Cationic Alkyl Complexes of the Rare‐Earth Metals: Synthesis, Structure, and Reactivity

Abstract

This contribution is dedicated to Dick Schrock on the occasion of his 60th birthday.Abstract: Cationic alkyl complexes of the rare-earth metals [LnR m ; L ¼ Lewis base) were virtually unknown species until recently. Because of their increased Lewis acidity/electrophilicity they should have considerable potential as homogeneous catalysts in olefin polymerization and in organic transformations. They can be generated by treating the neutral rare-earth metal precursors containing at least two alkyl groups R with suitable Lewis or Brønsted acids in the presence of weakly coordinating anions. Not only monocationic but also dicationic alkyl derivatives have been shown to be accessible. In the context of modeling homogeneous ethylene polymerization using a mixture consisting of LnR 3 /AlR 3 /[NMe 2 HPh][B(C 6 F 5 ) 4 ], such dications were discovered. Some thermally robust examples of mono-and dicationic alkyl complexes have been structurally characterized as solvent-separated ion pairs. Neutral and monoanionic macrocycles such as crown ethers or aza-crown ethers as well as amidinato, b-diketiminato, and substituted cyclopentadienyl ligands are suitable ancillary ligands to stabilize the cationic alkyl fragments.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

“…[287][288][289] It is also worth noting the effect that the choice of the WCA has upon the reactivity and, therefore,stability of these cationic species.T he [Ln(CH 2 SiMe 3 ) 2 (thf) x ] + cations can also be formed as the corresponding [B(Ph) 4 ] À salts,t hrough the use of [HNMe 2 Ph][B(Ph) 4 ]. [287][288][289] It is also worth noting the effect that the choice of the WCA has upon the reactivity and, therefore,stability of these cationic species.T he [Ln(CH 2 SiMe 3 ) 2 (thf) x ] + cations can also be formed as the corresponding [B(Ph) 4 ] À salts,t hrough the use of [HNMe 2 Ph][B(Ph) 4 ].…”

“…Ag ood example of the complementary reactivity of related Brønsted and Lewis acids is the formation of the cations [Ln(CH 2 SiMe 3 ) 2 (thf) x ] + (where Ln = Lu, X = 3; Ln = Y, X = 4) that can be formed either by abstraction of the alkyl residue by B(C 6 F 5 ) 3 or by protonolysis of the alkyl residue by [HNMe 2 Ph] + (Scheme 9). [287][288][289] It is also worth noting the effect that the choice of the WCA has upon the reactivity and, therefore,stability of these cationic species.T he [Ln(CH 2 SiMe 3 ) 2 (thf) x ] + cations can also be formed as the corresponding [B(Ph) 4 ] À salts,t hrough the use of [HNMe 2 Ph][B(Ph) 4 ]. These salts are isolable and thermally stable.I solation of the salts as analytically pure material with the less coordinating [B(C 6 F 5 ) 4 ] À WCA is not achieved, due to polymerization of the thf solvent, clearly demonstrating that the weaker coordination of the WCA to the cation increases the reactivity of the metal center.…”

“…These salts are isolable and thermally stable.I solation of the salts as analytically pure material with the less coordinating [B(C 6 F 5 ) 4 ] À WCA is not achieved, due to polymerization of the thf solvent, clearly demonstrating that the weaker coordination of the WCA to the cation increases the reactivity of the metal center. [287][288][289] Although these two routes typically offer complementary reactivity,there are examples where this is not observed and an example of this is the contrasting reactivity of the bis(imino)pyridineiron(II) dialkyl complex with [HNMe 2 Ph]-[B(Ph) 4 ]and B(C 6 F 5 ) 3 (Scheme 10). [290] Protonation results in elimination of the corresponding alkane and, in the presence of as uitable donor (e.g.E t 2 O or thf), the isolation of the donor-stabilized cations.I n contrast, the reaction with B-(C 6 F 5 ) 3 results in the abstraction of asilicon methide group followed by rearrangement.…”

Development of New Hydrogenations of Imines and Benign Reductive Hydroaminations: Zinc Triflate as a Catalyst

“…Compared to olefins, alkynes can undergo the reaction more easily because of steric reasons and their weaker p-bonds (70 kJ mol À1 ). After the pioneering work of Barluenga et al [17] using Hg and Tl catalysts, the hydroamination of alkynes is nowadays known to be catalyzed by a variety of early (Ti, Zr, V, Ta) [18] and late transition metal (Ru, Rh, Pd, Pt, Au, Ag), [19] lanthanide (Sm, Lu, Nd), [20] and actinide (U, Th) catalysts. [16] In the past few decades, different catalysts for inter-and intramolecular alkyne hydroaminations have been developed.…”

“…[16] In the past few decades, different catalysts for inter-and intramolecular alkyne hydroaminations have been developed. After the pioneering work of Barluenga et al [17] using Hg and Tl catalysts, the hydroamination of alkynes is nowadays known to be catalyzed by a variety of early (Ti, Zr, V, Ta) [18] and late transition metal (Ru, Rh, Pd, Pt, Au, Ag), [19] lanthanide (Sm, Lu, Nd), [20] and actinide (U, Th) catalysts. [21] Furthermore, heterogeneous catalysts have been explored, for example, zeolites, ionexchanged zeolites, and transition metal-exchanged montmorillonites.…”