Platinum , coupling

The reaction does not appear to be satisfactory with aliphatic aldehydes and also with some aryl aldehydes. Like platinum, coupled with various substances, palladium is also used to catalyse several organic reactions. One such combination is known as Lindlar s catalyst. 

In review problem 8.7 we saw two examples of one-bond phosphorus-platinum coupling constant of 2380 and 4640 Hz. These are exceptionally large J values. The one-bond coupling constants most commonly encountered in NMR are iCH, iCD, yCF, yCP, and yPH. In fact, we saw examples of some of these in Section 8.6, Example 8.19, and problems 8.1, 8.4, 8.5, and 8.6. These J values are generally positive and involve nuclei with positive y values. 

The valnes of the carbon-platinum coupling constants /pt c decrease along the sequence Cl > Br > I, which may be explained by a corresponding decrease of the Pt O a-bond strength in the same sequence. 

Limbeck, A., Rudolph, E., Hann, S., Koellensperger, G., Stingeder, G., Rendl, J. Flow injection on-line pre-concentration of platinum coupled with electrothermal atomic absorption spectrometry. J. Anal. At. Spectrom. 19, 1474—1478 (2004)... 

Graphite tubes afford an excellent protection to quartz or porcelain tubes on rare-metal couples, and are frequently used with base-metal couples for molten metals. Porcelain encased in a sheath of graphite can be used in molten aluminum. Platinum couples must be thoroughly protected against the vapors distilled from graphite or carbon and from the reducing atmosphere present near heated graphite. 

E. Davy discovered nitrosyl chloride, which he called chloronitrous gas and prepared from aqua regia. He thought it was formed by the combination of equal volumes of nitric oxide and chlorine without condensation , which would not agree with the formula NOCl, and the density (1-759 air = i, or 25 33 for H = i, instead of 32-7) he found was too low. He described an electrolytic method (deposition on platinum foil by a zinc-platinum couple) for the detection of arsenic, mercury, lead, and copper in poisoning, worked on corrosion of iron by sea water and on nicotin , which had been dis-... 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

In many applications tantalum can be substituted for platinum and gold, and there are some environments in which tantalum is more corrosion resistant than platinum. Table 3.37 lists the main chemicals for which tantalum is not a suitable substitute for platinum and, conversely, those for wliich tantalum is better than platinum. Tantalum is rapidly embrittled by nascent hydrogen even at room temperature. Therefore, it is very important to avoid the formation of galvanic couples between tantalum and other metals. 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

In Scheme 10, HMG-CoA reductase inhibitor 92 was synthesized via a Suzuki coupling approach. Hiyama s group also carried out a Hiyama coupling to make the same compound (93TL8263). Vinylsilane 119 was prepared by platinum-catalyzed reaction from terminal alkyne 89. 

Anilines have been reduced successfully over a variety of supported and unsupported metals, including palladium, platinum, rhodium, ruthenium, iridium, (54), cobalt, and nickel. Base metals require high temperatures and pressures (7d), whereas noble metals can be used under much milder conditions. Currently, preferred catalysts in both laboratory or industrial practice are rhodium at lower pressures and ruthenium at higher pressures, for both display high activity and relatively little tendency toward either coupling or hydrogenolysis,... 

Fused salts Molybdenum has excellent resistance to a wide range of fused salts and has been used in the fused salt electrolysis of magnesium, platinum, thorium and uranium. In the production of pure magnesium, molybdenum is used to couple graphite electrodes electrically. Molybdenum cathodes are... 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

It is a valve metal and when made anodic in a chloride-containing solution it forms an anodic oxide film of TiOj (rutile form), that thickens with an increase in voltage up to 8-12 V, when localised film breakdown occurs with subsequent pitting. The TiOj film has a high electrical resistivity, and this coupled with the fact that breakdown can occur at the e.m.f. s produced by the transformer rectifiers used in cathodic protection makes it unsuitable for use as an anode material. Nevertheless, it forms a most valuable substrate for platinum, which may be applied to titanium in the form of a thin coating. The composite anode is characterised by the fact that the titanium exposed at discontinuities is protected by the anodically formed dielectric Ti02 film. Platinised titanium therefore provides an economical method of utilising the inertness and electronic conductivity of platinum on a relatively inexpensive, yet inert substrate. 

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

---