Studies on the Coordination Patterns of Diamine-Bridged Tetraphenoxy Ligands with Group 3 and 4 Metals.

Alkane elimination reactions between the diamino- and dianilino-bridged tetrakis(phenolate) proligands 1a,b-H4 and precursors M(CH2SiMe3)3(THF)2, M(CH2C6H4-o-NMe2)3 (M = Sc and Y), and Hf(CH2Ph)4 were investigated. The diamino-bridged 1a-H4 afforded nonsymmetric complex 2a-Y2 incorporating two metal centers in different coordination environments. This one and other dinuclear compounds 2b-Sc2, 2a-Hf2, and 2b-Hf2 were characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction study (for 2a-Y2 and 2b-Sc2) and turned out to be symmetric in solution. Compound 2a-Y2, upon treatment with 2 equiv of 2-phenylpyridine, afforded symmetric bis(aryl) product 3a-Y2, which was authenticated by NMR spectroscopy and X-ray crystallography. The mechanism of its formation was studied by DFT computations and presumably involves a cooperative reorganization process within the nonsymmetric parent 2a-Y2 to afford a symmetric isomer prior to its reaction with 2-phenylpyridine.

Synthesis of Mono- and Dinuclear Aluminum Complexes Bearing Aromatic Amino-Phenolato Ligands: A Comparative Study in the Ring-Opening Polymerization of Cyclohexene Oxide.

Dinuclear aluminum methyl complexes bearing aromatic diamine-bridged tetra(phenolato) ligands and the mononuclear aluminum methyl complex with the phenylamine-bridged bis(phenolato) ligand have been synthesized and characterized. Structure determination revealed that the Al-Al distances in these dinuclear aluminum complexes are tunable by the choice of the suitable aromatic backbone of the diamine-bridged tetra(phenolato) ligands. The catalytic behaviors of these mono- and dinuclear aluminum complexes for cyclohexene oxide (CHO) polymerization were investigated. The activities of these dinuclear Al complexes were observed to increase with the decrease of Al-Al distances, and the dinuclear Al complexes appeared to have better catalytic activity than the mononuclear Al complex, even if the Al-Al distance is as long as 9.401 Å. Dinuclear aluminum complex 2, with the shortest Al-Al distance (7.236 Å), showed the highest activity toward CHO polymerization with TOFs up to 6460 h-1 in neat CHO at 30 °C. Furthermore, comparative kinetic studies revealed that the polymerization is first-order for CHO concentration, and the reaction orders for initiator concentration are different for the mono- and dinuclear Al complexes. The polymerization mechanism study revealed that both the methyl and phenolate groups were involved in the initiation process.

Synthesis of N-Methyl-o-phenylenediamine-Bridged Bis(phenolato) Lanthanide Alkoxides and Their Catalytic Performance for the (Co)Polymerization of rac-Butyrolactone and l-Lactide.

A series of lanthanide alkoxo complexes supported by ONNO salalen ligands were synthesized and characterized. A one-pot reaction of LH2 (L = (2-O-C6H2-tBu2-3,5)CH═N-C6H4-N(CH3)CH2(2-O-C6H2-tBu2-3,5)) with LnCp3(THF) in a 1:1 molar ratio followed by the addition of 1 equiv of ROH (R = Bn, iPr, and CF3CH2), afforded the dimeric lanthanide alkoxo complexes [LLn(µ-OCH2Ph)]2 [Ln = Lu (1), Yb (2), Sm (3), Nd (4)], [L2Yb(µ-OiPr)]2 (5), and [L2Yb(µ-OCH2CF3)]2 (6) in good isolated yields. All these lanthanide complexes were characterized by elemental analysis and FT-IR spectroscopy. In addition, complex 1 has been characterized by NMR spectroscopy. Single-crystal X-ray diffraction analysis of complexes 1, 2, 5, and 6 showed that these lanthanide alkoxo complexes are dimeric in the solid state. Complexes 1-6 showed good activity toward the homopolymerization of rac-butyrolactone (rac-BBL) to give atactic PHB, and ionic radii of central metals have profound influence on the polymerization. The polymerization behavior of l-lactide (l-LA) initiated by complex 2 was also explored. The kinetic study revealed that the polymerizations of rac-BBL and l-LA initiated by salalen lanthanide akoxide are first order for both the monomer and the initiator concentrations. Furthermore, it was found that complexes 1 and 2 showed good activity in the copolymerization of l-LA and rac-BBL, affording gradient copolymers.

