Variable nuclearity scorpionate-supported lanthanide polyhydrides: [(TpR,R′)LnH2]n (n=3, 4 and 6)

f-Block hydride complexes - synthesis, structure and reactivity

Complexes formed between the heaviest and lightest elements in the periodic table yield the f-block hydrides, a unique class of compounds with wide-ranging utility and interest, from catalysis to light-responsive materials and nuclear waste storage. Recent developments in syntheses and analytics, such as exploiting low-oxidation state metal ions and improvements in X-ray diffraction tools, have transformed our ability to understand, access and manipulate these important species. This perspective brings together insights from binary metal hydrides, with molecular solution phase studies on heteroleptic complexes and gas phase investigations. It aims to provide an overview of how the f-element influences hydride formation, structure and reactivity including the sometimes-surprising power of co-ligands to tune their behaviour towards a variety of applications.

Multi-electron redox reactivity of a samarium(ii) hydrido complex

Well-defined low-valent molecular rare-earth metal hydrides are rare, and limited to Yb2+ and Eu2+ centers. Here, we report the first example of the divalent samarium(ii) hydrido complex [(CpAr5)SmII(μ-H)(DABCO)]2 (4) (CpAr5 = C5Ar5, Ar = 3,5-iPr2-C6H3; DABCO = 1,4-diazabicyclooctane) supported by a super-bulky penta-arylcyclopentadienyl ligand, resulting from the hydrogenolysis of the samarium(ii) alkyl complex [(CpAr5)SmII{CH(SiMe3)2}(DABCO)] (3). Complex 4 exhibits multi-electron redox reactivity toward a variety of substrates. Exposure of complex 4 to CO2 results in the formation of the trivalent samarium(iii) mixed-bis-formate/carbonate complex [(CpAr5)SmIII(μ-η2:η1-O2CH)(μ-η2:η2-CO3)(μ-η1:η1-O2CH)SmIII(CpAr5)(DABCO)] (8), mediated by hydride insertion and reductive disproportionation reactions. Complex 4 shows four-electron reduction toward four equivalents of CS2 to afford the trivalent samarium(iii) bis-trithiocarbonate complex [(CpAr5)SmIII(μ-η2:η2-CS3)(DABCO)]2 (9). A mechanistic study of the formation of complex 8 was carried out using DFT calculations.

Synthesis and Reactivity of Bis-tris(pyrazolyl)borate Lanthanide/Aluminum Heterobimetallic Trihydride Complexes

Molecular heterobimetallic hydride complexes of lanthanide (Ln) and main-group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(μ-H)2Al(H)(N″)] [2-Ln; Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; N″ = N(SiMe3)2] have been synthesized by facile insertion of aminoalane [Me3N·AlH3] into the Ln-N amide bonds of [Ln(Tp)2(N″)] (1-Ln). Thus, this is a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts.

Utility of all-pyrazole heteroscorpionates in f-element chemistry

Since their discovery in 1966, scorpionate ligands have been utilized to make coordination compounds for a variety of applications such as: studying organometallic reactions, biomimetic complexes, light-emitting materials and single-ion magnets. The recent development of a solvent-free pyrazole substitution chemistry has yielded the quantitative synthesis of asymmetrically functionalized all-pyrazole heteroscorpionate ligands. In this frontier article, we highlight the utility of all-pyrazole heteroscorpionates, specifically, nitro-trispyrazolylborates, in f-element chemistry. They offer great versatility in coordinating ability, donor strength, steric bulk and even optical charge transfer properties, all of which can be used to tune the properties of resultant complexes with metal ions. We show how they can impart structural diversity, sensitize Ln3+ luminescence and engender magnetic anisotropy and slow magnetic relaxation in the ion they coordinate. Additionally, we comment on the future of functionalized trispyrazolyl scorpionates, which includes enabling post-synthetic modifications of f-element complexes and becoming a platform to study the electronic properties of low oxidation state actinides.

Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises

The synthesis, characterization, and solid-state structure of bulky alkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported. Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl, Y(OTr*)(CH₂SiMe₃)₂(THF)₂ (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl), resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H₂(THF)]₄ (1a). X-ray analysis revealed a highly symmetrical structure (4̅ site symmetry) with the four Y atoms located on the corners of a compressed tetrahedron, each bonded to an OTr* and tetrahydrofuran (THF) ligand and the cluster held together by four face-capping, μ₃-H, and four edge-bridging, μ₂-H, hydrides. DFT calculations on the full system with and without THF, but also on model systems, clearly show that the structural preference for complex 1a is controlled by the presence and coordination of THF molecules. Contrary to the exclusive formation of the tetranuclear dihydride, hydrogenolysis of the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH₂SiMe₃)₂(THF)₂ (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y₃(OAr*)₄H₅(THF)₄], polyhydride, 2b. Similar results, i.e., a mixture of tetra-/tri-nuclear products, were obtained from hydrogenolysis of the even bulkier Y(OAr^Ad2,Me)(CH₂SiMe₃)₂(THF)₂ compound. Experimental conditions were established to optimize the production of either the tetra- or trinuclear products. X-ray structure of 2b revealed a triangular array of three yttrium atoms with two face-capping μ₃-H and three edge-bridging μ₂-H hydrides, with one yttrium bonded to two aryloxy ligands while the other two have a complement of one aryloxy and two THF ligands; the solid-state structure is close to being C₂ symmetric, with the C₂ axis running through the unique Y and unique μ₂-H hydride. As opposed to 2a, which shows distinct ¹H NMR resonances for μ₃/ μ₂-H (δ = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchange on the NMR time scale. Their presence and assignment were secured at -40 °C from ¹H SST (spin saturation) experiment.

