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Selective benzylic C–H activation is achieved by the binary system of (PN3P)NiCl/KN(SiMe3)2.

We present the mechanistic understanding of an electrochemically-driven nickel-catalyzed coupling reaction. Computational analysis reveals that the spin density is mostly residing on the nickel (Ni) center when NiII is reduced to NiI. Ni-mediated halogen atom abstraction through outer-sphere electron-transfer pathway to yield coupling products under mild conditions is demonstrated. Importantly, we have elucidated the role of NiI and Ni0 for successive coupling of benzyl bromide and benzyl chloride derivatives, respectively, to corresponding bibenzyl products. The Ni-catalyst bearing a PN3P-ligand is an effective catalyst, producing a strong ligand effect on the reactivity and selectivity for the homocoupling reactions.

Polymerization of 2-(4-halophenyl)-1,3-butadiene (2-XPB) and their copolymerization with isoprene using a yttrium catalyst have been examined. The β-diketiminato yttrium bis(alkyl) complex (1) activated by [Ph3C][B(C6F5)4] and AliBu3 shows high cis-1,4-selectivity (>98%) for the polymerization of 2-XPB (2-XPB = 2-FPB, 2-ClPB and 2-BrPB) to afford halogenated plastic poly(dienes) with glass transition temperatures of 30–55 °C. Moreover, the copolymerization of 2-XPB with isoprene (IP) has also been achieved by this catalyst, and the insertion ratios of 2-XPB can be facilely tuned in a full range of 0%–100% simply by changing the 2-XPB-to-IP ratio. Quantitative hydrogenation of cis-1,4-poly(2-XPB) results in perfect alternating ethylene-halostyrene copolymers, and an alternating copolymer of 4-vinylbenzoic acid with ethylene is obtained by a consecutive reaction of ethylene-4-bromostyrene copolymer with nBuLi, CO2 and HCl.

Low-molecular-weight polystyrene (LMPS) is the precursor of a brominated styrene oligomer that is used for small, precise, thin electrical components and an environmentally benign flame-retardant m...

The ligand of an organometallic complex is typically considered as a spectator which can be modified to tune the steric and electronic properties of the coordination environment. We herein demonstarted a PN3P*Rh-CO pincer system where one of the C–H bonds of the pseudo-dearomatized pyridine ring was oxidized by O2 at room temperature to create an α,β-unsaturated carbonyl functionality on the ligand backbone. The resulting metal complex with the post-modified PN3P ligand readily reacts with thiophenol and 4-methylaniline to afford the corresponding oxidative Michael addition products.

In contrast to the typical C

Formic acid possesses many desirable properties as a promising hydrogen energy carrier. Density functional theory (DFT) was utilized to design new Rh complexes with 2,2′-biimidazoline ligands to explore their potential as catalysts for formic acid dehydrogenation. These designed complexes were prepared and examined as catalysts under various temperatures and ratios of formic acid to sodium formate. It was found that although our complexes gave a high TOF of 20,000 h−1 at 90 °C under certain formic acid/sodium formate ratios, the dehydrogenation reaction was prematurely deactivated once 30% of the formic acid was consumed. Possible deactivation mechanisms were investigated by NMR and HRMS. Our observations suggest that while guided rational design of catalysts by DFT has much potential, the method does have limitations as evidenced by our experimental comparisons and thus pure DFT design should be conducted with caution.

A PN3P pincer ligand (1) bearing dicyclopentylphosphine substituents reacts with [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) to produce the complex (PN3P)RhCl (2). Treatment of a benzene solution of 2 with KN(SiMe3)2 stimulates a dearomatization process, and C–H activation of benzene is achieved through the rearomatization of the central pyridine ring. This deprotonation/reprotonation of the NH arm of 2 gives the phenyl complex (PN3P)Rh(C6H5) (3). The subsequent introduction of CO gas into 3 yields the benzoyl complex (PN3P)RhCO (C6H5) (4), which can release benzaldehyde upon treatment with diluted HCl solution and regenerates 2.

H2 heterolysis to generate well-defined nickel hydride-proton complexes was achieved by the 2nd generation PN3P-pincer nickel platform. The regioselective protonation in the ligand framework was demonstrated for the first time to highlight the importance of in-plane hindrance during the H2 splitting process.

Well-defined ruthenium(II) PN3P pincer complexes were developed for the hydrogenation of carbon dioxide. Excellent product selectivity and catalytic activity with TOF (turnover frequency) and TON (turnover number) up to 13 000 h−1 and 33 000, respectively, in a THF/H2O biphasic system were achieved. Notably, effective conversion of carbon dioxide from air into formate was conducted in the presence of an amine, allowing easy product separation and catalyst recycling.

