XiaoFang Li

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Name: 李晓芳; XiaoFang Li

Organization: Beijing Institute of Technology

Department: Key Laboratory of Cluster Science of Ministry of Education, Department of Chemistry

Title: Associate Professor

TOPICS

Inorganic Chemistry

Rare Earth Metal Complex

Chemicals
References

Cis-1,4-Polymerization of Isoprene Catalyzed by 1,3-Bis(2-pyridylimino)isoindoline-Ligated Rare-Earth-Metal Dialkyl Complexes

Co-reporter:Pengfei Zhang, Hongyun Liao, Hanghang Wang, Xiaofang Li, Fanzhi Yang, and Shaowen Zhang

Organometallics July 10, 2017 Volume 36(Issue 13) pp:2446-2446

Publication Date(Web):June 22, 2017

DOI:10.1021/acs.organomet.7b00322

The two rare-earth-metal dialkyl complexes 1 and 2 (1, Ln = Sc; 2, Ln = Lu) were obtained via the same acid–base reaction between 1,3-bis(2-pyridylimino)isoindoline (BPI) ligand and rare-earth-metal trialkyl complexes. These complexes 1 and 2 were structurally characterized by X-ray diffraction. Both in the solid state and in the solution state, the mononuclear Sc dialkyl complex 1, containing one monoanionic tridentate C2-symmetric pincer-type BPI ligand, adopts a distorted-trigonal-bipyramidal configuration. When the Sc center of complex 1 was replaced by the larger Lu center, the intramolecular proton transfer from the isoindoline nitrogen atom to one of the imine nitrogen atoms could be observed in the tautomeric BPI ligand, which served as a monoanionic tetradentate ligand bridging two Lu centers and finally afforded the binuclear Lu dialkyl complex 2 with a cage-like symmetrical structure in the solid state. However, this binuclear Lu complex 2 could dissociate into a mononuclear structure in the solution state similar to the case for the scandium complex 1 since the same C2 symmetry was also observed in the 1H and 13C NMR spectra of the lutetium complex 2 in C6D6. In the presence of cocatalyst borate and AliBu3, these complexes 1 and 2 exhibited high activities (up to 1.9 × 106 (g of polymer)/(molLn h)) and high cis-1,4-selectivities (>99%) in the polymerization of isoprene in toluene, affording the cis-1,4-polyisoprenes with high molecular weights (Mn up to 610000 g mol–1) and narrow to moderate molecular weight distributions (Mw/Mn = 1.26–2.08).

A highly active chiral (S,S)-bis(oxazoline) Pd(II) alkyl complex/activator catalytic system for vinyl polymerization of norbornene in air and water

Co-reporter:Yingda Huang;Jianyun He;Zhanxiong Liu;Guilong Cai;Shaowen Zhang

Polymer Chemistry (2010-Present) 2017 vol. 8(Issue 7) pp:1217-1222

Publication Date(Web):2017/02/14

DOI:10.1039/C6PY01859A

A series of air/water-tolerant chiral Pd alkyl complexes (R2-(S,S)-BOZ)PdR′ (1–3, R = CH(CH3)2, Ph; R′ = Me, Ph) were synthesized and structurally characterized. In the presence of an excess of activator, such as [Ph3C][B(C6F5)4], [PhMe2NH][B(C6F5)4], or B(C6F5)3, the air/water-tolerant chiral Pd(II) alkyl complexes 1–3 could promote the norbornene polymerization in air and water using unpretreated technical grade solvent and monomer with extremely high activity up to 1.7 × 109 g PNB per molPd per h.

