Pyrrolide-Supported Lanthanide Alkyl Complexes. Influence of Ligands on Molecular Structure and Catalytic Activity toward Isoprene Polymerization

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine-Oxazoline-Ligated Cobalt Complexes

A family of pyridine-oxazoline-ligated cobalt complexes L2CoCl23a-h were synthesized and characterized. Determined via single-crystal X-ray diffraction, complexes 3a and 3d, ligated by two ligands, displayed a distorted tetrahedral coordination of a cobalt center. The X-ray structure indicated the pyridine-oxazoline ligands acted as unusual mono-dentate ligands by coordinating only to Noxazoline. Upon activation with AlEt2Cl (diethylaluminum chloride), these cobalt complexes all exhibited high catalytic activity (up to 2.5 × 106 g·molCo-1·h-1), affording cis-1,4-co-3,4-polyisoprene with molecular weights of 4.4-176 kg mol-1 and a narrow Ð of 1.79-3.42, suggesting a single-site nature of the active sites. The structure of cobalt catalysts and reaction parameters, especially co-catalysts and the reaction temperature, all have significant influence on the polymerization activity but not on the microstructure of polyisoprene.

Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises

The synthesis, characterization, and solid-state structure of bulky alkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported. Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl, Y(OTr*)(CH₂SiMe₃)₂(THF)₂ (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl), resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H₂(THF)]₄ (1a). X-ray analysis revealed a highly symmetrical structure (4̅ site symmetry) with the four Y atoms located on the corners of a compressed tetrahedron, each bonded to an OTr* and tetrahydrofuran (THF) ligand and the cluster held together by four face-capping, μ₃-H, and four edge-bridging, μ₂-H, hydrides. DFT calculations on the full system with and without THF, but also on model systems, clearly show that the structural preference for complex 1a is controlled by the presence and coordination of THF molecules. Contrary to the exclusive formation of the tetranuclear dihydride, hydrogenolysis of the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH₂SiMe₃)₂(THF)₂ (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y₃(OAr*)₄H₅(THF)₄], polyhydride, 2b. Similar results, i.e., a mixture of tetra-/tri-nuclear products, were obtained from hydrogenolysis of the even bulkier Y(OAr^Ad2,Me)(CH₂SiMe₃)₂(THF)₂ compound. Experimental conditions were established to optimize the production of either the tetra- or trinuclear products. X-ray structure of 2b revealed a triangular array of three yttrium atoms with two face-capping μ₃-H and three edge-bridging μ₂-H hydrides, with one yttrium bonded to two aryloxy ligands while the other two have a complement of one aryloxy and two THF ligands; the solid-state structure is close to being C₂ symmetric, with the C₂ axis running through the unique Y and unique μ₂-H hydride. As opposed to 2a, which shows distinct ¹H NMR resonances for μ₃/ μ₂-H (δ = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchange on the NMR time scale. Their presence and assignment were secured at -40 °C from ¹H SST (spin saturation) experiment.

Enantiomerically Pure Constrained Geometry Complexes of the Rare-Earth Metals Featuring a Dianionic N-Donor Functionalised Pentadienyl Ligand: Synthesis and Characterisation

We report the preparation of enantiomerically pure constrained geometry complexes (cgc) of the rare-earth metals bearing a pentadienyl moiety (pdl) derived from the natural product (1R)-(-)-myrtenal. The potassium salt 1, [Kpdl*], was treated with ClSiMe₂ NHtBu, and the resulting pentadiene 2 was deprotonated with the Schlosser-type base KOtPen/nBuLi (tPen=CMe₂ (CH₂ Me)) to yield the dipotassium salt [K₂ (pdl*SiMe₂ NtBu)] (3). However, 3 rearranges in THF solution to its isomer 3' by a 1,3-H shift, which elongates the bridge between the pdl and SiMe₂ NtBu moieties by one CH₂ unit. This is crucial for the successful formation of various monomeric C₁ - or dimeric C₂ -symmetric rare-earth cgc complexes with additional halide, tetraborohydride, amido and alkyl functionalities. All compounds have been extensively characterised by solid-state X-ray diffraction analysis, solution NMR spectroscopy and elemental analyses.

