Inorganic Chemistry Communications 35 (2013) 186–191

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Inorganic Chemistry Communications

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Monometallic rhenium(I) complexes as sensor for anions

Arumugam Ramdass a, Veerasamy Sathish a, Murugesan Velayudham b,1, Pounraj Thanasekaran b,⁎,Kuang-Lieh Lu b,⁎, Seenivasan Rajagopal a,⁎⁎a School of Chemistry, Madurai Kamaraj University, Madurai 625 021, Indiab Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan

⁎ Corresponding authors. Tel.: +886 2 27898518; fax⁎⁎ Corresponding author. Tel.: +91 452 2458246; fax:

E-mail addresses: ptsekaran@yahoo.com (P. Thanasekllu@gate.sinica.edu.tw (K.-L. Lu), rajagopalseenivasan@

1 Present address: Department of Inorganic and Physof Science, Bangalore 560 012, India.

1387-7003/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.inoche.2013.06.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 April 2013Accepted 13 June 2013Available online 20 June 2013

Keywords:AnionsRe(I) complexesColorimetric sensor

Three new tricarbonyl rhenium(I) polypyridine complexes containing amide, urea and thiourea derivatives of2,2′-bipyridine ligands L1–L3 have been synthesized, characterized and found to show significant UV–visibleabsorption spectral changes on the addition of anions CN−, F−, CH3COO− and H2PO4

−. These spectral changesand 1H NMR titrations of complexes 1–3with anions indicate strong binding of anions with the Re(I) complexesand the binding constants are in the range of 103–105 M−1. The strong binding is attributed to the H-bondformation/deprotonation of \NH proton in the ligand and the selectivity of anion is in the order CN− N F− N

CH3COO− N H2PO4−.

© 2013 Elsevier B.V. All rights reserved.

There has been upsurge of interest in the detection and quantifica-tion of anions in the last decade owing to the importance of anions invarious chemical, biological and environmental science processes andin the host–guest chemistry [1]. Anions are ubiquitous in biologicalsystem and their recognition is achieved through non-covalent interac-tions such as hydrophobic effect, electrostatic interactions, and halogenand hydrogen bonding [2]. Up to now, several anion receptors such asimidazoles [3], pyrroles [4], calixpyrroles [5], amides [6], carbamides[7], urea [8], and thiourea [9] have been developed and the labile pro-tons of the receptors are recognized through anions by either hydrogenbonding or deprotonation [10]. In this view, the design and synthesis ofreceptors sensitive to the anion–receptor interactions and capable ofshowing either a chromogenic and/or fluorogenic response are ofmajor concern [11]. In the past few years increasing attention hasbeen paid to the study of anion sensing using colorimetric sensor,because the change of color is discerned by ‘naked eye’ which is verysimple and does not require any equipment [12]. Although many re-ports are available for sensing of anions through organic compounds,transitionmetal based anion sensor is still a commendable practice [13].

Among the transition metal complexes, rhenium(I) tricarbonylcomplexes with d6 configuration have rich photophysical and photo-chemical behavior such as high stability and fairly strong emission inthe visible region. In addition, though the application of Re(I)tricarbonyl complexes towards sensors [14], catalysis [15], light-emitting devices [16], CO2 fixation [17], optical switches [18], and

: +886 2 27831237.+91 452 2459139.karan),yahoo.com (S. Rajagopal).ical Chemistry, Indian Institute

rights reserved.

imaging [19] has been developed enormously their role as the anionsensor is very limited [20]. Lees et al. reported that the luminescentrhenium(I) tricarbonyl pyridine complexes served as receptors foranions via intramolecular hydrogen bonding with high selectivity[21]. Tzeng and co-workers also reported the anion recognition capa-bility of Re(I)-based square containing dipyridyl-amide ligandthrough hydrogen bonding and it is realized using UV–vis absorptionand 1H NMR spectral techniques [22]. Herein we report the synthesisand characterization of three new tricarbonyl rhenium(I) complexescontaining amide, urea and thiourea derivatives of 2,2′-bipyridineligands and their role as sensor for anion through UV–vis absorptionand 1H NMR spectral techniques as well as naked eye.

