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“Two-Story” Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding Gael̈ De Leener,†,‡ Diana Over,‡ Coryse Smet,† Damien Cornut,† Ana Gabriela Porras-Gutierrez,§ Isidoro López,§ Bénédicte Douziech,§ Nicolas Le Poul,*,§ Filip Topić,∥ Kari Rissanen,∥ Yves Le Mest,§ Ivan Jabin,*,† and Olivia Reinaud*,‡

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Laboratoire de Chimie Organique, Université libre de Bruxelles (ULB), Avenue F. D. Roosevelt 50 CP160/06, B-1050 Brussels, Belgium ‡ Laboratoire de Chimie et de Biochimie Pharmacologiques et Toxicologiques, Université Paris Descartes, Sorbonne Paris Cité, CNRS UMR 8601, 45 rue des Saints Pères, 75006 Paris, France § UMR CNRS 6521, Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, CS 93837, 29238 Brest, France ∥ Department of Chemistry, University of Jyväskylä, Nanoscience Center, P.O. Box 35, 40014 Jyväskylä, Finland S Supporting Information *

ABSTRACT: A new “two-story” calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate ZnII complex with all of the carbonyl groups of the amido arms bound and a five-coordinate CuII complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the CuII/CuI couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the CuI complex to O2 at low temperature, a green species was obtained with a UV−vis signature typical of an end-on superoxide CuII complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a “one-story” calix[6]tren ligand.

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INTRODUCTION Metalloenzymes are very efficient natural catalysts involved in a great variety of reactions. Their active sites are organized in a way that maximizes their efficiency and selectivity through a specific and constrained environment allowing substrate recognition and nuclearity control of the metal center.1 In mononuclear enzymes, the metal ion is often buried in the heart of the protein in an active site that presents a cofacial triad of amino acid residues defining its first coordination sphere. For example, the copper enzymes dopamine β-monooxygenase, peptidylglycine α-hydroxylating monooxygenase, and nitrate reductase present a mononuclear copper ion buried in the enzyme active pocket.2 The second (and further) coordination sphere also plays a key role in the reactivity of the metal center and substrate binding and orientation. The classical biomimetic strategy for the conception of model compounds aims at mimicking the first coordination sphere of the metal site, with a polydentate ligand such as a tris(pyrazolyl)borate, 1,4,7triazacyclononane (tacn), tris(2-pyridylmethyl)amine (tmpa), or tris(2-aminoethyl)amine (tren) mimicking a poly(histidine) core.3 In order to tackle the question of the role and importance of the cavity defining the other coordination © 2017 American Chemical Society

spheres, we developed a supramolecular approach using calix[6]azacryptands as ligands, where a calix[6]arene is constrained into a cone conformation by covalent grafting of a nitrogenous cap to the small rim.4 Upon coordination of a metal ion, the ligand exerts a strong chelating effect, while the calixarene core ensures the mononuclearity and provides a welldefined second coordination sphere and an access channel for exogenous guests.5 One of the most advanced examples of these ligands is calix[6]tren, which presents a tren unit covalently bound to the small rim of the calix[6]arene core (Figure 1A).6 The corresponding ZnII and CuI/II f unnel complexes7,8 have been shown to display remarkable properties mimicking supramolecular aspects and reactivity patterns of metalloenzymes (Figure 1A). In order to vary the second coordination sphere, we envisioned the “two-story” calix[6]amido-tren ligand 1 bearing amido spacers (first story) between the tren cap (second story) and the calixarene core (Figure 1B). We expected this new ligand to display different host−guest and reactivity properties Received: May 12, 2017 Published: August 30, 2017 10971

DOI: 10.1021/acs.inorgchem.7b01225 Inorg. Chem. 2017, 56, 10971−10983

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Inorganic Chemistry

the anisol units (2.70 ppm) indicates that they are oriented toward the inside of the cavity. Such a conformation stands in contrast to the “one-story” calix[6]tren lacking the amido spacer units at its small rim (Figure 1A). This shows that the amido-tren cap is flexible enough to allow partial inclusion of these methyl substituents into the calixarene cone. Similar behavior was observed with other “two-story” calix[6]cryptands presenting aromatic xylenyl or pyridyl spacers between the tetra-aza cap and the calixarene core.11 The complete attribution of the 1H NMR resonances was achieved using 2D NMR spectra (COSY, HSQC, and HMBC). A protonation study with picric acid (PicH) allowed the identification of the mono- and trisprotonated derivatives, [1.(H)]+ and [1.(H)3]3+. Interestingly, the corresponding NMR spectra evidenced a conformational change of the calixarene core associated with the protonation of the tren cap (Figure 2b). The aromatic rings progressively flip relative to each other to yield a flattened cone with the methoxy groups projected inside the cavity. The NH2 protons of the trisprotonated tren cap display two wellseparated signals (δ+NH2 = 7.76 and 10.08 ppm). The significant downfield shift of one of them indicates its hydrogen bonding with the adjacent amido oxygen atom. As a result, the symmetry of the whole structure is decreased to C3, as shown by the splitting of the signals corresponding to diastereotopic protons. Hence, this preliminary study reveals that the amido spacers introduced between the tren cap and the calix core make ligand 1 highly flexible while still maintaining communication between the cap and the polyaromatic cavity. Synthesis and XRD Structure of the ZnII and CuII Complexes of Calix[6]amido-tren 1. Calix[6]amido-tren 1 was reacted with stoichiometric amounts of Zn(OTf)2 and triethylamine (TEA) in a CH2Cl2/MeOH (1:1 v/v) mixture to yield the corresponding ZnII complex [Zn(1)](OTf)2, which was isolated in 64% yield after precipitation in methanol. Single crystals suitable for X-ray diffraction (XRD) analysis were obtained by slow diffusion of diethyl ether at 5 °C into a solution of [Zn(1)](OTf)2 in CD3CN/CDCl3 (3:2 v/v). The X-ray structure shows a dicationic complex exhibiting a pseudooctahedral zinc center coordinated to the three secondary amino groups of the tren cap [dav(Zn−N) = 2.15 Å] and to the three carbonyl groups of the amido arms [dav(Zn−O) = 2.27 Å] (Figure 3). The distance between the metal center and the apical tertiary nitrogen atom of the tren cap [d(Zn−N) = 2.535(3) Å] is indicative of a strong dipole−charge interaction. The six NH groups of the calixarene cap are H-bonded to triflate anions or residual water molecules. The complex is homochiral at the level of the tren cap, with the three coordinated nitrogen atoms displaying the same absolute configuration. As a result, the coordinating tren core adopts a helical arrangement that wraps the metal center. The selfinclusion of a tBu substituent leads to a distortion of the calixarene cone and an overall asymmetric structure (C1). The three methoxy groups at the small rim are all projected toward the inside of the cavity. The CuII complex was synthesized using stoichiometric amounts of Cu(H2O)6(ClO4)2 and ligand 1 in a 1:1 (v/v) CH2Cl2/THF mixture. A blue crystalline solid corresponding to complex [Cu(1)](ClO4)2 was isolated in 95% yield. Elemental analyses showed a 1:1:2 ligand/Cu/ClO4 ratio, thereby indicating the formation of a dicationic complex. X-ray-quality crystals of [Cu(1)](ClO4)2 were obtained from a CH2Cl2/ EtOH solution. XRD analysis showed the presence of two

Figure 1. (A) Structure of the “one-story” calix[6]arene-based tren ligand and guest exchange of its corresponding metal funnel complexes. (B) Structure of the new “two-story” calix[6]arene-based tren ligand 1.

in spite of the presence of the same nitrogen-donor site (the tren cap) and the same pocket (the calix[6]arene macrocycle). Here we describe the synthesis of this new calix[6]amido-tren ligand 1 and its complexation to ZnII and CuII. The coordination properties of these complexes toward exogenous donors and the impact of the amido spacers introduced between the tren cap and the calixarene cavity are discussed in light of the previous studies related to “one-story” calix[6]tren complexes. Preliminary studies related to CuI and O2 activation are also presented.

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RESULTS Synthesis and Characterization of Calix[6]amido-tren 1. Calix[6]amido-tren 1 was synthesized in two steps from the known9 calix[6]tris(amine) 2 according to a macrocyclization strategy that previously proved its efficiency for calix[6]arenes capped at the small rim (Scheme 1).10 First, 2 was reacted with Scheme 1. Synthesis of Calix[6]amido-tren 1

bromoacetyl bromide to provide calix[6]tris(bromoamide) 3 in 76% yield. A subsequent [1 + 1] macrocyclization reaction between the two tripodal partners, 3 and tren (4 mM), led to the desired ligand 1 in 40% yield. The 1H NMR spectrum of calix[6]amido-tren 1 in CD3CN/ CDCl3 (3:2 v/v) at 298 K is characteristic of a C3v-symmetrical calix[6]arene that adopts an average cone conformation with aromatic units alternatively in endo and exo positions (Figure 2a). The high-field-shifted resonance of the methoxy groups of 10972


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Figure 2. 1H NMR spectra (CD3CN/CDCl3 3:2, 298 K, 300 MHz) of: a) calix[6]amido-tren 1, b) its tris-protonated derivative [1.(H)3]3+ obtained after addition of 7.3 equiv of PicH. Inset: schematized structure of [1.(H)3]3+ showing the intramolecular H-bonding interactions. s: solvent, w: water and G: grease.

