Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)−Dipyrrin Complexes Lauréline Lecarme,† Amélie Kochem,† Linus Chiang,‡ Jules Moutet,† Florian Berthiol,† Christian Philouze,† Nicolas Leconte,† Tim Storr,§ and Fabrice Thomas*,† †

Département de Chimie Moléculaire, UMR CNRS 5250, Université Grenoble Alpes, B.P. 53, 38041 Grenoble Cedex 9, France Department of Chemistry, University of the Fraser Valley, Abbotsford, British Columbia V2S 7M8, Canada § Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada ‡

S Supporting Information *

ABSTRACT: The sterically hindered bis(phenol)−dipyrrin ligands HLH3 and PhLH3 were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(HL)] and [Cu(PhL)]. Their X-ray crystal structures show relatively short C−O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl−phenoxyl radical species. Complexes [Cu(HL)] and [Cu(PhL)] exhibit rich electronic spectra, with an intense nearIR (NIR) band (ε > 6 mM−1 cm−1) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave (E1/21,ox = 0.05 and 0.04 V), as well as a reversible reduction wave (E1/21,red = −0.40 and −0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(HL)]+ and [Cu(PhL)]+) and anions ([Cu(HL)]− and [Cu(PhL)]−) were generated. They all display an axial (S = 1/2) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligandcentered redox processes in both reduction and oxidation. Complex [Cu(HL)](SbF6) was cocrystallized with [Cu(HL)]. Oxidation is accompanied by a slight contraction of both the C−O bonds (mean bond distance of 1.280 Å) and the C−C bonds connecting the peripheral rings to the dipyrrin. The cations show vis−NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)−dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions. center. Representative examples are phenolates,7,21,22 diaminobenzenes and anilines,23 oligopyrroles (such as porphyrins, dipyrrins, and tripyrrins24), aminophenols,25 and catechol moieties. Provided that they harbor electron-donating or sterically demanding substituents, most of the ligands listed above can be oxidized under mild conditions to radical species. In the past 5 years, we and others have developed first-row transition-metal complexes based on the sterically hindered bis(phenol)−dipyrrin (Chart 1)13,26−28 and bis(naphthol)− dipyrrin scaffold.29 We established that the bis(phenol)− dipyrrin ligand easily undergoes one-electron oxidation by air in the presence of stoichiometric amounts of a cobalt(II) or nickel(II) salt and base.27 This air oxidation affords a very stable radical complex with unprecedented mixed “phenoxyl− pyrrolyl” character; i.e., the ligand radical singly occupied

1. INTRODUCTION The coordination chemistry of “redox noninnocent” ligands currently attracts considerable interest.1−8 Apart from their peculiar spectroscopic behavior, they find a number of potential applications. For example, their ability to host spins makes them ideal candidates for the design of new magnetic materials,9 redox switches,10,11 and catalysts.1−5,8,12 Significant advances were achieved recently in this latter domain, leading to a classification based on the role of the ligand during catalysis:8 By shuttling between two redox states, the ligand can either act directly as an electron reservoir during turnovers1 or, alternatively, tune the Lewis acidity of the coordinated metal ion,13 enhancing the reactivity of the complex. This elegant modulation of the catalyst properties is not solely the domain of chemists because it is also found in a number of metalloenzymes, whose archetypes are the iron-containing peroxidases and cytochrome P450,14,15 as well as the copper enzymes galactose oxidase16−19 and glyoxal oxidase.20 Several ligand platforms or coordinating groups are recognized as “redox noninnocent” when chelated to a metal © XXXX American Chemical Society

Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: January 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b00044 Inorg. Chem. XXXX, XXX, XXX−XXX

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We establish that the ligands HLH3 and PhLH3 can adopt three stable oxidation states once they are metalated. The neutral copper radical complexes are very efficient functional models of galactose oxidase, capable of oxidizing both activated and unactivated alcohols.

Chart 1. Structures of the Ligands and Neutral Complexes

2. RESULTS AND DISCUSSION 2.1. Synthesis. The ligands HLH3 and PhLH3 were synthesized according to literature procedures.13,26 The copper complexes [Cu(HL)] and [Cu(PhL)] were prepared by mixing stoichiometric amounts of Cu(OAc)2·4H2O with the corresponding ligand. The reaction was conducted in air in the presence of 3 mol equiv of triethylamine. The complexes [Cu(HL)] and [Cu(PhL)] precipitate out as a dark-green powder. The cations [Cu(HL)]+ and [Cu(PhL)]+ were prepared quantitatively by reaction of the neutral precursor with 1.2 mol equiv of the one-electron oxidant AgSbF6. Crystallization of the cations was proven to be more difficult than the corresponding cobalt and nickel complexes.27 We, however, succeeded in isolating a crystalline material that contains an equimolar mixture of the precursor [Cu(HL)] and the cation [Cu(HL)](SbF6) for structural characterization. The anions were prepared in situ by adding cobaltocene to a solution of the neutral complexes [Cu( HL)] and [Cu(PhL)].31 Double integration of the electron paramagnetic resonance (EPR) signal of both the cation and anion confirmed the quantitative oxidation or reduction of the neutral precursor, respectively. 2.2. X-ray Crystal Structure of H LH 3 and the Complexes. The structure of the free ligand HLH3, whose synthesis26 but not crystal structure was previously described, is depicted in Figure 1. The two rings constituting the dipyrrin moiety are inequivalent, with an angle of 24° between them (Figure 1b). The H1A atom could be located close to the N1 atom, whereas the N2 atom is clearly sp2-hybridized, without a neighboring H atom. Further evidence for the different nature of the two rings of the dipyrrin comes from analysis of the C− N bonds: The C18−N2 bond distance is 1.336 Å, which is in the expected range for imines. The C7−N1, C10−N1, and C21−N2 bonds are comparatively much longer, at 1.363, 1.380, and 1.395 Å. The angle between the phenol and the adjacent pyrrole ring featuring the N1 atom is 24°. The angle between the other phenol and its adjacent pyrrole ring (incorporating the N2 atom) is 9°, suggesting increased conjugation on this side of the molecule. This fact is confirmed by the shorter C13−C18 bond distance (1.457 Å) compared to the opposite C2−C7 bond distance (1.467 Å). The O1, O2, N1, and N2 atoms are involved in a remarkable intramolecular hydrogenbonding network. The O1−H1A and N2−H1A distances are 2.365 and 2.478 Å, respectively. The H1A atom, therefore, acts as a donor in a weak bifurcated hydrogen-bonding network involving two pyrroles and one phenol. Furthermore, the N2− H2 distance is 1.786 Å, supporting a strong hydrogen bond between the phenolic H2 in the iminic N2. The neutral complexes were crystallized by the slow evaporation of a concentrated solution of [Cu(HL)] in CH2Cl2/ethanol or diffusion of pentane into a dichloromethane solution of [Cu(PhL)]. The crystal structures of complexes [Cu(HL)] and [Cu(PhL)] are depicted in Figure 2. They show an almost square-planar metal ion in both cases, which is coordinated to two N atoms (N1 and N2) of the dipyrrin and two O atoms (O1 and O2) of both phenolate groups. The angles between the opposite N1−Cu−O1 and N2−Cu−O2 planes are 14° and 10° for [Cu(HL)] and [Cu(PhL)],

