Silicon complexes with molybdenum

D. L. DuBois and J. A. Turner, Preparation and spectroelectrochemical characterization of silicon electrodes modified with molybdenum dinitrogen complexes, J. Am. Chem. Soc. 104, 4989, 1982. 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

The high-chromium irons undoubtedly owe their corrosion-resistant properties to the development on the surface of the alloys of an impervious and highly tenacious film, probably consisting of a complex mixture of chromium and iron oxides. Since the chromium oxide will be derived from the chromium present in the matrix and not from that combined with the carbide, it follows that a stainless iron will be produced only when an adequate excess (probably not less than 12% of chromium over the amount required to form carbides is present. It is commonly held, and with some theoretical backing, that carbon combines with ten times its own weight of chromium to produce carbides. It has been said that an increase in the silicon content increases the corrosion resistance of the iron this result is probably achieved because the silicon refines the carbides and so aids the development of a more continuous oxide film over the metal surface. It seems likely that the addition of molybdenum has a similar effect, although it is possible that the molybdenum displaces some chromium from combination with the carbon and therefore increases the chromium content of the ferrite. 

Attempts made to produce an alloy more resistant to hydrochloric acid have resulted in alloys containing 17-18% silicon or 14-5% silicon and chromium plus 3% molybdenum. The first is produced in Britain, and the second in the United States. The reason for the increase in resistance to hydrochloric acid of the Fe-18 Si alloy is thought to lie primarily in the increased density of the silica-rich film left on the metal by initial corrosion. The addition of 6% chromium with some molybdenum to Fe-14-5 Si causes the formation of extremely stable complex carbides with the consequent complete elimination of graphite plus the formation of a more penetration-resistant silica film, probably containing chromium in substantial quantity. 

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10... 

There are two groups of triarylcarbonium pigments inner salts of triphenyl-methane sulfonic acids, and complex salts with heteropolyacids containing phosphorus, tungsten, molybdenum, silicon, or iron. 

Many metal silyl complexes, especially those with early transition metals, have weak metal-silicon bonds. The molybdocene silyl hydride complexes seem to have molybdenum-silicon bonds of sufficient strength to withstand nucleophilic attack by strong hydride reagents (Eq. 4)... 

B. Semmens and A. B. Meggy, The reaction of kaolin with fluorides, J. Appl. Chem. 16, 122, 1966. D. L. DuBois, Modification of silicon and carbon electrodes with methacrylamide polymers containing covalently attached molybdenum dinitrogen complexes, Inorg. Chem. 23, 2047, 1984. 

Soluble monosilicic acid reacts with molybdic acid at pH 1-2, in the presence of at least a 0.05 M excess of molybdenum, to form the yellow soluble (l-molybdosilicic acid. The yellow colour is the basis of a rather insensitive spectrophotometric method for silicon [12-15]. The absorption maximum of the complex is in the ultraviolet. At 400 nm, the molar absorptivity is 2.2-10 (a - 0.08) (in the presence of acetone). 

Tungsten resembles molybdenum in showing a remarkable ability to form complex compounds. One molecule of an alkali oxide may be combined with 1, 2, 3, 4, 5, 6, or 8 molecules of WO3 while more complex molecules may contain as much as 5Mr20 condensed with varying amounts of WOa. There are also formed many series of complex tungstates in which W03 combines with varying proportions of the oxides of silicon, phosphorus, arsenic, antimony, vanadium, and boron,... 

We must however keep in mind that some of the above reactions may not be simple reactions at the silicon atom, since transition metal complexes show multicenter reactivity (metal atom, ligands) as exemplified in the chemistry of triphenylgermyl-carbene complexes of cobalt carbonyl (253). Thus, displacements of a silyl ligand may result from a multistep process and a thorough examination of these reactions has to be made. An example can be drawn from molybdenum-germanium chemistry (247). As shown in Scheme 59, germanium is displaced from complex 167 by HO with retention of configuration. Actually,... 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

Trichloro(tripyridine)chromium(III), synthesis 36 Trichloro (tripyridine) molybdenum (III), synthesis 39 Potassium octacyanotungstate(IV) 2-hydrate, synthesis 40 Chlorine(CH )-labeled thionyl chloride, silicon tetrachloride, boron chloride, germanium(IV) chloride, and phosphorus-(III) chloride, synthesis 44 Unipositive halogen complexes, synthesis 46 Monopyridineiodine(I) chloride, synthesis 47 Tris(3-nitroacetylacetonato)cobalt(III), synthesis 55 Inner complexes of cobalt(III) with diethylenetriamine, synthesis 56... 

---