2.1 Synthesis and spectroscopic measurements of 1

Similarly to the syntheses of L₁ and L₂, [9,10] the diamantyl functionalized ligand L₃ is synthesized from a Mannich condensation of triazacyclononane, paraformaldehyde, and 2-diamantyl-4-tert-butylphenol. The U(III) precursor complex [((^DiaArO)₃tacn)U] (1) is obtained by treating [U(N(SiMe₃)₂)₃] with L₃ in 1,2-dimethoxyethane (DME) yielding 1 as a red-brown solid (Scheme 2). Although X-ray diffraction (XRD) quality single crystals could not be obtained of 1, its spectroscopic behavior and reactivity are in accordance with the six-coordinate precursor complex.

For instance, the ¹H-NMR spectrum shows 13 resonances, consistent with a molecule possessing C₃ symmetry. Variable temperature (VT) magnetization data clearly identifies 1 as a trivalent species exhibiting magnetic moments of 1.26 B.M. and 3.04 B.M. at 5 K and 300 K, respectively (Fig. 3). The temperature dependence of the magnetic moment is similar to those seen in other U(III) precursors. [3] The electronic absorption spectrum exhibits a Laporte-allowed, metal-centered 5f ³ to 5f ²6d¹ transition in the visible region (460 nm, ɛ = 766 M⁻¹cm⁻¹) typically observed for complexes with U(III) ions (Fig. 3, bottom).

The CW X-band EPR spectrum of 1, recorded in frozen toluene solution at 6 K, exhibits an isotropic signal at g = 2.08 (Fig. 4), which is nearly identical to the one recorded for [((^RArO)₃tacn)U^III] (R = t-Bu, Ad) and similar to the one that was measured for the parent U(III) tris-amido system [U(N(SiMe₃)₂)₃] [10]. A feature clearly visible on the low-field part of the signal could not be simulated with a set of anisotropic g-values.

Furthermore, the observed reactivity of complex 1 with small molecules is also in accordance with trivalent uranium such as the one-electron activation of methylene chloride to form 2 and two-electron reduction of azidotrimethylsilane to form 3. Synthesis and spectroscopic characterization of both complexes are discussed in detail in the following sections.

2.2 Synthesis and molecular structure of 2

Treatment of a red-brown solution of complex 1 with methylene chloride in DME immediately results in decolorization of the reaction solution to green. The solution was filtered and volatiles were removed giving green solids of [((^DiaArO)₃tacn)U(Cl)] (2) (Scheme 3). Green crystals of XRD quality were obtained from diffusion of acetonitrile into a solution of 2 in methylene chloride. The molecular structure of 2 reveals a seven-coordinate complex where the chloride ligand is coordinated in the axial position and the uranium center is located 0.26 Å below the plane formed by the three phenolate oxygens (Fig. 5). The average UO_avg and UN_avg bond distances in complex 2 of 2.174(6) Å and 2.652(8) Å are comparable to those of the corresponding U(IV) chloride complex derived from the adamantyl functionalized ligand [((^AdArO)₃tacn)U(Cl)] (2.160(6) Å, 2.654(7) Å). [11] The UCl bond distance of 2.691(3) Å also compares well with that of the [((^AdArO)₃tacn)U(Cl)] complex (2.708(3) Å). [11] A notable feature observed in the molecular structure of 2 is the hydrogen bonding interaction (2.706 Å) between a co-crystallized dichloromethane hydrogen with the coordinated axial chloride (Fig. 5). The observation of hydrogen bonding suggests that non-polar small molecules such as methane could also be coaxed into the reactive pocket of U(V) terminal oxo species, a necessary pathway towards CH activation.

As expected, the three diamantyl substituents form a deep cavity in [((^DiaArO)₃tacn)U(Cl)] (2) of approximately 5.9 Å, significantly deeper than that of [((^AdArO)₃tacn)U(Cl)], where the adamantyl groups create a cavity depth of only 4.7 Å. The depth of the hydrophobic pocket can be expected to be even greater in the coordinatively unsaturated six-coordinate complex 1. Presumably, the strongly bound axial chloride ligand in complex 2 bends the diamantyl substituents away from the uranium center.

