Article pubs.acs.org/Organometallics
Half- and Mixed-Sandwich Uranium Permethylpentalene Compounds F. Mark Chadwick and Dermot M. O’Hare* Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA, U.K. S Supporting Information *
ABSTRACT: The synthesis and characterization of a series of U(IV) permethylpentalene complexes has been enabled by the development of an effective synthesis of the new Pn*U synthon [Pn*U(μ-Cl)4][Li(TMEDA)]2, which has allowed access to a set of half-sandwich Pn*U complexes. The compounds have been characterized via variable-temperature NMR and in some cases by structural analysis. The temperature dependence of the NMR solution spectra show a complex interplay between fluxional ligand dynamics and the effects of a paramagnetic f2 uranium center.
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INTRODUCTION Since the discovery of uranocene, U(COT)2 (COT = C8H8), by Streitwieser et al. in 1968 the cyclooctatetraene ligand has been a mainstay in organouranium chemistry.1 A considerable body of work has been undertaken utilizing this ligand, including the synthesis of half- and mixed-sandwich derivatives (i.e., compounds featuring both COT rings and Cp ligands; Cp = C5H5); in recent times the primary focus of these studies has been directed at the synthesis of U(III) complexes, due to the discovery of their ability to activate small molecules.2,3 Because of this, U(IV) mixed-sandwich complexes have been somewhat ignored, with only five examples reported to date in the literature.4−9 The pentalene ligand (C8H6, Pn) is isoelectronic with COT, related via a 1,5-transannular bond. However, due to difficulties with the synthesis of suitable precursors, the chemistry associated with this ligand has been severely neglected. To date there are five examples of pentalene-containing uranium compounds in the literature. The earliest report concerned the synthesis of the substituted pentalene uranocene equivalent UPnR2 (R = 1,4-SiiPr3).10 This species was the subject of an indepth DFT and PES analysis, but a crystal structure could not be attained. Two further papers utilizing this substituted pentalene ligand have been publishedone concerning a tris(pyrazolyl)borate pentalene half-sandwich species and the other a PnR/Cp* mixed-sandwich species (Cp* = C5Me5).11,12 Both of these complexes contain U(III); however, the latter was found to both reversibly reduce dinitrogen to form a U(IV)bridged dimer, [(PnR)UCp*]2(μ,η2:η2-N2), and recently enable the two-electron reduction of a phosphaalkyne.13 O’Hare and co-workers have reported the relatively facile syntheses of a pair of permethylated pentalene synthons (Li2Pn* and (SnMe3)2Pn*; Pn* = C8Me6).14,15 The former has been used to synthesize the permethylpentalene uranocene © 2014 American Chemical Society
analogue UPn*2, which has been structurally elucidated via single-crystal X-ray crystallography (demonstrating one of the advantages of permethylation).16 Recently the group has reported a series of group 4 half-sandwich Pn* complexes.17 Here we report the synthesis of the uranium permethylpentalene synthon and its subsequent use in the synthesis of three U(IV) mixed-sandwich complexes.
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RESULTS AND DISCUSSION
Synthesis and Characterization of [Pn*U(μ-Cl)4][Li(TMEDA)]2 (1). Our studies began by targeting an effective synthesis of a suitable half-sandwich Pn* synthon which would allow access to further UPn* chemistry. Initial attempts focused on direct addition of UCl4 to the previously synthesized sandwich compound UPn*2 in a redistribution reaction similar to U(COT) half-sandwich fomation;18 however, this led to no reaction. Combination of 1 equiv of UCl4 with Li2Pn*· xTMEDA also failed to yield the desired product and instead formed the bis-sandwich compound UPn*2. The limited solubility of the reagents in benzene was noted, and thus the UCl4 in benzene was solubilized by addition of just over 2 equiv of TMEDA to form UCl4(TMEDA)2 in situ.19 This was added in a slight excess directly to 1 equiv of Li2Pn*·xTMEDA. The solution promptly darkened, and following subsequent workup black crystals of [Pn*U(μ-Cl)4][Li(TMEDA)]2 (1) were isolated in good yield (71%; Scheme 1). 1 is soluble in aromatic and ethereal solvents and is stable indefinitely at room temperature under a N2 atmosphere. On application of a dynamic vacuum no TMEDA appears to be lost (via inspection Received: May 6, 2014 Published: July 15, 2014 3768