molecular orbital (SOMO) is delocalized over the entire ligand. The radical species shows remarkable optical properties, with intense absorptions in both the visible and near-IR (NIR) ranges. In contrast to the nickel(II) and cobalt(II) complexes, which rapidly react with dioxygen to give the radical species, the manganese(III) complex could be isolated under its bis(phenolate) form, in addition to the radical form.13 We showed that both the nonradical and radical complexes are capable of oxygenation of styrene, but the reaction was remarkably dependent on the initial oxidation state of the complex: the manganese(III) bis(phenolate) complex catalyzes the epoxidation of styrene, while the manganese(III) radical complex, because of the increased Lewis acidity of the metal center, was capable of anti-Markovnikov epoxide ring opening, producing the aldehyde. The ease of oxidation of the ligand when chelated to a divalent metal ion (radical formation is observed at potentials lower than those of the Tyr•/Tyr redox couple of galactose oxidase)16 and the possibility of controlling the chemoselectivity of a catalyzed reaction through the ligand oxidation state have encouraged us to investigate the ability of the bis(phenol)−dipyrrin platform to model the galactose oxidase active site. With this aim, we herein describe the coordination chemistry of HLH3 and PhLH3 with copper(II) (Figure 1). The

Figure 1. X-ray crystal structure of the ligand H3LH, shown with 30% thermal ellipsoids: (a) top view; (b) side view, without H atoms and tBu groups.

ligands HLH3 and PhLH3 differ by the meso substituent, which is either a benzene or an o,o′-diphenylbenzene. The choice of these substituents was motivated by the early reports on tetraphenylporphyrin (TPP) derivatives that showed that the planarity and subsequent redox properties of the porphyrin were directly correlated to the bulkiness of the o,o′ groups.30 B


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[Cu(PhL)], which shows C1−O1 and C12−O2 bond distances of 1.275 and 1.298 Å. The angle between the adjacent aromatic rings is smaller than that in the free ligand H3LH, with a maximum value of 19° in the case of [Cu(HL)]. Furthermore, the C2−C7 and C13−C18 bond distances (bonds connecting the phenol rings to the dipyrrin) are 1.416 and 1.418 Å, respectively, which are much shorter than those in H3LH. These structural features strongly support a sharing of the spin density between the phenolic and dipyrrin rings in neutral copper complexes, conferring it a mixed pyrrolyl−phenoxyl character (Scheme 1). Consistent with the dianionic radical nature of the ligand, the Cu ion adopts a (+II) oxidation state and lies within a preferred distorted square-planar geometry. The cationic complex [Cu(HL)]+ was prepared by reaction of the neutral precursor with 1.2 mol equiv of the single electron oxidant AgSbF6. Vis−NIR monitoring of the oxidation attests to the fact that the reaction proceeds quantitatively. Slow diffusion of pentane into a concentrated dichloromethane solution of this complex affords single crystals suitable for X-ray diffraction. The crystal cell contains two distinct molecules, A and B, which show the same overall geometry, and a single hexafluoroantimonate anion. It is therefore likely that the cation evolves during the crystallization process by oxidizing an unidentified impurity in the solvent, reforming the neutral precursor. From analysis of the metrical parameters within each complex present in the crystal cell (Table 1), it appears that the C1B−O1B and C12B−O2B bond distances (1.279 and 1.278 Å) in molecule B are shorter than the corresponding ones in molecule A (C1A−O1A and C12A−O2A at 1.287 and 1.290 Å, respectively). These latter values better fit with those measured in the crystal structure of the genuine neutral species. On the other hand, the C1B−O1B (and opposite C22B−O2B) and C6B−C7B (and opposite C16B−C1B) bond distances (molecule B) compare fairly well with those reported for the isostructural [Ni(HL)]+ species (1.285 and 1.279 Å), which was identified as a nickel(II) complex featuring an oxidized monoanionic ligand. A similar electronic structure is therefore likely in the case of [Cu(HL)]+, wherein the ligand is closedshell and adopts a quinoidal-like distribution of bonds (Scheme 2). This electronic structure of the cations, which excludes a copper(III) radical formulation, is fully supported by EPR spectroscopy (see below). Finally, it must be stressed that the angle between the meso ring and the mean plane of the

Figure 2. X-ray crystal structures of the neutral and cationic copper complexes shown with 30% thermal ellipsoids: (a) [Cu(HL)]; (b) [Cu(PhL)]; (c) [Cu(HL)]+ (molecule B). The H atoms are omitted for clarity. Note that the atom numbering is the same for the three structures.

respectively, indicative of only small tetrahedral distortions at the metal center. The coordination sphere is slightly asymmetric in both cases, with Cu−N1, Cu−N2, Cu−O1, and Cu−O2 bond distances of 1.918, 1.946, 1.946, and 1.896 Å, respectively, for [Cu(HL)] and 1.934, 1.920, 1.902, and 1.933 Å, respectively, for [Cu(PhL)]. The angles between the phenyl ring in the meso position and the mean plane formed by the dipyrrin moiety are 51° and 70° for [Cu(HL)] and [Cu(PhL)], respectively. The larger angle observed for [Cu(PhL)] likely results from the increased steric bulk provided by the additional o,o′ substituents of the phenyl group. The planarity of the dipyrrin core is also affected by the nature of the meso substituent: The angle between the pyrrole rings is indeed 8° in [Cu(PhL)], with a bend along the Cmeso−Cu axis, while it is 17° in [Cu(HL)] with a bend orthogonal to the Cmeso−Cu axis. It has been reported that the C−O bond distances within the phenolic moiety is a hallmark of the ligand oxidation state. The C−O bond distances in neutral [Cu(HL)] are 1.281 and 1.301 Å, which compare fairly well with the related radical species.7,13,27,28 The same observation can be made for Scheme 1. Canonical Structures of [Cu(HL)]a

a

Delocalization is only presented on a single side of the complex, but the same features are expected on the other side. C


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Inorganic Chemistry Table 1. Selected Metrical Parameters for the Ligand HLH3 and the Complexesa LH3

