Haruki Nagae

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities Christophe Copeŕet,*,† Aleix Comas-Vives,† Matthew P. Conley,† Deven P. Estes,† Alexey Fedorov,† Victor Mougel,† Haruki Nagae,†,‡ Francisco Nuñ́ez-Zarur,† and Pavel A. Zhizhko†,§ †Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1−5, CH-8093 Zürich, Switzerland ‡Department of Chemistry, Graduate School of Engineering Science, Osaka University, CREST, Toyonaka, Osaka 560-8531, Japan A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia

End-functionalized polymerization of 2-vinylpyridine through initial C-H bond activation of N-heteroaromatics and internal alkynes by yttrium ene-diamido complexes.

We successfully introduced end-capping functional groups to poly(2-vinylpyridine)s by initial introduction of the functional groups on yttrium catalysts through C-H bond activation of heteroaromatics and internal alkynes to the Y center via alkylyttrium-mediated σ-bond metathesis.

Synthesis, Characterization, and Lactide Polymerization Activity of Group 4 Metal Complexes Containing Two Bis(phenolate) Ligands

A series of group 4 metal complexes Zr-(1)(2), Zr-(2)(2), Zr-(3)(2), Zr-(4)(2), Zr-(5)(2), Hf-(1)(2), and Hf-(4)(2) containing two bridged bis(phenolate) ligands of the (OSSO)-type were prepared by the reaction of the corresponding bis(phenol) and group 4 metal precursor MX(4) (X = O(i)Pr, CH(2)Ph) and isolated as robust, colorless crystals. NMR spectra indicate D(2) symmetry, in agreement with the solid state structure determined by single crystal X-ray diffraction study of the complexes Zr-(1)(2), Hf-(1)(2), Zr-(3)(2), Zr-(4)(2), and Zr-(5)(2). The complexes with the 1,4-dithiabutanediyl bridged ligands exhibit a highly symmetric coordination around the metal center. The introduction of the rigid trans-1,2-cyclohexanediyl bridged ligands led to a distorted coordination around the metal center in Zr-(4)(2) and Zr-(5)(2) when the ortho substituent is tert-butyl and the para substituent is larger than methyl. The complexes Zr-(1)(2), Zr-(2)(2), Zr-(3)(2), Zr-(4)(2) as well as Hf-(1)(2) and Hf-(4)(2) initiated the ring-opening polymerization of meso-lactide at 100 °C to give heterotactic polylactide with pronounced heterotacticity (>70%) and varying polydispersity (1.05 < M(w)/M(n) < 1.61). As shown by kinetic studies, zirconium complex Zr-(1)(2) polymerized meso-lactide faster than the homologous hafnium complex Hf-(1)(2).

Aminomethylation reaction of ortho-pyridyl C-H bonds catalyzed by group 3 metal triamido complexes.

Tris[N,N-bis(trimethylsilyl)amido] complexes of group 3 metals, especially yttrium and gadolinium, served as catalysts for ortho-C-H bond addition of pyridine derivatives and N-heteroaromatics into the C═N double bond of nonactivated imines to afford the corresponding aminomethylated products. Addition of catalytic amounts of secondary amines, such as dibenzylamine, dramatically improved the catalytic activity through the formation of a mixed ligated complex such as [(Me3Si)2N]2Y(NBn2)(THF) (4). Furthermore, kinetic studies using the isolated complex 4 provided a plausible reaction mechanism by which coordination of two pyridine derivatives afforded a penta-coordinated species as a key step.