Dehydrogenative Coupling of Terminal Alkynes with O/N-Based Monohydrosilanes Catalyzed by Rare-Earth Metal Complexes

Newly synthesized rare-earth metal alkyl complexes bearing a tripyrrolyl ligand act as excellent precatalysts for the cross-dehydrogenative coupling between various terminal alkynes and O/N-based monohydrosilanes of HSi(OEt)₃/HSi(NMe₂)₃, leading to the formation of a variety of alkoxysilylalkyne and aminosilylalkyne derivatives in good to high yields. The precatalysts LRE(CH₂SiMe₃)(thf)₂ (RE = Y(1a), Er(1b), Yb(1c), L = 2,5-[(2-C₄H₃N)CPh₂]₂(C₄H₂NMe), thf = tetrahydrofuran) were easily prepared in high yields via the reactions of RE(CH₂SiMe₃)₃(thf)₂ with the proligand H₂L in a single step. Mechanistic studies reveal that treatment of 1 with phenylacetylene could generate the active catalytic species: dinuclear rare-earth metal alkynides (L(thf)_n[RE(μ-C≡CPh)]₂L) (RE = Y(5a), n = 1; Yb(5c), n = 0), which could react with HSi(OEt)₃ to produce the coupling product 4aa and the dinuclear rare-earth metal hydrides (L (thf)[RE(μ-H)]₂L) (RE = Y(6a); Yb(6c)). By contrast, prior treatment of 1c with HSi(OEt)₃ proceeds via cleavage of the Si-O bond to produce the dinuclear ytterbium alkoxide (LYb(μ-OEt))₂ 7c, which is inert in the dehydrogenative coupling reaction. The results of the mechanistic studies are consistent with the observation that the reaction is greatly influenced by the addition sequence of precatalyst/alkynes/silanes.

Potassium complexes containing bidentate pyrrole ligands: synthesis, structures, and catalytic activity for the cyclotrimerization of isocyanates

Bidentate pyrrolyl ligands, 2-(t-butyliminomethyl)pyrrole and 2-(t-butylaminomethyl)pyrrole, reacted with KH to give complexes [C₄H₃N(2-CH[double bond, length as m-dash]N^tBu)K(THF)]_n (1) and [C₄H₃N(2-CH₂NH^tBu)K]_n (2), respectively. Each has been characterized by C, H and N microanalysis, NMR spectroscopy, and single crystal X-ray structural analysis. The X-ray structure of complex 1 (n≥ 1) demonstrated that it exists as a 1D zig-zag coordination polymer in the solid state. Conversely, the structure of complex 2 (n≥ 1) showed that it is a 2D supramolecular network. They proved to be an effective class of catalysts for cyclotrimerization of isocyanate in excellent yield under mild conditions.

Alkali Blues: Blue-Emissive Alkali Metal Pyrrolates

2-Iminopyrroles [H^tBu L, 4-tert-butyl phenyl(pyrrol-2-ylmethylene)amine] are non-fluorescent π systems. However, they display blue fluorescence after deprotonation with alkali metal bases in the solid state and in solution at room temperature. In the solid state, the alkali metal 2-imino pyrrolates, M(^tBu L), aggregate to dimers, [M(^tBu L)(NCR)]₂ (M=Li, R=CH₃ , CH(CH₃ )CNH₂ ), or polymers, [M(^tBu L)]_n (M=Na, K). In solution (solv=CH₃ CN, DMSO, THF, and toluene), solvated, uncharged monomeric species M(^tBu L)(solv)_m with N,N'-chelated alkali metal ions are present. Due to the electron-rich pyrrolate and the electron-poor arylimino moiety, the M(^tBu L) chromophore possesses a low-energy intraligand charge-transfer (ILCT) excited state. The chelated alkali cations rigidify the chromophore, restricting intramolecular motions (RIM) by the chelation-enhanced fluorescence (CHEF) effect in solution and, consequently, switch-on a blue fluorescence emission.

Rare-earth metal bis(aminobenzyl) complexes supported by pyrrolyl-functionalized arylamide ligands: synthesis, characterization and styrene polymerization performance

Acid–base reaction between Ln(CH₂C₆H₄NMe₂-o)₃ and 2,5-Me₂C₄H₂NCH₂SiMe₂NHC₆H₄R (R = H, (HL1); R = Cl-p, (HL2) in 1 : 1 molar ratio gave L₁Ln(CH₂C₆H₄NMe₂-o)₂ (Ln = Sc (1), La (2), Lu (3) and L₂Ln(CH₂C₆H₄NMe₂-o)₂ (Ln = Sc (4), La (5), Lu (6). The catalyst systems of 2 or 5/[Ph₃C][B(C₆F₅)₄] were active for syndio-specific styrene polymerization.

A displacement-type fluorescent probe reveals active species in the coordinative polymerization of olefins

The correct identification of active species is an important prerequisite to study the mechanism of coordinative polymerization of olefins, which can afford important theoretical guidance for the design and synthesis of new organometallic catalysts and high-performance polyolefin materials.

