β-Diketiminato Rare-Earth Metal Complexes. Structures, Catalysis, and Active Species for Highly cis-1,4-Selective Polymerization of Isoprene

Synthesis of bis(oxazoline)-based rare-earth metal complexes and their catalytic performance in the polymerization of isoprene and polar ortho-methoxystyrene

A series of bis(oxazoline) rare-earth metal dialkyl complexes [(L)Ln(CH2SiMe3)2(THF)n] (L = L1 (dimethyl-substituted bis(oxazoline) ligand), Ln = Y (1-Y), Lu (1-Lu), Sc (1-Sc), n = 1; L = L2 (phenyl-substituted bis(oxazoline) ligand), Ln = Y (2-Y), Lu (2-Lu), Sc (2-Sc), n = 2) was successfully prepared. NMR spectroscopy and X-ray diffraction indicated that all the complexes ligated with a C2 symmetric bis(oxazoline) and two trimethylsilylmethyl ligands. In the presence of borate and triisobutyl aluminium, these complexes exhibited high catalytic activity for the polymerization of isoprene, yielding the polymer with high cis-1,4-regularity (up to 99.9%) and high molecular weight. Moreover, these ternary catalytic systems also served as efficient initiators for the polymerization of polar ortho-methoxystyrene. However, atactic polymers in all the cases were isolated despite the C2 symmetric geometry of bis(oxazoline) ligands.

Low-Coordinate Mixed Ligand NacNac Complexes of Rare Earth Metals

We report the synthesis and characterization of two types of new mixed-ligand rare earth complexes: tetracoordinate (NacNac^Mes)Ln(BIAN^dipp) (Ln = Dy (1), Er (2) and Y (3)) and pentacoordinate (NacNac^Mes)Ln(AP^dipp)(THF) (Ln = Dy (4), Er (5) and Y (6)). The first three compounds were prepared by the reaction of [(BIAN^Dipp)LnI] with potassium β-diketiminate. The salt metathesis of β-diketiminato-supported rare earth dichlorides (NacNac^Mes)LnCl₂(THF)₂ with sodium o-amidophenolate results in compounds 4-6. The crystal structures of complexes 1-6 were determined by single-crystal analysis. The combination of bulky monoanionic N-mesityl-substituted β-diketiminates with sterically hindered redox-active ligands led to the very low coordination numbers of rare earths and strong distortion of the chelate ligands.

Structural Diversity and Multielectron Reduction Reactivity of Samarium(II) Iodido-β-diketiminate Complexes Dependent on Tetrahydrofuran Content

The molecular structures of complexes [Sm(Nacnac)I(thf)_n] (Nacnac = HC(C(Me)Ndipp)₂^-, dipp = 2,6-diisopropylphenyl, thf = tetrahydrofuran) depending on the number of thf ligands are studied. The complete removal of thf from a known complex [Sm(Nacnac)I(thf)₂] leads to a tetranuclear product [Sm(Nacnac)I]₄ (4). The partial removal of thf results in mixtures of dinuclear [Sm₂(Nacnac)₂I₂(thf)] (2), trinuclear [Sm₃(Nacnac)₃I₃(thf)] (3), and tetranuclear [Sm₄(Nacnac)₄I₄(thf)₂] (4*) complexes and 4, depending on the conditions. The reaction of solvent-free SmI₂ with 1 equiv of K(Nacnac) results mainly in [Sm(Nacnac)₂] (1), while the interaction of 4 with certain amounts of thf allows obtaining pure 2 and 3 (with the admixture of 4*). Complex 4* is the exact dimer of 2, and both compounds are stable in solutions. Reactions with 3 and 4 as reductants are studied. 4 is oxidized by I₂ to stoichiometrically yield two products, mixed-valent tetranuclear [Sm₄(Nacnac)₄I₅] (5) and binuclear [Sm(Nacnac)I₂]₂ (6) complexes. In the reaction of 4 with ⁿBu₃PTe, a trinuclear complex [Sm₃(Nacnac)₃(μ-I)₃(μ₃-E)₂] (8, E = I or Te) is formed in small amounts, with the formation of 6 as the second product. 3 serves as a two-electron reductant in the reaction with ⁿBu₃PTe to yield a trinuclear complex [Sm₃(Nacnac)₃I₃(μ-Te₂)] (7). Complexes 2, 4, 4*, 5, 6, and 8 possess a unique flat Sm_xI_y core of heavy atoms, which is assumed to be a consequence of the Nacnac ligand geometry.

