I. General Introduction 1
6.10. Thermal Degradation of Peroxo Complexes
Scheme 6.9Formation of bis(µ-hydroxo)dicopper(ii) complex [(H{tBuBOX})(solv)CuII(µ -OH)]2(PF6)2, by either the reaction of the copper(i) complexCuItBuwith O2at r. t (left), or by the warming of the peroxo Cu2O2complextBuPto temperatures above∼ −50 °C or to r. t (right).
CuItBureacts with O2at ambient temperatures to yield the bis(µ-hydroxo)di-copper(ii) complex [(H{tBuBOX})(solv)CuII(µ-OH)]2(PF6)2(Scheme 6.9, left arrow).
»solv« is a loosely coordinated solvent molecule, H2O, THF and MeCN have been identified as ligands in the structurally characterised complexes considered here.
After warming to temperatures over∼ −50 °C or room temperature, solutions of the intensely violet peroxo Cu2O2complextBuPbleach and turn slowly bluish.
In the solid form,tBuPis quite stable even at room temperature, however after approximately one to two days at room temperature it turns light blue, indicative of the formation of the corresponding bis(hydroxo)-bridged Cu2(µ-OH)2complex.
Most of the Cu2O2compounds are thermally unstable and thus, the formation of a Cu2(OH)2complex as the decomposition product in such reactions is typical for peroxo Cu2O2complexes and also for copper(i) complexes reacting with O2at room temperature.[296–298]
The decomposition is also well evident in the Raman spectrum of this powder, which lacks the intense archetypicalν(O−O) andν(Cu···Cu) features oftBuP. Both peaks were present as intense features before thermal degradation at 731 cm−1 and 279 cm−1(Figure 6.36). The remaining weaker feature at∼250 cm−1might be assigned to the axial copper–solvent coordinationν(Cu−Nax/Oax).[70,259]
The large [(H{tBuBOX})2Cu2(OH)2(PF6)]+ion was identified in the high resol-ution ESI mass spectrum. A solresol-ution of18O-labelledtBuPat room temperature
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6.10. Thermal Degradation of Peroxo Complexes
200 400 600 800 1000
intensity
Raman shift (cm–1)
Figure 6.36.Raman spectrum (λex=632.8 nm) of the Cu2(µ-OH)2complex ( ) obtained upon leaving a powder sample of peroxo complextBuPat room temperature, indicated by a colour change from deep violet to light blue. For comparison the spectrum oftBuPis depicted ( ). See Figure 6.21a for details on Raman spectra for peroxo complextBuP.
afforded the corresponding18O-labelled bis(hydroxo) complex (Figure 6.37). How-ever,∼50 %16O/18O isotope scrambling was evident, and it is expected that an exchange with water from solvents or air is relatively fast. The IR spectrum of the Cu2(µ-OH)2complex features two sharp stretches at ˜ν(O–H) = 3583 and 3649 cm−1. The spectrum of a powder sample, prepared by leaving18O-labelled
tBuProom temperature and in air for some days, showed no difference in O−H stretch compared to the sample with natural abundance isotope composition. This also indicates easy isotope scrambling.
Structural Investigations
Single crystals of high quality could be isolated from two different solutions at room temperature upon slow concentration and were analysed by X-ray diffraction. The bis-hydroxo bridged [(H{tBuBOX})(solv)CuII(µ-OH)]2(PF6)2complexes crystallised in the monoclinic space groupsP21/n(solv≡H2O) andP21/c(solv≡MeCN/THF).
The elucidated structures are depicted in Figure 6.38 (solv≡THF/MeCN) and Figure 6.39 (solv≡H2O). Metric parameters of the CuII2(OH)2units (Table 6.12), are similar to those reported for other CuII2(OH)2dimers.[299]Both complexes (the complex with solv≡H2O and the complex with MeCN/THF) are structurally very similar. The dicationic complex is a dimer in close structural analogy to the peroxo Cu2O2complexes mainly discussed in this chapter. The Cu···Cu separation is with 3.04 Å (solv≡THF/MeCN) and 3.00 Å (solv≡H2O), significantly shorter
6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs
838 840 842 844 846 848 836 838 840 842 844 846 848
m/z
843.229, 843.228
842.233, 841.233
841.234, 841.230
840.235, 840.235
839.236, 839.231
m/z calculated abundance for [(C15H26N2O2)2Cu2(16OH)(18OH)PF6]+ HR-ESI-MS spectra
841.2231,841.2255
840.2303, 840.2287
839.2263, 839.2260
838.2319, 838.2303
837.2287, 837.2272
calculated abundance for [(C15H26N2O2)2Cu2(OH)2PF6]+
Figure 6.37.High-resolution ESI+mass spectra of [(H{tBuBOX})(solv)CuII(µ-OH)]2(PF6)2 from MeCN solutions. The magnified regions show the [(H{tBuBOX})2Cu2(OH)2(PF6)]+ion with calculated isotope patterns for the natural abundance Cu2(OH)2complex (pink) and the peak obtained from18O-labelled peroxo complextBuP(blue). The16O18O composition is presumably due to isotopic scrambling during the measurement/sample preparation.