Monomer-induced switching of stereoselectivity and limitation of chain growth in the polymerization of amine-containing para-substituted phenylacetylenes by [Rh(norbornadiene)Cl]2

Co-reporter:Yong Tian;Jianbing Shi;Bin Tong;Yuping Dong

Polymer Chemistry (2010-Present) 2017 vol. 8(Issue 37) pp:5761-5768

Publication Date(Web):2017/09/26

DOI:10.1039/C7PY01201B

A series of para-substituted phenylacetylenes bearing various amine-containing pendant groups 1–4 can serve as both a monomer and cocatalyst in the polymerization catalyzed by [Rh(norbornadiene)Cl]2 alone in CH2Cl2, affording polyphenylacetylenes (PPAs) with different stereoselectivities and molecular weights. The polymerization of monomer 4 having a long and flexible pendant group produces high molecular weight PPAs with high cis-transoid configurations, similar to those obtained from the coordination–insertion polymerization of these monomers 1–4 by using the [Rh(norbornadiene)Cl]2/cocatalyst systems. However, the stereospecific transformation from the cis-transoid to the trans-cisoid configuration and the growth limitation of the polymer chain are observed in the polymerization of monomers 1–3 containing short and rigid pendant groups. The resulting PPAs have trans-cisoid selectivities of up to 66% and low molecular weights. A metathesis mechanism is suggested, in which the steric repulsion between the pendant group of the monomer and the propagation chain originates from the successive 1,2-insertion of monomers 1–3 to the Rh–carbene active species gives a rational explanation for the tendency of stereospecific transformation and the growth limitation of the polymer chain.

1,4-Specific copolymerization of 1,3-cyclohexadiene with isoprene and their terpolymerization with styrene by cationic half-sandwich fluorenyl rare-earth metal alkyl catalysts

Co-reporter:Gaixia Du;Xinwen Yan;Pengfei Zhang;Hanghang Wang;Yuping Dong

Polymer Chemistry (2010-Present) 2017 vol. 8(Issue 4) pp:698-707

Publication Date(Web):2017/01/24

DOI:10.1039/C6PY01932C

1,4-Specific copolymerization of 1,3-cyclohexadiene (CHD) with isoprene (IP) and their terpolymerization with styrene (S) have been achieved for the first time by cationic half-sandwich fluorenyl rare-earth metal alkyl catalysts in situ generated from half-sandwich fluorenyl rare-earth metal dialkyl complexes Flu′Ln(CH2SiMe3)2(THF)n (1–10) in combination with an activator ([Ph3C][B(C6F5)4] (A), [PhNHMe2][B(C6F5)4] (B) and B(C6F5)3 (C)) and AliBu3. The copolymerization of CHD with IP affords 1,4-selective random CHD–IP copolymers containing different comonomer contents and sequence distributions unavailable previously (CHD content = 25–81 mol%, 1,4-CHD unit = 100%; 1,4-IP unit = 94–100%; cis-1,4-IP unit = 12–87%). Moreover, new 1,4-specific random CHD–IP–S terpolymers with different compositions and sequence distributions (CHD content = 31–64 mol%, IP content = 16–63 mol%, S content = 6–40 mol%, 1,4-CHD unit = 100%; 1,4-IP unit = 90–100%, cis-1,4-IP unit = 7–77%) are obtained via the terpolymerization of CHD, IP, with S. The activity and regio-/stereoselectivity of copolymerization as well as the comonomer content, sequence distribution, molecular weight and molecular weight distribution of copolymer can be easily controlled by modifying the substituted fluorenyl ligand, metal center, activator and molar ratio of comonomers. Residual CC bonds of the random CHD–IP copolymers and CHD–IP–S terpolymers are completely epoxidized by meta-chloroperoxybenzoic acid (mCPBA) at room temperature, affording high-performance copolymers with polar groups and reactive sites in the polymer backbone.

cis-1,4-specific carbocationic polymerization and copolymerization of 1,3-dienes initiated by (S,S)-bis(oxazolinylphenyl)amine chromium complexes

Co-reporter:

Journal of Polymer Science Part A: Polymer Chemistry 2017 Volume 55(Issue 7) pp:1250-1259

