Cationic Organometallic Complexes of Scandium, Yttrium, and the Lanthanoids

Cationic rare-earth-metal methyl complexes: a new preparative access exemplified for Y and Pr

A new procedure for the generation of cationic methyl complexes of rare-earth metals has been developed for yttrium and praseodymium as two examples from the rare-earth series. The reactions of the rare-earth-metal tetramethylaluminates [Ln(AlMe(4))(3)] of yttrium and praseodymium with three differently sized crown ethers in thf, namely [12]crown-4, [15]crown-5 and [18]crown-6, respectively, led to the compounds [YMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (1), [PrMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (2), [YMe([15]crown-5)(thf)(2)](2+)[AlMe(4)](-)(2) (3), [Pr([15]crown-5)(2)](3+)[AlMe(4)](-)(3) (4), [YMe([18]crown-6)(thf)](2+)[AlMe(4)](-)(2) (5) and [PrMe([18]crown-6)(thf)(2)](2+) [AlMe(4)](-)(2) (6). With the exception of 4, all compounds contain molecular di-cations with a Ln-CH(3) unit bonded to one (3, 4, 6) or two (1, 2) crown-ether molecules. The compounds were characterised by determination of their crystal structures, by elemental analyses and in the case of the yttrium compounds by variable temperature NMR spectroscopy. The latter gives insights into molecular dynamic processes and methyl group exchange between cations and anions in solution.

Cationic allyl complexes of the rare-earth metals: synthesis, structural characterization, and 1,3-butadiene polymerization catalysis

Monocationic bis-allyl complexes [Ln(eta(3)-C(3)H(5))(2)(thf)(3)](+)[B(C(6)X(5))(4)](-) (Ln = Y, La, Nd; X = H, F) and dicationic mono-allyl complexes of yttrium and the early lanthanides [Ln(eta(3)-C(3)H(5))(thf)(6)](2+)[BPh(4)](2)(-) (Ln = La, Nd) were prepared by protonolysis of the tris-allyl complexes [Ln(eta(3)-C(3)H(5))(3)(diox)] (Ln = Y, La, Ce, Pr, Nd, Sm; diox = 1,4-dioxane) isolated as a 1,4-dioxane-bridged dimer (Ln = Ce) or THF adducts [Ln(eta(3)-C(3)H(5))(3)(thf)(2)] (Ln = Ce, Pr). Allyl abstraction from the neutral tris-allyl complex by a Lewis acid, ER(3) (Al(CH(2)SiMe(3))(3), BPh(3)) gave the ion pair [Ln(eta(3)-C(3)H(5))(2)(thf)(3)](+)[ER(3)(eta(1)-CH(2)CH=CH(2))](-) (Ln = Y, La; ER(3) = Al(CH(2)SiMe(3))(3), BPh(3)). Benzophenone inserts into the La-C(allyl) bond of [La(eta(3)-C(3)H(5))(2)(thf)(3)](+)[BPh(4)](-) to form the alkoxy complex [La{OCPh(2)(CH(2)CH=CH(2))}(2)(thf)(3)](+)[BPh(4)](-). The monocationic half-sandwich complexes [Ln(eta(5)-C(5)Me(4)SiMe(3))(eta(3)-C(3)H(5))(thf)(2)](+)[B(C(6)X(5))(4)](-) (Ln = Y, La; X = H, F) were synthesized from the neutral precursors [Ln(eta(5)-C(5)Me(4)SiMe(3))(eta(3)-C(3)H(5))(2)(thf)] by protonolysis. For 1,3-butadiene polymerization catalysis, the yttrium-based systems were more active than the corresponding lanthanum or neodymium homologues, giving polybutadiene with approximately 90% 1,4-cis stereoselectivity.

Alternating and random copolymerization of isoprene and ethylene catalyzed by cationic half-sandwich scandium alkyls

The acid-base reactions between the scandium trialkyl complex Sc(CH(2)SiMe(3))(3)(THF)(2) and 1 equiv of Cp'-H afforded straightforwardly the corresponding mono(cyclopentadienyl)scandium dialkyl complexes Cp'Sc(CH(2)SiMe(3))(2)(THF) (Cp' = C(5)H(5) (1), C(5)MeH(4) (2), C(5)Me(4)H (3), C(5)Me(5) (4), C(5)Me(4)SiMe(3) (5)) in 65-80% isolated yields. The analogous half-sandwich complexes having a heteroatom-containing side arm, (C(5)Me(4)R)Sc(CH(2)SiMe(3))(2) (R = CH(2)CH(2)PPh(2) (6), C(6)H(4)OMe-omicron (7)), were obtained by the one-pot metathetical reactions of ScCl(3)(THF)(3) with 1 equiv of the potassium salts of the ligands and 2 equiv of LiCH(2)SiMe(3). The similar reactions of ScCl(3)(THF)(3) with KC(5)Me(4)(C(6)H(4)NMe(2)-omicron) and LiCH(2)SiMe(3) gave a methylene-bridged binuclear complex [{C(5)Me(4)(omicron-C(6)H(4)N(Me)CH(2)-mu}Sc(CH(2)SiMe(3))](2) (8). Complexes 1-8 were fully characterized by (1)H, (13)C NMR, X-ray, and microelemental analyses. The reactions of 5 and 7 with 1 equiv of [PhMe(2)NH][B(C(6)F(5))(4)] in THF afforded quantitatively the structurally characterizable cationic monoalkyl complexes [(C(5)Me(4)SiMe(3))Sc(CH(2)SiMe(3))(THF)(2)][B(C(6)F(5))(4)] (10) and [(C(5)Me(4)C(6)H(4)OMe-omicron)Sc(CH(2)SiMe(3))(THF)(2)][B(C(6)F(5))(4)] (11), respectively. In the presence of an activator such as [Ph(3)C][B(C(6)F(5))(4)], [PhMe(2)NH][B(C(6)F(5))(4)], or B(C(6)F(5))(3), all of the half-sandwich dialkyl complexes 1-7 were active for isoprene polymerization and isoprene-ethylene copolymerization, with the activity and selectivity being significantly dependent on the substituents at the cyclopentadienyl ligands to yield the corresponding homo- and copolymer materials with different microstructures and compositions. In the homopolymerization of isoprene, the less sterically demanding complexes 1 and 2 showed high cis-1,4 selectivity (up to 95%), whereas the more sterically demanding complexes 3-5 yielded 3,4-polyisoprene (51-65%) as a major product. The ether side arm coordinated complex 7 preferred trans-1,4-polyisoprene formation (60-79%), whereas the phosphine analogue 6 showed high cis-1,4 selectivity (84-90%) under the same conditions. In the copolymerization of isoprene and ethylene, complexes 1 and 2 afforded the random copolymers with high isoprene contents (85-92 mol %) and predominant cis-1,4-microstructures (up to 90%), thus constituting the first example of cis-1,4-selective copolymerization of isoprene with ethylene. In contrast, the copolymerization of isoprene and ethylene by 3, 4, 6, and 7 gave, for the first time, almost perfect alternating isoprene-ethylene copolymers. Possible mechanisms of the polymerization and copolymerization processes were proposed on the basis of the DFT calculations.