Formation and Spectral Optical Properties of R P

Oxygenation of the four colourless copper(i) complexesCu^IRat−78 °C in THF affords violet to purpleµ-η²:η²-peroxodicopper(ii) complexes^RP(Scheme 6.2).

The peroxo complexes are named^tBuP,^PhP,^MePand^HPin the following text, and addressed as »^RP« in general. UV-vis spectra of all four compounds are depicted in Figure 6.5, with intense optical absorption bands atλmax= 333 nm

N N

Scheme 6.2Activation of dioxygen with copper(i)–bis(oxazoline) complexesCu^IR(R = H, Me, Ph,tBu) to formµ-η²:η²-peroxodicopper(ii) complexes^HP,^MeP,^PhPand^tBuP.

120

6.3. Activation of Dioxygen and Spectroscopic Analysis

300 400 500 600 700

0 10 20 30 40 50

wavelength (nm)

H Me Ph

tBu

ε (mM–1cm–1)

30000 20000 15000

energy (cm^–1) 25000

Figure 6.5.Comparison of low temperature (−78 °C) UV-vis spectra ofµ-η²:η² -peroxodi-copper(ii) complexes^RPin THF (^PhPin acetone). While the concentrations/intensities of tBuPand^PhP(solid lines) are constant over&1 h,^HPand^MeP(dashed lines) precipitate during the formation and high concentration could not be reached (spectra with maximum absorption are depicted). See Table 6.2 for spectroscopic parameters and Figures 6.10 and 6.11 for spectra of complex formations.

(ϵ=48 mm^-1cm^-1) and 500 nm (2 mm^-1cm^-1) in the case of^tBuPand nearly identical features in the case of^PhP,^MePand^HP(Table 6.2). A photograph of an intense coloured^tBuPsolution is depicted in Figure 6.6.

The optical features are archetypical for a side-onµ-η²:η²-peroxodicopper(ii) species and are closely analogous to the spectroscopic features of oxy-hemocyanin and oxy-tyrosinase as introduced in the introduction, Section 2.2.1 (see also Table 2.2). These bands have previously been ascribed toπ_σ^∗ →d_x2−y²(in-plane) andπ_v^∗→d_x2−y²(out-of-plane) Cu^II←Operoxoligand-to-metal charge transfer (LMCT) transitions; the former at higher energy (∼333 nm) and being more intense due to a better overlap of orbitals.^[1,229]It is well evident that the optical charac-teristics are very similar between the four compounds and it can be predicted, that the structural properties are most likely identical. Absorptions of^HPand^MePare less intense since their solubility is very limited. Though similar to absorptions of previously reported systems, withλmax(ϵ, mm^-1cm^-1) =∼340–380 nm (∼18–25)

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

Table 6.2.

Spectroscopic data of dicopper–dioxygen complexes.^a UV-vis:λ_max, nm symbol ligand anion solvent (ϵ, mm^-1cm^-1) HP H{^HBOX} PF₆^– THF 330 (>30), 504 (>0.8)^b

PF₆^– solid 271, 332, 512,∼620sh^c MeP H{^MeBOX} PF₆^– THF 333 (>22), 496 (>0.5)^b

PF₆^– solid 261, 331, 493, 600^d PhP H{^PhBOX} PF₆^– aceton 334 (47), 496 (1.1)

PhO^e Li{^PhBOX} ^f aceton 338 (13), 400 (17),∼484sh(2),

∼600sh(1),∼750 (2) tBuP H{^tBuBOX} PF₆^– THF 333 (48.0), 509 (2.14)

PF₆^– aceton 333, 496

PF₆^– solid 263, 334, 520,∼620sh

– OTf^– THF 332 (40), 508 (0.6) 635 (0.8)

aSolutions at−78 °C, solids at r. t. ^bIncomplete formation/low solubility and precipitation.

csh= shoulder.^dSample was decomposed to some extent. ^ebis(µ-oxo)dicopper(iii), see Chapter 7. ^fNeutral complex.

Figure 6.6

Photograph of a^tBuPsolution in cooling bath with UV-vis im-mersion probe.

122

6.3. Activation of Dioxygen and Spectroscopic Analysis

and∼510–550 nm (∼1),^[34]both CT bands of^RPare at relatively high energy compared to known systems and are remarkably more intense (indicated also by a Beer’s law plot for^tBuP, discussed below).

The UV-vis bands of ^tBuPare similar in THF and in acetone solutions (cf.