A terminal nickel hydroxide complex (PN3P)Ni(OH) (3) bearing the 2nd generation phosphorus–nitrogen PN3P-pincer ligand has been synthesized and structurally characterized. As a nucleophile, 3 reacts with CO to afford the hydroxycarbonyl complex 4, (PN3P)Ni(COOH). 3 can also activate CO2 and CS2 to produce nickel bicarbonate (PN3P)Ni(OCOOH) (5) and bimetallic dithiocarbonate [(PN3P)NiS]2CO (6) respectively, as well as to promote aryl isocyanate and isothiocyanate insertion into the Ni–OH bond to give the corresponding (PN3P)NiEC(O)NHAr complexes (E = O, 7; E = S, 8). In addition, 3 catalyzes the nitrile hydration to various amides with well-defined intermediates (PN3P)Ni–NHC(O)R (R = Me, 9; R = Ph, 10).

A 2nd generation PN3P-pincer azido nickel complex (PN3P)Ni(N3) reacts with isocyanides to afford monosubstituted carbodiimides under irradiation, presumably via a transient nitrido intermediate. The resulting species can further generate unsymmetrical carboddimides and the PN3P nickel halide complex, accomplishing a synthetic cycle for a complete nitrogen atom transfer reaction.

A highly efficient strategy for the synthesis of polylactide with the UV absorption ability was established by employing a Salan-yttrium complex (acting as a fast runing catalyst) combined with large excess hydroxyl functionalized benzophenone, BP′-OH. During polymerization, BP′-OH, acting as the chain transfer agent, attached to the active rare-earth metal catalyst via a rapid-reversible exchange reaction to initiate the polymerization. Thus, more polyester chains appeared to grow from one active metal species, and the UV absorption fragments were incorporated into the polymer chains at specific sites, in situ. A high productivity up to 1000 molLA/mol(Salan-Y) was successfully achieved and 100 BP′-labeled PLA chains grew from each active metal center.

We report a ruthenium complex containing an N,N′-diimine ligand for the selective decomposition of formic acid to H2 and CO2 in water in the absence of any organic additives. A turnover frequency of 12 000 h–1 and a turnover number of 350 000 at 90 °C were achieved in the HCOOH/HCOONa aqueous solution. Efficient production of high-pressure H2 and CO2 (24.0 MPa (3480 psi)) was achieved through the decomposition of formic acid with no formation of CO. Mechanistic studies by NMR and DFT calculations indicate that there may be two competitive pathways for the key hydride transfer rate-determining step in the catalytic process.

H

The unprecedented highly cis-1,4 selective (>99%) coordination–insertion polymerization of the polar monomer 2-(4-methoxyphenyl)-1,3-butadiene (2-MOPB) has been achieved using a β-diketiminato yttrium bis(alkyl) complex to afford a hydrophilic plastic polymer P(2-MOPB) with a water contact angle of 87.7° and glass transition temperature of 34.2 °C. The copolymerization of polar 2-MOPB and non-polar isoprene has also been successfully realized for the first time to produce a type of highly modified cis-1,4 polyisoprene with a wide range of 2-MOPB content (8.2%–88.5%). The composition was adjusted by regulating the monomer feed ratio according to the copolymerization kinetics. Hydrogenation of P(2-MOPB) provided an unusual alternating copolymer, poly(4-methoxystyrene-alt-ethylene) that could not be obtained via any other methods.

An intensification and integration process of preparing highly thermal stable polylactide was reported. Calcium complex 1 [LCaN(SiMe3)2]2 (L = 2,5-bis-((pyrrolidin-1-yl)methene)-1H-pyrrole) was employed to synthesize the alcoholic chain-transfer agent and catalyze the immortal ring-opening polymerization of l -lactide in one pot. 1H NMR spectrum and MALDI-TOF mass spectrum analyses indicated that the polymer was end-capped by phosphate ester and hydroxyl groups. The resultant phosphate ester capped polylactide exhibited a Tonset of 57 °C higher than that ended with benzyloxy group, since the unzip degradation of PLLA initiated by back-biting was inhibited during the heating process.

