Yttrium Complexes of a Phenanthrene-Fused Cyclopentadienyl: Synthetic, Structural, and Reactivity Studies

A rare isocyanide derived from an unprecedented neutral yttrium(ii) bis(amide) complex

A room temperature stable complex formulated as Y(NHAr*)₂ has been prepared, where Ar* = 2,6-(2,4,6-(ⁱPr)₃C₆H₂)C₆H₃, by KC₈ reduction of ClY(NHAr*)₂. Based on EPR evidence, Y(NHAr*)₂ is an example of a d¹ Y(ii) complex with significant delocalization of the unpaired electron density from the metal to the ligand. The isolation of molecular divalent metal complexes is challenging for rare earth elements such as yttrium. In fact, stabilization of the divalent state requires judicious ligand design that allows the metal center to be coordinatively saturated. Divalent rare earth elements tend to be reactive towards various substrates. Interestingly, Y(NHAr*)₂ reacts as a radical donor towards ^t BuNC to generate an unusual yttrium isocyanide complex, CNY(NHAr*)₂, based on spectroscopic evidence and single-crystal X-ray diffraction data.

Access to Derivatizable Octahydrofluorenyl Ligand: Half-Sandwich Rare-Earth Metal Complexes for Highly Syndiospecific (Co)Polymerization of Styrene

A simple and facile synthetic pathway for accessing new derivatizable bulky-demanding octahydrofluorenyl (OHF) ligands has been developed, and a series of half-sandwich rare-earth metal (Sc, Y, Lu) complexes bearing the OHF ancillary ligands have been synthesized. In conjunction with a borate, the OHF-ligated Sc complexes exhibited high catalytic activity for styrene (co)polymerization to afford polymers with highly syndiotactic polystyrene sequence (>99% rrrr).

Carbonyl group and carbon dioxide activation by rare-earth-metal complexes

The rare-earth elements (Ln = Sc, Y, La-Lu) are widely used in stoichiometric and catalytic carbonyl group transformations. Sufficient availability, non-toxicity, high oxophilicity, tunable ion size/Lewis acidity and enhanced ligand exchangeability have been major driving factors for their successful implementation. Routinely employed reagents for stoichiometric carbonyl group transformations are divalent ytterbium and samarium compounds (e.g., ketone reduction), bimetallic CeCl3/LiR (C-C coupling), or ceric ammonium nitrate CAN (cyclic ketone oxidation). Rare-earth-metal triflates, and in particular Sc(OTf)3, are prominent examples of Lewis acid catalysts for versatile use in organic synthesis (e.g., Aldol and Michael reactions). Moreover, Ln(ii) and Ln(iii) complexes efficiently catalyze the (co)polymerization of carbonyl group-containing monomers including lactones, lactides, acrylates, and carbon dioxide. Featuring the most notorious greenhouse gas, CO2 is currently assessed as a cheap, abundant, and non-toxic C1 building block. Ln(iii) complexes are not only capable of efficient CO2 capture via reversible insertion but also of CO2 activation for catalytic conversions (copolymerization/cycloaddition with epoxides). This perspective focuses on structurally elucidated Ln complexes resulting from ketone or carbonyl derivative activation/insertion as well as carbon dioxide insertion products. The respective compounds will be sorted by structural motifs and, if applicable, details on reactivity and feasibility of catalytic reactions are presented. The article is subdivided in three parts: (i) donor and insertion products of ketones and aldehydes, (ii) redox-enhanced activation of carbonyl derivatives, and (iii) CO2 insertion/redox products and homogeneous catalytic conversion.

Synthesis, characterization, and reactivity of dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands: substituent effects on coordination and reactivity

