Synthesis, Structures, and Proton Self-Exchange Reaction of μ3

Crystallographic data for complexes Et3NH[1] and (Et3NH)2[2], atomic coordinates for optimized geometries of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3...
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Synthesis, Structures, and Proton Self-Exchange Reaction of μ3‑Oxido/Hydroxido Bridged Trinuclear Uranyl(VI) Complexes with Tridentate Schiff-Base Ligands Takashi Yoshimura,*,† Masayuki Nakaguchi,‡ and Keisuke Morimoto‡ †

Radioisotope Research Center, Osaka University, Suita 565-0871, Japan Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan

‡

S Supporting Information *

ABSTRACT: New μ3-hydroxido/oxido bridged trinuclear uranyl(VI) complexes with 3,5-di-t-butyl-N-salicylidene-2-aminophenolato (dbusap2−) ligands, Et3NH[(UO2)3(μ3-OH)(dbusap)3] (Et3NH[1]) and (Et3NH)2[(UO2)3(μ3-O)(dbusap)3] ((Et3NH)2[2]) were synthesized and characterized. Single-crystal X-ray structures of both complexes were determined. The oxygen atom on μ3-hydroxido center in [1]− is sp3 hybridized with an average U−(μ3-O)−U bond angle of 109.7(5)°; the μ3-oxido atom in [2]2− is sp2 hybridized with an average U−(μ3-O)−U bond angle of 118.0(10)°. U−(μ3-O) distances in [1]− are long (average of 2.43(1) Å) compared with those in [2]2− (average of 2.23(2) Å). The optimized geometries of the [(UO2)3(μ3-OH)]5+ core in [(UO2)3(μ3-OH)(sap)3]− and the [(UO2)3(μ3-O)]4+ core in [(UO2)3(μ3-O)(sap)3]2− (where sap = Nsalicylidene-2-aminophenolato) from density functional theory (DFT) calculations resemble those in [1]− and [2]2−, respectively. The U-(μ3-O) bond in [2]2− is significantly shorter than that in [1]−, because of the greater negative charge on the central μ3-oxido. A reversible structural conversion between [2]2− and [1]− was conducted by protonation and deprotonation of the μ3-oxido/hydroxido group. The activation enthalpy and entropy of the proton self-exchange reaction between [1]− and [2]2− determined from the temperature dependence of 1H NMR coalescence are ΔH⧧ = 23 ± 2 kJ mol−1 and ΔS⧧ = −77 ± 5 J K−1 mol−1.

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INTRODUCTION The chemical study of condensed uranyl(VI) complexes with UO22+ units is important for understanding the environmental behavior of uranium and for managing the disposal of nuclear waste.1−13 Monomeric UO22+ is stable in acidic solution (pH 3, as shown in eq 1.1−3,14−22

in such a trinuclear uranyl(VI) complex is expected to readily react with a proton. To our knowledge, protonation/ deprotonation reactions and associated structural changes have not been studied in μ3-oxido/hydroxido-bridged trinuclear uranyl(VI) complexes. Information on the protonation and structural conversion of [(UO2)3(μ3-O)]4+-type complexes is important for understanding fundamental processes in the environmental behavior of oligomeric uranyl(VI) complexes. In the present study, we focus on the protonation reaction of the central μ3-oxido in a trinuclear uranyl(VI) complex with a [(UO2)3(μ3-O)]4+ unit. To investigate this process, we have prepared a μ3-oxido bridged trinuclear uranyl(VI) complex with tridentate ligands, wherein μ2-phenolato units also bridge the uranyl(VI) ions. Thereby, protonation/deprotonation is limited to the central μ3-oxido/hydroxido site. According to the reported thermodynamic data of condensed uranyl(VI) species,1−3,16,17,20−22 various mononuclear, dinuclear, and trinuclear species coexist in weakly acidic aqueous solution. This suggests that small pH changes may lead to changes in the nuclearity of condensed uranyl(VI) species and promote the

m[UO2 ]2 + + nH 2O ⇄ [(UO2 )m (OH)n ]2m − n + nH+ (1) 2+

2+

Mononuclear [UO2] , dinuclear [(UO2)2(OH)2] , and trinuclear [(UO2)3(OH)5]+ and [(UO2)3(OH)4]2+ coexist at pH 3−5 in an aqueous solution.1,15,16,20−22 Based on spectroscopic data and density functional theory (DFT) calculations, [(UO2)3(OH)5]+ has been proposed to possess a triangular trinuclear uranyl(VI) structure with a μ3-oxido at the center of [(UO2)3(μ3-O)(μ2-OH)3(H2O)6]+.18,23−27 On the other hand, the structure of [(UO2)3(OH)4]2+ is undetermined. Evans proposed that [(UO2)3(OH)4]2+ exists as the hydroxidocentered triangularly arranged trinuclear uranyl(VI) complex, [(UO2)3(μ3-OH)(μ2-OH)3(H2O)6]2+.28 Recently, Zanonato et al. have also proposed the structure of [(UO2)3(OH)4]2+ to the μ3-hydroxido bridged trinuclear uranyl complex.22 The μ3-oxido © XXXX American Chemical Society

