Guomei He

Progress in the synthesis and reactivity studies of metallabenzenes

Metallabenzenes are metallacyclohexatriene complexes that are derived by replacement of one of the CH groups in benzene with a transition-metal fragment. Their synthesis and aromatic properties have recently attracted considerable attention. This paper summarizes the progress in the synthesis, aromatic property, and reactivity studies of metallabenzenes. We will describe the synthesis and reactivity of typical metallabenzenes with various metals (e.g. osmium, iridium, and ruthenium, etc.), the synthesis and structure of a recent reported example of metallanaphthalene and several rare examples of novel and stable metallabenzynes. Finally, the possible future developments in this field have also been suggested.

Synthesis, Characterization and Electrochemical Properties of Stable Osmabenzenes Containing PPh3 Substituents

Treatment of [OsCl(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond]CH/PPh(3) produces the osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)][OH] (2), which is air stable in both solution and solid state. The key intermediate of the one-pot reaction, [OsCl(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (3), and the related complex [Os(NCS)(2){CHC(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (7) have been isolated and characterized, further supporting the proposed mechanisms for the reaction. Reactions of 3 with PPh(3), NaI, and NaSCN give osmabenzene 2, iodo-substituted osmabenzene [Os{CHC(PPh(3))CHCICH}I(2)(PPh(3))(2)] (4), and thiocyanato-substituted osmabenzene [Os{CHC(PPh(3))CHC(SCN)CH}(NCS)(2)(PPh(3))(2)] (5) respectively. Similarly, reaction of [OsBr(2)(PPh(3))(3)] with HC[triple bond]CCH(OH)C[triple bond] CH in THF produces [OsBr(2){CH=C(PPh(3))CH(OH)C[triple bond]CH}(PPh(3))(2)] (9), which reacts with PPh(3)/Bu(4)NBr to give osmabenzene [Os{CHC(PPh(3))CHC(PPh(3))CH}Br(2)(PPh(3))(2)]Br (10). Ligand substitution reactions of 2 produce a series of new stable osmabenzenes 11-17. An electrochemical study shows that osmabenzenes 2, 12, and 14-17 have interesting different electrochemical properties due to the different co-ligand. The oxidation potentials of complexes 2, 12, 16, and 17 with Cl, NCS, and N(CN)(2) ligands gradually positively shift in the sequence of Cl<NCS<N(CN)(2). Among the six compounds, only 12 and 17 undergo a well-behaved, nearly reversible and a quasi-reversible reduction process, respectively, indicating that two NCS or N(CN)(2) ligands contribute to the stabilization of their reduced states.

Effects of substituents on the formation of rhenium carbyne and η2-vinyl complexes from the reactions of ReH5(PMe2Ph)3 with terminal alkynes

The reactions of the rhenium polyhydride complex ReH5(PMe2Ph)3 with HCCAr (Ar = p-C6H4CF3, o-C6H4CF3, o-C6H4Me and o-C6H4Br) have been carried out. Treatment of ReH5(PMe2Ph)3 with HCC(p-C6H4CF3) in the presence of two equiv. of HCl produces a mixture of the carbyne complex Re(CCH2(p-C6H4CF3))Cl2(PMe2Ph)3 and the η2-vinyl complex Re(η2-CH2C(p-C6H4CF3))Cl2(PMe2Ph)3. Treatment of ReH5(PMe2Ph)3 with HCCAr (Ar = o-C6H4CF3, o-C6H4Me, o-C6H4Br) in the presence of two equiv. of HCl produces only the carbyne complexes Re(CCH2Ar)Cl2(PMe2Ph)3. The mechanism of the reactions of ReH5(PMe2Ph)3 with HCCR has also been studied with the aid of DFT calculations, which suggests that the reactions are kinetically controlled and the pathways leading to the carbyne and η2-vinyl complexes are influenced by the steric and electronic effects of the substituents of the alkynes.

Fine-diameter microwave-absorbing SiC-based fiber

A fine-diameter silicon carbide (SiC) fiber is a promising reinforcing fiber for ceramic matrix composites with high temperature applications because of its high tensile strength and oxidation resistance. However, functional SiC fibers with microwave-absorbing properties are rarely reported. In this work, we report a new microwave-absorbing SiC-based fiber made from a new polymer containing titanium and boron (Si–C–Ti–B polymer) using a polymer precursor route. The evolution in morphology, microstructure and phase are studied by FT-IR, XRD, SEM, and TEM during the conversion of the polymer fiber into a ceramic fiber. The results show that the polymer fibers covert into inorganic fibers above 900 °C and maintain an amorphous state up to 1300 °C. The effect of pyrolysis-temperature (900–1300 °C) on the tensile strength, dielectric properties and microwave-absorption are also studied. The highest tensile strength of the obtained fiber is 1.2 GPa when produced at 1200 °C. The calculated reflection coefficient of the SiC-based fiber pyrolysed at 1200 °C with a thickness of 3.48 mm is less than −10 dB at the X band (8.2–12.4 GHz), which reveals that the obtained ceramic fiber has the potential to be a microwave-absorbing material. This study not only offers a new polymer precursor for new SiC-based fibers, but also provides a functional thermo-structural material.