dx.doi.org/10.1021/om500474h | Organometallics 2014, 33, 3768−3774
Organometallics
Article
within the range 2.67(2)−2.74(2) Å, comparable to both the uranium(IV) dimers (range 2.688(11)−2.775(9) Å for the N2 analogues and 2.723(6)−2.781(7) Å for the phosphaalkyne analogue) and its parent U(III) compound PnRUCp* (range 2.683(7)−2.733(7) Å); this is indicative of a δ-symmetry bonding interaction.10 The U−(μ-Cl)−Li moiety is relatively rare, with only 20 such structures registered in the CSD.21−34 The observed U−Cl distances (average of 2.762(4) Å) are typical for such an environment. Interestingly 1 opts to exists as a solvate complex (presumably to alleviate somewhat the coordinately unsaturated and electron-deficient uranium atom). This is in contrast to the analogous group 4 metal permethylpentalene chlorides, which exist as chloride-bridged dimers (with zirconium and hafnium incorporating a single LiCl into their dimeric structures).17 This could be because of the more strongly bound TMEDA stabilizing the presence of two lithium atoms within the structure. The 1H NMR spectrum of 1 consists of four resonances: two sharp bands at δ 10.61 and 8.16 ppm and two broad bands centered at δ 4.27 and 2.48 ppm, of integral ratio 12:6:24:8. These resonances can be assigned to the NWT methyl protons (δ 10.61 ppm; C9, C11, C12, C14), WT methyl protons (δ 8.16 ppm; C13, C10), the TMEDA methyl protons (δ 4.27 ppm; C15, C16, C19, C20, C21, C22, C25, C26), and the TMEDA ethyl protons (δ 2.48 ppm; C17, C18, C23, C24), respectively. In contrast to the other compounds reported within this paper, variable-temperature 1H NMR studies (203− 323 K) showed little change in line shape, indicating that the TMEDA molecules are not labile and the paramagnetism of the sample in solution does not obey simple Curie behavior. The Evans NMR method was employed to investigate the magnetic moment in solution; it was found that the effective magnetic moment decreased with increasing temperature from 2.81 μB at 293 K to 2.74 μB at 353 K. This behavior shows that, like UPn*2, 1 probably exists in a multiconfigurational ground state and therefore its magnetism cannot be easily modeled. The 13C{1H} NMR of 1 shows seven sharp resonances over a range of 700 ppm (δ −456.73 to +256.33 ppm). The TMEDA carbons can be assigned to the resonances at 47.65 ppm (Me) and 57.66 ppm. The WT-Me groups correspond to the 11.06 ppm resonance, while the NWT-Me groups correspond to the −78.28 ppm resonance. HMBC experiments allow the resonance at 256.33 ppm to be assigned to the bridgehead bond; however, it is impossible to distinguish which of the WT or NWT quaternary ring carbons correspond to the −456.73 and 157.61 ppm resonances. The 7Li NMR shows a single resonance at −2.01 ppm. Synthesis and Characterization of the Cp Derivatives [Pn*UCp(μ-Cl)2][Li(TMEDA)] (2) and Pn*U(Cp)2 (3). Having developed a high-yielding, multigram synthesis of the half-sandwich equivalent 1, further salt metathesis chemistry became accessible. Echoing previous work in the area with group 4 metals, both cyclopentadienyl and pentamethylcyclopentadienyl derivatives were targeted. The rapid addition of 0.9 equiv of NaCp to 1 in Et2O at room temperature was employed in order to avoid formation of Pn*UCp2 (3). [Pn*UCp(μCl)2][Li(TMEDA)] (2) was isolated as a black amorphous solid in moderate yield from a pentane extraction (42%; Scheme 2). 1H NMR spectroscopy revealed the presence of bound TMEDA, which could not be removed upon prolonged exposure to a vacuum ( 2σ(I)). CCDC 1000532. Crystal data for 4: UN2LiCl2C30H49, monoclinic (P21/c), a = 10.6213(1) Å, b = 43.4930(5) Å, c = 21.1430(3) Å, α = γ = 90°, β = 104.1914(5) °, V = 9469.0(2) Å3, Z = 12, λ = 0.71073 Å, T = 150(2) K, μ = 5.33 mm−1, Dcalcd = 1.586 Mg m−3, 20391 independent relections (R(int) = 0.045); R1 = 0.0589, wR2 = 0.0960 (I > 2σ(I)). CCDC 1000533.
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ASSOCIATED CONTENT
S Supporting Information *
More detailed figures of the variable-temperature 1H NMR spectra of 2 and 4, figures giving the 1H NMR spectra of 1, 2, and 4, and CIF files giving crystallographic data for 1 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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