Received: January 4, 2017

A

DOI: 10.1021/acs.inorgchem.6b03175 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

dbusap), 7.63 (s, 3H, phenyl, dbusap), 7.50 (s, 3H, phenyl, dbusap), 7.32 (dd, 3H, phenyl, dbusap), 6.92 (dd, 3H, phenyl, dbusap), 1.92 (br, 12H, Et3NH+), 1.61 (s, 27H, Bu, dbusap), 1.33 (s, 27H, Bu, dbusap), 0.35 (br, 18H, Et3NH+); in DMSO-d6/ppm: 9.60 (s, 3H, phenyl, dbusap), 7.66 (d, 3H, phenyl, dbusap), 7.60 (s, 3H, phenyl, dbusap), 7.59 (s, 3H, phenyl, dbusap), 7.21 (dd, 3H, phenyl, dbusap), 6.82 (d, 3H, phenyl, dbusap), 6.63 (dd, 3H, phenyl, dbusap), 1.74 (s, 27H, Bu, dbusap), 1.34 (s, 27H, Bu, dbusap), 0.96 (t, 18H, Et3NH+). UV-vis in CH2Cl2, λmax/nm (ε/M−1 cm−1): 518 (sh, 2.7 × 103), 424 (sh, 1.8 × 104), 376 (2.9 × 104), 287 (3.4 × 104); in CHCl3: 517 (sh, 3.0 × 103), 424 (sh, 1.8 × 104), 376 (3.0 × 104), 287 (3.4 × 104); in DMSO: 530 (sh, 3.7 × 103), 426 (sh, 2.0 × 104), 393 (3.4 × 104), 292 (3.2 × 104). IR (KBr pellet)/cm−1: 1608 (νCN), 895 (νOUO). Anal. Calcd for C75H107N5O13U3·2H2O: C, 44.23; H, 5.50; N, 3.44; Found: C, 44.29; H, 5.45; N, 3.57. Method 2. Triethylamine (1.4 mg, 0.014 mmol) in 1 mL of THF was added to a THF solution (0.5 mL) containing Et3NH[1] (30 mg, 0.015 mmol). The mixture was stirred for 10 min, and hexane (3.5 mL) was layered on the solution. The solution was left for several days to yield a solid, which was collected by filtration and dried under vacuum. Yield: 12 mg (40%). 1H NMR in CDCl3/ppm: 9.69 (s, 3H, phenyl, dbusap), 8.41 (d, 3H, phenyl, dbusap), 7.64 (d, 3H, phenyl, dbusap), 7.63 (s, 3H, phenyl, dbusap), 7.50 (s, 3H, phenyl, dbusap), 7.32 (dd, 3H, phenyl, dbusap), 6.92 (dd, 3H, phenyl, dbusap), 1.89 (br, 12H, Et3NH+), 1.61 (s, 27H, Bu, dbusap), 1.33 (s, 27H, Bu, dbusap), 0.32 (br, 18H, Et3NH+). Anal. Calcd for C75H107N5O13U3: C, 45.00; H, 5.39; N, 3.50; Found: C, 45.28; H, 5.15; N, 3.63. X-ray Crystallography. Single-crystal X-ray data were collected at −103 °C on a Rigaku RAXIS diffractometer with graphitemonochromated Mo Kα radiation. Cell parameters were retrieved using CrystalClear Software. Diffraction data were collected and processed using CrystalClear. Crystal structures were solved by the Patterson method (DIRDIF99-PATTY) or Direct method (SIR 92).30 Atomic coordinates and thermal parameters of non-hydrogen atoms were calculated by a full-matrix least-squares method (SHELXL-97 or SHELXL-2013).31 Calculations were performed using Crystal Structure 4.1.32 Crystallographic data are summarized in Table S1 in the Supporting Information. Physical Measurements. 1H NMR spectra were recorded on a JEOL Model ECS 500 MHz spectrometer. All peaks were referenced to the proton signal of Si(CH3)4 at δ = 0.00 in CDCl3 and DMSO-d6. Temperature was controlled using the probe cooler and heater. UV-vis spectra were measured by a JASCO Model V-550 spectrophotometer. Infrared (IR) spectra were recorded in KBr pellets on a JASCO Model FTIR-4100 spectrometer. Elemental analyses were performed using an Elementar vario MICRO cube. The IR and 1H NMR spectra of Et3NH[1] and (Et3NH)2[2] are exhibited in Figures S1−S3 in the Supporting Information. DFT Calculations. DFT calculations were performed with the Amsterdam Density Functional (ADF 2016) package.33,34 For geometry optimizations and calculation of frequencies, the initial geometries of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3-O)(sap)3]2− were taken from the X-ray structures with H substituted for the t-Bu moieties. Structural optimizations were carried out at the GGA level using PBE, the scalar relativistic ZORA, and the DZ for all atoms with a large frozen core approximation as provided in the ADF basis set library.35 Calculation of all positive frequencies confirmed the existence of the potential minimum. The atomic coordinates of the optimized geometries are listed in Tables S2 and S3 in the Supporting Information. The single point DFT calculation of [(UO2)3(μ3O)(sap)3]2− was performed using the optimized geometry at B3LYP, the scalar relativistic ZORA, the TZ2P for U and aug-DZP for O, N, C, H, and the COSMO solvent model36 for CHCl3, as provided in the ADF basis set library.

