Dimeric Titanium Research Articles

Alkyne Cyclotrimerisation, Acyclic Oligomerisation, and Transfer Hydrogenation Catalysed by a Titanium(IV) Phosphinoaryloxide Complex and its Redox Chemistry

AbstractThe reaction of sodium 2,4‐di‐tert‐butyl‐6‐(diphenylphos‐phino)phenolate (1, NaOArP) with TiIVCl4(THF)2 in a 2 : 1 molar ratio yielded TiIV(OArP)2Cl2 (2). The reduction of 2 with magnesium gave the dimeric titanium(III) compound [TiIII(OArP)2Cl]2 (3). In the presence of magnesium, compound 2 was found to catalyse the cyclotrimerization, acyclic oligomerisation, and hydrogenation of terminal alkynes. The complexes were characterised by single‐crystal X‐ray diffraction, optical spectroscopy, elemental analysis, and NMR or EPR spectroscopy, respectively.

Electronic State of Hydroxyl-terminated Dimeric Titanium Structure

酸化チタン光触媒の表面反応を理論計算で考察する時に，従来から用いられている平面型チタン二量体モデルの電子状態を調べた．用いた計算方法はB3LYP/6-311Gで，末端を水酸基化した初期構造(Ti2O6H4)を最適化計算した．水酸基の初期の配向によって結果が異なり，非平面構造を取り易いことがわかった．また，HOMOとLUMOのエネルギー準位の比較から，光触媒反応の酸化力と還元力に水素の位置が重要であることがわかった．チタン二量体モデルのみでは電子状態の記述に無理があり，周辺構造を含めた新しい表面モデルの構築が必要であることが示唆された．

Mechanistic Details of the Sharpless Epoxidation of Allylic Alcohols—A Combined URVA and Local Mode Study

In this work, we investigated the catalytic effects of a Sharpless dimeric titanium (IV)–tartrate–diester catalyst on the epoxidation of allylalcohol with methyl–hydroperoxide considering four different orientations of the reacting species coordinated at the titanium atom (reactions R1–R4) as well as a model for the non-catalyzed reaction (reaction R0). As major analysis tools, we applied the URVA (Unified Reaction Valley Approach) and LMA (Local Mode Analysis), both being based on vibrational spectroscopy and complemented by a QTAIM analysis of the electron density calculated at the DFT level of theory. The energetics of each reaction were recalculated at the DLPNO-CCSD(T) level of theory. The URVA curvature profiles identified the important chemical events of all five reactions as peroxide OO bond cleavage taking place before the TS (i.e., accounting for the energy barrier) and epoxide CO bond formation together with rehybridization of the carbon atoms of the targeted CC double bond after the TS. The energy decomposition into reaction phase contribution phases showed that the major effect of the catalyst is the weakening of the OO bond to be broken and replacement of OH bond breakage in the non-catalyzed reaction by an energetically more favorable TiO bond breakage. LMA performed at all stationary points rounded up the investigation (i) quantifying OO bond weakening of the oxidizing peroxide upon coordination at the metal atom, (ii) showing that a more synchronous formation of the new CO epoxide bonds correlates with smaller bond strength differences between these bonds, and (iii) elucidating the different roles of the three TiO bonds formed between catalyst and reactants and their interplay as orchestrated by the Sharpless catalyst. We hope that this article will inspire the computational community to use URVA complemented with LMA in the future as an efficient mechanistic tool for the optimization and fine-tuning of current Sharpless catalysts and for the design new of catalysts for epoxidation reactions.

Mechanism of Orbital Interactions in the Sharpless Epoxidation with Ti(IV) Peroxides: A DFT Study.