Figure 4. Side and top views of the crystal structure of the homochiral diastereoisomer of complex [Cu(1)](ClO4)2. Intramolecular H bonds are shown as black dashed lines [lengths (Å): N7C−O5D 2.72(3), N7D-O5E 2.815(4)]. Hydrogen atoms, counterions, and solvent of crystallization have been omitted for clarity. Selected bond lengths (Å) and angles (deg): N7D−Cu1 2.068(3), N7E−Cu1 2.138(3), O5C− Cu1 1.963(2), N1−Cu1 2.006(3), N7C−Cu1 2.021(7), N1−Cu1− N7E 85.4(1), N1−Cu1−N7D 85.2(1), N1−Cu1−N7C 83.2(2), N7C−Cu1−O5C 84.5(2), N7E−Cu1−O5C 102.3(1), N7D−Cu1− O5C 100.4(1).

Figure 3. Side and top views of the crystal structure of complex [Zn(1)](OTf)2. Hydrogen atoms, counterions, and solvent of crystallization have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−N1 2.535(3), N7D−Zn1 2.124(3), Zn1− N7C1 2.165(3), Zn1−O5C 2.191(2), Zn1−O5E 2.339(2), Zn1−O5D 2.274(2), N7E−Zn1 2.149(2), N1−Zn1−N7E 74.06(9), N1−Zn1− N7C1 73.15(9), N1−Zn1−N7D 74.6(1), N7D−Zn1−N7C1 111.6(1), N7C1−Zn1−N7E 111.8(1), N7E−Zn1−N7D 114.6(1), N7E−Zn1−O5E 75.27(9), N7C1−Zn1−O5C 78.15(9), N7D−Zn1− O5D 77.07(9).

whereas new resonances attributed to complex [Zn(1)](OTf)2 emerged. Full complexation was observed with a stoichiometric amount of Zn(OTf)2, denoting strong affinity (K > 3 × 105 mol−1·L). The resulting spectrum, which is identical to that of the isolated 1:1 complex (Figure 5b), displays a complicated pattern with many sharp resonances together with some relatively broad ones. Noteworthy is the presence of a broad peak in the high-field region, which corresponds to the inclusion of one tBu group inside the calixarene cone, as depicted in the XRD structure. Lowering the temperature to 268 K sharpened many resonances, in particular the one corresponding to the included tBu group, which further shifted to −0.16 ppm (Figure 5a). In contrast, at high temperature (398 K; Figure 5c), the spectrum simplified, becoming characteristic of average C3v symmetry as a result of the fast in−out exchange of the tBu group relative to the NMR time scale. Finally, the IR spectra of [Zn(1)](OTf)2 in solution and in the solid state show CO stretching bands shifted to lower wavenumbers (1659−1602 cm−1 range) compared with the free ligand 1 (1665 cm−1), which indicates coordination of the carbonyl groups to the metal center (Figures S20 and S21).13 All of these data show that the structure adopted by the complex in solution is very similar to that in the solid state.

different structures, one homochiral and the other heterochiral. The major difference between the two structures stems from the coordination of one nitrogen atom (N7C), which seems to be correlated to a different H-bonding network (Figure S19). The crystal structure of the homochiral complex is displayed in Figure 4. The metal ion is bound in the cap thanks to its coordination to the N4 core of the tren unit and one carbonyl group. The geometry around the five-coordinate metal center is intermediate between square-pyramidal (SBP) and trigonalbipyramidal (TBP) (τ = 0.61).12 The short distances between the secondary N atoms and the noncoordinated carbonyl oxygen atoms (2.82 and 2.72 Å) together with the relative orientation of the corresponding NH and CO moieties indicate intramolecular H-bonding. The self-inclusion of a tBu group in the calixarene cavity, associated with the expulsion of the corresponding methoxy group, breaks the symmetry of the whole structure. Solution Studies of the ZnII Complex. The complexation of ZnII by the ligand 1 was studied by 1H NMR spectroscopy. Upon stepwise addition of Zn(OTf)2 to a solution of 1 (7.5 mM) in CD3CN/CDCl3 (3:2 v/v) at 298 K, signals corresponding to the free ligand progressively vanished, 10973


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Figure 5. 1H NMR spectra of [Zn(1)](OTf)2: (a) 268 K (600 MHz, CD3CN/CDCl3 3:2 v/v); (b) 298 K (600 MHz, CD3CN/CDCl3 3:2 v/v); (c) 398 K (300 MHz, C2D2Cl4). Labels “s” and “w” denote solvent and water, respectively.

Table 1. UV−Vis Data and Electrochemical Parameters of Mono- and Dicationic Calix[6]amido-tren CuII Complexes E vs Fc+/Fc (V)b λmax/nma (ε/M−1 cm−1) solvent CH2Cl2 PhCN CH2Cl2/MeCN 1:1 CH2Cl2/MeOH 1:1 CH2Cl2/DMF 1:1 a

[Cu(1)](ClO4)2 711 nd 719 726 724

(100) (135) (105) (125)

[Cu(1)](ClO4)2

[Cu(1-H)](ClO4) 629 nd 636 636 625

(135) (180) (110) (160)

[Cu(1-H)](ClO4)

Epc

Epa

Epc

Epa

−0.90 −0.90 −0.78d −0.86 −0.93

−0.76 −0.77 −0.62d −0.75 −0.82

ndc −1.32 nd nd nd

nd +0.12 nd nd nd

The deconvoluted values are given in Table S2. bv = 100 mV/s. cNot determined. dIn pure MeCN.

d transitions centered at 711 nm (Figure 6A) and an associated EPR signature suggesting the presence of two diastereoisomers with a distorted square-pyramidal geometry (Figure 6B), as for the calix[6]tren−CuII complex.8a The IR spectrum (Figures S20 and S21) showed two different CO stretches, one of which has the same energy as the CO stretch observed for the free ligand (1668 cm−1), whereas the other one is of lower energy (1624 cm−1), consistent with its coordination to the metal center14 and in agreement with the XRD structure as described above (Figures 4 and S19). Cyclic voltammetry of the CuII complex in CH2Cl2 or PhCN with NBu4PF6 as the supporting electrolyte under anhydrous and anaerobic conditions showed a quasi-reversible system at E0 = −0.83 V vs Fc+/Fc (ΔEp = Epa − Epc = 140 and 130 mV, respectively, at v = 0.1 V/s), which is ascribed to the CuII/I redox couple. The system remains reversible over the studied scan rate range (0.02 V/s < v < 5 V/ s) (Figure S32). The cathodic peak potential (Epc = −0.90 V vs Fc+/Fc) is significantly lower than that obtained for the analogous [CuII(calix[6]tren)(H2O)]2+ complex (−0.76 V; Figure 7A). In addition, the complex displays radically different redox behavior in terms of reversibility, since reduction is irreversible for the calix[6]tren−CuII complex. Such a difference is explained by the ligand−metal binding in the Cu(I) redox state. For the calix[6]amido-tren−CuII complex, the coordinated carbonyl group of the amido arm replaces the water guest ligand, and its coordination in both redox states (possibly only transiently in the CuI state) ensures the reversibility of the electron transfer process. For calix[6]tren−CuII, reduction induces a drastic change in the coordination sphere leading to irreversibility: fast expulsion of the water guest ligand present in the CuII state, leading to a “cavity-free” CuI complex.

The interaction with various exogenous organic donors was examined by 1H NMR spectroscopy in CDCl3 or CD3CN/ CDCl3 (3:2 v/v). Whatever the donor tested, either neutral (amines, alcohols or amides) or anionic (hexanoate or acetylacetonate), no inclusion in the calixarene cavity could ever be detected, even at relatively high concentration (up to 20 equiv of the donor, i.e., 40 mM). The only observable changes were broadenings of some resonances that were more or less pronounced depending on the donor. Such behavior stands in strong contrast to that of the related “one-story” calix[6]tren− ZnII complex, which displays high affinities for neutral donors in the cavity (Figure 1A). Here the ZnII ion is coordinatively saturated since the three carbonyl groups of the amido spacers are coordinated in addition to the nitrogen donors of the tren cap. Reaction of the dicationic ZnII complex with bases was also explored. Whereas no reaction was observed upon addition of TEA (up to 30 equiv), the addition of a stronger base (1,8diazabicyclo[5.4.0]undec-7-ene, DBU, 1.4 equiv) led to a drastic change in the NMR spectrum. The new 1H NMR pattern indicated the expulsion of the self-included tBu group, while the symmetry of the complex remained C1. Such behavior suggests deprotonation of one coordinated amido arm, associated with a change in geometry of the complex. In view of the difficult interpretation of the NMR spectra, in-depth studies with bases and anions were conducted with the CuII complex instead. Solution Studies of the CuII Complex. The properties of the CuII complex in solution were explored using electron paramagnetic resonance (EPR), electronic, and vibrational spectroscopies as well as electrochemistry (Table 1). In dichloromethane, the complex [Cu(1)](ClO4)2 displayed d− 10974


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Figure 7. CVs (E/V vs Fc+/Fc, v = 0.1 V/s) at a glassy carbon electrode for (A) (a) [Cu(calix[6]tren)(H2O)]2+ and (b) [Cu(1)]2+ at ≈0.66 mM in PhCN/NBu4PF6 0.1 M and (B) at ≈0.66 mM [Cu(1)]2+ in CH2Cl2/NBu4PF6 0.2 M upon progressive addition of (a) 0, (b) 1, (c) 2, (d) 3, and (e) 4 equiv of NaN3 (solubilized in DMF). Figure 6. (A) UV−vis spectra of [CuII(1)](ClO4)2 (at room temperature) in (a) CH2Cl2, (b) CH2Cl2/MeCN 1:1, (c) CH2Cl2/ MeOH 1:1, and (d) CH2Cl2/DMF 1:1. (B) EPR spectra (X band) of [Cu(1)](ClO4)2 in (a) frozen CH2Cl2 (100 K) and (b−d) 1:1 (v/v) mixtures of CH2Cl2 with coordinating solvents.