decomposition of oligomeric forms by coordination of water and/or hydroxide molecules. To suppress such decomposition, we have used non- and weak-coordinating solvents to investigate μ3-oxido/hydroxido protonation and deprotonation in oligomeric uranyl(VI) complexes. Accordingly, we have synthesized the μ3-hydroxido trinuclear uranyl(VI) complex, [(UO2)3(μ3-OH)(dbusap)3]− ([1]−) (dbusap2− = 3,5-di-tbutyl-N-salicylidene-2-aminophenolato) and its deprotonated μ3-oxido complex, [(UO2)3(μ3-O)(dbusap)3]2− ([2]2−). Interestingly, the U−(μ3-O) bond distances and U−(μ3-O)−U bond angles differ significantly between [1]− and [2]2−. However, the structure of the trinuclear U(VI) core is retained upon protonation and deprotonation of the central μ3-oxido/ hydroxido site. The kinetics of the proton self-exchange reaction between [1]− and [2]2− also are investigated.

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EXPERIMENTAL SECTION

Materials. Caution: Uranium is a radioactive material! Uranyl nitrate hexahydrate (238UO2(NO3)2·6H2O) was used in this study. All commercially available reagents were used as received. N-(2hydroxyphenyl)-3,5-di-t-butylsalicylaldimine (dbusapH2) was prepared according to a literature procedure.29 Et3NH[(UO2)3(μ3-OH)(dbusap)3] (Et3NH[1]). Method 1. Triethylamine (125 mg, 1.24 mmol) and dbusapH2 (200 mg, 0.62 mmol) were mixed in THF (3 mL), and the resulting solution was added to UO2(NO3)2·6H2O (308 mg, 0.61 mmol). The mixture was stirred for 10 min, followed by the addition of a 30 mL hexane layer. The solution was left for several days, which resulted in the formation of a brown solid that was collected by filtration. The solid was recrystallized from CH2Cl2/hexane to give brown crystals, which were collected by filtration and dried under vacuum. Yield: 304 mg (78%). 1H NMR in CDCl3/ppm: 12.50 (br, 1H, μ3-OH), 9.80 (s, 3H, phenyl, dbusap), 8.37 (d, 3H, phenyl, dbusap), 7.71 (d, 3H, phenyl, dbusap), 7.62 (s, 3H, phenyl, dbusap), 7.55 (dd, 3H, phenyl, dbusap), 7.44 (s, 3H, phenyl, dbusap), 7.11 (dd, 3H, phenyl, dbusap), 1.49 (s, 27H, Bu, dbusap), 1.27 (s, 27H, Bu, dbusap), ca. 0 (br, Et3NH+). 1H NMR in DMSO-d6/ppm: 9.60 (s, 3H, phenyl, dbusap), 7.66 (d, 3H, phenyl, dbusap), 7.60 (s, 3H, phenyl, dbusap), 7.59 (s, 3H, phenyl, dbusap), 7.21 (dd, 3H, phenyl, dbusap), 6.82 (d, 3H, phenyl, dbusap), 6.63 (dd, 3H, phenyl, dbusap), 2.63 (br, 6H, Et3NH+), 1.74 (s, 27H, Bu, dbusap), 1.34 (s, 27H, Bu, dbusap), 1.00 (t, 9H, Et3NH+). UV-vis in CH2Cl2, λmax/nm (ε/M−1 cm−1): 523 (sh, 2.8 × 103), 424 (sh, 1.6 × 104), 374 (3.1 × 104), 294 (2.8 × 104); in CHCl3: 517 (sh, 3.3 × 103), 424 (sh, 1.7 × 104), 375 (3.1 × 104), 293 (2.9 × 104); in DMSO: 530 (sh, 3.7 × 103), 426 (sh, 2.0 × 104), 393 (3.4 × 104), 292 (3.2 × 104). IR (KBr pellet)/cm−1: 3620 (νO−H), 1607 (νCN), 918 (νOUO). Anal. Calcd for C69H92N4O13U3·0.25CH2Cl2: C, 43.28; H, 4.86; N, 2.92. Found: C, 43.03; H, 5.04; N, 3.29. Method 2. (Et3NH)2[2] (26 mg, 0.013 mmol) was added to 1 mL of THF containing p-toluenesulfonic acid monohydrate (2.4 mg, 0.013 mmol). The mixture was stirred for 10 min, and hexane (3 mL) was layered on the solution. The solution was left for several days to yield a brown solid, which was collected by filtration and dried under vacuum. Yield: 12 mg (48%). 1H NMR in CDCl3/ppm: 12.54 (br, 1H, μ3-OH), 9.80 (s, 3H, phenyl, dbusap), 8.37 (d, 3H, phenyl, dbusap), 7.71 (d, 3H, phenyl, dbusap), 7.62 (s, 3H, phenyl, dbusap), 7.55 (dd, 3H, phenyl, dbusap), 7.45 (s, 3H, phenyl, dbusap), 7.11 (dd, 3H, phenyl, dbusap), 1.49 (s, 27H, Bu, dbusap), 1.27 (s, 27H, Bu, dbusap), ca. 0 (br, Et3NH+). Anal. Calcd for C69H92N4O13U3·C4H8O: C, 46.14; H, 5.31; N, 2.95. Found: C, 46.27; H, 5.12; N, 3.16. (Et3NH)2[(UO2)3O(dbusap)3] ((Et3NH)2[2]). Method 1. A mixture of triethylamine (47 mg, 0.46 mmol) and dbusapH2 (50 mg, 0.15 mmol) in THF (3 mL) was added to UO2(NO3)2·6H2O (77 mg, 0.15 mmol). The solution was stirred for 10 min, followed by the addition of a 15 mL layer of hexane. The solution was left for several days, resulting in the formation of red crystals, which were collected by filtration. Yield: 94 mg (92%). 1H NMR in CDCl3/ppm: 9.69 (s, 3H, phenyl, dbusap), 8.41 (d, 3H, phenyl, dbusap), 7.64 (d, 3H, phenyl,