The M06-2X DFT functional has been employed to examine monomeric titanium(IV) hydroperoxo catalysts that model the individual steps in the dimeric titanium(IV)-catalyzed Sharpless reaction. This is the first example of a transition structure for titanium(IV) tert-butyl hydroperoxide-catalyzed epoxidation that describes the molecular motion required for oxygen atom transfer. These epoxidation catalysts have been examined for both bimolecular reactions with E-2-butene and the intramolecular epoxidation of allyl alcohol. The transition structure for the bimolecular peroxyacetic acid epoxidation of E-2-butene has been shown to be spiro in nature, and likewise, the intramolecular epoxidation of allyl alcohol is also nearly spiro. The significance of the O-C-C═C dihedral angle of allyl alcohol is examined for the Ti(IV) tert-butyl hydroperoxide epoxidation mechanism. Evidence is presented that supports a hexacoordinate titanium peroxo environment that exists in the dimeric form of the Sharpless catalyst. The mechanism for a 1,3-rearrangement of the alkoxide ligand in a titanium hydroperoxide to the Ti center in concert with oxygen atom transfer of the proximal oxygen to the C═C bond of the substrate is presented. The dimerization of Ti(IV)-(R,R)-diethyl tartrate-diisopropoxide and its hydrolysis have been calculated. The mechanism for rapid ligand exchange with alkyl hydroperoxides involving the Ti(O-i-Pr)4 precursor is examined to show how the active epoxidation catalyst is produced.

Stability of Hierarchically Formed Titanium(IV) Tris(catecholate ester) Helicates with Cyclohexyl Substituents in DMSO.

A cyclohexyl substituent strongly prefers the chair conformation with large substituents in equatorial positions, while other cycloalkyls are structurally more flexible. In hierarchically formed dimeric titanium(IV) tris(catecholates) equatorial versus axial connection of the cyclohexane to the ester results in either a more compact (axial) or more expanded (equatorial) structure. In DMSO solution the axial position results in a compact structure which minimizes solvophobic effects, leading to higher stability. However, computational investigations indicate that additionally intramolecular London dispersion interactions significantly contribute to the stability of the dimer. Thus, weak side-chain-side-chain interactions are responsible for the high stability of cyclohexyl ester derivatives with axial compared to equatorial ester connection.

Shedding Light on the Interactions of Hydrocarbon Ester Substituents upon Formation of Dimeric Titanium(IV) Triscatecholates in DMSO Solution.

The dissociation of hierarchically formed dimeric triple lithium bridged triscatecholate titanium(IV) helicates with hydrocarbyl esters as side groups is systematically investigated in DMSO. Primary alkyl, alkenyl, alkynyl as well as benzyl esters are studied in order to minimize steric effects close to the helicate core. The 1H NMR dimerization constants for the monomer–dimer equilibrium show some solvent dependent influence of the side chains on the dimer stability. In the dimer, the ability of the hydrocarbyl ester groups to aggregate minimizes their contacts with the solvent molecules. Due to this, most solvophobic alkyl groups show the highest dimerization tendency followed by alkenyls, alkynyls and finally benzyls. Furthermore, trends within the different groups of compounds can be observed. For example, the dimer is destabilized by internal double or triple bonds due to π–π repulsion. A strong indication for solvent supported London dispersion interaction between the ester side groups is found by observation of an even/odd alternation of dimerization constants within the series of n‐alkyls, n‐Ω‐alkenyls or n‐Ω‐alkynyls. This corresponds to the interaction of the parent hydrocarbons, as documented by an even/odd melting point alternation.

Chemoselective Claisen–Schmidt bis-substitutional condensation catalyzed by an alkoxy-bridged dinuclear Ti(IV) cluster

The highly efficient and chemoselective α,α′-bis-substitution of alkanones is important in organic synthesis. Herein, a dimeric titanium cluster, Ti2Cl2(OPri)6·2HOPri (Ti2), is used in the Claisen–Schmidt condensation reaction, for the selectively activation of symmetrical ketones containing α,α′-methylene groups and production of α,α′-bis-substituted alkanones in high efficiency and chemoselectivity. The high efficiency and chemoselectivity can be extended to a variety of typical alkanones and aromatic aldehydes. Both of the oxo-bridged dimeric motif of Ti2 and the ionic Ti–Cl bond are responsible for the high efficiency and chemoselectivity.