A terminal nickel hydroxide complex (PN3P)Ni(OH) (3) bearing the 2nd generation phosphorus-nitrogen PN3P-pincer ligand has been synthesized and structurally characterized. As a nucleophile, 3 reacts with CO to afford the hydroxycarbonyl complex 4, (PN3P)Ni(COOH). 3 can also activate CO2 and CS2 to produce nickel bicarbonate (PN3P)Ni(OCOOH) (5) and bimetallic dithiocarbonate [(PN3P)NiS]2CO (6) respectively, as well as to promote aryl isocyanate and isothiocyanate insertion into the Ni-OH bond to give the corresponding (PN3P)NiEC(O)NHAr complexes (E = O, 7; E = S, 8). In addition, 3 catalyzes the nitrile hydration to various amides with well-defined intermediates (PN3P)Ni-NHC(O)R (R = Me, 9; R = Ph, 10).

A 2nd generation PN3P-pincer azido nickel complex (PN3P)Ni(N3) reacts with isocyanides to afford monosubstituted carbodiimides under irradiation, presumably via a transient nitrido intermediate. The resulting species can further generate unsymmetrical carboddimides and the PN3P nickel halide complex, accomplishing a synthetic cycle for a complete nitrogen atom transfer reaction.

Treatment of heteroscorpionate zinc benzyloxy complex LZnOBn (1, L = (MePz)2CP(Ph)2NPh, MePz = 3,5-dimethylpyrazolyl) with phenylsilane (PhSiH3) gave a zinc hydride complex LZnH (2) containing a rare terminal hydride fragment. X-ray diffraction analysis and the DFT calculation confirm the zwitterionic structure of complex 2. The stoichiometric reaction of 2 with CS2 readily afforded a dithioformate complex LZnSCH(S) (3) of the C═S insertion into the Zn–H product. Moreover, complex 2 was an efficient catalyst for the hydrosilylation reaction of a series of silanes and aldehydes under mild conditions, featuring excellent functional group tolerance. The preliminary mechanistic study revealed that both zinc benzyloxy complex 1 and zinc hydride complex 2 were involved in the hydrosilylation process as the reaction intermediates.

(E)-1-(4-Methylphenyl)-1,3-butadiene (E-1-MPBD) synthesized via the “Wittig-type” reaction was polymerized with the ternary catalytic system (Flu-NHC)Lu(CH2SiMe3)2/AliBu3/[Ph3C][B(C6F5)4] (Flu-NHC = C13H8CH2CH2(NCHCHN(C6H2Me3-2,4,6)C) to afford a new product containing exclusively trans-3,4 (>99%) units with perfect syndiotacticity (rrrr > 99%). The regio-3,4 tacticity was proved by the IR and NMR (1H and 13C) spectroscopic analyses, while the 3,4-stereotacticity was confirmed by a model polymer with lower regularity (3,4 = 90.9%, rrrr = 49.3%) prepared by the binary catalytic system (Am-NHC)Lu(CH2SiMe3)2/[Ph3C][B(C6F5)4] (Am-NHC = 2,6-iPrC6H3N═C(C6H5)NCH2CH2(NCHCHN(C6H2Me3-2,4,6)C). The trans-planar conformation was uncovered through 2D-NOESY and 13C CPMAS NMR technologies. This represents the first stereo 3,4-syndioselective polymerization of 1,3-dienes achieved by a rare-earth metal based catalyst. Moreover, hydrogenating the resulting polymer gave the highly syndiotactic poly(4-methylphenyl-1-butene),...

Stereoselective coordination/insertion polymerization of the polar ortho-methoxystyrene has been achieved for the first time by using the cationic beta-diketiminato rare-earth-metal species. High activity and excellent isoselectivity (mmmm>99%) were acheived. The unmasked Lewis-basic methoxy group does not poison the Lewis-acidic metal center, but instead activates the polymerization through sigma-pi chelation to the active species together with the vinyl group, thus lower the coordination and activation energies as compared with those of styrene derivatives lacking the methoxy group.

Heterocyclic-fused cyclopentadienyl scandium bis(alkyl) complexes L1-4Sc(CH2SiMe3)(2)THF ((5-Me-1-Ph-cyclopenta[b]pyrrol-4-yl)Sc(CH2SiMe3)(2)THF (1), (2,5-Me-2-3-Ph-6H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)(2)THF (2), (2,4,5,6-Me-4-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)(2)THF (3), (2,3,4,5,6-Me-5-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)(2)THF) (4)) were facilely synthesized by alkane elimination reaction of Sc(CH2SiMe3)3(THF)2 with the heterocyclic-fused cyclopentadienyl ligands HL1-4 in high yields. Complexes 1-4 were characterized by H-1 and C-13 NMR spectroscopies and X-ray diffraction analyses as THF-solvated monomers, adopting a half-sandwich geometry. Upon activation of [Ph3C][B(C6F5)(4)]/AliBu(3), these half-sandwich scandium complexes displayed various activities toward the copolymerization of ethylene (E) and dicyclopentadiene (DCPD). Complex 1, supported by the phenyl-substituted pyrrole-fused cyclopentadienyl ligand, showed a slightly higher activity than the phenyl-substituted thiophene-fused cyclopentadienyl complex 2. Among the thiophene-fused cyclopentadienyl complexes 2-4, 4, bearing pentamethyl substituents, showed the highest activity of 2.9 x 10 (6) g/molSc.h.bar. The resultant copolymers had adjustable DCPD incorporation varying from 14.0 up to 46.1 mol %, of which the alternating poly(E-alt-DCPD) had a high Tg of 166 degrees C. In addition, no cross-linking was observed in the copolymers, suggesting that these catalytic systems were highly regioselective for the two active double bonds within DCPD.

