Water Activation by Methyllanthanides: Hydrogen Versus Methane Evolution

The lanthanide hydrocarbyl complexes often exhibit exceptional performance in organic synthesis, catalytic process, and small molecule activation, and they generally exhibit several valuable differences in reactivity which can be influenced by the metal center and its oxidation state. Decarboxylation of metal carboxylate precursor is a powerful means to obtain multitudinous organometallic complexes which are well suited for gas-phase investigation by employing electrospray ionization mass spectrometry (ESI-MS) experiments in combination with density functional theory (DFT) calculations.

The (CH₃CO₂)LnCl₃ ^- and (CH₃CO₂)₂LnCl₂ ^- (Ln = Sm, Eu, and Yb) precursor anions were produced in the gas phase via ESI of LnCl₃ and CH₃CO₂Na mixtures in methanol. Collision-induced dissociation (CID) technique was employed to obtain (CH₃)Ln^IIICl₃ ^- and (CH₃)Ln^IICl₂ ^- via fragmentation reactions of lanthanide acetate chloride anions. Activation of water molecule by (CH₃)Ln^IIICl₃ ^- or (CH₃)Ln^IICl₂ ^- via CH₄ or H₂ release was investigated by ion-molecule reaction (IMR) experiments. With the support of DFT calculations, the influences of lanthanide center and its oxidation state were also explored.

(CH₃)Ln^IIICl₃ ^- was generated via decarboxylation of (CH₃CO₂)LnCl₃ ^- (Ln = Sm, Eu, and Yb) upon CID, while (CH₃CO₂)₂LnCl₂ ^- underwent consecutive two-step CO₂/CH₃ or one-step CH₃CO₂·and CO₂ losses to give (CH₃)Ln^IICl₂ ^-. All three (CH₃)Ln^IIICl₃ ^- anions spontaneously reacted with H₂O to form Ln (OH)Cl₃ ^- accompanied by methane release, and the reaction extent is generally following as (CH₃)Sm^IIICl₃ ^- > (CH₃)Eu^IIICl₃ ^- > (CH₃)Yb^IIICl₃ ^-. For (CH₃)Eu^IIICl₃ ^- with H₂O, there was minor (CH₃O)EuCl₃ ^- produced through hydrogen loss reaction as well. Three (CH₃)Ln^IICl₂ ^- anions reacted much more quickly than (CH₃)Ln^IIICl₃ ^- with H₂O via CH₄ loss, and the reactivity enhancement extent is following as Yb > Eu > Sm, which agrees well with Ln-C bond elongation trend from (CH₃)Ln^IIICl₃ ^- to (CH₃)Ln^IICl₂ ^-. Moreover, (CH₃)Sm^IICl₂ ^- and (CH₃)Yb^IICl₂ ^- rather than (CH₃)Eu^IICl₂ ^- underwent H₂ loss reaction with H₂O, which is on the contrary with the cases of (CH₃)Ln^IIICl₃ ^-.

The reactivity of the methyllanthanide chloride complexes towards water is significantly influenced by the lanthanide center and its oxidation state. In general, the methyllanthanide (II) chloride complexes (CH₃)Ln^IICl₂ ^- is much more reactive than the methyllanthanide (III) chloride complexes (CH₃)Ln^IIICl₃ ^-. Besides, the reactivity of (CH₃)Ln^IIICl₃ ^- is following as Sm > Eu > Yb, while that of (CH₃)Ln^IICl₂ ^- is Eu > Sm ≈ Yb.