CONTENTS
Scheme 4.1.1: Synthesis of transition metal complexes of oxime 2.1.3
4.1.4: UV-vis and fluorescence spectroscopic studies of metal complexes of oxime with fluoride ion
Due to small size and high charge density on fluoride ion act as base to interact with fluorophore or chromophore containing phenols,9 ureas,10 thioureas,11 oximes,12 sulphonamides13 and amides.14 Such complexationsof the oxime derivatives to cause specific signal transductions. Fluorescent sensors based on high chemical affinity of the fluoride ions towards silicon15 or boron16 atoms are reported in literature. The silicon based receptors in aqueous solution, requires several minutes to complete the detection process,15 which is a disadvantage to use such compounds as receptors. In an another approach, the fluoride ions are detected on the basis of their specific tendency to coordinate to a hard metal ion for example, divalent calcium,17 trivalent iron18 or tetravalent zirconium ions.19 Such metal complexes require pre-treatment to avoid interference of other metal ions such trivalent aluminium or iron ions while detecting fluoride ions.18 Platinum(II) complexes possessing triarylboron group containing ligands show distinct phosphorescent response on interaction with fluoride ions.20Among the other metal complexes, ruthenium-2,2'-bipyridine,21 Fe(III)- thiourea complexes22 have been used todetect fluorides ions. Fluorescence receptor having a rhodamine unit senses fluoride ions and was used in imaging of fluoride ions in living cell.23 Conventionally, the ligand exchange reactions of fluoride ions from a complex help to detect of fluoride ions by spectroscopic tools. In such reactions, one or more ligands are released to generate spectroscopic or electrochemical signals. Despite of known utility of hydroxyaromaticaldoximes in the detection of fluoride ions,24 there are also many organic compounds25 which selectively interact with fluoride ion. Hydroxyaromatic oxime metal complexes have not been explored for such a purpose. UV-Vis spectra of these complexes were recorded in water, DMSO-water and aprotic polar solvents such as DMSO, DMF, DMA and THF with or without adding of different solution of tetrabutylammonium salts (chloride, bromide, acetate, fluoride, phosphate). The changes on addition of anions do not affect the spectra of the complexes in water or DMSO-water mixture (Fig. 4.1.15-4.1.16), but a definite change was found in each case in aprotic polar solvents with fluoride and hydroxide ions.
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Independent UV-vis spectra of copper, nickel and zinc complexes with different tetabutylammonium salts such as fluoride, chloride, bromide, phosphate and carbonate are shown in Fig. 4.1.7a-c.
(a) (b) (c)
(d) (e) (f)
Figure 4.1.7: UV-vis spectra of (a) copper, (b) nickel and (c) zinc complex (10-5 M) with different tetrabutylammonium salts (50 µl of 10-3 M) and UV-vis spectroscopic titration of (d) copper, (e) nickel and (f) zinc complex (10-5 M) with TBAF (10 μl aliquot of 10-3 M) in dimethylsuphoxide (DMSO).
It shows that only the terabutylammonium fluoride or hydroxide causes change in the UV-vis spectra of these complexes and effect on the rest of the salts are negligible (Fig. 4.1.7a-c).
The parent ligand as well as the zinc complex 4.1.7 has a strong absorption at 259 nm, whereas copper and nickel complex has absorption at 277 nm and 256 nm respectively. The changes caused by the fluoride ions are shown separately in Fig. 4.1.7d-f, which shows that the gradual increase of the fluoride ions yields a new absorption peak at 357 nm (for copper complex) or 382 nm (for nickel complex) or 343 nm (for zinc complex) and such a change passes through an isobestic point at 290 nm, 306 nm and 307 nm. Positions of the new absorptions peak in the case of the complexes 4.1.1-4.1.3 on interacting with tetrabutylammonium fluoride are independent of solvents of crystallization. However, the position of the new peak generated upon interactions with fluoride ions is dependent on the central metal ions. From the shifts in absorption of nickel, copper and zinc complexes has a definite trend, the zinc complex shows shift shorter than the copper and the nickel shows the highest shift. This is attributed to the electronic factors of these metal ions.
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The electron occupancies in d-orbitals follow the sequence Ni(II), Cu(II) and Zn(II). The metal complexes having polyhydroxy-groups have been shown to form hydrogen bonds with fluoride ions to cause a colour change.26 We have also shown earlier that the oxime 2.1.3 acts as fluoride sensor showing spectral changes in UV region.27 The UV-vis spectra of the copper complex 4.1.1 and the nickel complex 4.1.5 in acidic (pH = 6), neutral and basic (pH = 9) medium in dimethylsulphoxide. In basic medium complexes show a peak longer wavelength which is similar to the peak that was observed in presence of fluoride ions (Fig. 4.1.17).
