Solid-State Structures

High-quality single crystals ofCu^ItBuandCu^IPhwere obtained by letting an excess of dry Et₂O slowly diffuse over the gas phase into a∼0.2 m solution of H{^tBuBOX}orH{^PhBOX}and 1 eq. [Cu(MeCN)₄]PF₆in THF under an atmosphere of argon. Small turbid-white rods were obtained after some days and were subjected to X-ray diffraction.Cu^ItBuandCu^IPhcrystallise in the monoclinic space groups C2/candP21, respectively. Structural similarity between both complexes is well evident. The copper ions are coordinated in a trigonal planar fashion (Y shape) by the BOX ligands and an exogenous MeCN solvent molecule. The structure of Cu^ItBuis depicted in Figure 6.1 and the structure ofCu^IPhin Figure 6.2a, geometric parameters are listed in Table 6.1.

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6.2.Synthesis and Characterisation of Copper(i) Complexes of BOXs

O2B O2A

O1 N2

N1 Cu1

N3

PF6

(a)

(b)front view (c)side view (d)top view

Figure 6.1.X-ray solid-state molecular structure of [(H{^tBuBOX})Cu^I(MeCN)]PF6(Cu^ItBu).

( a )ORTEP plot with important atoms labelled. Displacement ellipsoids are drawn at the 50 % probability level; most hydrogen atoms have been omitted for the sake of clarity;

atoms located at a second site (B) due to crystallographic disorder are drawn in lighter shade and only one moiety of the disordered PF6^–anion is drawn.( b,c,d )Van-der-Waals representations for clarification of the coordination wedge. Significant interatomic distances and angles are listed in Table 6.1.

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

O2

N2 O1

Cu1 N1

N3 C18

(a) (b)

Figure 6.2. ( a ) X-ray solid-state molecular structure of [(H{^PhBOX})Cu^I (MeCN)]-PF6(Cu^IPh). 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 the sake of clarity. Significant interatomic distances and angles are listed in Table 6.1.( b )Overlay of the crystal structures of H{^tBuBOX} (blue) and H{^PhBOX} (red) from Figures 6.1 and 6.2a.

Table 6.1.

Significant geometric information of the solid-state structures ofCu^IPhandCu^ItBuin Figures 6.1 and 6.2a, respectively. In-teratomic distances (Å) and angles ( ° ).

Atoms 1,2 d1,2 ( Å ) Atoms 1,2,3 Angle 1,2,3 ( ° ) Cu^ItBu

Cu1−N1 1.9630(17) N1−Cu1−N2 93.56(7)

Cu1−N2 2.0037(18) N1−Cu1−N3 142.62(8)

Cu1−N3 1.8649(19) N2−Cu1−N3 123.64(8)

N3≡C16 1.1316(32) Cu1−N3≡C16 175.00(20)

Cu^IPh

Cu1−N1 2.0033(24) N1−Cu1−N2 93.42(10)

Cu1−N2 1.9554(29) N1−Cu1−N3 120.75(12)

Cu1−N3 1.8831(33) N2−Cu1−N3 145.73(12)

N3≡C18 1.1091(49) Cu1−N3≡C18 172.47(32)

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6.2.Synthesis and Characterisation of Copper(i) Complexes of BOXs

Both complexes adopt a boat-shaped coordination geometry and have very similar small ligand bite angles N−Cu−N of 93.6° (H{^tBuBOX}) and 93.4° (H{^PhBOX}) and both have nearly the same Cu−NBOXbond lengths of 1.96 and 2.00 Å. Due to the small bite angles, the trigonal-planar geometries deviate from the ideal N−Cu−N angles of 120°, while one of the remaining angles is at∼120°, the second is at around∼140°. In both complexes, one CMe₂group points more above and one more below the {N₃Cu} plane. Although, the structures are very similar, there are some notable differences between both complexes. While both oxazoline planes are relatively flat inH{^PhBOX}, inH{^tBuBOX}they are curved away from the tBu-backbone, an indication for the large sterical demand of this residue. Owing to the steric bulk of thetBu group, the angle between this residue R and the {N₃Cu} coordination plane is larger in the case of Cu^ItBu: ∼50° in the case of H{^PhBOX}and∼80° in the case ofH{^tBuBOX}(see the overlay of both structures in Figure 6.2b). InCu^IPhthe phenyl ring is rotated by only∼10°, while in the free ligand it is rotated by approximately 71° (cf. Section 4.2, p. 47). InCu^IPh, the Cu−NMeCNbond is 0.02 Å longer and the C N bond in the MeCN ligand is 0.02 Å shorter than inCu^ItBu, indicating that inCu^ItBu, MeCN is more loosely bound.