Two series of new dinuclear organo-rare-earth-metal alkyl complexes supported by 2-amidate-functionalized indolyl ligands with different haptic modes were synthesized and characterized. The treatment of [RE(CH2SiMe3)3(THF)2] with 1 equiv. of 2-(2,6-iPr2C6H3NHC[double bond, length as m-dash]O)C8H5NH (H2L1) and 2-(2-tBuC6H4NHC[double bond, length as m-dash]O)C8H5NH (H2L2) in toluene yielded the dinuclear organo-rare-earth-metal alkyl complexes {[η1:(μ2-η1:η1)-L1]RE(CH2SiMe3)(THF)2}2 [RE = Gd (1a), Dy (1b), Y (1c), Er (1d), and Yb (1e)] and {[η1:(μ2-η1:η1):η1-L2]RE(CH2SiMe3)(THF)2}2 [RE = Gd (2a), Dy (2b), Y (2c), Er (2d), and Yb (2e)] in good yields. When [RE(CH2SiMe3)3(THF)2] were treated with 2 equiv. of H2L1 or H2L2 in THF, the dinuclear organo-rare-earth-metal complexes {(η1:η1-HL)[η1:(μ2-η1:η1):η1-L]RE(THF)}2 (1ca: RE = Y, L = L1; 2ea: RE = Yb, L = L2) were obtained. The complexes could react with small organic molecules such as N,N'-diisopropylcarbodiimide (DIC), phenyl isocyanate, N-methylallylamine, phenylacetylene, pyridine, N-phenylimidazole, or 4-dimethylaminopyridine (DMAP) to yield a series of new complexes with different reactivity patterns along with the reported rare-earth-metal alkyl complexes. In the presence of cocatalysts, these dinuclear organo-rare-earth-metal alkyl complexes could initiate isoprene polymerization with high activity (100% conversion of 2000 equiv. of isoprene in 12 h), yielding polymers with high regioselectivity (1,4 polymers up to 96.1%).

Supersize Cp! Tetrabenzo[a,c,g,i]fluorenyl complexes of yttrium

The synthesis of tetrabenzo[a,c,g,i]fluorenyl (Tbf) yttrium dialkyl complexes, (Tbf)Y(CH2SiMe3)2(L) (L = tetrahydrofuran (THF), 1; L = bipy, 2), by direct protonolysis of the tris(alkyl) complex, Y(CH2SiMe3)3(THF)2, are reported. The X-ray crystal structures of 1 and 2 display the helical twisting typically observed for the Tbf ligand. Dynamic nuclear magnetic resonance (NMR) studies on 1 show a barrier to Tbf helical inversion (epimerization or “wagging”) of 38.1 ± 0.5 kJ mol−1. The reaction of 1 with acidic hydrocarbons such as 1,3-bis(trimethylsilyl)cyclopentadiene or trimethylsilylacetylene results in protonolysis to form the mixed Cp derivative [(Tbf){C5H3(SiMe3)2}Y(CH2SiMe3)(THF)] (3) or [(Tbf)Y(CCSiMe3)2(THF)]n (4), respectively. In the case of 4, a small amount of the trinuclear cluster (Tbf)Y3(μ3-CCSiMe3)2(μ2-CCSiMe3)3(CCSiMe3)3(THF)2 (5) was isolated and characterized by X-ray crystallography. Dialkyl 1 undergoes smooth insertion of trimethylsilyl isocyanate to afford [(Tbf)Y{κ2-(N,O)-Me3SiN(Me3SiCH2)CO}2(THF)] (6) but it does not react with alkenes. Treating 1 with [Ph3C]+[B(C6F5)4]− in bromobenzene generates a moderately active ethylene polymerization catalyst (36 kg mol−1 h−1 bar−1).

Bis(9,10-phenanthrenocyclopentadienyl)yttrium complexes: synthesis and solution behaviour

The synthesis of a number of yttrium metallocenes based on the phenanthrene-fused Cp ligand PCpR (R = H, Me, Ph) is reported. Acid−base (σ-bond metathesis) reactions between the parent HPCpR ligands and Y(CH2SiMe3)3(THF)2 afford the monomeric alkyl complexes Y(PCpR)2(CH2SiMe3)(THF) (R = H, 1; Me, 2; Ph, 3). Salt metathesis between Li+PCpMe− and YCl3 in THF similarly affords the monomeric chloride complex Y(PCpMe)2(Cl)(THF) (4), which reacts further with methyllithium to generate the bridging “ate” complex Y(PCpMe)2(μ-Me)2Li(THF)2 (5). Complex 3 undergoes rapid hydrogenolysis in the presence of phenylsilane to afford the crystalline bridging hydride dimer [Y(PCpPh)2]2(μ-H)2 (6). The X-ray structures of complexes 2 and 6 are reported along with the solution behaviour of 2.