1,721,069 research outputs found
Assembling Heterometals through Oxygen: An Efficient Way To Design Homogeneous Catalysts
No full textAssembling a molecule containing two metal centers with entirely different chemical properties remains a synthetic challenge. One of the major motivations for this chemistry is its ability to catalyze various organic transformations. The proximity between two different metals in a heterometallic complex allows more pronounced chemical communication between the metals and often leads to the modification of the fundamental properties of the individual metal atoms through the well-known cooperative interaction. Although various types of heterometallic Systems are known, the M-O-M' framework is particularly important because it brings the metals into close proximity with each. other. In this Account, we describe several suitable synthetic routes for the assembly of heterometals of entirely different chemical properties through an oxygen atom. The new synthetic strategies for the construction of heterobimetallic complexes take advantage of unprecedented syntheses of a number of hydroxide precursors of the type LMR(OH) [L = CH{N(Ar)(CMe)}(2), Ar = 2,6-iPr(2)C(6)H(3); M = Al, Ga, or Ge; R = alkyl, aryl, or lone pair of electrons], [LSr(mu-OH)](2) center dot (THF)(3) and CP (2)ZrMe(OH). We used the Bronsted acidic character of the proton in the M(O-H), Sr(O-H), or Zr(O-H) moiety, to build a new class of heterobimetallic complexes based on M-O-M' motif. This synthetic strategy assembles a main group element with another main group element, a transition metal, or a lanthanide metal. This synthetic development provides access to a new class of heterobimetallic complexes through oxygen bridging. In many cases these complexes prove to be excellent candidates for polymerization of monomers including e-caprolactone, ethylene, and styrene. Some of these catalysts bear a chemically grafted methylalumoxane (MAO) unit in the backbone of an active metal center, which led to efficient ethylene polymerizations at an unusually low MAO concentration. We attribute this reactivity both to the presence of a chemically grafted (Me)Al-O backbone in the active catalysts (a part of externally added cocatalyst, MAO) and to the enhanced Lewis acidity from the bridging oxygen at the active metal center. In addition, we have demonstrated the development of heterometallic systems having two catalytically active centers. Such structures could aid in the development of a catalytic system bearing two active centers with different chemistries
Main group chemistry of 9-hydroxophenalenone: Syntheses and structural characterization of the alkaline earth and zinc complexes
No full textHerein, we report the synthesis and characterization of 9-hydroxophenalenone based alkaline earth and zinc complexes. The reaction of 9-hydroxophenalenone (HO,O-PLY (1)) with one equivalent of KN(SiMe3)(2) and MI2 in THF yields heteroleptic complexes [(O,O-PLY)M(THF)(n)]I [M = Mg (2), Ca (3), Sr (4), Ba (5); n = 1-4], while use of two equivalents of KN(SiMe3)(2) in THF (with respect to PLY) produces homoleptic complex (O,O-PLY)(2)Mg(THF)(2) (6). Moreover, reaction between two equivalents of 1 with one equivalent of ZnMe2 in THF produces complex (O,O-PLY)(2)Zn(THF)(2) (7). All these complexes were characterized by NMR spectroscopy and elemental analyses. The solid state structures of complexes 2, 6 and 7 were established by single crystal X-ray diffraction analysis.IISER-Kolkata; CSIR, India [01 (2369)/10/EMR-II
Designing Molecular Catalysts with Enhanced Lewis Acidity
No full textOne of the key challenges in catalysis is the generation of catalytically active metal centers that are highly Lewis acidic so that the metal center can easily bind with a nucleophilic monomer to initiate a catalytic process. With this goal in mind, we pursued the designed synthesis of catalytically active metal centers with enhanced Lewis acidity, adopting two different synthetic strategies. One is the introduction of oxygen between two different metal atoms, and the other is the chemical attachment of highly electronegative fluorine around the catalytically active metal center. The attachment of the oxygen between the two metal centers also brings the metals into close proximity at the molecular level, resulting in a pronounced chemical communication between the metals. The compounds with different metals have often modified the fundamental properties of the individual metal atoms through the well-known "cooperative interaction" that is otherwise difficult to achieve. The synthetic strategy takes advantage of the Bronsted acidic character of the M(O-H) moiety in building up a new class of heterometallic complexes. Further, the discovery of Me3SnE as one of the most useful fluorinating reagents for organometallic complexes leads to the successful preparation of organometallic fluorides of Group-4 metals. This synthetic development has resulted in the availability of catalysts of a new class bearing enhanced Lewis acidic metal centers resulting either from oxygen bridging or from the attachment of a highly electronegative fluorine to a catalytically active metal center. In many cases, these complexes have proved to be excellent candidates for olefin polymerization, ring-opening polymerization of caprolactone, olefin epoxidation, and olefin hydroformylation. The improvement in the catalytic properties is a result of the presence of a more electrophilic metal center, which is essential for the catalysis
