Uv-visible and fluorescence spectra of the ligand 1,3-bis(2,2’:6’,2’’-ter- pyridyl-5-ymethylsulfanyl)propane L and it’s iron(II) complex have been investigated for analytical purposes. The two spectra of L and terpy are very similar which confirmed the ability of L to co-ordinate through the six N atoms of L with minimum distortion of the metal ion’s octahedral geometry. The ligand-based absorption band of L is shifted to t he longer wavelength. It was found that L is able to displace the two terpyridine groups in the complex to give [FeL] 2 . The high stability of the complex makes it good in spectrophotometry analysis of metals ions in solution. The fluorescence of L was progressively quenched with an increasing concentration of iron(II). This makes L a possible reagent for the quantitative analysis of metal by measuring fluorescence quenching.
Spectrophotometric and fluorescence methods are sensitive for metal ion analysis. The equipment is compact and easy to use. This has made the two methods popular, and they are used in routine laboratories all over the world. Any attempt to improve on detection limits of this type of equipment is valuable.
Spectroscopy analysis is based on the relationship between the degree of absorption and the concentration of the absorbing materials. Some ligands formed coloured complex with metals. The intense of the colour of the metal complexes can make spectrophotometer instruments to be able to detect very low concentration of metal, hence improving the detection limit.
Fluorescence is one of the several mechanisms by which a molecule can return to the ground state after being exited by absorption of radiation. All molecules have a potential to fluoresce but most do not, because their structure provides a radiationless pathway at a greater rate than the fluorescence emissions. Compounds that can fluoresce are often rigid molecules such as aromatics, compounds with conjugated double bonds, and heterocycles.
Inorganic fluorimetry is often based upon the reaction of an analyte with a chelating agent to form a complex that fluoresces or upon measurement of fluorescence quenching as a result of the analyte co-ordination. The fluorescence is measured at 90˚ to the excitation light. Fluorescent spectroscopy can often give high sensitivity and specificity. In favourable cases, it can measure as low as 10−9 mg/cm3 of analyte. The intense colour of the terpyridine metal complexes can make such instruments capable of detecting very low concentration of metal, hence improving the detection limit [
It has been recognised that metals sometimes react with ligands to form intensely coloured compounds. These ligands have been used for the determination of metal ions in solution at very low concentration. There are a number of ligands that are available for analysis of metals. Those for determining low concentration of iron(II) include 2.2’-bipyridyl, 4,7-diphenyl-10-phenanthroline, and 2,4,6-tri(2-pyridyl)-1,3,5-triazine; they react with iron(II) to form Fe(II) complexes with their molar extinction coefficient (εmax) of 800, 2240, 2260 dm3 mol−1∙cm−1 respectively as demonstrated by R. C. Denny et al. [
This ligand 1 has been recognised as a useful ligand in the fields of organic, organometallic and co-ordination chemistry. It is a good ligand for analytical chemistry since it forms bis(terpyridine) metal complexes with many metal ions e.g. the iron(II) complex 2 which has blood red colour. 2,2’:6’,2’’-Terpyridine co- ordinates strongly through the three nitrogen atoms, and remains planar in a meridonal mode of co-ordination.
Terpyridine ligands (terpy), and their metal complexes, can have very intense colours which arise from electronic transitions p®p* and metal to ligand charge transfer. The terpyridine ligand possess low-lying p* orbital which are able to accept electrons density from the metal as earlier explain by G. T. Morgan et al. [
It has been reported that 2.2’;6’2’’-terpyridine (terpy) and its substituted derivative form stable complexes with transition metals such as iron(II) and nickel (II) [
For example bis(2,2’:6’,2’’-terpyridine) iron(II) complex which is stable over a wide range of pH, and has a high molar extinction coefficient (ε552 12 500 dm3∙mol?1∙cm?1) as earlier reported by R. L. Morris [
The detection limit in the use of these ligands depends on the stability of the complexes that are formed, and on the magnitude of the molar extinction coefficients of the complexes.
In search for even more ligand that forms more stable complexes with metals we synthesis and characterise a new ligand (1,3-bis(2,2’:6’,2’’-terpyridyl-5-yl- methylsulfanyl) propane L as reported by Gleb et al. [
Therefore the objective of this study is to react ligand L with iron(II) and evaluate whether can be used in determining iron in solution by UV/Vis spectroscopy and Fluorescence method of analysis.
The ligand L was synthesis using the methods reported by Gleb et al. 2000 [
The electronic absorption spectra of L and terpy (3) are shown in
L and terpy show lmax at 238 nm and 280 nm respectively. Their molar extinction coefficients (εmax) are 46,000 dm3∙mol−1∙cm−1 and 18,000 dm3∙mol−1∙cm−1 respectively. It was noted that both terpy and L exhibit similar electronic transitions. Curve (L) indicates that one linked terpyridine has a higher absorbance than terpyridine alone, and the longest wavelength peak is shifted to a longer wavelength.
A uv/vis spectra of methanolic solutions of [Fe(terpy)2]2+ and [FeL]2+ was obtaine at concentrations of 1.43 × 10−5 mol∙dm−3 and 1.19 × 10−5 mol∙dm−3 respec-
tively and then the normalised. The two spectra were very similar which confirmed the ability of L to co-ordinate through the six N atoms of L with minimum distortion of the metal ion’s octahedral geometry. The ligand-based absorption band of L is shifted to the longer wavelength. They both exhibit a metal to ligand charge-transfer band at 552 nm of molar extinction coefficients.