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RESULTS AND DISCUSSION Synthesis and Characterization of Et3NH[(UO2)3(μ3OH)(dbusap)3] (Et3NH[1]) and (Et3NH)2[(UO2)3(μ3-O)(dbusap)3] ((Et3NH)2[2]). Treatment of UO2(NO3)2·6H2O B

DOI: 10.1021/acs.inorgchem.6b03175 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry with a 1:2 solution of dbusapH2 and Et3N in THF at room temperature gave a red-brown solution. A brown solid was obtained following the addition of hexane. Brown crystalline Et3NH[(UO2)3(μ3-OH)(dbusap)3] (Et3NH[1]) was obtained in good yield upon recrystallization from CH2Cl2/hexane. The μ3-hydroxido located at the center of the trinuclear uranyl(VI) complex presumably originates from the water molecule of UO2(NO3)2·6H2O, which become deprotonated upon formation of the complex. [1]− also was produced by protonation of the μ3-O center of [(UO2)3(μ3-O)(dbusap)3]2− ([2]2−) with p-toluenesulfonic acid. The IR spectrum of Et3NH[1] shows a ν3(OUO) band at 918 cm−1. The phenolic OH stretching bands of dbusapH2 disappear upon coordination. The νasym(CN) stretch of the azomethine group (1607 cm−1) in [1]− shifts to lower frequency, relative to that of the free ligand (1614 cm−1) upon coordination to U(VI). A shift to lower frequency typically is observed upon coordination of Schiff-base ligands to metal ions.29,37,38 A new band at 3620 cm−1 is assigned to the μ3-OH stretching band of the central hydroxido in the trinuclear complex. The band is not observed in (Et3NH)2[2], and the frequency of OH stretching calculated by DFT calculation is similar to the observed value, as described later. The 1H NMR chemical shifts of dbusap2− for [1]− in CDCl3 were observed at 0−10 ppm, indicating that [1]− is a diamagnetic complex. The phenolic proton signals of dbusapH2 at 5.88 and 12.69 ppm disappear in the 1H NMR spectrum of [1]− in CDCl3. The proton signal of −CHN− at 9.80 ppm shifts downfield by ca. 1 ppm compared to the value (8.71 ppm) for the free ligand. Thus, dbusap2− coordinates to the uranyl(VI) ion as an O−N−O tridentate ligand. The proton resonance of μ3-OH was observed at 12.5 ppm, and the integrated signal intensity ratio of 1:3 scales with the molar ratio of μ3-OH and dbusap2−. Observation of a single set of dbusap2− signals indicates that [1]− has C3 symmetry in CDCl3. The broadened ethyl resonances of Et3NH+ arise from the dynamics of intermolecular interaction between [1]− and Et3NH+. The ethyl signals of Et3NH+ sharpen in DMSO-d6, because the intermolecular interaction between [1]− and Et3NH+ is weaker in more polar DMSO. The integrated Et3NH+ to dbusap2− signal intensity ratio is 1:3 in DMSO-d6, which agrees with the result of single-crystal X-ray analysis. The μ3-oxido trinuclear complex, (Et3NH)2[(UO2)3(μ3O)(dbusap)3] ((Et3NH)2[2]), was obtained in high yield by reaction of UO2(NO3)2·6H2O with a molar equivalent of dbusapH2 and the excess amount of Et3N in THF at room temperature. In the synthesis of [2]2−, 2 equiv of Et3N per uranyl serve to deprotonate dbusapH2, and the remaining Et3N prevents protonation of the μ3-bridging O atom. This supports the conversion of [1]− to [2]2− observed by the reaction of [1]− with Et3N. (See Scheme 1.) The UV-vis absorption bands at 523 and 374 nm decrease and that at 423 nm increases upon the addition of Et3N with isosbestic points at 510, 397, and 330 nm in CH2Cl2 as illustrated in Figure 1. The spectrum after the addition of 1 equiv of Et3N to [1]− is consistent with that of [2]2−. The conversion was also monitored by the 1H NMR spectral measurement, as shown in Figure S4 in the Supporting Information. The signals of [1]− decrease and new signals whose chemical shift values corresponds to those of [2]2− increase upon addition of Et3N with broadening of the signals. Such broadening of the signals is due to the proton exchange reaction of [1]− and [2]2−, as described later. After the addition of 1 equiv of Et3N to [1]−, the signals of [1]− disappear and

