Transition-metal coordination mechanisms temperature

Transition metal coordination of Cu(II) carboxylate groups and pyridine groups was employed as a means of coupling a telechelic butadiene-base polymer with a randomly functionalized styrenic polymer. Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) indicated partial miscibility of the two polymers and Fourier transform infrared (FTIR) spectroscopy demonstrated that interactions occurred on a molecular level. When compared with blends of PSVP and the free acid derivative of CTB, the compositions based on the transition metal complex had improved dimensional stability at elevated temperatures, though there remains some question as to the stability of the copper salt to hydrolysis. Electron spin resonance (ESR) spectroscopy showed that only the... 

The cationic tantalum dihydride Cp2(CO)Ta(H)2]+ reacts at room temperature with acetone to generate the alcohol complex [Cp2(C0)Ta(H01Pr)]+, which was isolated and characterized [45]. The mechanism appears to involve protonation of the ketone by the dihydride, followed by hydride transfer from the neutral hydride. The OH of the coordinated alcohol in the cationic tantalum alcohol complex can be deprotonated to produce the tantalum alkoxide complex [Cp2(C0)Ta(01Pr)]. Attempts to make the reaction catalytic by carrying out the reaction under H2 at 60 °C were unsuccessful. The strong bond between oxygen and an early transition metal such as Ta appears to preclude catalytic reactivity in this example. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

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