[Cu(HL)]b

[Cu(PhL)]

[Cu(HL)] (A)c

[Cu(HL)]+ (B)c

1.383(2) 1.354(2) 1.467(3) 1.457(3) 1.399(3) 1.407(3)

1.918(4) 1.946(5) 1.946(4) 1.896(3) 1.281(5) 1.301(6) 1.410(8) 1.441(8) 1.438(6) 1.420(9)

1.934(3) 1.920(2) 1.902(2) 1.933(2) 1.298(4) 1.275(4) 1.420(4) 1.432(5) 1.435(5) 1.437(5)

1.922(3) 1.922(3) 1.921(3) 1.922(2) 1.287(3) 1.290(3) 1.420(6) 1.418(5) 1.438(5) 1.436(5)

1.916(3) 1.917(3) 1.919(3) 1.933(2) 1.279(3) 1.278(3) 1.397(5) 1.408(6) 1.463(5) 1.450(5)

H

Cu−N1 Cu−N2 Cu−O1 Cu−O2 C1−O1 C12−O2 C2−C7 C13−C18 C1−C2 C12−C13

a In angstroms. bGenuine crystals of [Cu(HL)]. cThe crystal cell contains two distinct molecules A and B assigned to the neutral and cationic complexes, respectively.

Scheme 2. Canonical Structures of [Cu(HL)]+

dipyrrin is mostly similar in molecules A and B (50−52°) and comparable to that measured in genuine crystals of [Cu(HL)]. 2.3. Electrochemistry. The electrochemical behavior of the ligands and complexes has been investigated by cyclic voltammetry (CV) and bulk electrolysis in CH2Cl2 containing tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte. The cyclic voltammograms of the free ligands are depicted in Figure 3a. Both ligands exhibit two successive oxidation waves, similarly to free porphyrins and tert-butylated salens (Table 2). The first oxidation wave is both monoelectronic and reversible. It is observed at relatively low potentials (E1/2ox,1 = 0.18 and 0.17 V for HLH3 and PhLH3, respectively) in comparison to H2TPP and sterically hindered salen ligands (Table 2). The dipyrrin bis(phenol) platform can thus be considered as a promising precursor of persistent radical species. The fact that the E1/2ox,1 values are very similar for both HLH3 and PhLH3 indicates that the electronic effect exerted by the meso substituent is the same. The free ligands HLH3 and PhLH3 also exhibit a reduction wave, but its reversibility is weak (Ipc/Ipa < 0.1). This low reversibility is in fact not surprising owing to the strong electron-donating effect of the di-tert-butylphenol moieties. The cyclic voltammograms of complexes [Cu(HL)] and [Cu(PhL)] are shown in Figure 3b. Both complexes exhibit two successive and reversible monoelectronic oxidation waves (E1/2ox,1 and E1/2ox,2), as well as two monoelectronic reduction waves (E1/2red,1 and E1/2red,2). The first oxidations are observed at E1/2ox,1 = 0.05 and 0.04 V versus ferrocenium/ferrocene (Fc+/Fc) for [Cu(HL)] and [Cu(PhL)], respectively. According to the X-ray diffraction study on [Cu(HL)] and [Cu(HL)]+, the first redox system located at E1/2ox,1 in the cyclic voltammogram of [Cu(HL)] is attributed to a ligand-centered process (Chart 2). The ligand radical unit within [Cu(HL)] is indeed monooxidized at E1/2ox,1 into a quinoidal monoanionic structure, which remains coordinated to the CuII ion. The similarity in the E1/2ox,1 values for both complexes shows that the first oxidation is only marginally affected by the nature of the meso substituent. In other terms, E1/2ox,1 is only slightly

Figure 3. CV curves of (a) HLH3 (black) and PhLH3 (red) and (b) [Cu(HL)] (black) and [Cu(PhL)] (red) in CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte. Reference: Fc+/Fc. Scan rate = 0.1 V s−1. T = 298 K.

affected by the geometry of the dipyrrin phenoxo−phenoxyl core. The E1/2ox,1 value is also mostly unaffected by the nature of the divalent metal ion because it has been measured at +0.03 and −0.01 V for the nickel and cobalt complexes, respectively (Table 2).27 The second oxidation is comparatively more affected by the meso group because it is measured at E1/2ox,2 = D


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Inorganic Chemistry Table 2. Electrochemical Properties of the Ligands and Complexesa ligand/complex

E1/2red,2

H

LH3 Ph LH3 H2TPP H2tBusalen [Cu(HL)] [Cu(PhL)] [Ni(HL)] [Co(HL)] [Cu(TPP)]c,d [Cu(tBusalen)]d

−2.07 −2.16 (0.11) −2.40b

−2.28

E1/2red,1

E1/2ox,1

−1.51 (0.17) −1.60 (0.17) −1.71

0.18 (0.15) 0.17 (0.17) 0.52 0.66 0.05 (0.08) 0.04 (0.09) 0.03 −0.01 0.50d 0.45d

−0.40 (0.07) −0.56 (0.09) −0.48 −0.61 −1.81

E1/2ox,2 0.68 0.66 0.77 0.80 0.95 0.99 0.58 0.73 0.65

(0.18) (0.14)

(0.09) (0.10)

reference this this 35 36 this this 27 27 35 37

work work

work work

a In CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte. All potentials refer to the regular Fc+/Fc redox potential (E1/2 = 0.19 V vs 0.01 M Ag/AgNO3; ΔEp = 0.13 V under our experimental conditions). T = 298 K. bIrreversible process; Epc is indicated. cTPP: deprotonated tetraphenylporphyrin. tBusalen: deprotonated Jacobsen’s ligand. dThe first oxidation of [Cu(TPP)] and [Cu(tBusalen)] corresponds to the first oxidation of the ligand to a ligand radical. The corresponding redox couple in [Cu(HL)] and [Cu(PhL)] is the first reduction process because the ligand is already under its radical form in the isolated neutral species.

