Synthesis of Poly(δ-caprolactone) via Bis(phenolate) Rare-Earth Metal Complexes Mediated Ring-Opening Polymerization and Its Chemical Recycling.

New amine-amino-bis(phenolate) ligands (H2LtBu and H2LCl) with a cyclic tertiary amine (pyrrolidine) as a side arm and tBu or Cl group on the phenolate ring have been prepared. The alkane elimination reaction between these free ligands and rare-earth tris(alkyl)s Ln(CH2SiMe3)3(THF)2 afforded the corresponding silylalkyl complexes LtBuLnCH2SiMe3(THF) (Ln = Y (1), Lu (2)) and LClYCH2SiMe3(THF) (3), where the solid-state structure of complex 1 was unambiguously confirmed by X-ray diffraction (XRD) analysis. These rare-earth metal complexes have been utilized as catalysts for the ring-opening polymerization (ROP) of biobased δ-caprolactone (δCL), either in the absence or presence of alcohols, to give poly(δ-caprolactone) (PδCL) with controlled molecular weight and narrow distribution (D < 1.2). The polymerization kinetics of δCL in toluene with yttrium complexes 1 and 3 were investigated. Oligomers prepared with complex 3 alone and the 3/PhCHMeOH binary catalyst system were well characterized with 1H NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Moreover, chemical recycling of the resultant PδCL was achieved with high yield in a solution at ambient temperature (>92%) or in bulk at 130 °C (>82%) by using commercial KOtBu as a promotor.

Bimetallic Arylamide-Ligated Rare-Earth Metal Complexes: Synthesis, Characterization, and Stereo-Selectively Switchable Property in 2-Vinylpyridine Polymerization.

Bimetallic complexes are expected to offer unique catalytic property, by facilitating cooperative effects between proximate functional groups or adjacent active metal centers, and thus have attracted increasing attention in the chemical community. Treatment of Ln(CH2SiMe3)3(THF)2 or Ln(CH2C6H4NMe2-o)3 with 1,4-(C6H5NH)2C6H4 in a 2:1 molar ratio in tetrahydrofuran (THF) generated a series of bimetallic arylamide-ligated rare-earth metal alkyl complexes [1,4-(C6H5N)2C6H4][Ln(CH2SiMe3)2(THF)2]2 (Ln = Sc (1), Lu (2), Y (3)), and aminobenzyl complexes [1,4-(C6H5N)2C6H4][Ln(CH2C6H4NMe2-o)2(THF)x]2 (Ln = Sc (4), x = 0; Lu (5), Y (6), x = 1) in 65-73% isolated yields. To reveal the polymerization difference between bimetallic and monometallic rare-earth metal complexes, the monoarylamide-ligated scandium bis(aminobenzyl) complex [(C6H5)2N]Sc(CH2C6H4NMe2-o)2 (7) was prepared by the reaction of Sc(CH2C6H4NMe2-o)3 with 1 equiv of diphenylamine (C6H5)2NH. All these rare-earth metal complexes were characterized by elemental analysis and NMR spectroscopy. The molecular structures of complexes 4 and 6 were authenticated by single-crystal X-ray diffraction. Complexes 2, 3, 5, and 6 alone were highly active for 2-vinylpyridine (2VP) polymerization at room temperature, giving poly-2VP with good iso-selectivity (mm). After activation with 2 equiv of [Ph3C][B(C6F5)4] or [PhNHMe2][B(C6F5)4], these complexes demonstrated an improved iso-selectivity (mm up to 96%) toward 2VP polymerization compared to their neutral analogues. In comparison, the bimetallic scandium complexes 1 and 4 showed relatively poor activity toward 2VP polymerization under the same conditions. However, the stereoselectivity of the polymerization could be switched from iso-tacticity to syndio-rich selectivity solely by tuning active species from only one scandium precatalyst. The catalyst system of complex 4/[PhNMe2H][B(C6F5)4] was able to promote a controlled syndio-specific polymerization of 2VP. The polymerization was experimentally verified to proceed via group transfer mechanism. Preliminary results indicated that the bimetallic rare-earth metal complexes showed a higher polymerization activity than the corresponding monometallic species mostly resulting from the cooperative effect.