Syntheses of Heterometallic Neodymium-Zinc Complexes and Their Performance in the Copolymerization of CO2 and Cyclohexene Oxide.

A series of Nd-Zn heterometallic complexes bearing o-phenylenediamine-bridged tris(phenolato) ligands (L) were synthesized and characterized. By tuning the backbones of ancillary tris(phenolato) ligands and initiating benzyloxy groups, a Nd-Zn heterometallic complex 12 (ClLNdZnOBnCF3) was found to be highly active for the copolymerization of CO2 and cyclohexene oxide (CHO) to produce perfect alternating poly(cyclohexene carbonate) with a high turnover frequency up to 5640 h-1 under the polymerization of 90 °C and 20 bar CO2 pressure. The kinetics study showed that CO2/CHO copolymerization catalyzed by 12 was the first order dependence of 12 and CHO concentration and the zero-order dependence of CO2 pressure. The reaction of 12 with CO2 generated a carbonate-coordinated [NdZnNd] trinuclear complex 13, which was believed to be the key intermediate to initiate CO2/CHO copolymerization. On the basis of some experiments, a plausible synergistic polymerization mechanism was proposed.

Synthesis, Characterization, and Catalytic Study of Amine-Bridged Bis(phenolato) Co(II) and Co(II/III)-M(I) Complexes (M = K or Na).

Co(II) complexes 1-3 bearing amine-bridged bis(phenolato) complexes have been synthesized through reactions of bis(phenols) with CoCl2 or Co(OAc)2. Oxidation of the Co(II) complex with air resulted in partial oxidation, generating mixed valence Co(II/III) complexes 4 and 5. In addition, due to the presence of alkali compounds (KOAc and NaOMe), 4 and 5 formed as Co-alkali metal heterometallic complexes, which are the first example of mixed valence Co(II/III)-M(I) (M = K or Na) complexes. Complexes 1-5 showed good activity in the cycloaddition of epoxides and CO2 under atmospheric pressure, generating cyclic carbonates in 40-99% yields. Co(II/III)-Na(I) complex 5 performed better in reactions of bulkier substrates, underlining the enhanced activity of mixed valence Co-alkali metal heterometallic complexes. On the contrary, complex 5 showed limited activity in copolymerization of epoxide and CO2.

Calcium-mediated C(sp3)-H Activation and Alkylation of Alkylpyridines.

Main-group metal calcium-mediated alkylpyridine benzylic C(sp3)-H activation and functionalization have been achieved. The reaction of a calcium hydride complex [{(DIPPnacnac)CaH(thf)}2] (DIPPnacnac = CH{(CMe)(2,6-iPr2-C6H3N)}2) with two equivalents of 2,6-lutidine rapidly yields a monomeric calcium alkyl complex with the release of dihydrogen. A hydride/carbon-bridged binuclear calcium complex [{(DIPPnacnac)Ca}2(µ-H){2-Me-6-(µ-CH2)-Py}(thf)] is obtained from an equimolar treatment of calcium hydride and 2,6-lutidine that is readily converted into mono- or binuclear calcium alkyl complexes upon subsequent addition of 2,6-lutidine. DFT calculations and kinetic studies are conducted to determine their reaction profiles. More significantly, this calcium hydride complex catalyzes regioselective benzylic C-H bond addition of alkylpyridines to a variety of alkenes, affording linear or branched alkylated pyridine derivatives.

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.

