Reversible Dihydrogen Activation in Cationic Rare-Earth-Metal Polyhydride Complexes

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.

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system.

Stabilizing hydrogen-mediated sextuple bonds by quintuple superatomic bonding and a bond

Multiple bond orders of four and five are frequently obtained for d-block elements. However, compounds with sextuple bonding (2σ, 2π, and 2δ), for instance, Cr₂, Mo₂ and W₂, are less stable and trapped only in the gas phase or inert matrices, probably resulting from large repulsion of two σ-type (σ_s and ) orbitals within the same zone. Herein, a superatomic bonding model is proposed to describe experimentally synthesized bridging hydride compounds. We theoretically predicted four unprecedented quintuple bridging hydride species with large E_HL values, among which [(Cp)₂Sc₂(μ-H)₅]^- and [(Cp)₂Mn₂(μ-H)₅]^- (Cp = cyclopentadienyl) contain quintuple superatomic bonding (σ, 2π, and 2δ). In particular, two other species, [(Cp)₂Ti₂(μ-H)₅]^- and [(Cp)₂V₂(μ-H)₅]⁺, were found to feature stable hydrogen-mediated sextuple bonding comprising a quintuple superatomic delocalized bond and an extra localized bond. Thereinto, the superatomic σ_s bond disperses to the outer space for better overlap with the orbitals of bridging H atoms, thus leaving the inner regions with low electron densities and providing adequate space for the bond. These two quintuple bridging hydride compounds are verified to have thermal stability by molecular dynamics simulations and expected to provide an effective strategy to synthesize molecules with stable hydrogen-mediated sextuple bonding under normal experimental conditions.

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.

Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Chemo- and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d- and f-block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti-Markovnikov products, while aryl-substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di- and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy- and siloxy-substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

Cationic magnesium hydride [MgH]+ stabilized by an NNNN-type macrocycle

A magnesium hydride cation [(L)MgH]⁺ supported by a macrocyclic ligand (L = Me₄TACD; 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) has been shown to react with Lewis acids as well as with unsaturated substrates including pyridine.

Zinc hydridotriphenylborates supported by a neutral macrocyclic polyamine

A neutral macrocyclic polyamine 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me₄TACD) stabilizes cationic zinc hydridotriphenylborates that react with CO₂(1 atm) under insertion into the B–H bonds.

Rare-earth metal hydrides supported by silicon-bridged boratabenzene fluorenyl ligands: synthesis, structure and reactivity

The first boratabenzene derivatives of rare-earth metal hydrides were synthesized, and their reactivity was studied.

Neutral binuclear rare-earth metal complexes with four μ2-bridging hydrides

The first neutral dimeric rare-earth metal hydride complexes containing quadruply bridged hydrido ligands were prepared, in which the intramolecular activation triggered by a THF molecule has been observed.