Mo-Mo Bond Research Articles

Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal.

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)3[PMo12O40] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo12/n products which were shown to be mixtures of electron-rich PMo12Lix species. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at n ∼ 8 and a significant structural change at n > 12. Successive MoVI reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one MoIV-MoIV bonded {MoIV3} triad together with MoV. Upon dissolution, the PMo12Lix species present in the solid PMo12/n products undergo electron exchange and single-peak 31P NMR spectra were observed for n = 1-12. For n ≥ 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with MoV-MoV and {MoIV3} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}+ and 2-electron-reduced PMo12 in (TBA)4[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n ≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.

Crystal structure of bis-[(η5-tert-butyl-cyclo-pentadien-yl)tri-carbonyl-molybdenum(I)](Mo-Mo).

The dinuclear mol-ecule of the title compound, [Mo2(C9H13)2(CO)6] or [Mo( t BuCp)(CO)3]2 where t Bu and Cp are tert-butyl and cyclo-penta-dienyl, is centrosymmetric and is characterized by an Mo-Mo bond length of 3.2323 (3) Å. Imposed by inversion symmetry, the t BuCp and the carbonyl ligands are in a transoid arrangement to each other. In the crystal, inter-molecular C-H⋯O contacts lead to the formation of layers parallel to the bc plane.

Skew trapezoidal bipyramidal distortion in MoS6 unit stabilizing distorted phases of 1T-MoS2 single layer

The exceptional electrocatalytic performance of the 1T phase of MoS2 in the hydrogen evolution reaction has motivated researchers to design methods for enhancing the stability of this phase. Herein, the electronic origin of the stability of 1T-MoS2 and its distorted phases: 1T' (zig-zag), 1T′′ (diamond chain), and 1T′′′ (triangle) was elucidated using first-principles calculations. The phase stability can be altered by the repeating MoS6 units that play a significant role in the generation of metal clusters. A novel skew trapezoidal bipyriamidal (STB) distortion was observed in the MoS6 unit, which increased the stability of the distorted 1T phases in MoS2. The highly distorted STB unit was found in the stable 1T' phase with an enhanced dyz orbital population. The short edges of the distorted STB increase the electron density fraction of the Mo atoms, promoting Mo-Mo bond formation. The 1T' phase exhibited superior stability due to stronger electron delocalization in the Mo-Mo bond compared to that in the 1T′′ and 1T′′′ phases. The nature of Mo-Mo bonding varied on different metal clusters for 1T' (σ, π, and δ), 1T′′ (π and σ), and 1T′′′ (σ) bond types. Therefore, the stability of the 1T'-MoS2 phase depends on the extent of distortion in the MoS6 unit, Mo-Mo bond nature, and layer thickness.

Analysis of chemical bonding of the ground and low-lying states of Mo2 and of Mo2Clx complexes, x = 2-10.

In this study, we perform accurate calculations via multireference configuration interaction and coupled cluster methodologies on the dimolybdenum molecule in conjunction with complete series of correlation and weighted core correlation consistent basis sets up to quintuple size. The bonding, the dissociation energies, and the spectroscopic parameters of the seven states that correlate with the ground state products are calculated. The ground state has a sextuple chemical bond, and each of the calculated excited states has one less bond than the previous state. The calculated values for the ground X1Σg + state of Mo2 have been extrapolated to the complete basis set limits. Our final values, re = 1.9324 Å and De (D0) = 4.502 ± 0.007(4.471 ± 0.009) eV, are in excellent agreement with the experimental values of re = 1.929, 1.938(9) Å and D0 = 4.476(10) eV. Mo2 in the Σg+13 state is a weakly bound dimer, forming 5s⋯5pz bonds, with De = 0.120eV at re = 3.53 Å. All calculated excited states (except Σg+13) have a highly multireference character (C0 = 0.25-0.55). The ordering of the molecular bonding orbitals changes as the spin is increased from quintet to septet state resulting in a change in energy separation ΔS,S-1 of the calculated states. The quite low bond dissociation energy of the ground state is due to the splitting of the molecular bonding orbitals in two groups differing in energy by ∼3eV. Finally, the bond breaking of Mo2, as the multiplicity of spin is increased, is analyzed in parallel with the Mo-Mo bond breaking in a series of Mo2Clx complexes when x is increased. Physical insight into the nature of the sextuple bond and its low dissociation energy is provided.