Facile synthesis of a dimeric titanium(iv) complex with terminal TiO moieties and its application as a catalyst for the cycloaddition reaction of CO2to epoxides

Dimeric terminal oxo-titanium complex1having no TiO→Ti bonds could be easily synthesized. The titanium complexes1and2could act as catalysts for cycloaddition between CO2and epoxides in the presence of cocatalysts.

The synthesis, structure, and ethylene polymerization of dimeric salicylaldiminato titanium complexes

A series of dimeric titanium complexes bearing salicylaldiminates, [4-R-6-tBu-2-(CH=NnBu)C6H2OTiCl2(µ-Cl)]2 [R = H (1); R = tBu (2); R = NO2 (3)], have been synthesized in high yield (>90%) by the reaction of corresponding 4-R-6-tBu-2-(CH=NnBu)C6H2OSiMe3 with TiCl4. The molecular structure of 2 has been confirmed by single-crystal X-ray diffraction analysis. When activated with methylaluminoxane, these complexes exhibit good catalytic activity for ethylene polymerization.

Effect of the solvent nature on the products of reactions of TiF4, ZrF4(DMSO)2, and HfF4(DMSO)2 with carbamoylmethylenephosphine oxides Ph2P(O)CH2C(O)NR2: stereoisomerism of dimeric titanium cations {(μ-F)[Ti(η2-L)F3]2}+

Effect of the solvent nature on the products of reactions of TiF4, ZrF4(DMSO)2, and HfF4(DMSO)2 with carbamoylmethylenephosphine oxides Ph2P(O)CH2C(O)NR2: stereoisomerism of dimeric titanium cations {(μ-F)[Ti(η2-L)F3]2}+

Determination of the Catalytic Active Species in the Polymerization of Propylene by Titanium Benzamidinate Complexes

The catalytic behavior of the monomeric titanium bis(benzamidinate) [η-C6H5-C(NSiMe3)2]2TiCl2 (1), the dimeric titanium mono(benzamidinate) {[η-C6H5-C(NSiMe3)2]TiCl3]}2 (2), and the monomeric titanium mono(benzamidinate) complex η-C6H5-C(NSiMe3)2]TiCl3]·THF (3) activated by methylalumoxane (MAO) has been compared in the polymerization of ethylene and propylene. Despite structural and symmetrical differences, the activities of all precatalysts were found to be alike, indicating that rearrangements toward similar active species are operative during the polymerization regardless of the starting materials. To shed some light on the mechanistic pathways for the formation of such active species from the different titanium benzamidinate complexes and on the role of the aluminum cocatalyst in the activation process, corresponding aluminum benzamidinate dichloro and dimethyl complexes were synthesized and compared to the titanium complexes. The formation of the different active sites was monitored using NMR and ESR spectroscopy, trapping experiments with [60]fullerene, and MALDI-TOF mass spectroscopy. The results obtained for the different benzamidinate titanium complexes and proposed mechanistic pathways for their activation reactions with MAO and their fate after the addition of the olefins are presented and discussed in this publication. In addition, viscoelastic and rheological mechanical properties of the polymers are disclosed.

Catalytic Intermolecular Hydroamination of Methylenecyclopropanes

The octahedral titanium catalyst Ti(Ph2PNpy)2(NEt2)2 (1) was found to be an efficient catalyst for the hydroamination of methylenecyclopropane with either aromatic or aliphatic amines. For a symmetrical methylenecyclopropane, only one product is obtained in high yield. The reaction of complex 1 with ethylamine and aniline produces the dimeric titanium complexes Ti2(Ph2PNpy)2(μ-NEt)2 and Ti2(Ph2PNpy)2(μ-NPh)2, respectively.

