, the distant coumarinyl-carboxamido group was assumed to have little effect on the anion complexation and, therefore, the simplest ligand 5 was used for the sake of computational efficiency. The minimum-energy structure obtained without constraints for the roughly C2symmetric [52(H2PO4)2]2− complex (Figure 16a) features an almost perfect parallel alignment of both pyrenyl mean planes (angle 2.2°) at a typical π-stacking distance (average 3.416 Å at pyrene ring centroids). Every H2PO4− anion strongly binds to N3 (dPOH···N3 = 1.839 Å; WBI = 0.068; ρ(r) = 3.95 × 10−2 e/ ao3) and with moderate strength to N2 (dPOH···N2 = 1.957 Å; WBI = 0.043; ρ(r) = 2.87 × 10−2 e/ao3) of one triazole and the acidic H5 atom of the second triazole unit (dPO···H5 = 2.039 Å; WBI = 0.021; ρ(r) = 2.11 × 10−2 e/ao3). In addition and close to the latter, the same dihydrogen phosphate O atoms form complementary weak H bonds with two other aryl CH groups belonging to the Cp (dPO···HC = 2.356 Å; WBI = 0.008; ρ(r) = 1.21 × 10−2 e/ao3) and pyrene rings (dPO···HC = 2.666 Å; WBI = 0.003; ρ(r) = 0.66 × 10−2 e/ao3). An optimal fit of all five acid (H-donor) and basic (H-acceptor) highly directional binding interactions on every ligand toward both amphoteric anion units requires rotation of the pyrene ring plane with respect to the triazole mean plane (43.0°) and the ferrocenyl ends are kept almost parallel (6.3°) at a π-stacking interacting distance (dCp1a···Cp2a = 3.303 Å) with lateral slippage of Cp units with respect to each other (1.503 Å). Ion pairing was modeled for the case of reaction of 6 with H2PO4− anion and Zn(ClO4)2. The most stable arrangement was found for the contact ion pair [Zn(H2PO4)]+-6-(ClO4)− complex, in which the perchlorate and dihydrogen phosphate are translocated (Figure 16b). In such a structure the cationic unit binds the basic side of the ligand through the carboxamide O atom (dZn−O = 1.948 Å; WBI = 0.219; ρ(r) = 8.33 × 10−2 e/ ao3) and the triazole N2 atom (dZn−N = 2.019 Å; WBI = 0.257; ρ(r) = 8.20 × 10−2 e/ao3) to complete the tetrahedral

Figure 14. Computed (COSMOMeCN/B3LYP-D3/def2-TZVP) most stable (a) and nonstacked (b) geometries for ligand 6.

a strong H bond (d = 1.881 Å; WBI = 0.034; ρ(r) = 3.30 × 10−2 e/ao3) that keeps a rigid framework, roughly coplanar with the ferrocenyl Cp group by virtue of an additional weaker HCp···O interaction with the amide carbonyl group (d = 2.409 Å; WBI = 0.002; ρ(r) = 1.34 × 10−2 e/ao3). The other pyrenyl− triazolyl side arm is located preferentially in a stacked conformation which is stabilized by a parallel heteroarene πstacking interaction between the coumarin and pyrene moieties (interplane angle 0.82°),36 as well as by H bonding between the amide carbonyl group and the triazolyl H5 atom (dCO···H = 2.612 Å; WBI = 0.002; ρ(r) = 0.80 × 10−2 e/ao3). In total, the interarm stabilizing interactions in 6 can be estimated to be 12.00 kcal/mol by comparison with the second most stable nonstacked conformer 6conf (Figure 14b), lacking both of them. Furthermore, from the hypothetical formamide derivatives 8 and 8conf (not shown) built up from frozen geometries of 6 and 6conf by replacing the coumarin moiety by an appropriately located H atom, and using bond lengths and angles obtained K


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Figure 16. Computed (COSMOMeCN/B3LYP-D3/def2-TZVP) most stable geometries for the model [52(H2PO4)2]2− complex (a) and the anion translocated ion pair [Zn(H2PO4)]+-6-(ClO4)− (b).