The MeCN is bound slightly tilted in both cases, the Cu−N C angles are 175°

(H{^tBuBOX}) and 172° (H{^PhBOX}).

The copper(i) coordination is shielded to some extent by the oxazoline-CMe₂ groups and by the backbone residue. This is illustrated by the Van-der-Waals representations in Figure 6.1b–d. The shielding oxazoline-CMe₂groups are in-tended to protect the copper(i) atom and possible copper−oxygen coordinations and to render the complexes more stable. The presence of tautomerism, in the ligandH{^PhBOX}(as described in Chapter 4) has apparently no influence in the coordination abilities.H{^PhBOX}inCu^IPhis present in the form of the diimine in contrast to the solid-state of the neat ligand, where it is present in the form of the iminoenamine tautomer.

6.2.3. Electrochemistry

Against the background of dioxygen activation (reduction of O₂, concomitant with the oxidation of Cu^Ito Cu^II), the redox properties ofCu^ItBuwere investigated electrochemically by cyclic voltammetry (CV) and the differential pulse technique, square-wave voltammetry (SWV), to get more insights in the complexesCu^IRas potential type 3 copper protein mimics.

The depicted CV curves (Figure 6.3) were recorded at a scan rate of 100 mV s⁻¹ in different scan directions, first measuring either cathodic or anodic scans. A

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

0.1 m solution of [NBu₄]PF₆as the supporting electrolyte in MeCN was used. The decamethylferrocene couple, [FeCp^∗₂]^+1/0(E1/2=−0.59 V vs. ferrocene, [FeCp2]^+1/0 in MeCN)^[224,225]was used as an internal standard to avoid the potential overlap of the FeCp2redox signal with other redox processes. The potential was furthermore converted toEvs. SCE (saturated calomel electrode) by adding 0.40 V.

Figure 6.3 shows cyclic voltammograms and a square-wave voltammogram of Cu^ItBuin MeCN under anaerobic conditions (Ar). A measurement over the broad range from−2.3 to +1.3 V vs. SCE shows redox processes, which are not present in the voltammogram of the neatH{^tBuBOX}ligand (cf. Figure 5.13, p. 100).Cu^ItBu shows an electrochemically irreversible reduction with a cathodic peak potential, Epc=−1.22 V vs. SCE. This reduction can most likely be assigned to the process Cu^I+e^– Cu⁰. An associated oxidation in the reverse anodic backscan with peak potentialEpa=−0.45 V vs. SCE (∆Ep= 0.77 V) likely corresponds to a »copper stripping peak« (Cu⁰ Cu^I+ e^–).

Furthermore, a quasi-reversible electrochemical oxidation of Cu^ItButakes place atEpa= 0.63 V vs. SCE and is assigned to the process Cu^I Cu^II + e^–. Some known factors influencing redox potentials of copper complexes include the types of donor atoms, the geometry in tetracoordinate complexes and the flexibility/rigidity of a chelating ligand.^[226]The absence of a proximal (∆Ep= 0.059 V) reduction peak renders the oxidationelectrochemicallyirreversible. The corresponding reduction in the reverse scan occurs atEpa= 0.33 V vs. SCE and

50 µA

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

Cu^I/Cu⁰

potential (V vs. SCE) Cu^II/Cu^I

Figure 6.3.Cyclic voltammograms ofCu^ItBuin MeCN containing 0.1 m [NBu4]PF6; scan rate 100mV s⁻¹( / ). Square-wave voltammogram ( ). Open circuit potential≈ 0.1 V.

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6.2.Synthesis and Characterisation of Copper(i) Complexes of BOXs

indicates that this process ischemicallyreversible (quasi-reversible). The large separation of the anodic and cathodic peak potentials (∆Ep= 0.30 V) for this couple on the cyclic voltammetry time scale reflects substantial rearrangement around the copper ion environment upon oxidation of Cu^I Cu^II. This is quite common for copper complexes as the coordination geometries around Cu^Iand Cu^II are usually significantly different.^[227]

Figure 6.4 shows additional measurements at varying scan rate (v), which support quasi-reversible mechanisms for both processes, since there is a shift of the peak potential with increasing scan rate and additionally, the peak currents ipincrease linearly as a function of the square root of the scan rate,ip ∝√

v

Figure 6.4.Reduction peaks (top left) and oxidation peaks (bottom left) in cyclic voltammo-grams ofCu^ItBuin MeCN containing 0.1 m [NBu₄]PF₆at varying scan ratev. Respective plots ofipvs.√

v(right), showing linear dependences (R¯²=0.997, top; 0.995, bottom).

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