Phenylenebis(ethynyl)-Tethered Bis-BINOL Ligands for Enantioselective Catalyst Based on Dimeric Ti(IV) Aggregate

Bis-BINOLs 7a-c in which two BINOL units are tethered by phenylenebis(ethynyl) groups react with titanium tetraisopropoxide (2 equiv) to form intramolecular dimeric titanium(IV) aggregates 2a-c. Of these, 2a,b with an o-phenylenebis(ethynyl) tether are stable in the presence of excess titanium tetraisopropoxide. Complex 2a exhibits a relatively high enantioselectivity in asymmetric addition of diethylzinc to an aldehyde. [structure: see text]

A study on the development of CVD precursors V – syntheses and characterization of new N-alkoxy-β-ketoiminate complexes of titanium

The synthesis and characterization of various new titanium N-alkoxy-β-ketoiminate complexes are reported. Reactions between N-alkoxy-β-ketoimine ligands and Ti(O- iPr) 4 resulted in dimeric [Ti(O- iPr) 2( N-alkoxy-β-ketoiminate)] 2 complexes or monomeric [Ti( N-alkoxy-β-ketoiminate) 2] ones depending on the amount of ligands. Terdentate N-alkoxy-β-ketoiminate ligands do not prevent dimer complexes from undergoing disproportional rearrangement to produce Ti(O- iPr) 4 and [Ti( N-alkoxy-β-ketoiminate) 2]. The mechanism of this behavior is too complicated but it may include the dissociation and recoordination of ligands. Crystal structures of [Ti( N-alkoxy-β-ketoiminate) 2] (MeC(O)CHC(Me)NC(Et)CH 2O ( 3f) and t-BuC(O)CHC(Me)NCH 2CH(Me)O ( 3k)) show that these are distorted octahedron and β-ketoiminate ligands appear to coordinate as a β-imino enolate. Two terdentate β-ketoiminate ligands coordinate meridionally and they are perpendicular to each other. Thermal characteristics of monomeric and dimeric titanium complexes were determined by TGA and DSC and these are reasonably volatile as potential precursors of TiO 2 thin films.

Supramolecular Structures of Titanium(IV)‐Substituted Wells–Dawson Polyoxotungstates

AbstractThe novel, dimeric titanium(IV)‐substituted phosphotungstate [(TiP2W15O55OH)2]14− (1) has been synthesized and characterized by IR and 31P NMR spectroscopy, elemental analysis, and single‐crystal Xray diffraction. The polyanion consists of two [P2W15O56]12− Wells–Dawson moieties linked through two titanium(IV) centers. Polyanion 1 is a dilacunary species and represents the first Ti‐containing sandwich‐type structure. The titanium centers are octahedrally coordinated by three oxygen atoms of each P2W15O56 subunit. The edge‐shared TiO6 units are symmetrically equivalent and have no terminal ligands. Polyanion 1 shows a chiral distortion within each P2W15Ti fragment. We also report on the structural characterization of the tetrameric, supramolecular species [{Ti3P2W15O57.5(OH)3}4]24− (2). Polyanion 2 is composed of four equivalent P2W15Ti3 fragments, fused together through terminal TiO bonds, leading to a structure with Td symmetry.

A facile route to hetero-bimetallic Ti(iv)-alkali metal calix[4]arene complexes

Deprotonation of calix[4]arenes by alkali metal in methanol using simple benchtop procedures is effective in the formation of novel monomeric and dimeric titanium(IV) complexes. For the larger alkali metal cations, K+ and Cs+, favourable complexation within the π-basic calix[4]arene cavity facilitates the formation of monomeric titanium(IV) complexes when oligomer formation is inhibited by an acetylacetonate ligand. In contrast, the smaller Li+ and Na+ ions preferentially form dimeric complexes with exo-bridging alkali and titanium(IV) metal centres. A dimeric complex is also obtained for potassium in the absence of acetylacetonate. The solid state structures of the K–Ti and Na–Ti calix[4]arene dimer complexes show different structural characteristics depending on the nature of the alkali metal.