A series of amidino N-heterocyclic carbene ligand supported rare-earth metal bis(alkyl) complexes (2,6-R12C6H3NCR2NCH2CH2(NCHCHN(C6H2Me3-2,4,6)C))Ln(CH2SiMe3)2 [R1 = iPr, R2 = Ph, Ln = Lu (1a); R1 = iPr, R2 = Ph, Ln = Sc (1b); R1 = iPr, R2 = tBu, Ln = Lu (2a); R1 = Me, R2 = Ph, Ln = Lu (3a)] were synthesized and characterized by NMR spectroscopy. On activation with [(Ph3C)][(B(C6F5)4)], complexes 1a, 2a and 3a exhibited high activities and high 3,4-selectivities (96.9%) for the polymerization of 2-phenyl-1,3-butadiene in a living mode to afford new plastic polydienes with high glass-transition temperatures (44.3–56.1 °C). The 3,4-selectivity decreased with increasing the polymerization temperature (25–60 °C), but increased with polarity of polymerization solvent in the order of chlorobenzene > toluene > cyclohexane. In contrast, complex 1b was almost inert for the polymerization of 2-phenyl-1,3-butadiene owing to its steric bulkiness.

Treatment of imidazolium iodide [R-NHC-CH2CH(nBu)OH]I (1a (R = methyl), 1b (R = isopropyl)) and benzimidazole iodide [R-NHC(C6H4)-CH2CH(nBu)OH]I (2a (R = methyl), 2b (R = isopropyl)), respectively, with ((trimethylsilyl)methyl)lithium (LiCH2SiMe3) followed by rare-earth metal tris(alkyl)s (Ln(CH2SiMe3)3(THF)2) (Ln = Sc, Y, Lu) afforded the alkoxy-modified N-heterocyclic carbene ligated rare-earth metal bis(alkyl) complexes {[R-NHC-CH2CH(nBu)OLn(CH2SiMe3)2]2 (3a (R = methyl, Ln = Sc), 3b (R = methyl, Ln = Lu), 4a (R = isopropyl, Ln = Y), 4b (R = isopropyl, Ln = Lu)) and [(CH3)2CH-NHC(C6H4)-CH2CH(nBu)OY(CH2SiMe3)2]2 (5)} via double-deprotonation reactions. All complexes were characterized by NMR spectroscopy, elemental analysis and X-ray crystallography. They are dimers in which two rare-earth metal ions are bridged by two μ2-O atoms. Under the activation of an organoborate, complex 3a showed cis-1,4 enriched regioselectivity for isoprene polymerization (84.3%).

A highly efficient strategy for synthesizing hydroxyl-end and amide-core, highly stereoregular, star-shaped polylactide was established by employing a lutetium complex via immortal ROP of rac-lactide with a polyamine as the chain transfer agent.

The polymerization of a new polar diene-based monomer 2-(2-methylidenebut-3-enyl)furan (MBEF) without masking is achieved by using the bis(phosphino)carbazoleide-ligated yttrium (Y) alkyl complex upon the activation of [Ph3 C][B(C6 F5 )4 ]. Under mild conditions, the polymerizations under the monomer-to-Y ratios ranging from 100:1 to 500:1 perform fluently in high yields. The afforded polydienes bearing pendant terminal furan groups have high cis-1,4-regularity up to 98.6% and molecular weights close to the theoretic values and narrow polymer dispersity index(PDI) (1.13-1.17) suggesting a livingness polymerization mode. In addition, this novel polydiene is an excellent building block for preparing functional rubber materials. For example, via Diels-Alder addition of furan groups under mild conditions, hydroxyl groups are successfully introduced on the side chains efficiently in a 75% conversion. Furthermore, the copolymerization of polar MBEF and nonpolar isoprene is also successfully realized by the bis(phosphino) carbazoleide-ligated scandium analog to access furan-modified cis-1,4 (>97%) polyisoprene with different MBEF contents (5.3%, 8.7%).

