Arene complexes of chromium

Unlike the ferrocenes, the arene complexes of chromium, in particular the arenechromi-umtricarbonyls, have seen much less use in asymmetric catalysis. This is beginning to change, however , and a number of synthetic transformations of arenechromiumtricar-bonyls owe their existence to the formation of planar chiral chromium complexes by... 

The advantage of using lithiated arene complexes of chromium as starting materials to synthesize o-,7r-bimetallic complexes of chromium and manganese was clearly demonstrated by reacting (77 -o-C6H4XLi)Cr(CO)3 (X = H, F) with Mn(CO),Br (305). The products, (77< -C6H4XC(0) Mn-... 

With the exception of the labile complexes of condensed aromatics, the metal-arene bond in (arene)Mn(CO)3 complexes is very robust. It is more difficult to cleave than that in the neutral arene complexes of chromium and molybdenum and requires strongly oxidative conditions such as DDQ in refluxing acetonitrile. 

The tendency for oxidation of arene complexes of chromium group metals M(arene)2 decreases in the series W Mo>Cr. The chromium compounds are conveniently oxidized to the Cr(arene) cations by air in the presence of water. 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Complexes of Cr, W, Mo, Fe, Ru, V, Mn and Rh form stable, isolable arene if -complexes. Among them, arene complexes of Cr(CO)3 have high synthetic uses. When benzene is refluxed with Cr(CO)6 in a mixture of dibutyl ether and THF, three coordinated CO molecules are displaced with six-7r-electrons of benzene to form the stable i/fi-benzene chromium tricarbonyl complex (170) which satisfies the 18-electron rule (6 from benzene + 6 from Cr(0) + 6 from 3 CO = 18). Complex formation is facilitated by electron-donating groups on benzene, and no complex of nitrobenzene is formed. Complex formation has a profound effect on reactivity of arenes, and the resulting complexes are used in synthetic reactions. The metal-free reaction products can be isolated easily after decomplexation by mild oxidation using low-valent Cr. Cycloheptatriene also forms a stable complex with Cr(CO)3 and its synthetic applications are discussed below. 

At present, there are only NMR data available for 7r-arene and TT-cycloheptatriene complexes of chromium, molybdenum, and tungsten (80,158,161) (see Table XXV). The chemical shifts of the complexed... 

Arenes form ri -complexes with a number of transition metals (e.g. Cr, Mo, W, Fe). Complexes of chromium have found widespread application because of their ease of synthesis, stability, easy removal of the ligands and usefulness in synthesis. 

Well-defined arene complexes of Group 4 metals in various oxidation states have been isolated. The air- and moisture-sensitive complexes Ti(r -arene)2 (56) have a sandwich structure similar to that of the related chromium compounds [176-178]. They have been used for deoxygenation of propylene oxide and coupling reaction of organic carbonyl compounds [179]. The first synthesis of 56 was cocondensation of metal vapor with arene matrix [176]. Two more convenient methods are reduction of TiCl4 with K[BEt3H] in arene solvent [180] and reaction of TiCl4(THF)2 with arene anions followed by treatment with iodine [170,176]. The latter method involves the formation of an anionic titanate complex, [Ti(ri -arene)2] (57), which can also be formed from KH and 56 [181]. 

Hexacarbonylchromium(O) is readily attacked by chlorine giving CrCR, CO, and COCI2. Bromine and iodine do not attack Cr(CO)e to any substantial extent at room temperature. The chromium tricarbonyl arene complexes, Cr(CO)z(r] -arene), are readily oxidized at room temperature by I2 to give Crlg this is a convenient preparative method for the anhydrous iodide. Although the oxidation of the tricarbonyl arene derivatives of chromium with I2 does not show evidence of intermediate carbonyl complexes in oxidation states >0, the corresponding molybdenum(O) compounds give a series of carbonyl iodides of molybdenum(II), for example, the ionic [Mo(CO)3( ) -arene)]I. 

Once it is recognized that cyclohexadienyl anionic complexes of chromium (41) can be generated by addition of sufficiently reactive nucleophiles and that simple oxidizing techniques convert the anionic intermediates to free substituted arenes, a general substitution process becomes available which does not depend on a specific leaving group on the arene [2]. The process is general for carban-ions derived from carbon acids with pK >22 or so only one example of a heteroatom nucleophile is reported [102]. 

The condensation of metal vapors with hexafluorobenzene and benzene gives unstable arene complexes of V, Cr, Mn, Fe, Co, Ni, and Pd, for instance, [Cr(C6H6)(C5F6)]. The condensation of vapors of chromium or iron with the mixture of vapors of benzene and PF3 or hexafluorobenzene and PF3 leads to the formation, with low yields, of the derivatives [Cr(C6F6)(PF3)3], [Cr(PhH)(PF3)3], and [Fe(PhH)(PF3)2]. Heteroleptic compounds of chromium, for example, [ (CgF ) (Pp3)3], are stable in contrast to [Cr(C6F6)2] which is unstable and explosive. The condensation method was also utilized for the preparation of hexahapto pyridine complexes of chromium. 

The reversible oxidation-reduction reaction Ar2Cr Ar2Cr" is characteristic of bis arene x-complexes of chromium. This property of chromium... 

Arene complexes of many of the (/-block transition metals may be prepared similarly. As with chromium, the initial products of the reducing Friedel-Crafts procedure are usually cationic the neutral bis-w-arene complexes are often obtained from them by reduction or by hydrolytic disproportionation. 

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