Interestingly, the stepwise addition of 1 then 2 molar equivalents of tetrabutylammonium chloride to the dicationic CuII complex led to two distinct events, as evidenced by UV− vis absorption spectra displaying sequential isosbestic points (Figure 8). Coordination of the first chloride anion yielded a new complex where the CuII ion is clearly sitting in a TBP environment, as corroborated by the corresponding EPR spectrum (Figure S27A). The chloride titration was also monitored by cyclic voltammetry: addition of 1 molar equivalent of NBu4Cl to a solution of [Cu(1)](ClO4)2 led to the loss of reversibility of the CV response accompanied by a negative shift of the reduction peak by 70 mV, whereas the corresponding reoxidation peaks appeared at much higher potential values (Figure S31). Such irreversible redox behavior indicates the formation of a new Cu(I) species exhibiting a very different coordination environment. After the addition of 2 molar equivalents of chloride, the cathodic peak does not shift any more (Epc = −1.07 V vs Fc+/Fc). The reactivity of the dicationic complex [Cu(1)](ClO4)2 toward bases was then scrutinized. Monodeprotonation of [Cu(1)]2+ was achieved by addition of 30 equiv of Et3N. The resulting complex, [Cu(1-H)]+ was isolated in good yield (83%) and characterized by electrospray ionization mass spectrometry (ESI-MS). The IR spectrum showed one CO stretch at a lower wavenumber (1582 cm−1) than for the dicationic complex (1624 cm−1), consistent with the deprotonation of the coordinated amido donor (Figures S20d and S21d). EPR spectroscopy displayed a different pattern than obtained for [Cu(1)]2+ (Figure 9A). Similar data were obtained by in situ preparation of the monodeprotonated complex (Figure S24). UV−vis spectroscopy showed a hypsochromic shift of the

As in the case of the ZnII complex, the CuII complex showed little sensitivity to exogenous neutral ligands. Addition of MeCN, MeOH, or DMF to dichlomethane solutions of the complex in the 10−100 mM range affected neither the spectroscopic signatures nor the CVs, whereas under the same conditions drastic changes were observed with calix[6]tren−CuII, attesting to their coordination.8a When MeCN and MeOH were used as a solvent or cosolvent, no appreciable changes in the EPR and UV−vis spectra were observed. Only DMF, in a 1:1 (v/v) mixture with dichloromethane, led to spectroscopic changes suggesting an interaction with the metal center (Figure 6 and Table 1). The coordination of two different anions, N3− and Cl−, was then investigated. The addition of 1 molar equivalents of N3− to a solution of [Cu(1)]2+ in CH2Cl2 led to the appearance of an intense absorption band at λ = 402 nm (MLCT, ε = 3180 M−1 cm−1) and d−d absorption bands at λ = 680 and 883 nm (Figure S28). Further addition of azide anions did not modify the spectra. Such data corroborate the formation of the monoazido adduct [Cu(1)(N3)]+. As shown in Figure 7B, the addition of azide also has a significant impact on the voltammetric behavior: loss of reversibility, a ca. 120 mV shift of the reduction peaks toward negative values (Epc = −1.02 V), and the appearance of a new oxidation peak at ca. −0.3 V. This result is consistent with the UV−vis data and confirms the coordination of the N3− anion to the cupric center. 10975


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= −1.16 V; Figure 9C) associated with a sluggish reoxidation peak at much higher potential (Epa = +0.02 V; not shown). The ensemble of data collected for the complex [Cu(1)](ClO4)2 indicate that (i) two labile coordination sites are accessible for anion binding (as observed with chlorides) in spite of the steric hindrance around the metal center, (ii) the coordinated amido arm is readily deprotonated by a base in the CuII state, and (iii) in all cases (addition of base or anions) no loss of EPR signal is observed, showing that the CuII complexes are robust and do not dimerize (no formation of anion-bridged dinuclear species). Copper(I) Complex Synthesis and Characterization. In order to investigate the oxygen chemistry of this new family of copper complexes, the cuprous complex [Cu(1)]+ was synthesized by the addition of 1 equiv of Cu(MeCN)4X [X = PF6 or BArF, BArF = tetrakis(pentafluorophenyl)borate] to a solution of ligand 1 in MeCN or acetone, respectively, under an inert atmosphere. As analyzed by 1H NMR spectroscopy in CD3CN, the CuI complex displays C3v symmetry, like the calix[6]tren-based cuprous complex.8b The presence of other resonances indicates the coexistence of other minor species devoid of symmetry (Figures S36 and S37). Upon bubbling of CO, a new NMR signature was obtained, indicative of the formation of the corresponding CO adduct (Figure S39). The corresponding νCO stretch value of 2083 cm−1 (Figure S40) lies between the values reported for tren-based complexes deprived of cavity (2098 and 2092 cm−1 for Me6tren15 and HIPT3tren (HIPT = 3,5-(2,4,6-iPr3C6H2)2C6H3),16 respectively) and that recorded for the [(calix[6]tren)CuI(CO)]+ complex (2075 cm−1).8b This suggests a sensitivity to the environment, as previously evidenced with tris(imidazole)-based calix[6]arene complexes.17 Reactivity of [Cu(1)]+ toward O2: Characterization of a Superoxo Cu:O2 Adduct. Next, the reactivity of the chemically synthesized CuI complex toward O2 was explored. At room temperature, the CuI complex readily reacted with dioxygen in all solvents to produce a light-blue solution indicative of the formation of CuII species, as previously observed for the calix[6]arene ligand. At low temperatures, solutions of the CuI complex in acetone (183 K) or MeCN (233 K) turned dark green immediately (Figure S41). Upon returning to room temperature, the solution turned light blue. In MeTHF (163 K), the low-temperature solution did not yield a green intermediate but only gradually turned pale blue. The formation and decomposition of the intermediate green species was then monitored by UV−vis spectroscopy at low temperatures in acetone (Figure 10A). At 183 K, its formation is immediate (t < 1 s) with the appearance of one intense band at λ = 398 nm and two less intense absorption bands at lower energy (λ = 596 nm and λ = 735 nm). Bubbling N2 into the solution did not modify the spectrum. Whereas the signal was stable for hours at 183 K, it slowly vanished at 233 K according to a first-order process with a half-life of ca. 15 min at this temperature (Figure 10B). The spectroscopic data obtained for the dark-green species at low temperature are consistent with the formation of an end-on superoxide complex, [CuII(1)(O 2 − )] + , as previously reported with tren and tmpa derivatives.16,18,19 Noticeably, the superoxo adduct displays good stability in comparison with other reported complexes.3b,16,18,21a,22 For instance, the Cu−O2 complex obtained with a methoxy-based tridentate diazacyclooctane ligand by Itoh and co-workers22e is characterized by a half-life of 889 s (ca. 15 min) at 213 K in

Figure 8. UV−vis spectra of the addition of NBu4Cl to [Cu(1)](ClO4)2 (1 × 10−3 M, NBu4ClO4 = 10 μM) in CH2Cl2: (A) 0 to 1.2 molar equivalents of NBu4Cl; (B) 1.2 to 9 molar equivalents of NBu4Cl.

Figure 9. Spectroscopic characterization of [Cu(1-H)]+ obtained upon addition of Et3N to a solution of [Cu(1)](ClO4)2: (A) EPR spectra (X band) in CH2Cl2 (100 K, 5 × 10−3 M) (a) before and (b) after addition of 15 equiv of Et3N; (B) UV−vis spectra in CH2Cl2/ MeCN (1:1 v/v, 5 × 10−3 M) upon stepwise addition of Et3N (0 to 30 equiv); (C) CV at a glassy carbon electrode (C ≈ 0.66 mM) in CH2Cl2/NBu4PF6 0.2 M (a) before and (b, c) after addition of (b) 1.0 and (c) 40 equiv of Et3N.

maximum absorption wavelengths from 719 and 1010 (sh) nm to 636 and 897 (sh) nm upon addition of 15 equiv of Et3N (Figure 9B). Further addition of triethylamine (up to 30 equiv) or of a stronger base (20 equiv of DBU) did not modify the spectroscopic signatures (Figure S25), indicating that deprotonation of a second amido donor does not occur. As in the case of the addition of Cl− and N3− to [Cu(1)](ClO4)2, cyclic voltammetry of the CuII/I system became irreversible upon addition of Et3N, with a reduction peak at lower potential (Epc 10976


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Figure 10. (A) UV−vis spectrum of a solution of [CuI(1)]PF6 in acetone after exposure to O2 at 183 K (estimated ε values: 3750, 1080, and 930 M−1 cm−1 at 398, 596, and 735 nm, respectively). (B) Decay of the absorbance at 398 nm at 233 K and curve fitting according to a first-order reaction (k = 7.8 × 10−4 s−1 and t1/2 = 14.8 min). Figure 11. (A) CVs at a glassy carbon electrode of ≈0.66 mM [Cu(1)](ClO4)2 in CH2Cl2/NBu4PF6 0.2 M (a) before and (b) after bubbling of O2. (B) Low-temperature (213 K) UV−vis spectroscopic monitoring (differential mode) of the formation of the superoxo adduct upon electrochemical reduction of [CuII(1)]2+ (C ≈ 0.3 mM) in acetone/NBu4PF6 0.05 M.

