Rare-Earth-Metal Methyl and Methylidene Complexes Stabilized by TpR,R′-Scorpionato Ligands─Size Matters

Homoleptic tetramethylaluminates Ln(AlMe4)3 react with KTptBu,Me (TptBu,Me = tris(3-tBu-5-Me-pyrazolyl)borato) to yield rare-earth-metal methylidene complexes (TptBu,Me)Ln(μ3-CH2)(μ-Me)AlMe22 (Ln = La, Ce, Nd). The lanthanum reaction is prone to additional C–H- and B–N-bond activation, affording coproducts LaHB(pzMe,tBu)(pzCMe2,Me)2(μ-CH2)(μ-Me)AlMe22 and La(μ-pztBu,Me)(AlMe4)22 (pztBu,Me = 3-tBu-5-Me-pyrazolato). The protonolysis reaction of Ln(AlMe4)3 and HpztBu,Me provides more efficient access to Ln(μ-pztBu,Me)(AlMe4)22 (Ln = La, Nd). Treatment of Ln(AlMe4)3 with KTpMe,Me led to methylidene complexes (TpMe,Me)Ln(μ3-CH2)(μ-Me)AlMe22 (Ln = Nd, Sm) or bis(tetramethylaluminate) complexes (TpMe,Me)Ln(AlMe4)2 (Ln = Y, Lu). The neodymium reaction generated methine derivative (TpMe,Me)Nd(μ4-CH)(AlMe2)2(μ-pz,Me,Me)(μ-Me)AlMe2 as a minor coproduct. The reaction of Ln(GaMe4)3 (Ln = Y, La, Ce, Nd, Sm, Ho) with HTptBu,Me gave methylidene complexes (TptBu,Me)Ln(μ3-CH2)(μ-Me)GaMe22 (Ln = La, Ce, Nd, Sm) and alkyl complexes (TptBu,Me)LnMe(μ-Me)GaMe3 (Ln = Y, Ho), while competing B–N bond activation reactions produced GaMe2BH(Me)(μ-pztBu,Me)2 and (TptBu,Me)Ln(η2-pztBu,Me)(μ-Me)GaMe3 (Ln = Y, Ho). The steric impact of the TpR,Me ligands was examined by cone angle calculations. Rare-earth-metal methylidene complexes (TptBu,Me)Ln(μ3-CH2)(μ-Me)EMe22 (E = Al, Ga) successfully promote carbonyl methylenation reactions upon addition of ketone.