Synthesis, characterization and molecular structure of a dinuclear uranyl complex supported by N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine

2.1 Syntheses and characterization of [(UO₂)₂L(OCMe₂)₂]

The ligand H₄L was prepared by a Schiff-base condensation of 3,5-di-tert-butyl-2-hydroxybenzaldehyde with 1,2,4,5-benzenetetramine tetrahydrochloride according to a literature procedure [20]. The reaction of the neutral, fully protonated form H₄L with uranyl nitrate in refluxing acetone provided a dark red precipitate of [(UO₂)₂L(OCMe₂)₂] which could be isolated in 91 % yield. [(UO₂)₂L(OCMe₂)₂] is highly soluble in a variety of organic solvents. The dinuclear uranium complex gave satisfactory elemental analysis and was further characterized by ESI-MS, IR, UV/Vis and NMR spectroscopy as well as cyclic voltammetry. Selected analytical data are listed in Table 1. Those of H₄L have been included for comparative purposes.

The high-resolution electrospray ionization mass spectrum of a dilute acetone solution of [(UO₂)₂L(OCMe₂)₂] (Fig. S1; Supporting Information available online; see note at the end of the paper for availability) displays a molecular ion peak at m/z = 1654.85197 with the correct isotopic distribution for the (radical) cation [(UO₂)₂L(OCMe₂)₂]⁺ ([M]⁺). The IR spectrum of H₄L shows a characteristic band at 1612 cm^–1 for the C=N stretching vibration, which shifts by 6 cm^–1 to lower frequencies upon coordination to the uranyl dication. The band at 914 cm^–1 is attributed to the asymmetric stretching vibration of the [O=U=O]²⁺ dication. The data for the present uranyl complex agree with those reported for other uranyl complexes supported by salophen ligands [22].

The UV/Vis spectrum of H₄L in chloroform (Fig. 2) displays two intense absorptions at 242 and 286 nm assigned to π→π* and n→π* transitions. The intense absorption at 382 nm, which entails into the visible region of the spectrum, results most probably from an azomethine n→π* transition. The UV/Vis spectrum of [(UO₂)₂L(OCMe₂)₂] in chloroform also shows intense intraligand n→π* and π→π* absorptions, but also intense phenolate-to-U^VI charge transfer transitions at 418 and 460 nm, as in other uranyl-salophen complexes [22–24]. The latter obscure the oxide-to-U(VI) LMCT transitions in this region. The azomethine n→π* transition shifts from 382 in H₄L to 333 nm in [(UO₂)₂L(OCMe₂)₂], and is also an indication for the presence of an uranyl salophen complex [23].

Fig. 2:

UV/Vis spectra of H₄L (dashed line) and [(UO₂)₂L(OCMe₂)₂] (solid line) in chloroform at ambient temperature.

The ¹H NMR spectrum for [(UO₂)₂L(OCMe₂)₂] in CD₂Cl₂ solution (Fig. S2) shows seven signals. All signals are shifted downfield upon coordination, particularly large shifts are seen for the imine protons. The three signals at 7.58, 7.74, and 7.87 ppm are assigned to the aromatic protons. The CHN (imine) protons resonate at 9.65 ppm, and the tert-butyl groups give rise to two signals at 1.39 and 1.71 ppm. A signal at 2.41 ppm is attributed to the methyl protons of the acetone coligands. The ¹³C NMR spectrum (Fig. S3) reveals ten signals in the aromatic region, two of which (110.4 and 146.9 ppm) are due to the central aromatic ring, six signals at 124.1, 130.2, 132.3, 139.7 (2×), and 168.1 belong to the phenolate rings, and two signals at 166.3 and 173.9 ppm are attributable to the C atoms of the imine function of H₄L and the carbonyl C atoms of the acetone co-ligands. The aliphatic signals at 29.7, 31.2, 33.9 and 35.2 can be assigned to the tert-butyl groups, and the remaining signal is due to the methyl groups of the acetone co-ligands. The data indicate that the solid-state structure of [(UO₂)₂L(OCMe₂)₂] is maintained in solution.