Traversing Double-Well Potential Energy Surfaces: Photoinduced Concurrent Intralayer and Interlayer Structural Transitions in XTe2 (X = Mo, W)

The microscopic arrangement of atoms and molecules is the determining factor in how materials behave and perform; i.e., the structure determines the property, a traditional paradigm in materials science. Photoexcitation-driven manipulation of the crystal structure and associated electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications; however, a generalized mechanism for such symmetry engineering is absent. Here, by ultrafast electron diffraction, structure factor calculation, and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phases of XTe2 (X = Mo, W). We discuss the modification of multiple quantum electronic states associated with the intralayer and interlayer structural transitions, such as the topological band inversion and the higher-order topological state. The twin structures and the stacking faults in XTe2 are also identified by ultrafast structural responses. The comprehensive study of the ultrafast structural response in XTe2 suggests the traversal of all double-well potential energy surfaces (DWPES) by laser excitation, which is expected to be an intrinsic mechanism in the field of photoexcitation-driven global/local symmetry engineering and also a critical ingredient inducing the exotic properties in the non-equilibrium state in a large number of material systems.

Tuning catalytic activity of dimolybdenum paddlewheel complexes by ligands: mechanism study on the radical addition reaction of CCl4 to 1-hexene

The detailed catalytic mechanism of a series of paddlewheel complexes [Mo2L4] featuring Mo-Mo quadruply-bond on radical addition of CCl4 to 1-hexene was studied using density functional theory. Different ligands of Mo-Mo bond are investigated to illustrate the ligand effect on the catalytic activity. The results show that the Mo-Mo quadruply-bond paddlewheel complexes have high catalytic activities on the title reaction. The whole reaction involves 4 steps. Firstly, the C-Cl bond of first CCl4 is activated by [Mo2L4] catalyst, and [Mo2L3Cl] and CH3COOCCl3 are obtained. Then the second CCl4 adds to [Mo2L3Cl] to produce [Mo2L3Cl2] and·CCl3 radical;·CCl3 radical interacts with 1-hexene to get an addition, the addition product which reacts with one Cl atom of [Mo2L3Cl2] to get the last product nBuCHClCH2CCl3 and regenerate [Mo2L3Cl]. The addition of the first CCl4 to [Mo2L4] catalyst is the rate-determining step of the whole reaction. Because this step is not in the catalytic cycle, the reaction would speed up after a certain period of time. The catalytic activity of dimolybdenum paddlewheel complex is depended on the natural population analysis (NPA) charge of Mo and the redox potential E(Mo24+/Mo25+). The higher NPA of Mo atom and higher E(Mo24+/Mo25+) of the catalyst, the higher catalytic activity it has. Our results provide an explanation for experimental observations and useful insights for further development of bimetallic catalysts in radical addition reactions.

From semi-metal to superconductor: The New Mo12 Cluster Sulfides Rb1+xMo12S14 (x = 0.08, and 1.65)

The novel Mo12 cluster sulfides Rb2.65Mo12S14 have been synthesized by direct solid-state reaction at a temperature of about 1500 °C in a welded Mo tubing. The crystal structure of Rb2.65Mo12S14 (space group P-31c: a = 9.2828(6) Å, c = 16.446(5) Å) was solved from X-ray single-crystal data. It is isomorphous with the K1+xMo12S14 compounds in which x ranges from 0 to 1.6. Consequently, the Mo-S network presents interconnected Mo12S14 cluster units delimiting large tunnels that are randomly occupied by a part of the Rb+ ions, the other ones being localized in large voids between two consecutive Mo12S14 units along the [001] direction. By topochemical redox reactions at around 90 °C, we could remove a part of the Rb+ ions from the channels and thus, form the oxidized phase Rb1.08Mo12S14. Electrical resistivity measurements performed on single crystals of Rb2.65Mo12S14 and Rb1.08Mo12S14 shows that the oxidized form is a semimetal while the reduced form is a metal and becomes superconducting below 4.07 K. The evolution of Mo-Mo bond lengths within the trioctahedral Mo12 cluster with respect to the cationic charge x is discussed and, compared with that found in the potassium series.

Cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery

Anion redox chemistry is an essential component of many high energy density electrode materials, while accompanying with voltage hysteresis and fade, which currently hinders its widespread use. Here we demonstrate cation-synergy stabilizing anion redox of Chevrel phase Mo6S8 in aluminum ion battery. EELS and XAS reveal that S is fully reduced, and Mo6 cluster is firstly oxidized and then reduced with Al3+ ions insertion, which originates from the contraction and elongation of Mo-Mo bond in Mo6 cluster verified by atomic-resolved imaging. DFT calculations uncover that the energy level of [Mo-Mo]* antibonding orbitals is lifted and fell by Mo-Mo bond evolution, resulting in the synergetic cationic and anionic redox, which contributes to the stable structure upon cycling. Our work figures out the electrochemical redox mechanism of Mo6S8 in aluminum ion batteries and provides implications generally for the design of materials employing anion redox chemistry.

Hydrothermal Synthesis and Structure of a Dinuclear Molybdenum(III) Hydroxy Squarate with a Mo-Mo Bond.

The reaction of molybdenum(II) and chromium(II) acetates with squaric acid in degassed and deionized water under hydrothermal conditions at 150 °C is described. The products have been formulated as M2(μ-OH)2(μ-C4O4)2(H2O)4·2H2O, where M = Cr (1) and Mo (2), based on combustion elemental analysis, infrared spectroscopy, magic angle spinning (MAS) solid-state carbon-13 nuclear magnetic resonance (NMR), and single-crystal X-ray diffraction. The edge-shared bioctahedral structures involve doubly bridging hydroxide ligands and μ-squarate ligands. The chromium compound lacks a direct metal–metal-bonding interaction, while in contrast the molybdenum compound contains a Mo–Mo bond [2.491(2) Å]. The nature of the Mo–Mo-bonding interaction is compared with that of other similar d3–d3 dimers.

Charge Distribution on S and Intercluster Bond Evolution in Mo6S8 during the Electrochemical Insertion of Small Cations Studied by X-ray Absorption Spectroscopy.

Mo6S8 is regarded as a promising cathode material in rechargeable Mg batteries. Despite extensive studies, some fundamental questions are still unclarified, including the origination of the chemical stability, key factors inducing the structural evolution, and the factors determining the electrochemical reversibility. Herein Mo L2,3 and S K-edge X-ray absorption spectroscopy are utilized to uncover the underlying mechanism. Two kinds of S with different effective charge are found, indicating the nonuniform charge distribution. With one cation inserted, the charge distribution becomes homogeneous, relevant to the chemical stability and electrochemical reversibility. The structural evolution is attributed to the change of bond length induced by the delocalization of inserted cations. Moreover, the evolution of intercluster Mo-Mo bond length can be revealed by the drastic change of the S K pre-edge and is closely related to the electrochemical reversibility. This study can shed light on the aforementioned questions and guide the development of Mg cathode material.

Mapping Bridge Conformational Effects on Electronic Coupling in Mo2-Mo2 Mixed-Valence Systems.