2.3 Magnetism and electronic absorption of 2

Variable temperature SQUID magnetization measurements are needed to identify U(III) f ³ and U(IV) f ² complexes since their magnetic moment values at room temperature are nearly indistinguishable (calc.: U(III): 3.69 μ_B; U(IV): 3.58 μ_B). Due to a non-magnetic singlet ground state at low temperatures, U(IV) f ² complexes exhibit temperature independent paramagnetism (TIP), resulting in low magnetic moments at low temperatures, typically ranging from 0.4–0.8 μ_B at 2 K. The VT SQUID plot of 2 clearly reveals a U(IV) species, where temperature dependent magnetic moments of 3.13 μ_B and 0.69 μ_B are observed at 300 K and 2 K, respectively (Fig. 6, top). By contrast, in U(III) f ³ complex 1, a steady decline from 3.04 to 1.26 μ_B is observed as the temperature decreases from 300 K to 5 K.

Two distinctive features observed in the electronic absorption spectrum further support that complex 2 is a U(IV) species. Upon oxidation of the uranium center, the color-giving f–d absorption band observed in the visible part of the spectrum found for the U(III) f ³ complex 1 shifts to the UV region and is no longer visible in the spectrum of 2, rendering the U(IV) species of this ligand system pale-colored. Only a series of weak absorption bands (ɛ ≈ 20–50 M⁻¹cm⁻¹) spanning the region of ca. 600–1600 nm are observed (Fig. 6, bottom). These bands arise from Laporte-forbidden f–f transitions and are characteristic of complexes containing tetravalent uranium centers [12].

2.4 Synthesis and molecular structure of 3

Precursor complex 1 undergoes two-electron redox chemistry when treated with azidotrimethylsilane to form a U(V) imido complex [((^DiaArO)₃tacn)U(NTMS)] (3) and dinitrogen (Scheme 4). Brown single crystals suitable for XRD analysis were obtained from a saturated solution of 3 in hexane. The molecular structure features a seven-coordinate species, where the trimethylsilylimide ligand is bound at the axial position and the uranium center is nearly coplanar with the three phenolate oxygens (0.066 Å below O₃ plane) (Fig. 7).

The average UO_avg and UN_avg bond lengths of 2.191(3) Å and 2.700(3) Å in complex 3 compare well with those of the previously published U(V) imido complex [((^AdArO)₃tacn)U(NTMS)] (2.211(2) Å and 2.696(2) Å). [11] Remarkably, the UN4 bond distance of 1.935(3) Å is significantly shorter in complex 3 compared to the unprecedentedly long uranium imido bond of 2.122(2) Å in [((^AdArO)₃tacn)U(NTMS)] and even shorter than that of [((^t-BuArO)₃tacn)U(NTMS)] (1.985(5) Å).[11] The length of the UNTMS is an indication of the strength and covalency of the uranium imido bond and is reflected by the UNSi angle. A more linear UNSi angle allows for better orbital overlap of the uranium with the imide nitrogen resulting in a shorter bond length. The UNSi angle in 3 is nearly linear, measuring 177.5(2)° compared to that of [((^AdArO)₃tacn)U(NTMS)] (162.6(1)°), [11] accounting for a significantly shorter UN_imido bond distance and a small displacement of the uranium center below the tris-aryloxide plane (0.066 Å). This latter U out-of-plane parameter shift is an exceedingly sensitive measure of the strength of the UX bond in these complexes. Concomitant with a short U≡N bond, the SiN4 bond length in complex 3 is 1.722(3) Å, considerably longer than that observed for the analogous [((^AdArO)₃tacn)U(NTMS)] (1.623(2) Å) [11]. In order to obtain an elusive and highly desirable uranium terminal nitride species, cleavage of the SiN is required and hence, a significantly weakened SiN bond is desirable. Strategies to accomplish such a synthesis are currently being undertaken. As observed for complex 2, the pocket in complex 3 of ∼ 5.4 Å is much deeper than that of [((^AdArO)₃tacn)U(NTMS)] (4.6 Å). The shallower cavity in complex 3 compared to complex 2 illustrates that the more the uranium center is bound to a stronger ligand (TMSN²⁻ > Cl⁻), the more the diamantyl groups are driven away from the uranium center. The difference in pocket depth (Δ_depth ≈ 0.5 Å) in the complexes of the diamantyl functionalized ligand is significantly larger than in the complexes supported by the adamantyl functionalized ligand (Δ_depth ≈ 0.1 Å). This suggests that although the diamantyl substituents are more sterically cumbersome, the ligand system is more flexible and accommodating than the adamantyl derivatized ligand system.