2.2 Description of the crystal structure of H₄L·5EtOH

It has been possible to grow single crystals of H₄L·5EtOH which allowed to determine its crystal and molecular structure. The crystal structure consists of well-separated H₄L molecules (Fig. 3) and ethanol solvate molecules. The structure reveals that H₄L exists in the phenol form, with the OH hydrogen atoms being involved in intramolecular hydrogen bonding interactions with the imine N atoms [25]. The salicylidene units are not coplanar with the central benzene ring as shown in Fig. 3. The distortion from planarity is best described by the C–C–N=C^imine torsional angles which are 42.7° (C33–C32–N1–C15) and 38.5° (C33′–C31–N2–C30).

Fig. 3:

Two perspective views of the molecular structure of H₄L in crystals of H₄L·5EtOH. Displacement ellipsoids are drawn at the 30 % probability level. Hydrogen atoms and EtOH solvent molecules are omitted for clarity. Intramolecular hydrogen bonding interactions are indicated by dashed lines. Symmetry code used to generate equivalent atoms: –x, 2 – y, –z. Selected bond lengths (Å): C1–O1 1.389(5), C16–O2 1.363(5), C15–N1 1.310(5), C30–N2 1.289(5); O1···N1 2.584(4), O2···N2 2.615(4). The atom labeling used here differs from that used in Scheme 1.

2.3 Description of the crystal structure of [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂

Red crystals of [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂ suitable for X-ray diffraction analysis were obtained from a saturated acetone solution by slow evaporation. The title compound crystallizes in the triclinic space group P1̅. The asymmetric unit contains two crystallographically independent but chemical identical formula units A (Fig. 4) and B, each of which exhibits crystallographic inversion symmetry. Table 2 lists selected bond lengths and angles.

Fig. 4:

Molecular structure of [(UO₂)₂L(OCMe₂)₂] (molecule A) in crystals of [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂. Displacement ellipsoids are drawn at the 30 % probability. Solvate molecules and H atoms are omitted for clarity. Symmetry code used to generate equivalent atoms: (′) 1 – x, 1 – y, –z.

The UO₂²⁺ groups are situated in the N₂O₂ binding pockets of the bis-salophen ligand. An acetone molecule completes the distorted pentagonal bipyramidal coordination geometry about the U atoms. The UO₂²⁺ groups are almost perfectly linear, with O=U=O angles of 177.62(4)° (molecule A) and 178.19(5)° (molecule B). The U=O bond lengths range from 1.870(1) to 1.782(1) Å, and are much shorter than the U–O^phenolato and U–N^imine bonds, which range from 2.237(8) to 2.281(9) Å and 2.498(1) to 2.532(1) Å, respectively. The U–O^acetone bonds at 2.489(1) Å and 2.474(1) Å are shorter than the U–O^phenolato bonds, as in related compounds. The equatorial N₂O₃ planes about the two Uranium atoms are planar, as manifested by the sum of the corresponding bond angles [360.0(3)° at U1 and 359.8(3)° at U2]. The N–U–N and N–U–O^phenolate angles average 64.3(3)° and 70.0(3)°, typical for uranyl salen compounds. The average O–U–O (phenolato-O, acetone-O) angle is 77.8(3)°. It is interesting to note that the C–C–N=C torsional angles in [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂ (31.0° (C4–N2–C1–C3′), 27.6° (C3–C2–N1–C19), 30.4 (C39–C38–N4–C55), 28.5° (C39′–C37–N3–C40)) are very similar to those in H₄L. However, while the free protonated ligand is in a “stepped” conformation (with the two salicylidene units on opposite sides of the central phenylenetetraamine plane), the deprotonated ligand in [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂ adopts a “boat” structure, where both salicylidene units are positioned on the same side of the central aromatic ring. A similar “boat” structure has been observed in [(UO₂)(salophen)(py)] (py = pyridine) [22].

2.4 Electrochemical properties

The electrochemical behavior of the complex [(UO₂)₂L(OCMe₂)₂] has been studied in dichloromethane at ambient temperature by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Figure 5 shows the results. All potentials are quoted relative to the ferrocenium/ferrocene couple (Fc⁺/Fc) [26]. The CV exhibits two quasi-reversible redox processes at E1/21 = 0.570 V and E1/22 = 0.822 V. These are attributed to two sequential one-electron oxidations, where two of the four coordinating phenolate groups are converted to phenoxyl radicals, as observed in other dinuclear metal complexes of the bissalophen ligand. In [Ni₂L], for example, there are two similar redox processes at E1/21 = 0.456 V and E1/22 = 0.540 V (vs. Fc⁺/Fc) [19]. Ikeda et al. have reported that UO₂²⁺ complexes of salophen can be electrochemically reduced at ca. –1.6 V vs. Fc⁺/Fc to form UO₂⁺ complexes [27–29]. In our case no features were observed in this regime.