(ε552 nm) of 11,500 dm3∙mol−1∙cm−1 [FeL]2+ and 9050 dm3∙mol−1∙cm−1 [Fe(terpy)2]2+.
The two complexes show two ligand-based absorption bands [FeL]2+ at 276 and 328 nm, and [Fe(terpy)2]2+ at 273 and 319 nm. The maximum molar extinction coefficients for each of the complexes are (ε328 nm) 41,800 dm3∙mol−1∙cm−1 [FeL]2+ and (ε319 nm) 41,200 dm3∙mol−1∙cm−1 [Fe(terpy)2]2+.
A solution of L (4.92 × 10−6 mol∙dm−3) was reacted with increasing concentrations of standardised Fe2+ ions of concentration between (2 × 10−6 - 2 × 10−5 mol∙dm−3) to form [FeL]2+. The fluorescence of these solutions was scanned in a 1 cm fluorescence cuvette from 350 - 600 nm with an excitation at 276 nm.
L is luminescent with a maximum in the emission spectrum at 350 nm.
A solution of iron(II) tetrafluoroborate (28.8 mg, 0.0835 mmol) in methanol (30
cm3) was added dropwise to a solution of L (50 mg, 0.0835 mmol) in methanol (refluxed to dissolve), and the resulting deep purple solution was stirred for 30 minutes. Excess methanolic ammonium hexafluorophosphate was added, and the volume reduced with a rotary evaporator to precipitate [FeL](PF6)2. The precipitate was filtered and washed with ether to give the complex as fine purple solid, (yield 41 mg, 53%).
13C NMR (100 MHz,CDCl3): d/p.p.m 161.26, 160.86, 158.92, 156.44, 155.45, 153.61, 141.82, 139.54, 139.47, 138.94, 128.14, 124.4, 124.04, 123.74, 118.14, 33.06, 31.05, 29.08.Elemental Analysis; observed C 42.1, H 3.85, N 8.06 and calculated for C35H30N6S2P2F12Fe 2.5H2O is C 42.5, H 3.56, N 8.49% FAB mass spectrum; calculated for C35H30N6S2P2F12Fe i.e. [FeL][PF6]2 m/z 944.6.
Found no molecular ion but daugther ion at m/z 799, 673, and 654.
Ferroin indicator was first prepared by dissolving FeSO4∙(NH4)2SO4∙6H2O (784 mg, 0.2 mmol) in deionised water (10 cm3). To this solution, 1,10-phenathroline monohydrate (120 mg, 0.6 mmol) was added to form [Fe(phen)3]2+ (ferroin) which has a red blood colour [
Each solution was transferred in turn to a quartz fluorescence cuvette and fluorescence emission spectrum was scanned between 200 - 450 nm with a Perkin Elmer LS50 luminescence spectrometer. The excitation wavelength was 276 nm (exit slit 5 nm and em. slit 10 nm). The spectrum was processed using a spread sheet.
The electronic absorption spectra of L and terpy (3) are shown in
L and terpy show lmax at 238 nm and 280 nm respectively. Their molar extinction coefficients (εmax) are 46,000 dm3∙mol−1∙cm−1 and 18,000 dm3∙mol−1∙cm−1 respectively. It was noted that both terpy and L exhibit similar electronic transitions. The Uv/Vis spectra of both ligand L and Terpy are very similar which confirmed the ability of L to co-ordinate through the six N atoms of L with minimum distortion of the metal ion’s octahedral geometry. The ligand-based absorption band of L is shifted to the longer wavelength. They both exhibit a metal to ligand charge-transfer band at 552 nm of molar extinction coefficients (ε552 nm) of 11,500 dm3∙mol−1∙cm−1 and 9050 dm3∙mol−1∙cm−1 respectively.
The stability of [FeL]2+ is demonstrated by the ability of L to displace both terpy ligands in [Fe(terpy)2]2+ which was as earlier discussed by Gleb et al. [
Volume of L/cm3 | Volume of Fe2+/cm3 | Volume of solvent (metha-nol)/cm3 | Final concentration of Fe2+/mol∙dm−3 |
---|---|---|---|
0.3 | 0.40 | 9.30 | 8 × 10−6 |
0.3 | 0.30 | 9.40 | 6 × 10−6 |
0.3 | 0.20 | 9.50 | 4 × 10−6 |
0.3 | 0.10 | 9.60 | 2 × 10−6 |
0.3 | 0.00 | 9.70 | 0 |
Uv spectra of both L and terpy are very similar.
Iron(II) is low spin when complex with L, which contributes to the high stability of the complex, therefore, making L a good reagent for the determination of iron(II) by spectrophotometric method of analysis.
The ligand L is fluorescent. From the results in
The use of L for the analysis of other Fe(II) ions in the presence of several other metal ions, e.g. in industrial waste water, needs further investigation.
Maritim, P.K. (2017) Evaluation of Hexadate Ligand 1, 3-bis(2,2’:6’,2’’-Terpyridyl-5-Ylmethylsulfany l) Propane in the Determination of Iron(II) in Solution by Spectrophotometric and Fluoremetric Methods of Analysis. Open Access Library Journal, 4: e3715. https://doi.org/10.4236/oalib.1103715