Reactivity with Dry Dioxygen

The reaction of 1aor1b(or shortly termed1in the following) with dry dioxygen was fol-lowed by UV/Vis spectroscopy. As oxo, peroxo and hydroxo model complexes in literature exhibit prominent spectroscopic signatures, these characteristics might help identifying intermediates in the reaction of 1 with dioxygen. Figure 5.3 depicts a recorded UV/Vis spectrum of 1aupon the addition of dioxygen in dry EtCN.

The black line represents the typical spectrum of low spin 1a at low temperatures in EtCN as described in Chapter 4 Section 4.3.2. The addition of dioxygen at –78^◦C yields a putative intermediate with very weak absorption patterns and no characteristic max-ima (orange line). Allowing this mixture to warm to ambient temperature leads to the appearance of a shoulder at 360-370 nm, which should account for an oxidized diiron(iii) species (red line). However, the strong absorption band at 417 nm in EtCN somewhat masks bands that might arise from the coordination of dioxygen. A solubility assay was performed and similar spectra were recorded in DCM and methanol as representatives for an apolar aprotic and a polar protic solvent. In these two solvents at low temperatures the absorption band at 417 nm for 1aand also the BF^–₄ analogue1bis not visible.

As described in detail in Chapter 4 the absorption band at 417 nm in nitriles can be associ-ated with a spin crossover process from high spin to low spin with decreasing temperature.

However, SCO behavior has only been observed exclusively in nitrile solutions for 1a. In methanol or DCM the spin state of 1aand 1bis not known.

When reacting a sample of 1b in DCM with dioxygen at –70^◦C and slowly warming to

N N N N

N N

N

N N

N N N N

N

Fe^II Fe^II N

(X)3

X = SO3CF3 1a X = BF₄ 1b

+ O2

Figure 5.3:UV/Vis spectrum of1ain EtCN at–78^◦C (black line). Formation of an oxygenated product after addition of O₂ and warming to ambient temperature (red line). The orange line indicates a possible diiron oxygen intermediate spectrum at –78^◦C with negligible spectroscopic signatures. The signal-to-noise ratio in all spectra was smoothed by a Savitzky-Golay fit. The arrows indicate the change of a respective band upon the addition of dioxygen at –78^◦C and subsequent warming to ambient temperature.

–8^◦C weak shoulders at 438 nm and approximately 360 nm appear (red line). At –70^◦C without subsequent warming the spectrum of displays a stable but featureless putative intermediate (orange line). In order to isolate the putative intermediate1b^Int, cold hexane was added to a solution of 1b at –70^◦C to yield an orange precipitate. To record a Mössbauer spectrum of 1b^Int, the solid was carefully transferred into a Mössbauer sample holder on a block of dry ice. The powder sample was flashfrozen and measured. Figure 5.4 depicts the described UV/Vis spectrum of 1b and the Mössbauer spectrum of the putative intermediate1b^Int isolated from a DCM solution.

Figure 5.4:Left: UV/Vis spectrum of 1bin DCM. The initial spectrum at ambient temperature is represented by the black line, putative intermediate at -70^◦C (orange line) and final oxygenated product (red line). Right: Zerofield Mössbauer spectrum of 1b^Int at 80 K. Isomer shifts δ and quadrupole splittings |∆E_Q| in [mm s^–1]. Red subspectrumδ= 0.49, |∆E_Q| = 1.10, blue subspec-trumδ= 0.72, |∆E_Q| = 3.01. Signal ratio: 44 %/56 %.

The Mössbauer spectrum of 1b^Int isolated at low temperatures exhibits two signals in an almost 1:1 ratio. The blue doublet with large quadrupole splitting and an isomer shift of 0.72 mm s^–1can most likely be assigned to a Fe(ii) species in a high spin state. The

assign-ment of the red doublet, however, is less trivial. From the isomer shift aferric species can be assumed. The prediction of possible binding modes of dioxygen though in accordance with the UV/Vis data at hand is challenging. In comparison with the Mössbauer spectrum of 3, it becomes apparent that the isomer shift of the blue subspectrum in3matches very well with theferric species (red subspectrum) found for1b^Int. Nevertheless, the isolated orange solid was found to be highly temperature sensitive, changing its color to dark red when warmed to room temperature. Thus it is assumed that this solid does not represent the comparatively stable species 3 despite partially similar Mössbauer parameters.

Very few examples in literature describe ferric dioxygen intermediates with absorption bands lower than 500 nm. One example represents oxy-hemerythrin with an absorp-tion band at 500 nm and Mössbauer shifts of δ= 0.50 mm s^–1 (|∆E_Q| = 2.02 mm s^–1) and δ= 0.51 mm s^–1 (|∆E_Q| = 1.01 mm s^–1).^[43] Recently discussed Fe(iii) superoxo interme-diates display absorptions below 500 nm.^[161–164] However, to date Mössbauer data are hardly available for these intermediates.

