Ti-Ti Bonds Research Articles

Structure and mechanical properties of novel lightweight refractory high entropy alloys NbMoTiZr-(Al/V): a combined first principles and experimental study

NbMoTaW refractory high entropy alloy (RHEA) has excellent high temperature mechanical properties, but low room temperature ductility and high density severely limit its practical applications. In the present work, to overcome this deficiency, the composition was adjusted, and the mechanical properties of four novel as-cast NbMoTiZr-(Al/V) lightweight refractory high entropy alloys (LRHEAs) were investigated by theoretical calculations combined with experimental methods. The first-principle calculations approach based on density functional theory predicts the mechanical properties of LRHEAs and explains the alloying effects at the atomic and electronic levels. The body-centered-cubic (BCC) phase structure of the alloys were predicted on the formation enthalpy and cohesive energy, as well as empirical parameters such as ΔSmix, ΔHmix, VEC, and the atomic size difference, in conjunction with CALPHAD and XRD. The microstructure of the LRHEAs were also analyzed by SEM. The experimental results, along with the calculated elastic constants and moduli, show significant improvement in the strength and ductility of NbMoTiZr-(Al/V) LRHEAs compared to NbMoTaW RHEA. LRHEAs exhibit elastic anisotropy and are therefore more suitable for use in engineering applications. The strengthening mechanisms of the alloys were analyzed in terms of total and partial densities of states, overlapping Mulliken population, charge density contour and atomic distances. The increased plasticity of LRHEAs is mainly due to the formation of Ti-Ti, Zr-Zr and Zr-Al metallic bonds in the alloys. The calculated results agree with the experimental results, indicating that first-principles calculations are an effective method for predicting the improved properties of lightweight refractory high-entropy alloys. The present work provides a good guideline for the design and research of lightweight refractory high entropy alloys.

Effect of hydrogen-vacancy cluster on the tensile strength of alpha-titanium twin boundaries: A first-principles investigation

This study investigated the formation of hydrogen-vacancy cluster at twin boundaries (TBs) in alpha-titanium and their impact on TB strength. Using first-principles calculations, it is found that a single vacancy (VA) on the {10−12} TB plane can trap up to five hydrogen atoms, while the single VA on the {10−11} TB plane can accommodate up to eight hydrogen atoms. The presence of VA weakens TBs by reducing the number of Ti-Ti bonds near the TBs. However, in the {10−12} TB with VAH5, the enhancement effect of Ti-H-Ti bonds dominates over the weakening effect of electron transfer, resulting in a slight increase in tensile strength compared to {10−12} TB with VA. On the other hand, in the {10−11} TB with VAH8, the weakening effect of electron transfer dominates, leading to a further reduction in tensile strength relative to {10−11} TB with VA. Therefore, the effect of hydrogen-vacancy cluster on the strength of twin boundaries is sensitive to the type of twin boundary. These findings provide valuable insights into the role of hydrogen in the embrittlement of alpha-titanium.

Enhanced strength and ductility in a powder metallurgy Ti material by the oxygen scavenger of CaB6

• There exists a continuous oxide layer on the irregular HDH Ti powder surface. • CaB 6 addition promotes the improved sintered density and refines the grain size. • The scavenging of O by CaB 6 addition makes a high strength and superior ductility for α-Ti alloy. The challenge of producing a high-ductility titanium (Ti) material using inexpensive high-oxygen hydride-dehydride (HDH) Ti powder is hereby addressed by the incorporation of CaB 6 oxygen-scavenger. The oxygen-scavenging behaviour, microstructure evolution, mechanical behavior and improvement mechanism were systematically investigated. A continuous TiO 2 oxide layer with a thickness of approximately 9.3 nm is presented on the HDH Ti powder surface. The oxide layer will dissolve into Ti matrix during sintering, making the increase of c / a value and leading to the Ti-Ti bonds developing from plastic metal bonds toward brittle covalent bonds. The CaB 6 addition can scavenge O impurity and make a significant increase in tensile ductility for α-Ti matrix. A small addition of 0.2 wt.% CaB 6 provides a superior tensile elongation of 22.2% for Ti material, almost three times as high as that of pure Ti (7.5%). The increase of deformation twining activity and grain refinement are responsible for the improved ductility. Furthermore, the CaB 6 oxygen-scavenger can react with the surface oxide layer to in-situ form rod-like TiB and granular CaTiO 3 reinforcements, refining the coarse near equiaxed grain of α-Ti matrix into fine equiaxed crystals. The multiple mechanisms of grain-boundary strengthening, load-bearing strengthening of TiB and Orowan strengthening of CaTiO 3 nanoparticles work together to increase the tensile strength of Ti/CaB 6 composites. This work offers an effective method to fabricate high-performance Ti material using inexpensive high-oxygen HDH Ti powder.