Rare-Earth Metal Complexes Supported by Polydentate Phenoxy-Type Ligand Platforms: C-H Activation Reactivity and CO2/Epoxide Copolymerization Catalysis.

Mono- and dinuclear group 3 metal complexes incorporating polydentate bis(imino)phenoxy {N2O}- and bis(amido)phenoxy {N2O}3- ligands were synthesized by alkane elimination reactions from the tris(alkyl) M(CH2SiMe3)3(THF)2 and M(CH2C6H4-o-NMe2)3 (M = Sc, Y) precursors. Complex 1a-Y was used for the selective C-H activation of 2-phenylpyridine at the 2'-phenyl position affording the corresponding bis(aryl) product 3a-Y, which was found to be reacted reluctantly with weak electrophiles (styrene, imines, hydrosilanes). The mechanism of formation of 3a-Y was established by DFT calculations, which also corroborated high stability of the complex toward insertion of styrene, apparently stemming from the inability to form the corresponding adduct. Copolymerization of cyclohexene oxide and CO2 promoted by 1a-Y (0.1-0.5 mol %) was demonstrated to proceed under mild conditions (toluene, 70 °C, PCO2 = 12 bar) giving polycarbonates with high efficiency (maximal TON of 460) and selectivity (97-99% of carbonate units).

Directing-Group-Free C7-Alkylations of N-Alkylindoles Mediated by Cationic Zirconium Complexes: Role of Brønsted Acid for Catalytic Manifold.

Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold.

Cycloaddition of Aziridine with CO2/CS2 Catalyzed by Amidato Divalent Lanthanide Complexes.

This is the first time that the amidato lanthanide amides ({L nLn[N(SiMe3)2]THF}2 ( n = 1, Ln = Eu (1); n = 2, Ln = Eu (3), Yb (4); HL1 = tBuC6H4CONHC6H3( iPr)2; HL2 = C6H5CONHC6H3( iPr)2) and {L3Eu[N(SiMe3)2]THF}{L32Eu(THF)2} (2) (HL3 = ClC6H4CONHC6H3( iPr)2)) were applied in the cycloaddition reactions of aziridines with carbon dioxide (CO2) or carbon disulfide (CS2) under mild conditions. The corresponding oxazolidinones and thiazolidine-2-thiones were obtained in good to excellent yields with good functional group tolerance.

Synthesis of Homo- and Heteronuclear Rare-Earth Metal Complexes Stabilized by Ethanolamine-Bridged Bis(phenolato) Ligands and Their Application in Catalyzing Reactions of CO2 and Epoxides.

A series of homonuclear rare-earth (RE) metal complexes (1Y, 2Yb, 3Nd, and 4La) and heteronuclear RE-Zn complexes (1Y-Zn, 3Nd-Zn, and 5Sm-Zn) stabilized by ethanolamine-bridged bis(phenolato) ligands was prepared and structurally characterized. Heteronuclear complexes are assembled through bridging acetate ligands, and their formation and characterization add to the diversity of 3d-4f complexes. Their activities in mediating reactions of CO2 and epoxides were evaluated and compared. Heteronuclear RE-Zn complexes found application in the copolymerization of cyclohexene oxide and CO2, giving rise to acetate-group-capped copolymers. Homonuclear complexes showed good activity in catalyzing the cycloaddition of variously monosubstituted epoxides and CO2 (1 bar), generating cyclic carbonates in 65-96% yield. For sterically hindered disubstituted epoxides, good yields of 60-91% were achieved in the presence of 10 bar CO2.

RE[N(SiMe3)2]3-Catalyzed Guanylation/Cyclization of Amino Acid Esters and Carbodiimides.

The example of rare-earth metal-catalyzed guanylation/cyclization of amino acid esters and carbodiimides is well-established, forming 4(3H)-2-alkylaminoquinazolinones in 65-96% yields. The rare-earth metal amides RE[N(TMS)2]3 (RE = Y, Yb, Nd, Sm, La; TMS = SiMe3) showed high activities, and La[N(TMS)2]3 performed best for a wide scope of the substrates.