A highly efficient strategy for synthesizing the first biocompatible polyesters with AIE characteristics has been established via immortal ring-opening polymerizations of cyclic esters bearing nonluminophores. In the process, the large excess hydroxyl-modified AIE active compound, acting as the chain transfer agent, attaches to the active rare-earth metal catalyst via the rapid-reversible exchange reaction to initiate the polymerization. Thus, more polyester chains appear to grow from one active metal species, and the AIE fragments are incorporated into the polymer chains at specific sites, in situ. The resultant polyesters have linear, block, or star-shaped microstructures mimicking those of the modified AIE compounds. The polymerization solutions can be directly fabricated to large-area thin solid films. The obtained PLA, for instance, emits fluorescence in water/THF mixtures owing to aggregation, the intensity of which is 2850-fold stronger than that in THF solution. This strategy avoids complicated pr...

The amidino-modified N-heterocyclic carbene ligated lutetium bis(alkyl) complex 1, (Am-NHC)Lu(CH2SiMe3)2, was synthesized by treatment of (AmH-NHC-H)Br ((2,6-iPrC6H3N═C(C6H5)NHCH2CH2(NCHCHN(C6H2Me3-2,4,6)CH)Br) with ((trimethylsilyl)methyl)lithium (LiCH2SiMe3) and lutetium tris(alkyls) (Lu(CH2SiMe3)3(THF)2) via double-deprotonation reactions and characterized by NMR spectroscopy and X-ray diffraction analysis. Under activation of an organoborate, complex 1 exhibited distinguished catalytic performance for the polymerization of isoprene with respect to high activity, 3,4-regioselectivity (99.3%), and livingness mode. In contrast to the systems reported to date, this system seemed not to be affected obviously by the polymerization temperature (0–80 °C), solvents, monomer-to-initiator ratios (500–5000), and type of organoborate. The resultant polymers have high glass-transition temperatures (38–48 °C) and moderate syndiotacticity (racemic enchainment triad rr 45%, pentad rrrr 20%). In addition, the living lu...

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.

Rare-earth metal bis(alkyl) complexes (Flu–NHC)Ln(CH2SiMe3)2 (Ln = Dy (1), Er (2), Sc (3)) attached by fluorenyl-modified N-heterocyclic carbene ligands ((Flu H–NHC–H)Br) have been synthesized by treatment of (FluH–NHC–H)Br with (trimethylsilylmethyl)lithium (LiCH2SiMe3) and rare-earth metal tris(alkyl)s (Ln(CH2SiMe3)3(THF)2) via double-deprotonation reactions in moderate to high yields. Under mild conditions (40 °C and normal ethylene pressure), the scandium precursor 3, upon activation of AliBu3 and [Ph3C][B(C6F5)4], showed high activity (4120 kg molSc–1 h–1 atm–1) for the copolymerization of ethylene and 1-hexene with moderate 1-hexene insertion ratio (20.2%), although the analogous complexes 1 and 2 were inert. In addition, this system displayed excellent catalytic performances for the copolymerization of ethylene and a higher α-olefin 1-octene with an activity of up to 3640 kg molSc–1 h–1 atm–1. The content of 1-octene could be controlled swiftly from 2.1% to 38.7% by varying the 1-octene feed ratio....

In the presence of a small molecular protic initiator, immortal ring-opening polymerization (ROP) of lactide (LA) is a highly efficient strategy to synthesize polylactide in a controllable manner, while using polymeric alcohol as an initiator has been less investigated. A series of polymeric alcohols (PS–OH) composed of styrene and 4.3%–18% hydroxyl functional styrene (diethyl(hydroxy(4-vinylphenyl)methyl)phosphonate, St–OH) were synthesized through reversible addition-fragmentation transfer (RAFT) polymerization. Using PS–OH as an initiator, the immortal ROP of rac-LA was catalyzed by dibutylmagnesium (MgnBu2) under various ratios of monomer to hydroxyl group within PS–OH to generate polystyrene-g-polylactide (PS–g–PLA) copolymers with different graft lengths. After thermal annealing at 115 °C, the PLA domain aggregated to nanospheres among the PS continuum. The size of the nanospheres, varying from 130.1 to 224.2 nm, was related to the graft density and length of PS–g–PLA. Nanoporous films were afforded through chemical etching of the PLA component.

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.