Electronic, Tensile, and Sliding Characteristics of the C/Ti Interface: A First-Principles Study.

In this paper, according to the C(111) surface and Ti(112̅0) surface relative positions, three stacking interface models were constructed by the first-principles method, and they were defined as 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0), respectively. After calculation, the work of interfacial adhesion of the 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0) interface models is found to be 9.689, 10.246, and 9.714 J/m2, respectively, and their interface energies are observed to be 1.064, 0.507, and 1.039 J/m2, respectively. Moreover, the electronic characteristics of C(111)/Ti(112̅0) interfaces are dominated by polar covalent bonds, supplemented by certain metallicity. When the strain reaches 13, 15, and 12%, respectively, the maximum tensile stress values of 1st-C(111)/Ti(112̅0), 2nd-C(111)/Ti(112̅0), and 4th-C(111)/Ti(112̅0) interface models are observed to be 16.207, 19.183, and 17.393 GPa, respectively. After all C(111)/Ti(112̅0) interfaces fracture under tension, the Ti atoms of the Ti(112̅0) surface are transferred to the C(111) surface, indicating that the strength of Ti-C bonds at the interface is higher than the strength of Ti-Ti bonds inside the Ti(112̅0) surface. The maximum value of the sliding potential energy surface is 1.709 J/m2; the maximum value of the potential energy curve is 0.445 J/m2; and the ideal shear strength of the C(111)/Ti(112̅0) interface is 0.386 GPa. In summary, the interfacial adhesion property of the 2nd-C(111)/Ti(112̅0) interface is better than those of 1st-C(111)/Ti(112̅0) and 4th-C(111)/Ti(112̅0) interfaces.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments.

Low-valent titanium species were prepared by reaction of [TiCp*X3 ] (Cp*=η5 -C5 Me5 ; X=Cl, Br, Me) with LiEH4 (E=Al, B) or BH3 (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X3 ] with LiAlH4 in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}2 {(μ-H)2 AlX(L)}2 ] (L=thf, X=Cl, Br; L=OEt2 , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH2 XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X3 ] with LiBH4 (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH4 )(μ-X)}2 ] (X=Cl, Br) and [{TiCp*(BH4 )(μ-BH4 )}2 ], respectively. The treatment of [TiCp*Me3 ] with 4 and 5 equiv. of BH3 (thf) produces the diamagnetic [{TiCp*(BH3 Me)}2 (μ-B2 H6 )] and paramagnetic [{TiCp*(μ-B2 H6 )}2 ] complexes, respectively.

Where is the unpaired transition metal in substoichiometric diboride line compounds?

The atomic structure and local composition of high quality epitaxial substoichiometric titanium diboride (TiB1.9) thin film, deposited by unbalanced magnetron sputtering, were studied using analytical high-resolution scanning transmission electron microscopy, density functional theory, and image simulations. The unpaired Ti is pinpointed to inclusion of Ti-based stacking faults within a few atomic layers, which terminates the {11¯00} prismatic planes of the crystal structure and attributed to the absence of B between Ti planes that locally relaxes the structure. This mechanism allows the line compound to accommodate off-stoichiometry and remain a line compound between defects. The planar defects are embedded in otherwise stoichiometric TiB2 and are delineated by insertion of dislocations. An accompanied decrease in Ti-Ti bond lengths along and across the faults is observed.