Fig. 5:

Cyclic and differential pulse voltammograms of [(UO₂)₂L(OCMe₂)₂] in dichloromethane at ambient temperature. Concentration 10^–3 m, supporting electrolyte concentration 0.1 m (tetrabutylammonium hexafluorophosphate), internal standard reference Fc/Fc⁺. Cyclic voltammetry (solid curve) recorded at a scan rate of 100 mV s⁻¹, initial scan direction anodic. Differential pulse voltammetry (dotted curve) step potential 5 mV, modulation amplitude 25 mV, modulation time 0.05 s, interval time 0.2 s.

4.1 Materials and methods

3,5-di-tert-butyl-2-hydroxybenzaldehyde was prepared as described in the literature [30]. N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine was synthesized according to a slightly modified literature procedure [20]. All other chemicals were commercially available and used without further purification. Melting points were determined on a Boëtius Mikro–Heiztisch and are uncorrected. ESI mass spectra were recorded on a Bruker Daltronics Impact II ESI-TOF-MS spectrometer. NMR spectra were recorded on a Bruker Avance DRX 300 spectrometer at 298 K. Chemical shifts refer to solvent signals. Electronic absorption spectra were recorded on a Jasco V-670 UV/Vis/near IR spectrophotometer. The infrared spectra were recorded as KBr discs using a Bruker Tensor 27 FT-IR spectrophotometer. Elemental analysis were carried out on a VARIO EL elemental analyzer. Cyclic voltammetry measurements were carried out at room temperature with an Autolab PGSTAT12 potentiostat/galvanostat. The cell contained a Pt working electrode, a Pt wire auxiliary electrode, and a Ag wire as reference electrode. Concentrations of solutions were 0.10 m in supporting electrolyte (tetrabutylammonium hexafluorophosphate) and ca. 1 × 10^–3 m in sample. Ferrocene was used as internal standard.

CAUTION! Care should be taken when handling uranium containing compounds because of their toxicity and radioactivity!

4.2 Synthesis of H₄L

A solution of technical grade 1,2,4,5-tetraaminobenzene tetrahydrochloride (3.00 g, 8.45 mmol), 3,5-di-tert-butyl-2-hydroxybenzaldehyde (7.92 g, 33.8 mmol) and sodium bicarbonate (3.19 g, 38.03 mmol) in ethanol (120 mL) was refluxed for 16 h. After cooling, the solution was cooled to −10 °C using a freezing mixture (urea/ice, 1:10, w/w). The yellow precipitate was filtered off and rapidly dissolved in dichloromethane (800 mL). The organic phase was treated with sodium bicarbonate, dried over magnesium sulfate, evaporated to dryness, and dried in an oven at 60 °C to constant weight to afford H₄L as a fine yellow-orange powder (6.24 g, 6.22 mmol, 74 %). M. p. 285–287 °C. – IR (KBr): ν = 3440 m ν(O–H), 2960–2871 ν(C–H), 1612 m ν(C=N), 1588 m ν(C=C) cm^–1. – UV/Vis (CHCl₃): λ_max (ε in L mol^–1 cm^–1) = 242 (32850), 286 (30560), 382 (39020) nm. – ¹H NMR (300 MHz, CD₂Cl₂, for atom labels, see Scheme 1): δ = 1.35 [s, 36 H, –C(C¹⁰H₃)₃], 1.46 [s, 36 H, –C(C⁸H₃)₃], 7.33 (d, 4 H, ⁴J_HH = 2.4 Hz, C²H), 7.50 (d, 4 H, ⁴J_HH = 2.4 Hz, C⁴H) 7.27 (s, 2 H, C¹³H), 8.85 (s, 4 H, C¹¹H), 13.55 (s, 4 H, C⁶–OH). – ¹³C{¹H} NMR (75 MHz, CD₂Cl₂): δ = 29.5 (–C(C⁸H₃)₃), 31.5 (–C(C¹⁰H₃)₃), 34.4 (–C⁹(CH₃)₃), 35.4 (–C⁷(CH₃)₃), 111.1 (C¹³H), 118.8 (C¹), 127.4 (C²H), 128.9 (C⁴H) 137.9 (C⁵), 141.1 (C³), 142.0 (C¹²), 158.8 (Ar, C⁶–OH), 165.1 (C¹¹). – MS [(+)-ESI, CH₂Cl₂/MeOH]: m/z (%) = 1003.6 (100) [M+H]⁺. – C₆₆H₉₀N₄O₄ (1003.47): calcd. C 79.00, H 9.04, N 5.58; found C 78.89, H 8.56, N 5.46.