Zinc-Catalyzed Hydrosilylation Copolymerization of Aromatic Dialdehydes with Diphenylsilane.

The hydrosilylation reaction of diphenylsilane and phthalaldehyde is catalyzed by heteroscorpionate zinc hydride complex LZnH (1, L = (Me Pz)2 CP(Ph)2 NPh, Me Pz = 3,5-dimethylpyrazolyl). Both terephthalaldehyde (tPAA) and isophthalaldehyde (iPAA) affords linear polymer; however, when PAA is used, a seven-membered cyclic silyl ether is formed. These products are characterized by 1 H and 13 C NMR spectra. Other properties of polymers are determined by gel permeation chromatography (GPC)-multiangle laser-light scattering, differential scanning calorimetry, and thermogravimetry.

Orange emissive carbon dots for fluorescent determination of hypoxanthine in fish.

Sensitive determination of hypoxanthine (HX), an indicator reflecting the degradation of meat and fish, is significantly important in monitoring food freshness. Herein, we design a novel sensor consisting of orange emissive carbon dots (O-CDs), nitrotetrazolium blue chloride (NTBC), and xanthine oxidase (XOD) for fluorescence turn-off detection of HX. O-CDs, possessing a high fluorescence quantum yield of 37%, are synthesized by hydrothermal treatment of 2,3-diaminopyridine in sulfuric acid. NTBC can react with HX/XOD-generated H2O2 and O2- to yield a violet-colored formazan, which remarkably quenches the orange fluorescence of O-CDs through inner filter effect. There is a linearity between the quenching efficiency and HX concentration in the range of 2-250 µM, and the limit of detection is 0.61 µM, lower than those of most reported HX sensors. In addition, the proposed method exhibits excellent selectivity, and can be applied to quantify HX in fish samples with satisfactory results.

Nickel-Catalyzed Reductive Csp2-Csp3 Cross Coupling Using Phosphonium Salts.

A nickel-catalyzed reductive cross coupling with phosphonium salts and allylic C(sp3)-O bond electrophiles, which granted direct construction of the C(sp2)-C(sp3) bond, is successfully developed. The protocol features broad substrate scope, high-functional-group tolerance, and heterocycle compatibility. Notably, the much more challenging reductive cross coupling with heterocyclic thiazolylphosphonium salts has also been accomplished for the first time.

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

An acid-base reaction between the rare-earth tris(o-dimethylaminobenzyl) complexes Ln(CH2C6H4NMe2-o)3 and the pyrrolyl-functionalized arylamide ligands 2,5-Me2C4H2NCH2SiMe2NHC6H4R (R = H, (HL1); R = Cl-p, (HL2)) was investigated. Treatment of HL1 and HL2 with 1 equiv. of Ln(CH2C6H4NMe2-o)3 in toluene at room temperature gave (2,5-Me2C4H2CH2NSiMe2NC6H5)Ln(CH2C6H4NMe2-o)2 (Ln = Sc (1), La (2), Lu (3)) and (2,5-Me2C4H2NCH2SiMe2NC6H4Cl-p)Ln(CH2C6H4NMe2-o)2 (Ln = Sc (4), La (5), Lu (6)) in good isolated yields. These complexes were characterized by elemental analysis and NMR spectroscopy. 2, 4 and 5 were structurally authenticated by single crystal X-ray diffraction. NMR and X-ray diffraction show that there are no interactions between the central metal and the pyrrolyl moiety in small sized metal complexes, while in large lanthanum complexes, besides the coordination of the arylamide moiety to La3+ in η1-bonding mode through a N atom, the pyrrolyl ring also has close contact with La3+ in 2 and 5. The binary systems of 1-6/[Ph3C][B(C6F5)4] were employed as catalysts for syndio-specific polymerization of styrene, and the pyrrolyl-ligated lanthanum complexes showed much higher activity than the complexes in which the pyrrolyl moiety had no coordination with the central metal.

Perfectly isoselective polymerization of 2-vinylpyridine promoted by ß-diketiminato rare-earth metal cationic complexes.