The large bridging ligand 9,10-anthracenedicarboxylate and its thiolated derivatives have been employed to assemble two dimolybdenum complex units and develop three Mo2 dimers, [Mo2(DAniF)3]2(μ-9,10-O2CC14H8CO2), [Mo2(DAniF)3]2(μ-9,10-OSCC14H8COS), and [Mo2(DAniF)3]2(μ-9,10-S2CC14H8CS2) (DAniF = N, N'-di( p-anisyl)formamidinate), for the study of conformation dependence of the electronic coupling between the two Mo2 centers. These compounds feature a large deviation of the central anthracene ring from the plane defined by the Mo-Mo bond vectors, with the torsion angles (ϕ = 50-76°) increasing as the chelating atoms of the bridging ligand vary from O to S. Consequently, the corresponding mixed-valence complexes do not exhibit characteristic intervalence charge transfer absorptions in the near-IR spectra, in contrast to the phenylene and naphthalene analogues, from which these systems are assigned to the Class I in Robin-Day's scheme. Together with the phenylene and naphthalene series, the nine total mixed-valence complexes in three series complete a transition from the electronically uncoupled Class I to the strongly coupled Class II-III borderline via moderately coupled Class II and permit a systematic mapping of the bridge conformation effects on electronic coupling. Density functional theory calculations show that the HOMO-LUMO energy gap, corresponding to the metal (δ) to ligand (π*) transition energy, and the magnitude of HOMO-HOMO-1 splitting in energy are linearly related to cos2 ϕ. Therefore, our experimental and theoretical results concur to indicate that the coupling strength decreases in the order of the bridging units: phenylene > naphthalene > anthracene, which verifies the through-bond superexchange mechanism for electronic coupling and electron transfer.

Aromaticity-Driven Molecular Structural Variation and Electronic Configuration Alternation: An Example of Cyclic π Conjugation Involving a Mo-Mo δ Bond.

We have synthesized and characterized the mixed-ligand dimolybdenum paddlewheel complex Na[(DAniF)3Mo2(C3S5)] (Na[1]; DAniF = N,N'-di-p-anisylformamidinate, dmit = 1,3-dithiole-2-thione-4,5-dithiolate), which has a six-membered chelating [Mo2S2C2] ring created by equatorial coordination of the dmit (C3S5) ligand to the Mo2 unit. One-electron oxidation of Na[1] using Cp2FePF6 yields the neutral complex [(DAniF)3Mo2(C3S5)] ([1]), and removal of two electrons from Na[1] using AgBPh4 gives [(DAniF)3Mo2(C3S5)]BPh4 ([1]BPh4). In the crystal structures, [1]- and [1] present dihedral angles of 118.9 and 142.3° between the plane defined by the Mo-Mo bond vector and the dmit ligand, respectively, while DFT calculations show that in [1]+ the Mo-Mo bond and the dmit ligand are coplanar. Complex [1] is paramagnetic with a g value of 1.961 in the EPR spectrum and has a Mo-Mo bond distance of 2.133(1) Å, increased from 2.0963(9) Å for [1]-. Consistently, a broad absorption band is observed for [1] in the near-IR region, which arises from charge transfer from the dmit ligand to the cationic Mo25+ centers. Interestingly, complex [1]+ has an aromatic [Mo2S2C2] core, as evidenced by a large diamagnetic anisotropy, in addition to the coplanarity of the core structure, which shifts downfield the 1H NMR signal of the horizontal methine proton (ArN-(CH)-NAr) but upfield those of the vertical protons, relative to the methine proton resonances for the precursor ([1]-). The magnetic anisotropy (Δχ = χ⊥ - χ∥) for the [Mo2S2C2] ring in [1]+ is -105.5 ppm cgs, calculated from the McConnell equation, which is about 2-fold larger than that for benzene. The aromaticity of the [Mo2S2C2] ring is supported by theoretical studies, including single-point calculations and gauge-including atomic orbital (GIAO) NMR spectroscopic calculations at the density functional theory (DFT) level. DFT calculations also show that the [Mo2S2C2] core in [1]+ possesses a set of three highest occupied and three lowest unoccupied molecular orbitals in π character, corresponding to those of benzene in symmetry, and six π electrons that conform to the Hückel 4n + 2 rule for aromaticity. Therefore, this study shows that an aromatic [Mo2S2C2] core is formed by coupling the δ orbital of the Mo≣Mo bond with the π orbital of the C═C bond through the bridging atoms (S), thus validating the equivalency in bonding functionality between δ and π orbitals.

