Trinuclear Titanium Research Articles

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Hydrodeoxygenative Cyclotetramerization of Carbon Monoxide by a Trinuclear Titanium Polyhydride Complex.

The reductive coupling of carbon monoxide (CO) by metal hydrides is of fundamental interest and practical importance. Herein we report an unprecedented hydrodeoxygenative cyclotetramerization of CO by a trinuclear titanium polyhydride complex [(C5Me4SiMe3)Ti]3(μ3-H)(μ2-H)6 (1). The reaction of CO with 1 at -78 °C gave an ethen-1,2-diyl species [CH═CH]2- through the hydrodeoxygenative dimerization of two molecules of CO, which upon cycloaddition to another two molecules of CO afforded a cyclobuten-3,4-diyl-1,2-diolate unit [C4H2O2]4-. The hydrogenolysis of the [C4H2O2]4- species with H2 yielded a tetrahydrocyclobuten-1,2-diolate species [C4H4O2]2-, which on heating at 100 °C gave a cyclobuten-2-yl-1-olate product [C4H4O]2-. The acidolysis of the [C4H2O2]4- and [C4H4O]2- species with HCl afforded γ-butyrolactone and cyclobutanone, respectively.

Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh6)-Bridged Titanium Complexes.

Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (Cp2Ti)3(μ3-HATNPh6) (2). On reaction of 2 with the ferrocenium salt [Cp2Fe]BF4, the cationic complexes [(Cp2Ti)3(μ3-HATNPh6)] n+ ( n = 1-3; 3-5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants Kc, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion-π interactions between the electron-deficient central ring of the HATNPh6 ligand and BF4- counterions were found.

Hydrodenitrogenation of pyridines and quinolines at a multinuclear titanium hydride framework

Investigation of the hydrodenitrogenation (HDN) of aromatic N-heterocycles such as pyridines and quinolines at the molecular level is of fundamental interest and practical importance, as this transformation is essential in the industrial petroleum refining on solid catalysts. Here, we report the HDN of pyridines and quinolines by a molecular trinuclear titanium polyhydride complex. Experimental and computational studies reveal that the denitrogenation of a pyridine or quinoline ring is easier than the ring-opening reaction at the trinuclear titanium hydride framework, which is in sharp contrast with what has been reported previously. Hydrolysis of the pyridine-derived nitrogen-free hydrocarbon skeleton at the titanium framework with H2O leads to recyclization to afford cyclopentadiene with the generation of ammonia, while treatment with HCl gives the corresponding linear hydrocarbon products and ammonium chloride. This work has provides insights into the mechanistic aspects of the hydrodenitrogenation of an aromatic N-heterocycle at the molecular level.

Mechanistic Insights into Ring Cleavage and Contraction of Benzene over a Titanium Hydride Cluster.

Carbon-carbon bond cleavage of benzene by transition metals is of great fundamental interest and practical importance, as this transformation is involved in the production of fuels and other important chemicals in the industrial hydrocracking of naphtha on solid catalysts. Although this transformation is thought to rely on cooperation of multiple metal sites, molecular-level information on the reaction mechanism has remained scarce to date. Here, we report the DFT studies of the ring cleavage and contraction of benzene by a molecular trinuclear titanium hydride cluster. Our studies suggest that the reaction is initiated by benzene coordination, followed by H2 release, C6H6 hydrometalation, repeated C-C and C-H bond cleavage and formation to give a MeC5H4 unit, and insertion of a Ti atom into the MeC5H4 unit with release of H2 to give a metallacycle product. The C-C bond cleavage and ring contraction of toluene can also occur in a similar fashion, though some details are different due to the presence of the methyl substituent. Obviously, the facile release of H2 from the metal hydride cluster to provide electrons and to alter the charge population at the metal centers, in combination with the flexible metal-hydride connections and dynamic redox behavior of the trimetallic framework, has enabled this unusual transformation to occur. This work has not only provided unprecedented insights into the activation and transformation of benzene over a multimetallic framework but it may also offer help in the design of new molecular catalysts for the activation and transformation of inactive aromatics.

Two-State Reactivity Mechanism of Benzene C-C Activation by Trinuclear Titanium Hydride.

