4 Results
4.4 Copper binding molecules such as anions alter the chelation rate
The effect of the perchlorate filter could actually be a result of UV light filtration by water as irradiation through a perchlorate (and phosphate) filter only changes the final spectrum slightly more than irradiation through 2 mm pure water (figure 4.23). Chloride has a larger effect than sulphate but even if the chloride concentration in the filter is quadrupled to 200 mM, it has a much smaller effect than 50 mM bromide.
Nitrate, with the highest cut off, has the largest inhibitory effect as expected.
Percentage of Cu
2+chelated by [2
4.3
1]adz after 30 min.
0%
100%
Water Perchlorate Phosphate Nitrate Water Perchlorate Phosphate Nitrate Water Perchlorate Phosphate Nitrate
25° C 40° C 55° C
Anion filters in front of UV light Added anions + UV light Added anions in darkness Netto effect of UV light
Figure 4.22: “3D” experiment investigating the effect of temperature and anions as filters and in solution.
The effect of NanX (X = ClO4-, PO43- or NO3-) on Cu2+ chelation by [24.31]adz was measured by spectrophotometry.
Concentrations after mixing: 0.10 mM Cu2+, 0.2 mM [24.31]adz and 20 mM X if X were added. If X were in the filter, the concentration was 50 mM. pH after mixing: 7.4 at 25 °C. Reaction temperatures: 25°, 40° and 55 °C. After 30 min 25 mL 10 mM HClO4 was added and the absorption spectrum measured. On the basis of the absorbance at 289 nm, the percentages of Cu(II) chelated by [24.31]adz were calculated.
4.4.2 Some anions can chemically affect the chelation rate
Besides having physical effect on the chelation rate by blocking the UV light, anions can coordinate free and/or chelated Cu(II) stabilising that state, they can react with activated molecules and they can stabilize the pH. Without addition of anions, the degree of chelation after 30 min in darkness at 55 °C was 46 % in the 3D experiment (figure 4.22). Phosphate is known to bind Cu2+ and as expected, it lowered the chelation to 31 % at 55 °C. At low temperatures, phosphate increases chelation when looking solely at the resulting spectra at 289 nm. However, addition of phosphate to the reactants increases their absorptivity at 289 nm, and
correcting for that reveals no chelation at 25 °C with phosphate in solution. At 40 °C, there is no increase in absorption at 600 nm in the phosphate sample, indicating that the chelation values for phosphate added in
darkness in figure 4.22 might be too high. It is only the phosphate spectra, which have a discrepancy between the absorption at 289 nm and 600 nm.
Nitrate, which in contrast only coordinates the complex, enhanced the chelation to 59 %, while non-coordinating perchlorate has no effect (47 %). At 55 °C the additive effect of nitrate and light seems to disappear.
UV spectra after irradiation for 1 hour
0 0.03 0.06 0.09 0.12 0.15 0.18
200 250 300 350 400
nm
Absorption
No filter Water Perchlorate Phosphate Sulfate Chloride 4xChloride Bromide Nitrate t = 0
Figure 4.23: Effect of 2.0 mm 50 mM sodium salt solutions as UV filters on Cu2+ chelation by [24.31]adz measured by spectrophotometry. Concentrations after mixing: 0.43 mM Cu2+ and 1.05 mM [24.31]adz irradiated for 1 hour at 45 °C
Since the inhibitory effect of nitrate is “blocking UV-light”, even small concentrations have a large effect, and increasing the nitrate concentration from 0.5 mM to 10 mM has no visible effect the first 200 seconds. In contrast, sulphate only has a minor effect at 0.5 mM. In fact, it even seems like it increases chelation after 1000 seconds. However, the shape of the curve indicates that this is artefact causes by increased absorption of the copper adamanzane complex if water is exchanged with sulphate as 5th ligand. The shape of the curve for 10 mM sulphate supports this, ending almost the same place as the control but with a much higher rate at 1000 seconds. Perchlorate is not expected to influence on chelation rate but it is possible that it could bind to the chelate at high concentrations thereby increasing the absorption at 285 nm. Most interesting is it that chloride in solution has an effect close to that of bromide even though the UV filtering effect is much lower.
At 0.5 mM there is only a small reduction of chelation; probably due to the light filter effect since bromide and chloride only bind marginally better as 5th ligand than water. At 10 mM chloride, chelation is
approximately halved, depending in how much chloride increases absorption as 5th ligand. Bromide has a larger effect but not enough to be explained solely by the light filter effect; so chloride must interfere with chelation in other ways than by absorbing UV light.
It is well known that basic solutions with access to atmospheric air absorb CO2 and accumulate -HCO3 - and -CO3
-2. When mixed with a Cu(II) solution the carbonates, like other Cu(II) binding anions, probably would
inhibit chelation. In all chelation experiments an acidic Cu(II) solution was mixed with a basic adamanzane asolution and the older these solutions were, the slower the chelation. Therefore, only solutions prepared the same day were compared.