Interstellar molecules: guides for new chemistry
No full textInterstellar space is among the most remarkable chemical laboratories in the universe. The existence of many unstable species with low-valent main group elements in the interstellar medium inspired us to investigate the feasibility of laboratory synthesis of such unstable molecules. Particularly the lighter Group 14 element carbon plays a very important role in space astrochemistry. Low-valent carbon as well as silicon were detected in the interstellar environment. This article describes our recent efforts in developing amazing chemistry of heavier low-valent Group 14 elements. This study unravels that the disproportionation pathway of the low-valent Group 14 elements can be arrested by using a sterically protected ligand, then one can artificially generate the situation observed in the interstellar surrounding where the chance of disproportionation is very low as the molecules are extremely dilute
Group 14 Hydrides with Low Valent Elements for Activation of Small Molecules
No full textTransition metal compounds are well known as activators of small molecules, and they serve as efficient catalysts for a variety of homogeneous and heterogeneous transformations. In contrast, there is a general feeling that main group compounds cannot act as efficient catalysts because of their inability to activate small molecules. Traditionally, the activation of small molecules is considered one of the key steps during a catalytic cycle with transition metals. As a consequence, researchers have long neglected the full range of possibilities in harnessing main group elements for the design of efficient catalysts. Recent developments, however, have made it possible to synthesize main group compounds with low-valent elements capable of activating small molecules. In particular, the judicious use of sterically appropriate ligands has been successful in preparing and stabilizing a variety of Group 14 hydrides with low-valent elements. In this Account, we discuss recent advances in the synthesis of Group 14 hydrides with low-valent elements and assess their potential as small-molecule activators. Group 14, which comprises the nonmetal C, the semimetals Si and Ge, and the metals Sn and Pb, was for years a source of hydrides with the Group 14 element almost exclusively in tetravalent form. Synthetic difficulties and the low stability of Group 14 hydrides in lower oxidation states were difficult to overcome. But in 2000, a divalent Sn(II) hydride was prepared as a stable compound through the incorporation of sterically encumbered aromatic ligands. More recently, the stabilization of GeH2 and SnH2 complexes using an N-heterocyclic carbene (NHC) as a donor and BH3 or a metal carbonyl complex as an acceptor was reported. A similar strategy was also employed to synthesize the Si(II) hydride. This class of hydrides may be considered coordinatively saturated, with the lone pair of electrons on the Group 14 elements taking part in coordination. We discuss the large-scale synthesis of hydrides of the form LMH (where M is Ge or Sn, L is CH(N(Ar)(CMe))(2), and Ar is 2,6-/Pr2C6H3), which has made It possible to test their reactivity in the activation of small molecules. Unlike the tetravalent Group 14 hydrides, the Ge(II) and Sn(II) hydrides were found to be able to activate a number of small molecules In the absence of any externally added catalyst. For example, the Ge(II) hydride and Sn(II) hydride can activate CO2, and the reaction results in the formation of Ge(II) and Sn(II) esters of formic acid. This product represents a prototype of a new class of compounds of Group 14 elements. Moreover, we examined the activation of carbonyl compounds, alkynes, diazo and azo compounds, azides, and compounds containing the C=N bond. These Group 14 hydrides with low-valent elements are shown to be able to activate a number of important small molecules with C C, C=O, N=N, and C=N bonds. The activation of small molecules is an Important step forward in the realization of main group catalyst development. Although it is not yet customary to assay the potential of newly synthesized main group compounds for small-molecule activation, our results offer good reason to do so