than it is in the peroxo complextBuP(3.51 Å by EXAFS analysis); and it is longer than in a bis(µ-oxo)dicopper(iii) (O) complex (∼2.8 Å).[34]However, the O···O separation is with 2.41 Å (solv≡THF/MeCN) and 2.45 Å (solv≡H2O), significantly longer than a peroxide O−O bond (∼1.4 Å) and longer than the O···O separation in anOcomplex (∼2.3 Å). In the H2O-ligated complex, the Cu···Cu separation is 0.04 Å longer and the O···O separation is 0.04 Å shorter than in the THF/MeCN-ligated complex. The BOX ligands coordinate copper with bite angles of 91.7°
(solv≡THF/MeCN) and 91.8° (solv≡H2O), the∠(HO-Cu-OH) angles are 76.7°
(THF/MeCN) and 78.5° (H2O).
The complex resides on a crystallographically imposed inversion centre, thus, the inversion centre is the central point of the Cu2(OH)2units. The bulkytBu ligand backbones are located on opposite sides of the Cu2(OH)2core. In analogy to
tBuP, solvent ligands are weakly bound (2.2–2.3 Å) in axial positions. Figure 6.38b
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6.10. Thermal Degradation of Peroxo Complexes
Figure 6.38.X-ray solid-state molecular structure of [(THF)0.85(MeCN)0.15(H{tBu BOX})Cu-(µ-OH)]2(PF6)2.( a )ORTEP plot with important atoms labelled. Displacement ellipsoids are drawn at the 50 % probability level; hydrogen atoms and PF6–anions have been omitted for the sake of clarity; MeCN located at a second site due to crystallographic disorder are drawn in lighter shade; symmetry transformations used to generate equivalent atoms (’):
1−x,1−y,1−z.( b )Detail of coordination sphere, viewing direction along the Cu1→ Cu1’ axis.( c )Van-der-Waals representations to illustrate the shielded Cu2(OH)2moiety;
side (left) and top (right) views; acetonitriles are omitted. Significant interatomic distances and angles are listed in Table 6.12.
shows a view along the Cu1→Cu1’ axis; the {Cu2O2} moiety is planar due to the molecule’s symmetry (torsion angleφ(Cu1-O3-O30-Cu10) = 180°). Such a coordination mode and symmetry is quite common for a Cu2(OH)2complex and the structural parameters are remarkably similar to square-planar coordinated bis(µ-hydroxo)dicopper(ii) dimers with bis-guanidine ligands.[299]In the structure, one BOX ligand coordinates to the Cu2(OH)2core from above and one from below the {Cu2O2} plane. The angles between the {Cu2O2} plane and the planes spanned by the {NCuN} coordinations are 16.4° (solv≡H2O) and 14.6° (solv≡THF/MeCN),
6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs
Figure 6.39.X-ray solid-state molecular structure of [(H2O)(H{tBuBOX})Cu(µ-OH)]2(PF6)2. ORTEP plot with important atoms labelled. Displacement ellipsoids are drawn at the 50 % probability level; PF6–anions and most hydrogen atoms have been omitted for clarity;
symmetry transformations used to generate equivalent atoms (’): 1−x,1−y,1−z. Significant interatomic distances and angles are listed in Table 6.12.
respectively. The BOX ligands are even stronger tilted with respect to the {Cu2O2} plane, the angles between the plane spanned by the BOX ligand’s C=N groups and the {Cu2O2} planes are 30.4° (solv≡H2O) and 29.3° (solv≡THF/MeCN), respectively. The ligand planes however, are coplanar with respect to each other.
The six-membered chelate rings adopt a boat conformation.