2.5 Magnetism and electronic absorption of 3

Variable temperature SQUID magnetization data can be used to clearly differentiate U(V) f ¹ complexes from the data of their f ² and f ³ counterparts. Based on the LS coupling scheme, the calculated μ_eff value for a U(V) f ¹ complex at room temperature is 2.54 μ_B. [13] The VT SQUID magnetization data for complex 3 shows a temperature dependent magnetic moment ranging from 2.55 μ_B to 1.26 μ_B in the temperature range of 300 K to 5 K, identifying 3 as a U(V) complex (Fig. 8, top).

From the ligand field theory for U(V) complexes, the ²F manifold of a 5 f electron will be split by the 5 f spin-orbit interaction (approx. 2200 cm⁻¹ for the free ion) into two J multiplets, J = 5/2 and 7/2. In C_3v symmetry, the J = 5/2 ground state splits into three magnetic doublets, two EPR active μ = ± 1/2 and one EPR inactive μ = ± 3/2 state, where μ is the crystal ground state number. [14] Despite their f ¹ electron configuration, the U(V) imido complex 3, [((^DiaArO)₃tacn)U(NTMS)], and all other reported U(V) imido complexes, are found to be EPR-silent. This observation suggests that the ground state crystal field of 3 must be μ = ± 3/2. Interestingly, the corresponding U(V) oxo complexes [((^RArO)₃tacn)U(O)] (R = t-Bu, Ad) are EPR active. These species show EPR spectra of axial symmetry; thus, confirming their μ = ± 1/2 ground state. Preliminary data of a U(V) oxo species, [((^DiaArO)₃tacn)U(O)], of the presented diamantane derivatized system are in agreement with this observation; the details of this species will be reported later.

Additionally, the electronic absorption spectrum of 3 displays absorption bands characteristic of U(V) imido complexes. The intense brown-orange color of 3 arises from intense LMCT transitions in the visible region ranging from ∼ 800 to 350 nm. Weaker Laporte-forbidden f–f transitions spanning from 850 nm to 2000 nm (ɛ ≈ 25–100 M⁻¹cm⁻¹) are also observed (Fig. 8, bottom). The absorption pattern of 3 is nearly identical to that of the corresponding U(V) imido complex bearing the adamantyl functionalized ligand [((^AdArO)₃tacn)U(NTMS)] (Fig. 8, bottom).

4.1 General methods

All experiments were performed under dry nitrogen atmosphere using standard Schlenk techniques or an MBraun inert-gas glovebox. Solvents were purified using a two-column solid-state purification system (Glasscontour System, Irvine, CA) and transferred to the glovebox without exposure to air. NMR solvents were obtained from Cambridge Isotope Laboratories, degassed, and stored over activated molecular sieves prior to use.

4.2 Spectroscopic methods

Magnetization data of crystalline powdered samples (20 – 30 mg) were recorded with a SQUID magnetometer (Quantum Design) at 10 kOe (5 – 300 K for 1 and 3) and (2 – 300 K for 2). Values of the magnetic susceptibility were corrected for the underlying diamagnetic increment (χ_dia = -878.98 × 10⁻⁶ cm³ mol⁻¹ (1), −888.08 × 10⁻⁶ cm³ mol⁻¹ (2), −906.48 × 10⁻⁶ cm³ mol⁻¹ (3)) by using tabulated Pascal constants and the effect of the blank sample holders (gelatin capsule/straw). Samples used for magnetization measurement were recrystallized multiple times and checked for chemical composition and purity by elemental analysis (C, H, and N) and ¹H NMR spectroscopy. Data reproducibility was also carefully checked on independently synthesized samples.

The EPR measurement was performed in a quartz tube with a J. Young valve. Frozen solution EPR spectrum were recorded on a JEOL continuous wave spectrometer JES-FA200 equipped with an X-band Gunn oscillator bridge, a cylindrical mode cavity, and a helium cryostat as well as a Bruker ELEXSYS E500 spectrometer equipped with a helium flow cryostat (Oxford Instruments ESR 910) and Hewlett-Packard frequency counter HP5253B. Spectral simulation was performed using the program QCMP 136 by Prof. Dr. Frank Neese from the Quantum Chemistry Program Exchange [15].

¹H NMR spectra were recorded on JEOL 270 and 400 MHz instruments operating at respective frequencies of 269.714 and 400.178 MHz with a probe temperature of 23 °C in C₆D₆ or CD₂Cl₂. Chemical shifts were referenced to protio solvent impurities (δ 7.15 (C₆D₆), δ 5.32 (CD₂Cl₂) and are reported in ppm.

