The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

Dinitrogen Binding at a Trititanium Chloride Complex and Its Conversion to Ammonia under Ambient Conditions

Reaction of [TiCp*Cl3 ] (Cp*=η5 -C5 Me5 ) with one equivalent of magnesium in tetrahydrofuran at room temperature affords the paramagnetic trinuclear complex [{TiCp*(μ-Cl)}3 (μ3 -Cl)], which reacts with dinitrogen under ambient conditions to give the diamagnetic derivative [{TiCp*(μ-Cl)}3 (μ3 -η1  : η2  : η2 -N2 )] and the titanium(III) dimer [{TiCp*Cl(μ-Cl)}2 ]. The structure of the trinuclear mixed-valence complexes has been studied by experimental and theoretical methods and the latter compound represents the first well-defined example of the μ3 -η1  : η2  : η2 coordination mode of the dinitrogen molecule. The reaction of [{TiCp*(μ-Cl)}3 (μ3 -η1  : η2  : η2 -N2 )] with excess HCl in tetrahydrofuran results in clean NH4 Cl formation with regeneration of the starting material [TiCp*Cl3 ]. Therefore, a cyclic ammonia synthesis under ambient conditions can be envisioned by alternating N2 /HCl atmospheres in a [TiCp*Cl3 ]/Mg(excess) reaction mixture in tetrahydrofuran.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments

Low-valent titanium species were prepared by reaction of [TiCp*X₃ ] (Cp*=η⁵ -C₅ Me₅ ; X=Cl, Br, Me) with LiEH₄ (E=Al, B) or BH₃ (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X₃ ] with LiAlH₄ in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}₂ {(μ-H)₂ AlX(L)}₂ ] (L=thf, X=Cl, Br; L=OEt₂ , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH₂ XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X₃ ] with LiBH₄ (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH₄ )(μ-X)}₂ ] (X=Cl, Br) and [{TiCp*(BH₄ )(μ-BH₄ )}₂ ], respectively. The treatment of [TiCp*Me₃ ] with 4 and 5 equiv. of BH₃ (thf) produces the diamagnetic [{TiCp*(BH₃ Me)}₂ (μ-B₂ H₆ )] and paramagnetic [{TiCp*(μ-B₂ H₆ )}₂ ] complexes, respectively.

Preparation of Dimeric Monopentamethylcyclopentadienyltitanium(III) Dihalides and Related Derivatives

The synthesis, crystal structure, and reactivity of a series of half-sandwich titanium(III) dihalide complexes [Ti(η⁵-C₅Me₅)X₂] (X = Cl, Br, I) and several of its Lewis base derivatives were investigated. The reaction of the trihalides [Ti(η⁵-C₅Me₅)X₃] (X = Cl (1), Br (2), I (3)) with LiAlH₄ (≥1 equiv) in toluene at room temperature results in the formation of the halide-bridged dimers [{Ti(η⁵-C₅Me₅)X(μ-X)}₂] (X = Cl (4), Br (5), I (6)). The treatment of 4 with [Li{N(SiMe₃)₂}] (≥2 equiv) at room temperature affords the precipitation of the amido titanium(III) complex [{Ti(η⁵-C₅Me₅)(μ-Cl){N(SiMe₃)₂}}₂] (7), but analogous reactions of 4 with other lithium reagents [LiR] (R = Me, CH₂SiMe₃, NMe₂) lead to disproportionation into titanium(IV) [Ti(η⁵-C₅Me₅)R₃] and presumably titanium(II) derivatives. Similarly, complex 4 in solution at temperatures higher than 100 °C undergoes disproportionation as demonstrated by its reactions with cobaltocene and N-(4-methylbenzylidene)aniline yielding the ionic paramagnetic compound [Co(η⁵-C₅H₅)₂][Ti(η⁵-C₅Me₅)Cl₃] (8) and the diamagnetic diazatitanacyclopentane [Ti(η⁵-C₅Me₅)Cl{N(Ph)CH(p-tolyl)}₂], respectively. Treatment of complex 4 with 2 equiv of 2,6-dimethylphenylisocyanide or tert-butylisocyanide in toluene at room temperature affords the paramagnetic titanium(III) dinuclear adducts [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)(CNR)}₂] (R = 2,6-Me₂C₆H₃ (9), tBu (10)). Magnetic studies for polycrystalline 9 show that it displays a weak intramolecular antiferromagnetic coupling between the Ti ions, which is consistent with the long Ti-Ti distance of 3.857(1) Å determined by X-ray diffraction. The isocyanide ligands in complex 10 undergo a reductive coupling reaction in toluene to give the titanium(IV) iminoacyl derivative [{Ti(η⁵-C₅Me₅)Cl₂}₂(μ-η²:η²-tBuN═C-C═NtBu)] (11). Whereas an analogous dinuclear structure was found in the aqua titanium(III) complex [{Ti(η⁵-C₅Me₅)Cl(μ-Cl)(OH₂)}₂] (12), resulting from the reaction of 4 with adventitious amounts of water, compound 4 reacts with excess ammonia to give a mononuclear adduct [Ti(η⁵-C₅Me₅)Cl₂(NH₃)₂] (13) with a robust layered pattern in the solid state.