Influence of interactions between hydrogen and [formula omitted] twin boundary on hydrogen embrittlement in α-Ti

Ti and Ti alloys are used in several areas thanks to their good mechanical properties, however, the large affinity of hydrogen during industrial application would limit their service life. This study serves as a comprehensive investigation of the embrittlement effects induced by additional hydrogen in detail by means of first-principles calculations with the Pseudopotential Plane-wave method within the framework of the Rice-Wang thermodynamic model. The existence of the twin boundary markedly influences segregation energy, the activation barriers for diffusion between sites, and intergranular embrittlement. We demonstrate that segregation energy and the barriers in the region near the boundary are consistently lower than in bulk, indicating that the sites located at the boundary are more attractive to hydrogen than bulk. Further, the intergranular strength of a Ti grain boundary is reduced with hydrogen segregation. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the weakening effect of hydrogen is due to the breaking of the interfacial Ti-Ti bonds, which significantly decrease twin boundary adhesion and its resistance to cracking.

∑3 Twin Boundaries in Gd2Ti2O7 Pyrochlore: Pathways for Oxygen Migration

Understanding the chemistry at the twin boundaries is a well-recognized challenge which could enable the capabilities to manipulate the functional properties in complex oxides. The study of this atomic imperfection becomes even more important, as the presence of twin boundaries has been widely observed in materials, regardless of the dimensionalities, due to the complexities in the growth methods. In the present study, we demonstrate the atomic scale insights on an Ʃ3(111 ̅) <11 ̅0> twin boundary present in pyrochlore-structured Gd2Ti2O7 using the atomic-resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (111 ̅) with 71o angle between the two symmetrically arranged crystals. We observe distortions (~3-5% strain) in the atomic structure at the TB with an increase in Gd-Gd (0.66± 0.03nm) and Ti-Ti (0.65± 0.02 nm) bondlengths in the (11 ̅0) plane, as compared to 0.63 nm in the ordered structure. Using atomistic modeling, we further calculate the oxygen migration barrier for vacancy hoping at 48f-48f sites in the pyrochlore structure which is the primary diffusion pathway fast oxygen transport. The mean migration barrier is lowered by ~25% to 0.9 eV at the TB as compared to 1.23 eV in the bulk, suggesting the ease in the oxygen transport through the Ʃ3 twin boundaries. Overall, these results offer a critical understanding of the atomic arrangement at the twin boundaries in pyrochlores, leading to control of the interplay between defects and properties.

Tailoring the structural properties and electronic structure of anatase, brookite and rutile phase TiO2 nanoparticles: DFTB calculations

In this study, we perform a theoretical investigation using the density functional tight-binding (DFTB) approach for the structural analysis and electronic structure of anatase, brookite and rutile phase TiO2 nanoparticles (NPs). Our results show that the number of Ti-O bonds is greater than that of O-O, while the number of Ti-Ti bonds is fewer. Thus, large amounts of O atoms prefer to connect to Ti atoms. The increase in the temperature of the NPs contributes to an increase in the interaction of Ti–O bonding, but a decrease in the O-O bonding. The segregation of Ti and O atoms shows that Ti atoms tend to co-locate at the center, while O atoms tend to reside on the surface. Increasing temperature causes a decrease of the bandgap from 3.59 to 2.62 eV for the brookite phase, which is much more energetically favorable compared to the bulk, while it could increase the bandgap from 3.15 to 3.61 eV for anatase phase. For three-phase TiO2 NPs, LUMO and Fermi levels decrease. The HOMO level of anatase phase NP decreases, but it increases for brookite and rutile phase TiO2 nanoparticles. An increase in the temperature contributes to the stabilization of anatase phase TiO2 NP due to a decrease in the HOMO energies.

Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal-Assisted Intercalation.