Symmetric and asymmetric ß-diketiminato rare-earth metal dialkyl complexes L1Ln(CH2SiMe3)2(THF) (L1 = (2,6-Et2C6H3)NC(Me)CHC(Me)N(2,6-Et2C6H3), Ln = Y (1a), Lu (1b)) and L2Ln(CH2SiMe3)2(THF) (L2 = (2,6-iPr2C6H3)NC(Me)CHC(Me)N(C6H5), Ln = Y (2a), Lu (2b)) have been synthesized by the reaction of the precursor Ln(CH2SiMe3)3(THF)2 with the corresponding pro-ligands (HL1 and HL2) in good yield. These complexes promote the polymerization of 2-vinylpyridine (2-VP) to produce isotactic-biased poly(2-vinylpyridine) (P2VP) (mm: 40%) in quantitative yield. The in situ generated cationic species with the addition of borate ([Ph3C][B(C6F5)4], [PhNHMe2][B(C6F5)4]) prior to the polymerization display distinct results, and the symmetric ones afford perfectly isotactic P2VP with an mm value up to 99%, whilst the asymmetric ones produce atactic P2VP (mm: 26%). The 1H NMR spectrum and MALDI-TOF mass analysis of an oligomer prepared with 1a/[Ph3C][B(C6F5)4] indicate that the polymerization is initiated by coordination insertion of 2-VP into the Y-CH2SiMe3 bond of the in situ generated cationic species.

Controlled iso-specific polymerization of 2-vinylpyridine catalyzed by arylamide-ligated rare-earth metal aminobenzyl complexes.

Treatment of rare-earth tris(o-dimethylaminobenzyl) complexes Ln(CH2C6H4NMe2-o)3 with one equivalent of the pyrrolyl-functionalized arylamide ligands 2,5-Me2C4H2NCH2CH2NHC6H5 and 2,5-Me2C4H2NSiMe2NHC6H5 in toluene generated arylamide rare-earth metal bis(aminobenzyl) complexes (2,5-Me2C4H2NCH2CH2NC6H5)Ln(CH2C6H4NMe2-o)2 (Ln = Sc (1), La (2), Lu (3)) and (2,5-Me2C4H2NSiMe2NC6H5)Ln(CH2C6H4NMe2-o)2 (Ln = Lu (5), Y (6)) in good isolated yields. Reaction of Y(CH2C6H4NMe2-o)3 with 2,5-Me2C4H2NCH2CH2NHC6H5 in 1 : 1 or 1 : 2 molar ratio specifically yielded bis(arylamide) yttrium mono(aminobenzyl) complex (2,5-Me2C4H2NCH2CH2NC6H5)2Y(CH2C6H4NMe2-o) (4). Complexes 1-6 were characterized by elemental analysis and NMR spectroscopy. Complexes 2, 4 and 5 were structurally validated by single crystal X-ray diffraction. These complexes were highly active for polymerization with 2-vinylpyridine to afford iso-tactic polymers with high molecular weight and narrow molecular weight distribution. Complex 4 promoted the polymerization in a controlled fashion, and the iso-selectivity of the polymerization reached up to a Pm value of 0.98. Analyses of the oligomer by NMR and MALDI-TOF showed that the polymerization proceeded via a group transfer polymerization mechanism.

Neutral binuclear rare-earth metal complexes with four µ2-bridging hydrides.

The first neutral rare-earth metal dinuclear dihydrido complexes [(NPNPN)LnH2]2 (2-Ln; Ln = Y, Lu; NPNPN: N[Ph2PNC6H3((i)Pr)2]2) bearing µ2-bridging hydride ligands have been synthesized. In the presence of THF, 2-Y undergoes intramolecular activation of the sp(2) C-H bond to form dinuclear aryl-hydride complex 3-Y containing three µ2-bridging hydride ligands.

Isoselective ring-opening polymerization of rac-lactide initiated by achiral heteroscorpionate zwitterionic zinc complexes.

A highly isotactic stereoblock PLA has been obtained using the achiral heteroscorpionate zwitterionic zinc complexes as catalysts, for the first time, via a chain-end control mechanism.

Intramolecular C-H bond activation induced by a scandium terminal imido complex.

A CpPN-based scandium terminal imido complex was isolated, which could induce the intramolecular C-H bond activation of a phenyl group even at room temperature.