First-principles investigation of new structure, mechanical and electronic properties of Mo-based silicides

The adjustment of strength and ductility of high-temperature ceramics is still a big challenge. Although Mo-based silicides are promising high-temperature materials, the influence of Mo concentration on the mechanical and electronic properties of Mo-based silicides is unclear. In addition, it is necessary to explore the novel Mo-based silicides. In this paper, we present results of novel phases, mechanical and electronic properties of the stable Mo-based silicides within various stoichiometries. Two new Mo-based silicides: MoSi (Cmcm and Pnma) and Mo2Si (I4/mcm) are predicted. The calculated results show that the volume deformation resistance of Mo-based silicides increases with increasing Mo concentration. MoSi2 shows the strongest elastic stiffness and shear deformation resistance due to the strong Mo-Si bonds. The calculated intrinsic hardness of MoSi2 (37.7GPa) is much larger than that of other Mo-based silicides. In particular, MoSi2 and MoSi show brittle behavior. However, other silicides exhibit ductility. We further find that high concentration of Mo can improve the electronic properties of Mo-based silicides because of the formation of Mo-Mo metallic bond. Finally, our works indicate that the adjustment of the Mo stoichiometric ratio to improve the mechanical and electronic properties of Mo-based silicides.

Investigation of Structural Changes in Carbon-Coated Li2MoO3-LiCrO2 Cathode Material By Synchrotron-Based X-Ray Techniques

In recent years, layered Li2MoO3 has attracted considerable attention as a cathode material for Li-ion batteries due to its high theoretical capacity (339 mAh g-1), the multiple electron transfer capability of Mo4+/6+ without oxygen release and the unique structure evolution behavior during electrochemical cycling [1,2]. In 2014, Zhou et al. studied the unusual expansion of a(b) lattice parameters during delithiation of Li2MoO3 and found its origin from the expansion of Mo-Mo bond that is not the case of conventional LiMO2 (M=Ni, Co, Mn) layered cathode materials in which the a(b) parameters shrink due to contraction of redox active ionic radii [2]. Li2MoO3 exhibited much smaller c lattice expansion than typical layered cathode materials during charge and did not collapse at all even at high voltage of 4.8V. These were explained on the basis of stabilizing effect of Mo ions in the Li layers. It is known that some amount of cation mixing occurs in Li2MoO3 during delithiation. Meanwhile, one important point is that such structural change behavior can be varied in some materials where Li2MoO3 is incorporated with other components, for example, in the case of Li2MoO3-LiM’O2 (M’=transition metal, etc.) system or Li2MoO3/C composite [3,4,5]. It has been reported that some of these Li2MoO3 related materials more easily disordered than pure Li2MoO3 to form similar to cation disordered rock salt structure [4,5]. Carbon coated Li2MoO3-LiCrO2 (0.7Li4/3Mo2/3O2-0.3LiCrO2; LMCO/C) was first reported by J. Lee et al. in 2014 [4]. It was found that this material has unique structure evolution behavior in which the original layered structure transforms to a disordered rock salt-like structure in the first cycle. Despite of severe cation mixing, LMCO/C showed high and stable capacity over 260mAh/g during the initial 10 cycles in voltage range of 1.5 - 4.3V. Interestingly, very small variation of lattice parameter was observed during delithiation in disordered LMCO/C (Volume change less than 1%). Small volume change can be beneficial in terms of structural stability by introducing less stress along the cathode particles during repeated cycling. Such small volume change implies that the unit cell dimensional change of LMCO/C is not simply affected by the oxidation of transition metal ions but strongly correlated with the local structure including Mo-Mo bonding. In this study, we revisited LMCO/C cathode material and performed systematic analysis using synchrotron radiation based characterization techniques in order to better understand the detailed redox mechanism and structure evolution. By utilizing combination of XRD and XAS (XANES and EXAFS analysis), we demonstrate that how each transition metal and their local environments affect the overall structural evolution during charge/discharge process. Figure 1 shows the variation of XRD patterns with corresponding voltage profiles during first cycle and second charge process (C/20, 1.4 - 4.3V). It can be seen that disappearing of (003) peak and merging of (018), (110) peaks during first charge are almost irreversible. This indicates that most of the transition metal cations migrating into the Li sites during charge do not return to their original sites during discharge. However, as shown in Figure 2, reversible changes in the Mo K-edge EXAFS spectra indicate that reversible reaction processes exist in local level during electrochemical cycling. The changes in Mo-O peaks during charge indicate that Mo-O6 octahedra are significantly distorted during first charge. These are accompanied by severe cation mixing. During successive discharge/charge, Mo-O6 octahedra restored similar to the original state and distorted again whereas bulk structure remains disordered. From these results, we will discuss structure evolution behavior of cation disordered LMCO/C cathode material. More detailed results and discussion including reaction process of both Cr and Mo in LMCO/C will be presented in the meeting. [1] J. Ma, Y. N. Zhou, Y. R. Gao, X. Q. Yu, Q. Y. Kong, L. Gu, Z. X.Wang, X. Q. Yang, and L. Q. Chen, Chem. Mater., 26, 3256 (2014). [2] Y. N. Zhou, J. Ma, E. Y. Hu, X. Q. Yu, L. Gu, K. W. Nam, L. Q. Chen, Z. X. Wang, and X. Q. Yang, Nat. Commun., 5 (2014). [3] K. S. Park, D. Im, A. Benayad, A. Dylla, K. J. Stevenson and J. B. Goodenough, Chem. Mater., 24, 2673 (2012). [4] J. Lee, A. Urban, X. Li, D. Su, G. Hautier, and G. Ceder, Science, 343, 519 (2014). [5] S. Kumakura, Y. Shirao, K. Kubota, and S. Komaba, Phys. Chem. Chem. Phys., 18, 28556 (2016). Figure 1