Switching the Lactide Polymerization Activity of a Cerium Complex by Redox Reactions

Polylactide (PLA) is a biodegradable polymer suitable for various biomedical applications, and it can also be used as a packaging material. 2] A plethora of single-site polymerization catalysts have been developed to produce PLA with a defined molecular weight and tacticity by ring-opening polymerization (ROP) of lactide, a biomass-derived monomer. One tool to control the polymerization rate of lactide monomers is redox-switchable catalysis, as introduced by Gibson et al. in 2006. Here, control over the rate of polymerization of rac-lactide was achieved by redox processes at the ethynylferrocene-substituted salen-type ligand within a titanium complex. Although several examples of ligand-centered redoxswitchable catalysis have been reported, only a few examples of redox-switchable catalysis by reactions at the metal center are known. Recently, Matyjaszewski et al. reported the first application of a metal-centered redox process with a Cu/ Cu system to mediate the atom-transfer radical polymerization of methyl acrylate. Subsequently, Diaconescu et al. showed that cerium complexes with a tetradentate salen-type ligand can exert redox control over the polymerization of l-lactide by changing the oxidation state of the cerium center. We report here that ROP of meso-lactide can be controlled by switching between an anionic Ce complex and a neutral Ce complex with two (OSSO)-type bis(phenolate) ligands. Cerium complexes 3 a and 3 b with two (OSSO)-type bis(phenolate) ligands were readily prepared from the reaction of tetra(isopropoxy)cerium (1) with two equivalents of ligand precursor 2 a or 2 b and isolated as purple powders in 83 and 63 % yield, respectively (Scheme 1). Cerium complexes with only one (OSSO)-type ligand were not accessible even from reactions of 3 a with 1, which gave a mixture of 3 a and unreacted 1. The H NMR spectrum of 3 a features diagnostic broad signals consistent with a diamagnetic Ce complex. For the eight tert-butyl groups in 3 a, four signals are observed, and this is indicative of the high symmetry of complex 3 a. Crystals suitable for X-ray diffraction were grown from n-hexane. The molecular structure of 3 a shows the Ce center coordinated by four phenolate and four thioether moieties of two bis(phenolate) ligands in a distorted square antiprismatic geometry (Figure 1). Complex 3 a shows D2 symmetry in solution and in the solid state. The average Ce O bond length of 2.171 is within the literature range for eight-coordinated cerium(IV) complexes with phenolate ligands (Ce O 2.087–2.237 , Ce S 3.055– 3.058 ). 14] Cyclic voltammetry of 2 a in CH2Cl2 at 25 8C indicated no reduction of 2 a in the electrochemical window of the solvent (Figure 2). An oxidation wave was observed at EOx = 1.248 V (vs. ferrocene). For complex 3 a, a reduction wave at ERed = 1.513 V was observed, which can be attributed to the reduction of Ce to Ce, which is followed by an oxidation wave of the reduced species at EOx = 1.01 V; this is similar to the observation of Diaconescu et al. with a salen-type ligand. A second oxidation wave was observed at EOx = 0.67 V, and it can be attributed to ligand oxidation. Electrochemical oxidation of the phenolate moiety of bis(phenolate)Fe complexes to give a phenoxy radical was previously reported. 16] Prompted by the electrochemical reduction of Ce to Ce, we screened for a suitable chemical reduction agent. An NMR-scale experiment in [D8]THF showed that upon addition of one equivalent of cobaltocene to a purple solution of 3 a an orange paramagnetic compound was formed instantly. Reduction of complex 3 a by addition of one equivalent of cobaltocene proceeded smoothly to give the corresponding Ce complex 4 a in 76 % yield. Oxidation of reduced species 4 a by addition of one equivalent of tungsten hexachloride, iodine, Ag[BPh4], or [FeCp2][B(C6F5)4] (Scheme 2) to form 3 a was observed by H NMR spectroscopy. Single crystals of 4 a suitable for X-ray analysis were grown from a THF/n-pentane solution at 25 8C. The molecular structure of the anion of 4 a is isostructural to the molecular structure of neutral complex 3 a (Figure 1). The cobaltocenium cation is located close to two phenyl rings of the (OSSO)-type ligands. The average Ce O and Ce S distances in 4 a (2.313, 3.178 , respectively) are Scheme 1. Synthesis of 3 a and 3 b by protonolysis of 1.