Ti3C2T x (MXene) exhibits attractive properties in different applications. However, traditional synthesis leads to unsatisfactory yield of two-dimensional (2D) Ti3C2T x, e.g., lower than 20%, which stems from the strong interactions of potential Ti-Ti bonds and residual Ti-Al bonds between the adjacent Ti3C2 layers, hindering the effective intercalation and delamination. Herein, we propose a facile hydrothermal-assisted intercalation (HAI) strategy to boost the yield of 2D sheets, achieving a record high value of 74%. This HAI assists the diffusion and intercalation of reagent effectively, promoting the subsequent delamination; meanwhile, an antioxidant is applied to protect these Ti3C2T x from oxidation during the HAI process. Therefore, massive Ti3C2T x 2D sheets can be easily synthesized. Thanks to the synergistic effect of high conductivity and substantial terminated functionalities, these Ti3C2T x 2D sheets show promising application in supercapacitor, providing a high capacitance of 482 F g-1. Besides, the ultrafast carrier dynamics results of Ti3C2T x 2D sheets clearly imply the promising application in photocatalysis due to the relatively long bleaching relaxation time. Our work not only paves the way for the mass production of Ti3C2T x 2D sheets but also provides insights into their electronic and optical properties.

Synthesis, and Crystal and Electronic Structures, of the Titanium-Rich Bismuthides AE3Ti8Bi10 ( AE = Sr, Ba, Eu).

Three isotypic compounds with the chemical formula AE3Ti8Bi10 ( AE = Sr, Ba, Eu) have been obtained via both high-temperature solid state and flux growth reactions. Their crystal structure, representing a new type (space group P63/ mmc, Pearson symbol hP42), features an open framework composed of interlinked TiBi5 square pyramids and TiBi6 octahedra. The Ti-Bi substructure is penetrated by infinite columns of face-sharing AE6 polyhedra centered by Bi atoms. First-principle calculations and physical property measurements indicate metallic behavior and absence of localized magnetic moments on the Ti atoms. Analysis of the chemical bonding reveals strong Ti-Bi and Ti-Ti bonds. The latter demonstrate classic two-center, as well as multicenter, interactions.

Theoretical investigation on elastic and thermal properties of layered ternary ceramic-Ti4AlN2.89 under high pressure

Using pseudopotential plane-waves method, we calculated the structural, elastic properties and hardness for layered ternary ceramic Ti4AlN2.89 in the pressure range from 0 to 200 GPa. The elastic properties were derived to discuss the stability and bonding composition of Ti4AlN2.89 under high pressure. It is observed that this crystal is stable in our studied pressure range, and there are four typical bonds in Ti4AlN2.89. They are Ti-N bonds, Ti-Ti, Al-Al and Ti-Al bonds, and Ti-N bonds are much stronger than Ti-Ti, Al-Al and Ti-Al bonds. In addition, we investigated the bond stiffness for Ti4AlN2.89 under high pressure and found its damage tolerance and fracture toughness is improved at high pressure. Besides, we employed a semiempirical method considering the role of metallic components to evaluate the hardness of this compound at different pressures. Results showed that the Vickers hardness of Ti4AlN2.89 increases as the pressure is increased. What’s more, by calculating the Debye temperatures under high pressure, we found the melting point of Ti4AlN2.89 increases as the pressure is increasing. In a word, combined the results of mechanical properties with hardness and Debye temperature, we got the conclusion that the performance of Ti4AlN2.89 might be improved at a higher pressure.

Influencing Mechanism of Carbonyl and Thiophene on Cranial Nerve Based on Their Bridging and Steric Hindrance Effects on Titanium Coordination Compound

This paper aims to disclose the damage or repair mechanism of carbonylation and thiophene on cranial nerves. To this end, the bonding pattern of binuclear titanium carbonyls with thiophene was investigated by two methods of density functional theory (DFT), namely, the B3LYP method and the BP86 method. Then, 8 stable isomer structures were found for binuclear titanium carbonyls with thiophene, including 5 doublet structures and 3 triplet structures. These structures were discussed in details and the energy needed to dissociate each of them into mononuclear fragments was calculated through the thermochemistry analysis. The results show that the Ti2(C4H3S)m(CO)n structures are linked by 2.8~3.6 A-long Ti-Ti bonds. The bonds are relatively long and considered as formal single bonds. Every structure of the binuclear titanium carbonyls with thiophene is bridged by CO group or C4H3S group and sometimes dibridged by both the CO group and C4H3S group. The energy to dissociate Ti2(C4H3S)2(CO)6 into Ti(C4H3S)2(CO)2 and Ti (CO)4 fragments is 71.4kcal/mol (B3LYP)/ 89.6kcal/mol (P86), higher than that to dissociate the other Ti2(C4H3S)m(CO)n structures into Ti(C4H3S)i(CO)j fragments. The electron structure of the Ti atom in Ti2(C4H3S)2(CO)6 is 17-electron structure, the nearest to the 18-electron structure. The research findings provide important insights into the influencing mechanism of carbonyl and thiophene on caranial nerve.