acetone. More specifically, previously described complexes based on other tren derivatives are much less stable: the half-life of the [(trenMe6)CuO2]+ adduct at 223 K in EtCN (extracted from the kinetics data obtained at [Cu] = 0.211 mM) is shorter than 0.1 s.20 With calix[6]tren, the O2 adduct could not be accumulated, even at 173 K.8c For the calix[6]amido-tren derivative, the stabilization is likely due to the microenvironment surrounding the copper center that prevents dimerization and possibly stabilizes the bound superoxide anion through Hbonding to an amido arm. Indeed, the architecture and flexibility of the system enables H-bonding to each O atom of the superoxide, as shown by simple molecular modeling (Figure S42). Investigations were also performed by starting from a solution of the CuII complex. Hence, the CuI complex can be electrochemically generated at the electrode surface on a short time scale (a few seconds) and instantly reacted with dioxygen in the reaction layer. As shown in Figure 11A, the CV of [Cu(1)]2+ became irreversible upon bubbling of dioxygen at room temperature. This evidences a fast (t < 5 s) interaction between the electron-rich CuI center and O2. Spectroelectrochemical experiments under thin-layer conditions at room temperature to characterize the Cu:O2 adduct were unsuccessful. However, low-temperature (213 K) spectroelectrochemical studies under the same conditions clearly showed the formation of the superoxo complex. Application of a potential value allowing reduction of the CuII complex, but not of freely diffusing O2, at the working electrode led to the appearance of absorption bands at λ = 400 and 592 nm, as shown in Figure 11B. These values are consistent with the data obtained with the CuI complex in the presence of dioxygen. Furthermore, the steady-state value of the absorbance signal at 400 nm indicates that the superoxo complex is stable for at least several minutes under these experimental conditions. This also shows that reduction of [Cu(1)(O2)]+ occurs at a lower potential than reduction of [Cu(1)]2+ (E = −0.90 V vs Fc+/Fc at 298 K).

Reactivity of [Cu(1)]+ toward O2: Analysis of the Final Products. The EPR spectra of the room-temperature products resulting from the reaction of [Cu(1)]+ with dioxygen showed the formation of several CuII complexes. Interestingly, ESI-MS evidenced the presence of oxygenated species with two new peaks at M + 14 and M + 28 that correspond to (M + O − 2H) and (M + 2O − 4H), respectively (Figures S43 and S44). The same oxygenation products were previously observed with the [(calix[6]tren)CuI]+ complex after exposure to O2 in a noncoordinating solvent.8c All of these observations can be interpreted in the following way: at low temperature, the CuI complex readily reacts with O2 to form a superoxide complex, [CuII(1)(O2−)]+, which undergoes decomposition with partial oxygenation of methylene groups of the ligand, as previously observed with calix[6]tren, to give oxo derivatives as major products (Scheme 2). Such selectivity in favor of the oxo products (no hydroxylated products) was proposed to stem from hydrogen abstraction by the superoxide intermediate followed by the formation of an alkylhydroperoxide derivative8c,23 that undergoes final and selective decomposition into a keto product. This four-electron redox process primarily leads to a CuI product that can then either go through a second cycle to yield an M + 28 product (M + 2O − 4H) or undergo oneelectron oxidation with O2 (associated with the release of superoxide) to give the final CuII species (Scheme 2). The highest percentage of oxygenated products was obtained in MeCN (ca. 30% vs ca. 10% in acetone or MeTHF; Table S3). Finally, the addition of thioanisole to the green solution of the oxygenated complex at low temperature resulted in instant bleaching. MS analyses indicated only traces of oxygenated ligand, and no sulfoxide product was detected. As proposed for 10977


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Inorganic Chemistry Scheme 2. Proposed Reaction Pathways for the Oxygenation of Complex [CuI(1)]+ upon Exposure to Dioxygen

a bulky derivative of a tren−CuI complex,16 coordination of the thioether to the Cu complex likely triggers displacement of O2 from the copper center, which is followed by slow autoxidation of CuI into CuII at higher temperature. Further characterization of this new superoxo species, either spectroscopically (Raman and EPR) or through reactivity studies with a larger panel of substrates under different conditions, is now underway in our laboratory and will be the subject of another publication.

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to the embedded metal center is preserved and plays a crucial role in the structuration of the complexes. As neutral donors, they interact with ZnII, CuII, and CuI ions, whereas the divalent metal ions can still bind one or two anions. In the presence of a base, an amido arm is readily deprotonated, giving rise to monocationic complexes. Thus, in spite of the constraints imposed by the calixarene macrocycle, the amido substituents linked to the N4 capping core remain available for coordination to the metal center. Comparison of Calix[6]tren and Calix[6]amido-tren M Complexes (Scheme 3). The major structural difference between the calix[6]tren and calix[6]amido-tren ligands stems from the introduction of the amido spacers between the calixarene cavity and the tren cap. This has important consequences, as it modifies the first and second coordination spheres and introduces flexibility relative to the cavity. In the classical “one-story” calix[6]tren system, both ZnII and CuII centers are five-coordinate (distorted TBP geometry for CuII, τ = 0.70) with the methoxy groups projected away from the cavity toward the solvent.8a In noncoordinating solvents, these complexes have a coordinated water molecule bound at the endo position at the level of the calixarene small rim, which is readily substituted by neutral ligands fitting into the cavity. In contrast, the carbonyl groups of the “two-story” calix[6]amidotren ligand complete the coordination sphere of ZnII (to Oh) and CuII (to distorted TBP, τ = 0.61) in most solvents, and the methoxy groups are projected toward the inside of the cavity. As a result, calix[6]amido-tren-based metal complexes show very low sensitivity toward neutral donors such as EtOH or MeCN under conditions where they are bound in the “onestory” system (a few molar equivalents at millimolar concentrations).7a,8a By electrochemistry, the “one-story” complex [(calix[6]tren)Cu](ClO4)2 displayed a completely irreversible system in dichloromethane and benzonitrile and required a good guest ligand for the CuI state (such as MeCN) to become reversible.8a,c On the contrary, electrochemical studies of the calix[6]amido-tren−CuII complex [Cu(1)](ClO4)2 showed a reversible signature in the same solvents as a result of the intramolecular coordination of an amido arm in both oxidation states. The classical “one-story” complexes are reluctant to bind anions because of the nature of their second coordination sphere provided by the oxygen atoms present at the small rim of the calixarene: their lone pairs point toward the pseudo-C3 axis of the complex, strongly disfavoring the coordination of anionic guests through electrostatic repulsion (Scheme 3A).30

DISCUSSION

Comparison with Tren-Based Complexes Deprived of Cavity. Tren-based ligands have been extensively used in the preparation of Zn and Cu enzyme mimics, as the tren unit reproduces basic features of the cofacial polyhistidine core found in the natural systems. However, the system faces problems of nuclearity control. With tren itself, bridged dimeric complexes of ZnII and CuII are readily produced by reaction with anions.24 The CuI complexes also appear to be difficult to stabilize, as they have a strong tendency to disproportionate in this electron-rich environment.25 In the presence of dioxygen, they readily react to form superoxo intermediates that evolve into peroxo and hydroxo CuII dinuclear species.26 As the chelating ligand becomes more sterically hindered, CuI complexes and the corresponding mononuclear superoxo CuII adducts become better stabilized.16,27 The metal complexes obtained with ligand 1 described in this study are all mononuclear. This shows that the calixarene−amido structure provides enough steric hindrance around the metal ion to prevent the formation of dinuclear species in spite of the high flexibility of the structure and the presence of two coordination sites accessible to exogenous donors. Comparison of Calix[6]amido-tren MII Complexes with MII Complexes Obtained with “Classical” Polydentate N-Donor Ligands Bearing an Amido Substituent. To the best of our knowledge, no amido-substituted tren ligands have been reported, but other N-donor chelates presenting one or more appended amido arms have been described (tacn14,28 and tmpa13,29). In almost all metal complexes, the coordination of at least one amido carbonyl group was observed in the dicationic state. They were also shown to stabilize extra anionic donors through H-bonding, as in the cases of superoxo 22a and hydroperoxo 29a,b Cu II complexes of tmpa ligands bearing one or two amido substituents, respectively. When deprotonated, the amido donor can also act as a protection against dimerization of the complex. With ligand 1, the coordination of the amido groups 10978


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Scheme 3. (A) Overall View of the Coordination Properties of Complexes [Zn(1)]X2 (X = OTf−, ClO4−) and [Cu(1)](ClO4)2 (Potentials Measured at 0.66 mM in CH2Cl2/NBu4PF6 0.2 M vs Fc+/Fc unless Otherwise Stated); (B) Reactivity of [Cu(1)]BArF toward O2 (LH2 = Ligand 1); (Insets) Comparison to Calix[6]tren Complexes