Enantioselective Reduction of Ketones Catalyzed by Rare-Earth Metals Complexed with Phenoxy Modified Chiral Prolinols.

Enantioselective reduction of ketones and α,ß-unsaturated ketones by pinacolborane (HBpin) has been well-established by using chiral rare-earth metal catalysts with phenoxy modified prolinols. A number of highly optically active alcohols were obtained from reduction of simple ketones catalyzed by ytterbium complex 1 [L4Yb(L4H)] (H2L4 = ( S)-2- tert-butyl-6-((2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)phenol). Moreover, α,ß-unsaturated ketones were selectively reduced to a wide range of chiral allylic alcohols with excellent yields, high enantioselectivity, and complete chemoselectivity, catalyzed by a single component chiral ytterbium complex 2 [L1Yb(L1H)] (H2L1 = ( S)-2,4-di- tert-butyl-6-((2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)phenol).

n-Butyllithium Catalyzed Selective Hydroboration of Aldehydes and Ketones.

Highly efficient and selective hydroboration of aldehydes and ketones with HBpin is achieved by using the simple and convenient n-BuLi as a catalyst. The reaction proceeds rapidly with low catalyst loading (0.1-0.5 mol %) under mild conditions. Key features include the high catalytic efficiency, exceptional functional group compatibility, ample substrate scope, and high selectivity for aldehydes over ketones. Computational studies were carried out to provide a mechanistic insight into the n-BuLi catalyzed hydroboration of aldehydes/ketones with HBpin.

Synthesis and Characterization of Dinuclear Salan Rare-Earth Metal Complexes and Their Application in the Homo- and Copolymerization of Cyclic Esters.

Four rare-earth-metal aryloxo complexes stabilized by a tetradentate Salan ligand were prepared, and their catalytic properties for the (co)polymerization of lactides and ε-caprolactone were elucidated. The proton-exchange reactions of (C5H5)3Ln(THF) with the Salan ligand N, N'-(CH2Ph)2- N, N'-[CH2(2-OH-C6H2-Me2-3,5)]2 (LH2) in a 1:1 molar ratio, and subsequently with 1 equiv of p-methylphenol, gave the rare-earth-metal aryloxides [LLn(OC6H4-4-CH3)(THF) n]2 [ n = 0 and Ln = Y (1), Sm (2), and Nd (3); n = 1 and Ln = La (4)] in good isolated yields. These complexes were fully characterized by elemental analysis, IR, and NMR spectroscopy (for complexes 1 and 4). Solid-state structures of complexes 1-4 were confirmed by single-crystal X-ray diffraction analysis. Complexes 1-4 have dinuclear solid-state structures, with a Ln2O2 core bridging the Salan ligands. The coordination geometry around each of the metals is a slightly distorted octahedron in complexes 1-3, whereas it is a capped trigonal prism in complex 4. It was found that complexes 1-4 can initiate efficiently the homopolymerization of l-lactide (l-LA) and rac-lactide ( rac-LA) at 30 °C in tetrahydrofuran. The increasing activity of these complexes is in agreement with increasing ionic radii. A kinetic study revealed that seven-coordinated lanthanum complex 4 is more active for rac-LA polymerization compared with l-LA. A further study revealed that complex 4 was also an efficient initiator for the random copolymerization of l-LA and ε-caprolactone with the simultaneous addition of these two monomers, and the Tg values of the copolymers obtained increase linearly from -30.2 to +38.3 °C with an increase of the percentage of LA units. A mechanism study revealed that transesterification plays a crucial role in the formation of a random copolymer.

Addition of C-H Bonds of Pyridine Derivatives to Alkenes Catalyzed by Zirconium Complexes Bearing Amine-Bridged Bis(phenolato) Ligands.