Electrochemical studies carried out with receptor 7 demonstrated that only addition of Hg2+ cations caused variation in the redox response of the receptor, whereas in the presence of anions only F−, H2PO4, HP2O73−, AcO−, and BzO− caused perturbation in the oxidation peak. Notable changes in the absorption spectrum are observed in the presence of Hg2+ cations and H2PO4−, AcO−, and BzO− anions, whereas F−, and HP2O73− clearly induced deprotonation with a concomitant color change. The monomer emission band is redshifted to the excimer emission band position in the presence of H2PO4−, AcO−, and BzO− anions, whereas addition of Hg2+ cations induced a progressive quenching of the emission band. In summary, the formation of the key intermediate ferrocene−triazole−pyrene triad 3 allows the formation of a bottom arm densely decorated by different nitrogen-binding sites end-capped by photoactive units which give rise to multichannel ferrocenyl recognition receptors for anions, cations, and ion pairs.

coordination sphere around the metal. At the other −amphoteric−side, the carboxamido NH group is intramolecular stabilized by H-bonding with the coumarine carbonyl group. The ClO4− anion interacts tightly with the ligand owing to its ability to form several hydrogen-bonding interactions with the coumarin NH group (dO···HN = 2.285 Å; WBI = 0.005; ρ(r) = 1.22 × 10−2 e/ao3), the triazole C−H atom (dO···HC = 2.393 Å; WBI 0.007; ρ(r) = 1.03 × 10−2 e/ao3), and several other weak interactions with ferrocene and pyrene H atoms. Such an unusual anion contact ion pair arrangement is energetically favored (by ca. 29.80 kcal/mol) with respect to the isomer keeping the original Zn(ClO4)+ group and entails moderate ligand strain (6.68 kcal/mol). Conclusions. Three different 1,1′-disubstituted ferrocene derivatives with a common pyrene−triazole-based upper arm and decorated with a bottom arm bearing a nitrogen-based binding site, aldimine, amide, and urea, linked to substituents displaying different optical properties have been synthesized. Addition of Pb2+ and Zn2+ metal cations to an electrochemical solution of receptor 4 induced an anodic shift (ΔE1/2 = 200 mV for Pb2+ and 45 mV for Zn2+) of the redox potential of the ferrocene/ferrocenium redox couple, and the addition of Cu2+ and Hg2+ promotes the oxidation of the ferrocene moiety, whereas the addition of F−, H2PO4−, HP2O73−, AcO−, and C6H5COO− anions promoted a remarkable cathodic shift (ΔE1/2 ranging from −75 mV for C6H5COO− anions to −112 mV for HP2O73− anions). The UV/vis spectrum of receptor 4 undergoes perturbation in the presence of Pb2+ and Zn2+ accompanied by a noticeable color change of the solution, while addition of the aforementioned anions did not induce any type of change. However, the emission spectrum of receptor 4, in the presence of these anions, undergoes a remarkable decrease in the intensity of the monomer emission band with concomitant increase of the pyrene excimer emission band. No emission changes were observed upon addition of metal cations. Receptor 6 is shown to be a rare ion pair recognition receptor; it demonstrates a dramatic enhancement of Mg2+, Ca2+, Cd2+, and Zn2+ cation binding by cobound H2PO4− anions, whereas no affinity of the free receptor by these divalent metal cations is observed. Receptor 6 exhibits a perturbation of the redox potential of the ferrocene unit (ΔE1/2 = 29−48 mV), a dramatic enhancement of the monomeric emission band and the quantum yield, resulting in a 9-fold increase, and a noticeable color change in the presence of the aforementioned cations when the H2PO4− anion is bound to the anion binding site.

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EXPERIMENTAL SECTION

General Methods. 1,1′-Bis(azido)ferrocene15 was prepared as described previously in the literature. Melting points were determined on a hot-plate melting point apparatus and are uncorrected. 1H and 13 C spectra were recorded on a 300, 400, or 600 MHz apparatus. The following abbreviations have been used for stating the multiplicity of the signals: s (singlet), bs (broad singlet), d (doublet), dd (doublet of doublets), ddd (doublet of doublets of doublets), t (triplet), pt (pseudotriplet), td (triplet of doublets), q (quaternary carbon). Chemical shifts refer to signals of tetramethylsilane in the case of 1H and 13C spectra. CV and OSWV techniques were performed with a conventional three-electrode configuration consisting of platinum working and auxiliary electrodes and a Ag/AgCl reference electrode. The experiments were carried out with a 3 × 10−4 to 5 × 10−4 M solution of sample in an adequate solvent containing 0.1 M TBAHP as the supporting electrolyte. Ferrocene was used as an internal reference both for potential calibration and for reversibility criteria. Under similar conditions, ferrocene has E1/2 = 0.39 V vs SCE and the anodic−cathodic peak separation is 67 mV. Deoxygenation of the solutions was achieved by bubbling nitrogen for at least 10 min, and the working electrode was cleaned after each run. The cyclic voltammograms were recorded with a scan rate increasing from 0.05 to 1.00 V s−1, while the OSWV curves were recorded at a scan rate of 100 mV s−1 with a pulse height of 10 mV and a step time of 50 ms. The guest under investigation was added as a 2.5 × 10−2 M solution in the appropriate solvent using a microsyringe while the electrochemical properties of the solution were monitored. UV−vis and fluorescence spectra were obtained in the solvents and concentrations stated in the text and in the corresponding figure captions. UV−vis spectra were carried out in a UV−vis−NIR spectrophotometer using a dissolution L


