Half-Sandwich Rare Earth Research Articles

Half-Sandwich Rare-Earth metal complexes bearing 4,5,6,7-Tetrahydroindenyl ligands for highly Syndiospecific (Co)Polymerization of styrene

As Cp-derived ligands, substituted 4,5,6,7-tetrahydroindene (THI) ligands with tunable steric bulk have been synthesized, and their ligated half-sandwich rare-earth metal complexes were also sucessfully prepared and tested for styrene (co)polymerization. In conjunction with [Ph3C][B(C6F5)4], the THI-ligated scandium complexes exhibited high catalytic activity for highly syndiotactic polymerization of styrene (>99% rrrr). THI3Me-Sc/[Ph3C][B(C6F5)4] binary system also promoted the copolymerization of styrene and isoprene/butadiene by both the concurrent addition and sequentical addition of monomers, affording the copolymers with block microstructure as deterimined by NMR (1H and 13C) spectra. The huge difference in reactivity ratios betewen styrene and isoprene/butadiene (rIP / rSt = 31.1 and rBD / rSt = 173.6) revealed in kinetics investigation further proved the predominantly block structure of resultant copolymers.

Regiodivergent C-H Alkylation of Quinolines with Alkenes by Half-Sandwich Rare-Earth Catalysts.

The regiodivergent catalysis of C-H alkylation with alkenes is of great interest and importance but has remained hardly explored to date. We report herein the first regiodivergent C-H alkylation of quinolines with alkenes by half-sandwich rare-earth catalysts. The regiodivergence was achieved by fine-tuning the metal/ligand combination or steric and electronic properties of the catalysts. The use of the C5Me5-ligated scandium catalyst Sc-3 for the reaction of quinolines with styrenes and that of the C5Me4H-ligated yttrium catalyst Y-2 for the reaction with aliphatic olefins exclusively afforded the corresponding C8-H alkylation products, thus constituting the first example of direct C8-H alkylation of neutral quinolines. In contrast, the Sc-3-catalyzed reaction of 2-arylquinolines with aliphatic olefins and the Y-2-catalyzed reaction with styrenes selectively gave the 2-aryl o-C-H alkylation products. On the basis of the catalyst/substrate-controlled regiodivergence, the sequential regiospecific dialkylation of quinolines with two different alkenes has also been achieved. DFT studies revealed that the C-H activation of 2-phenylquinoline at both the C8 position and an ortho position of the 2-phenyl substituent was possible, and these two types of initially formed C-H activation products were interconvertible through the coordination and C-H activation of another molecule of quinoline. The regioselectivity for the C-H alkylation reactions was governed not only by the ease of the initial formation of the C-H activation products but also by the energy barriers for their interconversions, as well as by the energy barriers or steric and electronic influences in the subsequent alkene insertion processes. This work has not only constituted an efficient protocol for the selective synthesis of diversified quinoline derivatives but also offered unprecedented insights into the C-H activation and transformation of quinolines and may help in the design of more efficient, selective, or complementary catalysts.

Diastereodivergent [3 + 2] Annulation of Aromatic Aldimines with Alkenes via C-H Activation by Half-Sandwich Rare-Earth Catalysts.

Stereodivergent catalysis is of great importance, as it can allow efficient access to all possible stereoisomers of a given product with multiple stereocenters from the same set of starting materials. We report herein the first diastereodivergent [3 + 2] annulation of aromatic aldimines with alkenes via C-H activation by half-sandwich rare-earth catalysts. This protocol provides an efficient and general route for the selective synthesis of both trans and cis diastereoisomers of multisubstituted 1-aminoindanes from the same set of aldimines and alkenes, featuring 100% atom efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The diastereodivergence is achieved by fine-tuning the sterics or ligand/metal combination of the half-sandwich rare-earth metal complexes.

C-H Polyaddition of Dimethoxyarenes to Unconjugated Dienes by Rare Earth Catalysts.