Scheme 1

Figure 1. UV-vis absorption spectra for the mixture of [1]− (0.021 mM) and various concentrations of Et3N (0 (black trace), 0.011 mM (red trace), 0.016 mM (blue trace), and 0.021 mM (magenta trace)) in CH2Cl2.

change to those of [2]2−. These results suggest that the conversion of [1]− to [2]2− involves simple deprotonation of μ3-hydroxido in the trinuclear uranyl(VI) complex. The IR spectrum of (Et3NH)2[2] shows the OUO stretching band at 895 cm−1. The coordination of dbusap2− to uranyl(VI) was confirmed by the shift of the azomethine νasym(CN) frequency from 1614 cm−1 in the free ligand to 1608 cm−1 in the complex and the disappearance of the phenolic OH stretching band. The 1H NMR spectrum in CDCl3 exhibits signals for coordinated dbusap2− and Et3NH+. The dbusap2− chemical shifts differ from those in [1]−. Observation of signals at 0−10 ppm indicates that the compound is diamagnetic. The spectrum of (Et3NH)2[2] shows a single set of the resonances for dbusap2−, suggesting that [2]2− has C3 symmetry. The integrated Et3NH+ to dbusap2− signal intensity ratio is 2:3, which is consistent with the result of the single-crystal X-ray analysis. The UV-vis spectra of Et3NH[1] and (Et3NH)2[2] in CH2Cl2 and CHCl3 are exhibited in Figures S5 and S8 in the Supporting Information. The absorption band intensities increase linearly with increasing complex concentration in both solvents, as shown in Figures S6 and S7 and Figures S9 and S10 in the Supporting Information. The spectral features of [1]− and [2]2− are similar, but their spectra are distinct. This suggests that both complexes are structurally stable in CH2Cl2 and CHCl3. The bands at ca. 400 and 520 nm are assigned to C

DOI: 10.1021/acs.inorgchem.6b03175 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

in the X-ray structural analysis. The μ3 oxygen atom is greatly displaced from the mean plane defined by the three U atoms (0.802(9) Å), and the U3(μ3-OH) moiety composes a triangular pyramid. The U−(μ 3 -O) bond lengths are 2.417(9)−2.436(8) Å (average of 2.43(1) Å). The distances are similar to those in the reported μ3-OH bridged trinuclear uranyl(VI) complex.49 The U···U distances are 3.9546(8)− 3.9808(9) Å (average of 3.971(2) Å). The distances between uranium and nonbridging phenoxido (Onb), U1−O5, U2−O9, and U3−O13, are 2.190(9)−2.216(11) Å (average of 2.20(2) Å), which are similar to the U−Onb distances in the reported uranyl(VI) complexes with Schiff-base ligands.50−52 The bond distances between uranium and the bridging phenoxido (Ob) of 2.426(8)−2.466(10) Å (average of 2.45(2) Å), are longer than the U−Onb in [1]−. A similar trend is observed in dinuclear uranyl(VI) complexes with salophen ligands.53 The triethylammonium ion participates in intermolecular hydrogen bond interactions (2.96(2)−2.97(2) Å) with the O atoms of UO22+, as shown in Figure S14. Single crystals of (Et3NH)2[2] were obtained from THF/ hexane. X-ray analysis shows that four triethylammonium cations, two complex anions, and solvent molecules are contained in the asymmetric unit, indicating that the net charge of the complex anion is −2. Two complex anions with almost identical structure are included in the asymmetric unit. The molecular structure of [2]2− is shown in Figure 3. The selected bond distances and angles in [2]2− are summarized in Table 1, as well as Figure S15. The geometry around the U atoms and two triethylammonium cations are illustrated in Figure S16 in the Supporting Information. The X-ray structure analysis shows that [2]2− contains three UO2(dbusap) units and an O atom. Each tridentate dbusap2− coordinates to equatorial positions on uranium with one phenolato moiety bridging to a neighboring U atom. An O atom bridges three U atoms at the center of the molecule. The U atoms exhibit distorted pentagonal bipyramidal geometry with a N atom and four O atoms at the equatorial positions. The overall X-ray structure of [2]2− is isostructural with that of [1]−, but the bond distances and angles around the μ3-O atom differ significantly. The U−(μ3-O)−U bond angles (116.6(4)°− 119.2(4)°, average of 118.0(10)°) are close to 120°, and the μ3O atom displacement from the U3 mean plane (0.25(1) and 0.38(1) Å) is small, indicating that the μ3-O in [2]2− is sp2hybridized. The U−(μ3-O)−U bond angles are similar to those of μ3-oxido bridged trinuclear uranyl(VI) complexes reported in the literature.23,43−48 The fact that the U−(μ3-O) distances (2.184(10)−2.274(8) Å, average of 2.23(2) Å) are much shorter than those in [1]− supports the oxido character of the μ3-O atom, because a greater negative charge creates a stronger electrostatic interaction with the U atoms. Hydrogen bonding interactions involving the μ3-oxido are not observed in [2]2−. The UO (1.711(13)−1.792(11) Å, average of 1.76(4) Å) and U−N (2.513(14)−2.579(13) Å, average of 2.54(3) Å) distances and OUO angles (174.6(5)°−179.2(5)°, average of 176.5(12)°) are similar to those in [1]−. The U−Onb distances (2.248(10) − 2.294(9) Å, average of 2.27(2) Å) are longer than those in [1]− (2.190(9)−2.216(11) Å, average of 2.20(2) Å). Because the Onb atoms are located approximately trans to μ3-oxido, the U−Onb bonds are lengthened by the trans influence and significant shortening of the U−(μ3−O) bond. The U···U distances (3.7965(9)−3.8771(8) Å, average of 3.827(2) Å) are shorter by ca. 0.1 Å than those in [1]−. The bond distances and angles around the U atoms resemble those