Structural and Chemical Effects of the PtBu2 Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and Hydrocarbyl Ligands.

A high-yield synthetic route for the preparation of the unsaturated anion [Mo2Cp2(μ-PtBu2)(μ-CO)2]- (2) was implemented, via two-electron reduction of the chloride complex [Mo2Cp2(μ-Cl)(μ-PtBu2)(CO)2] (1). Reaction of 2 with [NH4][PF6] led to the formation of the 30-electron complex [Mo2Cp2(H)(μ-PtBu2)(CO)2] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σ2δ2δ2 configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCH2Ph gave the alkyl-bridged complexes [Mo2Cp2(μ-κ1:η2-CH2R)(μ-PtBu2)(CO)2] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [Mo2Cp2(μ-κ1:η2-CH3)(μ-PtBu2)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [Mo2Cp2(μ-CPh)(μ-PtBu2)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [Mo2Cp2(H)(μ-CHPh)(μ-PtBu2)(CO)] (10), a product which evolves slowly by H2 elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [Mo2Cp2(μ-CH)(μ-PtBu2)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central Mo2PC rings of these molecules.

Mo-Mo Quintuple Bond is Highly Reactive in H-H, C-H, and O-H σ-Bond Cleavages Because of the Polarized Electronic Structure in Transition State.