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C (0.6 active sites/W). This high activity at low temperature broadens the scope of these catalysts to functionalized substrates. This activation process can also be applied to classical industrial catalysts. We provide evidence for the formation of a metallacyclopentane intermediate and propose how the active species are formed.

Mechanistic understanding of alkyne cyclotrimerization on mononuclear and dinuclear scaffolds: [4 + 2] cycloaddition of the third alkyne onto metallacyclopentadienes and dimetallacyclopentadienes

Cyclotrimerization of alkynes catalyzed by transition metal complexes is a straightforward synthetic method for constructing a benzene skeleton in organic synthesis. Not only mononuclear complexes, but also multinuclear complexes act as catalysts for alkyne cyclotrimerization, and their reaction mechanisms have been intensively investigated toward developing highly efficient and regio- and chemo-selective catalysts. In this review, we summarize stoichiometric and catalytic alkyne coupling reactions on mononuclear and dinuclear scaffolds in relation to the reaction mechanism of alkyne cyclotrimerization, including our recent mechanistic approaches using dinuclear tantalum motifs.

Organomagnesium‐Catalyzed Isomerization of Terminal Alkynes to Allenes and Internal Alkynes

Organomagnesium complexes 2 were synthesized from N,N-dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3-Aryl-1-propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium-catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl-, allenyl-, and propargylmagnesium species.

Lanthanide Complexes Supported by a Trizinc Crown Ether as Catalysts for Alternating Copolymerization of Epoxide and CO2: Telomerization Controlled by Carboxylate Anions

A new family of heterometallic catalysts based on trimetalated macrocyclic tris(salen) ligands and rare-earth metals was prepared and structurally characterized. The LaZn3 system containing anionic ligands such as acetate plays a critical role in catalyzing the alternating copolymerization of cyclohexene oxide (CHO) and CO2 with a high proportion of carbonate linkages. Among the lanthanide metals, the CeZn3 system exhibits high catalytic activity with a turnover frequency (TOF) of over 370 h-1 . NMR analysis of the complex and end-group analysis of the polymer suggest that the acetate ligands are rapidly exchanged, not only among coordinated acetates, but also between coordinated acetates and added carboxylate anions. These unique properties make this the first example of telomerization for the copolymerization of CHO and CO2 .

Tunable Ligand Effects on Ruthenium Catalyst Activity for Selectively Preparing Imines or Amides by Dehydrogenative Coupling Reactions of Alcohols and Amines

Selective dehydrogenative synthesis of imines from a variety of alcohols and amines was developed by using the ruthenium complex [RuCl2 (dppea)2 ] (6 a: dppea=2-diphenylphosphino-ethylamine) in the presence of catalytic amounts of Zn(OCOCF3 )2 and KOtBu, whereas the selective dehydrogenative formation of amides from the same sources was achieved by using another ruthenium complex, [RuCl2 {(S)-dppmp}2 ] [6 d: (S)-dppmp=(S)-2-((diphenylphosphenyl)methyl)pyrrolidine], in the presence of catalytic amounts of Zn(OCOCF3 )2 and potassium bis(trimethylsilyl)amide (KHMDS). Our previously reported ruthenium complex, [Ru(OCOCF3 )2 (dppea)2 ] (8 a), was the catalyst precursor for the imine synthesis, whereas [Ru(OCOCF3 )2 {(S)-dppmp}2 ] (8 d), which was derived from the treatment of 6 d with Zn(OCOCF3 )2 and characterized by single-crystal X-ray analysis, was the pre-catalyst for the amide formation. Control experiments revealed that the zinc salt functioned as a reagent for replacing chloride anions with trifluoroacetate anions. Plausible mechanisms for both selective dehydrogenative coupling reactions are proposed based on a time-course study, Hammett plot, and deuterium-labeling experiments.