The C-H polyaddition of dimethoxyarenes such as 1,4-dimethoxybenzene and 4,4'-dimethoxybiphenyl to unconjugated dienes such as norbornadiene and 1,4-divinylbenzene has been achieved for the first time by using cationic half-sandwich rare earth alkyl catalysts. This protocol afforded novel polymer materials consisting of dimethoxyarene moieties and nonpolar hydrocarbon structure motifs (cyclic, linear, and aromatic) in perfectly alternating sequences that are otherwise difficult to make. The reaction proceeded via C═C double bond insertion into a C-H bond ortho to each of the two methoxy groups in a step-growth polymerization fashion.

Regio- and Stereochemical Control in Ocimene Polymerization by Half-Sandwich Rare-Earth Metal Dialkyl Complexes.

The polymerization of ocimene has been first achieved by half-sandwich rare-earth metal dialkyl complexes in combination with activator and Al(i) Bu3 . The regio- and stereoselectivity in the ocimene polymerization can be controlled by tuning the cyclopentadienyl ligand and the central metal of the complex. The chiral cyclopentadienyl-ligated Sc complex 1 prepares syndiotactic cis-1,4-polyocimene (cis-1,4-selectivity up to 100%, rrrr = 100%), while the corresponding Lu, Y, and Dy complexes 2-4 and the achiral pentamethylcyclopentadienyl Sc, Lu, and Y complexes 5-7 afford isotactic trans-1,2-polyocimenes (trans-1,2-selectivity up to 100%, mm = 100%).

Enantioselective C-H bond addition of pyridines to alkenes catalyzed by chiral half-sandwich rare-earth complexes.

Cationic half-sandwich scandium alkyl complexes bearing monocyclopentadienyl ligands embedded in chiral binaphthyl backbones act as excellent catalysts for the enantioselective C-H bond addition of pyridines to various 1-alkenes, leading to formation of a variety of enantioenriched alkylated pyridine derivatives in high yields and excellent enantioselectivity (up to 98:2 er).

Rare-Earth Half-Sandwich Dialkyl and Homoleptic Trialkyl Complexes for Rapid and Stereoselective Polymerization of a Conjugated Polar Olefin

Under ambient conditions, discrete half-sandwich rare-earth (RE) dialkyls, [η5-(1,3-(SiMe3)2C9H5)]]RE(CH2SiMe3)2(THF) (RE = Sc, Y, Dy, Lu), catalyze rapid and stereoselective coordination polymerization of β-methyl-α-methylene-γ-butyrolactone (βMMBL), a conjugated polar olefin and a member of the naturally occurring or biomass-derived methylene butyrolactone family. Within the present RE series, the complex of the largest ion (Dy3+) exhibits the highest activity, achieving a high turnover frequency of 390 min–1, and also produces the highly isotactic polymer PβMMBL (mm = 91.0%). This stereoregular polymer is thermally robust, with a high glass-transition temperature of 280 °C, and is resistant to all common organic solvents. Other half-sandwich RE catalysts of the series are also highly active and produce polymers with a similarly high isotacticity. Intriguingly, even simple homoleptic hydrocarbyl RE complexes, RE(CH2SiMe3)3(THF)2 (RE = Sc, Y, Dy, Lu), also afford highly isotactic polymer PβMMBL, despite their much lower polymerization activity, except for the Lu complex, which maintains its high activity for both types of complexes. Computational studies of both half-sandwich and simple hydrocarbyl yttrium complexes have revealed a stereocontrol mechanism that well explains the observed high stereoselectivity of βMMBL polymerization by both types of catalysts. Specifically, the experimental stereoselectivity can be well rationalized with a monometallic propagation mechanism through predominantly chain-end stereocontrol in the coordination–addition polymerization. In this mechanism, formation of an isotactic polymer chiefly originates from interactions between the methyl groups on the chiral β-C atom of the five-membered ring of both the coordinated monomer and the last inserted βMMBL unit of the chain, and the auxiliary ligand on the metal makes a negligible contribution to the stereocontrol of the polymerization.