Atomic-scale analysis of cation ordering in reduced calcium titanate

The phenomenon of cation ordering is closely related to certain physical properties of complex oxides, which necessitates the search of underlying structure-property relationship at atomic resolution. Here we study the superlattices within reduced calcium titanate single crystal micro-pillars, which are unexpected from the originally proposed atomic model. Bright and dark contrasts at alternating Ti double layers perpendicular to b axis are clearly observed, but show no signs in corresponding image simulations based on the proposed atomic model. The multi-dimensional chemical analyses at atomic resolution reveal periodic lower Ti concentrations at alternating Ti double layers perpendicular to b axis. The following in-situ heating experiment shows no phase transition at the reported Tc and temperature independence of the superlattices. The dimerization of the Ti-Ti bonds at neighboring double rutile-type chains within Ti puckered sheets are directly observed, which is found to be not disturbed by the cation ordering at alternating Ti double layers. The characterization of cation ordering of complex oxides from chemical and structural point of view at atomic resolution, and its reaction to temperature variations are important for further understanding their basic physical properties and exploiting potential applications.

Ductile and electronic properties of La-doped gamma-TiAl systems based on density functional theory

Because of the low density, high specific strength and excellent performance at high temperature, -TiAl based alloy has become a new generation of materials in the aeronautic field. However, its poor ductility at room temperature set a limitation to its wide applications. In this paper, the crystal structures, stabilities and ductilities of La-doped -TiAl systems are investigated by using first principles method based on density functional theory, in which Ti or Al is substituted by La and the impurity content values are 1.85 at.%, 2.78 at.%, 4.17 at.%, 6.25 at.%, 8.33 at.% and 12.5 at.%, respectively. The results show that all of the La-doped alloys have good energy stabilities, namely they can be prepared experimentally, when the impurity concentration x of system is less than or equal to 12.5 at.%. And the density of the La-doped system is less than 4.6 gcm-3. La doping induces the lattice parameters and the axial ratio of the alloy system to change. The axial ratio of La-doped system with low impurity concentration (x6.25 at.%) is closer to 1, which is very beneficial to improving the ductility of the materials. It is predicted that the system Ti11LaAl12 would have the best ductility among those of the investigated systems, for its axial ratio is the closest to 1. The electronic effect about the ductility of La-doped system is discussed through the comparisons of the populations, charge densities and densities between the states of systems Ti11LaAl12 and Ti12Al12. It is found that the system Ti11LaAl12 presents a state of electron redistribution in valence electron orbitals of Al and Ti due to an atom of titanium substituted with that of lanthanum. The charge numbers of Ti-d and Al-p orbitals and the numbers of electrons can be delocalized by reducing the p-d orbital hybridization. Thus, the intensity of p-d orbital hybridization is weakened, the resistance of dislocation movement is reduced, and the ductility of TiAl systems can be improved. Actually, the new electron redistribution shows different properties of some chemical bonds, in which some of covalent AlTi bonds are replaced by ionic AlLa bonds and some of covalent TiTi bonds are replaced by metallic TiLa bonds. Therefore, the covalent and directional properties of chemical bonds are reduced distinctly while the metallic properties of materials are strengthened. The average intensity of AlAl bonds decreases and those of AlTi and TiTi bonds are increased in the La-doped -TiAl system (Ti11LaAl12). As a result, the differences between the three kinds of chemical bonds diminish and the degree of isotropy of the crystal structure increases, which can greatly improve the ductility of -TiAl alloy.

