Xiaobo Pan

Iso-Selective Ring-Opening Polymerization of rac-Lactide Catalyzed by Crown Ether Complexes of Sodium and Potassium Naphthalenolates

Two crown ether complexes of sodium and potassium naphthalenolates were synthesized and entirely characterized. The two complexes can iso-selectively catalyze the ring-opening polymerization (ROP) of rac-lactide at room temperature and afford polylactides with desired molecular weights and narrow PDIs; the best isotacticity (Pm) achieved was 0.73.

Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-lactide) Synthesis.

A high degree of chain end control in the isoselective ring-opening polymerization (ROP) of rac-lactide is a challenging research goal. In this work, eight highly active sodium and potassium phenolates as highly isoselective catalysts for the ROP of rac-lactide are reported. The best isoselectivity value of Pm = 0.94 is achieved. The isoselective mechanism is chain-end control through the analysis of the stereoerrors in the microstructure of a final polymer; thus, isotactic multiblock structure polymers are obtained, and the highest melt point can reach 192.5 °C. The donating group in phenolate can clearly accelerate the ROP reaction, potassium complexes are more active than the analogous sodium complexes, and the big spacial hindrance of the ligand can decrease the activity. The high isoselectivities of these complexes mostly result from their sandwich structure constructed by the plane of the crown and the plane of the anthryl group.

Stepwise Reduction of 9,10‐Bis(dimesitylboryl)anthracene

Stepwise reduction of 9,10-bis(dimesitylboryl)anthracene afforded an radical anion and a dianion, accompanied by stepwise changes of the aromaticity of the anthracene moiety. The radical has a planar semiquinoidal structure, while the dianion has a puckered quinoidal structure. The alteration of the geometries of the 9,10-bis(dimesitylboryl)anthracene upon reduction is rationalized by the nature of the bonding. These results have been confirmed by cyclic voltammetry, X-ray crystallography, NMR, EPR, and UV-vis-NIR spectroscopy, as well as DFT calculations.

Isoselective mechanism of the ring-opening polymerization of rac-lactide catalyzed by chiral potassium binolates

In this work, two enantiopure potassium complexes were synthesized and characterized. These two complexes show good isoselectivity for the ring-opening polymerization (ROP) of rac-lactide (Pm = 0.80 at room temperature for complex 1 and Pm = 0.80 at −60 °C for complex 2) and display very different activities for the ROP of rac-lactide due to the presence of an intramolecular hydrogen bond in complex 1. Meanwhile, a ligand assisted monomer activated mechanism for the ROP of lactide is rationalized in the presence of alcohol for this system. The stereoselective mechanism is ascribed to a chain end control mechanism via three experimental methods (the analysis of the stereo microstructure of the polymer, the activity of one enantiopure catalyst towards the ROP of L-lactide, D-lactide, and rac-lactide, and the enantiomeric excess values of the residual monomers and polymers via optical rotation measurements at different ROP conversions).

Multimetallic synergic sedation of a labile sodium atrane: synthesis and characterization of a tetranuclear sodium atrane cation complex.

A series of sodium and aluminum atrane complexes of Na(3)L(THF)(5) (1), [AlLMe][Na(4)L(THF)(6)] (2), AlL(THF) (3), AlNaLMe(THF)(2) (4), and AlNaLOBn(THF)(2) (5), wherein L = tris(2-oxy-4,6-di-tert-butyl-benzyl)amine, were synthesized and characterized by NMR, X-ray crystallography, and elemental analysis. The trinuclear sodium atrane complex of Na(3)L(THF)(5) (1) is labile at room temperature; however, the tetranuclear sodium atrane cation in complex 2 can be stabilized by a multimetallic synergetic effect due to a firm interaction ring of -[Na-O-benzene](3)-. Complex 2 is also the first example of a sodatrane and alumatrane ion-paired complex in which both the cationic and anionic moieties contain an atrane ligand.

Bulky bisphenol ligand-supported aluminum-sodium inverse crown ether complex.