The cleavage of inert C-C bonds is a central challenge in modern chemistry. Multinuclear transition metal complexes would be a desirable alternative because of the synergetic effect of multiple metal centers. In this work, carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride were investigated using density functional theory. The reaction occurs via a novel "two-state reactivity" mechanism. The important elementary steps consist of hydride transfer, benzene coordination, dehydrogenation, oxidative addition, hydride-proton exchange, and reductive elimination. Most importantly, the ground-state potential energy surface switches from nearly degenerate triplet and antiferromagnetic singlet states to a closed-shell singlet state in the dearomatization of benzene, which effectively decreases the activation barrier. Furthermore, the roles of the transition metal centers and hydrides were clarified.

ChemInform Abstract: Heterometallic Cube‐Type Molecular Nitrides

AbstractReview: [emphasis on nitrido complexes with cuboidal [MTi3N4] cores; 96 refs.].

Heterometallic Cube‐Type Molecular Nitrides

AbstractPolynuclear transition‐metal nitrido complexes constitute a class of molecular cage compounds with fascinating structures and interesting bonding properties. However, there is a lack of systematic strategies for the rational construction of aggregates with desired structures and compositions. This article provides a brief overview of the structure and bonding modes of polynuclear nitrido complexes, the most common synthetic approaches used to generate such aggregates, and a systematic review of the development of a family of heterometallic nitrido complexes with [MTi3N4] cube‐type cores. The rational entry to those well‐defined systems is based on the incorporation of transition metals, lanthanides, and main‐group metals into the incomplete cube structure of the trinuclear titanium(IV) imido nitrido complex [{TiCp*(µ‐NH)}3(µ3‐N)] (Cp* = η5‐C5Me5). The great versatility of [{TiCp*(µ‐NH)}3(µ3‐N)] as a preorganized metalloligand is also confirmed by the preparation of complexes with heterometals in low (e.g., Mo0, IrI, and Pt0) and high oxidation states (e.g., ZrIV, TaV, and PtIV).

Improved Reactivity in the Ring-Opening Polymerization of ε-Caprolactone with a Trinuclear Titanium(IV) Oxochloroneopentoxide as Initiator

The complex [Ti3(O)(Cl)(ONep)9] and two iron(II)-containing products, [Fe4Cl8(thf)6] and [Fe2Cl3(thf)6][FeCl4], were synthesized by a redox reaction involving [Ti3(OiPr)11][FeCl4] and neopentanol; the structures of compounds [Ti3(O)(Cl)(ONep)9] and [Fe2Cl3(thf)6][FeCl4] have been confirmed by X-ray diffraction methods. Variable-temperature 1H NMR analysis of [Ti3(O)(Cl)(ONep)9] revealed signals for three non-equivalent neopentoxides in the 1:1:1 integration ratio, indicating the maintenance of the trinuclear structure in solution. Alkoxide [Ti3(O)(Cl)(ONep)9] was employed as initiator in both bulk and solution polymerization of e-caprolactone (e-CL) at various temperatures, monomer/initiator molar ratios and reaction times. [Ti3(O)(Cl)(ONep)9] produced poly(e-caprolactone) in high yields, with molecular weights ranging from 7100 to 9800 g mol-1 and PDI (polydispersity index) values between 1.3 to 1.5. The reactivity of [Ti3(O)(Cl)(ONep)9] in the ring-opening polymerization (ROP) of e-CL was compared to those of [Ti3(OiPr)11][FeCl4] and [Ti(OiPr)4]. The better polymerization reactivity shown by [Ti3(O)(Cl)(ONep)9] seems to be associated with structural features, as well as with the stronger Lewis basicity of -ONep over -OiPr ligands and the maintenance of the trimetallic structure in the reaction media.

Carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride.

The cleavage of carbon-carbon (C-C) bonds by transition metals is of great interest, especially as this transformation can be used to produce fuels and other industrially important chemicals from natural resources such as petroleum and biomass. Carbon-carbon bonds are quite stable and are consequently unreactive under many reaction conditions. In the industrial naphtha hydrocracking process, the aromatic carbon skeleton of benzene can be transformed to methylcyclopentane and acyclic saturated hydrocarbons through C-C bond cleavage and rearrangement on the surfaces of solid catalysts. However, these chemical transformations usually require high temperatures and are fairly non-selective. Microorganisms can degrade aromatic compounds under ambient conditions, but the mechanistic details are not known and are difficult to mimic. Several transition metal complexes have been reported to cleave C-C bonds in a selective fashion in special circumstances, such as relief of ring strain, formation of an aromatic system, chelation-assisted cyclometallation and β-carbon elimination. However, the cleavage of benzene by a transition metal complex has not been reported. Here we report the C-C bond cleavage and rearrangement of benzene by a trinuclear titanium polyhydride complex. The benzene ring is transformed sequentially to a methylcyclopentenyl and a 2-methylpentenyl species through the cleavage of the aromatic carbon skeleton at the multi-titanium sites. Our results suggest that multinuclear titanium hydrides could serve as a unique platform for the activation of aromatic molecules, and may facilitate the design of new catalysts for the transformation of inactive aromatics.