Synthesis and Structures of the C5Me4SiMe3‐Supported Polyhydride Complexes over the Full Size Range of the Rare Earth Series

The acid-base reaction of [Ln(CH(2)SiMe(3))(3)(thf)(2)] with Cp'H gave the corresponding half-sandwich rare earth dialkyl complexes [(Cp')Ln(CH(2)SiMe(3))(2)(thf)] (1-Ln: Ln=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Cp'=C(5)Me(4)SiMe(3)) in 62-90% isolated yields. X-ray crystallographic studies revealed that all of these complexes adopt a similar overall structure, in spite of large difference in metal-ion size. In most cases, the hydrogenolysis of the dialkyl complexes in toluene gave the tetranuclear octahydride complexes [{(Cp')Ln(μ-H)(2)}(4)(thf)(x)] (2-Ln: Ln=Sc, x=0; Y, x=1; Er, x=1; Tm, x=1; Gd, x=1; Dy, x=1; Ho, x=1) as the only isolable product. However, in the case of Lu, a trinuclear pentahydride [(Cp')(2)Lu(3)(μ-H)(5)(μ-CH(2)SiMe(2)C(5)Me(4))(thf)(2)] (3), in which the C-H activation of a methyl group of the Me(3)Si unit on a Cp' ligand took place, was obtained as a major product (66% yield), in addition to the tetranuclear octahydride [{(Cp')Lu(μ-H)(2)}(4)(thf)] (2-Lu, 34%). The use of hexane instead of toluene as a solvent for the hydrogenolysis of 1-Lu led to formation of 2-Lu as a major product (85%), while a similar reaction in THF yielded 3 predominantly (90%). The tetranuclear octahydride complexes of early (larger) lanthanide metals [{Cp'Ln(μ-H)(2)}(4)(thf)(2)] (2, Ln=La, Ce, Pr, Nd, Sm) were obtained in 38-57% isolated yields by hydrogenolysis of the bis(aminobenzyl) species [Cp'Ln(CH(2)C(6)H(4)NMe(2)-o)(2)], which were generated in-situ by reaction of [Ln(CH(2)C(6)H(4)NMe(2)-o)(3)] with one equivalent of Cp'H. X-ray crystallographic studies showed that the fine structures of these hydride clusters are dependent on the size of the metal ions.

Half-Sandwich Rare Earth Metal Complexes as Novel Catalysts for Olefin Polymerization and Other Chemical Transformations

Abstract This article describes our recent studies on the synthesis and chemistry of mono(cyclopentadienyl)-ligated rare earth metal alkyl and hydride complexes. The half-sandwich mono- and dialkyl rare earth metal complexes are prepared either by alkane elimination reactions between trialkyl rare earth complexes and the neutral cyclopentadiene derivatives or by metathetical reactions of rare earth trichlorides with the alkali metal salts of the corresponding ligands. The reaction of the dialkyl complexes with an equivalent of a borate compound such as [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4] generates easily the corresponding cationic monoalkyl species, which serve as excellent catalysts for the polymerization and copolymerization of a variety of olefins to yield a new family of polymer materials that show novel properties but are difficult to prepare by other catalysts. The half-sandwich monoalkyl rare earth complexes bearing the silylene-linked cyclopentadienyl–arylamido ligands act as excellent catalysts for the catalytic dimerization of terminal alkynes, cross coupling of terminal alkynes with isocyanides, and the catalytic addition of terminal alkyne C–H, amine N–H, and phosphine P–H bonds to carbodiimides. The hydrogenolysis of the dialkyl rare earth complexes with H2 affords a new class of polynuclear rare earth metal polyhydride complexes, which show unique features in both structure and reactivity.

Half-Sandwich Indenyl Rare Earth Metal Dialkyl Complexes: Syntheses, Structures, and Catalytic Activities of (1,3-(SiMe3)2C9H5)Ln(CH2SiMe3)2(THF)

Reactions of 1,3-bis(trimethylsilyl)indene with Ln(CH2SiMe3)3(THF)2 gave half-sandwich rare earth metal complexes (1,3-(SiMe3)2C9H5)Ln(CH2SiMe3)2(THF) (Ln = Y, Lu, Dy). These complexes were characterized by single-crystal X-ray diffraction, which showed the η5 hapticity of indenyl ligands in all of the complexes. The catalytic behaviors of the complexes for intramolecular hydroamination of aminoalkenes were investigated. The catalytic activity increased with increasing the metal ion size, and the Y and Dy complexes showed excellent activities to a variety of aminoalkenes.