[Cu(HL)]−, which is found to be delocalized over the dipyrrin core (see the Supporting Information).34 It is also supported by the observation of a reduction wave at a similar potential in the cyclic voltammogram of the nickel complex [Ni(HL)] (−2.15 V, irreversible wave, not shown).27 Because of its low reversibility and very low potentials, we did not investigate further. It is significant that radical formation in the bis(phenolate)− dipyrrin series is observed in a more negative potential range than that for copper complexes of sterically hindered salens, anilinosalens, and TPPs. This shift is a consequence of the trianionic nature of the fully deprotonated bis(phenolate)− dipyrrin ligand, as opposed to the dianionic charge of the other ligands. The low E1/2red,1 also explains the spontaneous oxidation of the copper bis(phenolate) precursors in air to the corresponding radical species and their exceptional stability. 2.4. Theoretical Analysis. We further examined the electronic structure of the neutral complexes, as well as the monooxidized and monoreduced forms using DFT calculations. The computational results for [Cu(HL)] and [Cu(PhL)] are similar, and thus we focus the discussion here on [Cu(HL)]. The data for [Cu(PhL)] are available in the Supporting Information. Three electronic states are possible for [Cu(HL)]: (i) a CuII d9 center ferromagnetically coupled to the HL2− ligand radical (S = 1), (ii) a CuII d9 center antiferromagnetically coupled to the HL2− ligand radical (S = 0), or (iii) a CuIII d8 center coordinated by a diamagnetic HL3− ligand (S = 0). All three possibilities were explored by DFT, with both ii and iii initial guesses converging to the CuIII electronic state. Importantly, the triplet (S = 1) solution was predicted to be lowest in energy by 16.3 kcal mol−1, which is in agreement with the experimental data (see below). A symmetric structure was predicted for [Cu(HL)] with metrical parameters within ±0.04 Å of the experimental values (Table 3). The spin-density plot of [Cu(HL)] depicts two unpaired electrons (S2 = 2.03) distributed over both the Cu center and ligand framework, consistent with a S = 1 description (Figure 4). Three electronic states were explored for the one-electronoxidized species [Cu(HL)]+: (i) a CuII d9 center coordinated by a diamagnetic HL− ligand (S = 1/2), (ii) a CuIII d8 center coordinated by a HL2− ligand radical (S = 1/2), and (iii) a CuII center ferromagnetically coupled to a diradical HL− ligand (S = 3 /2). The CuIII initial guess ii converged to the S = 1/2 CuII solution i, demonstrating the stability of the latter electronic

Chart 2. Redox Processes for the Neutral Complexes Showing the Ligand Oxidation States (R = H, Ph)

0.95 and 0.99 V for [Cu(HL)] and [Cu(PhL)], respectively. It may be assigned to the oxidation of CuII to CuIII or, alternatively, further oxidation of the ligand.32 The first reduction of the neutral complexes is reversible and occurs at E1/2red,1 = −0.40 V ([Cu(HL)]) and −0.56 V ([Cu(PhL)]). This redox wave is assigned to the ligandcentered process RL•2−/RL3− (R = H, Ph) because a metalcentered process would be accompanied by geometrical changes at the Cu ion, which would alter the reversibility of the redox wave. This assignment is corroborated by a comparison with the derivatives [Ni(HL)] and [Co(HL)], which are reduced in the same potential range (−0.48 and −0.61 V, respectively; Table 2),27 as well as density functional theory (DFT) calculations and EPR spectroscopy (see below). The shift in the RL•2−/RL3− potential upon going from [Cu(HL)] to [Cu(PhL)] deserves further comment. The related redox wave in the free (triprotonated) ligands corresponds to E1/2ox,1, and it is observed at identical potentials for both HLH3 and PhLH3. Thus, the shift in E1/2red,1 between [Cu(HL)] and [Cu(PhL)] does not arise from a change in the electronic properties of the meso substituent but rather originates from the differences in geometry within the complexes. It must be stressed that, in the absence of metal, the ligand has large degrees of freedom to adopt a lower-energy conformation, unlike highly conjugated macrocyclic porphyrins. In our case, the influence of the meso substituent on the ligand structure is probably weak and the oxidation is observed at the same potential for HLH3 and PhLH3. In the complexes, the ligand wraps around the metal center, minimizing the degrees of freedom of the organic framework and maximizing the influence of the meso substituent. This geometrical effect can be visualized through the difference in planarity between the pyrrole units within [Cu(HL)] and [Cu(PhL)] (see above), which is known to affect the oxidation potential in metalated porphyrins.30 The second reduction is quasi-reversible for [Cu(HL)] (E1/2red,2 = −2.16 V) but irreversible for [Cu(PhL)] (Epc = −2.40 V). The quite negative value33 suggests again a ligand-centered redox process. This assignment is corroborated by an inspection of the lowest unoccupied molecular orbital of E


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is a consequence of a spatial proximity between the radical and the Cu center. Because EPR was uninformative for unraveling the spin state of the compounds, we measured their magnetic susceptibility by NMR (Evans’ method). The spectra were broad and expanded beyond the diamagnetic window, while the calculated numbers of unpaired electrons were 1.5 and 2 for complexes [Cu(HL)] and [Cu(PhL)], respectively. These data unequivocally rule out a copper(III) formulation of the complexes, which would have been diamagnetic. Thus, they fully support a triplet spin state, i.e., copper(II) radical formulation, of the neutral compounds, as predicted by DFT calculations. In contrast to the neutral complexes, both the cations and anions are EPR-active (Figures 5 and S1). They all show axial

Table 3. Comparison of the Selected Metrical Parameters in the Geometry-Optimized and X-ray Crystal Structures [Cu(HL)]

a

[Cu(HL)]−

[Cu(HL)]+ b

bond

X-raya

DFT (S = 1)a

X-raya

DFT (S = 1/2)a

DFTb (S = 1 /2)a

Cu−N1 Cu−N2 Cu−O1 Cu−O2 C1−O1 C12−O2

1.918(4) 1.946(5) 1.946(4) 1.896(3) 1.281(5) 1.301(6)

1.931 1.931 1.922 1.922 1.296 1.296

1.916(3) 1.917(3) 1.919(3) 1.933(2) 1.279(3) 1.278(3)

1.923 1.923 1.922 1.922 1.278 1.278

1.933 1.935 1.929 1.922 1.313 1.313

In angstroms. Section.

b

b

For computational details, see the Experimental

Figure 5. X-band EPR spectra of the chemically generated (a) [Cu(HL)]+ and (b) [Cu(HL)]− in CH2Cl2. The solid black lines are experimental spectra, and the dotted red lines are simulations using the parameters given in Table 4. Microwave frequency = 9.43 GHz. Power = 5 mW. Modulation frequency = 100 kHz. Amplitude = 0.3 mT. T = 100 K.