Diferric oxo-bridged complexes on the other hand exhibit also characteristic absorption bands in the 300-400 nm region. Mössbauer shifts and quadrupole splittings of the "oxo-dimer" region match as well with the obtained spectra.^[160] Also hydroxo-bridged dimers are known to feature similar UV/Vis and Mössbauer parameters.[160,165,166] Therefore the obtained spectra cast doubt on whether diferrous 1b reacts via an intermediate to its oxygenated pendant. The orange solid could just as well represent a mixture of not fully oxygenated 1b instead of two distinct iron sites within one molecule. On the other hand, the Mössbauer spectrum of 1b^Intdepicts despite the high signal to noise ratio two subspectra with similar integral areas. For a product mixture this would be unlikely.

As a result the following conclusions can be drawn from the obtained spectra: From UV/Vis data, the formation of a peroxo intermediate does not seem feasible as most per-oxo intermediates exhibit absorption bands above 500 nm. An per-oxo, hydper-oxo or superper-oxo diiron core seems reasonable in this respect. As two Mössbauer doublets with very different parameters were obtained for1b^Int, a putative intermediate would not adopt a symmetric geometry with two equal iron sites. From a structural perspective though, an oxo-bridged diiron core seems unlikely as already the hydoxo-bridged diiron site in 3 is very unstable and contracted in its coordination geometry. Thus by elimination of possibilities the most likely species observed in the oxidation of 1b are a superoxo and/or a hydroxo species.

For comparison of the isolated putative intermediate 1b^Int and the final oxygenation product 1b^Ox a Mössbauer spectrum of 1b after oxidation and warming up to ambient temperature was recorded. For that purpose a sample of 1bwas dissolved in dry, degassed DCM. Dry dioxygen was added and the mixture was allowed to stirr for a while. Finally the solvent was removed. A powder sample of 1b^Ox was submitted for the measurement.

The spectrum is depicted in Figure 5.5.

Both subspectra of 1b^Ox can be assigned to Fe(iii) species. Isomer shifts and quadrupole splittings differ slightly from the parameters found for 1b^Int and3. The red subspectrum in1b^Oxcould be comparable to the red subspectrum in1b^Intand the blue subspectrum in

Figure 5.5:Mössbauer spectrum of1b^Oxat 80 K. Isomer shiftsδand quadrupole splittings |∆E_Q| in [mm s^–1]. Red subspectrum δ= 0.47 |∆E_Q| = 0.89, blue subspectrum δ= 0.54 |∆E_Q| = 1.84.

Signal ratio: 73 %/27 %.

3, but since also in this Mössbauer spectrum the signal to moise ratio is very high a direct comparison is purely speculative. Evaluating the obtained spectra it seems reasonable to assume that an oxo and/or even more likely a hydroxo-bridged diiron complex as found for 3 forms upon oxygenation. As Mössbauer shifts of peroxo, oxo and hydroxo bridged complexes are often very similar from these parameters alone it is impossible to assign structural features for the final oxygenation product 1b^Ox. Table 5.1 summarizes all Mössbauer parameters.

Table 5.1:Comparison of Mössbauer parameters for putative oxygenation products and interme-diates.

1b^Int 1b^Ox 3

δ [mm s^–1] 0.49, 0.72 0.47, 0.54 0.49, 0.45

|∆E_Q| [mm s^–1] 1.10, 3.01 0.89, 1.84 0.67, 1.11

Another widely established method to gain insight into the nature of diiron oxygen in-termediates and thus reaction pathways is resonance Raman spectroscopy as peroxo and superoxo O–O bonds as well as Fe–O bonds feature characteristic vibrational modes.

However, attempts to perform these experiments with1b^Int failed due to the fast decom-position of 1b^Intand overlaps with solvent bands in solution spectra. Mass spectra of the oxidized material1b^Ox did not lead to further insights as none of the signals could be as-signed properly. Thus in the framework of this work the nature of a putative intermediate and the final oxygenated species1^Ox remains unclear.

Also unknown is the ability of1^Oxto oxygenate substrates at low temperatures. A first at-tempt was made by adding triphenylphosphine to the putative intermediate1b^Intin DCM at–40^◦C. ESI analysis of the reaction solution revealed the presence of triphenylphosphine oxide as well as triphenylphosphine and the diiron complex. This can be seen as a first indication that 1b^Int might be capable to oxygenate triphenylphosphine. However, fur-ther clarification is needed to provide a more detailed insight into this reaction, e.g., by labeling studies with¹⁸O₂ and³¹P-NMR analysis of the products.