Article pubs.acs.org/IC
Extending Stannyl Anion Chemistry to the Actinides: Synthesis and Characterization of a Uranium−Tin Bond Matthew S. Winston,† Enrique R. Batista,*,‡ Ping Yang,‡ Aaron M. Tondreau,† and James M. Boncella*,† †
Chemistry Division and ‡Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
ABSTRACT: We have synthesized a rare example of a uranium(IV) stannyl (κ4-N(CH2CH2NSi(iPr)3)3U(SnMe3), 1) via transmetalation with LiSnMe3. This complex has been characterized crystallographically and shown to have a U−Sn bond length of 3.3130(3) Å, substantially longer than the only other crystallographically observed U−Sn bond (3.166 Å). Computational studies suggest that the U−Sn bond in 1 is highly polarized, with significant charge transfer to the stannylate ligand. We briefly discuss plausible mechanistic scenarios for the formation of 1, which may be relevant to other transmetalation processes involving heavy main group atoms. Furthermore, we demonstrate the reducing ability of [SnMe3]− in the absence of strongly donating ligands on U(IV).
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INTRODUCTION Although the synthesis and reactivity of transition metal stannyl complexes are well understood,1 f-block stannyls are comparatively rare. Within the f-block, reported lanthanide−tin bonds,2 which are installed by salt metathesis with stannylates (M−SnR3, M = group I) or protonolysis with tin hydrides (HSnR3), are more common than their actinide congeners, of which only one has been unambiguously reported.3 Porchia demonstrated that amido protonolysis of Cp3U−NEt2 (Cp = cyclopentadienyl) by HSnPh3 cleanly afforded Cp3U−SnPh3, which was characterized crystallographically and shown to have a U−Sn bond (3.166 Å) within the sum of each atoms’ covalent radius (3.35 Å).4 Notably, a salt metathesis approach from Cp3UCl and LiSnPh3 resulted in the formation of multiple products. Through calorimetric studies, Marks has shown that the U−Sn bond in Cp3U−SnPh3 is decidedly weak (bond disruption enthalpy = 37.2(4.0) kcal/mol in toluene),5 while Cummins predicts that the U−Sn bond disruption enthalpy in tris(amide)-stabilized U(IV) is somewhat larger (44 kcal/ mol).6 These insights suggest that the lack of isolable uranium(IV) stannyls may be attributed to, in part, a small or negative thermodynamic driving force for their formation. The robust and versatile Cp ligand and its derivatives can stabilize high and low oxidation states7 and render potentially weak U−X bonds kinetically inert; as a result, Cp ligands are amenable to the isolation and characterization of novel U− metal/heteroatom bonds.3,8,9 Recently, bulky and easily derivatized amido ligands have been used to characterize U− metal/heteroatom bonds in non-Cp-stabilized species. For © XXXX American Chemical Society
instance, Cummins has reported the synthesis of the uranium(IV) silyl (Ar[tBu]N)3U−Si(SiMe3)3 (Ar = 3,5-C6H3Me2),6 while Liddle has had tremendous success characterizing U−X bonds of varied saturation (X = N,10 P,11 As,12 Ga13) within the U-N-silyl-TREN manifolds (TREN = tris(2-amidoethyl)amine). Therefore, it is surprising that U−Sn bonds have yet to be characterized in non-Cp systems. Herein we present the synthesis and characterization of a U−Sn bond within a TRENtype coordination environment. We further discuss the reducing behavior of tin anions with “ligand-free” actinides.
