Cobalt radical anions

Table 4.5 E Electron spin densities in cobalt(o) radical anions...

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

For instance, the (pentaphenylcyclopentadienyl) cobalt dicarbonyl anion-radical complex [(q-C5Ph5)Co(CO)2] has (n + 1) metal orbital populated with an unpaired electron, according to calculations by Connelly et al. (1986). In contrast, reduction of (bpy)Cr(CO)4 (bpy = 2,2 -bipyridyl) to its anion radical is known to occur without any major change in its structure or composition. 

Another possible precursor to conduct free radical reactions is the glycosyl-cobait(III) dimethylglyoximato complex 33 [22,23], These organometallic compounds can readily be prepared by the displacement of the halide atom in 17 with the highly nucleophilic cobalt(I) anion 32. The latter can be generated from the dimeric Co(II) complex 31 under reducing conditions. 

We have mentioned that the structural parameters of C2H4 bridged compounds can vary over a wide range. Whereas most examples reported do not have metal-metal bonds, there is one conspicuous exception. Theopold and Bergman succeeded in synthesizing the propane-1,3-d iyl cobalt derivative 125 from the radical anion [(t) ,-C5H5)Co(/z-CO)12 and 1,3-dibromopropane (98, 295) in 40 5 yield. This compound is best described as a dimetallacyclopentane, and its chemistry (thermolysis and reaction with CO and phosphines Scheme 34) supports this view. Formation of cyclopropane (100°C or I2/25°C) is probably the most remarkable feature of this cyclic system. Simple C—C bond formation has never been observed before in ligand-induced or thermal reactions of either mono- or binuclear cyclopentadienylcobalt complexes. The architectural details of... 

The cyclization reactions of organocobalt complexes are very useful, and they offer an excellent alternative to the tin hydride method when reduced products are not desired. Most cobalt cyclizations have been conducted with nucleophilic radicals. Precursors are prepared by alkylation of cobalt(I) anions, and are usually (but not always) isolated. One suspects that alkylcobalt precursors should be useful for slow cyclizations because there are no rapid competing reactions that would consume the initial radical (coupling of the initial radical with cobalt(II) regenerates the starting complex). 

The room-temperature chemistry of high-surface-area CoO-MgO solid solutions is dominated by the adsorption of CO on edges and steps. Co-ordinatively unsaturated Co2+ and O2 ions react primarily as 02 Co2+02 triplets with formation of [(CCh CoCO]2- species. In samples with high Co contents, the large amounts of clustered cobalt guest species are easily reduced by CO, even at room temperature, with formation of Co(CO)4 and carbonate-like species (377). The formation of polymeric radical anions of CO on high-surface-area CoO-MgO solid solution has also been reported (378). 

It is important to note that even certain phase-transfer catalysts can be carbonylated to carboxylic acids, not by cobalt tetracarbonyl anion catalysis, but by acetylcobalt tetracarbonyl. This is a slow but high-yield reaction that occurs for quaternary ammonium salts that are capable of forming reasonably stable free radicals. For example, phenylacetic acid is formed in 95% yield from benzyltriethylammonium chloride (benzyl radi-... 

The complex [Co3(/A3-CPh)2Cp3] (76) (E = E = CPh) is reduced by potassium metal to a radical anion whose ESR spectrum suggests a symmetrical structure with a half-filled orbital constructed from cobalt 3d atomic orbitals. The cation [76]" (E = E = CPh), detected by cyclic voltammetry ( = 0.34 V) and prepared by electrolytic oxidation, has the unpaired electron in a degenerate orbital, and a structural Jahn-Teller distortion is again expected 189). 

Notably, when the same cyclisation was carried out using sodium cobalt(I)salophen, the reaction became selective for toddaquinoline methyl ether <00TL6681>. This apparent diehotomy was attributed to the formation of a Lewis acid - Lewis base complex between cobalt(II)salophen and the pyridine moiety. Loss of bromide from the radical anion 151 generates aryl radieal 152 which adds to the proximal pyridine giving 153. Dehydrocobaltation to toddaquinoline methyl ether 149 completes the sequence (Scheme 42). Notably, as the pyridine ring is activated by complexation to the Lewis acidic Co(II), the eyelisation is more akin to a Minisci reaction. Consequently, cyclisation to C6 is promoted in this case <01T4447>. 

In reductive acylation and dimerization, the cathode is often superior to dissolving metal or radical anions reductants. So a, j6-unsaturated ketones or esters can be acylated in high yield to 1,4-dicarbonyl compounds at the mercury cathode [39], but the corresponding reaction with sodium in tetrahydrofuran (THE) fails [40]. On the other hand, reductive acylation of double bonds becomes possible in high yield, when vitamin Bj2 is used as mediator [41]. Here cobalt-alkyl complexes play a decisive role as intermediates. 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

In contrast to the cobalt-based system, small amounts of H2 and no CO are produced when nickel cyclam or other saturated 14-membered tetraazamacrocycles (L) in Figure 3 are used to replace the cobalt complex in the above system [22]. Flash photolysis studies indicate that the electron-transfer rate constant (kn) for the reaction of the />-terphenyl radical anion with Nil (cyclam)2 is 4.3 x 10 M s. However, when CO2 is added to the solution, the decay of the TP anion becomes slower Flash photolysis studies of the acetonitrile solutions... 

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