The ligands were synthesized in good yield by a simple condensa-tion reaction between 4-methyl-2,2′-bipyridine-4-carboxaldehydeand corresponding amine in ethanol medium. The formation of ligandswas confirmed by NMR, IR, Mass spectral and elemental analysistechniques. The formation of imine linkage of the ligands L1–L3 wascharacterized by a signal at δ 8.27, 8.25, and 8.24 (\N_CH) in 1HNMR spectrum and a band at 1661, 1679 and 1593 in IR spectrumrespectively. The HR-ESI-MS analysis showed a molecular ion peak atm/z = 317.14, 332.15, and 348.13 with the experimental isotopepattern matching the calculated values for the ligands L1–L3(Figs. S1–S3). The reaction of starting material Re(CO)5Br with thebidentate ligands L1–L3 refluxed in toluene leads to the formation offac-[Re(CO)3(N–N)Br] species which are air and moisture stable, kinet-ically inert and the structure of their complexes are shown in Chart 1.These complexes 1–3 were characterized by NMR, IR, Mass spectraland elemental analysis techniques. In IR spectra, the carbonyl regionshows three peaks at 2023, 1917, and 1900 cm−1 which correspondto A′(1) symmetric, A′(2) asymmetric and A″ symmetric stretchingfrequencies [23]. The HR-ESI-MS analysis showed a molecular ionpeak at m/z = 664.98, 679.99, and 695.97 with the experimental

N

N

N

HN

CH3

ORe

Br

OCCO

OC N

NN

HN

CH3

HN

ORe

Br

OCCO

OC

N

NN

HN

CH3

HN

SRe

Br

OCCO

OC

1 2

3

Chart 1. Structure of the rhenium(I) complexes.

187A. Ramdass et al. / Inorganic Chemistry Communications 35 (2013) 186–191

isotope pattern matching the calculated values for the complexes 1–3(Figs. S4–S6).

The absorption spectra of ligands L1–L3 and Re(I)-tricarbonyl com-plexes 1–3 were measured and the absorption maximum values aregiven in Table 1. The electronic absorption spectra of Re(I)-tricarbonylcomplexes reveal strong absorption bands at 194–352 nm and lessintense absorption shoulders at ca. 390 nm (Table 1). The high-energyintense band in the UV region is assigned to the ligand centered (LC)π–π* transition and it is also observed for the free ligands (Fig. S7).The low energy absorption band at 390 nm is assigned to spin allowedmetal to ligand charge transfer (MLCT) transition from the Re dπ- orbit-al to the π* orbital of the ligand [dπ (Re) → π* (ligand)] (Fig. 1) [24].However, in complex 3 the ligand centered band was shifted to thetune of 57 nm from 295 to 352 nm compared to that in complexes 1and 2 due to the presence of electron rich sulfur atom in thioureamoiety which may facilitate delocalization. Our complexes are non-luminescent in nature because these complexes bear imine N-atomand similar type of Re(I) complexes is reported earlier [25].

The anion-sensing ability of complexes 1–3 in acetonitrile hasbeen studied by UV–vis absorption spectral titration experiments.The detectable change in the absorption spectrum can be noticed onthe addition of tetrabutylammonium (TBA) salt solution of CN−, F−,CH3COO− and H2PO4

− to the acetonitrile solution of complexes 1–3.Conversely, little absorption spectral change was observed on theaddition of Cl−, Br−, I−, SCN−, ClO4

− and NO3− ions (Fig. 2 and Figs. S8,

S9). With the addition of incremental amounts of CN− ions to the solu-tion of complex 1, the absorption peak at 250 nm is slightly blue shiftedto 245 nm and the peak at 316 nm is blue shifted by 19 nm to 297 nm.In addition a newpeak appears in the region at 462 nm (Fig. 3). The for-mation of new band may be due to the hydrogen bond formation withamide proton or deprotonation of amide proton [26]. The previous liter-ature and 1H NMR studies show that the formation of new band is dueto the increase in the extent of π-conjugation of the deprotonatedamide moiety [26]. Similar type of results was also observed with

Table 1Absorption spectral data of ligands and complexes in CH3CN medium.

Compounds λabs, nm (εmax, dm3 mol−1 cm−1)

1 194 (66,495), 250 (26,500), 316 (27,604), 390 (8000)2 196 (79,565), 235 (36,863), 295 (23,981), 390 (8000)3 194 (66,850), 248 (28,000), 300 (22,500), 352 (28,731), 391 (19,500)L1a 205 (48,266), 246 (40,135), 291 (58,113)L2a 205 (58,046), 240 (58,960), 286 (53,056), 303 (53,234)L3a 208 (61,923), 246 (40,135), 282 (38,308), 326 (67,826)

a CH3CN: THF (98:2).

other anions such as F−, CH3COO−, and H2PO4− with complex 1. Some

interesting results have been observed with complex 1 on increasingconcentration of F− anion (Fig. S10). The formation of hydrogen fluo-ride (HF) from the reaction of complex 1 with F− anion by deproton-ation of NH proton is responsible for these spectral changes. Morebasicity of fluoride is responsible for selective recognition amonghalides, which is corroborated by previous report [27]. The observationof well-defined isosbestic point at 350 nm in this UV–vis absorptionspectral changes is indicative of ground state complex formationbetween the anion with the amide moiety of the complex 1 [28].