Figure 4. Spin-density plot of [Cu(HL)] (S = 1). See the Experimental Section for calculation details.

state in agreement with the experimental data. The S = 3/2 solution is predicted to be 9.7 kcal mol−1 higher in energy in comparison to the S = 1/2 CuII solution. A symmetric structure was predicted for [Cu(HL)]+ with coordination bond distances within ±0.02 Å of the experimental values (Table 3). The spindensity plot of [Cu(HL)]+ shows spin covalency between the Cu dx2−y2 orbital and the coordinating atoms (Figure S2). Similarly, three electronic states were explored for the oneelectron-reduced species [Cu(HL)]‑: (i) a CuII d9 center coordinated by a diamagnetic HL3− ligand (S = 1/2), (ii) a CuI d10 center coordinated by a HL2− ligand radical (S = 1/2), and (iii) a CuII center ferromagnetically coupled to a diradical HL‑ ligand (S = 3/2). The CuI initial guess ii converged to the S = 1 /2 CuII solution i, confirming the stability of the CuII electronic structure, as observed experimentally. The S = 3/2 solution is clearly ruled out because it is predicted to be 27.7 kcal mol−1 higher in energy to this S = 1/2 CuII solution. Similar to [Cu(HL)]+, the spin-density plot of [Cu(HL)]− shows spin covalency between the Cu dx2−y2 orbital and the coordinating atoms (Figure S2). The corresponding spin-density plots for [Cu(PhL)] and the associated cation and anion are shown in Figure S3. 2.5. EPR Spectroscopy. The copper complexes were investigated by EPR spectroscopy. Both [Cu(HL)] and [Cu(PhL)] were found to be X-band EPR-silent or difficult to detect at the X-band frequency at both 10 and 100 K. Such behavior was already reported for phenoxyl radical complexes and assigned to either antiferromagnetic coupling, large zerofield-splitting parameters in the triplet species, fast relaxation, or a combination of both of the latter phenomena.38 This behavior

Table 4. Spin-Hamiltonian Parameters of the copper Complexesa complex

g values

H

[Cu( L)] [Cu(PhL)] [Cu(HL)]+ [Cu(PhL)]+ [Cu(HL)]− [Cu(PhL)]−

g⊥ = 2.043, 2.213 g⊥ = 2.048, 2.202 g⊥ = 2.044, 2.204 g⊥ = 2.044, 2.204

g∥ = g∥ = g∥ = g∥ =

ACu (mT) EPR-silent EPR-silent A⊥ = 3.5, A∥ 19.2 A⊥ = 3.5, A∥ 19.9 A⊥ = 3.5, A∥ 20.3 A⊥ = 3.5, A∥ 20.3

A2N (mT)

=

A⊥ = A∥ = 1.5

=

A⊥ = 1.5, A∥ = 1.6 A⊥ = 1.6, A∥ = 1.6 A⊥ = 1.6, A∥ = 1.6

= =

a

Simulation of the spectra in a 0.5 mM CH2Cl2 solution at 100 K. The same spectra were obtained at 10 K.

EPR signals that are typical for mononuclear copper(II) species. The signal exhibits hyperfine splitting due to interaction of the electronic spin with the copper nuclear spins. We determined the following spin-Hamiltonian parameters by simulation: g∥ = 2.213, g⊥ = 2.043, ACu∥ = 19.2, and A∥2N = 3.5 mT for [Cu(HL)]+ and g∥ = 2.202, g⊥ = 2.048, ACu∥ = 19.9, and A∥2N = 3.5 mT for [Cu(PhL)]+. The parameters slightly differ between [Cu(HL)]+ and [Cu(PhL)]+, in line with the small differences in the crystal structures of the neutral precursors. The spin-Hamiltonian parameters for [Cu(HL)]− are identical with those for [Cu(PhL)]− but differ slightly from those of the cations g∥ = 2.204, g⊥ = 2.044, ACu∥ = 20.3, and A∥2N = 3.5 mT. This behavior is congruent with the fact that F


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780, and 612 nm. While the bands at 732 and 617 nm show similar phenoxyl-to-dipyrrin donation, the donor and acceptor orbitals for the transition at 999 nm are delocalized over the ligand framework (Table 6). The spectra of the cations are characterized by intense NIR transitions reminiscent of quinoidal or radical compounds.10,27,40−46 Their positions differ significantly from those of the neutral species, especially the low-energy transitions that are blue-shifted. As an example, the most intense NIR band is observed at 1084 nm (16.47 mM−1 cm−1) in the case of [Cu(HL)]+ versus 1343 nm (7.07 mM−1 cm−1) for [Cu(HL)]. TD-DFT calculations consistently predict this shift with excitations at 927 nm ( fosc = 0.2757) and 836 nm ( fosc = 0.0708), corresponding to the experimental bands at 1084 and 960 nm. Using NTO analysis, both excitations correspond to LLCT, with donation from the quinone/ phenoxide groups to the dipyrrin unit (Table S1). The electronic absorption spectra of [Cu(HL)]− and [Cu(PhL)]− are simpler than those of the corresponding neutral and cationic species. It is confined to the UV−vis region, consistent with the copper(II) bis(phenolato) characterand neither radical nor quinoidal natureof the anions. The most intense transition is observed at 677 nm (38.67 mM−1 cm−1) and 686 nm (41.10 mM−1 cm−1) for [Cu(HL)]− and [Cu(PhL)]−, respectively. TD-DFT analysis predicts an intense band at 640 nm for [Cu(HL)]−, which compares well with the experimental transition at 677 nm. For this LLCT transition, the acceptor orbital is localized to the dipyrrin unit (Table S2). In addition, no bands are predicted at low energy, matching the experimental spectrum (Figure 6c). The corresponding NTOs for [Cu(PhL)] and the associated cation and anion are shown in Tables S3−S5. 2.7. Catalysis. The neutral complexes [Cu(HL)] and [Cu(PhL)] show an electronic structure, e.g., a copper radical core, reminiscent of that found in the active site of galactose oxidase.47 We therefore investigated their reactivity toward aerobic alcohol oxidation. Before undertaking this study, we first ensured that the complexes were capable of simple reactivity, such as direct electron transfer. Thus, [Cu(HL)] and [Cu(PhL)] were reacted with an excess of tri-tert-butylphenolate, and the corresponding tri-tert-butylphenoxyl was formed quantitatively. Having established with this simple substrate that one-electron reactivity could be achieved by [Cu(HL)] and [Cu(PhL)], we investigated the catalytic activity toward the aerobic oxidation of alcohols, i.e., a biomimetic two-electron redox chemistry. Because the solubility of [Cu(PhL)] is higher than that of [Cu(HL)] in concentrated CH2Cl2/alcohol solutions, the reactivity was preferentially scrutinized with the first complex. Thus, complex [Cu(PhL)] catalyzes very efficiently the oxidation of benzyl alcohol to benzaldehyde, with 300 turnover numbers (TONs) achieved in 24 h for a catalyst loading of 0.08 mol % (entry 1, Table 7) in the presence of 1.2 equiv of potassium tertbutoxide. The reaction proceeds very rapidly because 240 TONs were already measured after only 1 h of workup at 298 K. For a higher catalyst loading (1 mol %), the TON decreases significantly (18 after 24 h, entry 2), suggesting that neither dimerization or oligomerization of the complex is required for catalysis. Deprotonation of the substrate is essential for the activity, as revealed by two series of experiments. First, the TON does not exceed 100 when potassium tert-butoxide is used in a substoichiometric amount (0.2 equiv, entries 3 and 4). Second, the replacement of potassium tert-butoxide by weaker

the redox-active units are directly bound to the metal center and that they undergo redox changes upon going from the cation to the anion, while the CuII oxidation state is maintained. 2.6. Vis−NIR Spectroscopy and Time-Dependent DFT (TD-DFT) Calculations. The neutral complex [Cu(HL)] is intensely green-colored (Figure 6 and Table 5), exhibiting a

Figure 6. Vis−NIR spectra for the copper complex [Cu(HL)] under various oxidation states in CH2Cl2: (a) [Cu(HL)]; (b) [Cu(HL)]+; (c) [Cu(HL)]−. T = 298 K.