DFT Studies on cis-1,4-Polymerization of Dienes Catalyzed by a Cationic Rare-Earth Metal Complex Bearing an Ancillary PNP Ligand

Dnsity functional theory (DFT) calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand. It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route. The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically. The chain growth follows the π-allyl-insertion mechanism. The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with a metal center and the deformation energy of butadiene moiety. The electronic factor of the central metal has a decisive influence on the cis- vs. trans-insertion and the regioselectivity (cis-1,4- vs. cis-1,2-insertion) is mainly determined by steric hindrance. Tetrahydrofuran (THF) coordination made monomer insertion less favorable compared with THF-free case and had more noticeable impact on the trans-monomer insertion compared with the cis case. During the chain propagation, cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.

Di and trinuclear rare-earth metal complexes supported by 3-amido appended indolyl ligands: synthesis, characterization and catalytic activity towards isoprene 1,4-cis polymerization

Different di and trinuclear rare-earth metal complexes supported by 3-amido appended indolyl ligands were synthesized and their catalytic activities towards isoprene polymerization were investigated. Treatment of [RE(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-(CyN[double bond, length as m-dash]CH)C₈H₅NH in toluene or in THF afforded dinuclear rare-earth metal alkyl complexes having indolyl ligands in different hapticities with central metals {[η²:η¹-μ-η¹-3-(CyNCH(CH₂SiMe₃))Ind]RE-(thf)(CH₂SiMe₃)}₂ (Cy = cyclohexyl, Ind = Indolyl, RE = Yb (1), Er (2), Y (3)) or {[η¹-μ-η¹-3-(CyNCH(CH₂SiMe₃))Ind]RE-(thf)₂(CH₂SiMe₃)}₂ (RE = Yb (4), Er (5), Y (6), Gd (7)), respectively. These two series of dinuclear complexes could be transferred to each other easily by only changing the solvents in the process. Reaction of [Er(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-t-butylaminomethylindole 3-(^tBuNHCH₂)C₈H₅NH in THF afforded the unexpected trinuclear erbium alkyl complex [η²:η¹-μ-η¹-3-(^tBuNCH₂)Ind]₄Er₃(thf)₅(CH₂SiMe₃) (8), which can also be prepared by reaction of 3 equiv. of [Er(CH₂SiMe₃)₃(thf)₂] with 4 equiv. of 3-(^tBuNHCH₂)C₈H₅NH in THF. Accordingly, complexes [η²:η¹-μ-η¹-3-(^tBuNCH₂)Ind]₄RE₃(thf)₅(CH₂SiMe₃) (RE = Y (9), Dy (10)) were prepared by reactions of 3 equiv. of [RE(CH₂SiMe₃)₃(thf)₂] with 4 equiv. of 3-(^tBuNHCH₂)C₈H₅NH in THF. Reactions of [RE(CH₂SiMe₃)₃(thf)₂] with 1 equiv. of 3-t-butylaminomethylindole 3-(^tBuNHCH₂)C₈H₅NH in THF, followed by treatment with 1 equiv. of [(2,6-ⁱPr₂C₆H₃)N[double bond, length as m-dash]CHNH(C₆H₃ⁱPr₂-2,6)] afforded, after workup, the dinuclear rare-earth metal complexes [η¹-μ-η¹:η¹-3-(^tBuNCH₂)Ind][η¹-μ-η¹:η³-3-(^tBuNCH₂)Ind]RE₂(thf)[(η³-2,6-ⁱPr₂C₆H₃)NCHN(C₆H₃ⁱPr₂-2,6)]₂(RE = Er (11), Y (12)) having the indolyl ligands bonded with the rare-earth metal in different ligations. All new complexes 1-12 were fully characterized by spectroscopic methods and elemental analyses, and their structures were determined by X-ray crystallographic analyses. It was found that, except for complexes 1, 4, 11 and 12, all complexes were highly efficient catalysts for selective isoprene polymerization (up to 99% 1,4-cis selectivity) with the cooperation of co-catalysts, and the trinuclear complexes displayed advantages over dinuclear complexes in terms of molecular weight of polymers.

Synthesis, characterization and reactivity study of aluminum compounds incorporating bi- and tri-dentate pyrrole–piperazine ligands

A series of Al derivatives are afforded incorporating substituted bi- and tri-dentate pyrrole–piperazine precursors. The reactivity with small organic molecules and the associated mechanistic details are also reported.

Polymerization of conjugated dienes with a homogeneous Ziegler–Natta catalytic system based on a carbon-bridged bis(phenolate) yttrium alkyl complex

1,3-Diene polymerization with some controllable characteristics was carried out and catalyzed by a triisobutylaluminum modified MAO activated complex [(EDBP)Y(CH₂SiMe₃)(THF)₃].

