Synthesis and structure of [{N(CH₂CH₂NSiMe₃)₃}URe(η⁵-C₅H₅)₂]: a heterobimetallic complex with an unsupported uranium–rhenium bond

Abstract

The first structurally authenticated molecular uranium–transition metal bond is reported; DFT studies show σ- and π-components in the U–Re bond and this is the first time that the latter component has been reported in an unsupported f-element–transition metal bond.

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The study of molecular metal–metal bonds is fundamentally important to understanding chemical bonding.¹ There is continued debate over the extent of covalency in f-element bonding, so it is important to develop an understanding of the role of the metal valence orbitals. For example, f-orbital–ligand interactions have been observed in carbon dioxide,²carbon monoxide,³dinitrogen,⁴arene,⁵ and alkane⁶ derivatives, and ligands which engage in covalent bonding have been shown to be selective for the extraction of actinides over lanthanides in solution.⁷ The identification of discrete metal–metal bonds has become widespread throughout the d- and p-blocks, and they are now even known in the s-block.⁸ However, despite the proclivity of metal–metal complexes to exhibit novel bonding, complexes containing unsupported thorium/uranium–metal bonds are rare.⁹ Only four unsupported uranium–metal/metalloid bonds have been structurally authenticated:¹⁰ the ‘heavy alkyls’ [{(Ar)(Bu^t)N}₃USi(SiMe₃)₃] (Ar = 3,5-Me₂C₆H₃)¹¹ and [(η⁵-C₅H₅)₃USnPh₃];¹² the donor–acceptor complex [(η⁵-C₅H₄SiMe₃)₃UAl(η⁵-C₅Me₅)];¹³ and the gallyl complex [(Tren^TMS)U{Ga(NAr′CH)₂}(THF)] [1, Ar′ = 2,6-Prⁱ₂C₆H₃; Tren^TMS = N(CH₂CH₂NSiMe₃)₃].¹⁴ However, uranium–transition metal bonds have not been structurally characterised, so little is known of their nature.

Since our report of 1, we have turned our attention towards the synthesis of unsupported uranium–transition metal bonds. In search of suitable transition metal anion candidates our attention was drawn to rhenocene hydride,¹⁵ formally, a d⁴rhenium(III) complex. However, it has been shown that [(η⁵-C₅H₅)₂ReH] reacts with butyllithium to give alkane elimination and a complex formulated as [(η⁵-C₅H₅)₂ReLi], containing a formal d⁶ rhenium(I) centre.¹⁶ Since a metal-coordinated [(η⁵-C₅H₅)₂Re]⁻ fragment would be expected to be bent, re-hybridisation of the e_2g and a_1g orbitals in the linear free anion to form a 1a₁, b₂ and 2a₁ configuration would be anticipated, thus presenting a manifold of three ‘lone pairs’ around the equatorial girdle for potential bonding interactions.¹⁷ Surprisingly, there are only two reports of rhenium–metal bonds using [(η⁵-C₅H₅)₂Re]⁻: [(η⁵-C₅H₅)₂Re–Ln(η⁵-C₅H₅)₂] (Ln = Y or Yb),¹⁸ and [(η⁵-C₅H₅)₂Re–Sn(Cl)_n(Me)_3−n] (n = 1, 3).¹⁹ Herein, we report the synthesis and characterisation of the first structurally authenticated uranium–transition metal bond which is also the first example of a molecular complex to feature an actinide–rhenium bond and the first report of an actinide–transition metal bond featuring σ- and π-components.

We targeted a uranium–rhenium bond using the Tren-ligand as it is clearly effective at supporting such linkages.¹⁴ Although [(Tren^TMS)U(Cl)(THF)] (2) was an effective precursor to 1,¹⁴ we prepared the corresponding iodide complex as we postulated that it might be more favourable to eliminate KI than KCl. The new uranium(IV) complex [(Tren^TMS)U(I)(THF)] (3) was prepared from 2 and Me₃SiI and isolated as green crystals in 85% yield.†‡

The crystal structure of 3§ is illustrated in Fig. 1 with selected bond lengths and angles. Complex 3 is very similar to 2 in gross structural terms, and bond lengths and angles are unexceptional.²⁰

Fig. 1 Molecular structure of 3. Thermal ellipsoids are set at 40% probability and hydrogen atoms are omitted for clarity. Selected bond lengths (Å): U(1)–I(1) 3.1430(4), U(1)–N(1) 2.241(4), U(1)–N(2) 2.233(4), U(1)–N(3) 2.256(4), U(1)–N(4) 2.574(4), U(1)–O(1) 2.552(4).