π−π* in the ligand and to LMCT transitions, respectively.39 The UV-vis spectra of Et3NH[1] and (Et3NH)2[2] in DMSO are shown in Figure S11 in the Supporting Information. Absorbance increases linearly with increasing concentration for both complexes in DMSO (Figures S12 and S13 in the Supporting Information). However, the spectra are identical at all wavelengths measured, and the 1H NMR chemical shifts of [1]− and [2]2− in DMSO-d6 are the same. The phenoxidobridged dinuclear uranyl(VI) complex decomposes in DMSO to the mononuclear species containing coordinated DMSO.40 Therefore, our trinuclear uranyl(VI) complexes might decompose in DMSO to produce a mononuclear complex. Crystal Structures of [(UO2)3(μ3-OH)(dbusap)3]− ([1]−) and [(UO2)3(μ3-O)(dbusap)3]2− ([2]2−). Figure 2 shows the

Figure 2. Molecular structure of the complex anion and numbering scheme for Et3NH[(UO2)3(μ3-OH)(dbusap)3] (Et3NH[1]). Hydrogen atoms are omitted for clarity.

single-crystal X-ray structure of the complex anion, [1]−. Figure S14 in the Supporting Information shows the geometry around the U atoms and includes a triethylammonium cation and a dichloromethane molecule. Table 1 and Figure S15 in the Supporting Information summarizes the selected bond distances and angles in [1]−. The X-ray analysis shows that one triethylammonium cation, one complex anion, and solvent molecules are contained in the asymmetric unit, indicating that the net charge of the complex anion is −1. The complex anion is composed of three UO2(dbusap) units and an O atom. The U atoms in [1]− are triangularly arranged with the μ3-oxygen atom bridging the three U atoms by coordination at equatorial positions on the uranyls. The OUO angles and UO bond distances are 178.2(4)−179.4(5)° (average of 178.9(8)°) and 1.751(9)−1.802(9) Å (average of 1.78(2) Å), respectively, and fall within the range reported in the literature.41,42 Each tridentate dbusap2− ligand coordinates to equatorial positions on uranium with one phenolato moiety bridging to a neighboring U atom in μ2-fashion. A N atom and four O atoms occupy the equatorial positions about each U atom, forming a distorted pentagonal bipyramidal geometry. The U− (μ3-O)−U bond angles in [1]− (average of 109.7(5)°) are much smaller than those in [2]2− (average of 118.0(10)°) and in the μ3-oxido bridged trinuclear uranyl(VI) complexes reported (114.8°−120.56°).23,43−48 We assigned that the μ3oxygen at the center of the molecule is an sp3 hybridized hydroxido. This is supported by the presence of a hydrogen bond between the μ3-OH and the Cl atom of dichloromethane (O···Cl = 3.25(1) Å), as shown in Figure S14, although the hydrogen atom on μ3-OH was not found by Fourier synthesis D

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Table 1. Selected Bond Distances and Angles of Et3NH [1] and (Et3NH)2 [2], and Calculated Bond Distances and Angles of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3-O)(sap)3]2− Observed Et3NH[1]

Calculated −

(Et3NH)2[2]

[(UO2)3(μ3-OH)(sap)3]

[(UO2)3(μ3-O)(sap)3]2−

a

a

U−(μ3-O)

2.417(9)−2.436(8) avg 2.43(1)

Bond Distances (Å) 2.184(10)−2.274(8) avg 2.23(2)

2.423−2.427 avg 2.435

2.247−2.248 avg 2.247

UO

1.751(9)−1.802(9) avg 1.78(2)

1.711(13)−1.792(11) avg 1.76(4)

1.823−1.836 avg 1.830

1.836−1.838 avg 1.837

U−Ob

2.426(8)−2.466(10) avg 2.45(2)

2.401(10)−2.503(12) avg 2.44(4)

2.443−2.494 avg 2.468

2.462−2.499 avg 2.476

U−Onb

2.190(9)−2.216(11) avg 2.20(2)

2.248(10)−2.294(9) avg 2.27(2)

2.232−2.235 avg 2.234

2.307−2.310 avg 2.309

U−N

2.524(12)−2.535(11) avg 2.53(2)

2.513(14)−2.579(13) avg 2.54(3)

2.549−2.553 avg 2.551

2.594−2.600 avg 2.598

U···U

3.9546(8)−3.9808(9) avg 3.971(2)

3.7965(9)−3.8771(8) avg 3.827(2) Bond Angles (deg) 116.6(4)−119.2(4) avg 118.0(10)

4.011−4.020 avg 4.015

3.890−3.895 avg 3.891

111.7−112.0 avg 111.8

119.9−120.2 avg 120.0

177.5−177.8 avg 177.5

171.79−171.91 avg 171.9

U−(μ3-O)−U

109.2(3)−110.1(3) avg 109.7(5)

