I. General Introduction 1
7. A Bis(µ-oxo) Complex by Stepwise Conversion of a Peroxo Species177
7.2. Preparation and Spectral Properties of Ph O
A Bis( µ -oxo) Complex by Stepwise Conversion of a Peroxo Species
7.1. Introduction
I
n the preceding chapters, the coordinating abilities of the four bis(oxazoline) ligandsH{RBOX}(R≡H, Me, Ph,tBu) towards copper(i) and copper(ii) have been investigated in detail, and it has been shown that all form stable complexes with CuI, as well asµ-η2:η2-peroxodicopper(ii) (SP) complexes upon reaction with O2, relatively stable at−78 °C. In contrast, the BOX ligands are readily oxidised at room temperature by CuCl2acting as oxidant. This reactivity was expressed by dimerisation and oxygenation of the least bulkyH{HBOX}andH{MeBOX}, while more bulkyH{PhBOX}showed only an oxygenation reaction. Differing from this, the ligand with the bulkiest residueH{tBuBOX}forms a stable CuIIcomplex. It is assumed that the residue R is shielding the backbone-proton and also prevents the ligand from adopting a planar conformation. Due to this reason, the deprotonation of theH{tBuBOX}ligand was not feasible.In spite of these problems, it was intended to establish copper complexes and copper−oxygen complexes also with theH{RBOX}ligands in their deprotonated condition as bis(oxazolinate) {RBOX−}. Since the least-bulky ligands suffer from the oxidative dimerisation reactions and it was not feasible to deprotonate the H{tBuBOX}ligand,H{PhBOX}was selected as the only ligand of the four, which might be suitable for a deeper investigation using it in its deprotonated form.
7.2. Preparation and Spectral Properties of
PhO
Deprotonation of H{PhBOX} and its CuIComplex
The complex was prepared starting fromH{PhBOX}ligand (Scheme 7.1).H{PhBOX}
was deprotonated by the addition of 1.0 eq.n-butyl lithium/hexane solution to
7. A Bis(µ-oxo) Complex by Stepwise Conversion of a Peroxo Species
Scheme 7.1Synthesis of bis(µ-oxo)dicopper(iii) complexPhOfrom the CuIcomplex of Li{PhBOX} in THF.
the ligand in THF under an atmosphere of nitrogen at−78 °C. This deprotona-tion was also conducted in a stepwise manner followed by UV-vis spectroscopy (see Figure 7.1). The spectrum ofH{PhBOX}( ) is characterised by an intense absorption at aroundλmax= 280 nm (ϵ= 34 mm-1cm-1), assigned to aπ →π* transition; the »double peak« shape originates most likely from the presence of both, diimine and iminoenamine tautomers of H{PhBOX}(cf. Chapter 4 for details onH{PhBOX}tautomerism). The spectral changes upon deprotonation of H{PhBOX}are however only small and the spectrum of {PhBOX}−( ) is quite similar to that ofH{PhBOX}.
After warming the Li{PhBOX} solution to room temperature, 1.0 eq. [Cu(MeCN)4 )]-PF6was added. The colourless solution turned yellow, indicated also by a shift of the intense ligand peak to slightly lower energy (300 nm,ϵ = 35 mm-1cm-1, ) and a shoulder at∼370 nm (ϵ≈1.5 m-1cm-1). After the full dissolution of [Cu(MeCN)4]PF6, the mixture was recooled to−78 °C.
The bis(µ-oxo)dicopper(iii) complexPhO
ThePhOcomplex was obtained upon injection of dry O2gas into the solution at
−78 °C. The formation ofPhO( ) is well evident by a very rapid and intense colour change of the copper(i) solution to a very dark green colour (Figure 7.1).
Interestingly, noPhOformation is observed when the [(MeCN)CuI(H{PhBOX})]
CuIPhcomplex is prepared first andnBuLi is added to this complex in a second step, followed by O2at−78 °C.
The UV-vis spectrum ofPhOfeatures strong transitions atλmax= 288, 334 and 395 nm (ϵ= 40, 13 and 8 mm-1cm-1) and further weaker and broad features around 500 and 700 nm. While the strong absorption of the copper−oxygen complex at 288 nm most likely arises from the BOX ligand’sπ →π* transition and might obscure transitions arising from a copper−oxygen species in this region, the intense
178
7.2. Preparation and Spectral Properties ofPhO
300 400 500 600 700 800
0 10 20 30 40
ε (mM–1cm-1)
wavelength (nm)
0.0 eq base 0.1–0.9 eq 1.0 eq CuI(PhBOX)
PhO
decomposition
*
40000 30000 20000
energy (cm–1)
Figure 7.1.UV-vis spectra (THF,−78 °C) of the H{PhBOX} ligand ( ) and its stepwise ( ) deprotonation to {PhBOX}−( ). The corresponding CuI complex ( ) from Scheme 7.1,c=0.58 mm(yellow solution) and its bis(µ-oxo) complexPhO( ), which is formed upon injection of O2into this solution. Upon warming to r. t., the complex decays, indicated by vanishing of spectral features ( ). The asterisk denotes the presence of the correspondingµ-η2:η2-peroxo isomer. Equivalents of base are per Cu atom.
feature at 400 nm was not observed before. Absorptions at∼300 and∼400 nm are indicative for a bis(µ-oxo)dicopper(iii) (O) species. The 300 nm features arise from bis(µ-oxo)πσ∗ →Cudx2−y2charge-transfer transitions, similar to the∼350 nm transition inSPcomplexes, while the 400 nm features are unique inOspecies and arise from bis(µ-oxo)σ∗→Cudx2−y2CT transitions and are quite intense due to the good orbital overlap between both orbitals. The high intensity was additionally ascribed to the significant covalency of the Cu−O bonds.[34]The additional feature in the spectrum at 333 nm is presumably originating from the presence of the correspondingSPisomer, in equilibrium with theOspecies (Scheme 7.2). Such an equilibrium was evidenced for some complexes with different ligand systems (see the Introduction, Section 2.2.2, p. 22).
7. A Bis(µ-oxo) Complex by Stepwise Conversion of a Peroxo Species
Scheme 7.2 Equilibrium of µ-η2:η2 -peroxodicopper(ii)SPand bis(µ -oxo)di-copper(iii)Oisomers.
Solutions ofPhOcan be stored at−80 °C for at least four weeks without changes of colour. A UV-vis measurement showed the same spectral features as found in a fresh sample. ThePhOcopper−oxygen complex is temperature sensitive and decays upon warming to e. g. room temperature. The decay is accompanied by bleaching of the solution to colourless or light green and disappearance of the spectral features ( , in Figure 7.1). The resulting UV-vis spectrum is somewhat reminiscent to that of the corresponding CuIcomplex with additional weak ab-sorptions in the lower energy region (∼600–800 nm). Upon cooling again to−78 °C no sign of repeated copper−oxygen complex formation was observed.