Whereas, the UV-vis spectra of the complexes in acidic medium tally with the peak of the ligand, suggesting that the complexes decompose in acid medium. These experiments also show that the fluoride detection by these complexes is possible only at a neutral condition in aprotic medium.
The zinc complex 4.1.7 has a fluorescence emission at 346 nm on excitation at 270 nm.
However, a new fluorescence emission at 415 nm (Fig. 4.1.8) on addition of fluoride ions to the complex 4.1.7 was observed. This emission peak was very selective to the fluoride ions among different ions tested (Fig. 4.1.8). Further to these we have examined the fluorescence emission of the zinc complex 4.1.7 in different pH (Fig. 4.1.17e-f). The zinc complex is very weakly fluorescent in neutral and in acidic medium. To the solution of this complex at pH = 6 when tetrabutylammonium fluoride was added, no change in fluorescence emission took place.
(a) (b)
Figure 4.1.8: The changes in the fluorescence emission spectra of zinc complex 4.1.7 (10-5 M solution in DMSO) on addition of (a) tetrabutylammonium salts (fluoride, chloride, bromide, phosphate, carbonate) (50 µl of 10-3M) and (b) tetrabutylammonium fluoride (10 μl aliquot of 10-3 M in DMSO).
This suggests that the complex is stable at such pH and does not respond to fluoride ions. But this complex at pH = 9 showed a strong emission at 415 nm. This showed that a substrate have ability to abstract the acidic hydrogen of the ligand cause a similar effect with the
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complexes. The emission peak at 415 nm was associated with a shoulder at 375 nm (Fig.
4.1.8b). At lower concentration of fluoride ions the shoulder of this peak is not observed but peak at 415 nm was observed. Hence the peak at 375 nm is attributed to hydrogen bond formation. As the concentration of fluoride ions were increased proton transfer took place showing the prominent peak at 415 nm arising from the anionic form of the complex. At lower concentration of fluoride ions, there is equilibrium between hydrogen bonded assembly of metal complex and deprotonated metal complex caused by fluoride ions. Once more amounts of fluoride ions are added the equilibrium shifts toward deprotonated metal complex which emits at 415 nm.
A series of anions were tested for changes in fluorescence emission with the solution of complex 4.1.7 and also the relative sensitivity of the complexes were tested. The emission profiles are shown (Fig. 4.1.18a, experimental section, page-122) by a bar graph diagram.
Beside this, the effect of the sensitivity of the emission caused by various anions were checked and found that the fluoride ions can be easily detected in the presence of other ions such as acetate, biphosphate, bromide, carbonate, chloride, perchlorate etc (Fig. 4.1.18b).
Hence, complex 4.1.7 is a highly sensitive fluorophore for the detection of fluoride ions.
Comparing the fluorescence emission spectra of the complex 4.1.7 in basic medium with the medium in presence fluoride ions showed that deprotonation of hydroxy group of the ligand was responsible for such an increase in intensity of emission with a shift of fluorescence towards longer wavelength. Hence, a highly sensitive zinc-oxime complex for the detection of fluoride ions is identified. The complexation of divalent metal ions having a d10 configuration through deprotonation of ligands generally causes fluorescence enhancement of fluorophoric units.8 The detection limit of the complex 4.1.7 was found to be 4.86×10-6 M which was calculated by using 3α/k as the detection limit, whereas, α is the standard deviation of the complex and k is the slope of intensity vs. concentration plot (Fig. 4.1.19a, experimental section, page-123). Thus, the basic behavior of the fluoride ion deprotonate all these complexes to form resonance stabilized anions shown in Scheme 4.1.2. Divalent zinc possess a d10 configuration, generates less resonance stabilized anion; whereas, the d9 and d8 configuration of the central metal ions have comparatively higher delocalization, hence the absorption occurs at a longer wavelength than the corresponding zinc complex. Phenols shows absorption changes on deprotonation by fluoride ions,10 but such a change may occur without a proton transfer.11 Specific changes in absorption by the fluoride ions in the present case but not caused by the respective acetate or hydroxide tetrabutylammonium salt showed that proton-transfer is not necessarily required for such a process. In basic medium these
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complexes are deprotonated, hence fail to detect fluoride ions. Based on these observations, Scheme 4.1.2 is proposed to explain the absorption changes.28
Scheme 4.1.2: Tautomeric structures generated from the complexes by the interactions of