Publication Date(Web):2017/04/01

DOI:10.1002/pola.28491

ABSTRACTTwo new chiral (S,S)-bis(oxazolinylphenyl)amine chromium dichloride complexes have been synthesized and structurally characterized. In combination with 2 equiv. of borate and an excess of AlR3, such Cr complexes serve as effective cationic initiators in the stereoregular carbocationic polymerization of 1,3-dienes such as isoprene (IP) and myrcene (MY), affording cyclized cis-1,4-PIPs/PMys (cis-1,4-selectivity up to 96%) with cyclic sequence contents ranging from 26% to 87%. Moreover, these Cr initiator systems also exhibit an unprecedented control over sequence distribution of comonomers in the carbocationic copolymerization of IP and MY, preparing novel copolymers with different microstructures from mainly cyclized cis-1,4-specific statistical copolymers to cyclic olefin copolymers. The nature of Cr complex, borate, AlR3, temperature, molar ratio of comonomers has considerable effect on the (co)polymer's yield, stereoselectivity, cyclization, and comonomer sequence distribution. A plausible mechanism is suggested, which gives a new strategy for biomimetic synthesis of natural rubber. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 55, 1250–1259

On-Water Polymerization of Phenylacetylene Catalyzed by Rh Complexes Bearing Strong π-Acidic Dibenzo[a,e]cyclooctatetraene Ligand

Co-reporter:

Journal of Polymer Science Part A: Polymer Chemistry 2017 Volume 55(Issue 4) pp:716-725

Publication Date(Web):2017/02/15

DOI:10.1002/pola.28417

ABSTRACTA series of new mononuclear neutral and water-soluble cationic rhodium (Rh) complexes bearing strong π-acidic dibenzo[a,e]cyclooctatetraene (dbcot) diene ligand have been synthesized and structurally characterized. In the polymerization of phenylacetylene, the dbcot Rh complex exhibits higher catalytic activity than the corresponding cod-based Rh complex in both of organic solvent and aqueous media, affording the high cis-transoidal PPAs with up to 99% of cis-contents, moderate molecular weights, and moderate to broad molecular weight distributions. Moreover, on-water polymerization of substituted phenylacetylenes is achieved by these complexes under air atmosphere, in which 3- to 163-fold acceleration of the polymerization rate is observed in aqueous polymerization compared to that in organic solvents. The nature of the Rh complex, solvent, polymerization temperature, and substituted group on the phenylacetylene impact on the polymer's yield, stereoselectivity, molecular weight, and molecular weight distribution. In addition, the water-soluble cationic Rh complexes can be reused for three times. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 716–725

Palladium-catalyzed C(sp3)–C(sp2) cross-coupling of homoleptic rare-earth metal trialkyl complexes with aryl bromides: efficient synthesis of functionalized benzyltrimethylsilanes

Co-reporter:Guilong Cai, Yingda Huang, Tingting Du, Shaowen Zhang, Bo Yao and Xiaofang Li

Chemical Communications 2016 vol. 52(Issue 31) pp:5425-5427

Publication Date(Web):15 Mar 2016

DOI:10.1039/C6CC01312K

The first C(sp3)–C(sp2) cross-coupling of rare-earth metal alkyl complexes with aryl bromides has been developed. This reaction was conducted at low catalyst loading (0.5 mol%) and exhibited a broad substrate scope, thus providing a facile method for the synthesis of benzyltrimethylsilanes with diverse functional groups.

Transition metal complexes bearing tridentate ligands for precise olefin polymerization

Co-reporter:Deqian Peng, Xinwen Yan, Chao Yu, Shaowen Zhang and Xiaofang Li

Polymer Chemistry 2016 vol. 7(Issue 15) pp:2601-2634

Publication Date(Web):07 Mar 2016

DOI:10.1039/C6PY00040A

Tridentate chelating ligands, which possess three coordination sites and six bonding electrons to coordinate a metal center, are considered as an alternative to the cyclopentadienyl ligand. Such tridentate ligands can stabilize a variety of metal oxidation states of a complex and form two four-, five-, six- or seven-membered chelate rings. In addition, fine tuning of its electronic and steric properties by the modification of coordination atoms, ligand skeleton, and substituents on the skeleton of the tridentate ligand will enable control of the catalytic activity and stereochemistry of its complexes in olefin polymerization. This review introduces the recent progress on transition metal catalysts bearing tridentate chelating ligands for olefin polymerization. Special emphasis is placed on the effects of ligand modifications on the polymerization activity, the stereochemistry, comonomer incorporation, and molecular weight of the resulting polymers.