Organometallics

Article 3

J = 7.8 Hz), 7.45 (1H, d, 3J = 8.7 Hz), 7.20 (1H, d, 3J = 8.7 Hz), 5.18 (2H, pt, Fc), 4.92 (2H, pt, Fc), 4.53 (2H, pt, Fc), 4.45 (2H, pt, Fc). 13 C NMR (CD2Cl2, 101 MHz): δC 156.3 (q, imine), 131.8 (q), 130.6 (q), 129.9 (q), 129.8 (CH), 129.3 (q), 128.8 (q), 127.8 (q), 127.7 (q), 127.3 (CH), 127.2 (CH), 127.1 (CH), 126.6 (CH), 126.5 (CH), 126.4 (CH), 126.4 (CH), 126.3 (CH), 125.7 (CH), 125.3 (CH), 125.1 (CH), 125.0 (CH), 124.9 (CH), 124.8 (q), 124.5 (q), 124.4 (q), 124.2 (CH), 124.0 (CH), 123.9 (q), 123.6 (CH), 123.5 (q), 123.4 (CH), 123.3 (q) 123.0 (q), 122.5 (q), 121.4 (CH, triazole), 105.9 (q, Fc), 99.7 (q, Fc), 68.6 (CH, Fc), 67.0 (CH, Fc), 64.0 (CH, Fc), 62.4 (CH, Fc). ESI-MS: m/z (relative intensity) 681.2 (M+ + 1, 100). Anal. Calcd for C45H28FeN4: C, 79.42; H, 4.15; N, 8.23. Found: C, 79.18; H, 4.43; N, 8.02. Synthesis of 1-[4-(1-Pyrenyl)-1,2,3-triazol-1-yl]-1′(acetylamino)ferrocene (5). A solution of 1-[4-(1-pyrenyl)-1,2,3triazol-1-yl]-1′-aminoferrocene (50 mg, 0.11 mmol)8c and acetic anhydride (5 mL) in anhydrous THF (10 mL) was stirred at room temperature under a nitrogen atmosphere for 4 h. Then, H2O (20 mL) was added and the THF was removed under vacuum. The resulting mixture was extracted with CH2Cl2 (2 × 100 mL) and the organic phase washed with a saturated solution of sodium bicarbonate (100 mL). The organic phase was dried over anhydrous MgSO4 and the solvent removed under vacuum to give a yellow solid (0.048 g, 88% yield) which was crystallized from CH2Cl2. Mp: 190−192 °C. 1H NMR (CD2Cl2, Me4Si, 600 MHz): δH 8.86 (d, 1H, 3J = 9.2 Hz), 8.35 (d, 1H, 3J = 8.1 Hz), 8.30 (s, 1H), 8.29 (d, 1H, 3J = 8.1 Hz), 8.25 (d, 1H, 3J = 7.4 Hz), 8.24 (d, 1H, 3J = 7.4 Hz), 8.20 (d, 1H, 3J = 9.2 Hz), 8.15 (d, 2H, 3J = 13.8 Hz), 8.13 (d, 1H, 3J = 13.8 Hz), 8.06 (t, 1H, 3J = 7.4 Hz), 6.83 (s, 1 H), 5.01 (pt, 2H, Fc), 4.74 (pt, 2H, Fc), 4.37 (pt, 2H, Fc), 4.14 (pt, 2H, Fc), 1.78 (s, 3H). 