Dorothea Schädle

Methylaluminum‐Supported Rare‐Earth‐Metal Dihydrides

Compounds combining the large rare-earth-metal (Ln) centers with the smallest anionic ligand, H (hydrido), continue to pose challenging questions both in fundamental and applied chemistry. The inherent bonding properties in solid-state binary LnHx phases (e.g., causing metallic behavior) as well as in ligand-supported molecular counterparts (revealing unique cluster chemistry, see Supporting Information) have been the focus of extensive research. Moreover, heterobimetallic solid-state materials, such as Ni5LaHx, feature approved rechargeable battery components or, such as LnAlH6 (obtained from LnCl3 and NaAlH4 by the release of hydrogen), are discussed as intermediate-temperature hydrogen-storage materials. On the other hand, the quest for soluble molecular hydrides has triggered immense research efforts. In the meantime, mono and dihydrido derivatives “L2LnH” and “LLnH2” (L = monoanionic ligand), respectively, are assigned a crucial role in a variety of stoichiometric and catalytic transformations, whereas complexes of type [LnH3(Do)x] (Do = neutral donor ligand) are still elusive. While mono hydride complexes can exist as monomers, e.g., [(C5H2tBu3)2CeH], [4] dihydrido species “LLnH2”, carrying only one ancillary ligand per lanthanide center, tend to form polynuclear complexes (see Supporting Information) containing as few as two and up to six lanthanide metal centers. Several types of ancillary ligands have been employed in an effort to stabilize complexes of low nuclearity, including sterically demanding cyclopentadienyl derivatives such as C5Me4SiMe3 [6] tris(pyrazolyl)borato scorpionates, tetraazacycloamido, bis(phosphinophenyl)amido pincer, and pyridylamido ligands as well as chelating diamido ligands (see Supporting Information). However, the synthesis of a monomeric rare-earth-metal dihydride was not successful to date. The group of Takats used the sterically demanding hydrotris(3-tert-butyl-5-methylpyrazolyl)borato ligand (Tp) to stabilize Ln centers in species such as alkyls, carbenes, amides, halides, 13] or hydrides and was also able to obtain lanthanide dihydride complexes using the less-bulky dimethyl, diisopropyl, or unsubstituted derivative of the Tp ligand, but reported the formation of a mixture of products for the more bulky Tp ligand because of possible side reactions involving the ligand tertbutyl group. 15] Since tetrameric [(TpLnH2)4] as well as the other dihydride clusters reported were all synthesized from alkyl precursors by the addition of H2 or silanes, we tried to adopt a different route using HAlMe2 as hydride source. For example, this reaction pathway could yield the desired [TpLnH2] complex as a mononuclear species owing to the steric bulk of the ligand by a direct alkyl hydrido exchange generating trialkyl aluminum as byproduct or result in the formation of a bimetallic adduct complex. As use of the super-bulky tris(pyrazolyl)borato ligand Tp had enabled the isolation of soluble monomeric rareearth-metal dimethyl complexes we treated complex [TpLuMe2] with two equivalents of HAlMe2 in toluene at ambient temperature (Scheme 1). Formation of a precip-

Reactivity of halfsandwich rare-earth metal methylaluminates toward potassium (2,4,6-tri-tert-butylphenyl)amide and 1-adamantylamine

The equimolar reaction of potassium (2,4,6-tri-tert-butylphenyl)amide with Cp*Ln(AlMe4)2 (Cp* = 1,2,3,4,5-pentamethyl cyclopentadienyl) yielded {Cp*Ln(AlMe4)[NH(mes*)]}x (Ln = Y, La; mes* = C6H2tBu3-2,4,6). The treatment of Cp*Ln(AlMe4)[NH(mes*)] with tetrahydrofuran led to intramolecular C–H bond activation of a tBu group with the formation of Cp*YMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2)(thf). A similar methyl-anilide species CpQLuMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2) (CpQ = 2,3,4,5-tetramethyl-1-(8-quinolyl)cyclopentadienyl) with a C–H bond activated ligand backbone formed by the reaction of CpQLu(AlMe4)2 and K[NH(mes*)]. The reactivity of CpQY(AlMe4)2 toward H2NAd (Ad = adamantyl) ultimately led to the methyl–amide complex CpQYMe[NH(Ad)](AlMe3), corroborating the presence of competing deprotonation and donor-induced methylaluminate cleavage reactions. The halfsandwich complexes CpQLu(AlMe4)2, Cp*Y(AlMe4)[NH(mes*)], Cp*YMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2)(thf), CpQLuMe{NH[C6H2tBu2-2,4-(CMe2CH2)-6]}(AlMe2), and CpQYMe[NH(Ad)](AlMe3) as well as the side-product AlMe3(H2NAd) were fully characterized by NMR/FTIR spectroscopy, elemental analysis, and X-ray crystallography.

Rare-Earth Metal Diimide Complexes via Alkylaluminate Templating, including a Ceric Derivative

Protonolysis of lanthanide tris(tetramethylaluminate)s with two equivalents of 2,6-diisopropylaniline affords LaIII and CeIII diimide compounds Ln[(μ-NC6 H3 iPr2 -2,6)2 AlMe2 ](thf)4 featuring a bidentate AlMe2 -linked diimido ligand. As revealed for the corresponding Ce(GaMe4 )3 -reaction, formation of the diimide complexes proceeds via tetrametallic complexes of the type [Ce{(μ-NC6 H3 iPr2 -2,6)(HNC6 H3 iPr2 -2,6)(MMe3 )}]2 (Me=Al, Ga). Oxidation of the cerium(III) complex with hexachloroethane leads to a neutral CeIV diimide species. Partial protonolysis with phenylacetylene and hydrogenolysis via H3 SiPh give conclusive insights into the reactive coordination sites of such diimide complexes.