Phenalenyl-based ligand for transition metal chemistry: Application in Henry reaction
No full textWe report the synthesis and characterization of the first transition metal complex of a phenalenylbased ligand. The reaction of Cu(OAc)2.H2O with 9-N-methylamino-1-N -methylimino-phenalene (LH) in 1:1 stoichiometric ratio results in the formation of a mononuclear copper complex [LCu(OAc)] (1). The molecular structure of 1 was established by X-ray crystallography. The electrochemistry of 1 indicates the formation of an anionic radical by one electron reduction into the non-bonding molecular orbital of the phenalenyl system. The complex 1 efficiently catalyses the C–C bond forming Henry reaction
Phenalenyl-based ligand for transition metal chemistry: Application in Henry reaction
No full textWe report the synthesis and characterization of the first transition metal complex of a phenalenylbased ligand. The reaction of Cu(OAc)2.H2O with 9-N-methylamino-1-N -methylimino-phenalene (LH) in 1:1 stoichiometric ratio results in the formation of a mononuclear copper complex [LCu(OAc)] (1). The molecular structure of 1 was established by X-ray crystallography. The electrochemistry of 1 indicates the formation of an anionic radical by one electron reduction into the non-bonding molecular orbital of the phenalenyl system. The complex 1 efficiently catalyses the C–C bond forming Henry reaction
Phenalenyl-Based Organozinc Catalysts for Intramolecular Hydroamination Reactions: A Combined Catalytic, Kinetic, and Mechanistic Investigation of the Catalytic Cycle
No full textHerein, we report the synthesis and characterization of two organozinc complexes that contain symmetrical phenalenyl (PLY)-based N,N-ligands. The reactions of phenalenyl-based ligands with ZnMe2 led to the formation of organozinc complexes [N(Me),N(Me)-PLY]ZnMe (1) and [N(iPr),N(iPr)-PLY]ZnMe (2) under the evolution of methane. Both complexes (1 and 2) were characterized by NMR spectroscopy and elemental analysis. The solid-state structures of complexes 1 and 2 were determined by single-crystal X-ray crystallography. Complexes 1 and 2 were used as catalysts for the intramolecular hydroamination of unactivated primary and secondary aminoalkenes. A combined approach of NMR spectroscopy and DFT calculations was utilized to obtain better insight into the mechanistic features of the zinc-catalyzed hydroamination reactions. The progress of the catalysis for primary and secondary aminoalkene substrates with catalyst 2 was investigated by detailed kinetic studies, including kinetic isotope effect measurements. These results suggested pseudo-first-order kinetics for both primary and secondary aminoalkene activation processes. Eyring and Arrhenius analyses for the cyclization of a model secondary aminoalkene substrate afforded ?H?=11.3 kcal?mol-1, ?S?=-35.75 cal?K-1?mol-1, and Ea=11.68 kcal?mol-1. Complex 2 exhibited much-higher catalytic activity than complex 1 under identical reaction conditions. The in situ NMR experiments supported the formation of a catalytically active zinc cation and the DFT calculations showed that more active catalyst 2 generated a more stable cation. The stability of the catalytically active zinc cation was further supported by an in situ recycling procedure, thereby confirming the retention of catalytic activity of compound 2 for successive catalytic cycles. The DFT calculations showed that the preferred pathway for the zinc-catalyzed hydroamination reactions is alkene activation rather than the alternative amine-activation pathway. A detailed investigation with DFT methods emphasized that the remarkably higher catalytic efficiency of catalyst 2 originated from its superior stability and the facile formation of its cation compared to that derived from catalyst 1.IISER-Kolkata; CSIR, India [01(2369)/10/EMR-II]; DST, Indi
Synthesis, structural characterization, catalytic properties, and theoretical study of compounds containing an Al-O-M (M = Ti, Hf) core
No full textTwo single oxygen-bridged heterobimetallic oxides of Al(III) with group 4 metals (Ti, Hf) have been prepared. The reaction of LAlMeOH (1) [L = CH(N(Ar)(CMe))(2), Ar = 2,6-iPr(2)C(6)H(3)] with dimethylmetallocenes of Ti and Hf in toluene (80 degrees C) and ether (room temperature), respectively, resulted in the formation of LAl(Me)(mu-O)M(Me)Cp-2 [M = Ti (2), Hf (3)] in moderate to good yield. Compounds 2 and 3 were characterized by elemental analysis, IR, NMR (H-1 and C-13), EI-MS, and single-crystal X-ray structural analysis. Furthermore, compound 2 showed good catalytic activity in ethylene and styrene homopolymerization, while compound 3 is less active in ethylene polymerization. The styrene polymerization yields atactic polystyrene
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