The copper centres are coordinated in asquare pyramidalfashion, the Cu atoms are slightly moved outside of the {N2CuO2} planes and into the coordination pyramids. The copper atoms have angular structural parametersτ5of 0.02 (solv
≡THF or MeCN) and 0.10 (solv≡H2O), which characterises the geometries as square pyramidal with slight distortion in the case of the H2O ligated complex;
see Section A.1, p. 269, for a definition ofτ5. Theτvalue is not influenced by the position of the apical ligands, but is a parameter of the planarity of the {N2CuO2} moiety. However, while H2O is located above the middle of the pyramid, as well as THF is roughly in the middle, MeCN is located more towards the {Cu2O2} moiety.
Figure 6.38c shows a Van-der-Waals representation of the THF/MeCN-coordinated complex. The structures, especially the THF-coordinated one, are quite compact and are of an overall spherical shape and the Cu2(OH)2core is completely shielded by the organic residues.
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6.10. Thermal Degradation of Peroxo Complexes
Table6.12. Significantgeometricinformationofthesolid-statestructuresof[(solv)(H{tBuBOX})Cu(µ-OH)]2(PF6)2 withsolv≡THF/MeCNorH2OinFigures6.38and6.39.Interatomicdistances(Å),angles(°)andτ parameters. Atoms1,2d1,2(Å)Atoms1,2,3Angle1,2,3(°) solvsolv THF/MeCNH2OTHF/MeCNH2O Cu1···Cu103.0435(6)2.9968(7)N1−Cu1−N291.65(9)91.81(11) O3···O302.4058(22)2.4502(26)O3−Cu1−O3076.65(7)78.54(8) Cu1−N11.9794(25)1.9804(26)N1−Cu1−O3094.54(9)93.75(9) Cu1−N21.9863(23)1.9852(28)N2−Cu1−O395.16(8)93.85(9) Cu1−O31.9376(16)1.9349(19)N1−Cu1−O3(α)166.15(9)163.65(9) Cu1−O301.9420(17)1.9360(16)N2−Cu1−O30(β)167.64(8)169.71(9) Cu1−O42.2885(39)2.2463(28)N1−Cu1−O494.40(12)100.09(10) Cu1−N32.3478(374)N2−Cu1−O492.35(11)95.87(10) O3−Cu1−O497.35(11)94.59(8) O30−Cu1−O497.82(11)91.67(9) N1−Cu1−N399.74(90) N2−Cu1−N3101.46(71) O3−Cu1−N390.72(86) O30−Cu1−N388.04(64) O4−Cu1−N3a10.78(83) Cu1−O3−Cu10103.35(8)101.46(8) ϑ(Cu1-O3-O30-Cu10)180180 τ5valueb0.020.10 aNotethatthisbondangleisbetweentwodisorderedmolecules.bτ5=(β−α)/60°,[300]seep.269;trigonal bipyramidal,τ=1;squarepyramidal,τ=0.
6. Biomimetic Activation of O2by Copper(i) Complexes of BOXs
The DFT computed structure oftBuPis remarkably similar to the X-ray struc-tures of the Cu2(OH)2complexes. Although keeping in mind that the Cu2(OH)2 structural parameters have been exploited as starting coordinates for the calcula-tion of thetBuPstructure, the calculations had not required a drastic alteration of geometric parameters, other than the parameters of the Cu2O2moiety, of course.
It therefore might be deduced, that the Cu2(OH)2complexes are good structural models for the analogous (µ-η2:η2-O2)Cu2complex.
6.10.2. Ligand Oxygenation
No indication for any oxidative degradation of theH{tBuBOX}ligand itself was found in course of this work. This is important, considering that intramolecular ligand oxygenation or oxidation has often been observed in Cu2O2complexes with other capping ligands,[60,301]and regarding the degradation by CuCl2coordination, which was investigated in detail for theH{RBOX}ligands in focus of this work in Chapter 5. The UV-vis spectroscopic trace ofHPformation however suggested degradation of the Cu2O2complex already upon formation from the respective CuIcomplexes and O2(see Figure 6.11, p. 130). Since the curve shape is similar to the kinetic trace of a consecutive A→B→C reaction, it was attempted to fit the spectroscopic trace with this kinetic model. The plot in Figure 6.40 shows the best fit and clearly excludes this supposedHPdegradation pathway. It can be assumed, that the characteristic curve shape results from precipitation of
HPonly. Precipitation is however only well evident for larger amounts, as the precipitated powder is very fine; the observed yields additionally indicate no significant degradation uponHPformation.
N N
Scheme 6.10Thermal degradation ofHPalong with ligand oxygenation.
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