Uranium Tetrakis-Aryloxide Derivatives Supported by

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Uranium Tetrakis-Aryloxide Derivatives Supported by Tetraazacyclododecane: Synthesis of Air-Stable, CoordinativelyUnsaturated U(IV) and U(V) Complexes Julian Hümmer, Frank W. Heinemann, and Karsten Meyer* Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstraße 1, 91058Erlangen, Germany S Supporting Information *

ABSTRACT: We present the synthesis, characterization, and one-electron oxidation of two uranium(IV) complexes, coordinated to the cyclen-anchored tetrakis(aryloxide) ligands tetrakishydroxybenzyl-1,4,7,10 tetraazacyclododecane, (R,RArOH)4cyclen; R = tBu, Me. The new uranium(IV) and (V) complexes exhibit an eight-coordinate, tetragonal ligand environment, effecting exceptional stability of the coordinatively unsaturated uranium compounds. Cyclic voltammetry studies reveal redox events ranging from tri- to hexavalent species, covering an electrochemical window of ∼4 V.

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INTRODUCTION Aryloxide functionalized, tacn-based ligands (e.g., 1,4,7-tris(3,5dialkyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, (R,R′ ArOH)3 tacn), have been developed and employed extensively by the Meyer group, since they stabilize highly reactive uranium ions in various oxidation states.1−6 The molecular architecture of the tacn-based uranium complexes significantly influences the reactivity of the uranium metal center and varies greatly depending on the steric substituents of the aryloxide ligand arms.3,7 In continuation of this work, the research reported herein aims at probing the effect of changing anchoring polyamine frameworks on the physical and chemical properties of the coordination compounds. Extension of the tacn-chelator by an additional amine results in the well-known 1,4,7,10-tetraazacyclododecane (cyclen) macrocyclic ligand, which provides not only an additional N-donor but also allows for the introduction of another functionalized aryloxide pendant arm. Such N-functionalized cyclen-based ligands are widely utilized in coordination chemistry of transition-metal ions8,9 and, especially, lanthanides.10 Highly labile lanthanide ions that are cyclen coordinated have various applications, including as magnetic resonance imaging contrast agents or in radionuclide therapy. However, so far, successful attempts of cyclen-based ligand coordination to actinides remain scarce.11,12 To explore the potential of these ligands for uranium coordination, we synthesized a ligand derivative, namely, (t‑Bu,t‑BuArOH)4cyclen, (tetrakis(3,5-di-t-butyl-2-hydroxybenzyl)-1,4,7,10-tetraaza-cyclododecane), carrying four 2-hydroxybenzyl-substituted pendant arms at the polyamine anchor for eight-coordinate uranium complex [((t‑Bu,t‑BuArO)4cyclen)U] © 2016 American Chemical Society

formation (Scheme 1). Employing a Mannich-type reaction, Ogo and Nakai et al. recently synthesized a similar ligand system with two methyl substituents on the aryloxide pendant arms, namely, (Me,MeArOH)4cyclen.13 They designed the ligand Scheme 1. Synthesis of (t‑Bu,t‑BuArOH)4cyclen, Complexation of [UIII(N(SiMe3)2)3] to Form [((t‑Bu,t‑BuArO)4cyclen)UIV] (1), and Subsequent Oxidation to Yield [((t‑Bu,t‑BuArO)4cyclen)UV][SbF6] (2)

Received: September 13, 2016 Published: November 16, 2016 3201

DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206

Article

Inorganic Chemistry for the examination of an oxygen-sensitive, luminescent terbium complex.13 Although the Tb3+ ion of this complex is wellencapsulated by the N4O4 octadentate ligand environment, the metal center is seven-coordinate, in an N4O3 heptadentate fashion, with one of the phenolic arms remaining uncoordinated.