Reactivity with Electrophiles of Imido Groups Supported on Trinuclear Titanium Systems

Several trinuclear titanium complexes bearing amido μ-NHR, imido μ-NR, and nitrido μn-N ligands have been prepared by reaction of [{Ti(η(5)-C5Me5)(μ-NH)}3(μ3-N)] (1) with 1 equiv of electrophilic reagents ROTf (R = H, Me, SiMe3; OTf = OSO2CF3). Treatment of 1 with triflic acid or methyl triflate in toluene at room temperature affords the precipitation of compounds [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NH2)(OTf)] (2) or [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)(μ-NH2)(μ-NMe)(OTf)] (3). Complexes 2 and 3 exhibit a fluxional behavior in solution consisting of proton exchange between μ-NH2 and μ-NH groups, assisted by the triflato ligand, as could be inferred from a dynamic NMR spectroscopy study. Monitoring by NMR spectroscopy the reaction course of 1 with MeOTf allows the characterization of the methylamido intermediate [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NHMe)(OTf)] (4), which readily rearranges to give 3 by a proton migration from the NHMe amido group to the NH imido ligands. The treatment of 1 with 1 equiv of Me3SiOTf produces the stable ionic complex [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NHSiMe3)][OTf] (5) with a disposition of the nitrogen ligands similar to that of 4. Complex 5 reacts with 1 equiv of [K{N(SiMe3)2}] at room temperature to give [Ti3(η(5)-C5Me5)3(μ3-N)(μ-N)(μ-NH)(μ-NHSiMe3)] (6), which at 85 °C rearranges to the trimethylsilylimido derivative [Ti3(η(5)-C5Me5)3(μ3-N)(μ-NH)2(μ-NSiMe3)] (7). Treatment of 7 with [K{N(SiMe3)2}] affords the potassium derivative [K{(μ3-N)(μ3-NH)(μ3-NSiMe3)Ti3(η(5)-C5Me5)3(μ3-N)}] (8), which upon addition of 18-crown-6 leads to the ion pair [K(18-crown-6)][Ti3(η(5)-C5Me5)3(μ3-N)(μ-N)(μ-NH)(μ-NSiMe3)] (9). The X-ray crystal structures of 2, 5, 6, and 8 have been determined.

Dinitrogen Cleavage and Hydrogenation by a Trinuclear Titanium Polyhydride Complex

Both the Haber-Bosch and biological ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong N≡N triple bond and forming an N-H bond. This has spurred investigations of the reactivity of molecular multimetallic hydrides with dinitrogen. We report here the reaction of a trinuclear titanium polyhydride complex with dinitrogen, which induces dinitrogen cleavage and partial hydrogenation at ambient temperature and pressure. By (1)H and (15)N nuclear magnetic resonance, x-ray crystallographic, and computational studies of some key reaction steps and products, we have determined that the dinitrogen (N2) reduction proceeds sequentially through scission of a N2 molecule bonded to three Ti atoms in a μ-η(1):η(2):η(2)-end-on-side-on fashion to give a μ2-N/μ3-N dinitrido species, followed by intramolecular hydrogen migration from Ti to the μ2-N nitrido unit.

Computational tools for mechanistic discrimination in the reductive and metathesis coupling reactions mediated by titanium(IV) isopropoxide

