Resonance Raman Spectroscopy

Besides the archetypical optical spectroscopic properties, inherent to the different copper−dioxygen species (see preceding sections) and besides structural data (see following section), a direct spectroscopic evidence of reduced O₂can be obtained through vibrational spectroscopy. Resonance Raman ( rR ) spectroscopy is particu-larly advantageous over IR spectroscopy for copper−dioxygen systems, since O−O, Cu−O and Cu···Cu vibrational modes can be selectively enhanced and symmetric vibrational modes may be detected. Furthermore, the shift of a vibrational feature upon¹⁸O₂isotopic substitution allows unambiguous assignment of the bands and is strong proof of the O₂reduction level present.

Inµ-η²:η²-peroxo Cu₂O₂complexes, the O−O bond is significantly weakened compared to H₂O₂( ˜ν(O−O) = 880 cm⁻¹) due to back-bonding. See Section 2.1.2, 16, in the introduction. Known systems have characteristic Resonance Raman (rR) frequencies and isotope shifts of the intraperoxide stretch, ˜ν(O−O) = 730–

760 cm⁻¹(∆[¹⁸O]≈40 cm⁻¹). See Figure 6.19 for a representation of the

dia-O O

Cu Cu

O O

Cu Cu

A_g A_g

ν(O–O) ν(Cu–Cu)

Figure 6.19. The two diagnostic in-plane normal vibrational modes of a µ-η²:η² -peroxodicopper(ii) ^SP system. Atom motions are represented by red arrows. Addi-tional modes have previously been identified and were sometimes observed in other compounds.^[229]

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6.6. Resonance Raman Spectroscopy

gnostic vibrational modes. In contrast, 1,2-trans-peroxodicopper(ii) systems have ν(O˜ −O)≈830 cm⁻¹(∆[¹⁸O]≈46 cm⁻¹),^[34]a 1,2-cis-peroxodicopper(ii) system has ˜ν(O−O) = 800 cm⁻¹ (∆[¹⁸O] = 45 cm⁻¹)^[44] and bis(µ-oxo)dicopper(iii) sys-tems, lacking the O−O stretch, have however ˜ν(Cu−O)≈600 cm⁻¹ (∆[¹⁸O]≈ 28 cm⁻¹).^[34]

An intense isotopically insensitive feature at∼280 cm⁻¹ is additionally dia-gnostic for the side-on peroxo complex and has been assigned as the fundamental symmetric Cu₂O₂core vibration,ν(Cu···Cu), see Figure 6.19. It has been calculated that theν(O−O) stretch at∼740 cm⁻¹is composed of>90 % O−O motion, while theν(Cu···Cu) vibration at∼280 cm⁻¹is mostly (90 %) copper motion. A very small¹⁸O isotopic shift of 0.2 cm⁻¹was calculated for this vibration as a result of predominant Cu···Cu motion.^[229,247]

tBuP: rR spectroscopy (λex= 632.8 nm) of a THF solution of^tBuPwas conducted with the help of a special cryo measuring cell at 213 K. The cooled copper(i) com-plexCu^ItBuwas oxygenated inside the cryostat’s sample stage by the injection of O₂gas. The sample showed a Raman shift of 729 cm⁻¹(Figure 6.20), typical for the weakν(O−O) stretch of theµ-η²:η²-bound peroxide. The intense feature at 278 cm⁻¹is the Cu₂O₂core vibration,ν(Cu···Cu).

Due to problematic solubility, thermal lability and the technically demanding set-up, further experiments were conducted with solid samples. rR spectra of the solid^tBuPare depicted in Figure 6.21a (blue area), the spectrum shows the presence ofµ-η²:η²-coordinated peroxide with ˜ν(O−O) = 731 cm⁻¹and ˜ν(Cu···Cu)

300 400 500 600 700 800

ν(O–O) ν(Cu–Cu)

729

Raman intensity

rRaman shift (cm⁻¹)

278

*

Figure 6.20. Resonance Raman spectrum of peroxo complex^tBuPwith 632.8 nm laser excitation. Solution spectrum;ν˜(¹⁶O−¹⁶O) =729 cm⁻¹,ν˜(Cu···Cu) =278 cm⁻¹; THF at 213 K; no rR in solution with¹⁸O isotopic substitution is provided due to problematic solubility. The asterisk at745 cm⁻¹indicates PF6^–.