Cationic zirconium complexes in situ generated from zirconium dibenzyl complexes bearing amine-bridged bis(phenolato) ligands have been developed to catalyze addition of C(sp2)-H and C(sp3)-H bonds of pyridine derivatives to alkenes. A series of zirconium complexes bearing different ligands have been synthesized, and their activities in catalyzing addition of C(sp3)-H bonds of pyridine derivatives to alkenes have been studied and compared. Both reaction activity and regioselectivity are influenced by electronic and steric properties of ligand backbones. In addition, a cationic zirconium complex has been isolated and structurally characterized to shed some light on reaction mechanism.

Neutral and Cationic Zirconium Complexes Bearing Multidentate Aminophenolato Ligands for Hydrophosphination Reactions of Alkenes and Heterocumulenes.

Zirconium complexes supported by multidentate aminophenolato ligands were synthesized and characterized. The catalytic activities of neutral zirconium complexes and their cationic derivatives in the hydrophosphination of alkenes as well as heterocumulenes have been investigated and compared. Neutral complex 1 bearing a multidentate amino mono(phenolato) ligand exhibited high activity in hydrophosphination of simple alkenes, and anti-Markovnikov products were obtained in 37-94% yields at room temperature. Cationic species generated in situ from complex 3 stabilized by a bis(phenolato) ligand were found to be more active for hydrophosphination of heterocumulenes, i.e., carbodiimides and isocyanates, and gave phosphaguanidines and phosphaureas in 67-93% yields. The Lewis acidity and coordination space of metal centers are modified through changes in the ligand structure, which is found to significantly influence catalytic activity. These complexes are among the most active group 4 metal-based catalysts for hydrophosphination reactions.

Highly efficient hydroboration of carbonyl compounds catalyzed by tris(methylcyclopentadienyl)lanthanide complexes.

Homoleptic lanthanide complexes coordinated by a Me-substituted Cp ligand [(MeCp)3Ln] demonstrate unprecedentedly high efficiency in catalyzing the hydroboration of aldehydes and ketones with pinacolborane. This protocol is also applicable for the hydroboration of aryl-substituted imines. In addition, broad functional group compatibility and excellent chemoselectivity is also achieved. DFT calculations are employed to shed light on the reaction mechanism.

Frustrated Lewis Pair-Like Reactivity of Rare-Earth Metal Complexes: 1,4-Addition Reactions and Polymerizations of Conjugated Polar Alkenes.

Three rare-earth aryloxide ion pairs {[L1REOAr]+ /[B(C6 F5 )4 ]- ; L1=CH3 C(2,6-iPr2 C6 H3 N)CHC(CH3 )(NCH2 CH2 PPh2 ); RE=Sc, Y, Lu; Ar=2,6-tBu2 C6 H3 } were reported that feature rare-earth/phosphorus (RE/P) combinations exhibiting frustrated Lewis pair (FLP)-like 1,4-addition reactions towards conjugated carbonyl substrates (e.g., enone, ynone, and acrylic substrates). Furthermore, these RE/P complexes were found to be effective catalysts for the polymerization of conjugated polar alkene monomers. Mechanistic studies revealed that the rare-earth metal-catalyzed polymerizations were initiated by new FLP-type 1,4-additions rather than traditional and ubiquitous covalent RE-E (E=H, C, N, etc.) bond insertion or single-electron transfer.

Recyclable Single-Component Rare-Earth Metal Catalysts for Cycloaddition of CO2 and Epoxides at Atmospheric Pressure.

Ionic rare-earth metal complexes 1-4 bearing an imidazolium cation were synthesized, which, as single-component catalysts, showed good activity in catalyzing cyclic carbonate synthesis from epoxides and CO2. In the presence of 0.2 mol % catalyst, monosubstituted epoxides bearing different functional groups were converted into cyclic carbonates in 60-97% yields under atmospheric pressure. In addition, bulky/internal epoxides with low reactivity yielded cyclic carbonates in 40-95% yields. More importantly, the readily available samarium complex 2 was reused for six successive cycles without any significant loss in its catalytic activity. This is the first recyclable rare-earth metal-based catalyst in cyclic carbonate synthesis.