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RESULTS AND DISCUSSION For our purposes, cyclen was functionalized with 4 equiv of 1bromomethyl-3,5-di-tert-butylphenol 1 4 to yield the (t‑Bu,t‑BuArOH)4cyclen, in an SN2-type reaction (Scheme 1). Deprotonation of this ligand with potassium bis(trimethylsilyl) amide in benzene in situ and subsequent addition of [UIV(I)4(dioxane)2] results in formation of a pale green complex, albeit in relatively low yields (20%). The limiting factor of complex formation is likely incomplete ligand deprotonation and was circumvented by employing an alternative route, starting from the U(III) precursor material [U(N(SiMe3)2)3]. The internal base of this compound is easily replaced in a protonolysis reaction (Scheme 1). Despite the different oxidation states of the starting materials, both reaction routes result in the same U(IV) product, namely, [((t‑Bu,t‑BuArO)4cyclen)UIV] (1), which was characterized and studied by CHN elemental analysis, 1H NMR, UV/vis/NIR spectroscopy, SQUID magnetization measurements, and electrochemistry. In case of the trivalent uranium tris(amide) starting material, we propose that, in the first step, the sevencoordinate U(III) species is formed, which subsequently undergoes a one-electron oxidation, accompanied by reduction of the remaining ligand phenol proton to yield 0.5 equiv of H2 and 1 in excellent yields (yield 90%). The reductive activation of alcohols, phenols, and amines by trivalent U species, with subsequent formation of H2, is well-known and documented in the literature.15−18 Formation of H2 also was confirmed by gas chromatography-thermal conductivity detector measurements (see Supporting Information). Single crystals of 1 were obtained by tetrahydrofuran (THF) diffusion into a concentrated toluene solution. The X-ray diffraction analysis reveals the C2 symmetry of the molecule in the solid state (C4 symmetry in solution) as well as an unexpected unsaturated coordination polyhedron. The saturated uranium center is coordinated by all four nitrogen atoms of the cyclen anchoring unit (d(U−N)av =2.778(2) Å) and, thus, is effectively shielded from further coordination. The four oxygen atoms of the ligand aryloxides are occupying the equatorial positions (d(U−O)av =2.216(2) Å), leaving one free axial coordination site trans to the cyclen anchor. The oxygen and nitrogen atoms define one plane each, which are parallel to each other. The overall geometry of the polyhedron is best described as a distorted square antiprism with the two planes twisted against each other (Figure 1, right). The aspects of the stereochemistry of the eight-coordinate complexes, in which the molecular geometry departs from the idealized polyhedron (cf. square antiprism), were analyzed according to a procedure originally established by Porai-Koshits and Aslanov and further amplified by Muetterties and Guggenberger (see Supporting Information).19,20 The uranium ions’ out-of-plane shift (Uoop) of −0.88 Å is identical to the coordinatively unsaturated tacn-based U(III) complex [((t‑Bu,AdArO)3tacn)UIII], with a Uoop of −0.88 Å. The Uoop parameter specifies the displacement of the U ion below the plane defined by the four oxygen atoms.3,4 Given the similarity of the Uoop parameters of complex 1 and the tacn-

Figure 1. Solid-state molecular structure of [((t‑Bu,t‑BuArO)4cyclen)UIV] (1) in crystals of 1·5 THF, and the coordination polyhedron of the hetero atoms. Hydrogen atoms and cocrystallized solvent molecules are omitted for clarity. Thermal ellipsoids are at the 50% probability level.