OUO

178.2(4)−179.4(5) avg 178.9(8)

174.6(5)−179.2(5) avg 176.5(12)

Ob: bridged oxygen atom, Onb: nonbridged oxygen atom.

listed in Tables S2 and S3, respectively, in the Supporting Information. The calculated bond distances around the U atoms in [(UO2)3(μ3-OH)(sap)3]− are similar to the values obtained by the crystal structure analysis of [1]−. The calculated angles for U−(μ3-O)−U (111.7°−112.0°, average of 111.8°) are similar those in the X-ray structure (109.2(3)°−110.1(3)°, average of 109.7(5)°), and the values are consistent with sp3 hybridization. The calculated ν3(OUO) band is observed at 917 cm−1, which is similar to the observed value of 918 cm−1 for [1]−. The calculated frequency of μ3−OH stretching in [(UO2)3(μ3OH)(sap)3]− is 3605 cm−1, which is in good agreement with the observed frequency of 3620 cm−1. These results support μ3hydroxido coordination to three UO2(dbusap) units in [1]−. In the geometrically optimized structure of [(UO2)3(μ3O)(sap)3]2−, the U−(μ3-O) and U···U distances are 2.247− 2.248 Å (average of 2.247 Å) and 3.890−3.895 Å (average of 3.891 Å), respectively. These distances are similar to the experimental values of [2]2− and are ca. 0.1 Å shorter than the relevant distances in [(UO2)3(μ3-OH)(sap)3]−. [U−(μ3-O) = 2.423−2.427 Å (average of 2.435 Å), U···U = 4.011−4.020 Å (average of 4.015 Å).] The calculated U−(μ3-O)−U bond angles (119.9°−120.2°, average of 120.0°) suggest that the central μ3-O is sp2-hybridized. The calculated UO, U−Ob, U−Onb, and U−N distances are similar to their corresponding values in the X-ray structure of [2]2−. The bond distance and bond angle differences between [1]− and [2]2− are dependent on whether the complex contains a μ3-hydroxido or μ3-oxido center. A triangular μ3-oxido bridged array is well-known in carboxylate bridged trinuclear d-block metal complexes.57 In the RuIIIRuIII2(μ3-O) complexes, the pz orbital of the μ3-oxido conjugates with the d-orbitals of three Ru ions to give a cluster-

Figure 3. Molecular structure of the complex anion and numbering scheme for (Et3NH)2[(UO2)3(μ3-O)(dbusap)3] ((Et3NH)2[2]). Hydrogen atoms are omitted for clarity.

observed in previously reported the μ3-oxido bridged trinuclear uranyl(VI) complexes.9,23,43−48,54−56 The triethylammonium cations are hydrogen-bonded at 2.77(2)−2.86(2) Å to the O atoms of UO22+, as illustrated in Figure S16. DFT Calculations. To simplify computation, N-(2phenolato)salicylaldiminato (sap2−) was substituted for dbusap2− for geometry optimization and electronic structure calculations. The geometry calculations of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3-O)(sap)3]2− were performed using DFT. Table 1 contains the calculated bond distances and angles of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3-O)(sap)3]2−, together with the observed distances and angles in [1]− and [2]2−. The atomic coordinates of optimized geometries for [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3-O)(sap)3]2− are E

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Inorganic Chemistry based π-molecular orbital system.57,58 Thus, it is of interest to know if a similar π-bonding motif exists for μ3-oxido and uranyl(VI). A single point DFT calculation was performed on [(UO2)3(μ3-O)(sap)3]2−. Figure 4 shows the contour plot of

spectra at various temperatures; the right-hand side contains the computer-simulated best-fit spectra. A computer program based on the modified Bloch equation for two-site exchange59 was used for the simulation. The chemical shifts and half-widths at each temperature were taken from the spectra of the individual species. The second-order rate constants obtained by computer simulation are listed in Table 2. As shown in Figure Table 2. Second-Order Rate Constants for the Proton SelfExchange Reaction between Et3NH[1] and (Et3NH)2[2] in CDCl3 temperature, T (K)

rate constant, k (× 104 M−1 s−1)