The role of natural rubber endogenous proteins in promoting the formation of vulcanization networks

Non-rubber components are critical in the formation of the natural rubber (NR) vulcanization network, which leads to outstanding mechanical properties of NR. This study reports the effect of NR endogenous proteins (C-serum protein/lutoid protein [CSP/LP]) on the formation of vulcanization networks at the molecular level. Results indicate that CSP/LP has a positive effect on vulcanization. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses demonstrate that the decrease in vulcanization time of CSP/LP is ascribed to coordination interaction between Zn2+ and amide bond. The interaction increases the availability of ZnO in the matrix, thereby promoting the formation of the vulcanized network. CSP/LP also participates in the construction of the vulcanization network as a new crosslinking point, thus increasing crosslinking density and improving the mechanical properties of the NR. This study provides new research ideas for studying the relationship among component–structure–property of NR materials and developing high-strength and high-toughness elastomer materials.

Rare-Earth-Metal Complexes Bearing an Iminodibenzyl-PNP Pincer Ligand: Synthesis and Reactivity toward 3,4-Selective Polymerization of 1,3-Dienes

A PNP-pincer ligand provides a versatile ligation framework, which is highly useful in organometallic chemistry and catalytic chemistry. In this work, by a de novo strategy, a simple and efficient synthetic pathway, has been developed to prepare the new iminodibenzyl-based PNP pincer proligand ^imin-RPNP(Li or H) (R = isopropyl, phenyl). By employing salt metathesis or direct alkyl elimination, we successfully synthesized a series of iminodibenzyl-PNP rare-earth-metal (Ln = Sc, Y, Dy, Ho, Er, Tm, Lu) complexes and characterized them by NMR and X-ray diffraction analyses. Upon addition of a borate and triisobutylaluminum (TIBA), the rare-earth-metal complexes 2-Y, 2-Dy, 2-Ho, 2-Er, and 2-Tm bearing the ^imin-PhPNP ligand exhibited unexpectedly high 3,4-selectivity (up to 95%) for the polymerization of 1,3-dienes (isoprene and myrcene); in particular, the chosen yttrium complex 2-Y promoted the 1,3-diene polymerization in a living manner. A computational study suggested that the sterically congested configuration around the metal center imposed by the ^imin-RPNP ligand might be the main reason for this unusual selectivity.

Controlled, one-pot synthesis of recyclable poly(1,3-diene)-polyester block copolymers, catalyzed by yttrium β-diketiminate complexes

The one-pot synthesis of well-defined block copolymers of olefins/1,3-dienes and polar monomers, such as cyclic esters and acrylates has long been the focus of intense research. Cationic alkyl rare earth metal catalysts, activated by organoborates, have shown to be promising for the polymerization of isoprene or styrene and ε-caprolactone. In this study, we synthesize a series of yttrium bis(alkyl) complexes supported by simple β-diketiminate ancillary ligands. Subtle changes have been made to the β-diketiminate ligand framework to elucidate the effect of ligand structure on the rate and selectivity of olefin/1,3-diene and cyclic ester polymerization, with small ligand changes having a large impact on the resulting polymerizations. Generation of the active cationic species was easily streamlined by identification of appropriate catalyst : organoborate ratios, allowing for high catalyst efficiencies. Notably, we demonstrate the first cationic rare earth metal alkyl-initiated polymerization of δ-valerolactone and ε-decalactone as well as introduced five new block copolymer morphologies. In addition, selective degradation of the ester block in poly(isoprene-b-caprolactone) enabled recovery of the polyisoprene block with identical spectroscopic and thermal properties. Significantly, recopolymerization of the recovered poly(1,3-diene) with fresh ε-caprolactone reproduced the desired diblocks with nearly identical thermal and physical properties to those of virgin copolymer, illustrating a plausible recycling scheme for these materials.

Ligand features that promote one-pot block copolymerization of 1,3-dienes and cyclic esters were realized with yttrium β-diketiminate complexes. Depolymerization and repolymerization of the polyester block introduced a plausible recycling strategy.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions

Metalation of β-diketiminato rare-earth metal complexes L^nacnacLn(PhNCH₂PPh₂)₂ (Ln = Y, Yb, Lu) with (COD)Pd(CH₂SiMe₃)₂ afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph₃P)₂PdI(Ph), imine PhNCH₂, and a β-diketiminato yttrium diiodide. In the related reaction of L^nacnacY(PhNCH₂PPh₂)₂ with (Ph₃P)₂PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

Rigid Acridane-Based Pincer Supported Rare-Earth Complexes for cis-1,4-Polymerization of 1,3-Conjugated Dienes

