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

Homoleptic tetramethylaluminates Ln(AlMe ) react with KTp (Tp = tris(3- Bu-5-Me-pyrazolyl)borato) to yield rare-earth-metal methylidene complexes (Tp )Ln(μ -CH )[(μ-Me)AlMe ] (Ln = La, Ce, Nd). The lanthanum reaction is prone to additional C-H- and B-N-bond activation, affording coproducts La[HB(pz )(pz ) ][(μ-CH )(μ-Me)AlMe ] and [La(μ-pz )(AlMe ) ] (pz = 3- Bu-5-Me-pyrazolato). The protonolysis reaction of Ln(AlMe ) and Hpz provides more efficient access to [Ln(μ-pz )(AlMe ) ] (Ln = La, Nd). Treatment of Ln(AlMe ) with KTp led to methylidene complexes (Tp )Ln(μ -CH )[(μ-Me)AlMe ] (Ln = Nd, Sm) or bis(tetramethylaluminate) complexes (Tp )Ln(AlMe ) (Ln = Y, Lu). The neodymium reaction generated methine derivative (Tp )Nd[(μ -CH)(AlMe ) (μ-pz )][(μ-Me)AlMe ] as a minor coproduct. The reaction of Ln(GaMe ) (Ln = Y, La, Ce, Nd, Sm, Ho) with HTp gave methylidene complexes (Tp )Ln(μ -CH )[(μ-Me)GaMe ] (Ln = La, Ce, Nd, Sm) and alkyl complexes (Tp )LnMe[(μ-Me)GaMe ] (Ln = Y, Ho), while competing B-N bond activation reactions produced GaMe [BH(Me)(μ-pz ) ] and (Tp )Ln(η -pz )[(μ-Me)GaMe ] (Ln = Y, Ho). The steric impact of the Tp ligands was examined by cone angle calculations. Rare-earth-metal methylidene complexes (Tp )Ln(μ -CH )[(μ-Me)EMe ] (E = Al, Ga) successfully promote carbonyl methylenation reactions upon addition of ketone.