A theoretical study has been carried out at the B3LYP/LANL2DZ level to compare the reactivity of phenyl isocyanate and phenyl isothiocyanate towards titanium(IV) alkoxides. Isocyanates are shown to favour both mono insertion and double insertion reactions. Double insertion in a head-to-tail fashion is shown to be more exothermic than double insertion in a head-to-head fashion. The head-to-head double insertion leads to the metathesis product, a carbodiimide, after the extrusion of carbon dioxide. In the case of phenyl isothiocyanate, calculations favour the formation of only mono insertion products. Formation of a double insertion product is highly unfavourable. Further, these studies indicate that the reverse reaction involving the metathesis of N,N′-diphenyl carbodiimide with carbon dioxide is likely to proceed more efficiently than the metathesis reaction with carbon disulphide. This is in excellent agreement with experimental results as metathesis with carbon disulphide fails to occur. In a second study, multilayer MM/QM calculations are carried out on intermediates generated from reduction of titanium(IV) alkoxides to investigate the effect of alkoxy bridging on the reactivity of multinuclear Ti species. Bimolecular coupling of imines initiated by Ti(III) species leads to a mixture of diastereomers and not diastereoselective coupling of the imine. However if the reaction is carried out by a trimeric biradical species, diastereoselective coupling of the imine is predicted. The presence of alkoxy bridges greatly favours the formation of the d,l (±) isomer, whereas the intermediate without alkoxy bridges favours the more stable meso isomer. As a bridged trimeric species, stabilized by bridging alkoxy groups, correctly explains the diastereoselective reaction, it is the most likely intermediate in the reaction. Computational studies carried out at the B3LYP/LANL2DZ level throw light on the contrasting reactivity of isocyanates and isothiocyanates towards titanium(IV) alkoxides. While both mono and double insertion reactions are feasible with isocyanates, only the mono insertion reaction appears to be thermodynamically feasible with isothiocyanates. Multilayer calculations are also performed on multinuclear titanium intermediates in diastereoselective coupling reactions initiated by low valent titanium species. These studies clearly predict that the coupling of a trinuclear titanium complex involvng alkoxy bridges gives rise to diastereoselective coupling. Non bridged forms and dimeric species would not result in the observed diastereoselectivity.

Tris(4-hydroxy-3,5-diisopropylbenzyl)amine as a new bridging ligand for novel trinuclear titanium complexes

Tris(4-hydroxy-3,5-diisopropylbenzyl)amine ( LH 3 ) was synthesized by the reaction of 2,6-diisopropylphenol and hexamethylenetetramine in the presence of p-toluenesulfonic acid or paraformaldehyde. Its solid state structure was determined by single crystal X-ray diffraction. Its fully deprotonated specie, (4-O-3,5- i-Pr 2PhCH 2) 3N ( L), was used to form novel trinuclear half-sandwich titanocene complexes, namely [(η 5-C 5Me 5)TiCl 2] 3L ( 1) and [(η 5-C 5Me 5)Ti(OMe) 2] 3L ( 2), which were then tested for the syndiospecific polymerization of styrene in the presence of methylaluminoxane (MAO) cocatalyst. Their catalytic properties were directly compared with those of trichloro(pentamethylcyclopentadienyl)titanium(IV) ( 3) and dichloro(2,6-diisopropylphenolato)(pentamethylcyclopentadienyl)titanium(IV) ( 4). 1/MAO and 2/MAO systems showed higher activities towards styrene polymerization than the mononuclear catalytic systems 3/MAO and 4/MAO, giving syndiotactic polystyrene of high molecular weight.

Simple Synthesis of (Triphenylphosphoniomethylidene)(pentamethylcyclopentadienyl)titanium(II) Dichloride and Its Unexpected Reduction To Form a Trinuclear Titanium Cluster

The novel titanium phosphoniomethylidene complex [Cp*TiCl2(−CH═PPh3)] (2; Cp* = η-C5Me5) was obtained via a transylidation reaction of Cp*TiCl3 (1) with 2 equiv of the phosphorus ylide Ph3P═CH2. The trinuclear titanium(III) cluster [Cp*Ti(μ-Cl)Cl]3 (3) was formed through reduction of complex 2 with CO. Complexes 2 and 3 were structurally characterized by X-ray diffraction.

Reaction of the titanocene alkyne complex Cp2Ti(η2-Me3SiC2SiMe3) with methanol: Preparation and characterization of a novel trinuclear titanium complex [{Cp2Ti(OMe)}2{Ti(OMe)4}

The titanocene alkyne complex Cp2Ti(η2-Me3SiC2SiMe3) (1) reacts with methanol to yield depending on the reaction conditions (solvent, temperature) different titanium methoxides [Cp2Ti(OMe)]2 (2a) and [{Cp2Ti(OMe)}2{Ti(OMe)4}] (3). The structure of the trinuclear complex 3, consisting of titanium III and IV, was confirmed by X-ray studies.