anchored [((t‑Bu,AdArO)3tacn)UIII], the steric demand of the additional aryloxide arm of the cyclen ligand derivative appears to be very similar to the overall steric bulk of the adamantyl groups of the tacn-based ligand systems, both forcing the uranium center below the aryloxide plane, thereby severely constricting access to the axial coordination site. Nevertheless, likely due to the differences in oxidation state and coordination number, the reactivity of the tacn and cyclen-based complexes is significantly different. While derivatives of [((R,R′ArO)3tacn)UIII], like most low-valent uranium complexes, are very reactive and activate and functionalize a variety of small molecules,21 1 exhibits extraordinary stability, even in air and aqueous solution. Attempts to preparatively obtain the more reactive U(III) anionic complex reductively were not successful. Instead, the synthesis of the oxidized analogue of 1 was achieved by one-electron oxidation with Ag salts, for example, AgSbF6, yielding 2 as a black solid in quantitative yield. The cyclic voltammetry of 1 in THF solution and [N(n-Bu)4][PF6] electrolyte (see Supporting Information) shows redox chemistry spanning from U(III) to U(VI). The preparatively accessible U(IV)→U(V) oxidation is electrochemically observed as a reversible redox event centered at a half-wave potential of E1/2 = −0.28 V against Fc+/Fc. Two more quasireversible redox waves, assigned to the U(IV/III) and U(VI/V) redox couples, are located at −3.2 and +0.79 V, respectively. Overall, the observed redox potentials are spanning a remarkable wide potential range of ∼4 V. The magnetic moments of tetravalent 1 and pentavalent 2 were determined by SQUID magnetization measurements, in the temperature range from 2 to 300 K, with an applied magnetic field of 1 T (data for two independently synthesized samples are shown for reproducibility. For complexes 3, 4, and 5, see Supporting Information). The effective magnetic moment μeff (averaged over two independent measurements) of U(IV) species 1 is strongly temperature-dependent and varies from 0.53 μB at 2 K to 2.62 μB at 300 K. The magnetic moment of U(V) complex 2 is less temperature-dependent, ranging from 1.34 (2 K) to 2.02 μB (300 K). It is noteworthy that despite the significantly different molecular structures of the cyclen and tacn-based systems in [((R,RArO)4cyclen)UIV/V] and [((R,RArO)3tacn)U(X)] (with X = halide (UIV), imide(UV)), and their resulting distinctive crystal fields, the temperature dependency, and absolute values of μeff of the U(IV) and U(V) complexes are surprisingly similar, as expected for U(IV) f 2 and U(V) f 1 complexes. That is, to emphasize, despite the tetrakis(aryloxide) U ligand environment in 1−5 3202

DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206

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Inorganic Chemistry and the trigonal-pyramidal tris(aryloxide) U−X coordination in tacn-based complexes.22 The electron paramagnetic resonance (EPR)-active doublet ground state of the uranium(V) f 1 system of 2 is in accordance with the (slightly) rhombic signal observed by X-band EPR spectroscopy (Figure 2).

Figure 3. Top-view space-filling models of (from left to right): [((t‑Bu,t‑BuArO)4cyclen)UIV] (1), [((Me,MeArO)4cyclen)UIV] (3), and [((Me,MeArO)4cyclen)UV][SbF6] (4).

uranium(III) remained elusive, and attempts to coordinate additional ligands have not met with success. The one-electron oxidation with Ag salts is feasible, however, and oxidation with AgSbF6 resulted in [((Me,MeArO)4cyclen)UV][SbF6] (4) in near-quantitative yield (95%). Overall, U(IV) complex 3 is still surprisingly unreactive, but unlike 1, which has not shown any observable air-oxidation, 3 is slowly oxidized by air in the course of several weeks. This oxidation is observable by a color change from green to brown and was monitored by UV/vis absorption spectroscopy (see Supporting Information). Under optimized experimental conditions, the oxidation can be accelerated to be complete within 4 d. To achieve this transformation, a THF solution of 3 was saturated with O2 gas, and sodium tetraphenylborate was added to provide a counterion. The resulting complex, [((Me,MeArO)4cyclen)UV][BPh4] (5; see Figure 4), was isolated in 71% yield as an air-

Figure 2. Temperature-dependent magnetic susceptibility data of two independently synthesized samples of 1 (black) and 2 (red) plotted as μeff vs T (left) (for 3, 4, and 5 see Supporting Information). X-band EPR spectrum of 2 at 7 K (right), recorded in 10 mM frozen benzene solution (black trace) and the simulation (red trace) to fit the experimental data. Experimental conditions: microwave frequency ν = 8.960 GHz, microwave power = 1 mW, modulation frequency = 100 kHz, time constant = 0.1 s, modulation width = 2 mT. Simulation parameters: g values at g1 = 2.05, g2 = 1.83, g3 = 1.45, line width of W1 = 20.9 mT, W2 = 38.7 mT, W3 = 69.7 mT.