Table 5. Vis−NIR Data for the Complexes complex H

[Cu( L)] [Cu(PhL)] [Cu(HL)]+ [Cu(PhL)]+ [Cu(HL)]− [Cu(PhL)]−

λmax, nm (ε, mM−1 cm−1)a 386 (19.77), 428 (21.51), 460sh (15.66), 612 (17.71), 710sh (6.90), 782 (17.93), 997 (3.83), 1200sh (4.36), 1343 (7.07) 390 (20.68), 432 (20.78), 470sh (15.11), 619 (16.24), 730sh (7.58), 792 (18.01), 986 (3.93), 1200sh (4.27), 1322 (6.71) 440 (20.83), 480sh (11.39), 636 (11.53), 830sh (9.95), 960 (17.18), 1084 (16.47) 430 (18.73), 449 (19.23), 540sh (11.10), 500sh (11.21), 653 (7.69), 840sh (9.58), 964 (17.00), 1089 (16.55) 443 (15.73), 621sh (17.0), 677 (38.67) 443 (15.73), 629sh (17.0), 686 (41.10)

a

In CH2Cl2 at 298 K. The cations and anions were chemically generated.

rich vis−NIR spectrum, with intense low-energy bands at 612 nm (17.71 mM−1 cm−1), 782 nm (17.93 mM−1 cm−1), 997 nm (3.83 mM−1 cm−1), and 1343 nm (7.07 mM−1 cm−1). Complex [Cu(PhL)] shows essentially the same features, with dominant bands at 619 nm (16.24 mM−1 cm−1), 792 nm (18.01 mM−1 cm−1), 986 nm (3.93 mM−1 cm−1), and 1322 nm (6.71 mM−1 cm−1), suggesting similar electronic structures. On the basis of their high intensity, all of these bands are assigned to ligandcentered electronic transitions. In order to gain insight into the origin of the bands, we conducted TD-DFT calculations. Because only marginal differences were observed between [Cu(HL)] and [Cu(PhL)], we will limit our discussion to [Cu(HL)]. A number of low-energy transitions of significant intensity were predicted, and natural transition orbitals (NTOs)39 contributing to the transitions are shown in Table 6. Thus, an electronic excitation is predicted at 1267 nm (fosc = 0.1045) for [Cu(HL)], which corresponds to the band experimentally observed at 1346 nm. It is assigned to a ligand-to-ligand charge transfer (LLCT) with donation from the phenoxyl rings to the dipyrrin unit (Table 6). Three additional electronic transitions were calculated at 999 nm (fosc = 0.0557), 732 nm ( fosc = 0.0912), and 617 nm (fosc = 0.2748), which account for the bands experimentally detected at 996, G


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a

See the Experimental Section for calculation details.

bases such as N-methylimidazole (NMI) or cesium carbonate (Cs2CO3), which are not capable of deprotonating the alcohol without the assistance of a Lewis acid, gave almost no reactivity (entries 5 and 6). For example, we could not detect benzaldehyde when using NMI, while only 6 TONs were measured when using Cs2CO3. We next investigated the reactivity of [Cu(PhL)] with two other alcohols under the best experimental conditions (0.08 mol % catalyst and 1.2 equiv of potassium tert-butoxide). The secondary alcohol 1-phenylethanol was oxidized with essentially the same efficiency as benzyl alcohol (210 TONs after 1 h and 270 TONs after 24 h, entry 7), showing that [Cu(PhL)] has a broader substrate scope than galactose oxidase and biomimetic complexes. The unactivated primary alcohol 2-phenylethanol was also oxidized, albeit with a reduced efficiency because 33 TONs were achieved after 24 h (entry 8). Complex [Cu(HL)] was found to be roughly as efficient as [Cu(PhL)] for all of the reactions investigated. The TONs for benzyl alcohol oxidation reaches 325 after 24 h in CH2Cl2 (entry 9). 1-Phenylethanol is also oxidized very efficiently, with 390 TONs in 24 h (entry 10), but the most remarkable result concerns 2-phenylethanol (entry 11). Indeed, 95 TONs were achieved in 24 h for this alcohol, making [Cu(HL)] one of the best copper radical catalysts for the aerobic oxidation of unactivated alcohols. As a comparison, the copper bis(iminosemiquinone) complex [Cu(ISQ2L)] (Chart 3, entry 12), based on N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-

diaminobiphenyl reported by Wieghardt et al., achieves 65 TONs for the aerobic oxidation of benzyl alcohol (1 mol % catalyst and 0.9 equiv of nBu4NOH; [PhCH2OH] = 0.05 M),48 while 95 TONs were reported for benzyl alcohol oxidation with the copper complex of 2,2′-selenobis(4,6-di-tert-butylphenol) ([(Et3N)Cu(LSe)]; Chart 3, entry 13), under somewhat different conditions (0.1 mol % catalyst and 0.02 equiv of nBu4NOCH3; [PhCH2OH] = 2 M).49 The binaphthylsalen complex [Cu(BSPL)](BF4) (Chart 3, entries 14 and 15) reported by Stack et al. reaches 60 TONs for the oxidation of 1-phenylethanol and 1300 TONs for benzyl acohol (0.1 mol % catalyst and 2% of sodium alkoxide; neat substrate).50 We also included in Table 7 the CuIOTf/bpy/TEMPO/NMI system, which has been popularized by Hoover and Stahl.51 While very active at high concentrations (total conversion in less than 15 min at 5 mol %, not shown), the catalyst efficiency decreases significantly at lower concentrations (entries 16 and 17). One has to emphasize that a further comparison with our systems is obscured by the fact that the optimal base is not the same. We recently reported that the copper(III) complex of the deprotonated (R,R)-N,N′-bis(3,5-di-tert-butyl-2-aminobenzylidene)-1,2-diaminocyclohexane, namely, [Cu( NL)](SbF 6 ) (Chart 3, entry 18), was an efficient catalyst for the aerobic oxidation of benzyl alcohol, with 236 TONs achieved in 24 h under conditions identical with those reported here (catalyst 0.08 mol % and 1.2 equiv of tBuOK).52 Its higher activity in comparison to one-electron-oxidized salen complexes built H