6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs

0

250 500 750 1000 1250 1500 1750 2000

Intensity

16O2 – ¹⁸O2 Difference

ν (O−O) 278

ν (C=N) PF6

279

692

rRaman shift (cm⁻¹)

16O2 18O2 731

ν (Cu···Cu)

(a)rR spectra of ^tBuP.ν˜(O−O) =731 cm⁻¹ (∆[¹⁸O] =39 cm⁻¹),ν˜(Cu−Cu) = 278 cm⁻¹(∆[¹⁸O] =1 cm⁻¹).

0

150 200 250 300 350 400 450 500 550 600 650 700 750 800

16O2 18O2

Intensity

Difference

282 (0) ν (Cu···Cu) 16O2 – ¹⁸O2

ν (O–O) 703 742

rRaman shift (cm^–1)

PF6

(b)rR spectra of^HP.ν˜(O−O) =742 cm⁻¹(∆[¹⁸O] =39 cm⁻¹),ν˜(Cu−Cu) =282 cm⁻¹ (∆[¹⁸O] =0 cm⁻¹).

Figure 6.21. Resonance Raman (rR) spectra of solid samples of peroxo complexes^tBuP and^HPwith 632.8 nm laser excitation; with¹⁶O¹⁶O (blue) and¹⁸O¹⁸O (magenta) isotopic composition.¹⁶O minus¹⁸O difference spectra ( ). No¹⁶O/¹⁸O isotope scrambling is detected.

144

6.6. Resonance Raman Spectroscopy

= 278 cm⁻¹, essentially unchanged compared to the solution features. Isotopic substitution with¹⁸O₂shifts the peroxo stretch to 692 cm⁻¹(∆[¹⁸O] = 39 cm⁻¹) as expected (Figure 6.21a, pink area). The Cu···Cu stretch is, just as expected, only slightly affected by the¹⁸O₂isotopic substitution (∆[¹⁸O]≈1 cm⁻¹). The¹⁶O−¹⁶O vibration is slightly obscured by the 745 cm⁻¹PF₆^–stretch, the¹⁶O minus¹⁸O difference spectrum (Figure 6.21a, red solid line) however clarifies the assignments.

Furthermore, rR and IR spectroscopies confirm the absence of MeCN and the presence of PF₆^–counter ions in the solid material (Figure 6.22) in addition to the elemental analysis. No other peaks which shift upon isotopic substitution were found in the Raman spectra between 150 and 2000 cm⁻¹. The diagnostic Raman vibrations for all investigated samples ofµ-η²:η²-peroxodicopper(ii) complexes

RPare listed in Table 6.7.

HP: The rR spectrum of^HPresembles the preceding spectra (Figure 6.21b). The Cu···Cu vibration is roughly at equal energy (282 cm⁻¹) and not affected by¹⁸O₂ isotopic substitution. Compared to^tBuP, the intraperoxo stretchν(O−O) is at

500 1000 1500 2000 2500 3000

500 1000 1500 2000 2500 3000

ν(O–O)

ν(Cu–Cu)

PF6

ν(C=N)

*

**

Raman intensity transmission

wavenumber (cm⁻¹)

rRaman shift (cm⁻¹)

IR Raman

* ^ν(C–H)

Figure 6.22.Comparison of¹⁶O₂labelled infrared (IR, ) and rR spectra (λ_ex= 632.8 nm, ) of solid^tBuP. Assigned vibration modes are indicated; a KBr disc was measured in case of IR, while for rR a neat sample was used; the asterisks indicate vibrations arising from PF6^–counterions.