According to the 1H NMR studies, the sterically encumbered complex 2 as well as the U(IV) precursor 1 show enhanced air stability over several weeks. Recently, Evans and Marthur et al. employed bidentate heteroaryl-substituted, fluorinated alkenol ligands to prepare a new class of air-stable U(IV) compounds, also featuring an N4O4 coordination.23 It is therefore reasonable to assume that not only the sterics but also the electronic structure play a major role in the stabilization of the herereported complexes. To investigate the influence of varying steric demand on the complexes’ reactivity, the t-butyl groups of the cyclen-anchored aryloxides were changed to methyl groups. We speculated that this change ought to grant better access to the metal center in the axial position and, thus, should generate a more reactive molecule. The envisioned ligand derivative was synthesized in a Mannich reaction analogous to the procedure reported by Ogo and Nakai et al.13 Complexation was achieved with [UIII(N(SiMe3)2)3] in THF, as described above, and resulted in the pale green complex [((Me,MeArO)4cyclen)UIV] (3) in near-quantitative yield. The 1 H NMR spectroscopy and single-crystal X-ray diffraction analysis revealed the same symmetry, coordination polyhedron, and bond lengths found for 1 (Table 1). However, the molecular structure of complex 3 exhibits a significantly wider axial cavity at the uranium ion (Figure 3). Even though compound 3 exhibits less steric bulk in the axial position, compared to the t-butyl substituted analogue 1, the overall reactivity is comparably low. For example, reduction to

Figure 4. Solid-state molecular structure of [((Me,MeArO)4cyclen)UV][BPh4] (5) in crystals of 5·pyridine and the coordination polyhedron of the heteroatoms. Hydrogen atoms and cocrystallized, solvent molecules are omitted for clarity. Thermal ellipsoids are at the 50% probability level.

stable, black solid. To the best of our knowledge, this is the first report of a fully characterized (including CHN elemental analysis) air- and moisture-stable uranium(V) compound. The +5 oxidation state of 5 was confirmed by UV/vis and X-band EPR spectroscopy as well as SQUID magnetization measurements (see Supporting Information). Analogous to the entire series of new uranium complexes reported herein, the XRD analysis of 5 confirmed that the distorted square antiprismatic geometry of the tetragonal ligand environment is retained upon O2 oxidation (see Supporting Information). Noteworthy, no axial ligand (e.g., O2) is coordinated, and the complex remains

