Synthesis, Structure, and Bonding Nature of Ethynediyl-Bridged Bis(silylene) Dinuclear Complexes of Tungsten and Molybdenum

Techniques in the synthesis of organometallic compounds of tungsten

Tungsten is an elegant substance, and its compounds have great significance because of their extensive range of applications in diverse fields such as in gas sensors, photocatalysis, lithium ion batteries, H2production, electrochromic devices, dyed sensitized solar cells, microchip technology, and liquid crystal displays. Tungsten compounds exhibit a more efficient catalytic behavior, and tungsten-dependent enzymes generally catalyze the transfer of an oxygen atom to or from a physiological donor/acceptor with the metal center. Furthermore, tungsten has an n-type semiconductor band gap. Tungsten forms complexes by reacting with several elements such as H, C, N, O, and P as well as other numerous inorganic elements. Interestingly, all tungsten reactions occur at ambient temperature, usually with tetrahydrofuran and dichloromethane under vacuum. Tungsten has extraordinarily high-temperature properties, making it very useful for X-ray production and heating elements in furnaces. Tungsten coordinates with diverse nonmetallic elements and ligands and produces interesting compounds. This article describes an overview of the synthesis of various organometallic compounds of tungsten.

Enhancement of the Catalytic Activities of Heteronuclear Bimetallic Cations for the C-H Bond Activation of Cyclohexane

Heterometallic cations NiCu⁺ and CoNi⁺ can easily induce triple dehydrogenation of cyclohexane with high yield, and monometallic cations Ni⁺ and Co⁺ only give rise to double dehydrogenation with low yield. Reaction mechanisms of the six C-H bond activations for cyclohexane are systematically investigated by comparing the difference between bimetallic cations and monometallic ones. Fragment molecular orbital analysis clearly indicates that charge transfer (CT) occurs from the occupied interacting orbital of the metallic cation to the σ*-antibonding orbital of the first, third, and fifth activated C-H bonds in transition states. The synergistic effects of heteronuclear bimetallic cations result in the destabilization of the occupied interacting orbital in bimetallic cations, which raise the reactivity of bimetallic cations and enhance the CT between catalysts and substrates. Contrary to the absence of the third dehydrogenation product in the mononuclear metallic cation catalytic reaction, a significant amount of the third dehydrogenation product is observed in the presence of heteronuclear cations (NiCu⁺ and CoNi⁺). π back-bonding between Ni of heteronuclear metallic cations and the substrate cyclohexadiene plays an essential role in lowering the energies of transition states, which accelerate the third dehydrogenation. The reasons why heteronuclear bimetallic cations are more reactive than monometallic ones are discussed in detail.

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 (CT_Mo→XH) from the Mo-Mo quintuple bond to the X-H (X = H, C, or O) bond and that (CT_XH→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 CT_Mo→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.

Phosphinosilylenes as a novel ligand system for heterobimetallic complexes

The first heterobimetallic complexes comprising interconnected silylene and phosphine donors are reported. In a stepwise fashion, first the silylene coordinates to iron and subsequently the phosphine coordinates to tungsten. Another heterobimetallic complex can be obtained by the insertion of platinum into the P–H bond.