13C NMR (CD2Cl2, Me4Si, 600 MHz): δC 168.7 (q, CO), 147.5 (q), 131.8 (q), 131.3 (q), 128.8 (q), 128.5, 128.3, 127.7, 127.4, 126.6, 125.8, 125.6, 125.4 (q), 125.3, 125.3 (q), 125.2, 125.1 (q), 125.0 (q), 123.5, 96.7 (q, Fc), 94.8 (q, Fc), 67.8 (CH, Fc), 66.5 (CH, Fc), 63.6 (CH, Fc), 63.4 (CH, Fc), 24.1. ESI-MS: m/z (relative intensity) 511.4 (M+ + 1, 100). Anal. Calcd for C30H22FeN4O: C, 70.60; H, 4.34; N, 10.98. Found: C, 70.49; H, 4.59; N, 10.70. Synthesis of 1-[4-(1-Pyrenyl)-1,2,3-triazol-1-yl]-1′-[(coumarin-3-carbonyl)amino]ferrocene (6). A mixture of thionyl chloride (0.4 mL, 5.5 mmol) and coumarin-3-carboxylic acid (0.029 g, 0152 mmol) was stirred at 100 °C under a nitrogen atmosphere for 2.5 h. Then, the remaining thionyl chloride was removed under vacuum and a solution of 1-[4-(1-pyrenyl)-1,2,3-triazol-1-yl]-1′-aminoferrocene (0.05 g, 0.107 mmol) in THF (10 mL) was added, under a nitrogen atmosphere. The resulting solution was stirred at room temperature for 3 h, and afterward, the solvent was removed under vacuum. The resulting solid was washed with dichloromethane and chromatographed on a silica gel column using CH2Cl2/AcOEt (9/1) as eluent (Rf = 0.6), yielding a deep red solid which was crystallized from CH2Cl2 (0.056 g, 82% yield). Mp: 240−243 °C dec. 1H NMR (CD2Cl2, Me4Si, 400 MHz): δH 9.80 (1H, s, H13), 8.98 (1H, d, 3J = 9.3 Hz, H24), 8.23 (1H, s, H17), 8.18 (1H, d, 3J = 7.6 Hz, H31 or H33) 8.17 (1H, d, 3J = 8 Hz, H23), 8.16 (1H, d, 3J = 7.6 Hz, H31 or H33), 8.11 (1H, d, 3J = 0.6 Hz, H41), 8.07 (1H, d, 3J = 9.3 Hz, H25), 8.03 (1H, t, 3J = 7.6 Hz, H32), 8.02 (1H, d, 3J = 8.0 Hz, H22), 8.01 (1H, d, 3J = 8.9 Hz, H29 or H30), 7.92 (1H, d, 3J = 8.9 Hz, H29 or H30), 6.52 (ddd, 1H, 3J = 8.3 Hz, 3J = 7.6 Hz, 4J = 1.6 Hz, H44), 6.43 (1H, dd, 3J = 7.6 Hz, 4J = 1.6 Hz, H42), 6.25 (1H, ddd, 3J = 8.3 Hz, 4J = 1 Hz, 4J = 0.6 Hz, H45), 6.21 (1H, td, 3J = 7.6 Hz 4J = 1 Hz, H43), 5.19 (2H, pt, Fc), 5.04 (2H, pt, Fc), 4.40 (2H, pt, Fc), 4.21 (2H, pt, Fc). 13C NMR (101 MHz; CD2Cl2): δC 161.6 (q, CO, 37), 160.0 (q, CO, 34), 158.3 (q, 36), 153.4 (q), 147.7 (CH, 41), 147.51 (q, 16), 132.8 (CH, 44), 131.7 (q), 131.3 (q), 131.1 (q), 128.5 (CH, 42), 128.4 (CH, 25), 127.9 (CH, 29 or 30), 127.7 (q), 127.5 (CH, 29 or 30), 126.4 (CH, 22) 126.3 (CH, 31 or 33), 125.6 (CH, 31 or 33), 125.4 (CH, 23), 125.3 (CH, 24), 125.1 (CH, 32), 124.8 (q), 124.6 (q), 123.9 (CH, 43) 121.9 (CH, 17), 117.3 (q, 38), 116.9 (q, 40), 115.2 (CH, 45), 96.6 (q, Fc), 95.0 (q, Fc), 67.4 (CH, Fc), 66.5 (CH, Fc), 62.9 (CH, Fc), 62.5 (CH, Fc) ppm. ESI-MS: m/z (relative intensity) 640.1 (M+ + 1, 100). Anal. Calcd for