Reaction of 3 with [(η⁵-C₅H₅)₂ReK], prepared in situ from rhenocene hydride and benzyl potassium in THF, afforded a turbid red solution. Filtration and work-up afforded red crystals of [(Tren^TMS)URe(η⁵-C₅H₅)₂] (4) in 65% yield (Scheme 1).†‡ The CHN analyses and ¹H NMR spectrum of 4 support the proposed formulation; the latter exhibits resonances in the range +8 to −12 ppm which is typical for uranium(IV)–Tren compounds. The infrared spectrum of 3 is featureless in the region where bridging hydrides would be expected;²¹ treatment of a solution of 4 in C₆D₆ with 1–5 equivalents of CCl₄ did not result in the formation of CHCl₃. The molecular ion of 4 was not seen in the EI mass spectrum, but [M⁺] − C₅H₅ was observed at m/z 849 (3%). The magnetic moment of 4 in benzene at 298 K (Evans method), μ_eff = 2.88 μ_B, is below the theoretical free ion value of 3.58 μ_B expected for the ³H₄ ground state of f²uranium,^22a although uranium(IV) compounds usually fall in the range 2.5–2.9 μ_B.^22b

Scheme 1 Synthesis of 4.

An X-ray diffraction study of 4§ was performed and the molecular structure of 4 is depicted in Fig. 2. The geometry at uranium is best described as distorted tetrahedral, with the face defined by the three amide centres capped by the amine centre, which therefore essentially resides trans to rhenium. The U–Re bond distance of 3.0475(4) Å is without precedent, but it is ∼0.42 Å shorter than the sum of the covalent radii of uranium and rhenium (U + Re = 3.47 Å).²³ The U–N_amido and U–N_amine bond lengths of 2.266(6) (av.) and 2.680(6) Å are typical of U(IV)–N_amide and U(IV)–N_amine bond distances,^14,20 but are longer than that observed in 1, which implies that the rhenocene anion is a stronger donor than the gallyl ligand in 1 since, additionally, the uranium centres are 5- and 6-coordinate in 4 and 1, respectively.

Fig. 2 Molecular structure of 4. Thermal ellipsoids are set at 40% probability and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): U(1)–Re(1) 3.0475(4), U(1)–N(1) 2.282(6), U(1)–N(2) 2.274(6), U(1)–N(3) 2.241(6), U(1)–N(4) 2.680(6); Ct–Re(1)–Ct 166.7, Re(1)–U(1)–N(4) 169.74(14).

We carried out unrestricted DFT calculations on 4 using a ZORA/TZP all electron basis set from ADF2007.01.† Calculated bond distances and angles compare very well to the experimental structure; we thus conclude that the DFT calculations of 4 provide a qualitative description of the electronic structure of 4. Mulliken population analysis shows that the spin density on U (+2.2710) is above that for a formal 5f²U(IV) centre and this, with the small negative spin densities across the Re (−0.1175) and N (av. −0.0482 amides; −0.0172 amine) centres in 4, is consistent with charge donation from the ligands. A spin density less than +2 for U would indicate metal to ligand charge transfer, so we rule out π-type back donation in 4. The Mulliken charge of +2.2041 for U(1) is considerably less than the +4 expected for a U(IV) centre and the Re charge of −0.0634 is significantly lower than that for the Re centre (+0.2650) in the calculated [(η⁵-C₅H₅)₂Re]⁻ fragment, indicating significant charge donation from the N and Re centres.

The Mayer bond orders reveal a U–Re bond order of 0.896 which is larger than the U–N_amide bonds (av. 0.732) and much greater than the U–N_amine bond (0.241). We examined the U–Re bond in 4 using an energy decomposition analysis which gave a calculated U–Re interaction energy of −561.31 kJ mol⁻¹, ESI†, and whilst there is a dominant and strong electrostatic contribution (68%) to the total attractive interaction between the [(Tren^TMS)U]⁺ and [(η⁵-C₅H₅)₂Re]⁻ fragments, the orbital contribution is not insignificant (32%). The HOMO (219a) and HOMO−1 (218a) SOMOs, Fig. 3, are localised on uranium and possess essentially exclusive 5f character consistent with a ³H₄U(IV) centre.^22b HOMO−3 (216a) and HOMO−4 (215a), together with their β-spin counterparts, are the principal molecular orbitals involved in the U–Re interaction, Fig. 3. HOMO−4 contains 48% Re 5d_z², 6% U 5f_z³, 5% Re 5d_yz, 5% U 6d_z², 1% U 5f_z²y and 1% Re 6p_z character and can be considered as σ-donation from the Re 5d_z² orbital into vacant uranium valence orbitals. HOMO−3 contains 54% Re 5d_yz, 8% U 5f_z²y, 5% Re 5d_z², 2% U 6d_yz and 1% U 5f_z³ character and involves a weak π-type donation from the filled Re 5d_yzπ-orbital into principally the empty U 5f_z²y orbital. The extent of covalency in the U–Re bond in 4 should not be overstated, but it is clear that σ- and π-components are present, and this is the first time that the latter component has been reported in an unsupported f-element–transition metal bond. This is similar to the U–Ga π-interaction in 1;¹⁴ however, in 4 the orbital contribution from U to the π-bond is ca. half that calculated in 1, but it contrasts with [(η⁵-C₅H₅)₂YRe(η⁵-C₅H₅)₂], where the Y–Re bonding interaction was described as electrostatic,¹⁸ which highlights the differences between group 3/lanthanide and actinide bonding interactions.