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

Co-reporter:Deqian Peng;Gaixia Du;Pengfei Zhang;Bo Yao;Shaowen Zhang

Macromolecular Rapid Communications 2016 Volume 37( Issue 12) pp:987-992

Publication Date(Web):

DOI:10.1002/marc.201600102

Copolymerization of isoprene with ethylene catalyzed by cationic half-sandwich fluorenyl scandium catalysts

Co-reporter:Gaixia Du;Jiaping Xue;Deqian Peng;Chao Yu;Hanghang Wang;Yuening Zhou;Jiaojiao Bi;Shaowen Zhang;Yuping Dong

Journal of Polymer Science Part A: Polymer Chemistry 2015 Volume 53( Issue 24) pp:2898-2907

Publication Date(Web):

DOI:10.1002/pola.27769

ABSTRACT

Isoprene polymerization and copolymerization with ethylene can be carried out by using cationic half-sandwich fluorenyl scandium catalysts in situ generated from half-sandwich fluorenyl scandium dialkyl complexes Flu'Sc(CH₂SiMe₃)₂(THF)_n, activator, and AlⁱBu₃ under mild conditions. In the isoprene polymerization, all of these cationic half-sandwich fluorenyl scandium catalysts exhibit high activities (up to 1.89 × 10⁷ g/mol_Sc h) and mainly cis−1,4 selectivities (up to 93%) under similar conditions. In contrast, these catalysts showed different activities and regio-/stereoselectivities being significantly dependent on the substituents of the fluorenyl ligands in the copolymerization of isoprene with ethylene under an atmosphere of ethylene (1 atm) at room temperature, affording the random copolymers with a wide range of cis−1,4-isoprene contents (IP content: 64 − 97%, cis−1,4-IP units: 65 − 79%) or almost alternating copolymers containing mainly 3,4-IP-alt-E or/and cis−1,4-IP-alt-E sequences. Moreover, novel high performance polymers have been prepared via selective epoxidation of the vinyl groups of the 1,4-isoprene units in the IP-E copolymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2898–2907

1,4-Selective Polymerization of 1,3-Cyclohexadiene and Copolymerization with Styrene by Cationic Half-Sandwich Fluorenyl Rare Earth Metal Alkyl Catalysts

Co-reporter:Gaixia Du, Yingyun Long, Jiaping Xue, Shaowen Zhang, Yuping Dong, and Xiaofang Li

Macromolecules 2015 Volume 48(Issue 6) pp:1627-1635

Publication Date(Web):March 11, 2015

DOI:10.1021/acs.macromol.5b00037

The regioselective coordination–insertion polymerization of 1,3-cyclohexadiene (CHD) and copolymerization with styrene (S) could be achieved by cationic half-sandwich fluorenyl rare earth metal alkyl catalysts generated by treating half-sandwich fluorenyl rare earth metal dialkyl complexes Flu′Ln(CH2SiMe3)2(THF)n (1–10) with an activator (such as [Ph3C][B(C6F5)4] (A), [PhNHMe2][B(C6F5)4] (B), or B(C6F5)3 (C)) and AliBu3. The homopolymerization of CHD afforded poly(CHD)s with complete 1,4 selectivity (1,4 selectivity up to 100%). The copolymerization of CHD with styrene gave new random CHD–S copolymers with CHD content ranging from 22 to 74 mol % containing 1,4-linked CHD–CHD, alternating CHD–S, and syndiotactic S–S sequences unavailable previously. The activity of the copolymerization and the comonomer compositions and sequences of the resulting CHD–S copolymers could be easily controlled by changing the substituted fluorenyl ligand, the metal center, the activator, the temperature, and the molar ratio of comonomers. The residual C–C double bonds of the random CHD–S copolymers could be further epoxidized by meta-chloroperoxybenzoic acid (mCPBA) at room temperature to prepare high-performance polymers with polar groups and reactive sites in the polymer backbone. Such functionalization could improve the solubility, dying, acidity, and surfactivity of these copolymer materials.