Monomere Kupfer(I)-Alkyle mit β-Wasserstoffatomen und Kupfer(I)-Aryle mit kondensierten Aromaten; die Festkörperstruktur von [(η 5-C 5H 4SiMe 3) 2Ti(CCSiMe 3) 2]Cu nC 4H 9

The synthesis of heterobimetallic titanium(IV)–copper(I) complexes of type {[Ti](CCSiMe 3) 2}CuR { 3a, R= i C 3H 7; 3b, R= n C 4H 9; 3c, R= c C 5H 9; 3d, R=9-C 13H 9; 5a, R=1-C 10H 7; 5b, R=9-C 14H 9; [Ti]=(η 5-C 5H 4SiMe 3) 2Ti} in which monomeric low-valent copper(I) organyls (CuR) are stabilised by the chelating effect of the organometallic π-tweezer [Ti](CCSiMe 3) 2 are accessible by the reaction of {[Ti](CCSiMe 3) 2}CuSC 6H 4CH 2NMe 2-2 ( 1) with equimolar amounts of ER (E=Li, BrMg). These species feature monomeric copper alkyls or aryls, bearing β-hydrogen atoms (complexes 3a– 3c) or condensed aromatic π-systems (complexes 5a and 5b). While 3a– 3d, 5a and 5b are stable at low temperature in the solid-state, it appeared that in solution 3a– 3c undergo β-hydride elimination, affording propene ( 3a), 1-butene ( 3b) or cyclopentene ( 3c). Next to these species, HSiMe 3 ( 8) along with the dimeric titaniumcopper acetylide {[Ti](CCSiMe 3)(CuCC)} 2 ( 6) is also formed. In contrast, 5b decomposes on heating (30 °C) to afford the tweezer molecule [Ti](CCSiMe 3) 2 ( 9) and (C 14H 9) 2 ( 10). Possible reaction mechanism for the latter reactions will be discussed. The solid-state structure of 3b is reported. Complex 3b crystallizes in the triclinic space group P 1 ̄ with cell parameters a=13.659(2), b=17.270(3), c=18.106(4) Å, α=107.64(3), β=100.11(3), γ=108.51(3)°, Z=4 and V=3681(1) Å 3. The most striking structural feature of 3b, which will be discussed, is that the n C 4H 9 moiety is orientated out of the Ti(CCSi) 2Cu plane.

( t-Bu-DAD)Ti(μ,η 2-OCPh 2)] 2Mg(μ-Cl) 2(THF): synthesis and molecular structure of a dimeric titanium ketone complex

Reduction of [( t-Bu-DAD)TiCl(μ-Cl)] 2 ( 1) ( t-Bu-DAD: t-BuNCHCHN t-Bu) with magnesium in the presence of benzophenone yields the MgCl 2-bridged dimeric titanium ketone complex [( t-Bu-DAD)Ti(μ,η 2-OCPh 2)] 2Mg(μ-Cl) 2(THF) ( 2) whose molecular structure largely conforms with the assumptions about the structure of the first step of the reductive coupling of ketones at low-valent titanium.

A Non-Metallocene Hydride of Titanium(III)

Several titanium(m) complexes incorporating the chelating diamidoamine ligand [(Me3SiNCH2CH2)2NSiMe3]2-[NN2] are described. The reaction between Li2[NN2] and TiCl3(THF)3 (THF = tetrahydrofuran) resulted in the formation of the dimeric titanium(III) chloride {TiCl[NN2]}2 1, from which the monomeric titanium(III) alkyl Ti{CH(SiMe3)2}[NN2] 2 could be synthesized via a salt metathesis route. The alkyl complex 2 was found to react with hydrogen gas to form the thermally stable, dimeric, diamagnetic titanium(III) hydride {TiH[NN2]}2 3; density functional calculations on a simplified model system of 3 indicated the presence of a weak Ti-Ti s- interaction. The solid-state structures of 1, 2, and 3 are described.

Density functional study of the primary events on TiO2 photocatalyst

Density functional calculations at the B3LYP/6-311G**//B3LYP/6-311G level of theory were used to study the initial process of ethylene degradation on the TiO2 photocatalyst by adopting the dimeric titanium structure Ti2O6H4 as a model of the catalyst surface. Adsorption energies of water and ethylene were calculated to be 31.9 and 20.4 kcal mol−1. The photogenerated OH radical does not desorb from the catalyst surface and the further reaction with ethylene proceeds since the adsorption energy was estimated to be 39.3 kcal mol−1. Our calculation also indicated that under steady illumination, ethylene directly attacks the OH radical bound to the TiO2 surface even though the surface has vacant sites available for ethylene adsorption.