Rare-earth-metal half-sandwich complexes incorporating methyl, methylidene, and hydrido ligands

Complexes Cp*₃Ln₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ (Ln = Y, Dy; Cp* = C₅Me₅) are formed via elimination of CH₄ and C₂H₄ from the respective trinuclear polymethyl complexes [Cp*Ln(μ₂-CH₃)₂]₃, involving the methyl/methylidene complex Cp*₃Ln₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-CH₃)(thf)₂ as an intermediate. Compound Cp*₃Y₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ displays "Schrock-type" nucleophilic carbene reactivity, converting ketones into the corresponding terminal alkenes. Treatment of Cp*₃Y₃(μ₂-CH₃)₃(μ₃-CH₂)(μ₃-H)(thf)₃ with [D₄]methanol proved accessibility of all small-group functionalities as indicated by the simultaneous formation of deuterated HD and deuterated methanes CH₃D and CH₂D₂.

CO, CO2 and CS2 activation by divalent ytterbium hydrido complexes

Treatment of a divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (1) with CO, CO₂ and CS₂ resulted in the formation of a divalent ytterbium ethenediolate complex [(Tp^Ad,iPr)Yb]₂(cis-OCHCHO) (2), a formate complex [(Tp^Ad,iPr)Yb(κ²-O₂CH)(THF)] (3), and a trivalent ytterbium ethenetetrathiolate complex [(Tp^Ad,iPr)Yb^III]₂(C₂S₄) (4), respectively. DFT calculations were carried out to elucidate the reaction profiles of complexes 3 and 4.

A mononuclear divalent ytterbium hydrido complex supported by a super-bulky scorpionate ligand

The first mononuclear divalent ytterbium hydride complex [(Tp^Ad,iPr)Yb(H)(THF)] (Tp^Ad,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) (2) bearing a terminal hydrido ligand was obtained by hydrogenolysis of the benzyl precursor in hexane. Complex 2 exhibited two different reaction patterns towards allenes: Yb-H addition with cyclohexylallene and deprotonation of 1,1-dimethylallene.

Scandium and lanthanum hydride complexes stabilized by super-bulky penta-arylcyclopentadienyl ligands

Hydrogenolysis of the half-sandwich penta-arylcycopentadienyl-supported rare-earth metal dibenzyl complexes [(Cp^Ar5)Ln(p-CH₂-C₆H₄-Me)₂(THF)] (Cp^Ar5 = C₅Ar₅, Ar = 3,5-ⁱPr₂-C₆H₃; Ln = Sc, La) afforded a bimetallic scandium complex [(Cp^Ar5)Sc(H)(μ-OC₄H₉)]₂ (2) with two terminal hydrido ligands, and a double-sandwich bimetallic lanthanum hydride complex [(Cp^Ar5)La(μ-H)]₂ (4) bearing the reduced Cp^Ar5 ligand. DFT calculations were conducted to elucidate the reaction profiles.

Photoluminescence of Homoleptic Lanthanide Complexes With Tris(benzotriazol-1-yl)borate

Bright photoluminescent neutral complexes having general formula [Ln(tbtz)₃] (Ln = Eu, Tb; tbtz = tris(benzotriazol-1-yl)borate) were obtained by reacting K[tbtz] with EuCl₃ and TbCl₃. The emissions in the visible range, related to the f-f transitions of the trivalent lanthanide ions, are observable upon excitation with wavelengths shorter than 350 nm. The most intense emission bands correspond to the ⁵D₀ → ⁷F₄ transition at 699 nm for the europium complex and to the ⁵D₄ → ⁷F₅ transition at 542 nm for the terbium derivative. The luminescence is in all the cases mostly associated with the antenna-effect from the coordinated tbtz ligands. The synthetic approach was successfully extended to the preparation of the analogous yttrium and gadolinium derivatives. Tricapped trigonal prismatic geometry was attributed to the complexes on the basis of luminescence data and DFT calculations. Highly photoluminescent plastic materials were obtained by embedding small amounts of [Eu(tbtz)₃] or [Tb(tbtz)₃] in poly(methyl methacrylate).

Monomeric thorium dihydrido complexes: versatile precursors to actinide metallacycles

The monomeric actinide dihydrido complex [(Cp^Ar*)(Cp*)ThH₂(THF)] (2) and actinide metallacyclopentyne [(Cp^Ar*)(Cp*)Th(PhCH–CC–CHPh)] (4) were obtained for the first time.

Trinuclear alkyl hydrido rare-earth complexes supported by amidopyridinato ligands: synthesis, structures, C–Si bond activation and catalytic activity in ethylene polymerization

[(Ap^9MeLu)₃(μ²-H)₃(μ³-H)₂(CH₂SiMe₃)(thf)₂] was synthesized. For Y and Yb C–Si bond activation occurs affording [(Ap^9MeLu)₃(μ²-H)₃(μ³-H)₂(CH₂SiMe₃)(thf)₂] and [(Ap^9MeLn)₃(μ²-H)₃(μ³-H)₂(CH₂SiH₂Ph)(thf)₂].