A mixed aluminum-sodium phenolate ring compound with an oxo core has been synthesized and crystallographically characterized as a new inverse crown ether complex, together with some of its precursors. The experimental results showed that the Al-Na inverse crown ether can be obtained from two different approaches: (1) hydrolysis of [(EDBP)Al(CH(3))(2)Na(THF)(2)] (1), followed by condensation with [(EDBP)Al(CH(3))(mu(2)-OH)Na(THF)(3)] (2); (2) fast redox reaction by oxidation of a methyl group on complex 1 and reduction of molecular oxygen to oxide.

Synthesis and X-ray crystallography of diverse metal complexes derived from xanthone-crown ether

A series of sandwich, monomeric, dimeric and polymeric complexes supported with 1,8-xanthone-18-crown-5 (L) were synthesised. Mass spectrum experiments suggested the existence of sandwich and monomeric complexes in solution. And the structure characterisations of six new complexes by single-crystal X-ray diffraction show the strong coordination of xanthone-18-crown-5 carbonyl oxygen with alkaline earth metal cation, which results high fluorescent increase in alkaline earth metal complexes.

Isoselective Polymerization of rac-Lactide Catalyzed by Ion-Paired Potassium Amidinate Complexes

Three potassium crown ether complexes supported with bulky amidinate ligands were synthesized for the ring-opening polymerization (ROP) of rac-lactide. The side polymerization reaction initiated directly by ligand anion was suppressed well in the presence of alcohol as our design, and the synthesis of linear polylactide with a molecular weight as high as 117.7 kg/mol was successful together with an isoselectivity value of Pm = 0.88 at -70 °C. In this system, lactide can be deprotonated by amidinate anion to give lactide enolate, which can initiate the ROP of lactide as a side reaction in the absence of alcohol; however, this side reaction can also be suppressed well in the presence of alcohol by a decrease in temperature. An interesting anti-Arrhenius-like behavior in the polymerization was discovered, which can be attributed to the fact that the active catalyst can be converted to a less active lactide enolate potassium complex at a high temperature.

Synthesis of Bis-Cycloborate Olefin and Butatriene Derivatives through the Reduction of Alkynyl-Bridged Diboryl Compounds

A series of bis-cycloborate olefin and butatriene derivatives were synthesized from alkynyl-bridged diboryl compounds using a simple reduction strategy. In the reduction route of 1,2-bis[2-(dimesitylboranyl)phenyl]ethyne (1), bis-cycloborate olefin (4) to diborate-bridged stilbene (5) can be obtained selectively by varying the reaction temperature. In addition, a thermal isomerization from 4 to 5 was found as a result of the rearrangement of double bonds. The generality of this reaction was further verified in similar reduction reactions. In the direduction of 1,2-bis[8-(dimesitylboranyl)naphthalen-1-yl]ethyne (2), the bis-cycloborate olefin (6) was isolated in high yield and demonstrated no thermal isomerization. In the two-electron reduction of 1,8-bis{[2-(dimesitylboranyl)phenyl]ethynyl}naphthalene (3), the bis-cycloborate butatriene (7) was obtained unexpectedly because of the departure of the naphthyl group. When using Na as the reductant and diethyl ether as the solvent, 1,2-bis[( Z)-2-(dimesitylboranyl)benzylidene]-1,2-dihydroacenaphthylene (8) was isolated after adding 1 or 2 drops of H2O to the reduction reaction filtrates. Meanwhile, the related mechanisms for radical cyclization, thermal isomerization, and formation of bis-cycloborate butatriene were also discussed.

Acetonitrile­triaqua­[3-eth­oxy-1,8-(3,6,9-trioxaundecane-1,11-diyldi­oxy)-9H-xanthen-9-one]terbium(III) tris­(perchlorate)

In the title compound, [Tb(CH3CN)(C23H26O8)(H2O)3](ClO4)3, the Tb3+ atom is eight-coordinated by one N atom of an acetonitrile molecule, three water O atoms and four ligand O atoms. The Tb3+ atom is located on one side of the macrocycle and the carbonyl oxygen coordinated to the terbium [Tb1—O1= 2.210 (3) Å] is bent out of the xanthone plane by 0.514 (3) Å. The geometry around terbium is a distorted two-capped trigonal prism.