Table 1. Selected Structural Parameters structural parametera

1

3

4

5

d(U−O)av d(U−N)av Uoop shift

2.216(2) 2.778(2) −0.88

2.204(2) 2.733(2) −0.86

2.103(2) 2.671(3) −0.85

2.100(2) 2.669(3) −0.83

a

Distances in angstroms. 3203

DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206

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Inorganic Chemistry

radioactive uranium fluoride compounds (e.g., UF6), we areby regulation and restrictionsnot allowed to conduct elemental analyses on fluoride-containing uranium complexes. The reported yields refer to isolated, microcrystalline material (or material in powder form but not recrystallized, single-crystalline material), where purity was confirmed by NMR spectroscopy and/or elemental analysis (where applicable). Single-crystal X-ray structure data were collected using a Bruker Kappa Appex 2 IμS Duo diffractometer. Electrochemical experiments were performed using a three-electrode setup with a rotating glassy carbon electrode and platinum rods as counterand reference electrodes. The potentiostat was an Autolab type-III. EPR spectra were recorded on a JEOL CW spectrometer JESFA200 equipped with an X-band Gunn diode oscillator bridge, a cylindrical mode cavity, and a helium cryostat. The spectra were simulated with the program W95EPR.3 Caution! Depleted uranium (DU, primary isotope 238U) is radioactive (weak α-emitter, 4.197 MeV) with a half-life of 4.47 × 109 years. Exposure should be minimized, and incorporation must be prevented. Accordingly, manipulations and reactions must be performed with appropriate care in monitored f ume hoods or inert-atmosphere dryboxes. Synthesis of Tetrakis(3,5-di-t-butyl-2-hydroxybenzyl)1,4,7,10-tetraazacyclododecane (t‑Bu,t‑BuArOH)4cyclen. To a solution of 1,4,7,10-tetraazacyclododecane (0.230 g, 1.34 mmol) and Et3N (0.600 g, 5.90 mmol) in 60 mL of dichloromethane, 1bromomethyl-3,5-di-t-butylphenol (1.60 g, 5.36 mmol) was added dropwise over a period of 30 min at room temperature. After complete addition, the solvent was removed in vacuo, and the residue was received from Et2O to yield a colorless solution and a white precipitate that was filtered off. After the precipitate was washed with Et2O, the combined organic phases were evaporated in vacuo at 40 °C to obtain the product as a white solid. Yield: 93% (1.31 g, 1.25 mmol). 1H NMR (25 °C, 270 MHz, CDCl3): δ = 9.92 (br. s, 4H, Ar−OH), 7.20− 7.19 (d, 4JHH = 2.7 Hz, 4H, Ar-H), 6.72−6.71 (d, 4JHH = 2.7 Hz, 4H, Ar-H), 3.64 (s, 8H, CH2), 2.85 (s, 16H, CH2), 1.38 (s, 36H, t-Bu), 1.23 (s, 36H, t-Bu) ppm. 13C NMR (25 °C, 67.8 MHz, CDCl3): δ = 153.6 (Ar, 4 C), 140.9 (Ar, 4 C), 135.7 (Ar, 4 C), 123.7 (Ar, 4 C), 123.2 (Ar, 4 C), 120.8 (Ar, 4 C), 60.6 (CH2, 4 C), 50.5 (cyclen, 8 C), 34.8 (t-Bu, 8 C), 34.1 (t-Bu, 8 C), 31.7 (t-Bu, 24 C), 29.6 (t-Bu, 24 C) ppm. Anal. Calcd for C54H69O3N: C 83.14%; H 8.91%; N 1.80%. Found: C 83.34%; H 8.94%; N 1.65%. Synthesis of [((t‑Bu,t‑BuArO)4cyclen)UIV] (1). Method A. Tetrakis(3,5-di-t-butyl-2-hydroxybenzyl)-1,4,7,10-tetraazacyclododecane (25 mg, 0.024 mmol) was dissolved in THF, and a solution of potassium bis(trimethylsilyl)amide (21 mg, 0.11 mmol) in THF was added dropwise. This mixture was stirred for 2 h, then [(dioxane)2UIV(I4)] (22 mg, 0.024 mmol), dissolved in THF, was added dropwise, and the reaction was stirred overnight. The solvent of the nearly colorless suspension was removed, and the residue received with benzene and filtered over a diatomaceous earth pad. Evaporation of the filtrate and washing with n-pentane yielded a pale green solid. Yield: 20% (6.0 mg, 0.0047 mmol). 1H NMR (25 °C, 270 MHz, C6D6): δ = 54.39 (4H), 21.29 (4H), 17.21 (4H), 13.81 (4H), 13.59 (4H), 4.78 (36H), −6.81 (36H), −11.07 (4H), −15.01 (4H), −60.14 (4H) ppm. Method B: UIII[N(SiMe3)2)] (35 mg; 0.048 mmol) was dissolved in benzene and added dropwise to a solution of (t‑Bu, t‑BuArOH)4cyclen (50 mg; 0.048 mmol) in benzene. The reaction was allowed to proceed overnight. After evaporation of the solvent the solid was washed with Et2O and dissolved in toluene to filter the solution. Removal of the solvent in vacuo yielded the product as a pale green solid. The complex is soluble in toluene and slightly soluble in benzene and THF. Single crystals were obtained by THF diffusion into a concentrated toluene solution. Yield: 90% (55 mg, 0.043 mmol). 1H NMR (25 °C, 270 MHz, C6D6): δ = 54.47 (4H), 21.31 (4H), 17.22 (4H), 13.81 (4H), 13.61 (4H), 4.78 (36H), −6.80 (36H), −11.06 (4H), −15.04 (4H), −60.06 (4H) ppm. Anal. Calcd for C68H104O4N4U: C 63.83%; H 8.19%; N 4.38%. Found: C 63.78%; H 8.10%; N 4.52%. Synthesis of [((t‑Bu,t‑BuArO)4cyclen)UV][SbF6] (2). A 20 mL scintillation vial was charged with [((t‑Bu,t‑BuArO)4cyclen)UV] (24 mg, 0.019 mmol) dissolved in 2 mL toluene and AgSbF6 (8.0 mg, 0.023 mmol) was added and stirred for 1 h. Filtration over a diatomaceous