In Situ Formation of Heterobimetallic Salen Complexes Containing Titanium and/or Vanadium Ions

A combination of high-resolution electrospray mass spectrometry and (1)H NMR spectroscopy has been used to prove that when a mixture of [(salen)TiO]2 complexes containing two different salen ligands (salen and salen') is formed, an equilibrium is established between the homodimers and the heterodimer [(salen)TiO2Ti(salen')]. Depending upon the structure and stereochemistry of the two salen ligands, the equilibrium may favor either the homodimers or the heterodimer. Extension of this process to mixtures of titanium(salen) complexes [(salen)TiO]2 and vanadium (V)(salen') complexes [(salen')VO] (+)Cl (-) allowed the in situ formation of the heterobimetallic complex [(salen)TiO2V(salen')] (+)X (-) to be confirmed for all combinations of salen ligands studied except when the salen ligand attached to titanium contained highly electron-withdrawing nitro-groups. The rate of equilibration between heterobimetallic complexes is faster than that between two titanium complexes as determined by line broadening in the (1)H NMR spectra. These structural results explain the strong rate-inhibiting effect of vanadium (V)(salen) complexes in asymmetric cyanohydrin synthesis catalyzed by [(salen)TiO]2 complexes. It has also been demonstrated for the first time that the titanium and vanadium complexes can undergo exchange of salen ligands and that this is catalyzed by protic solvents. However, the ligand exchange is relatively slow (occurring on a time scale of days at room temperature) and so does not complicate studies aimed at using heterobimetallic titanium and vanadium salen complexes as asymmetric catalysts. Attempts to obtain a crystal structure of a heterobimetallic salen complex led instead to the isolation of a trinuclear titanium(salen) complex, the formation of which is also consistent with the catalytic results obtained previously.

Synthesis and crystal structure of a trinuclear titanium(IV) β -diketonate complex with a six-membered [Ti–(μ-O)]3 ring

The trinuclear titanium(IV) β-diketonate complex, [Ti3(μ-O)3(dppd)6] · (DMF)(0.5EtOH) (1) (H-dppd = 1,3-diphenylpropane-1,3-dione) has been synthesized and its crystal structure and spectroscopic properties have been studied. Complex 1 crystallizes in monoclinic system with space group C2/c and a = 20.954(5) Å, b = 18.705(4) Å, c = 24.617(6) Å, β = 111.957(4)°, Z = 4. The complex consists of a six-membered [Ti–(μ-O)]3 ring, of which three titanium(IV) ions are linked together by bridging oxygen atoms. Each titanium(IV) possesses an octahedral geometry, with four oxygen atoms from the two β-diketonate ligands and two bridging oxygen atoms.

Selective Oxidation and Reduction of Trinuclear Titanium(II) Hexaazatrinaphthylene ComplexesSynthesis, Structure, and Electrochemical Investigations

Titanocene complexes with chelating N-heterocyclic ligand bridges react with ferrocenium salts as selective oxidants to afford air-stable cationic complexes and allow the preparation of exceptional mixed valence hexaazatrinaphthylene complexes [(Cp2Ti)3(mu3-HATNMe6)]n+ (1n+) (n=1, 2, 3, 4). Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) show that nine oxidation states of 1 are generated without decomposition. Comproportionation constants Kc have been calculated in order to determine the extent of electronic communication between the titanium centers. The Kc values of the mixed valence states are indicative of uncoupled (14+), moderately coupled (15+), and strongly coupled (1-, 1+, and 12+) systems. Small but significant structural changes occurring upon oxidation of neutral 1 are observed by X-ray structural analysis on 1+-14+. Anion-pi interactions between the electron-deficient central ring of the HATNMe6 moiety and PF6- and BF4- counterions, respectively, are found for 12+, 13+, and 14+. The short cation-anion contacts cause interesting molecular allignments in terms of molecular architecture. For 12+ the assembly of an one-dimensional (1D) polymer is observed. Electrochemical investigations on the mononuclear cationic titanocene complexes [(Cp2Ti)(L)]+ (L=2,2'-biquinoline (2+), 4,4'-dimethyl-2,2'-biquinoline (3+), and 5,8'-dimethyl-2,3'-biquinoxaline (4+)) showed similar oxidation and reduction characteristics among each other. Conversion to monoanionic, neutral, and dicationic states is enabled. As found for the trinuclear compounds 1n+, the molecular structures of 2+-4+ reveal significant differences compared to their neutral parents.

A Novel Trinuclear Titanium(IV) Complex with a C3 Axis Along Ti1—Ti2—Ti3 Containing 3‐[(1H‐1,2,4‐Triazol‐1‐yl)methyl]‐BINOLate Ligands: Synthesis, Structure, and Reactivity.

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