First-principles investigation of oxygen diffusion mechanism in -titanium crystals

How impurity atoms move through a crystal is a fundamental and renewed issue in condensed matter physics and materials science. Diffusion of oxygen (O) in titanium (Ti) affects the formation of titanium-oxides and the design of Tibased alloys. Moreover, the kinetics of initial growth of titania-nanotubes via anodization of a titanium metal substrate also involves the diffusion of oxygen. Therefore, the understanding of the migration mechanism of oxygen atoms in -Ti is extremely important for controlling oxygen diffusion in Ti alloys. In this work, we show how the diffusion coefficient can be predicted directly from first-principles studies without any empirical fitting parameters. By performing the first-principles calculations based on the density functional theory (DFT) through using the Vienna ab initio Simulation Package (VASP), we obtain three locally stable interstitial oxygen sites in the hexagonal closed-packed (hcp) lattice of titanium. These sites are octahedral center (OC) site, hexahedral center (HE) site, and TiTi bond center crowdion (CR) site with interstitial energies of -2.83, -1.61, and -1.48 eV, respectively. From the interstitial energies it follows that oxygen atom prefers to occupy the octahedral site. From electronic structure analysis, it is found that the TiO bonds possess some covalent characteristics and are strong and stable. Using the three stable O sites from our calculations, we propose seven migration pathways for oxygen diffusion in hcp Ti and quantitatively determine the transition state and diffusion barrier with the saddle point along the minimum energy diffusion path by the climbing image nudged elastic band (CI-NEB) method. The microscopic diffusion barriers (E) from the first-principles calculations are important for quantitatively describing the temperature dependent diffusion coefficients D from Arrhenius formula D = L2v* exp(-((E)/(kBT)), where v* is the jumping frequency and L is the atomic displacement of each jump. The jumping frequency v* is determined from where vi and vj are the vibration frequency of oxygen atom at the initial state and the transition state respectively. This analysis leads to the formula for calculating the temperature dependent diffusion coefficient by using the microscopic parameters (vi and E) from first-principles calculations without any fitting parameters. Using the above formula and the vibration frequencies and diffusion barriers from first-principles calculations, we calculate the diffusion coefficients among different interstitial sites. It is found that the diffusion coefficient from the octahedral center site to the available site nearby is in good agreement with the experimental result, i.e., the diffusion rate D is 1.046510-6 m2s-1 with E of 0.5310 eV. The jump from the crowdion site to the octahedral interstitial site prevails over all the other jumps, as a result of its low energy barrier and thus leading to markedly higher diffusivity values. The diffusion of oxygen atoms is mainly controlled by the jump occurring between OC and CR sites, resulting in high diffusion anisotropy. This finding of oxygen diffusion behavior in Ti provides a useful insight into the kinetics at initial stage of oxidation in Ti which is very relevant to many technological applications of Ti-based materials.

Metal-Metal Bonding in Trinuclear, Mixed-Valence [Ti3X12](4-) (X = F, Cl, Br, I) Face-Shared Complexes.

Metal-metal bonding in structurally characterized In4Ti3Br12, comprising linear, mixed-valence d(1)d(2)d(1) face-shared [Ti3Br12](4-) units with a Ti-Ti separation of 3.087 Å and strong antiferromagnetic coupling (Θ = -1216 K), has been investigated using density functional theory. The antiferromagnetic configuration, in which the single d electron on each terminal Ti(III) (d(1)) metal center is aligned antiparallel to the two electrons occupying the central Ti(II) (d(2)) metal site, is shown to best agree with the reported structural and magnetic data and is consistent with an S = 0 ground state in which two of the four metal-based electrons are involved in a two-electron, three-center σ bond between the Ti atoms (formal Ti-Ti bond order of ∼0.5). However, the unpaired spin densities on the Ti sites indicate that while the metal-metal σ interaction is strong, the electrons are not fully paired off and consequently dominate the ground state antiferromagnetic coupling. The same overall partially delocalized bonding regime is predicted for the other three halide [Ti3X12](4-) (X = F, Cl, I) systems with the metal-metal bonding becoming weaker as the halide group is descended. The possibility of bond-stretch isomerism was also examined where one isomer has a symmetric structure with identical Ti-Ti bonds while the other is unsymmetric with one short and one long Ti-Ti bond. Although calculations indicate that the latter form is more stable, the barrier to interconversion between equivalent unsymmetric forms, where the short Ti-Ti bond is on one side of the trinuclear unit or the other, is relatively small such that at room temperature only the averaged (symmetric) structure is likely to be observed.