Chiral Palladium(II) and Nickel(II) Complexes with C2-Symmetrical Tridentate Bis(oxazoline) Ligands: Synthesis, Characterization, and Catalytic Norbornene Polymerization

Co-reporter:Jianyun He, Zhanxiong Liu, Gaixia Du, Yinxia Fu, Shaowen Zhang, and Xiaofang Li

Organometallics 2014 Volume 33(Issue 21) pp:6103-6112

Publication Date(Web):October 1, 2014

DOI:10.1021/om5007616

A series of chiral palladium(II) and nickel(II) complexes bearing a C2-symmetric monoanionic tridentate bis(oxazoline) ligand, (R2-(S,S)-BOZ)M(X) (1–6: R = CH(CH3)2, M = Pd, X = OAc (1); R = CH(CH3)2, M = Pd, X = Cl (2); R = Ph, M = Pd, X = Cl (3); R = Ph, M = Ni, X = Cl (4); R = CH(CH3)2, M = Ni, X = Cl (5); R = CH(CH3)2, M = Pd, X = OTf (6)), have been synthesized and structurally characterized. The experimental results demonstrate that such chiral palladium(II) and nickel(II) complexes bearing C2-symmetric tridentate ligands in which the monoanionic group is located inside are effective for norbornene polymerization. In the presence of various cocatalysts such as MAO, MMAO, and activator/AlR3, these chiral palladium(II) complexes exhibit much higher activities of up to 4.8 × 108 g of PNB (mol of Pd)−1 h–1 for the vinylic polymerization of norbornene, affording insoluble polynorbornenes with high packing density. In contrast, the chiral nickel(II) complexes show relatively low activities of ca. 4.5 × 107 g of PNB (mol of Ni)−1 h–1 and produce both insoluble polynorbornenes and soluble high-molecular-weight polynorbornenes with moderate molecular weight distributions.

Cationic Tropidinyl Scandium Catalyst: A Perfectly Acceptable Substitute for Cationic Half-Sandwich Scandium Catalysts in cis-1,4-Polymerization of Isoprene and Copolymerization with Norbornene

Co-reporter:Siqian Liu, Gaixia Du, Jianyun He, Yingyun Long, Shaowen Zhang, and Xiaofang Li

Macromolecules 2014 Volume 47(Issue 11) pp:3567-3573

Publication Date(Web):June 2, 2014

DOI:10.1021/ma500740m

Different nonmetallocene rare earth metal alkyl complexes such as monotropidinyl (Trop) scandium dialkyl complex (Trop)Sc(CH2SiMe3)2(THF) (1), ditropidinyl yttrium alkyl complex (Trop)2Y(CH2SiMe3)(THF) (3) as well as binuclear lutetium alkyl complex bearing one tetradentate dianionic 6-N-methyl-1,4-cycloheptadienyl (NMCH) ligand [(NMCH)Lu(CH2SiMe3)(THF)]2 (2) have been synthesized in high yields via one-pot acid–base reaction by using of the tris(trimethylsilylmethyl) rare earth metal complexes with the readily available natural product tropidine. The polymerization experiments indicate that the monotropidinyl scandium dialkyl complex 1 displays reactivity akin to that of the analogous monocyclopentadienyl scandium dialkyl complexes. In the presence of activator and a small amount of AlMe3, complex 1 exhibits similar activities (up to 1.6 × 106 g molSc–1 h–1) but higher cis-1,4-selectivities (up to 100%) than (C5H5)Sc(CH2SiMe3)2(THF) (cis-1,4-selectivity as 95%) in the isoprene polymerization, yielding the pure cis-1,4-PIPs with moderate molecular weights (Mn = 0.5–11.2 × 104 g/mol) and bimodal molecular weight distributions (Mw/Mn = 1.48–6.07). Moreover, the complex 1/[Ph3C][B(C6F5)4/AliBu3 ternary system also shows good comonomer incorporation ability in the copolymerization of isoprene and norbornene similar to the [C5Me4(SiMe3)]Sc(η3-CH2CHCH2)2/activator binary system, affording the random isoprene/norbornene copolymers with a wide range of isoprene contents around 57–91 mol % containing cis-1,4 configuration up to 88%.