Group 1 and group 2 metal complexes supported by a bidentate bulky iminopyrrolyl ligand: synthesis, structural diversity, and ε-caprolactone polymerization study

We report here a series of alkali and alkaline earth metal complexes, each with a bulky iminopyrrolyl ligand [2-(Ph3CN=CH)C4H3NH] (1-H) moiety in their coordination sphere, synthesized using either alkane elimination or silylamine elimination methods or the salt metathesis route. The lithium salt of molecular composition [Li(2-(Ph3CN=CH)C4H3N)(THF)2] (2) was prepared using the alkane elimination method, and the silylamine elimination method was used to synthesize the dimeric sodium and tetra-nuclear potassium salts of composition [(2-(Ph3CN=CH)C4H3N)Na(THF)]2 (3) and [(2-(Ph3CN=CH)C4H3N)K(THF)0.5]4 (4) respectively. The magnesium complex of composition [(THF)2Mg(CH2Ph){2-(Ph3CN=CH)C4H3N}] (5) was synthesized through the alkane elimination method, in which [Mg(CH2Ph)2(OEt2)2] was treated with the bulky iminopyrrole ligand 1-H in 1 : 1 molar ratio, whereas the bis(iminopyrrolyl)magnesium complex [(THF)2Mg{2-(Ph3CN=CH)C4H3N}2] (6) was isolated using the salt metathesis route. The heavier alkaline earth metal complexes of the general formula {(THF)nM(2-(Ph3CN=CH)C4H3N)2} [M = Ca (7), Sr (8), and n = 2; M = Ba (9), n = 3] were prepared in pure form using two synthetic methods: in the first method, the bulky iminopyrrole ligand 1-H was directly treated with the alkaline earth metal precursor [M{N(SiMe3)2}2(THF)n] (where M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent at ambient temperature. The complexes 7-9 were also obtained using the salt metathesis reaction, which involves the treatment of the potassium salt (4) with the corresponding metal diiodides MI2 (M = Ca, Sr and Ba) in 2 : 1 molar ratio in THF solvent. The molecular structures of all the metal complexes (1-H, 2-9) in the solid state were established through single-crystal X-ray diffraction analysis. The complexes 5-9 were tested as catalysts for the ring-opening polymerization of ε-caprolactone. High activity was observed in the heavier alkaline earth metal complexes 7-9, with a very narrow polydispersity index in comparison to that of magnesium complexes 5 and 6.

Cyclometalation and coupling of a rigid 4,5-bis(imino)acridanide pincer ligand on yttrium

An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan, H[AIm2] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene. Reaction of intensely orange H[AIm2] with LiCH2SiMe3 formed deep blue Li(x)[AIm2]x (x = 2 in the solid state), while reaction with [Y(CH2SiMe3)3(THF)2] (0.5 equiv.) afforded deep blue [Y(AIm2)(AIm)] (1; AIm = an AIm2 ligand cyclometalated at the ortho-position of one of the phenyl rings). Compound 1 slowly isomerizes to form green-brown 2, which contains a single trianionic, hexadentate ligand that features one amine, two imine, and three amido donors. The acridanide backbone and one imine group in each of the original AIm2 ligands is intact, but the two acridanide backbones are now linked by an isoindoline heterocycle. Yttrium in 2 is coordinated to six nitrogen donors and the ortho carbon of an isoindoline phenyl substituent. The intense colours of H[AIm2], Li(x)[AIm2]x and 1 were shown by TD-DFT calculations to arise from charge transfer transitions from the HOMO, which is localized on the acridanide ligand backbone, to the LUMO and LUMO+1, which are localized on the imine substituents. The conversion of 1 to 2 was studied by UV-Visible absorption spectroscopy and is first-order with a half-life of 7.8 hours at room temperature.

Heterometallic rare-earth metal complexes with imino-functionalized 8-hydroxyquinolyl ligands: synthesis, characterization and catalytic activity towards hydrophosphinylation of trans-β-nitroalkene

The heterobimetallic complexes exhibited high catalytic activities on the hydrophosphinylation of β-nitroalkenes.

Reactivity of functionalized indoles with rare-earth metal amides. Synthesis, characterization and catalytic activity of rare-earth metal complexes incorporating indolyl ligands

The synthesized rare-earth metal amido complexes displayed different catalytic activity for intramolecular hydroamination of aminoalkenes.

From C,N- and N,N-chelated chloroboranes to substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines: a straightforward route to five-membered rings containing the B–N or N–B–N moiety

The reaction of C,N- and N,N-chelated boranes with lithium reagents resulted in the formation of substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines.