Cyanocobaltates

Kobaltozyankalium, n. potassium cobalto cyanide, potassium cyanocobaltate(II). 

Alkali metal boratabenzenes may be liberated from bis (boratabenzene) cobalt complexes 7 and 13 by reductive degradation with elemental Li, sodium amalgam, or Na/K alloy (60), or alternatively by degradation with cyanides (61). The latter method has been developed in detail (Scheme 4). It produces spectroscopically pure ( H-NMR control) solutions of the products 26 the excess alkali metal cyanide and the undefined cyanocobalt compounds produced are essentially insoluble in acetonitrile. 

Atom Transfer Atom Transfer (AT) takes place typically in the case of d7 complexes, which abstract the halogen atom from RX. The radical formed combines then with a second metal [193, 194]. A classical example of this mechanism is the hydrodehalogenation with cyanocobaltates(II) (see Section 18.2.1) [8, 9], but an analogous pathway was suggested recently for the Co(II) corrin-catalyzed dechlorination of CC14 in the presence of S2 /cysteine as reductant (Eqs. (11)—(12))... 

For practical hydrogenation of olefins four classes of metal complexes are preferred (a) Rh complexes, the RhCl(PPh3)3, the so-called Wilkinson catalyst and the [Rh(diene)-(PR3)2]+ complexes, (b) a mixture of Pt and Sn chlorides, (c) anionic cyanocobalt complexes and (d) Ziegler catalysts, prepared from a transition metal salt and an alkylaluminum compound. 

The protonation of the a-allylic cyanocobaltate complexes has been reported by Kwiatek and Seyler 50) to proceed with the liberation of the corresponding olefin. Thus the complex prepared from butadiene [Eq. (35)] on treatment with aqueous HCl liberates 1-butene. The carbonium ion which probably forms first can cleave directly to 1-butene or it may first rearrange to a Tr-olefin complex, from which the olefin is then displaced with either HgO or chloride ... 

The formal potentials of solid hexa-cyanometalates can also be correlated with the lattice constants, that is, with a parameter that depends mainly on the radii of the two metal ions forming the framework of the compounds. This theoretically derived dependence can be verified when the nitrogen coordinated metal ion and the inserting metal ions are kept constant and the carbon coordinated metal ions are varied, for example, when hexacyanoferrate, hexa-cyanocobaltate, hexacyanomanganate, and so on are compared [53, 55]. The equation... 

T he absorption of molecular hydrogen by aqueous solutions of cyanocobaltate(II) was first reported by Iguchi in 1942 (5). Since then, several groups of workers have sought to determine the nature of the activating species and the product of its hydrogenation. 

Mills, Weller, and Wheeler suggested (12) that the final product obtained via either heterolytic or homolytic cleavage of hydrogen was the pentacyano-cobaltate(I) anion. They observed that cyanocobaltate(II) solutions lost a por-... 

Reduction with Deuterium. Cyanocobaltate(II) (42.6 ml. of solution, 0.15M cobalt, CN/Co = 5.1) was formed in an atmosphere containing equimolar quantities of deuterium and butadiene and stirred for 15 minutes, at which time a sample of the atmosphere was taken for analysis, trans-2-Butene (26%), cis-2-butene (0.51%), ana 1-butene (4.1%) as well as unreacted butadiene (53%) were separated by vapor phase chromatography and each fraction was submitted for mass spectrographic analysis. The presence of di-, mono-, and nondeuterated species was detected in each butene fraction, while the butadiene was shown to contain small quantities of mono- and dideutero species. 

Reduction of H202. Cyanocobaltate(II) was formed in a hydrogen atmosphere, 200 ml. of solution (0.15M cobalt, CN/Co = 5.1, 0.45M KOH) absorbing 272 ml. of H2. A solution of 30% hydrogen peroxide (Fisher reagent) was injected incrementally and hydrogen absorption noted as follows ... 

Reduction of K3Fe(CN)6. An aqueous solution of potassium ferricyanide was injected into a cyanocobaltate (II) solution similar to that described in the previous example but containing NaOH rather than KOH and initially absorbing 323 ml. of H2. The following hydrogen absorptions were obtained ... 

Reduction of Nitrobenzenes. Nitrobenzene was injected in small increments into 200 ml. of prehydrogenated (258 ml. of H2) cyanocobaltate(II) solution (0.15M cobalt, GN/Co = 5.1). After an induction period of approximately 4 minutes, hydrogen absorption commenced ... 

The reduction of acrylic acid was attempted at elevated temperatures. Surprisingly, the reaction was found to yield not only propionic acid, but also the dimer, a-methylglutaric acid. When the reaction was conducted in the absence of hydrogen, the product obtained was 3-methylglutaconic acid, which apparently is the precursor of the saturated dimer formed in a hydrogen atmosphere. Similarly, methacrylic acid yielded a-methylene-y,y-dimethylglutaric acid when heated with cyanocobaltate (II) in the absence of hydrogen. Its structure was established via ozonolysis. Similar dimerizations have been reported for acrylic acid (I, 14), methacrylate ester (7, 11), crotonic acid (13), and its diethylamide (15). 

Absorption of Butadiene by Cyanocobaltate(II). Of the various substrates reduced by this catalyst system, butadiene was especially convenient for use as a model substrate in a study of mechanism, since its absorption, as well as desorption of product butenes, could be readily followed using a gas buret, and the products formed were easily analyzed by vapor phase chromatography. 

The formation of cyanocobaltate(II) solutions in a butadiene atmosphere resulted in absorption of the gas in varying quantities, depending on the cyanide-cobalt ratio employed (Figure 2). Maximal absorptions were observed at CN/Co values of 3.5 and 6.0 (C4H6/Co = 0.34 at the latter ratio), the absorptions being... 

Since the aging reaction of cyanocobaltate(II) results in the formation of hydrido complex, the question arises as to which cobalt species is involved in the absorption of butadiene. If the hydride is the reactive species, absorption would be expected to increase with time. In Figure 3 it may be seen that the absorption of butadiene by cyanocobaltate(II) does increase with time in a manner paralleling the decrease in hydrogen absorption capacity (12). 

Reactions with Hydrido Complex. Upon injection of a prehydrogenated cyanocobaltate(II) solution (0.15M cobalt, CN/Co = 6.0) into an atmosphere of butadiene, the gas was rapidly absorbed, 0.92 mole of butadiene being taken up for each hydrogen atom previously absorbed. Similarly, when the injection was made into a butadiene-saturated cyanocobaltate(II) solution in a butadiene atmosphere, 1.08 moles of butadiene were absorbed. These results provide evidence of the addition of butadiene to the hydrido complex in the following manner ... 

Proposed Mechanism for Butadiene Reduction. The above results are compatible with the reaction sequence illustrated below. In the absence of a hydrogen atmosphere, CoH, formed via the aging reaction of cyanocobaltate(II), reacts reversibly with butadiene to yield Co(C4H7) which reacts further with CoH and/ or undergoes hydrolysis to yield butenes. The over-all result is oxidation of cyano-cobaltate(II) to cyanocobaltate(III) with concomitant reduction of butadiene to butenes. 

In the presence of a hydrogen atmosphere, CoH is formed mainly by reaction of cyanocobaltate(II) with hydrogen, and the role of hydrolysis, if such exists in... 

It is possible that a small portion of the hydroxo complex is also formed by the reaction of pentacyanocobaltate(II) with hydrogen peroxide, which is known to be almost quantitative (4). No cyanocobaltate(III) species is known to activate hydrogen, and we have observed that the addition of hexacyanocobaltate(III) to CoH (H2 atmosphere) does not result in absorption of hydrogen. 

Equation 7 shows the interaction of ferricyanide and cobaltocyanide to form a binuclear complex as described by Haim and Wilmarth (4). It is probable that the hydrogen evolution noted occurs via displacement of the equilibrium shown in Equation 6. Equation 8 defines the role of alkali, the presence of which is required to effect the catalytic reduction of ferricyanide. The hydroxo complex so obtained may then undergo the reverse aging process shown in Equation 5 to reform cyanocobaltate(II), which then absorbs hydrogen. The over-all result is reduction of ferri- to ferrocyanide by hydrogen. 

Benzoquinone. The formation of cyanocobaltate(II) in a hydrogen atmosphere and in the presence of excess benzoquinone resulted in the absorption of only 40% of that amount of hydrogen normally taken up in the formation of CoH, and the substrate was not catalytically reduced addition of alkali did not activate the system. When excess benzoquinone was added to CoH (H2 atmosphere), hydrogen was not absorbed and, again, alkali did not activate the system. The addition of less than stoichiometric quantities of substrate also resulted in no hydrogen absorption. 

However, when small increments of substrate were added to CoH containing added alkali (KOH, 3X cobalt concentration), 1.4 atoms of hydrogen were absorbed per mole of quinone. This effect of alkali is similar to that noted in the reduction of ferricyanide. However, with benzoquinone, the addition of excess substrate to CoH containing added alkali still resulted in the absorption of hydrogen, the hydrogen atom to substrate ratio being reduced to 0.98. Furthermore, the presence of excess quinone during the formation of cyanocobaltate(II) with added alkali did not prevent catalytic reduction. 

Equation 9 indicates the addition of benzoquinone to CoH to form a new complex which cannot react further with CoH. Equation 10 defines the role of excess alkali in effecting the catalytic reduction of benzoquinone. As shown in previous examples, the hydroxo complex may then undergo the reverse aging process, leading to hydrogen absorption. The over-all result is reduction of benzoquinone to hydroquinone when limited amounts of substrate are available, and to quinhydrone when excess substrate is available. Equation 11 is an attempt to explain the lowered amount of hydrogen absorption noted when cyanocobaltate(II) is prepared in the presence of excess benzoquinone. Displacement of reduced substrate from this binuclear complex by alkali is assumed, since quinone was catalyti-cally reduced when the above procedure was carried out in the presence of added alkali. 

As was the case with benzoquinone, the presence of excess substrate during the formation of cyanocobaltate(II) with added alkali did not prevent catalytic reduction, while addition of alkali to CoH-substrate(excess) did not activate the system. 

Nitrobenzene. Observations made on the formation of cyanocobaltate(II) in the presence of excess nitrobenzene, and on the addition of an excess of this substrate to the prehydrogenated complex, were identical to those made with benzoquinone as the substrate. However, a difference was noted when less than stoichiometric quantities of nitrobenzene were added. After a short induction period of approximately 4 minutes, hydrogen absorption commenced, 3.3 atoms of hydrogen being absorbed per mole of substrate (no absorption occurred with benzoquinone in the absence of added alkali). Further additions of small incre-... 

Since it was observed that absorption ceased after 3.3 atoms of hydrogen were taken up per mole of nitrobenzene, Equation 14 is shown as producing 4 moles of cyanocobaltate(II) per mole of substrate via reaction of the latter with CoH. Since further absorption of hydrogen occurred only upon introduction of alkali, it is implied that an intermediate complex, X, is formed which is not subject to further reaction with CoH but may be decomposed by alkali. The stoichiometry of this equation requires formulation of complex X as [Co(CN)5(C6H5NH)]—3. However, since absorption ceased after two atoms of hydrogen had been absorbed per atom of cobalt present, it is implied that a binuclear complex is formed, perhaps involving phenylhydroxylamine, azobenzene, or some other reduction intermediate. 

Equation 14 actually represents the result of several consecutive reactions involving additions of CoH to nitrobenzene and intermediates such as nitroso-benzene to form complexes subject to further interaction with CoH to yield reduction products in a manner similar to that postulated for the hydrogenation of butadiene (see Equations 1 and 3). Equation 15 defines the role of alkali whereby reduction products are released and the hydroxo complex so formed is able to undergo the reverse aging process as discussed in other examples. Equation 16 is similar to that shown for benzoquinone (Equation 11) and indicates a possible interaction of the substrate with nonhydrogenated cyanocobaltate(II). 

In addition to conjugated dienes, cyanocobalt catalysts also hydrogenate the C=C bond of activated alkenes.52 Carvene, mesityl oxide, 2-cyclohexenone, benzalacetone and an androstenone derivative were reduced in this way.53... 

In early studies, a catalyst solution believed to contain a cyanocobalt(II)-chiral amine complex was prepared (Fig. 22). The chiral amines (-)-(/ )-l, 2-propanediamine (Pn) or (+)-(S)-N, N -dimethyl-1,2-propanediamine (diMPn) were used. It was suggested that the catalytically active species might resemble a previously characterized compound, p-ethylenediaminebis[tetracyanocobaltate-(II)] (compound VI). Whatever the precise structure of the active species, the catalyst solution did effect the asymmetric reduction of atropic acid, but with low asymmetric induction (Fig. 23). 

FIG. 22. Preparation of a cyanocobalt(II)-chiral diamine catalyst solution. The cobalt species in solution may resemble the ethylenediamine complex (VI). 

FIG. 23. Asymmetric hydrogenation with cyanocobalt(II)-chiral diamine solutions. Pn = 1,2-propanediamine diMPn = 7V,./V-dimethyl-l,2-propanediamine % ee = percent enantiomeric excess. 

Place a few drops of the neutral (litmus) test solution upon potassium hexa-cyanocobaltate(III) (or cobalticyanide, Rinmann s green) test paper. Dry the paper over a flame and ignite in a small crucible. Observe the colour of the ash against a white background part of it will be green. 

It has been found quite recently that the isomerization to carbonyl compounds of oxiranes containing a rr-electron system is catalyzed by certain metal complexes. The experimental data acquired so far suggest that only the penta-cyanocobalt complexes are active towards the isomerization of aliphatic and alicyclic oxiranes. ... 

Among the complexes which may function in this way are pentacyano-cobaltate ion, iron pentacarbonyl, the platinum-tin complex, and iridium and rhodium carbonyl phosphines. It has been suggested that with tristriphenylphosphine Rh(I) chloride, a dihydride is formed and that concerted addition of the two hydrogen atoms to the coordinated olefin occurs (16). There are few examples of the homogeneous reduction of other functional groups besides C=C, C=C, and C=C—C=C penta-cyanocobaltate incidentally is specific in reducing diolefins to monoolefins. 

Nitriles. a-HALONiTRiLES. Considering the results obtained with alkyl iodides, it was assumed that the reaction of a-halonitriles with penta-cyanocobaltate(II) would result in halogen abstraction to form a resonance-stabilized radical. The fate of the radical would be expected to depend on the structure of the particular nitrile used. The products obtained from six a-halonitriles [a-chloroacetonitrile previously reported (22)] are shown in Table VII. 

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