6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs

slightly higher energy (742 cm⁻¹), yet is equally shifted upon isotopic substitution (∆[¹⁸O] = 39 cm⁻¹to 703 cm⁻¹. The¹⁶O−¹⁶O vibration is partially covered by the 745 cm⁻¹PF₆^–stretch. With the help of the¹⁶O minus¹⁸O difference spectrum (Figure 6.21b, red solid line) it was however possible to precisely determine the location of the¹⁶O−¹⁶O peak. Theν(O−O) stretch is located higher in energy in

HPcompared to^tBuP indicating a stronger O−O bond in^HP. This might suggest that the {N₂CuO₂} moieties are more planar in^HPthan they are in^tBuP.

MeP and^PhP: A solid sample of^MePwas also subjected to Raman spectroscopy (Figure 6.23, ), verifying the indicated partial degradation by the presence of

Table 6.7.

Raman spectroscopic features ofµ-η²:η²-peroxodicopper(ii) complexes^RP(λ_ex= 632.8 nm).

ν, cm˜ ⁻¹(∆[¹⁸O₂])

sample Cu···Cu O−O

tBuP solution 278 729

solid 279 (1) 731 (39)

HP solid 282 (0) 742 (39)

MeP solid^a 282 735

PhP solid^b n/a n/a

aPartly decomposed complex. ^bDecomposed complex.

600 650 700 750 800 850

Intensity

rRaman shift (cm^–1)

H Me Ph tBu PF₆

Figure 6.23. Powder rRaman spectra of partially degraded^MeP(ν˜(O−O) =735 cm⁻¹, ν˜(Cu···Cu) =282 cm⁻¹) and the decomposition product of^PhP, compared to the spectra of tBuPand^HP(λ_ex= 632.8 nm, solid samples). Note the absence of the O−O stretch in the case of^PhPand the low intensity in the case of^MeP.

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6.6. Resonance Raman Spectroscopy

only a low intensityν(O−O) stretch at 735 cm⁻¹(in accordance with elemental analysis, solid state UV-vis and EXAFS data). The energy of the O−O bond interestingly is in between those of the other two peroxo complexes. Overall, the O−O bond energy is weaker, the more bulky the residue R is. This indicates that the bond energy might be directly related to the sterical demand of the residue R.

Due to degradation,^PhPcould not be isolated and in accordance with EXAFS data, no feature which could be attributed to an O−O stretch is present in the Raman spectrum (Figure 6.23, ). This indicates that there is no residue of peroxo complex from the solution present anymore upon isolation of the precipitated complex.

Isotope Effects in Vibrational Spectroscopy

The frequency of a vibration of a bond can accurately be described by theclassical harmonic oscillator model

in which ˜νis the fundamental vibration frequency (here expressed in wavenum-bers) with the speed of lightc,kis the force constant of the bond andµis the reduced mass

µ= m_a·m_b

(ma+m_b) (6.5)

in whichmaandm_bare the mono-isotopic masses of the two vibrating atoms.

Substituting the isotope of an atom changesµ, but does not change the bond strength i. e. the force constant. From equation 6.4 follows, that the ratio between two frequencies ˜ν1and ˜ν2is

With equation 6.6 the wavenumber of the¹⁸O₂peroxo stretch can be calculated from the ˜νmeasured for the¹⁶O₂stretch. A factor ˜ν(¹⁸O₂)/ν(˜ ¹⁶O₂) =0.9428 follows from the harmonic oscillator model.

In the case of ^tBuPa isotope shift of the stretch ˜ν(¹⁶O₂) = 731 cm⁻¹gives

˜

ν(¹⁸O₂) = 689 cm⁻¹. This theoretical shift of∆[¹⁸O₂] = 41.8 cm⁻¹is in good agreement with the shift of 39 cm⁻¹, experimentally observed. In^HP, similar agreement is observed: experimental∆[¹⁸O₂]=39 cm⁻¹vs. theoretical 42.4 cm⁻¹.

6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs

6.7. Structural Investigations: XAS, XANES and EXAFS

Im Dokument Biomimetic Copper(I)-Mediated Activation of Dioxygen and Redox Non-Innocence in Copper(II) Complexes of Bis(oxazoline)s (Seite 158-164)