The recently reported high reactivity of the Mo-Mo quintuple bond of Mo2(N∧N)2 (1) {N∧N = μ-κ2-CH[N(2,6-iPr2C6H3)]2} in the H-H σ-bond cleavage was investigated. DFT calculations disclosed that the H-H σ-bond cleavage by 1 occurs with nearly no barrier to afford the cis-dihydride species followed by cis-trans isomerization to form the trans-dihydride product, which is consistent with the experimental result. The O-H and C-H bond cleavages by 1 were computationally predicted to occur with moderate (ΔG°⧧ = 9.0 kcal/mol) and acceptable activation energies (ΔG°⧧ = 22.5 kcal/mol), respectively, suggesting that the Mo-Mo quintuple bond can be applied to various σ-bond cleavages. In these σ-bond cleavage reactions, the charge-transfer (CTMo→XH) from the Mo-Mo quintuple bond to the X-H (X = H, C, or O) bond and that (CTXH→Mo) from the X-H bond to the Mo-Mo bond play crucial roles. Though the HOMO (dδ-MO) of 1 is at lower energy and the LUMO + 2 (dδ*-MO) of 1 is at higher energy than those of RhCl(PMe3)2 (LUMO and LUMO + 1 of 1 are not frontier MO), the H-H σ-bond cleavage by 1 more easily occurs than that by the Rh complex. Hence, the frontier MO energies are not the reason for the high reactivity of 1. The high reactivity of 1 arises from the polarization of dδ-type MOs of the Mo-Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the dδ-MO of the Mo-Mo bond and the σ*-antibonding MO of the X-H bond to facilitate the CTMo→XH and reduce the exchange repulsion between the Mo-Mo bond and the X-H bond. This polarized electronic structure of the transition state is similar to that of a frustrated Lewis pair. The easy polarization of the dδ-type MOs is one of the advantages of the metal-metal multiple bond, because such polarization is impossible in the mononuclear metal complex.

Thermodynamic calculations and dynamics simulation on thermal-decomposition reaction of MoS2 and Mo2S3 under vacuum

In this paper, thermal decomposition process of molybdenum concentrate in vacuum was thermodynamically analyzed, and decomposition mechanism of MoS system in molybdenum was mainly studied. To further investigate and analyze the electronic properties in different temperature, the simulation calculations of MoS2(0 0 1) surface and Mo2S3(0 0 1) surface density of states, electron density difference, electronic orbital and Mulliken overlap population were carried out by density functional theory(DFT) formalism. The dynamics simulations results of MoS2(0 0 1) surface and Mo2S3(0 0 1) surface were obtained the temperature range of phase transforming are 1573 K–1673 K and from 1473 K to 1573 K at 20Pa, respectively. The interaction of MoS2 and Mo2S3 shows that thermal-decomposition of MoS2 and Mo2S3 does react, when new SS bonds and MoMo bond are formed instead of MoS bond fracture. The experimental results of the thermal decomposition of MoS2 and Mo2S3 were in accordance with the thermodynamics theoretical calculation results and ab-initio molecular dynamics simulation results.

Reactivity of Cyclopentadienyl Molybdenum Compounds towards Formic Acid: Structural Characterization of CpMo(PMe3)(CO)2H, CpMo(PMe3)2(CO)H, [CpMo(μ-O)(μ-O2CH)]2, and [Cp*Mo(μ-O)(μ-O2CH)]2.

The molecular structures of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H have been determined by X-ray diffraction, thereby revealing four-legged piano-stool structures in which the hydride ligand is trans to CO. However, in view of the different nature of the four basal ligands, the geometries of CpMo(PMe3)(CO)2H and CpMo(PMe3)2(CO)H deviate from that of an idealized four-legged piano stool, such that the two ligands that are orthogonal to the trans H-Mo-CO moiety are displaced towards the hydride ligand. While CpRMo(PMe3)3-x(CO)xH (CpR = Cp, Cp*; x = 1, 2, 3) are catalysts for the release of H2 from formic acid, the carbonyl derivatives, CpRMo(CO)3H, are also observed to form dinuclear formate compounds, namely, [CpRMo(μ-O)(μ-O2CH)]2. The nature of the Mo···Mo interactions in [CpMo(μ-O)(μ-O2CH)]2 and [Cp*Mo(μ-O)(μ-O2CH)]2 have been addressed computationally. In this regard, the two highest occupied molecular orbitals of [CpMo(μ-O)(μ-O2CH)]2 correspond to metal-based δ* (HOMO) and σ (HOMO-1) orbitals. The σ2δ*2 configuration thus corresponds to a formal direct Mo-Mo bond order of zero. The preferential occupation of the δ* orbital rather than the δ orbital is a consequence of the interaction of the latter orbital with p orbitals of the bridging oxo ligands. In essence, lone-pair donation from oxygen increases the electron count so that the molybdenum centers can achieve an 18-electron configuration without the existence of a Mo-Mo bond, whereas a Mo═Mo double bond is required in the absence of lone-pair donation.