cell with 10 mm pathlength, and they were recorded with the spectra background corrected before and after sequential additions of different aliquots of cations/anions in the same solvent (c = 2.5 × 10−2 M). Fluorescence spectra were carried out in a fluorescence spectrophotometer using a 10 mm fluorescence cell. Before the spectra were recorded, the samples were deoxygenated, to avoid fluorescence quenching via oxygen, by bubbling nitrogen for at least 5 min. All of the spectra were recorded before and after the sequential additions of different aliquots of a solution of cations/anions (c = 2.5 × 10−2 M). Quantum yield values were measured with respect to anthracene as the standard (Φ = 0.27 ± 0.01) using the equation Φx/Φs = (Sx/Ss)[(1 − 10−As)/(1 − 10−Ax)](ns2/nx2), where x and s indicate the unknown and standard solutions, respectively, Φ is the quantum yield, S is the area under the emission curve, A is the absorbance at the excitation wavelength, and n is the refractive index.37 The association constants were obtained using the computer program SPECFIT.18 For their calculation the corresponding titrations were carried out three or four times in order to test the reliability of the results and the calculation of the associated error that are included in Table 2. X-ray Structural Analysis. Suitable single crystals for X-ray diffraction were mounted in a loop fiber and transferred to a Bruker D8 QUEST diffractometer. Data were recorded at 100(2) K using multilayer-monochromated Mo Kα radiation (λ = 0.71073 Å) in ωscan mode. Multiscan absorption correction was applied. Crystal structures were solved by direct (compound 5) or Patterson (compound 3) methods, and all non-hydrogen atoms were refined anisotropically on F2 using the program SHELXL-97. Hydrogen atoms were refined using a standard riding model unless otherwise stated. Special Features and Exceptions. For compound 5, the N4−H04 hydrogen atom was refined freely. One dichloromethane molecule is disordered over three positions of equal occupancy, and all the atoms of the components were refined isotropically. Large residual electron density peaks are present near the disordered dichloromethane and may be due to a more complicated disorder effect. Computational Details. Quantum chemical calculations were performed with the ORCA electronic structure program package.38 All geometry optimizations were run with tight convergence criteria using the B3LYP39 functional together with the new efficient RIJCOSX algorithm40 and the def2-TZVP basis set.41 In all optimizations and energy evaluations, the latest Grimme semiempirical atom-pairwise correction (DFT-D3), accounting for the major part of the contribution of dispersion forces to the energy, was included.42 Solvent effects (acetonitrile) were taken into account via the COSMO solvation model.43 From these geometries all reported data were obtained by means of single-point (SP) calculations using the same functional as well as the more polarized def2-TZVPP41,44 basis set. Reported energies are uncorrected for the zero-point vibrational term. The topological analysis of the electronic charge density, ρ(r), was conducted using AIM2000 software,45 and the wave functions (electron density) were generated with the Gaussian09 software package.46 Wiberg bond indices (WBI)47 were obtained from a natural bond orbital (NBO) population analysis.48 Synthesis of 1-[4-(1-Pyrenyl)-1,2,3-triazol-1-yl]-1′(pyrenylmethyleneamino)ferrocene (4). To a solution of 1-[4(1-pyrenyl)-1,2,3-triazol-1-yl]-1′-azidoferrocene (50 mg, 0.10 mmol) in anhydrous THF (40 mL) was added a 1 M solution of trimethylphosphine (0.5 mL, 0.5 mmol) under a nitrogen atmosphere at room temperature. The mixture was stirred for 30 min, and a solution of 1-pyrenecarboxaldehyde (23.3 mg, 0.10 mmol) in anhydrous THF (10 mL) was gradually added. Then, the reaction mixture was stirred at room temperature for 6 h. Afterward, the solvent was removed under vacuum and the orange solid obtained was crystallized from CH2Cl2, yielding 38.5 mg (56%) of 4. Mp: 243−246 °C. 1H NMR (CD2Cl2, Me4Si, 600 MHz): δH 9.26 (s, 1H, imine), 8.77 (1H, d, 3J = 9.3 Hz), 8.30 (1H, d, 3J = 7.9 Hz), 8.27 (1H, d, 3J = 9.1 Hz), 8.20 (1H, s, H-triazole), 8.13 (1H, d, 3J = 7.5 Hz), 7.94 (1H, d, 3J = 9 Hz), 7.93 (1H, t, 3J = 7.5 Hz), 7.88 (1H, d, 3J = 9.3 Hz), 7.84 (1H, d, 3J = 7.5 Hz), 7.83 (1H, d, 3J = 7.2 Hz), 7.76 (1H, d, 3J = 7.2 Hz), 7.69 (1H, t, 3J = 7.2 Hz), 7.68 (1H, d, 3J = 9 Hz), 7.56 (1H, d, 3J = 7.8 Hz), 7.50 (1H, d, 3J = 9.1 Hz), 7.49 (1H, d, 3J = 7.9 Hz), 7.47 (1H, d, M