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Chart 3. Structures of (a) [Cu(ISQ2L)], (b) [(Et3N)Cu(LSe)], (c) [Cu(NL)], and (d) [Cu(BSPL)]+

Table 7. Aerobic Alcohol Oxidation Catalyzed by the Copper(II) Radical Complexesa

entry

substrate

1

benzyl alcohol

2

benzyl alcohol

3

benzyl alcohol

4

benzyl alcohol

5

benzyl alcohol

6

benzyl alcohol

7

1-phenylethanol

8

2-phenylethanol

9

benzyl alcohol

10

1-phenylethanol

11

2-phenylethanol

12b

benzyl alcohol

13c

benzyl alcohol

14d

benzyl alcohol

15d

1-phenylethanol

16

benzyl alcohol

17

benzyl alcohol

18e

benzyl alcohol

catalysta Ph

[Cu( L)], (0.08 mol %) [Cu(PhL)], (1 mol %) [Cu(PhL)], (0.08 mol %) [Cu(PhL)], (1 mol %) [Cu(PhL)], (0.08 mol %) [Cu(PhL)], (0.08 mol %) [Cu(PhL)], (0.08 mol %) [Cu(PhL)], (0.08 mol %) [Cu(HL)], (0.08 mol %) [Cu(HL)], (0.08 mol %) [Cu(HL)], (0.08 mol %) [Cu(ISQ2L)], (1 mol %) [(Et3N)Cu(LSe)] (0.125 mol %) [Cu(BSPL)](BF4) (0.01 mol %) [Cu(BSPL)], (0.01 mol %) CuIOTf/bpy/ TEMPO (0.5 mol %) CuIOTf/bpy/ TEMPO (0.08 mol %) [Cu(NL)](SbF6) (0.08 mol %)

base 1.2 equiv of tBuOK 1.2 equiv of tBuOK 0.2 equiv of tBuOK 0.2 equiv of tBuOK 1.2 equiv of NMI 1.2 equiv of Cs2CO3 1.2 equiv of tBuOK 1.2 equiv of tBuOK 1.2 equiv of tBuOK 1.2 equiv of tBuOK 1.2 equiv of tBuOK 0.9 equiv of nBu4NOH 4% MeONBu4 2% sodium alkoxide 2% sodium alkoxide 1.2 equiv of NMI

TON 300 18 100 7 0 6 270

alcohol oxidation, supporting our working hypothesis. Counterintuitively, we did not observe a significant increase in activity upon going from [Cu(HL)] to [Cu(PhL)] in spite of a lowering of E1/2red,1. We interpret this behavior by an enhanced steric bulk created by the additional phenyl substituents.

33 325 390

3. CONCLUSION In summary, we prepared the complexes [Cu(HL)] to [Cu(PhL)], which belong to an original family of copper(II) radical species. The SOMO is delocalized over the whole ligand, confering to the compounds a mixed pyrrolyl−phenoxyl radical character and remarkable optical properties (absorption over the whole UV−vis window that extends to the NIR region). The complexes can undergo reversible monoelectronic reduction and oxidation, which are both ligand-centered. Apart from their peculiar spectroscopic properties, these complexes are efficient catalysts for aerobic alcohol oxidations. Activated alcohols such as benzyl alcohol and 1-phenylethanol are rapidly oxidized in a basic medium, with high TONs (>270 in 24 h). Unactivated alcohols (2-phenylethanol) are also oxidized, with somewhat more modest TONs (33−95 in 24 h).

95 6548 9549 130050 6050 43

1.2 equiv of NMI

4

1.2 equiv of tBuOK

23652

a

4. EXPERIMENTAL SECTION

TONs given after 24 h. Conditions, unless otherwise indicated. Solvent: CH2Cl2. [benzyl alcohol] = 0.2 or 0.6 M. 1 bar of dioxygen. T = 298 K. See the Experimental Section for further details. bLigand ISQ2 L: bis(iminosemiquinone) radical form of the deprotonated N,N′bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diaminobiphenyl. [complex]: 0.5 mM. [nBu4NOH] = 45 mM. Solvent: CH2Cl2. [PhCH2OH] = 0.05 M. In air. T = 295 K. TON after 15 h. cLigand H2LSe: 2,2′selenobis(4,6-di-tert-butylphenol). [benzyl alcohol] = 1 M. [complex]: 1.25 mM. [MeONBu4] = 40 mM. Solvent: CH3CN. 1 bar of dioxygen. T = 295 K. dLigand H2BSPL: N,N′-bis(3-thioisopropyl-5-tertbutylsalicylidene)-2,2′-diaminobinaphthyl. Solvent: benzyl alcohol or 1-phenylethanol. [complex]: 1 mM. [base] = 0.2 M. 1 bar of dioxygen. T = 295 K. TON after 20 h. eLigand H2NL: (R,R)-N,N′-bis(3,5-di-tertbutyl-2-aminobenzylidene)-1,2-diaminocyclohexane.

4.1. Chemicals. All chemicals were commercially available and used as received. Anhydrous dichloromethane was purchased from Acros and used without any further purification. 4.2. Instrumentation. NMR spectra were recorded on a Brüker Avance 300, 400, or 500 spectrometer (1H at 300, 400, or 500 MHz and 13C at 75, 100, or 126 MHz). 1H and 13C chemical shifts are given in ppm relative to solvent residual peaks. Mass spectra were recorded on a Bruker Esquire 3000 spectrometer (electrospray ionization/ion trap). Microanalysis was performed using an apparatus designed by the Service Central d’Analyze du CNRS (Lyon, France). UV−vis spectra were recorded on a PerkinElmer Lambda 1050 spectrophotometer in quartz cells (Hellma) of 1.00 mm path length. X-band EPR spectra were recorded on a Bruker EMX Plus spectrometer controlled with the Xenon software and equipped with a Bruker teslameter. A Bruker nitrogen-flow cryostat connected to a high-sensitivity resonant cavity was used for 100 K measurements, while an Oxford Instruments helium-flow cryostat connected to a dual-mode resonator was used for measurements at 10 K and below. The spectra were simulated using the Simfonia software. 4.3. Chemicals and Synthesis. All chemicals were commercially available and were used as received. Anhydrous dichloromethane was purchased from Acros and used without any further purification. [Cu(HL)]. Under air and at room temperature, Cu(OAc)2·H2O (32 mg, 0.16 mmol, 1.0 equiv) and Et3N (90 μL, 0.65 mmol, 4.0 equiv) were added to a stirred solution of HLH3 (100 mg, 0.16 mmol, 1.0

from the same diamine was tentatively assigned to an easier regeneration of the active catalyst in air: The oxidation potential of the copper(II) bis(anilido) complex is indeed −0.14 versus 0.45 V for the copper(II) bis(phenolato) derivative. The corresponding potential in the present series is E1/2red,1, and it is even lower (−0.40 and −0.56 V for [Cu(HL)] and [Cu(PhL)], respectively). With such values, the copper(II) bis(phenolato) complex is unstable in air and easily undergoes oxidation to the copper(II) radical species potent for I