Rare Earth/d-Transition Metal Heteromultimetallic Polyhydride Complexes Based on Half-Sandwich Rare Earth Moieties

The reaction of half-sandwich rare earth metal bis(alkyl) complexes Cp′Ln(CH2SiMe3)2(THF) (Cp′ = η5-C5Me4SiMe3; Ln = Y, Lu) with 1 equiv of the binuclear tungsten octahydride complex [Cp*WH4]2 (Cp* = η5-C5Me5) readily gave the corresponding Ln/W2 heterotrimetallic hexahydride complexes Cp′Ln(THF)(μ-H)6(WCp*)2 (Ln = Y (1a), Lu (1b)) with release of SiMe4. A similar reaction of (C5Me4R)Ln(CH2SiMe3)2(THF) with the binuclear osmium tetrahydride complex [Cp*OsH2]2 gave the Ln/Os2 heterotrimetallic trihydride complexes [(Cp*Os){(μ-η5:η1-C5Me4CH2)Os}Ln(C5Me4R)(THF)(μ-H)3] (2a, Ln = Y, R = SiMe3; 2b, Ln = Lu, R = Me), in which one methyl carbon atom of the Cp* ligand on an Os center is bonded to the Ln atom through activation of the C—H bond. In contrast, the analogous reaction of (C5Me4R)Ln(CH2SiMe3)2(THF) with the binuclear ruthenium tetrahydride complex [Cp*RuH2]2 gave the corresponding Ln/Ru4 heteropentametallic heptahydride complexes (Cp*Ru)4(C5Me3RCH2)Ln(μ-H)7 (3a, Ln = Y, R = SiMe3; 3b, Ln =Lu, R = Me) as a result of removal of two protons from two molecules of [Cp*RuH2]2 by one molecule of (C5Me4R)Ln(CH2SiMe3)2(THF) followed by oxidative addition of a methyl C−H bond of the (C5Me4R)Ln unit to a Ru center. The 1H NMR spectra of 3a and 3b showed that oxidative C−H addition and C−H reductive elimination at the Ru atom are reversible in solution. Hydrogenolysis of 3a and 3b gave the decahydride complexes (Cp*Ru)4(Cp′Y)(μ-H)10 (4a) and (Cp*Ru)4(Cp*Lu)(μ-H)10 (4b), respectively. The molecular structures of 1b, 2b, 3a, 3b, 4a, and 4b were determined by X-ray analyses.

Catalytic Addition of Terminal Alkynes to Carbodiimides by Half-Sandwich Rare Earth Metal Complexes

The catalytic addition of terminal alkynes to carbodiimides has been achieved for the first time by use of half-sandwich rare earth metal complexes, such as {Me2Si(C5Me4)(NPh)}Y(CH2SiMe3)(THF)2, which offers a straightforward, atom-economic route to the N,N'-disubstituted propiolamidines which contain a conjugated C-C triple bond, a new family of amidines which were difficult to prepare by other means. A rare earth metal amidinate species was confirmed to be a true catalytic species in this process, thus demonstrating for the first time that an amidinate unit, though being often used as an ancillary ligand for various organometallic complexes, can itself participate in a catalytic reaction under appropriate conditions.

Half-Sandwich Rare Earth Metal Catalysts for Olefin Polymerization and Copolymerization

This article describes the synthesis and polymerization catalysis of rare earth metal bis (alkyl) complexes bearing mono (cyclopentadienyl) ligands. Polymerization and copolymerization of ethylene, 1-hexene, styrene, norbornene, dicyclopentadiene, 1, 3-hexadiene, cyclohexene oxide, and carbon dioxide are reported. Many of these reactions show unprecedented regio-and stereoselectivity, and afford a series of new polymers that were difficult to be prepared previously by other catalysts.