253.3 263.3 273.3 283.3 293.3 303.3 313.3 323.3

0.93 1.4 2.0 2.7 3.9 5.8 9.0 14

S17 in the Supporting Information, the Eyring plot results in a good straight line, from which the activation parameters were estimated to be ΔH⧧ = 23 ± 2 kJ mol−1 and ΔS⧧ = −77 ± 5 J K−1 mol−1. The second-order rate constant at 298 K based on these parameters is 5.4 × 104 M−1 s−1. Proton transfer in organic compounds is fast, because the O atom retains sp 3 hybridization upon protonation and deprotonation.60,61 However, the bridging oxido ligand in dblock metal ion complexes interacts with the metal ions via πbonding.61 Protonation of the oxido ligand is accompanied by a change from sp2 to sp3 hybridization and cleavage of the πbonding interaction. Thus, protonation of the bridged oxido ligand is slow, because of a large kinetic barrier.61 The range of second-order proton transfer rate constants in oxido bridged dinuclear manganese(III) complexes is 1920−11200 M−1 s−1.61 The activation parameters for protonation of [(6methylbispicen)Mn(μ2-O)]22+ to [(6-methylbispicen)Mn(μ2O)(μ2-OH)Mn(6-methylbispicen)]3+ (k = 5440 M−1 s−1 at 298 K) are ΔH⧧ = 67 kJ mol−1 and ΔS⧧ ≈ 0 J K−1 mol−1.61 The large activation enthalpy is responsible for the slow rate of reaction.61 Because the protonation rate constant of the trinuclear uranyl(VI) complex in this study is marginally larger than those of the manganese complexes at 298 K, we assigned its exchange reaction to the “slow proton transfer” regime. The activation enthalpy of protonation in the uranyl complex is significantly smaller than that in the manganese complex. As shown by single-crystal X-ray analysis, significant bond distance and bond angle differences exist between [1]− and [2]2−. The small activation enthalpy of the proton self-exchange reaction suggests that the structural reorganization energy of conversion between [1]− and [2]2− is small. A small reorganization energy may arise from the absence of π-bonding between the μ3-oxido and uranium, as described in the section entitled “DFT Calculations”. Hybridization of the O atom on the μ3-oxido/ hydroxido center can readily change between sp2 and sp3 in the case of weak or no π interaction between the μ3-oxido and metal ions. The negative activation entropy value may be due to formation of a pair of the complex anions at the transition state. The proton exchange reaction is assigned to an interchange mechanism, because the small positive activation enthalpy and the negative activation entropy were observed. The observation

Figure 4. A contour plot of HOMO−9 for [(UO2)3(μ3-O)(sap)3]2−. The isodensity surfaces are drawn at the 0.05 a.u. level.

HOMO-9. The pz orbital of the μ3-oxido makes a large contribution to HOMO-9, whereas the f-orbital contribution is small in this molecular orbital. This suggests that the pz orbital electrons of the μ3-oxido exist as a lone pair. Therefore, the significant shortening of the U−(μ3-O) bond in [2]2−, compared with [1]−, is due to the increase in negative charge upon conversion of μ3-OH− to μ3-O2−. The addition or elimination of a proton at the central μ3-O atom of the triangular triuranyl(VI) complex is primarily responsible for its change between sp3 and sp2 hybridization and the structural change of the U3(μ3-O) moiety between triangular pyramidal and planar shapes. Proton Self-Exchange Reaction Kinetics. The kinetics of proton self-exchange at the μ3-oxido/hydroxido center on [1]− and [2]2− was investigated by 1H NMR line broadening. When [1]− and [2]2− are mixed in CDCl3, the separate signals from the two complexes at low temperature begin to coalesce as the temperature is increased. The proton exchange rate constant is evaluated by analyzing the temperature dependence of this behavior. Figure 5 shows the temperature dependence of the tbutyl signals of [1]− and [2]2−. The left-hand side contains the

Figure 5. Variable-temperature 1H NMR spectra of 1.0 mM Et3NH[1] and 1.1 mM (Et3NH)2[2] in CDCl3 (at 253 K (black spectrum), 273 K (red spectrum), 283 K (blue spectrum), 293 K (magenta spectrum), 303 K (green spectrum), 323 K (deep blue spectrum)). Computersimulated spectra are shown on the right. F

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of isosbestic points in the UV-vis spectra during deprotonation of [1]− to [2]2− and the absence of intermediates in the 1H NMR line broadening experiment support this mechanism. The activation enthalpy is comparable to the energy required to dissociate an O−H···O hydrogen bond.62 This suggests that the distance between the central O atom and the proton approaches the length of a hydrogen bond at the transition state.

CONCLUSION μ3-Oxido and μ3-hydroxido bridged trinuclear uranyl(VI) complexes with tridentate dbusap2− ligands were produced selectively in high yield by changing in the proportion of triethylamine in the synthesis. Because of steric hindrance from the t-butyl substituents on one phenol ring in dbusap2−, the phenolato moiety without t-butyl groups selectively bridges uranyl(VI) ions to give a triangular trinuclear uranyl(VI) complex. [1]− and [2]2− interconvert upon protonation/ deprotonation of the μ3-oxido/hydroxido. Trinuclear geometry is retained in [1]− and [2]2−, despite significant changes in bond distances and angles resulting from this reaction. Retention of the triangular uranium core in the present complexes, despite large changes in bond distances and angles, may be due to structural flexibility at the equatorial positions on uranium(VI). The μ3-oxido atom in the trinuclear uranyl(VI) complex act as a nucleophile, because it effectively contains a lone pair of electrons. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03175. Crystallographic data for complexes Et3NH[1] and (Et3NH)2[2], atomic coordinates for optimized geometries of [(UO2)3(μ3-OH)(sap)3]− and [(UO2)3(μ3O)(sap)3]2−, figures of IR and 1H NMR, and UV-vis spectra, trinuclear core structures and hydrogen bonding interactions for Et3NH[1] and (Et3NH)2[2], and Eyring plot of proton self-exchange reaction of Et3NH[1] and (Et3NH)2[2] (PDF) X-ray crystallographic information (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takashi Yoshimura: 0000-0002-9216-9043 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japanse Governments for the support of the research (No. 25420908, Grant-in-Aid for Scientific Research (C)) to T.Y. G

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b03175 Inorg. Chem. XXXX, XXX, XXX−XXX