Chelation of Cu2+ by [24.31]adz with anions in solution
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 200 400 600 800 1000
Seconds
Absorption at 285 nm
No anions added 0.5 mM bromide 10 mM bromide 0.5 mM sulphate 10 mM sulphate 0.5 mM chloride 10 mM chloride 0.5 mM nitrate 10 mM nitrate 92 mM perchlorate
Figure 4.24: Effect on Cu2+ chelation by [24.31]adz of adding sodium salt solutions measured by the absorption at 285 nm. The samples were irradiated continuously without filter for 1000 seconds. Concentrations after mixing: 0.10 mM Cu2+, 0.10 mM [24.31]adz. The initial absorption at 285 nm of the samples has been subtracted.
4.4.3 Heat enhances light induced chelation and induces light independent chelation At 40 °C in the 3D experiment, there is a significant chelation in darkness but in addition to inducing chelation independent of light, heat also enhances the UV light induced chelation. In all samples, the light induced chelation (light effect) is increased when the temperature is raised from 25 °C to 40 °C. At 55 °C, some samples reach 100 % chelation before the incubation is over. As a result, the effect of irradiation seems to be lower for the water and perchlorate samples at 55 °C but that is an artefact, which would not have been seen if chelation time had been shorter.
When identical samples are irradiated in a thermostated spectrophotometer cell, the initial chelation rate is doubled for every 10 °C the temperature is raised (data not shown).
4.4.4 Dissolved O2
Since the light induced chelation is dependent on irradiation of the reactants in the LMCT band, it is likely that induction involves electron transfer between Cu(II) and its surroundings. Furthermore, copper redox chemistry is known to be dependent on the oxygen concentration and photo induced formation of a Cu(I) complex by reduction of Cu(II) is inhibited by O2 [152]. Therefore, it is possible that O2 also can inhibit light
induced chelation of Cu(II) by adamanzanes, if redox chemistry is involved. To test this, a volume from a solution of [24.31]adz and from a Cu(ClO4)2 solution was mixed and irradiated in a spectrophotometer for 300 seconds. Then both the copper and the adamanzane solution had oxygen removed by bubbling argon through them before identical volumes were mixed and irradiated.
Chelation of Cu2+ by old [24.31]adz
0 0.1 0.2 0.3 0.4 0.5
0 60 120 180 240 300
Seconds
Absorption at 285 nm
Chelation rate of Cu2+ by old [24.31]adz
0 0.02 0.04 0.06 0.08
0 60 120 180 240 300
Seconds
ΔAbs./min. at 285 nm
Under atm.
Under Argon
Figure 4.25: Effect of dissolved gases on chelation, and the rate per minute of 200 µM Cu2+ and 300 µM [24.31]adz measured by the absorption at 285 nm. Samples from a copper and an adamanzane solution was mixed and measured twice a second for 300 seconds. Then the copper and adamanzane solutions had dissolved gases removed by bobbling argon through them for an hour before repeating the mixing and measurement.
The samples under argon initially had a 1.9-fold higher absorption at 285 nm, increasing during irradiation to almost 2.3 fold (figure 4.25). The rate of the samples under argon was initially 4.3 fold higher decreasing to 2.8 fold after 30 seconds. The rate of all samples dropped fast the first 60 seconds followed by a slower more steady decrease. This is peculiar, since a slower rate was expected to change the shape of the rate curve as in figure 4.15 c. However, it should be remembered that only the very first part of the chelation curve is seen and the first 300 seconds in figure 4.15 c looks similar. Since the rate flattens after a minute in both samples it is unlikely be due to product formation or decreasing concentrations of reactants. The chelation rate is quadrupled under argon, pointing to an important role of redox chemistry on the chelation rate.
Table 4.3: Chelation rate (ΔAbs/min. at 285 nm)
Seconds Under atm. Under argon
30 0.011 0.044
120 0.008 0.027
The UV spectra before and after 300 seconds of irradiation also show the effect of oxygen (figure 4.26). The UV spectrum before irradiation increased significantly by argon removal of oxygen (brown line, left figure).
As expected from the rates, the effect of irradiation is much smaller on the untreated sample (olive green line) compared to the effect on the argon treated sample (red line). Moreover, the irradiation caused spectral increase in the argon treated sample peaks at 285 nm, where it should, whereas the spectral increase in the untreated sample peaks at 280 nm indicating a secondary process - possibly due to O2.
Spectra of Cu2+ and [24.31]adz
0 0.05 0.1 0.15 0.2
230 280 330 380
nm
Absorption -UV -Ar
-UV +Ar +UV -Ar +UV +Ar
Change due to irradiation or Argon
0 0.05 0.1 0.15 0.2
230 280 330 380
nm
ΔAbsorption Ar -UV
Ar +UV UV -Ar UV +Ar
Figure 4.26: Effect of removal of dissolved gases on the spectra of 200 µM Cu2+ and 300 µM [24.31]adz before (-UV) and after 300 seconds of UV irradiation (+UV). The samples where gases have been removed by argon are named
“+Ar”, the other “Ar”. To the right, the effects of argon treatment and UV irradiation on the spectra are shown. “Ar -UV” is the effect of argon treatment before irradiation and “Ar +-UV” is the effect of argon treatment after irradiation.
Likewise is “UV -Ar” the effect of irradiation without argon treatment and “UV +Ar” the effect of irradiation after argon treatment.
4.5 N,N’-(CH2COOH)2[24.31]adz becomes partly decarboxylated by UV-irradiation