Fig. 3 Kohn–Sham α-spin frontier orbitals of 4: (a) HOMO (−2.786 eV), (b) HOMO−1 (−2.805 eV), (c) HOMO−3 (−4.043 eV), HOMO−4 (−4.214 eV).

To conclude, we have reported the first structurally authenticated molecular uranium–transition metal bond which contains σ- and π-components. We are investigating the synthesis, structure, and reactivity of 4 and other f-element–metal complexes and will report on their bonding, and reactivity in due course.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Full synthetic, spectroscopic, crystallographic, and computational details for 3 and 4. CCDC 723115 and 723116. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b906554g

‡ Preparation of 3: Me₃SiI (2.13 ml, 15.00 mmol) was added to a cold (0 °C) mixture of [(Tren^TMS)U(Cl)(THF)] (2) (10.58 g, 15.00 mmol). The mixture was allowed to warm to room temperature and was stirred for 1 h. Volatiles were removed at reduced pressure and the resulting green solid was extracted into warm toluene (12 ml). THF (1 ml) was added and the solution was stored overnight to afford 3 as green blocks. Yield: 10.16 g, 85%. Anal. Calcd for C₁₉H₄₇IN₄OSi₃U: C, 28.63; H, 5.95; N, 7.03%. Found: C, 29.48; H, 6.32; N, 7.45%. ¹H NMR (C₆D₆, 295 K) δ 27.35 (4H, s, THF), 15.02 (6H, s, br, CH₂), 7.46 (27H, s, SiMe₃), 2.48 (4H, s, THF), −43.76 (6H, s, br, CH₂). FTIR (Nujol): ν 676.9 (m), 720.4 (m), 774.4 (m), 833.8 (s), 907.9 (s), 927.7 (s), 1021.1 (m), 1060.7 (m), 1081.9 (m), 1247.6 (m). μ_eff (Evans method, C₆D₆, 295 K): 2.79 μ_B.Preparation of 4: THF (30 ml) was added to a cold (−78 °C) mixture of [(Tren^TMS)U(I)(thf)] (0.797 g, 1.00 mmol) and [(η⁵-C₅H₅)₂ReK] (0.335 g, 1.00 mmol). The resulting dark brown suspension was allowed to slowly warm to ambient temperature while stirring. The reaction mixture was then stirred for a further 12 h and after being allowed to settle over 1 h, a dark red solution and white precipitate were afforded. The solution was filtered and volatiles were removed in vacuo to give a red solid. Recrystallisation at −30 °C over 12 h from hot (70 °C) toluene (2 ml) afforded single crystals of 4 suitable for X-ray diffraction. Yield: 0.59 g, 65%. Anal. Calcd for C₂₅H₄₉N₄ReSi₃U: C, 32.85; H, 5.40; N, 6.13%. Found: C, 32.84; H, 5.60; N, 6.15%. ¹H NMR (C₆D₆, 295K) δ 7.65 (6H, s, br, CH₂), 0.20 (27H, s, br, SiMe₃), −0.02 (10H, s, C₅H₅), −11.50 (6H, s, br, CH₂). FTIR (Nujol): ν 799 (m), 836 (w), 1020 (m), 1047 (m), 1095 (m), 1260 (m), 1299 (w). (MS/EI) m/z: 318 [(η⁵-C₅H₅)₂ReH]⁺, 630 [(η⁵-C₅H₅)₂ReUH₂SiMe₃]⁺, 849 [M⁺] − C₅H₅. μ_eff (Evans method, C₆D₆, 295 K): 2.88 μ_B.

§ Single crystal data for 3: C₁₉H₄₇IN₄OSi₃U, M = 796.81, tetragonal, space groupP4₃2₁2, a = b = 12.8637(3), c = 36.1910(13) Å, U = 5988.7(3) ų, Z = 8, D_c = 1.768 g cm⁻³, μ = 6.588 mm⁻¹ (MoKα, λ = 0.71073 Å), T = 150 K, R (F² > 2σ) = 0.0279, R_w (F², all data) = 0.0530, goodness-of-fit = 1.060 for all 6891 unique data (53 401 measured, R_int = 0.0740, 2θ < 55°. CCDC 723115.Single crystal data for 4: C₂₅H₄₉N₄ReSi₃U, M = 914.18, monoclinic, space groupP2₁/c, a = 15.805(2), b = 10.2483(12), c = 19.910(3) Å, β = 100.734(2)°, U = 3168.5(7) ų, Z = 4, D_c = 1.916 g cm⁻³, μ = 9.055 mm⁻¹ (MoKα, λ = 0.71073 Å), T = 150 K, R (F² > 2σ) = 0.0396, R_w (F², all data) = 0.0948, goodness-of-fit = 1.019 for all 7190 unique data (19 270 measured, R_int = 0.0584, 2θ < 55°. CCDC 723116. Bruker SMART APEX CCD diffractometer). Programs: standard Bruker AXS control and integration software and SHELXTL.²⁴

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