Electronic absorption spectra were recorded from 200 to 2000 nm (Shimadzu (UV-3101PC)) in the indicated solvent.

Results from elemental analysis were obtained from the Analytical Laboratories at the Friedrich-Alexander-University Erlangen-Nürnberg (Erlangen, Germany) on Euro EA 3000.

4.3 Starting materials

Precursor complexes [(THF)₄UI₃] and [U(N(SiMe₃)₂)₃] were prepared as described by Clark et al. [16,17] Uranium turnings were purchased from Oak Ridge National Laboratory (ORNL) and activated according to literature procedures. [16] The 4-hydroxydiamantane (99+%) was obtained from Chevron Technology Ventures as a generous gift and used as received. Thionyl chloride (> 99%) and azidotrimethylsilane (95%) were purchased from Aldrich and used as received. The 4-tert-butylphenol (97%) was purchased from Acros Organics and also used as received.

4.4 Synthesis of 4-chlorodiamantane

In a flask fitted with a reflux condenser, thionyl chloride (18.0 mL, 0.0248 mol) was added to 4-hydroxydiamantane (5.00 g, 0.0245 mol. The mixture was heated to reflux at 90 °C for 2 h. At room temperature, the remaining thionyl chloride is removed in vacuo. The residue is taken up in dichloromethane (50 mL) and washed three times (50 mL) with distilled water. The volatiles in the organic fraction were removed in vacuo to yield a white powder. Yield: 5.24 g (0.0235 mol, 96%). Elemental analysis (%) calcd: C, 75.49; H, 8.60. Measured: C, 75.67; H, 8.65.

4.5 Synthesis of 2-diamantyl-4-tert-butylphenol

In a flask equipped with a reflux condenser, 4-chlorodiamantane (5.00 g, 0.0224 mol) and 4-tert-butylphenol (8.41 g, 0.056 mol) were heated to reflux at 140 °C. The melt was stirred for 12 hours. At room temperature, the solid mass was placed on a Kugelrohr device at 100 °C for 20 hours or until a pure white product was obtained, checked by ¹H-NMR. Yield: 5.88 g (0.0175 mol, 78%). ¹H-NMR (400 MHz, dichloromethane-d₂, 20 °C) δ = 7.27 (d, 1H), 7.07 (dd, 1H), 6.58 (d, 1H), 4.80 (s, 1H) 2.20 – 1.70 (m, 19H).

4.6 Synthesis of (^DiaArOH)₃tacn (L₃)

In a flask fitted with a reflux condenser, triazacyclononane (0.43 g, 3.3 mmol) and paraformaldehyde (0.30 g, 9.9 mmol) were heated to 80 °C in 1-propanol. After 2 hours, the 2-diamantyl-4-tert-butylphenol (5.00 g, 14.9 mmol) was added and the reaction mixture was allowed to stir at 80 °C for an additional 14 hours. During the cooling process to room temperature, a white precipitate started to form. A few drops of water were added to the reaction mixture to facilitate complete precipitation of the product. The white precipitate was collected by filtration and washed three times (10 mL) with ethanol. Yield: 2.83 g (2.4 mmol, 72%). ¹H-NMR (400 MHz, dichloromethane-d₂, 20 °C) δ = 7.19 (d, 3H), 6.79 (d, 3H), 3.70 (s, 6H), 2.77 (s, 12H), 2.22 – 1.68 (m, 57H), 1.27 (s, 27H).

4.7 Synthesis of [((^DiaArO)₃tacn)U] (1)

A solution of [U(N(SiMe₃)₂)₃] (0.643 g, 0.89 mmol) in 1,2-dimethoxyethane (∼8 mL) was added dropwise to a stirring solution of (^DiaArOH)₃tacn (L₃) (1.00 g, 0.85 mmol) in DME (∼7 mL). Within 10 minutes, a red-brown solution is observed. The reaction was allowed to stir for 4 hours. The reaction mixture was filtered through Celite and the volatiles were removed in vacuo to obtain 1 as a red-brown powder. Yield: 0.823 g (0.70 mmol, 82%).

4.8 Synthesis of [((^DiaArO)₃tacn)U(Cl)] (2)

Dichloromethane (7 μL, 0.11 mmol) diluted in DME (∼2 mL) was added dropwise to a stirring solution of 1 (0.150 g, 0.11 mmol) in DME (∼6 mL). Immediately the red-brown reaction solution turned pale green. The reaction was allowed to proceed at room temperature for 5 hours. The reaction mixture was filtered and volatiles were removed yielding 2 as pale green solids. Yield: 0.123 g (0.085 mmol, 77%).