coordinatively unsaturated. As expected, and compared to their tetravalent precursor, the average U−O and U−N bond distances decrease from 2.204(2) and 2.733(2) Å in 3 to 2.103(2) and 2.671(3) Å in 4 and to 2.100(2) and 2.669(3) Å in 5. Overall, the metric parameters determined for the cyclenanchored tetrakis(aryloxide) complexes agree well with those found in the related tacn-based tris (aryloxide) uranium complexes.4

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CONCLUSION In summary, we report here the coordination of uranium ions in various oxidation states to newly developed N 4 O 4 octadentate ligand systems, resulting in extraordinarily stable, coordinatively unsaturated uranium complexes. The compounds exhibit remarkable air and water stability, varying only slightly depending on the steric demand of the aryloxide ligand substituents. The redox chemistry is restricted to the reversible redox event of the U(IV/V) couple. Oxidation reactions to U(VI)even with the strongest available oxidants such as tungsten(VI) hexachloride or ceric(IV) ammonium nitratewere not observed. We conclude that the newly developed cyclen-anchored tetrakis(aryloxide) ligand favors and stabilizes the U(IV) and U(V) oxidation states, as seen in the lack of reduction to U(III) and oxidation routes to highervalent U(VI) species. To the best of our knowledge, this is the first report of air- and water-stable uranium(V) compounds. The exceptional stability and redox chemistry may render these complexes suitable materials for applications in remediation technologies and redox-flow batteries.

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive experiments were performed under dry dinitrogen atmosphere, using standard Schlenk techniques or an MBraun inert-gas glovebox containing an atmosphere of purified dinitrogen. The glovebox is equipped with a−35 °C freezer. Solvents were purified using a twocolumn, solid-state purification system (Glass Contour System, Irvine, CA), transferred to the glovebox without exposure to air, and stored over molecular sieves and sodium (where appropriate). NMR solvents were obtained packaged under argon and stored over activated molecular sieves and sodium (where appropriate) prior to use. All reagents for synthesis of the organic ligands were obtained from commercial suppliers and were used without further purification. Precursor complexes [UIV(I)4(dioxane)2] and [UIII(HMDS)3] were prepared as described in literature.24 1H NMR spectra were recorded on a JEOL ECX 400 or 270 MHz instrument at a probe temperature of 25 °C. Chemical shifts, δ, are reported relative to residual 1H resonances of the solvent in ppm. Because of to the complexity of the spectra, these are shown as images. Electronic absorption spectra were recorded from 250 to 2200 nm (Shimadzu, UV-3600) in the indicated solvent at room temperature. IR vibrational spectra were recorded from 3500 to 400 cm−1 (Shimadzu, IRAffinity-1) as KBr pellets at room temperature. Magnetization data of microcrystalline and powdered samples (15−30 mg) were recorded with a SQUID magnetometer (Quantum Design) at different temperatures (2−300 K) at 1 T. Values of the magnetic susceptibility were corrected for the underlying diamagnetic increment by using tabulated Pascal constants and the effect of the blank sample holders (gelatin capsule/straw).25 Samples used for magnetization measurement were checked for chemical composition and purity by elemental analysis (C, H, and N) and 1H NMR spectroscopy. Data reproducibility was carefully checked on two independently synthesized and measured samples. Elemental analyses were obtained using Euro EA 3000 (Euro Vector) and EA 1108 (Carlo-Erba) elemental analyzers in the Chair of Inorganic and General Chemistry at Friedrich-Alexander University Erlangen− Nürnberg (FAU). Because of the possible formation of (volatile) 3204

DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206

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earth pad and evaporation of the filtrate yields the product as black solids. The complex is soluble in benzene and slightly soluble in toluene. Yield: 85% (24 mg, 0.016 mmol). 1H NMR (25 °C, 270 MHz, C6D6): δ = 27.47 (4H), 12.57 (4H), 11.02 (4H), 9.94 (4H), 3.91 (4H), 2.78 (36H), −2.80 (36H), −4.09 (4H), −22.31 (4H) ppm. Synthesis of [((Me,MeArO)4cyclen)UIV] (3). UIII[N(SiMe3)2)] (0.168 g; 0.233 mmol) was dissolved in THF and added dropwise to a solution of (Me,MeArOH)4cyclen (0.150 mg; 0.212 mmol) in THF. The reaction was allowed to proceed for 1 h. After evaporation of the solvent the solid was washed with hexane and dissolved in THF to filter the solution. Removal of the solvent in vacuo yielded the product as a pale green solid. The complex is soluble in THF and pyridine and slightly soluble in benzene. Single crystals were obtained by hexane diffusion into a concentrated THF solution. Yield: 96% (0.192 mg, 0.203 mmol). 1H NMR (25 °C, 270 MHz, C6D6): δ = 52.14 (4H), 24.10 (4H), 17.10 (4H), 15.16 (4H), 12.98 (4H), 6.24 (12H), −0.75 (12H), −10.83 (4H), −16.31 (4H), −63.43 (4H) ppm. Anal. Calcd for C44H56O4N4U: C 56.04%; H 5.99%; N 5.94%. Found: C 55.93%; H 5.66%; N 6.13%. Synthesis of [((Me,MeArO)4cyclen)UV][SbF6] (4). A 20 mL scintillation vial was charged with [((Me,MeArO)4cyclen)UV] (20 mg, 0.021 mmol) dissolved in 1 mL THF and AgSbF6 (8.0 mg, 0.023) was added and stirred for 1 h. Filtration over a diatomaceous earth pad and evaporation of the filtrate yields the product as black solids. The complex is soluble in THF and slightly soluble in benzene. Single crystals were obtained by hexane diffusion into a concentrated THF solution. Yield: 95% (24 mg, 0.020 mmol). 1H NMR (25 °C, 270 MHz, C6D6): δ = 24.23 (4H), 14.71 (4H), 12.31 (4H), 11.57 (4H), 9.79 (4H), 5.75 (12H), 2.78 (4H), −0.17 (12H), −4.81 (4H), −22.43 (4H) ppm. Synthesis of [((Me,MeArO)4cyclen)UV][BPh4] (5). [((Me,MeArO)4cyclen)UIV] (45 mg, 0.048 mmol) was dissolved in THF and the solution saturated with oxygen gas. Afterward, NaBPh4 (18 mg, 0.053 mmol) was added and the resulting mixture stirred for 3 days at room temperature. Subsequent filtration over a medium porosity frit and washing with THF yields the product as a black residue. The complex is only soluble in pyridine. Single crystals were obtained by diffusion of pentane into a concentrated pyridine solution. Yield: 71% (43 mg, 0.034 mmol). 1H NMR (25 °C, 270 MHz, C5D5N): δ = 17.27 (4H), 10.82 (4H), 10.71 (4H), 8.04 (8H), 7.29 (8H), 7.12 (4H), 6.76 (4H), 6.42 (4H), 5.93 (12H), 3.88 (12H), −2.77 (4H), −5.08 (4H), −12.32 (4H) ppm. Anal. Calcd for C73H81BN5O4U: C 65.37%; H 6.09%; N 5.22%. Found: C 65.37%; H 6.03%; N 5.04%.

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ACKNOWLEDGMENTS This work was supported by funds of the German Federal Ministry of Education and Research (BMBF 2020+ support codes 02NUK012C and 02NUK020C), the Joint DFG-ANR projects (ME1754/7-1, ANR-14-CE35-0004-01), as well as the FAU Erlangen-Nürnberg.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site at DOI: http://pubs.acs.org/doi/ abs/10.1021/acs.inorgchem.6b02123. 1 H NMR, UV−vis spectroscopy, SQUID magnetization measurements, X-ray crystallography, and cyclic voltammograms (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Julian Hümmer: 0000-0003-1126-782X Karsten Meyer: 0000-0002-7844-2998 Notes

The authors declare no competing financial interest. 3205

DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206

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DOI: 10.1021/acs.inorgchem.6b02123 Inorg. Chem. 2017, 56, 3201−3206