Tungsten complexes arene

Molybdenum and tungsten atoms seem to react with alkylbenzenes more efficiently than chromium atoms yields of 30 to 50% are reported (113). Conventional routes to the synthesis of tungsten-arene complexes are difficult and inefficient so that the ability to prepare these compounds in high yield via tungsten atoms is of special significance. Unfortunately, tungsten has a very high vaporization temperature and the scale of work with its vapor is necessarily limited. 

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

Some examples have been reported of tethered r 6-arene complexes of molyb-denum(II) and tungsten(II) that incorporate oxygen as the auxiliary donor atom.37 40 Reaction of neat 2,6-diphenylphenol with [MH4(PMePh2)4] (M = Mo, W) at 150 °C gives the tethered hydrido(aryloxo)metal(II) complexes 30 and 31.36 A metal-hydride bond is probably first cleaved by the phenolic oxygen with loss of H2 coordination of the pendant arene then induces loss of another equivalent of H2. 

A variety of transition metals, for example, chromium, molybdenum, tungsten, iron, vanadium, manganese, and rhodium can be used to prepare relatively stable j -arene complexes (see Arene Complexes). Reactions of j -arene chromium tricarbonyl complexes have been extensively examined, and numerous reviews are available. Although chromium complexes are by far the most utilized in organic synthesis, complexes of iron and manganese are emerging as potentially useful alternatives. 

There are several classic examples of the use of FTIR spectroelectrochemistry in elucidating the EC reactions of oxidized carbonyl complexes. These include the isomerization of 17e complexes for example, the isomerization of m-[Mo(CO)2(P-P)2]+ to the trans-isomer.139 Similarly, the cis-isomer of [Re(CO)2(P P)2]+ or [Re(CO)(P—P)2X] will isomerize on oxidation as monitored in a reflection IR cell.140 One-electron oxidation of [IrH(CO)(PPh3)3] is reversible, but further oxidation to the dication induces hydride oxidation and the appearance of bands due to the 16e complex [Ir(CO)(PPh3)3]+.141 Oxidation of arene tricarbonyls of Group 6 metals is frequently irreversible, especially in coordinating solvents at ambient temperature. However, the mesitylene tungsten tricarbonyl complex is oxidized by two electrons with the reversible take up of MeCN.142... 

Radius, U. Attner. J. Dinuclear molybdenum(III) and tungsten(III) calix[4]arene complexes — Metal-metal triple 27. bonds supported by bridging calix[4]arene ligands. Eur. J. 

Arene complexes are usually prepared by the displacement of more weakly bound, monodentate ligands, by oxidation or evolution of CO ligands, or by reducing a metal from a higher oxidation state in the presence of an arene. These methods have been reviewed. - In some cases, one arene will displace another. For example, hexamethylbenzene replaces p-diisopropyl benzene in the tungsten complex of Equation 2.13. This reaction is imdoubt-edly controlled by steric effects. The mechanisms of these arene exchange reactions have been clarified by Traylor and are discussed in Section 4.55. 

T -Arene complexes containing late metals have been known for many years, but the scope and utility of these complexes have increased in recent years. Copper(I) and silver form labile arene complexes of various stoichiometries that are apparently T -arene complexes. A few of these complexes have been structurally characterized. More recently, a large number of V-arene and heteroarene complexes of osmium, rhenium, molybdenum, and tungsten have been prepared for the purpose of dearomatization of the arene or heteroarenes. Two examples are shown in Figure 2.33. This dearomatization creates a diene or vinyl unit that imdergoes the organic chemistry of ttiese isolated units, instead of the chemistry of an arene. n -Arene complexes of rhodium and platinum have been characterized structurally and studied in the context of their likely intermediacy in the oxidative addition of arene C-H bonds. ... 

Chromium, Molybdenum and Tungsten.- The use of metal vapour syntheses continues to dominate the reports of new arene complexes including new complexes of Cr, W and Re with benzene and... 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Methyl benzoate, anisole, and diphenyl ether each give sandwich compounds with chromium vapor, although in rather low yield (32, 55, 110). Chromium appears to attack alkyl ethers and this deoxygenation probably competes with complexation with the aromatic oxygen compounds. No simple product has been isolated from chromium atoms and aniline, but bis(7V,7V-dimethylaniline)chromium has been prepared (32). The behavior of molybdenum and tungsten vapors closely resembles that of chromium in reactions with oxygen- and nitrogen-substituted arenes (113). 

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