4.3 Synthesis and characterization of [(UO₂)₂L(OCMe₂)₂]

H₄L (400 mg, 498 μmol) was dissolved in acetone (150 mL). Uranyl nitrate hexahydrate (600 mg, 1.2 mmol) in 50 mL acetone was added and the resulting mixture was refluxed for 2 h. After cooling, three-fourths of the volume were evaporated and the mixture was kept in a freezer at ≤5 °C overnight. The dark red-colored precipitate was filtered off, washed with a small amount of cold acetone and dried in an oven at 60 °C until constant weight to afford [(UO₂)₂L(OCMe₂)₂] as dark red powder (563 mg, 366 μmol, 91 %). M. p. 105–107 °C (decomp.). – IR (KBr): ν = 2975–2869 m, 1606 m, 1586 m, 915 cm^–1. – UV/Vis (CHCl₃): λ_max (ε in L·mol^–1·cm^–1) = 241 (37330), 333 (27200), 418 (26180), 460 nm (27050). – ¹H NMR (300 MHz, CD₂Cl₂): δ = 1.39 [s, 36 H, –C(C¹⁰H₃)₃], 1.71 [s, 36 H, –C(C⁸H₃)₃], 2.41 (s, br 12 H, –CH₃, acetone coligand) 7.58 (d, 4 H, ⁴J_HH = 2.4 Hz, C²H), 7.74 (s, 2 H, C¹³H), 7.87 (d, 4 H, ⁴J_HH = 2.4 Hz, C⁴H), 9.65 (s, 4 H, C¹¹H). – ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ = 29.7 (–C(C⁸H₃)₃), 31.2 (–C(C¹⁰H₃)₃), 31.3 (–CH₃, acetone coligand), 33.9 (–C(C⁹H₃)₃), 35.2 (–C⁷(CH₃)₃), 110.7 (C¹³H), 124.4 (C¹), 130.2 (C²H), 132.3 (C⁴H), 139.6 (C⁵), 139.7 (C³), 146.9 (C¹²), 166.3 (C¹¹), 168.1(C⁶), 173.9 (C=O, acetone coligand). – HRMS [(+)-ESI, CH₂Cl₂]: m/z = 1654.85197 calcd. 1654.82933 for C₇₂H₉₈N₄O₁₀U₂, [M]⁺ (see Fig. S1). – C₇₂H₉₈N₄O₁₀U₂ (1655.65): calcd. C 52.23, H 5.97, N 3.38; found C 52.19, H 5.94, N 3.35.

4.4 Crystal structure determinations

Single crystals of H₄L·5EtOH were obtained by slow evaporation of a EtOH-CH₂Cl₂ (1:1, v/v) solution. Crystals of [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂ were obtained by slow evaporation of a saturated acetone solution. Crystallographic data are collected in Table 3. The data sets were collected on a Stoe IPDS-1 diffractometer for H₄L·5(C₂H₅OH) or Stoe IPDS-2T diffractometer for [(UO₂)₂L(OCMe₂)₂]·1.5OCMe₂ using graphite monochromated MoK_α radiation (λ = 0.71073 Å). The intensity data were processed with the program Stoe x-area [31]. Structures were solved by direct methods [32] and refined by full-matrix least-squares on the basis of all data against F² using Shelxl-97 [33]. Platon was used to search for higher symmetry [34, 35]. Hydrogen atoms, except H1 and H2 in H₄L·5EtOH, were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. Ortep-3 was used for the artwork of the structures [36, 37].

CCDC 904825 and CCDC 904826 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.