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RESULTS AND DISCUSSION
Synthesis and Characterization of Uranium(IV) Stannyl 1. Actinide halides are more accessible than actinide amides, and salt metathesis with stannyl lithium is potentially a more general route to stannyl actinides. Stannyl lithium solutions are typically prepared by reduction of distannanes (R3Sn−SnR3) or stannyl chlorides (Cl−SnR3)14 or by deprotonation of a tin hydride (H−SnR3) with a strong amide base.15 Nevertheless, these methods require excess Sn reagent and suffer from reduced yields, often due to side reactions and product instability in solution.16 We reasoned that impurities in traditionally prepared stannyl lithium solutions contribute to decomposition of the reactive actinide product. Wang and Uchiyama17 have reported a high-yielding preparation of THF solutions of stannyl lithium (Li−SnR3, R Received: March 4, 2016
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DOI: 10.1021/acs.inorgchem.6b00543 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry = Me, nBu, Ph), from either distannanes or stannyl chlorides, lithium metal, and catalytic naphthalene (Scheme 1). By Scheme 1. Preparation of Stannyl Lithium Solutions in THF and Salt Metathesis Reaction of Uranium(IV) Chlorides
Figure 1. Selected region of the 1H NMR spectrum of 1 in C6D6 at 298 K. The signal for Si(CH(CH3)2)3 has been vertically truncated. * and ⧧ correspond to unidentified impurities and NMR solvent isotopomers, respectively. See Figure S1 for a full spectrum.
titration, >99% of the Sn source is lithiated, and the only other product is lithium naphthalenide in catalytic quantities; importantly, these solutions are stable for up to several weeks, highlighting the inertness of pure lithium organostannylate solutions, which may contribute to predictable and wellbehaved reactivity. Since its report, this method has not been used to install metal−tin bonds via salt metathesis (Scheme 1), but we reasoned that the high purities of LiSnR3 solutions might aid in isolation of stannyl actinide complexes. Indeed, addition of a green solution of LiSnMe3 in THF to a green solution of ( TIPS TREN)UCl ( TIPS TREN = N(CH2CH2NSi(iPr)3)3)10a in toluene at −30 °C instantly resulted in a deep red solution, which was stirred for 1 h while warming to room temperature (Scheme 2). After
broad charge-transfer band at 558 nm (ε = 1200 M−1 cm−1) (Figure S2). Complex 1 crystallizes in the monoclinic space group P21/n. Its structure was characterized by X-ray diffraction (Figure 2A),
Scheme 2. Synthesis of U(IV) Stannyl 1 and Cyclometalated Complex 2 Figure 2. (A) Thermal ellipsoid representation of 1 at the 50% probability level. Hydrogen atoms and some iPr substituents on Si have been omitted for clarity. Selected bond lengths: U−Sn (3.3130(3) Å); U−N(1) (2.246(3) Å); U−N(2) (2.227(3) Å); U− N(3) (2.237(3) Å); U−N(4) (2.635(2) Å). (B) Uranium−tin bonding molecular orbital of 1 from natural bond order analysis.
unambiguously revealing a pseudo-C3v stannyl complex with a U−Sn bond (3.3130(3) Å) just under the sum of U−Sn covalent radii. The U−Sn bond in Cp3U−SnPh3 is substantially shorter (3.166 Å), perhaps reflecting the greater steric congestion in the TIPSTREN ligand around the metal center relative to Cp. As expected, the Sn atom is highly pyramidalized (∑C−Sn−C = 293.0(3)°). Density functional theory (DFT) and natural bond orbital (NBO)18 calculations were carried out to characterize the frontier orbitals and U−Sn bonding in 1, using the DFT-M0619 functional and the Stuttgart relativistic pseudopotentials for U20 and Sn.21 A 6-31+G* basis set was used for all other atoms. The two unpaired spins (f2) are located in nonbonding U orbitals primarily of f-orbital character (96%) (see Supporting Information). NBO analysis of the U−Sn bond suggests a single, polarized σ-bond, with 25% and 75% of the electron density on U and Sn, respectively. Because the contributions to the U−Sn bonding molecular orbital are mostly the U dz2 and [SnMe3]− 2a1 atomic orbitals, the interaction is appreciably directional (Figure 2B). Congruent with the spectroscopic data, there is significant charge transfer from Sn, resulting in a primarily ionic (75%) interaction. Maron and Liddle22 have observed ligand activation to the cyclometalated complex [U{N(CH 2 CH 2 NSi( i Pr) 3 ) 2 (CH 2 CH 2 NSi( i Pr) 2 C(H)MeCH 2 )} (2) upon treating (TIPSTREN)UCl with KBn, presumably via the unobservable U(IV) benzyl intermediate; DFT calculations point toward a
concentration, the product was dissolved in hexane, filtered, and cooled to isolate large red crystals of 1 in 76% yield. The mother liquor also contained
