UV-vis Spectroscopy

Figure 5.9: UV-vis spectra of ^Na[Cu^II2L³(O2)]^OTf at RT in EtCN (2.5 × 10⁻⁴M). The product decayed rapidly (see Section 6.3), and the spectrum displayed above was recorded within seconds of sample insertion into the

spectrophotometer. The above spectrum is indistinguishable from those of sodium cation free samples.

The experiments described below were conducted at RT in order to facilitate easier handling and allow for high throughput. The results were qualitatively equivalent in both EtCN and Me2CO. Compared with that of ^Na[Cu^II2L³(O2)]^OTf, no considerable difference in the colour of the dioxygen adduct ^−M[Cu^II2L³(O2)]^OTf was readily apparent upon visual inspection, however, the corresponding UV-vis spectrum was found to subtly but significantly differ. The two main peaks are red shifted to 530 and 653 nm, the latter has gained considerable intensity such that it is now clearly resolved as a discrete band, and a new feature at approximately 450 nm is visible as a shoulder on the 530 nm absorption. While no concentration dependence was evident for solutions of ^−M[Cu^II2L³(O2)]^OTf, further investigation revealed that the aforementioned spectra of ^Na[Cu^II2L³(O2)]^OTf (Section 4.4.2) are observed only for relatively concentrated solutions. Thus, while in good agreement with the solid state reflectance data at high concentrations (≥ 5.0 × 10⁻³M), at lower concentrations (≤ 2.5 × 10⁻⁴

M) solution spectra of ^Na[Cu^II2L³(O2)]^OTf are indistinguishable from those of ^−M[Cu^II2L³(O2)]^OTf (Figure 5.9). An additive spectrum with an intermediate λmax value resulting from overlap of both peak sets was observed at concentrations between these two limiting spectra.

Figure 5.10: Overlay of three RT UV-vis spectra of ^Na[Cu^II₂L³(O₂)]^OTf (5.0 × 10⁻⁴M) in Me₂CO with increasing NaOTf presence (total of 1, 10 or 16 equivalents). Each spectra was generated with the same stock solution of

Na[Cu^II2L³(O2)]^OTf. The spectra were chosen arbitrarily from within the first minutes after reaction with oxygen.

The influence of sodium cation concentration was then investigated. Formation of

Na[Cu^II2L³(O2)]^OTf at low (2.5 × 10⁻⁴ and 5.0 × 10⁻⁴ M) concentrations in the presence of increasing amounts of NaOTf or NaBPh4 thus resulted in spectra with λmax values progressively shifted toward higher wavelength. The extent of the shift correlated with the amount of excess sodium salt added, up to the 500 nm maximum value observed for the solid state ([Cu^II2L³(O2)]^OTf·NaOTf) and concentrated solutions of ^Na[Cu^II2L³(O2)]^OTf (Figure 5.10). Interestingly, NaBPh4 was less effective at inducing the change in both solvents, bringing into question the role of triflate anions. Hence, up to nine equivalents of [Bu4N]OTf were added to dilute solutions of ^Na[Cu^II2L³(O2)]^OTf under identical conditions to those used above, which caused no change whatsoever in the resulting UV-vis spectra. This was confirmed by ¹⁹F NMR diffusion-ordered spectroscopy (DOSY) at −30°C. Solutions of

Na[Cu^II2L³(O2)]^OTf in CD3CN or (CD3)2CO displayed diffusion coefficients identical to that of [Bu4N]OTf, indicating complete dissociation of the triflate anions. Furthermore, the lack of change in the UV-vis spectra upon [Bu4N]OTf addition rules out alteration of the ionic strength as a causal factor, which could otherwise conceivably perturb the absorption profile as a result of changes in the solvent permittivity, akin to solvatochroism.²⁰⁷ Taken together, the above results indicate that sodium ions specifically bind to the peroxide moiety in solution in a concentration dependent equilibrium, without the involvement of triflate (Scheme 5.2).

Scheme 5.2: Sodium cation binding equilibrium and associated dominant UV-vis feature.

The equilibrium behaviour described above was qualitatively demonstrated in both Me2CO and EtCN at RT. The observation that NaBPh4 is less effective than NaOTf at perturbing the equilibrium was more pronounced in Me2CO, and might be attributed to the tetraphenylborate anion. Interaction between the phenyl rings and sodium cations in NaBPh4 has been observed in the solid state, such that it can be described as an organosodium compound,²⁰⁸ and pairing is known to occur between the two ions in Me2CO/Me2SO solvent mixtures.²⁰⁹ Furthermore, stabilisation of a Me6tren supported copper(I) complex was recently reported when tetraphenylborate was employed as the counteranion, implying an interaction between the two in solution.³² It is thus feasible that BPh4−

anions might hinder sodium ion binding to ^−M[Cu^II2L³(O2)]^OTf in solution, either by providing alternative cation binding sites, or through formation of a tight ion pair.

Figure 5.11: UV/Vis spectra of NaOTf titration of ^Na[Cu^II₂L³(O₂)]^OTf at −30 °C in Me₂CO. The isosbestic point is at 507 nm. The inset shows the changes in absorbance as a function of NaOTf added. The red curves in the inset

show the best fit used to derive the binding constant.

In order to quantify the strength of sodium ion binding, pre-formed ^Na[Cu^II2L³(O2)]^OTf was then titrated with NaOTf in (Me)2CO at −30 °C. Preliminary measurements indicated that at lower temperatures the binding affinity is significantly enhanced, so smaller relative amounts of NaOTf were required to shift the equilibrium. Thus, the decrease in intensity of the two CT features of ^−M[Cu^II2L³(O2)]^OTf at 530 and 653 nm was monitored as a function of

NaOTf added (Figure 5.11), and fitting of the resulting binding isotherm gave an association constant (Ka) of 1700 (± 10%) M⁻¹ for acetone at −30 °C. Although potassium ion binding in solution was not quantitatively assessed by UV-vis spectroscopy at this point in time, it also clearly occurs. Weaker association is expected, as evidenced by the intermediate frequencies and intensities in the rR spectra, the longer distance in the solid state structures of [Cu^II2L³(O2)]^OTf·KOTf and [Cu^II2L³(O2)]^ClO4·KClO4, and preliminary UV-vis data which show a lack of significant change in the absorption spectrum under conditions which the sodium cation has a considerable effect.