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C38H24FeN4O3: C, 71.26; H, 3.78; N, 8.75. Found: C, 71.54; H, 3.89; N, 8.57. Synthesis of 1-[4-(1-Pyrenyl)-1,2,3-triazol-1-yl]-1′-{3(E)-[(4nitrophenyl)diazenylphenyl]ureido}ferrocene (7). A solution of 1-[4-(1-pyrenyl)-1,2,3-triazol-1-yl]-1′-aminoferrocene (50 mg, 0.11 mmol) and (E)-1-isocyanato-4-(4-nitrostyryl)benzene (58 mg, 0.22 mmol) in anhydrous THF (10 mL) was stirred at room temperature under a nitrogen atmosphere for 2 h. Then, the solvent was removed to dryness under vacuum and the resulting solid was sequentially washed with diethyl ether (1 × 100 mL) and chloroform (2 × 1 mL). The solid was crystallized from chloroform, giving rise to a deep red solid (47 mg, 60% yield). Mp: 184−186 °C. 1H NMR (CD2Cl2; Me4Si, 600 MHz): δH 9.54 (1H, s, urea), 9.05 (1H, d, 3J = 9 Hz), 9.04 (1H, s, H-triazole), 8.56 (1H, s, urea), 8.45 (2H, d, 3J = 8.6 Hz), 8.29 (1H, d, 3J = 8 Hz), 8.23 (1H, d, 3J = 7.8 Hz), 8.16 (1H, d, 3J = 8 Hz), 8.15 (1H, d, 3J = 9 Hz), 8.04 (1H, d, 3J = 7.8 Hz), 7.99 (1H, d, 3J = 8.4 Hz), 7.98 (1H, t, 3J = 7.8 Hz), 7.90 (1H, d, 3J = 8.4 Hz), 7.79 (2H, d, 3 J = 8.6 Hz), 6.99 (2H, d, 3J = 8.4 Hz), 6.89 (2H, d, 3J = 8.4 Hz), 5.23 (2H, pt, Fc), 4.76 (2H, pt, Fc), 4.40 (2H, pt, Fc), 4.13 (2H, pt, Fc). 13 C NMR (CD2Cl2, 101 MHz): δC 154.7 (q, CO), 151.5 (q), 147.1 (q), 146.1 (q), 144.9 (q), 142.9 (q), 130.1 (q), 129.8 (q), 129.7 (q), 129.6 (q), 127.0 (CH), 126.6 (CH), 126.4 (CH), 126.0 (CH), 125.4 (CH), 124.6 (CH), 124.5 (CH), 124.4 (CH), 124.2 (CH), 124.1 (CH), 124.0 (q), 123.6 (q), 123.2 (q), 122.8 (CH), 122.5 (CH), 122.3 (CH), 116.2 (CH), 98.1 (q, Fc), 93.5 (q, Fc), 66.2 (CH, Fc), 64.5 (CH, Fc), 61.4 (CH, Fc), 60.1 (CH, Fc). ESI-MS: m/z (relative intensity) 737 (M+ + 1). Anal. Calcd for C41H28FeN8O3: C, 66.86; H, 3.83; N, 15.21. Found: C, 66.87; H, 3.87; N, 15.10.

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and CIF and xyz files giving NMR spectra for all compounds, X-ray characterization data for 3 and 5, electrochemical, UV−vis, fluorescence, and 1H NMR titration data, and all computed Cartesian coordinates in a format for convenient visualization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.T.: [email protected]. *E-mail for P.M.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the MICINN-Spain, Project CTQ2011/27175, FEDER. We also want to dedicate this paper to our beloved Prof. Alan R. Katrizky (University of East Anglia, U.K., and Gainesville, Florida, USA), who recently died.

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REFERENCES

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Organometallics

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Organometallics

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