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Dichloromethane was added, followed by the appropriate amount of substrate. The tube was filled with oxygen using a syringe (purging three times). The mixture was next shaken at room temperature, and aliquots of 30 μL were taken and dissolved directly into CDCl3 for determination of the conversion by 1H NMR. Typically, the final concentration of the complex is 0.5 mM, and the substrate concentration is 0.6 M when using 0.08 mol % catalyst. When using 1 mol % catalyst, the final concentration of the complex is 1 mM and the substrate concentration is 0.1 M. For every condition and substrate, a blank without the complex was realized. As an example for 0.6 M benzyl alcohol and 1.2 equiv of tBuOK, the raw conversion after 24 h is 5% without catalyst but 28% with 0.08 mol % catalyst. The values given in Table 7 have been systematically corrected from the blanks.

equiv) in a CH2Cl2/methanol (MeOH; 2:1, v/v) mixture (30 mL). After the mixture was refluxed for 2 h, the greenish-black precipitate was filtered through a frit, washed with MeOH, and dried under vacuum. Yield: 70%. Dark crystals were grown by evaporation at room temperature of a solution of the complex in ethanol/CH2Cl2 (1:1, v/ v). MS (ESI): m/z 688 ([M]−). IR (ν, cm−1): 2950, 2902, 2865, 1515, 1424, 1369, 1245, 1096, 1087, 1005, 905. Anal. Calcd for C43H49CuN2O2: C, 74.91; H, 7.16; N, 4.06. Found: C, 74.78; H, 7.28; N, 3.93. [Cu(PhL)]. Under air and at room temperature, Cu(OAc)2·H2O (26 mg, 0.13 mmol, 1.0 equiv) and Et3N (70 μL, 0.52 mmol, 4.0 equiv) were added to a stirred solution of PhLH3 (100 mg, 0.16 mmol, 1.0 equiv) in a CH2Cl2/MeOH (2:1, v/v) mixture (30 mL). The mixture was heated at 50 °C for 5 h and allowed to cool to room temperature. The solution was concentrated to one-third under reduced pressure and cooled to 5 °C with an ice bath. The black precipitate was collected by filtration, washed with MeOH, and dried under vacuum. Yield: 92%. Dark crystals were grown at room temperature by vapor diffusion of pentane into a solution of the complex in dichloromethane. MS (ESI): m/z 840 ([M]−). IR (ν, cm−1): 2956, 2905, 2865, 1524, 1467, 1364, 1257, 1091, 1009, 905, 754. Anal. Calcd for C55H57CuN2O2: C, 78.49; H, 6.83; N, 3.33. Found: C, 78.66; H, 6.99; N, 3.26. Generation of the Cations and Anions. The cations were generated by adding 1.2 equiv of silver hexafluoroantimonate to a CH2Cl2 solution of the neutral complex. After 1 h of stirring under argon at room temperature, the solution was filtered through Celite and collected in a measuring flask for spectroscopic characterization. The same protocol was employed for generating the anions, except that silver hexafluoroantimonate was replaced by colbatocene. 4.4. Electrochemical Measurements. CV and rotating-diskelectrode voltammetry curves were recorded with either a Biologic SP300 or a CH Instruments 620 potentiostat. Experiments were performed in a standard three-electrode cell under argon. In a typical measurement, the complex concentration was ca. 0.5 mM in a CH2Cl2 solution containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte. A glassy carbon disk electrode (3 mm diameter), which was polished with 1 mm diamond paste, was used as the working electrode. The auxiliary electrode was a platinum wire, while 0.01 M Ag/AgNO3 was used as the reference electrode. For electrolysis, the working electrode was replaced by carbon felt. 4.5. Crystal Structure Analysis. Crystals were mounted on a Kappa Nonius CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and a cryostream cooler. The collected reflections were corrected for absorption (SADABS), solved by direct methods, and refined with Olex software.53 All non-H atoms were refined with anisotropic thermal parameters. H atoms were generated in idealized positions, riding on the carrier atoms, with isotropic thermal parameters. CCDC 1587929−1587932 contain the crystallographic data for HLH3, [Cu(HL)], [Cu(PhL)], and [Cu(HL)]2(SbF6), respectively. 4.6. DFT Calculations. Geometry optimizations were carried out with the Gaussian 16 software (revision A.03),54 using the B3LYP functional55,56 with a polarized continuum model (PCM) for CH2Cl2 (dielectric e = 8.94),57−60 and the 6-31g* basis set61 for all atoms. This combination of functional and basis set has been previously used for structurally similar bisphenolate−dipyrrin complexes, providing good matches to experimental metrical parameters.13,27 Frequency calculation at the same level of theory was systematically carried out in order to ensure that the optimized structures were located at a minimum of the potential energy surface. Single-point calculations for energetic analyses, as well as NTO analyses, were performed using the B3LYP functional and the TZVP basis set of Ahlrichs62,63 on all atoms with a PCM for CH2Cl2. The optical properties of the 30 lowestenergy electronic transitions were investigated by TD-DFT64 at the B3LYP/TZVP level with a PCM for CH2Cl2.63 4.7. Catalysis. The catalytic activity of the complexes has been investigated by using a previously described protocol.52,65,66 In a typical experiment, the catalyst (concentrated CH2Cl2 solution) and solid potassium tert-butoxide were introduced to a glass tube.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00044. Additional spectroscopic data, spin-density plots, NTOs, and computational details (PDF) Accession Codes

CCDC 1587929−1587932 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nicolas Leconte: 0000-0001-5959-709X Tim Storr: 0000-0002-3163-6218 Fabrice Thomas: 0000-0002-6977-5192 Author Contributions

The manuscript was written with contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for financial support from the Labex Arcane (ANR-11-LABX-0003-01) and for analytical support from the ICMG Chemistry Nanobio Platform, Grenoble, France. L.C. and T.S. thank Compute Canada for access to computational resources.

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REFERENCES

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