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

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Abstract
Homoleptic tetramethylaluminates Ln(AlMe4)(3) react with KTp(tBu,Me) (Tp(tBu,Me) = tris(3-tBu-5-Me-pyrazolyl)borato) to yield rare-earth-metal methylidene complexes (Tp(tBu,Me))Ln(mu(3)-CH2)[(mu-Me)AlMe2](2) (Ln = La, Ce, Nd). The lanthanum reaction is prone to additional C-H- and B-N-bond activation, affording coproducts La[HB(pz(Me,tBu))(pz(CMe2,Me))(2)][(mu-CH2)(mu-Me)AlMe2](2) and [La(mu-pz(tBu,Me))(AlMe4)(2)](2) (pz(tBu,Me) = 3-tBu-5-Me-pyrazolato). The protonolysis reaction of Ln(AlMe4)(3) and Hpz(tBu,Me) provides more efficient access to [Ln(mu-pz(tBu,Me))(AlMe4)(2)](2) (Ln = La, Nd). Treatment of Ln(AlMe4)(3) with KTp(Me,Me) led to methylidene complexes (Tp(Me,Me))Ln(mu(3)-CH2)[(mu-Me)AlMe2](2) (Ln = Nd, Sm) or bis(tetramethylaluminate) complexes (Tp(Me,Me))Ln(AlMe4)(2) (Ln = Y, Lu). The neodymium reaction generated methine derivative (Tp(Me,Me))Nd[(mu(4)-CH)(AlMe2)(2)(mu-pz(,Me,Me))][(mu-Me)AlMe2] as a minor coproduct. The reaction of Ln(GaMe4)(3) (Ln = Y, La, Ce, Nd, Sm, Ho) with HTp(tBu,Me) gave methylidene complexes (Tp(tBu,Me))Ln(mu(3)-CH2)[(mu-Me)GaMe2](2) (Ln = La, Ce, Nd, Sm) and alkyl complexes (Tp(tBu,Me))LnMe[(mu-Me)GaMe3] (Ln = Y, Ho), while competing B-N bond activation reactions produced GaMe2[BH(Me)(mu-pz(tBu,Me))(2)] and (Tp(tBu,Me))Ln(eta(2)-pz(tBu,Me))[(mu-Me)GaMe3] (Ln = Y, Ho). The steric impact of the Tp(R,Me) ligands was examined by cone angle calculations. Rare-earth-metal methylidene complexes (Tp(tBu,Me))Ln(mu(3)-CH2)[(mu-Me)EMe2](2) (E = Al, Ga) successfully promote carbonyl methylenation reactions upon addition of ketone.

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