The situation is very different for calix[6]amido-tren: the arms of the ligand are more flexible, the structure is more open to the solvent, and the oxygen atoms of the small rim are quite far away from the metal center. Furthermore, the amido arms can provide H-bonding to the coordinated anions. As a result, the metal ion readily binds one or two anions. Finally, both “one-” and “two-story” calixarene Cu I complexes exhibited high reactivity toward dioxygen (Scheme 3B). However, while in the case of calix[6]tren no intermediate could be accumulated at low temperature upon exposure to O2, a green species could be accumulated with ligand 1 in MeCN or acetone. Its UV−vis absorption signature is very similar to those previously recorded for CuII−superoxo complexes obtained with N4 ligands. This [Cu(1)O2]+ adduct undergoes decomposition at room temperature. Oxygenation of the ligand was detected by mass spectrometry, thus showing the capacity of the CuI complex to activate dioxygen to perform selectively four-electron (+O − 2H) oxidations (Figures S43 and S44). A similar process was observed with the calix[6]tren−CuI

complex, but only in noncoordinating solvents. This was explained by the very fast substitution of the transient superoxide by the solvent (MeCN) due to its strong affinity for the cavity.8a Hence, the fact that an intermediate was detected with 1 as a ligand and not with calix[6]tren whereas their CuI/dioxygen reactivities lead to the same reaction pattern (+O − 2H) is significant and can be ascribed mainly to two features: • Different second coordination spheres: in the case of calix[6]tren, the inward-oriented oxygen lone pairs of the calixarene structure destabilize the transiently bound superoxide anion; in contrast, the amido arms of the “two-story” ligand 1 may well stabilize it through Hbonding (Figure S42). • Different hosting properties: because of the very strong affinity of calix[6]tren for neutral and linear guest ligands (e.g., MeCN), the transiently produced superoxide anion is readily displaced by the solvent MeCN, a pathway that is much less favored with 1 since the amido spacers and 10979


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CH2Cl2 (0.2 mL) upon addition of diethyl ether (0.8 mL). Calix[6]amido-tren 1 was obtained as a white solid (111.5 mg, 40% yield). Mp: 247 °C (dec.). 1H NMR (CD3CN/CDCl3 3:2, 300 MHz, 298 K): δ = 0.96 (s, 27H, tBu), 1.16 (s, 27H, tBu), 2.70 (m, 21 H, OCH3, CH2N, CH2CH2N), 3.30 (s, 6H, NHCOCH2NH), 3.35 (d, J = 14.9 Hz, 6H, ArCH2eq), 3.56 (bs, 6H, OCH2CH2NHCO), 3.86 (bs, 6H, OCH2), 4.52 (d, J = 15.0 Hz, 6H, ArCH2ax), 6.87 (s, 6H, ArH), 7.08 (s, 6H, ArH), 8.28 ppm (bs, 3H, NHCO). 13C NMR (CDCl3/ CD3CN 2:3, 75 MHz, 298 K): δ = 29.7, 31.5, 31.7, 34.5, 34.6, 40.0, 47.3, 53.1, 53.3, 60.8, 71.8, 125.3, 127.4, 133.4, 133.8, 146.4, 152.1, 154.6, 173.0 ppm. IR (ATR): ν = 3334, 2961, 1667, 1482, 1363, 1294, 1200, 1121 cm−1. HRMS (ESI-TOF): calcd for C87H124N7O9 [M + H]+, 1410.9461; measured, 1410.9496. Synthesis of [Zn(1)](OTf)2. To a solution containing calix[6]amido-tren 1 (10 mg, 7.1 μmol, 1 equiv) in distilled dichloromethane (0.5 mL) was added Zn(OTf)2 (2.6 mg, 7.1 μmol, 1 equiv) dissolved in distilled CH3OH (0.5 mL) under an inert atmosphere. After addition of triethylamine (0.95 μL, 7.1 μmol, 1 equiv), the reaction mixture was stirred at room temperature for 1 h. After concentration to a third of the volume by bubbling of argon through the solution, a crystalline product precipitated. It was separated from the solvent by centrifugation, washed with cold CH3OH, and dried under vacuum to give [Zn(1)](OTf)2 (8.1 mg, 64% yield) as a white solid. Mp: >250 °C. 1H NMR (C2D2Cl4, 400 MHz, 398 K): δ = 0.95 (s, 27H, tBu), 1.38 (s, 27H, tBu), 3.00 (m, 6H, CH2Ncap), 3.18 (m, 6H, CH2Ncap), 3.48 (s, 6H, COCH2NH), 3.53 (d, J = 15.9 Hz, 6H, ArCH2eq), 3.63 (bs, 6H, 9H, OCH3), 3.67 (m, 6H, OCH2CH2), 3.79 (m, 6H, OCH2), 4.31 (bs, 3H, NHcap), 4.49 (d, J = 15.8 Hz, 6H, ArCH2ax), 6.79 (s, 6H, ArH), 7.28 (s, 6H, ArH), 7.44 ppm (bs, 3H, NHCO). IR (CH2Cl2): ν = 3350−3210, 2966, 1666, 1650, 1629, 1606, 1481, 1383, 1279, 1250, 1205, 1116, 1030 cm−1. HRMS (ESI-Orbitrap): calcd for C87H123N7O9Zn [M]2+, 736.9331; measured, 736.9342. Synthesis of [Cu(1)](ClO4)2. Under argon, a solution of Cu(ClO4) 2(H2O)6 (5.3 mg, 0.0142 mmol, 1 equiv) in dry tetrahydrofuran (0.8 mL) was added to a solution of calix[6]amidotren 1 (20.0 mg, 0.0142 mmol, 1 equiv) in distilled CH2Cl2 (1.3 mL), and the reaction mixture was stirred at rt for 1 h. After removal of the solvents under reduced pressure, the light-blue solid was redissolved in CH2Cl2 (1 mL), and the solution was filtered through Celite. The solvent was evaporated to yield pure [Cu(1)](ClO4)2 as a light-blue powder (22.6 mg, 95% yield). Mp: >250 °C. IR (ATR): ν = 3290− 3270, 2958, 1670, 1624, 1481, 1362, 1291, 1204, 1105, 624 cm−1. HRMS (ESI-Orbitrap): calcd for C87H123N7O9Cu [M]2+, 736.4339; measured, 736.4319. Anal. Calcd for C87H123Cl2CuN7O17(CH2Cl2)(H2O)2: C, 59.15; H, 7.27; N, 5.09. Found: C, 58.9; H, 7.25; N, 5.46. Synthesis of [Cu(1-H)](ClO4). Under argon, a solution of Cu(ClO4) 2(H2O)6 (5.3 mg, 0.0142 mmol, 1 equiv) in dry tetrahydrofuran (0.8 mL) was added to a solution of calix[6]amidotren 1 (20.0 mg, 0.0142 mmol, 1 equiv) in distilled CH2Cl2 (1.3 mL), and the reaction mixture was stirred at rt for 1 h. After removal of the solvents under reduced pressure, the solid was dissolved in CH2Cl2 (1 mL), and triethylamine (57.4 μL, 30 equiv) was added. The mixture was stirred for 30 min at rt and washed with water (1 × 1 mL, 15 min), resulting in the formation of a precipitate. This precipitate was isolated by centrifugation, giving pure [Cu(1-H)](ClO4) as an intensely blue powder (18.6 mg, 83% yield). Mp: >250 °C. IR (ATR): ν = 3352, 3236, 2960, 1656, 1582, 1481, 1362, 1296, 1203, 1114, 627 cm−1. HRMS (ESI-Orbitrap): calcd for C87H122N7O9Cu [M]+, 1471.8606; measured, 1471.8556. Synthesis of [Cu(1)]PF6 and [Cu(1)](BArF). In a glovebox under an argon atmosphere, anhydrous calix[6]amido-tren 1 (10.0 mg, 0.0071 mmol, 1 equiv) was suspended in acetonitrile or acetone (600 μL). A solution containing Cu(MeCN)4PF6 (2.6 mg, 0.0071 mmol, 1 equiv) in acetonitrile or Cu(MeCN)4BArF (3.9 mg, 0.0071 mmol, 1 equiv, synthesized according to a previously published procedure31) in acetonitrile or acetone (600 μL), respectively, was added to the suspension of 1 in the corresponding solvent, and the mixture was stirred at room temperature for 10 min to form the complex [Cu(1)(CD3CN)]PF6 or [Cu(1)(CD3CN)]BArF, respectively. The complexes can be used in situ without further purification or

the self-included methoxy groups decrease its hosting properties and thus its affinity for the coordinating solvent.

CONCLUSION All of the complexes formed with the new “two-story” calix(6)amido-tren ligand 1 are mononuclear, just as are those with the previously described “one-story” calix[6]tren ligand. However, they present very different properties. On the one hand, the introduction of the amido spacer decreases the host−guest properties of the complexes. On the other hand, the resulting modification of the second coordination sphere associated with the opening of a second coordination site available to exogenous donors opens new routes in terms of reactivity and biomimetism. This is well-exemplified by the preliminary results related to CuI/O2 reactivity presented herein. Indeed, upon exposure of the CuI complex of ligand 1 to O2 at low temperature, a green intermediate could be accumulated for the first time in the calixarene family of copper complexes. Its UV−vis signature fits with the formation of an end-on superoxide intermediate, which was previously postulated for the calix[6]tren-based CuI complexes as a transient species responsible for oxygen insertion reactions onto the ligand according to an unusual four-electron oxidative pathway. In-depth studies of its formation, characterization, and reactivity are currently underway in our laboratories. These studies are of high interest for the conceptualization of metalloenzyme mimics.