The Reductive Activation of CO2 Across a Ti=Ti Double Bond: Synthetic, Structural, and Mechanistic Studies.

The reactivity of the bis(pentalene)dititanium double-sandwich compound Ti2Pn†2 (1) (Pn† = 1,4-{SiiPr3}2C8H4) with CO2 is investigated in detail using spectroscopic, X-ray crystallographic, and computational studies. When the CO2 reaction is performed at −78 °C, the 1:1 adduct 4 is formed, and low-temperature spectroscopic measurements are consistent with a CO2 molecule bound symmetrically to the two Ti centers in a μ:η2,η2 binding mode, a structure also indicated by theory. Upon warming to room temperature the coordinated CO2 is quantitatively reduced over a period of minutes to give the bis(oxo)-bridged dimer 2 and the dicarbonyl complex 3. In situ NMR studies indicated that this decomposition proceeds in a stepwise process via monooxo (5) and monocarbonyl (7) double-sandwich complexes, which have been independently synthesized and structurally characterized. 5 is thermally unstable with respect to a μ-O dimer in which the Ti–Ti bond has been cleaved and one pentalene ligand binds in an η8 fashion to each of the formally TiIII centers. The molecular structure of 7 shows a “side-on” bound carbonyl ligand. Bonding of the double-sandwich species Ti2Pn2 (Pn = C8H6) to other fragments has been investigated by density functional theory calculations and fragment analysis, providing insight into the CO2 reaction pathway consistent with the experimentally observed intermediates. A key step in the proposed mechanism is disproportionation of a mono(oxo) di-TiIII species to yield di-TiII and di-TiIV products. 1 forms a structurally characterized, thermally stable CS2 adduct 8 that shows symmetrical binding to the Ti2 unit and supports the formulation of 4. The reaction of 1 with COS forms a thermally unstable complex 9 that undergoes scission to give mono(μ-S) mono(CO) species 10. Ph3PS is an effective sulfur transfer agent for 1, enabling the synthesis of mono(μ-S) complex 11 with a double-sandwich structure and bis(μ-S) dimer 12 in which the Ti–Ti bond has been cleaved.

New titanium carbonyls: Ti2(CO)10, Ti2(CO)11, and Ti2(CO)12.

Structural and energetic features of the binuclear titanium carbonyls Ti2(CO)n (n = 12, 11, 10) have been examined using density functional theory. The lowest-energy Ti2(CO)12 structure is a singlet structure consisting of two Ti(CO)6 units linked by Ti═Ti double bonds of lengths 3.0-3.2 Å. A similar slightly higher energy triplet Ti2(CO)12 structure is found with longer Ti-Ti bonds (3.37-3.62 Å), considered to be formal single bonds. The energy required for the dissociation of Ti2(CO)12 into two Ti(CO)6 fragments is 18.5 ± 2 kcal/mol higher than the energy required for the dissociation of V2(CO)12 into two V(CO)6 fragments. For the unsaturated Ti2(CO)11 system, the lowest-energy structures contain a four-electron-donor bridging η(2)-μ-CO group and 10 terminal CO groups with formal Ti═Ti double bonds in the singlet structures and formal Ti-Ti single bonds in the triplet structures. Similarly, the most favored geometries for the more highly unsaturated Ti2(CO)10 contain two four-electron-donor bridging η(2)-μ-CO groups with formal Ti═Ti double bonds for the singlet structures and formal Ti-Ti single bonds for the triplet structures. Higher-energy triplet Ti2(CO)10 structures are found with two or three two-electron-donor semibridging CO groups and formal Ti≡Ti triple bonds of length 2.7-2.8 Å.

Reductive deoxygenation of CO2 by a bimetallic titanium bis(pentalene) complex

The bimetallic bis(pentalene) complex (μ:η(5),η(5)-Pn(†))2Ti2 reductively splits CO2 to form a bis(oxo) bridged dimer [(η(8)-Pn(†))Ti(μ-O)]2, in which the Ti-Ti bond has been cleaved, and the dicarbonyl complex (μ:η(5),η(5)-Pn(†))2[Ti(CO)]2.