Contrasting the group 6 metal-metal bonding in sodium dichromate(ii) and sodium dimolybdate(ii) polymethyl complexes: synthetic, X-ray crystallographic and theoretical studies.

Extending the class of group 6 metal-metal bonded methylate compounds supported by alkali metal counter-ions, the first sodium octamethylmolybdate(ii) complex [(TMEDA)Na]4Mo2Me8 and heptamethylchromate(ii) relations [(donor)Na]3Cr2Me7 (donor is TMEDA or TMCDA) are reported. The former was made by treating [(Et2O)Li]4Mo2Me8 with four equivalents of NaOtBu/TMEDA in ether; whereas the latter resulted from introducing TMEDA or TMCDA to ether solutions of octamethyldichromate [(Et2O)Na]4Cr2Me8. X-ray crystallography revealed [(TMEDA)Na]4Mo2Me8 is dimeric with square pyramidal Mo centres [including a short Mo-Mo interaction of 2.1403(3) Å] each with four methyl groups in a mutually eclipsed conformation. In dinuclear [(TMCDA)Na]3Cr2Me7 trigonal bi-pyramidal Cr centres each bond to three terminal methyl groups and one common Me bridge, that produces a strikingly short Cr-Cr contact of 1.9136(4) Å. Broken symmetry density functional theoretical calculations expose the multiconfigurational metal-metal bonding in these compounds with a Mo-Mo bond order of 3 computed for octamethylmolybdate(ii). This is contrasted by the single Cr-Cr bond in heptamethylchromate(ii) where the singlet ground state is derived by strong antiferromagnetic coupling between adjacent metal ions.

An Unsaturated Four-Coordinate Dimethyl Dimolybdenum Complex with a Molybdenum-Molybdenum Quadruple Bond.

We describe the synthesis and the molecular and electronic structures of the complex [Mo2 Me2 {μ-HC(NDipp)2 }2 ] (2; Dipp=2,6-iPr2 C6 H3 ), which contains a dimetallic core with an Mo-Mo quadruple bond and features uncommon four-coordinate geometry and has a fourteen-electron count for each molybdenum atom. The coordination polyhedron approaches a square pyramid, with one of the molybdenum atoms nearly co-planar with the basal square plane, in which the trans coordination position with respect to the Mo-Me bond is vacant. The other three sites are occupied by two trans nitrogen atoms of different amidinate ligands and the methyl group. The second Mo atom occupies the apex of the pyramid and forms an Mo-Mo bond of length 2.080(1) Å, consistent with a quadruple bond. Compound 2 reacts with tetrahydrofuran (THF) and trimethylphosphine to yield the mono-adducts [Mo2 Me(μ-Me){μ-HC(NDipp)2 }2 (L)] (3⋅THF and 3⋅PMe3 , respectively) with one terminal and one bridging methyl group. In contrast, 4-dimethylaminopyridine (dmap) forms the bis-adduct [Mo2 Me2 {μ-HC(NDipp)2 }2 (dmap)2 ] (4), with terminally coordinated methyl groups. Hydrogenolysis of complex 2 leads to the bis(hydride) [Mo2 H2 {μ-HC(NDipp)2 }2 (thf)2 ] (5⋅THF) with elimination of CH4 . Computational, kinetic, and mechanistic studies, which included the use of D2 and of complex 2 labelled with 13 C (99 %) at the Mo-CH3 sites, supported the intermediacy of a methyl-hydride reactive species. A computational DFT analysis of the terminal and bridging coordination of the methyl groups to the Mo≣Mo core is also reported.