A convenient synthetic route has been developed for preparing the novel rigid 4,5-(PR₂)2-2,7,9,9-tetramethylacridane-based pincer ligands (^acri-RPNP; R = ⁱPr and Ph), and the first rare-earth (Ln = Y, Lu) alkyl complexes bearing the ^acri-RPNP ligands were synthesized by a salt metathesis reaction (for the isopropyl-substituent ^acri-iPrPNP complexes, 1-Ln) or direct alkylation (for the phenyl-substituent ^acri-PhPNP complexes, 2-Ln). For both 1-Ln and 2-Ln, the NMR spectroscopy and X-ray diffraction study confirmed the successful coordination of the ^acri-RPNP ligand to the central metal ion in a tridentate manner via the two phosphine and the nitrogen donors. In contrast to 1-Ln that are solvent-free complexes, the metal centers in 2-Ln are each coordinated with one tetrahydrofuran molecule. Upon activation by [Ph₃C][B(C₆F₅)₄], 1-Y and 2-Lu could catalyze the living polymerization of isoprene and β-myrcene with high catalytic activity and high cis-1,4-selectivity (up to 92.3% for isoprene and 98.5% for β-myrcene). Moreover, the 1-Y/[Ph₃C][B(C₆F₅)₄] catalytic system also could promote the polymerization of butadiene and its copolymerization with isoprene to produce copolymers with high cis-1,4-selectivity and narrow polydispersity.

Unsymmetrical diarylamido-based rare-earth alkyl complexes: their synthesis and catalytic performance in isoprene polymerization

A family of rare-earth complexes bearing diarylamido-based pincer ligands with phosphino-, phenylthio- and quinolino-sidearms have been synthesized and fully characterized. Upon activation by [Ph₃C][B(C₆F₅)₄], the scandium (P-Sc and S-Sc) and yttrium complexes (P-Y and S-Y) could catalyze the polymerization of isoprene with cis-1,4 selectivity (up to 98.8%), while the lutetium analogues P-Lu and S-Lu produced trans-1,4 selective polyisoprene (up to 83.3%). Density functional theory (DFT) calculations were carried out to clarify the mechanisms for the metal-dependent stereoselective (cis to trans) polymerization of isoprene catalyzed by P-Sc, P-Y and P-Lu, suggesting that kinetically and thermodynamically more favorable insertion-isomerization during chain propagation is the key reason for P-Lu catalyzed trans-stereoselective isoprene polymerization.

Neodymium catalysts for polymerization of dienes, vinyl monomers, and ε-caprolactone

This review discusses various neodymium catalysts for stereospecific polymerization of dienes, vinyl monomers, and ε-caprolactone.

Lutetium and yttrium complexes supported by an anilido-oxazoline ligand for polymerization of 1,3-conjugated dienes and ε-caprolactone

Lutetium and yttrium complexes supported by an anilido-oxazoline ligand exhibit highcis-1,4 stereoselectivity for 1,3-conjugated diene polymerization and high activity for ring-opening polymerization of ε-caprolactone.

Regulation of cis and trans microstructures of isoprene units in alternating copolymers via “space-limited” living species in anionic polymerization

Space-limited living species provide new insight into the regulation of the microstructure of isoprene units in alternating copolymers composed of isoprene and 1,1-bis(4-dimethylsilylphenyl)ethylene (DPE-2SiH).

Bis(alkyl) scandium and yttrium complexes coordinated by an amidopyridinate ligand: synthesis, characterization and catalytic performance in isoprene polymerization, hydroelementation and carbon dioxide hydrosilylation

Highly versatile and robust organolanthanides as catalysts or catalyst precursors for a variety of challenging transformations.

Anilido-oxazoline-ligated rare-earth metal complexes: synthesis, characterization and highly cis-1,4-selective polymerization of isoprene

Anilido-oxazoline-ligated rare-earth metal dialkyl complexes have been synthesized and structurally characterized. The complexes exhibited strong fluorescence emissions and good catalytic performance on isoprene polymerization with high cis-1,4-selectivity. The treatment of anilido-oxazoline precursors ortho-C6H4[NH(2,6-R12C6H3)][C[double bond, length as m-dash]NC(R2,R3)CH2O] (R1 = R2 = R3 = Me (HL1); R1 = iPr, R2 = R3 = Me (HL2); R1 = R2 = iPr, R3 = H (HL3); and R1 = iPr, R2 = R3 = H (HL4)) with an equimolar amount of Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y) afforded rare-earth metal complexes L1-4-Ln(CH2SiMe3)2(THF)n (L1-Sc (1), n = 1; L2-Sc (2), n = 0; L1-Y (3), n = 1; L2-Y (4), n = 1; L3-Y (5), n = 1; and L4-Y (6), n = 1) in good yields. The complexes are stable in both the solid state and solution. Single crystal X-ray diffraction study showed that complexes 1, 3 and 4 exhibit a distorted trigonal bipyramidal configuration, while complex 2 is pseudo-tetrahedral without coordinated THF. The luminescence properties of complexes 1-4 were investigated and the emission maxima were found in the range of 465-477 nm. DFT and TD-DFT studies were carried out to explore their characteristic electronic structures and gain insight into their optical properties. Upon activation with organic borates, the reported complexes exhibited high activity and cis-1,4-selectivity for isoprene polymerization. The nature of the central metal and substituent groups in oxazoline have an influence on the cis-1,4-selectivity.

Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment

Aminopyridine iron(ii) chloride complexes coordinating a type of flexible ligand are reported as precursors for controlled polymerization of isoprenes.

Influences of Fluorine Substituents on Iminopyridine Fe(II)- and Co(II)-Catalyzed Isoprene Polymerization

A series of iminopyridine complexes of Fe(II) and Co(II) complexes bearing fluorinated aryl substituents were synthesized for the polymerization of isoprene. The structures of complexes 3a, 2b and 3b were determined by X-ray diffraction analysis. Complex 3a contained two iminopyridine ligands coordinated to the iron metal center forming an octahedral geometry, whereas 2b adopted a chloro-bridged dimer, and 3b featured with two patterns of cobalt centers bridged via chlorine atoms. Complexes 2b and 3b represented rare examples of chlorine bridged bimetallic Co(II) complexes. The fluorine substituents effects, particularly on catalytic activity and polymer properties such as molecular weight and regio-/stereo-selectivity were investigated when these complexes were employed for isoprene polymerization. Among the Fe(II)/methylaluminoxane (MAO) systems, the 4-CF₃ substituted iminopyridine Fe(II) complex 1a was found as a highly active isoprene polymerization catalyst exhibiting the highest activity of 10⁶ g·(mol of Fe)-1·h-1. The resultant polymer displayed lower molecular weight (Mn = 3.5 × 10⁴ g/mol) and moderate polydispersity index (PDI = 2.1). Furthermore, the ratio of cis-1,4-/3,4 was not affected by the F substituents. In the series of Co(II)/AlEt₂Cl binary systems, complexes containing electron-withdrawing N-aryl substituents (R = 4-CF₃, 2,6-2F) afforded higher molecular weights polyisoprene than that was obtained by the complex containing electron-donating N-alkyl substituents (R = octyl). However, ternary components system, complex/MAO/[Ph₃C][B(C₆F₅)₄] resulted in low molecular weight polyisoprene (Mn < 2000) with high trans-1,4-unit (>95%).

Synthesis of high cis-1,4 polybutadiene with narrow molecular weight distribution via a neodymium-based binary catalyst

Complex Nd(CF₃SO₃)₃*3TOP with good solubility was synthesized to prepare polybutadiene with high cis-1,4-unit content and narrow molecular weight distribution.

Mixed Allyl Rare-Earth Borohydride Complexes: Synthesis, Structure, and Application in (Co-)Polymerization Catalysis of Cyclic Esters

A series of new trivalent rare-earth allyl-borohydride complexes with the formula [RE(BH₄ )₂ (C₃ H₅ )(thf)_x ] (RE=Sc (1), x=2; RE=Y (2) and La (3), x=3) were synthesized by reaction of the corresponding rare-earth trisborohydrides [RE(BH₄ )₃ (thf)_x ] with half an equivalent of bis(allyl)magnesium. The complexes were fully characterized by determining their X-ray structure. Similar to their previously described Nd (4) and Sm (5) analogues, these complexes display a monomeric structure with two terminal trihapto BH₄ groups, one π-η³ allyl ligand, three THF molecules for complexes 2 and 3, and two THF molecules for complex 1. The catalytic behavior of complexes 1-5 toward the ring-opening polymerization (ROP) of l-lactide (l-LA) and ϵ-caprolactone (ϵ-CL) was assessed. The Nd complex featured the best activity toward l-LA (turnover frequency (TOF)=1300 h^-1 ) and the order was Nd>La>Sm>Y>Sc. Complexes 1-3 were found very active for the ROP of ϵ-CL (TOF=166 000 h^-1 ), which is in line with the already established exceptionnally high performance of complexes 4 and 5. With both monomers, it was shown that the borohydride moiety was the preferentially initiating group, rather than the allyl one. The random copolymerization of l-LA and ϵ-CL was performed with complexes 1-5, in the absence or in the presence of benzyl alcohol as a chain-transfer agent, affording copolymers with ϵ-caprolactone up to 62 % inserted. The copolymers synthesized display a variety of microstructures, that is, blocky, random, or quasi-alternating.