Bis(imino)diphenylamido rare-earth metal dialkyl complexes: synthesis, structure, and catalytic activity in living ring-opening ε-caprolactone polymerization and copolymerization with γ-butyrolactone

Co-reporter:Gaixia Du, Yanling Wei, Wei Zhang, Yuping Dong, Zhengguo Lin, Huan He, Shaowen Zhang and Xiaofang Li

Dalton Transactions 2013 vol. 42(Issue 4) pp:1278-1286

Publication Date(Web):23 Oct 2012

DOI:10.1039/C2DT31932B

Bis(imino)diphenylamido rare-earth metal dialkyl complexes [o-(2,6-iPr2-C6H3–NC–C6H4)2–N]Ln(CH2SiMe3)2 (1: Ln = Sc; 2: Ln = Lu; 3: Ln = Y) have been synthesized in good yields and structurally characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction studies. They serve as highly efficient single-component catalysts both for the living ring-opening ε-caprolactone polymerization and random copolymerization with γ-butyrolactone, with the activity being dependent on the steric hindrance around the metal center, yielding high molecular weight PCLs or P(CL-co-BL)s with narrow molecular weight distributions.

Aluminum Effects in the Syndiospecific Copolymerization of Styrene with Ethylene by Cationic Fluorenyl Scandium Alkyl Catalysts

Co-reporter:Xiaofang Li, Xiaoying Wang, Xin Tong, Hongxia Zhang, Yuanyuan Chen, Ying Liu, Hui Liu, Xiaojie Wang, Masayoshi Nishiura, Huan He, Zhenguo Lin, Shaowen Zhang, and Zhaomin Hou

Organometallics 2013 Volume 32(Issue 5) pp:1445-1458

Publication Date(Web):January 28, 2013

DOI:10.1021/om3011036

A series of half-sandwich fluorenyl (Flu′) scandium dialkyl complexes Flu′Sc(CH2SiMe3)2(THF)n (1, Flu′ = C13H9, n = 1; 2, Flu′ = 2,7-tBu2C13H7, n = 1; 3, Flu′ = 9-SiMe3C13H8, n = 1; 4, Flu′ = 2,7-tBu2-9-SiMe3C13H6, n = 1; 5, Flu′ = 9-CH2CH2NMe2C13H8, n = 0; 6, Flu′ = 2,7-tBu2-9-CH2CH2NMe2C13H6, n = 0) have been synthesized and structurally characterized. In comparison with the well-known cyclopentadienyl-ligated scandium catalyst system [(C5Me4SiMe3)Sc(CH2SiMe3)2(THF)]/[Ph3C][B(C6F5)4], the analogous combinations of the fluorenyl-ligated, THF-containing complexes 1–4 with [Ph3C][B(C6F5)4] show relatively low activities, albeit with similar syndioselectivities for styrene polymerization and styrene–ethylene copolymerization. However, on treatment with 15 equiv of AliBu3, the 1–4/[Ph3C][B(C6F5)4] combinations show a dramatic increase in catalytic activity without changes in the stereoselectivity. In contrast, the combinations of complexes 5 and 6, which have an amino group attached to the fluorenyl ring and intramolecularly bonded to the metal center, exhibit very low activity, no matter whether or not AliBu3 is present, affording syndiotactic polystyrenes with broad molecular weight distributions. The DFT calculations of the activation mechanism by using the representative catalysts suggest that AliBu3 can capture the THF molecule from the catalyst precursors 1–4 at first,and then the new, THF-free cationic half-sandwich scandium active species [Flu′Sc(CH2SiMe3)][B(C6F5)4] with less steric hindrance around the metal center is generated in the presence of an activator such as [Ph3C][B(C6F5)4]. The DFT calculations on the syndioselectivity of styrene (co)polymerization catalyzed by [Flu′Sc(CH2SiMe3)][B(C6F5)4] have also been carried out, thus shedding new light on the mechanistic aspects of the (co)polymerization processes.