The urea group contains two acidic NH protons which can beenvisaged as good binding sites for anions [29]. Urea moiety is presentin complex 2. Upon addition of incremental amounts of F− anions tothe solution of complex 2, the absorption peak at 235 nm is slightlyred shifted to 245 nm and peak at 295 nm is marginally blue shiftedby 4 nm to 291 nm indicating the binding of anion with the Re(I) com-plex. In addition to that, the formation of newpeak at 486 nm (Fig. S11)is due to the deprotonation of NH proton of the urea moiety. Similarkind of results was also observed with other anions such as CN−,CH3COO−, and H2PO4

− with complex 2. The well-defined isosbesticpoint at 353 nm in the UV–vis absorption spectral region is indicativeof ground state interaction between the anion with the urea moiety ofthe complex 2. Fabbrizzi and co-workers [30], reported intensively on

Fig. 1. Absorption spectra of complexes 1–3 (20 μM) in CH3CN medium.

Fig. 4. Absorption spectral changes of complex 3 (20 μM) with increasing concentrationof F− (0–200 μM) anion in CH3CN medium.

Fig. 2. Absorption spectral studies of complex 1 (20 μM) with different anions (200 μM)in CH3CN medium.

188 A. Ramdass et al. / Inorganic Chemistry Communications 35 (2013) 186–191

elucidating the nature of urea–fluoride interactions in urea based sys-tems, both mono- and double deprotonation by F− anion.

Thiourea based receptors are also important binding moieties ofrelatively basic anions and the binding can lead to a deprotonationof one, or in some cases even two amidic NH, with interesting anduseful consequences for colorimetric anion sensing [31]. In our case,the complex 3 contains thiourea moiety. Upon gradual addition ofstandard solution of F− anion to the solution of complex 3, the ab-sorption peak at 352 nm is red shifted from 31 nm to 383 nm(Fig. 4). The formation of new band at 502 nm is due to deproton-ation of NH proton of the thiourea moiety. The observation of anisosbestic point at 410 nm indicates the formation of a stable complexor new species with unique spectroscopic properties as a result of in-teraction between the anion with the thiourea moiety of the complex3. This large shift in isosbestic point of 410 nm is attributed to thepresence of thiourea moiety in complex 3. This thiourea moiety ismore acidic than urea and it is expected to form stronger hydrogenbond/deprotonation leading to formation of intense red color whenthe anion is added to complex 3. This stronger hydrogen bond forma-tion/deprotonation leads to substantial red shift in the absorptionmaxima of 3. Similar results have already been reported by Amendolaet al. and Evans et al. [32]. Similar result was also observed with otheranion such as CN−, CH3COO−, and H2PO4

− with complex 3. Lees

Fig. 3. Absorption spectral changes of complex 1 (20 μM) with increasing concentrationof CN− (0–200 μM) anion in CH3CN medium.

and co-workers [33], reported on elucidating the nature of thiourea–cyanide interactions in thiourea based systems, and clearly explainedthe deprotonation mechanism by CN− anion.

The binding constant of complexes 1–3 with anions is calculatedthrough the Benesi–Hildebrand equation [34] and the values arecollected in Table 2. The Benesi–Hildebrand plot is shown in Fig. S12.The absorption spectral changes indicate strong binding of anionswith the Re(I) complexes and the binding constants are in the rangeof 103–105 M−1. Among these four anions CN− has the highest bindingconstant, probably due to high negative charge density andmore basic-ity of CN− anion [35]. The Job's plot is applied for the determination ofthe stoichiometric ratio between the Re(I) complex and anion guests(Fig. 5). This plot shows 1:1 ground state complex formation betweenanion and Re(I) complex.

To establish the mechanism of recognition process through eitherhydrogen bonding interaction or deprotonation we carried out detailed1H NMR spectroscopic titrations on complexes 1–3 in DMSO-d6. 1HNMR studies are effective for obtaining useful information on the siteof binding of anions with Re(I) complex [20]. Fig. S13 shows thechanges observed in the 1H NMR spectrum of complex 1 upon theaddition of F− anion. In the 1H NMR of free complex 1, the imideproton signal of 1H appears at 12.48 ppm. Upon gradual addition ofthefluoride salt, the amideproton signal first gets broadened andfinallydisappeared. This result indicates the abstraction of the imide proton ofcomplex 1 to form HF [27].

The binding properties of complex 3 for F− anion were also exam-ined by 1H NMR spectral titration studies in DMSO-d6. The 1H NMRspectra of complex 3 recorded in the absence and presence of F− areshown in Fig. 6. In the 1H NMR of free complex 3, two \NH protons ina different chemical environment appear at 12.46 (H2) and 10.44 (H1)ppm. The addition of F− to complex 3 resulted first in the broadeningand finally disappearance of signals suggesting an interaction of the

Table 2Binding constant values of complexes 1–3 with anions using absorption spectraltechnique.

Anion 1 2 3

K ± 0.2, M−1 K ± 0.2, M−1 K ± 0.2, M−1

F− 2.4 × 104 6.2 × 104 7.1 × 104

CN− 3.0 × 104 6.5 × 104 8.7 × 104

CH3COO− 7.2 × 103 8.9 × 103 1.1 × 104

H2PO4− 4.6 × 103 5.1 × 103 5.8 × 103

Fig. 5. Job's plot for complex 1 with F− anion.

Fig. 7. Color change observed in (A) complex 1, (B) complex 2, and (C) complex 3 inthe presence of various anions in CH3CN.

189A. Ramdass et al. / Inorganic Chemistry Communications 35 (2013) 186–191

anion with NH groups. The disappearance of NH proton suggests theabstraction of two protons by F− anion to form HF2− which appearedas triplet at 16 ppm [36]. But this peak is not present in our studiesdue to the presence of little amount of water in DMSO-d6 [37]. Thebinding properties of complex 3with CH3COO− anionwere also studiedby 1H NMR spectral titration in DMSO-d6 (Fig. S14). In complex 3, twoNH protons in a different chemical environment appear at 12.46 (H2)and 10.44 (H1) ppm. The addition of CH3COO− to complex 3 resulted

Fig. 6. Partial 1H NMR (300 MHz, d6-DMSO) spectral changes of complex 3 with the addition of [Bu4N]F.

190 A. Ramdass et al. / Inorganic Chemistry Communications 35 (2013) 186–191

first in the broadening and finally disappearance suggesting deproton-ation of two protons from NH groups.

The photograph in Fig. 7 shows the dramatic color changes thatoccur when the CH3CN solution of the receptors 1–3 is treated withCN−, F−, CH3COO− andH2PO4− ions as it is in the formof TBA salt. Con-versely, no color change was observed on the addition of Cl−, Br−, I−,SCN−, ClO4

− and NO3− ions. In general, the color change was from a

light yellow to red. The origin of the color change in the presence ofCN−, F−, CH3COO− and H2PO4

− ions could be ascribed to the deproton-ation of the NH protons of the amide/urea/thioureamoiety of the recep-tors 1–3. It is interesting to note that the intensity of color of Re(I)–anion complex/adduct depends on the strength of binding i.e. on thebinding constant value. The color of the product is more intense in thepresence of F−/CN− whereas it is less intense in the presence ofCH3COO−/H2PO4

−.Conclusion. A series of new three Re(I) metal complexes with

amide, urea and thiourea derivatives of 2,2′-bipyridine ligands havebeen synthesized and characterized by various spectral techniques.These complexes were found to show significant spectral changesupon addition of anions such as CN−, F−, CH3COO−, and H2PO4

− in-vestigated by UV–vis absorption and 1H NMR spectroscopy. This sens-ing of anions is attributed to the deprotonation of NH proton in theamide/urea/thiourea. It is interesting to note that thiourea unit hasthe strongest hydrogen bonding ability with anions, followed byurea and amide moiety has the least ability. Thus the change in thestructure of the ligand in the Re(I) complex changes the capabilityof its role as the sensor for anions. The strong binding affinity andcolor changes towards anions make Re(I) complexes as promisingcandidates for sensor application.

Acknowledgment

We thank the Department of Science and Technology (DST) andthe Council of Scientific and Industrial Research (CSIR), New Delhifor the generous funding in the form of a project. Prof. Lu acknowl-edges the financial support from the Academia Sinica and the Nation-al Science Council of Taiwan. A.R. is the recipient of UGC MeritoriousFellowship under the Basic Scientific Research (BSR) scheme.

Appendix A. Supplementary material

Experimental section, mass spectral data of complexes 1–3, UV–vis spectral data of ligands L1–L3 and 1H NMR spectroscopic titrationsof complexes 1 and 3 with anions are available. Supplementary datato this article can be found online at http://dx.doi.org/10.1016/j.inoche.2013.06.024.

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