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EXPERIMENTAL SECTION

Caution! Although we have not encountered any problems, it is noted that perchlorate salts of metal complexes with organic ligands are potentially explosive and should be handled only in small quantities with appropriate precautions. Synthesis of Calix[6]tris(bromoamide) 3. Bromoacetyl bromide (0.8 mL, 9.06 mmol, 6 equiv) was diluted in anhydrous dichloromethane (26 mL) under an inert atmosphere. The mixture was cooled to −63 °C, and a solution of calix[6]tris(amine) 2 (1.7 g, 1.51 mmol, 1 equiv) and triethylamine (1.2 mL, 9.04 mmol, 6 equiv) in anhydrous dichloromethane (34 mL) was added dropwise. The mixture was stirred at −63 °C for 1 h. The reaction mixture was then slowly warmed to room temperature (rt), stirred for 1 h at rt, and washed with water (3 × 10 mL). The combined organic layers were evaporated under reduced pressure. The crude residue was purified by trituration and centrifugation at −10 °C in EtOH (3 × 4 mL), and calix[6]tris(bromoamide) 3 was isolated as a white powder (1.72 g, 76% yield). Mp: 242 °C. 1H NMR (CDCl3/CD3CN 3:2, 300 MHz, 298 K): δ = 0.66 (s, 27H, tBu), 1.09 (s, 27H, tBu), 2.30 (bs, 9H, OCH3), 3.21 (d, J = 15.1 Hz, 6H, ArCH2eq), 3.42 (bs, 6H, CH2NH), 3.66 (m, 12H, OCH2, CH2Br), 4.29 (d, J = 15.2 Hz, 6H, ArCH2ax), 6.54 (s, 6H, ArH), 7.00 (s, 6H, ArH), 7.16 ppm (bs, 3H, NHCO). 13C NMR (CDCl3/CD3CN 3:2, 75 MHz, 298 K): δ = 29.5, 29.8, 31.3, 31.6, 34.3, 34.4, 40.7, 60.6, 71.1, 124.5, 127.9, 133.2, 133.6, 146.3, 154.4, 166.8 ppm. IR (KBr): ν = 3297, 2961, 1663, 1534, 1482, 1362, 1294, 1202 cm−1. HRMS (ESI-TOF): calcd for C80H107Br3N3O9 [M + H]+, 1505.5742; measured, 1505.5744. Synthesis of Calix[6]amido-tren 1. Under argon, calix[6]tris(bromoamide) 3 (0.300 g, 0.199 mmol, 1 equiv) was dissolved in anhydrous chloroform (20 mL) and anhydrous acetonitrile (30 mL). Tris(2-aminoethyl)amine (tren) (0.06 mL, 0.398 mmol, 2 equiv) and potassium carbonate (0.165 g, 1.19 mmol, 6 equiv) were added, and the reaction mixture was stirred at 55 °C for 2 h 30 min. After evaporation under reduced pressure, the crude residue was dissolved in CH2Cl2 (10 mL) and washed with water (3 × 10 mL). The combined organic layers were evaporated under reduced pressure, and the residue was purified by precipitation from a minimum volume of 10980


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evaporated to dryness under reduced pressure to give [Cu(1)(CD3CN)]PF6 or [Cu(1)(CD3CN)]BArF as a white solid that could be redissolved in various solvents. Data for [Cu(1)(CD3CN)]PF6: 1H NMR (CD3CN, 500 MHz, 350 K): δ = 1.07 (s, 27H, tBu), 1.19 (s, 27H, tBu), 2.82 (bs, 6H, CH2N or CH2CH2N), 2.85 (bs, 6H, CH2N or CH2CH2N), 3.08 (s, 9H, OCH3), 3.41 (s, 6H, NHCOCH2NH), 3.48 (d, J = 15.3 Hz, 6H, ArCH2eq), 3.60 (m, 6H, OCH2CH2NHCO), 3.80 (t, J = 5.1 Hz, 6H, OCH2CH2NH), 4.54 (d, J = 15.8 Hz, 6H, ArCH2ax), 7.00 (s, 6H, ArH), 7.11 ppm (s, 6H, ArH). IR (CH3CN): ν = 3634, 3542, 2958, 2869, 1673, 1625, 1523, 1479, 1362, 1298, 1242, 1120, 1060, 1008 cm−1. Data for [Cu(1)(CD3CN)]BArF: 1H NMR (CD3CN, 500 MHz, 298 K): δ = 0.97 (s, 27H, tBu), 1.20 (s, 27H, tBu), 2.73 (bs, 6H, CH2N or CH2CH2N), 2.77 (bs, 6H, CH2N or CH2CH2N), 2.89 (s, 9H, OCH3), 3.35 (s, 6H, NHCOCH2NH), 3.44 (d, J = 15.0 Hz, 6H, ArCH2eq), 3.59 (m, 6H, OCH2CH2NHCO), 3.80 (bs, 6H, OCH2CH2NH), 4.49 (d, J = 14.1 Hz, 6H, ArCH2ax), 6.90 (s, 6H, ArH), 7.15 (s, 6H, ArH), 7.58 ppm (s, 3H, NHamide).

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REFERENCES

(1) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Structural and functional aspects of metal sites in biology. Chem. Rev. 1996, 96, 2239−2314. (2) Solomon, E. I.; Heppner, D. E.; Johnston, E. M.; Ginsbach, J. W.; Cirera, J.; Qayyum, M.; Kieber-Emmons, M. T.; Kjaergaard, C. H.; Hadt, R. G.; Tian, L. Copper Active Sites in Biology. Chem. Rev. 2014, 114, 3659−3853. (3) (a) Itoh, S. Developing mononuclear copper-active-oxygen complexes relevant to reactive intermediates of biological oxidation reactions. Acc. Chem. Res. 2015, 48, 2066−74. (b) Lewis, E. A.; Tolman, W. B. Reactivity of dioxygen-copper systems. Chem. Rev. 2004, 104, 1047−1076. (c) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Structure and spectroscopy of copper-dioxygen complexes. Chem. Rev. 2004, 104, 1013−1045. (4) (a) Le Poul, N.; Le Mest, Y.; Jabin, I.; Reinaud, O. Supramolecular Modeling of Mono-copper Enzyme Active Sites with Calix[6]arene-based Funnel Complexes. Acc. Chem. Res. 2015, 48, 2097−2106. (b) Rebilly, J.-N.; Reinaud, O. Calixarenes and resorcinarenes as scaffolds for supramolecular metallo-enzyme mimicry. Supramol. Chem. 2014, 26, 454−479. (c) Rebilly, J.-N.; Colasson, B.; Bistri, O.; Over, D.; Reinaud, O. Biomimetic cavity-based metal complexes. Chem. Soc. Rev. 2015, 44, 467−489. (5) For MII complexes based on the smaller calix[4]arene scaffold, the cavity does not play the role of a funnel for ligand hosting. For example, see: (a) Joseph, R.; Chinta, J. P.; Rao, C. P. Calix[4]areneBased 1,3-Diconjugate of Salicylyl Imine Having Dibenzyl Amine Moiety (L): Synthesis, Characterization, Receptor Properties toward Fe2+, Cu2+, and Zn2+, Crystal Structures of Its Zn2+ and Cu2+ Complexes, and Selective Phosphate Sensing by the [ZnL]. Inorg. Chem. 2011, 50, 7050−7058. (b) Kumar Pathak, R.; Kumar Hinge, V.; Rai, A.; Panda, D.; Pulla Rao, C. Imino−Phenolic−Pyridyl Conjugates of Calix[4]arene (L1 and L2) as Primary Fluorescence Switch-on Sensors for Zn2+ in Solution and in HeLa Cells and the Recognition of Pyrophosphate and ATP by [ZnL2]. Inorg. Chem. 2012, 51, 4994− 5005. (6) Jabin, I.; Reinaud, O. First C3v-Symmetrical Calix[6](aza)crown. J. Org. Chem. 2003, 68, 3416−3419. (7) (a) Darbost, U.; Rager, M.-N.; Petit, S.; Jabin, I.; Reinaud, O. Polarizing a Hydrophobic Cavity for the Efficient Binding of Organic Guests: The Case of Calix[6]tren, a Highly Efficient and Versatile Receptor for Neutral or Cationic Species. J. Am. Chem. Soc. 2005, 127, 8517−8525. (b) Darbost, U.; Zeng, X.; Rager, M.-N.; Giorgi, M.; Jabin, I.; Reinaud, O. X-ray and Solution Structures of the First Zn Funnel Complex Based on a Calix[6]aza-cryptand. Eur. J. Inorg. Chem. 2004, 2004, 4371−4374. (8) (a) Izzet, G.; Douziech, B.; Prange, T.; Tomas, A.; Jabin, I.; Le Mest, Y.; Reinaud, O. Calix[6]tren and copper(II): a third generation of funnel complexes on the way to redox calix-zymes. Proc. Natl. Acad. Sci. U. S. A. 2005, 102, 6831−6. (b) Izzet, G.; Rager, M.-N.; Reinaud, O. Insights into the binding properties of a cuprous ion embedded in the tren cap of a calix[6]arene and supramolecular trapping of an intermediate. Dalton Trans. 2007, 771−780. (c) Izzet, G.; Zeitouny, J.; Akdas-Killig, H.; Frapart, Y.; Ménage, S.; Douziech, B.; Jabin, I.; Le Mest, Y.; Reinaud, O. Dioxygen Activation at a Mononuclear Cu(I) Center Embedded in the Calix[6]arene-Tren Core. J. Am. Chem. Soc. 2008, 130, 9514−9523. (9) Darbost, U.; Giorgi, M.; Hucher, N.; Jabin, I.; Reinaud, O. A calix[6]arene receptor rigidified by a self-assembled triammonium cap: X-ray and NMR characterization of the binding of polar neutral guests. Supramol. Chem. 2005, 17, 243−250. (10) (a) Le Gac, S.; Jabin, I. Synthesis and study of calix[6]cryptamides: A new class of heteroditopic receptors that display versatile host-guest properties toward neutral species and organic associated ion-pair salts. Chem. - Eur. J. 2008, 14, 548−557. (b) Ménand, M.; Jabin, I. Synthesis of the First Calix[6]crypturea via a Versatile Tris-azide Precursor. Org. Lett. 2009, 11, 673−676. (c) Lascaux, A.; Le Gac, S.; Wouters, J.; Luhmer, M.; Jabin, I. An allosteric heteroditopic receptor for neutral guests and contact ion

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01225. Experimental details; 1D and 2D NMR spectra of all new organic compounds; protonation NMR studies of compound 1; HRMS and IR characterizations and NMR studies of [Zn(1)](OTf)2; HRMS, IR, EPR, UV−vis, and CV experiments on [Cu(1)](ClO4)2 and [Cu(1-H)](ClO4); IR, NMR, and HRMS studies of complexes [Cu(1)]X (X = PF6, BArF) (PDF) Accession Codes

CCDC 1541407−1541408 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, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.L.P). *E-mail: [email protected] (I.J.). *E-mail: [email protected] (O.R.). ORCID

Nicolas Le Poul: 0000-0002-5915-3760 Filip Topić: 0000-0003-3811-6036 Kari Rissanen: 0000-0002-7282-8419 Olivia Reinaud: 0000-0002-2600-5331 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Fonds de la Recherche Scientifique - F.R.S.-FNRS (Project FRFC 2.4.617.10.F and a Ph.D. grant to G.D.L.), the Agence Nationale de la Recherche (ANR10-BLAN-714 Cavity-zyme(Cu) Project), and the De Brouckère-Solvay and Michel Kaisin Funds (travel grants to G.D.L.) and was undertaken within the framework of COST Action CM-1005 “Supramolecular Chemistry in Water”. 10981


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reactions of Cu−O2 and Cu2−O2 complexes. Coord. Chem. Rev. 2013, 257, 187−195. (c) Kim, S.; Saracini, C.; Siegler, M. A.; Drichko, N.; Karlin, K. D. Coordination Chemistry and Reactivity of a Cupric Hydroperoxide Species Featuring a Proximal H-Bonding Substituent. Inorg. Chem. 2012, 51, 12603−12605. (d) Schatz, M.; Becker, M.; Walter, O.; Liehr, G.; Schindler, S. Reactivity towards dioxygen of a copper(I) complex of tris(2-benzylaminoethyl)amine. Inorg. Chim. Acta 2001, 324, 173−179. (e) Kunishita, A.; Kubo, M.; Sugimoto, H.; Ogura, T.; Sato, K.; Takui, T.; Itoh, S. Mononuclear Copper(II)− Superoxo Complexes that Mimic the Structure and Reactivity of the Active Centers of PHM and DβM. J. Am. Chem. Soc. 2009, 131, 2788− 2789. (f) Abe, T.; Morimoto, Y.; Tano, T.; Mieda, K.; Sugimoto, H.; Fujieda, N.; Ogura, T.; Itoh, S. Geometric Control of Nuclearity in Copper(I)/Dioxygen Chemistry. Inorg. Chem. 2014, 53, 8786−8794. (g) Fry, H. C.; Scaltrito, D. V.; Karlin, K. D.; Meyer, G. J. The Rate of O2 and CO Binding to a Copper Complex, Determined by a “Flashand-Trap” Technique, Exceeds that for Hemes. J. Am. Chem. Soc. 2003, 125, 11866−11871. (h) Kakuda, S.; Rolle, C. J.; Ohkubo, K.; Siegler, M. A.; Karlin, K. D.; Fukuzumi, S. Lewis Acid-Induced Change from Four- to Two-Electron Reduction of Dioxygen Catalyzed by Copper Complexes Using Scandium Triflate. J. Am. Chem. Soc. 2015, 137, 3330−3337. (i) Lee, Y.; Lee, D.-H.; Narducci Sarjeant, A. A.; Zakharov, L. N.; Rheingold, A. L.; Karlin, K. D. Thioether Sulfur Oxygenation from O2 or H2O2 Reactivity of Copper Complexes with Tridentate N2Sthioether Ligands. Inorg. Chem. 2006, 45, 10098− 10107. (j) Saracini, C.; Liakos, D. G.; Zapata Rivera, J. E.; Neese, F.; Meyer, G. J.; Karlin, K. D. Excitation Wavelength Dependent O2 Release from Copper(II)−Superoxide Compounds: Laser FlashPhotolysis Experiments and Theoretical Studies. J. Am. Chem. Soc. 2014, 136, 1260−1263. (k) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler, S.; Jacobson, R. R.; Tyeklár, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.; Zubieta, J.; Karlin, K. D. Copper(I) Complexes, Copper(I)/O2 Reactivity, and Copper(II) Complex Adducts, with a Series of Tetradentate Tripyridylalkylamine Tripodal Ligands. Inorg. Chem. 2001, 40, 2312−2322. (23) (a) Melia, C.; Ferrer, S.; Rezac, J.; Parisel, O.; Reinaud, O.; Moliner, V.; de la Lande, A. Investigation of the Hydroxylation Mechanism of Noncoupled Copper Oxygenases by Ab Initio Molecular Dynamics Simulations. Chem. - Eur. J. 2013, 19, 17328− 17337. (b) Thiabaud, G.; Guillemot, G.; Schmitz-Afonso, I.; Colasson, B.; Reinaud, O. Solid-State Chemistry at an Isolated Copper(I) Center with O2. Angew. Chem., Int. Ed. 2009, 48, 7383−7386. (24) (a) Bieksza, D. S.; Hendrickson, D. N. Magnetic exchange in transition metal dimers. 9. Copper (II) dimers with single end-to-end cyanide bridges and unusual electron paramagnetic resonance spectra. Inorg. Chem. 1977, 16, 924−929. (b) Felthouse, T. R.; Laskowski, E. J.; Bieksza, D. S.; Hendrickson, D. N. Structural and magnetic properties of copper (II) dimers bridged by oxalate, azide, and cyanide ions; Xray structures of [Cu2{EtN(CH2CH2NEt2)2}2(C2O4)][BPh4]2 and [Cu2{MeN(CH2CH2NMe2)2}2(N3)2][BPh4]2. The role of the transition-metal ion ground state in magnetic exchange interactions. J. Chem. Soc., Chem. Commun. 1976, 777−778. (c) Laskowski, E. J.; Duggan, D. M.; Hendrickson, D. N. Magnetic exchange interactions in transition metal dimers. VII. X-ray structural characterization of three outer-sphere copper(II) dimers [Cu2(tren)2X2](BPh4)2 where X− = chloride(1−), isocyanate(1−), and isothiocyanate(1−). Outer-sphere electron exchange interactions. Inorg. Chem. 1975, 14, 2449−2459. (d) Parker, R. J.; Spiccia, L.; Moubaraki, B.; Murray, K. S.; Skelton, B. W.; White, A. H. Cyano bridged dinuclear Cu(II) complexes. Inorg. Chim. Acta 2000, 300−302, 922−931. (e) Rodríguez-Fortea, A.; Alemany, P.; Alvarez, S.; Ruiz, E.; Scuiller, A.; Decroix, C.; Marvaud, V.; Vaissermann, J.; Verdaguer, M.; Rosenman, I.; Julve, M. Exchange Coupling in Cyano-Bridged Homodinuclear Cu(II) and Ni(II) Complexes: Synthesis, Structure, Magnetism, and Density Functional Theoretical Study. Inorg. Chem. 2001, 40, 5868−5877. (f) Duggan, D. M.; Hendrickson, D. N. X-ray structural characterization of an outersphere copper(II) dimer, dicyanobis(2,2′,2″-triaminotriethylamine)dicopper(II) tetraphenylborate. Hydrogen-bonding between two trigonal-bipyramidal copper cations. Inorg. Chem. 1974, 13, 1911−

pairs with a remarkable selectivity for ammonium fluoride salts. Org. Biomol. Chem. 2010, 8, 4607−4616. (11) (a) Zeng, X.; Bornholdt, C.; Over, D.; Reinaud, O. Synthesis of “Two-Story” Calix[6]aza-Cryptands. Org. Lett. 2011, 13, 5660−5663. (b) Over, D.; Zeng, X.; Bornholdt, C.; Marrot, J.; Reinaud, O. Coordination of Lead(II) in the Supramolecular Environment Provided by a ″Two-Story″ Calix[6]arene-based N6 Ligand. Inorg. Chem. 2013, 52, 14089−14095. (12) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen−sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349−1356. (13) Rivas, J. C. M.; de Rosales, R. T. M.; Parsons, S. Internal hydrogen bonding and amide co-ordination in zinc (II) complexes of a tripodal N4 ligand: structural, spectroscopic and reactivity studies. Dalton Trans. 2003, 2156−2163. (14) Berreau, L. M.; Halfen, J. A.; Young, V. G.; Tolman, W. B. Synthesis and copper coordination chemistry of hindered 1, 4, 7triazacyclononane ligands with amide appendages. Inorg. Chem. 1998, 37, 1091−1098. (15) Chu, L.; Hardcastle, K. I.; MacBeth, C. E. Transition Metal Complexes Supported by a Neutral Tetraamine Ligand Containing N,N-dimethylaniline Units. Inorg. Chem. 2010, 49, 7521−7529. (16) Kobayashi, Y.; Ohkubo, K.; Nomura, T.; Kubo, M.; Fujieda, N.; Sugimoto, H.; Fukuzumi, S.; Goto, K.; Ogura, T.; Itoh, S. Copper(I)Dioxygen Reactivity in a Sterically Demanding Tripodal Tetradentate tren Ligand: Formation and Reactivity of a Mononuclear Copper(II) End-On Superoxo Complex. Eur. J. Inorg. Chem. 2012, 2012, 4574− 4578. (17) Rondelez, Y.; Sénèque, O.; Rager, M.-N.; Duprat, A. F.; Reinaud, O. Biomimetic Copper(I)−CO Complexes: A Structural and Dynamic Study of a Calix[6]arene-Based Supramolecular System. Chem. - Eur. J. 2000, 6, 4218−4226. (18) (a) Maiti, D.; Lee, D.-H.; Gaoutchenova, K.; Würtele, C.; Holthausen, M. C.; Narducci Sarjeant, A. A.; Sundermeyer, J.; Schindler, S.; Karlin, K. D. Reactions of a Copper(II) Superoxo Complex Lead to C-H and O-H Substrate Oxygenation: Modeling Copper-Monooxygenase C-H Hydroxylation. Angew. Chem., Int. Ed. 2008, 47, 82−85. (b) Maiti, D.; Fry, H. C.; Woertink, J. S.; Vance, M. A.; Solomon, E. I.; Karlin, K. D. A 1:1 copper-dioxygen adduct is an end-on bound superoxo copper(II) complex which undergoes oxygenation reactions with phenols. J. Am. Chem. Soc. 2007, 129, 264−265. (19) Spectroscopic features of mononuclear Cu:O2 adducts identified as end-on superoxide CuII complexes [λ/nm (ε/M−1 cm−1)]: Me6tren20 412 (4800), 747 (1000); HIPT3tren16 434 (3850), 549 (1350), 675 (1910), 790 (3520); (TMG)3tren18a,21 447, 680, 780; tmpa20 410 (4000); NMe2tmpa18b 418 (4300), 615 (1100), 767 (840). (20) Weitzer, M.; Schindler, S.; Brehm, G.; Schneider, S.; Hörmann, E.; Jung, B.; Kaderli, S.; Zuberbühler, A. D. Reversible Binding of Dioxygen by the Copper(I) Complex with Tris(2dimethylaminoethyl)amine (Me6tren) Ligand. Inorg. Chem. 2003, 42, 1800−1806. (21) (a) Schatz, M.; Raab, V.; Foxon, S. P.; Brehm, G.; Schneider, S.; Reiher, M.; Holthausen, M. C.; Sundermeyer, J.; Schindler, S. Combined Spectroscopic and Theoretical Evidence for a Persistent End-On Copper Superoxo Complex. Angew. Chem., Int. Ed. 2004, 43, 4360−4363. (b) Wü rtele, C.; Gaoutchenova, E.; Harms, K.; Holthausen, M. C.; Sundermeyer, J.; Schindler, S. Crystallographic Characterization of a Synthetic 1:1 End-On Copper Dioxygen Adduct Complex. Angew. Chem., Int. Ed. 2006, 45, 3867−3869. (22) (a) Peterson, R. L.; Himes, R. A.; Kotani, H.; Suenobu, T.; Tian, L.; Siegler, M. A.; Solomon, E. I.; Fukuzumi, S.; Karlin, K. D. Cupric Superoxo-Mediated Intermolecular C−H Activation Chemistry. J. Am. Chem. Soc. 2011, 133, 1702−1705. (b) Fukuzumi, S.; Karlin, K. D. Kinetics and thermodynamics of formation and electron-transfer 10982


Article

Inorganic Chemistry 1916. (g) Duan, Q.; Cao, Y.; Li, Y.; Hu, X.; Xiao, T.; Lin, C.; Pan, Y.; Wang, L. pH-Responsive Supramolecular Vesicles Based on WaterSoluble Pillar[6]arene and Ferrocene Derivative for Drug Delivery. J. Am. Chem. Soc. 2013, 135, 10542−10549. (h) Haddad, M. S.; Hendrickson, D. N. Magnetic properties of binuclear copper (II) complexes with one hydroxo bridge. Inorg. Chim. Acta 1978, 28, L121−L122. (25) (a) Becker, M.; Heinemann, F. W.; Schindler, S. Reversible Binding of Dioxygen by a Copper(I) Complex with Tris(2dimethylaminoethyl)amine (Me6tren) as a Ligand. Chem. - Eur. J. 1999, 5, 3124−3129. (b) Becker, M.; Heinemann, F. W.; Knoch, F.; Donaubauer, W.; Liehr, G.; Schindler, S.; Golub, G.; Cohen, H.; Meyerstein, D. Syntheses, Structures and Properties of Copper(I) and Copper(II) Complexes of the Ligand N,N′-Bis[2′-(dimethylamino)ethyl]-N,N′-dimethylethane1,2-diamine (Me6trien). Eur. J. Inorg. Chem. 2000, 2000, 719−726. (26) (a) Karlin, K. D.; Kaderli, S.; Zuberbühler, A. D. Kinetics and Thermodynamics of Copper(I)/Dioxygen Interaction. Acc. Chem. Res. 1997, 30, 139−147. (b) Zhang, C. X.; Kaderli, S.; Costas, M.; Kim, E.i.; Neuhold, Y.-M.; Karlin, K. D.; Zuberbühler, A. D. Copper(I)− Dioxygen Reactivity of [(L)CuI]+ (L = Tris(2-pyridylmethyl)amine): Kinetic/Thermodynamic and Spectroscopic Studies Concerning the Formation of Cu−O2 and Cu2−O2 Adducts as a Function of Solvent Medium and 4-Pyridyl Ligand Substituent Variations. Inorg. Chem. 2003, 42, 1807−1824. (c) Würtele, C.; Sander, O.; Lutz, V.; Waitz, T.; Tuczek, F.; Schindler, S. Aliphatic C−H Bond Oxidation of Toluene Using Copper Peroxo Complexes That Are Stable at Room Temperature. J. Am. Chem. Soc. 2009, 131, 7544−7545. (27) (a) Fujisawa, K.; Tanaka, M.; Moro-oka, Y.; Kitajima, N. A Monomeric Side-On Superoxocopper(II) Complex: Cu(O2)(HB(3tBu-5-iPrpz)3). J. Am. Chem. Soc. 1994, 116, 12079−12080. (b) Hikichi, S.; Komatsuzaki, H.; Akita, M.; Moro-oka, Y. Aliphatic C−H Bond Oxygenation by the CoIIOOX Species with the Hindered Hydrotris(pyrazolyl)borate Ligand (X = Co(II), Alkyl, H). J. Am. Chem. Soc. 1998, 120, 4699−4710. (28) (a) Watson, A. A.; Willis, A. C.; Fairlie, D. P. Organization of Amino Acids Using a Metallotriazacyclononane Template. Inorg. Chem. 1997, 36, 752−753. (b) Weyhermüller, T.; Weighardt, K.; Chaudhuri, P. Nitrogen versus oxygen co-ordination of carboxamidefunctionalized triazacyclononane ligands in transition metal ion complexes. J. Chem. Soc., Dalton Trans. 1998, 3805−3814. (29) (a) Wada, A.; Harata, M.; Hasegawa, K.; Jitsukawa, K.; Masuda, H.; Mukai, M.; Kitagawa, T.; Einaga, H. Structural and Spectroscopic Characterization of a Mononuclear Hydroperoxo−Copper(II) Complex with Tripodal Pyridylamine Ligands. Angew. Chem., Int. Ed. 1998, 37, 798−799. (b) Yamaguchi, S.; Wada, A.; Funahashi, Y.; Nagatomo, S.; Kitagawa, T.; Jitsukawa, K.; Masuda, H. Thermal Stability and Absorption Spectroscopic Behavior of (μ-Peroxo)dicopper Complexes Regulated with Intramolecular Hydrogen Bonding Interactions. Eur. J. Inorg. Chem. 2003, 2003, 4378−4386. (c) Kakuda, S.; Peterson, R. L.; Ohkubo, K.; Karlin, K. D.; Fukuzumi, S. Enhanced Catalytic FourElectron Dioxygen (O2) and Two-Electron Hydrogen Peroxide (H2O2) Reduction with a Copper(II) Complex Possessing a Pendant Ligand Pivalamido Group. J. Am. Chem. Soc. 2013, 135, 6513−6522. (30) Izzet, G.; Zeng, X.; Akdas, H.; Marrot, J.; Reinaud, O. Drastic effects of the second coordination sphere on neutral vs. anionic guest binding to a biomimetic Cu(ii) center embedded in a calix[6]azacryptand. Chem. Commun. 2007, 810−812. (31) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D. A Bis-Acetonitrile Two-Coordinate Copper(I) Complex: Synthesis and Characterization of Highly Soluble B(C6F5)4− Salts of [Cu(MeCN)2]+ and [Cu(MeCN)4]+. Inorg. Chem. 2002, 41 (8), 2209−2212.

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