Quasi-Living trans-1,4-Polymerization of Isoprene by Cationic Rare Earth Metal Alkyl Species Bearing a Chiral (S,S)-Bis(oxazolinylphenyl)amido Ligand

Co-reporter:Hui Liu, Jianyun He, Zhanxiong Liu, Zhengguo Lin, Gaixia Du, Shaowen Zhang, and Xiaofang Li

Macromolecules 2013 Volume 46(Issue 9) pp:3257-3265

Publication Date(Web):April 17, 2013

DOI:10.1021/ma4005549

A series of chiral mononuclear dialkyl complexes [(S,S)-BOPA]Ln(CH2SiMe3)2 (1, 2) (BOPA = (S,S)-bis(oxazolinylphenyl)amido; Ln = Sc (1); Ln = Lu (2)) and binuclear alkyl complexes [ο-(S)-OPA–C6H4–(CH2SiMe3)C═N–CH(iPr)CH2–O]Ln(CH2SiMe3)}2 (3,4) (OPA = (oxazolinylphenyl)amine; Ln = Y (3); Ln = Tm (4)) have been synthesized in moderate yields via one-pot acid–base reactions by use of the tris(trimethylsilylmethyl) rare earth metal complexes with the chiral tridentate (S,S)-bis(oxazolinylphenyl)amine ligand. In the presence of activator with or without a small amount of AliBu3, the dialkyl complexes 1 and 2 exhibit very high activities (up to 6.8 × 105 g molLn–1 h–1) and trans-1,4-selectivity (up to 100%) in the quasi-living polymerization of isoprene, yielding the trans-1,4-PIPs with moderate molecular weights (Mn = (0.2–1.0) × 105 g/mol) and narrow molecular weight distributions (Mw/Mn = 1.02–2.66).

Direct synthesis of cyclic carbonates from olefins and CO2 catalyzed by a MoO2(acac)2-quaternary ammonium salt system

Co-reporter:Fawang Chen, Tao Dong, Tiegang Xu, Xiaofang Li and Changwen Hu

Green Chemistry 2011 vol. 13(Issue 9) pp:2518-2524

Publication Date(Web):18 Jul 2011

DOI:10.1039/C1GC15549K

A highly efficient synthesis of cyclic carbonates from olefins and CO2 has been achieved by the use of a molybdenyl acetylacetonate (MoO2(acac)2)–quaternary ammonium salt catalytic system with tert-butyl hydroperoxide as an oxidant through a one-pot multistep process. This simple and cheap method can be applied to various olefins, such as 1-octene, 1-hexene, allyl chloride, cyclohexene and styrene, affording the highest yields in comparison with the data reported previously except for styrene. A plausible mechanism is proposed based on the results.

Tetrabutylammonium salts of tritransition-metal-substituted A-α-tungstogermanate: Halogen-free and single-component catalysts for the coupling reaction of carbon dioxide and epichlorohydrin

Co-reporter:Fa Wang Chen, Tao Dong, Xiao Fang Li, Tie Gang Xu, Chang Wen Hu

Chinese Chemical Letters 2011 Volume 22(Issue 7) pp:871-874

Publication Date(Web):July 2011

DOI:10.1016/j.cclet.2011.01.004

The green synthesis of chloropropylene carbonate via the coupling reaction of carbon dioxide and epichlorohydrin had been achieved using halogen-free and single-component catalysts tetrabutylammonium salts of tritransition-metal-substituted A-α-tungstogermanate [(n-C4H9)4N]3H7GeW9M3(H2O)3O37 (M = CuII, NiII, CoII and MnII) without any solvent. The catalytic activity was significantly depended on the transition metal introduced in polyoxometalates. [(n-C4H9)4N]3H7GeW9Mn3(H2O)3O37 exhibited the highest catalytic activity with 94.9% conversion for epichlorohydrin and 98% selectivity for chloropropylene carbonate in 3 h. Plausible mechanism was proposed based on the results.