Table 6.2, Figure 6.9a), while no peroxo complex formation was observable in CH₂Cl₂. The reactions of all fourCu^IRwith O₂were found to follow first-order kinetics under the applied conditions (Section 6.4). While^tBuPand^PhPare both soluble in THF and sufficiently stable at low temperatures for an extended time, the solubility of^HPand^MePis rather limited and they were found to precipitate from their solutions already during the oxygenation reaction (discussed below). While the quite limited solubility of all four complexes is problematic to analysis and experiments in solution, it is advantageous for isolation of the peroxo complexes as solids (Section 6.5). Exchange of PF₆^–counterion to triflate (OTf^–) in^tBuPgives a Cu₂O₂complex with analogous spectroscopic features, but an additional peak at 635 nm is present in the UV-vis spectrum (Table 6.2).

Experiments with the dimerised ligand^MeBOX₂(cf. Chapter 5), [Cu(MeCN)₄]PF₆ and O₂, in analogy toCu^IRshowed no formation of a copper–oxygen complex at low temperatures, which might be due to the larger steric hindrance of this ditopic ligand. Finally, it is noteworthy that all spectroscopy of^RPgives no indication for the traceable presence of the corresponding bis(µ-oxo)dicopper(iii) (O) species, which is often evidenced to be in equilibrium with a^SPcomplex.^[34]Interestingly, anOcomplex was observed with the deprotonated ligand^PhBOX⁻, as described in detail in the following Chapter 7.

Self-assembly

Synthetic copper−dioxygen complexes are usually not only quite temperature-sensitive and reactive towards exogenous and endogenous substrates (or the ligands itself), and their formation is usually highly sensitive to the oxygenation conditions, too.^[230]The oxygenation ofCu^IRcould be achieved either by the

O Cu^II O Cu^II

RP

bubble O₂ inject Cu^IR

Cu^IR O₂

a b

Scheme 6.3Formation of Cu₂O₂complexes. Self-assembly of peroxo complexes^RPmay proceed by two distinct sequences with similar yields. Either dry O2is bubbled through a

−78 °CCu^IRsolution (a), or theCu^IRsolution is injected into a−78 °C O₂-saturated solvent (b).

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

injection of O₂gas into solutions of Cu^IRin THF (at−78 °C under Ar or N₂ atmosphere; Scheme 6.3, sequence a) or by the slow injection of theseCu^IR/THF solutions into O₂-saturated solvent at−78 °C (Scheme 6.3, sequence b).

It is usual that the injection of a copper(i) complex solution directly into cooled pre-oxygenated solvent (sequence b) gives the highest yields of copper−dioxygen complex.^[74]However, the yields of peroxo complexes^RPare comparable, when O₂ is bubbled through cooled copper(i) (Cu^IR) solutions (sequence a), demonstrating the robust self-assembly of the peroxo-core in the BOX ligand-supported systems examined here.

Beer’s law plot

To determine the peak intensities more precisely, solutions of varying concentra-tions of^tBuP(0.1−0.3 mm) in THF were prepared by injection ofCu^ItBusolutions into chilled, saturated solutions of O₂in THF at−78 °C. The solutions turned slowly purple; the development of the full spectra (250−700 nm) was monitored with a 0.1 cm path length immersion probe until they remained unchanged.

The slopes of the absorbances at 333 and 500 nm vs. concentration of^tBuP give extinction coefficients of approximately 48 and 2.1 mm^-1cm^-1, respectively (Figure 6.7, Table 6.3). The high intensity of the 333 nm transition for^tBuPis only comparable to model complexes with ethylene diamine-derived ligand systems (∼34–40 mm^-1cm^-1). Peroxodicopper(ii) complexes of those bidentate ligands usually feature undefined weak axial solvent or counterion ligation; intimate interaction with the counteranion was observed spectroscopically.[99,108,109]This

Figure 6.7

Beer’s law plots, absorbance vs.

tBuPconcentration (i. e. per Cu₂O₂ unit), forλmax = 333(black) and 500 nm (blue) in THF at −78 °C.

2 mm( ) and5 mm() stock solu-tions were used. The slopes of the linear fits ( , ) give the

6.3. Activation of Dioxygen and Spectroscopic Analysis

Table 6.3.

Molar extinction coefficients (ϵ) of^tBuPin THF @−78 °C, obtained from Beer’s law plots in Figure 6.7 and fit parameters.

λ_max 333 nm 500 nm

ϵ(mm^-1cm^-1) 48.0±1.1 2.14±0.20

R¯² 0.998 0.963

suggests the presence of weakly bound axial solvent ligands also in^RP, in accor-dance with EXAFS data (see below, Section 6.7).

The high intensity of the 333 nmπ_σ^∗ →d_x2−y² (in-plane) feature might be explained by the BOX ligands playing a role in this charge-transfer transition.

The C=N bonds of the ligands are in plane with the Cu₂O₂core and a relevant orbital interaction can be expected. Theoretical investigations of the excited electronic states of^tBuP, using time-dependent density-functional theory (TD-DFT) calculations (Section 6.8), support this assumption, as charge from the ligand is transferred in addition to charge from the peroxide in course of this electronic excitation.