Cyanocobaltate complexes

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 ... 

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. ... 

The isomer distribution of the butene product obtained from butadiene is dependent on the cyanide-to-cobalt ratio employed in formation of the catalyst (4). Thus, at low ratios, as much as 86%trans-butene-2 has been obtained, while at high ratios, 87%butene-1 has been found, the greatest change in isomer distribution occurring between cyanide-to-cobalt ratios of 5,5 and 6,0, This effect has been ascribed (4) to the possible intermediacy of a 1-methyl-TT-allyl cyanocobaltate complex in reductions carried out at low cyanide-to-cobalt ratios, and a sigma-bonded methylallyl cyanocobaltate complex at high ratios. Evidence for such intermediates, as well as the relationship of diene and ally lie halide reductions, is now presented. 

The PMR spectra of the o - and tt -allyl complexes correspond very well with the spectra of the corresponding manganese and cobalt carbonyl complexes (9). Although the exact location of the tt -allyl group with respect to the metal is not known, the reaction with cyanide ion indicates that the TT -allyl group may be considered to be bidentate, a conclusion in full accord with the displacement of carbon monoxide in the conversion of a -to-tt-allyl cobalt and manganese carbonyls (9), and with the coordination of dimethyl-sulfoxide in the conversion of 7r-to-o -allyl palladium chloride (10). Structure(I) is tentatively proposed for the tt -allyl cyanocobaltate complex. 

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))... 

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 ... 

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. 

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. 

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 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). 

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. 

Inorganic substrates that are reduced by the [Co(CN)5] /H2 system include H2O2, O2, Sg, halogens, FeCCN) , Mn04", 8263 , Cr207 , N02, NHjOH, and penta-cyanocobaltate(III) complexes . The [Co(CN)5] hydrogenation system is poisoned by excess O2 however, the poisoning is reversible if the O2 is removed. ... 

The complex obtained from the reaction of phenacyl bromide with penta-cyanocobaltate(II) is assigned structure(II), a Tt-oxaallyl type (11), on the basis of absence of carbonyl absorption (IR) and vinylidene hydrogen (NMR) acidification releases acetophenone. Evidence is being sought for the possible intermediacy of a ir -homoallyl complex (structure III) in the reduction of bicyclo[2,2, l]heptadiene. 

See the general references in the Introdnction, specifically [116], [121] and [313], and some more-specialized books [2-5], Some articles in journals discuss DF theory for [Rli6(PH3)6Hm] , m = 12, 14 or 16 [6] reductions of Co by metallic ions [7] iridium [8] mononuclear cyanocobalt(lll) complexes [9] Ii chloro and bromo species [10] and metal-metal bonding in Rh [11],... 

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