Living 3,4-Polymerization of Isoprene by Cationic Rare Earth Metal Alkyl Complexes Bearing Iminoamido Ligands

Co-reporter:Gaixia Du, Yanling Wei, Lin Ai, Yuanyuan Chen, Qi Xu, Xiao Liu, Shaowen Zhang, Zhaomin Hou, and Xiaofang Li

Organometallics 2011 Volume 30(Issue 1) pp:160-170

Publication Date(Web):December 6, 2010

DOI:10.1021/om100971d

Treatment of rare earth metal trialkyl complexes Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Lu, and Y) with 1 equiv of α-diimine ligands 2,6-R2C6H3N═CH−CH═NC6H3R2-2,6 (R = iPr, Me) affords straightforwardly monoanionic iminoamido rare earth metal dialkyl complexes [2,6-R2C6H3N−CH2−C(CH2SiMe3)═NC6H3R2-2,6]Ln(CH2SiMe3)2(THF) (1: Ln = Sc, R = iPr; 2: Ln = Lu, R = iPr; 3: Ln = Y, R = iPr; 4: Ln = Sc, R = Me; 5: Ln = Lu, R = Me; 6: Ln = Y, R = Me) in 65−85% isolated yields. X-ray analyses show these complexes have decreasing steric hindrance in the coordination spheres of the metal centers in the order 1 > 2 > 3 > 4 > 5 > 6. A mechanism involving intramolecular alkyl and hydrogen migration is supported on the basis of DFT calculations to account for ligand alkylation. Activated by [Ph3C][B(C6F5)4], all of these iminoamido rare earth metal dialkyl complexes are active for living polymerization of isoprene, with activity and selectivity being significantly dependent on the steric hindrance around the metal center to yield homopolyisoprene materials with different microstructures and compositions. The sterically crowded complexes 1−3 give a mixture of 3,4- and trans-1,4-polyisoprenes (3,4-selectivities: 48−82%, trans-1,4-selectivities: 50−17%), whereas the less sterically demanding complexes 4−6 show high 3,4-selectivities (3,4-selectivities: 90−100%). In the presence of 2 equiv of AliBu3, the complexes 1−6/activator systems exhibit higher activities and 3,4-selectivities in the living polymerization of isoprene. A similar structure−reactivity relationship in polymerization catalysis can be also observed in these ternary systems. A possible mechanism of the isoprene polymerization processes is proposed on the basis of the DFT calculations.

Living regioselective copolymerization of ethylene with nonconjugated bicyclic dienes using (salicylaldiminato)(-enaminoketonato) titanium catalysts

Co-reporter:Ying-Yun Long;Yong-Xia Wang;Jing-Yu Liu;Xiao-Fang Li;Yue-Sheng Li

Journal of Polymer Science Part A: Polymer Chemistry 2011 Volume 49( Issue 21) pp:4626-4638

Publication Date(Web):

DOI:10.1002/pola.24906

Abstract

A series of heteroligated (salicylaldiminato)(β-enaminoketonato)titanium complexes [3-^tBu-2-OC₆H₃CHN(C₆F₅)] [PhNC(CF₃)CHCRO]TiCl₂ [3a: R = Ph, 3b: R = C₆H₄Cl(p), 3c: R = C₆H₄OMe(p), 3d: R = C₆H₄Me(p), 3e: R = C₆H₄Me(o)] were synthesized and characterized. Molecular structures of 3b and 3c were further confirmed by X-ray crystallographic analyses. In the presence of modified methylaluminoxane as a cocatalyst, these unsymmetric catalysts displayed favorable ability to incorporate 5-vinyl-2-norbornene (VNB) and 5-ethylidene-2-norbornene (ENB) into the polymer chains, affording high-molecular weight copolymers with high-comonomer incorporations and alternating sequence under the mild conditions. The comonomer concentration in the polymerization medium had a profound influence on the molecular weight distribution of the resultant copolymer. At initial comonomer concentration of higher than 0.4 mol/L, the titanium complexes with electron-donating groups in the β-enaminoketonato moiety mediated room-temperature living ethylene/VNB or ENB copolymerizations. Polymerization results coupled with density functional theory calculations suggested that the highly controlled living copolymerization is probably a consequence of the difficulty in chain transfer of VNB (or ENB)-last-inserted species and some characteristics of living ethylene polymerization under limited conditions. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011