4.9 Synthesis of [((^DiaArO)₃tacn)U(NTMS)] (3)

Azidotrimethylsilane (14 μL, 0.11 mmol) diluted in hexane (∼2 mL) was added dropwise to a stirring solution of 1 (0.150 g, 0.11 mmol) in DME (∼6 mL). The reaction solution turns dark brown-orange and evolution of dinitrogen was observed. Stirring was continued for 4 hours. The reaction mixture was filtered and volatiles were removed to obtain 3 as brown-orange solid. Yield: 0.121 g (0.081 mmol, 76%).

4.10 Crystallographic details for 2

Green block crystals, grown from slow diffusion of acetonitrile into a dichloromethane solution of 2 at room temperature, were coated with isobutylene oil on a microscope slide. A crystal of approximate dimensions 0.25 × 0.21 × 0.14 mm³ was selected and mounted on a nylon loop. A total of 87361 reflections (–30 ≤ h ≤ 30, –15 ≤ k ≤ 16, –32 ≤ l ≤ 32) were collected at T = 150(2) K in the θ range from 3.29 to 25.68°, of which 16647 were unique (R_int = 0.0726) and 13714 were observed [I > 2σ(I)] on a Bruker-Nonius KappaCCD diffractometer using MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXTL NT 6.12, Bruker AXS, Inc., 2002). All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated idealized positions. One of the t-Bu groups is disordered, with two refined alternative positions being occupied by 36(2) and 64(2)% for C54 – C56 and C54A – C56A, respectively. The compound crystallizes with a number of solvent molecules of which the positions of one acetonitrile and two dichloromethane molecules were located. Apart from this the crystal structure contains two solvent accessible voids that are occupied by heavily disordered solvents. Here, the Squeeze algorithm was applied [18]. The residual peak and hole electron density were 3.293 and –3.787e.Å⁻³. The absorption coefficient was 2.026 mm⁻¹. The least-squares refinement converged normally with residuals of R₁ = 0.1045, wR₂ = 0.2070, and GOF = 1.241 (all data). C₈₅H₁₁₅N₄O₃Cl₅U, monoclinic, space group P2₁/c, a = 24.996(5), b = 13.380(2), c = 27.038(3) Å, β = 101.300(9)°, V = 8867(2) ų, Z = 4, ρ_calcd = 1.240 Mg/m³, F(000) = 3416, R₁(F) = 0.0910, wR₂(F²) = 0.2014 [I > 2σ(I)]. CCDC reference number: 763284.

4.11 Crystallographic details of 3

Brown plate crystals, grown from a saturated solution of 3 in hexane at room temperature, were coated with Paratone N oil on a microscope slide. A crystal of approximate dimensions 0.13 × 0.10 × 0.04 mm³ was selected and mounted on a glass fiber. A total of 112504 reflections (–14 ≤ h ≤ 14, −23 ≤ k ≤ 23, –30 ≤ l≤ 30) were collected at T = 150(2) K in the θ range from 3.15 to 26.73°, of which 18581 were unique (R_int = 0.1249) and 14448 were observed [I > 2σ(I)] on a Bruker-Nonius KappaCCD diffractometer using MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXTL NT 6.12, Bruker AXS, Inc., 2002). All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated idealized positions. One of the t-Bu groups is disordered, with two refined alternative positions being occupied by 38(3) and 62(3)% for C79 – C81 and C79A – C81A, respectively. The asymmetric unit contains two molecules of n-hexane both of which are disordered. The residual peak and hole electron density were 1.348 and –0.752 e.Å⁻³. The absorption coefficient was 1.917 mm⁻¹. The least-squares refinement converged normally with residuals of R₁ = 0.0748, wR₂ = 0.0885, and GOF = 1.055 (all data). C₉₆H₁₄₅N₄O₃SiU, triclinic, space group P-1, a = 11.3040(6), b = 18.194 (2), c = 24.048(2) Å, α = 111.877(6)°, β = 91.959(7)°, γ = 105.210(7)°, V = 4380.6(6) ų, Z = 2, ρ_calcd = 1.266 Mg/m³, F(000) = 1758, R₁(F) = 0.0458, wR₂(F²) = 0.0797 [I > 2σ(I)]. CCDC reference number: 763285.